spacer gif spacer gif spacer gif spacer gif spacer gif
QUICK SEARCH:   [advanced]


spacer gif
     Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents    

First published online February 29, 2008
Journal of Experimental Biology 211, 1000-1011 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.015222
This Article
Right arrow Summary Freely available
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Blitz, D. M.
Right arrow Articles by Nusbaum, M. P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Blitz, D. M.
Right arrow Articles by Nusbaum, M. P.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati   Add to Twitter  
What's this?

A newly identified extrinsic input triggers a distinct gastric mill rhythm via activation of modulatory projection neurons

Dawn M. Blitz1, Rachel S. White1, Shari R. Saideman1,*, Aaron Cook1, Andrew E. Christie2,3, Farzan Nadim4,5 and Michael P. Nusbaum1,{dagger}

1 Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA
2 Department of Biology, University of Washington, Seattle, WA 98195-1800, USA
3 Mount Desert Island Biological Laboratory, Salisbury Cove, ME 04672, USA
4 Department of Mathematical Sciences, New Jersey Institute of Technology, Newark, NJ 07102, USA
5 Department of Biological Sciences, Rutgers University, Newark, NJ 07102, USA

{dagger} Author for correspondence (e-mail: nusbaum{at}mail.med.upenn.edu)

Accepted 14 January 2008


   Summary
TOP
 Summary
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Neuronal network flexibility enables animals to respond appropriately to changes in their internal and external states. We are using the isolated crab stomatogastric nervous system to determine how extrinsic inputs contribute to network flexibility. The stomatogastric system includes the well-characterized gastric mill (chewing) and pyloric (filtering of chewed food) motor circuits in the stomatogastric ganglion. Projection neurons with somata in the commissural ganglia (CoGs) regulate these rhythms. Previous work characterized a unique gastric mill rhythm that occurred spontaneously in some preparations, but whose origin remained undetermined. This rhythm includes a distinct protractor phase activity pattern, during which a key gastric mill circuit neuron (LG neuron) and the projection neurons MCN1 and CPN2 fire in a pyloric rhythm-timed activity pattern instead of the tonic firing pattern exhibited by these neurons during previously studied gastric mill rhythms. Here we identify a new extrinsic input, the post-oesophageal commissure (POC) neurons, relatively brief stimulation (30 s) of which triggers a long-lasting (tens of minutes) activation of this novel gastric mill rhythm at least in part via its lasting activation of MCN1 and CPN2. Immunocytochemical and electrophysiological data suggest that the POC neurons excite MCN1 and CPN2 by release of the neuropeptide Cancer borealis tachykinin-related peptide Ia (CabTRP Ia). These data further suggest that the CoG arborization of the POC neurons comprises the previously identified anterior commissural organ (ACO), a CabTRP Ia-containing neurohemal organ. This endocrine organ thus appears to also have paracrine actions, including activation of a novel and lasting gastric mill rhythm.

Key words: neuromodulation, central pattern generator, projection neurons, neuropeptide, Cancer borealis


   INTRODUCTION
TOP
 Summary
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Neuromodulation enables single motor circuits to generate multiple distinct activity patterns by changing the intrinsic and synaptic properties of circuit neurons (Marder et al., 2005Go; LeBeau et al., 2005Go; Kiehn, 2006Go; Gordon and Whelan, 2006Go; Tryba et al., 2006Go). Further flexibility in the output of these motor circuits is afforded by modulatory actions at the level of the projection neurons that drive circuit activity (Di Prisco et al., 2000Go; Beenhakker and Nusbaum, 2004Go; Blitz et al., 2004Go; McLean and Sillar, 2004Go; Brocard et al., 2005Go; Smetana et al., 2007Go). However, the extrinsic inputs that provide these modulatory influences on projection neurons are not well-documented in most systems.

We are using the stomatogastric nervous system (STNS) of the crab Cancer borealis to identify the extrinsic input responsible for the activation of a previously identified version of the gastric mill (chewing) rhythm (Wood et al., 2004Go). The stomatogastric nervous system is an extension of the decapod crustacean CNS that includes the unpaired stomatogastric (STG) and oesophageal (OG) ganglia plus the paired commissural ganglia (CoGs) (Nusbaum and Beenhakker, 2002Go; Marder and Bucher, 2007Go). Overlapping sets of the 26 neurons in the C. borealis STG contribute to the gastric mill and pyloric (filtering of chewed food) rhythms (Marder and Bucher, 2007Go). In C. borealis, these rhythms are regulated by input from no more than 20 projection neurons, most of which are present as single neurons within each CoG (Coleman et al., 1992Go; Nusbaum et al., 2001Go). In addition, extrinsic inputs that convey sensory and other information modify these rhythms by influencing circuit neurons and/or projection neurons (Meyrand et al., 1994Go; Combes et al., 1999Go; Christie et al., 2004Go; Beenhakker and Nusbaum, 2004Go; Blitz et al., 2004Go).

We have identified a novel extrinsic input to the STNS of C. borealis. This input, called the post-oesophageal commissure (POC) neurons, consists of bilateral peptidergic fiber bundles that project through the post-oesophageal commissure (poc) and circumoesophageal connectives (cocs) to innervate the CoGs.

Extracellular poc stimulation drives the POC neurons to trigger a long-lasting activation of CoG projection neurons, which in turn drive the gastric mill rhythm. Two of these projection neurons are modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2) (Coleman and Nusbaum, 1994Go; Norris et al., 1994Go). Interestingly, despite the likely participation of MCN1 and CPN2 in the POC-triggered gastric mill rhythm, the POC-triggered activity pattern of these projection neurons and the associated gastric mill rhythm are distinct from previous versions of this rhythm that are activated by these same two projection neurons (Beenhakker and Nusbaum, 2004Go; Blitz et al., 2004Go). Our data further suggest that the POC excitation of MCN1 and CPN2 is mediated by the neuropeptide transmitter Cancer borealis tachykinin-related peptide Ia (CabTRP Ia). The POC neurons also appear to be the source of the CabTRP Ia-containing anterior commissural organ (ACO), a dense neurohemal structure in the CoG neuropil (Messinger et al., 2005Go).

Some of this work was published previously in abstract form (White et al., 2005Go).


   MATERIALS AND METHODS
TOP
 Summary
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Animals
Male Jonah crabs (Cancer borealis Stimpson) were obtained from Commercial Lobster and Seafood Co., Boston, MA, USA and the Marine Biological Laboratory, Woods Hole, MA, USA. Before experimentation, crabs were housed in commercial tanks containing recirculating, filtered and aerated artificial seawater (10°C). Crabs were cold anesthetized by packing in ice for at least 30 min prior to dissection. The STNS was dissected as described previously (Blitz et al., 2004Go). Briefly, the foregut was first removed and pinned down in a Sylgard 170 (KR Anderson, Morgan Hill, CA, USA, or World Precision Inc., Sarasota, FL, USA)-coated glass bowl in chilled C. borealis saline. The poc was bisected under a dissecting microscope, after which the stomach was bisected ventrally and pinned flat with the interior stomach wall against the Sylgard. The STNS, including all four ganglia (two CoGs, OG, STG) plus their connecting and peripheral nerves (Fig. 1), was next dissected from the surface of the foregut and pinned in a Sylgard 184 (KR Anderson)-coated Petri dish. The foregut and nervous system were maintained in chilled (10–13°C) saline throughout the dissection and subsequent experiment.


Figure 1
View larger version (13K):
[in this window]
[in a new window]

  		Fig. 1. Schematic of the isolated stomatogastric nervous system, including the axon projections of MCN1 and CPN2 to the STG. The two lines with arrowheads projecting posteriorly from the STG neuropil represent the projection pattern of most STG motor neurons. Ganglia: CoG, commissural ganglion; OG, oesophageal ganglion; STG, stomatogastric ganglion; TG, thoracic ganglion. Nerves: cocTG, circumoesophageal connective from the CoG to the TG; cocB, circumoesophageal connective from the CoG to the brain; dgn, dorsal gastric nerve; dpon, dorsal posterior oesophageal nerve; ion, inferior oesophageal nerve; lgn, lateral gastric nerve; lvn, lateral ventricular nerve; mvn, medial ventricular nerve; pdn, pyloric dilator nerve; poc, post-oesophageal commissure; son, superior oesophageal nerve. Neurons: CPN2, commissural projection neuron 2; MCN1, modulatory commissural neuron 1.

 
All C. borealis used for fiber counting, tracing the POC axons to the thoracic ganglion (TG) and axon diameter measurement were collected by hand at Mount Desert Island Biological Laboratory (Salisbury Cove, ME, USA) and maintained in flow-through natural seawater tanks at ambient water temperature (10–14°C). For ease of dissection and immunoprocessing, these animals were smaller than those used for electrophysiological experiments. For tissue collection, as above, these crabs were first anesthetized by packing in ice for at least 30 min. The dorsal carapace was then removed and the thoracic ganglion, with the cocs and CoGs attached, were isolated by microdissection in chilled (approximately 10°C) C. borealis saline.

Solutions
Cancer borealis saline for dissections had the following composition (in mmol l–1): 440 NaCl, 26 MgCl2, 13 CaCl2, 11 KCl, 10 Trizma base, and 5 maleic acid (pH 7.4–7.6). During recording, 5 mmol l–1 dextrose was added to the saline. In high-divalent cation saline (Hi-Di), MgCl2 and CaCl2 were raised to 130 mmol l–1 and 65 mmol l–1, respectively. Phosphoramidon (Sigma, St Louis, MO, USA) and CabTRP Ia (Biotechnology Center, University of Wisconsin, Madison, WI, USA) were stored as frozen aliquots and diluted in C. borealis saline immediately prior to use.

Electrophysiology
Extracellular recordings were made by isolating a section of nerve with petroleum jelly (Vaseline: Medical Accessories and Supply Headquarters, Alabaster, AL, USA) and placing one stainless steel wire of a pair inside the Vaseline compartment and the other wire in the main bath compartment. These recordings were amplified in a 2-stage process (stage 1: AM Systems Model 1700 AC Amplifier, Carlsborg, WA, USA; stage 2: Brownlee Precision Model 410 Amplifier, Santa Clara, CA, USA). To facilitate intracellular recordings, ganglia were desheathed and viewed with light transmitted through a darkfield condenser (Nikon, Tokyo, Japan). Intracellular recordings were accomplished using borosilicate microelectrodes filled with 0.6 mol l–1 K2SO4 plus 10 mmol l–1 KCl (20-25 M{Omega}). Intracellular signals were amplified using Axoclamp 2B amplifiers (Molecular Devices, Sunnyvale, CA, USA) and digitized at ~5 kHz using a Micro 1401 data acquisition interface and Spike2 software (Cambridge Electronic Design, Cambridge, England).

Network and projection neurons were identified based on their activity patterns, synaptic connectivity and axonal projection patterns (Weimann et al., 1991Go; Norris et al., 1994Go; Coleman and Nusbaum, 1994Go; Beenhakker and Nusbaum, 2004Go; Saideman et al., 2007aGo; Saideman et al., 2007bGo). In some experiments, the activity of the projection neuron CPN2 was monitored indirectly, via the presence of excitatory postsynaptic potentials in the gastric mill (GM) protractor motor neuron (Norris et al., 1994Go).

Each half of the bisected poc was surrounded by a Vaseline well. Axons in the poc were stimulated extracellularly using a Grass S88 stimulator (AstroMed, West Warwick, RI, USA) and stimulus isolation unit (SIU5, AstroMed). The poc was stimulated tonically, using a range of voltages (4–15 V), at 15–30 Hz for 15–30 s. All poc stimulations were unilateral. To activate the gastro-pyloric receptor 2 neuron (GPR2) (Katz et al., 1989Go), the gastro-pyloric nerve was stimulated tonically at 10 Hz for 4 s. The ventral cardiac neurons (VCNs) (Beenhakker et al., 2004Go) were activated by stimulating the dorsal posterior oesophageal nerve in a rhythmic pattern (burst duration: 6 s, interburst freq.: 0.06 Hz, intraburst freq.: 15 Hz) (Beenhakker et al., 2004Go; Beenhakker and Nusbaum, 2004Go). CabTRP Ia was pressure ejected (10–4 mol l–1, 41–69 kPa, 0.5–10 s) from a Picospritzer II device (General Valve Corporation, Fairfield, NJ, USA) into the desheathed CoG neuropil. The dorsal aspect of the CoG is covered with neuronal somata, and the neuropil is underneath these somata. Therefore, to focally apply CabTRP Ia into the CoG neuropil, we inserted the peptide-containing pipette through the soma layer and into the depth of the anterior neuropil (Blitz and Nusbaum, 1999Go). The endopeptidase inhibitor phosphoramidon (10–5 mol l–1) was superfused to the anterior portion of the STNS, which was isolated from the STG by a Vaseline wall built across the recording dish. No data collection was made until phosphoramidon superfusion had occurred for at least 25 min.

Immunocytochemistry
Whole mounts of the isolated STNS and the thoracic ganglion (TG) with attached cocs and CoGs were fixed in 4% paraformaldehyde (Electron Microscopy Sciences, Hatfield, PA, USA) for 12–24 h, rinsed at least five times, at 1-h intervals, in phosphate (P) buffer (0.1 mol l–1) with 0.3% Triton X-100 (P-Triton) and then incubated for 24–72 h with a monoclonal rat anti-substance P antibody (1:300; Accurate Chemical and Scientific Corporation, Westbury, NY, USA; Abcam Incorporated, Cambridge, MA, USA) that has been used previously on this system (Goldberg et al., 1988Go; Christie et al., 1997Go; Blitz et al., 1999Go; Messinger et al., 2005Go). The nervous system was then again rinsed in P-Triton, five times at 1 h intervals, after which the STNS preparations were incubated with goat anti-rat Alexa Fluor 488 or 647 (1:300; Invitrogen Corporation, Carlsbad, CA, USA) for 12–16 h. In preparations where the TG was studied, the nervous system was incubated with donkey anti-rat IgG conjugated with either FITC or Rhodamine Red-X (Jackson ImmunoResearch, West Grove, PA, USA). In both cases, the preparations were then rinsed at least five times at 1-h intervals with P buffer and then mounted in 80% glycerol/20% 20 mmol l–1 sodium carbonate and a cover slip was place on top. For the STNS preparations, fluorescence was visualized and photographed with a Leica DMRB microscope, a Leica DC 350 FS camera, and Image-Pro Express software (Leica, version 4.5.1.3) using a L4 or Y5 (Leica) filter set (Leica Microsystems Inc., Bannockburn, IL, USA). The thoracic-CoG preparations were imaged using a Zeiss LSM 510 Meta confocal system (Carl Zeiss MicroImaging Inc., Thornwood, NY, USA), equipped with a Zeiss Observer.Z1 inverted microscope and argon and helium-neon lasers. Imaging was done using Zeiss EC plan-NEOFLUAR 10x/0.3 dry, Plan-Apochromat 20x/0.8 dry, EC plan-NEOFLUAR 40x/1.30 oil and Plan-Apochromat 63x/1.4 oil objective lenses, standard FITC and Rhodamine filter sets, and manufacturer-supplied software.

Data analysis
Data analysis was performed with custom written macros using Spike2 (`The Crab Analyzer', freely available at http://www.uni-ulm.de/~wstein/spike2/index.html). Gastric mill cycle period was measured as the duration from the onset of a lateral gastric (LG) neuron burst to the onset of the subsequent LG burst. An average of 10 consecutive cycles was obtained in each condition. Control MCN1 and CPN2 firing frequencies were measured continuously for 30 seconds prior to stimulation. MCN1 and CPN2 firing frequencies after stimulation were quantified during 10 consecutive protraction and retraction phases of the gastric mill rhythm in each preparation, as the average frequency across the entire protraction or retraction phase. MCN1 pyloric-timed activity was measured as the percentage of time it was active during each pyloric cycle, defined as the duration from the onset of a pyloric dilator (PD) neuron burst until the onset of the subsequent PD burst, for the pyloric cycles occurring during 10 consecutive protraction and retraction phases in each preparation.

The coc is a bilateral fiber bundle that connects the TG to the supra-oesophageal ganglion (brain), with the CoG being an outpocketing of the coc between its two termination points (Fig. 1). The poc connects the cocs on the TG side of the CoG. To refer specifically to a region of the coc relative to the CoG, we label the region of the coc projecting from the CoG towards the TG as the cocTG, and the region of the coc projecting from the CoG towards the brain as cocB (Fig. 1).

Figures were made using Spike2, CorelDraw (Corel Corporation, Ottawa, ON, Canada) and Igor Pro (Wavemetrics, Portland, OR, USA). Statistical analysis was performed with SigmaStat (Systat Software, San Jose, CA, USA). The Paired Student's t-test or repeated-measures (RM) one-way ANOVA followed by multiple comparisons using the Student–Newman–Keuls method were used as indicated. Significance was considered to be P<0.05. Data are expressed as mean ± s.e.m.


   RESULTS
TOP
 Summary
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
The gastric mill rhythm is a two-phase motor pattern driven by descending input
The gastric mill rhythm (cycle period: 5–20 s) drives the rhythmic protraction and retraction movements of the teeth in the gastric mill stomach compartment, thereby enabling the chewing of food (Heinzel, 1988Go; Heinzel et al., 1993Go). In C. borealis there are eight different types of gastric mill neurons, seven of which are motor neurons (Weimann et al., 1991Go; Saideman et al., 2007bGo; Stein et al., 2007Go). Four of these gastric mill neurons are protractor motor neurons, including the LG, GM, medial gastric (MG) and inferior cardiac (IC) neurons, although the IC and MG neurons can also exhibit retractor phase activity during some versions of the gastric mill rhythm (Beenhakker and Nusbaum, 2004Go; Blitz et al., 2004Go; Wood et al., 2004Go; Saideman et al., 2007bGo). There are also three retractor motor neurons, the dorsal gastric (DG), anterior median (AM) and ventricular dilator (VD) motor neurons, plus interneuron 1 (Int1), which is also active during the retractor phase and is the sole interneuron in this circuit. There is a single neuron of each type per STG, except the GM neurons of which there are four functionally equivalent copies.

In the isolated STNS of C. borealis, some of the gastric mill neurons (Int1, MG, IC, VD) are spontaneously active in pyloric rhythm-time, even in the absence of the gastric mill rhythm (e.g. VD and IC in Fig. 2, left panel) (Weimann et al., 1991Go; Blitz and Nusbaum, 1997Go). The pyloric rhythm (cycle period 0.5–2 s), which controls the filtering of chewed food in the posterior (pyloric) stomach compartment, is generated by a second motor circuit in the STG and is continuously active both in vitro and in vivo (Marder and Bucher, 2007Go).


Figure 2
View larger version (24K):
[in this window]
[in a new window]

  		Fig. 2. The gastric mill rhythm is triggered by poc nerve stimulation. (Left) Prior to poc stimulation, there was an ongoing pyloric rhythm [medial ventricular nerve (mvn) and pdn], but no gastric mill rhythm (dgn, lgn). The large, tonically active unit in the dgn corresponds to the activity of the anterior gastric receptor (AGR) neuron. AGR is a muscle tendon proprioceptor neuron that is spontaneously active in the isolated STNS (Combes et al., 1995Go; Smarandache and Stein, 2007Go). (Middle) 2 min after tonic poc stimulation (15 Hz, 30 s), the gastric mill rhythm was triggered, as is evident from the rhythmic bursting in the protractor LG neuron that alternated with the retractor phase activity of the DG, VD and IC neurons. Note the pyloric-timed bursting in the LG neuron. (Right) This expanded section of the middle panel shows more explicitly that each protractor LG burst is time-locked to the pyloric rhythm. Each period of inactivity in LG starts with a pyloric dilator (PD) neuron burst (grey bars). Pro., protraction, Ret., retraction.

 
The gastric mill rhythm is usually silent, in the isolated STNS as well as in vivo, unless the projection neurons that drive it are activated (Fleischer, 1981Go; Heinzel et al., 1993Go; Nusbaum et al., 2001Go; Beenhakker and Nusbaum, 2004Go; Blitz et al., 2004Go). However, Wood et al. (Wood et al., 2004Go) characterized a version of the gastric mill rhythm that occurred in some preparations without any experimental manipulation of projection neuron activity. This gastric mill rhythm was unusual in that the gastric mill LG neuron and projection neuron MCN1 exhibited a pyloric rhythm-timed activity pattern during the protraction phase, instead of the tonic firing pattern that they exhibit during protraction in all other characterized gastric mill rhythms in C. borealis (Coleman and Nusbaum, 1994Go; Beenhakker et al., 2004Go; Blitz et al., 2004Go; Christie et al., 2004Go; Saideman et al., 2007bGo). This spontaneously active gastric mill rhythm was driven largely by an unusually high level of spontaneous activity in the projection neuron MCN1. This MCN1 activity was not only pyloric rhythm-timed, but the resulting gastric mill rhythm was largely replicated by pyloric rhythm-timed extracellular stimulation of MCN1 (Wood et al., 2004Go).

POC stimulation triggers a long-lasting gastric mill rhythm
Stimulating the poc nerve (15 Hz tonic stimulation, 30 s duration) consistently triggered the gastric mill rhythm, beginning soon after the stimulation was terminated (N=39). In the example shown in Fig. 2, this rhythm started approximately 2 min after the end of poc stimulation and, as is typical for gastric mill rhythms, there was rhythmic alternating bursting of the protractor (LG) and retractor (DG, VD) neurons. It is also noteworthy that, during these rhythms, the IC neuron was mostly active during the retractor phase instead of the protractor phase (Fig. 2). Across preparations, the poc-triggered gastric mill rhythm started approximately 1 min after the end of poc stimulation (mean latency post-stimulation: 0.91±0.05 min, N=39). These rhythms exhibited a cycle period of 13.1±0.9 s (N=20).


Figure 3
View larger version (19K):
[in this window]
[in a new window]

  		Fig. 3. The poc-triggered gastric mill rhythm is long-lasting. (Left) Before poc stimulation, there was an ongoing pyloric rhythm (pdn) but no gastric mill rhythm (lgn, dgn). (Middle) 2 min after tonic poc stimulation (15 Hz, 30 s), the gastric mill rhythm had been triggered and was ongoing. Note the pyloric-timed LG bursts. (Right) This rhythm persisted for more than 15 min after poc stimulation.

 
During each protractor phase, the LG neuron exhibited pyloric rhythm-timed bursts (Fig. 2). Within each pyloric-timed burst, LG activity alternated with activity in the pyloric pacemaker neurons (e.g. PD neuron in Fig. 2), as also occurred in the spontaneously active rhythm characterized by Wood et al. (Wood et al., 2004Go). The protractor GM motor neuron also exhibited pyloric-timed bursts in many preparations (not shown). In all other previously studied gastric mill rhythms, the protractor neuron bursts instead exhibited a tonic firing pattern (Beenhakker and Nusbaum, 2004Go; Blitz et al., 2004Go; Christie et al., 2004Go; Saideman et al., 2007bGo).

The poc-triggered gastric mill rhythm was also long-lasting. After a 30 s poc stimulation, the gastric mill rhythm tended to persist for many minutes, and sometimes for more than 1 h (N=39). Specifically, in a few preparations this rhythm lasted for less than 5 min (N=4/39), but it often persisted for 5–20 min (N=22/39) or longer (N=13/39). The pattern was consistent for the duration of the triggered gastric mill rhythm. For example, there was stable, alternating bursting between the retractor (e.g. DG) and protractor (e.g. LG) neurons, with consistent pyloric-timed interruptions in each LG burst (Fig. 3).

POC stimulation indirectly activates the gastric mill rhythm
Extrinsic inputs can alter STG circuit activity via synaptic actions on circuit neurons and/or descending projection neurons (Hooper and Moulins, 1990Go; Katz and Harris-Warrick, 1990Go; Meyrand et al., 1994Go; Combes et al., 1999Go; Beenhakker and Nusbaum, 2004Go; Blitz et al., 2004Go). To determine whether the input(s) activated by poc stimulation influenced the gastric mill circuit directly or indirectly, we selectively superfused the CoGs with high divalent cation (Hi-Di: 5xCa2+/5xMg2+) saline while continuing to supply normal C. borealis saline to the STG. The Hi-Di saline raises action potential threshold and reduces the likelihood of polysynaptic transmission (Blitz and Nusbaum, 1999Go). This allowed us to reversibly reduce the ability of any poc-stimulated synaptic actions to activate CoG projection neurons and thereby determine whether this input activated the gastric mill rhythm via direct actions on STG neurons.

After determining that poc stimulation triggered a gastric mill rhythm in control conditions (Fig. 4A), Hi-Di saline was superfused selectively to the CoGs to suppress poc activation of CoG projection neurons. Under these conditions, stimulating the poc did not activate the gastric mill rhythm (N=6), even when the stimulation voltage was increased by 2 V (Fig. 4B). To ensure that the inability of poc stimulation to activate the gastric mill rhythm was not a consequence of a dysfunctional gastric mill circuit, we used extracellular stimulation of the inferior oesophageal nerve to drive this rhythm via selective activation of the projection neuron MCN1 (Bartos et al., 1999Go). Tonic MCN1 stimulation elicits a distinct gastric mill rhythm from the one triggered by poc stimulation, but both rhythms involve the same gastric mill circuit neurons (Coleman et al., 1995Go; Bartos et al., 1999Go; Wood et al., 2004Go; Saideman et al., 2007bGo). Extracellular MCN1 stimulation consistently elicited the gastric mill rhythm despite the presence of high-divalent cation saline to the CoGs (N=3, data not shown). Furthermore, after washing out the Hi-Di saline, poc stimulation again triggered the gastric mill rhythm (Fig. 4C; N=5/6). Thus, axons in the poc appear to project into the CoGs to activate projection neurons and thereby indirectly activate the gastric mill rhythm. We have designated the poc input that triggers the gastric mill rhythm as the POC neurons (see below).


Figure 4
View larger version (32K):
[in this window]
[in a new window]

  		Fig. 4. The poc-triggered gastric mill rhythm requires the activation of CoG projection neurons. (A) During normal saline superfusion of the CoGs, tonic poc stimulation (15 Hz, 30 s) triggered the gastric mill rhythm. (B) During superfusion of 5x Mg2+/5x Ca2+ saline selectively to the CoGs and OG (grey shading in STNS schematic), the same poc stimulation did not trigger the gastric mill rhythm. (C) After washout of the 5x Mg2+/5x Ca2+ saline, poc stimulation again triggered the gastric mill rhythm. Note that the black bar in each STNS schematic represents a Vaseline wall that enabled separate saline superfusion of the anterior (CoGs, OG) and posterior (STG) aspects of the STNS. All panels are from the same preparation.

 
The POC neurons excite the projection neurons MCN1 and CPN2
Two previously identified CoG projection neurons in C. borealis, MCN1 and CPN2, are necessary and sufficient for driving two previously characterized gastric mill rhythms that are elicited by stimulation of a mechanosensory (VCN neurons) or proprioceptor (GPR neuron) input (Beenhakker and Nusbaum, 2004Go; Blitz et al., 2004Go). Furthermore, the spontaneously active gastric mill rhythm studied by Wood et al. (Wood et al., 2004Go) was largely mimicked by direct stimulation of MCN1. We therefore examined the activity of MCN1 and CPN2 before and after poc stimulation, and found that this stimulation consistently triggered a long lasting excitatory response in both projection neurons (N=39). This excitatory response included an increased firing rate and pyloric-timed activity (Fig. 5).


Figure 5
View larger version (25K):
[in this window]
[in a new window]

  		Fig. 5. Activation of the CoG projection neurons CPN2 and MCN1, as well as the gastric mill rhythm, is triggered by poc stimulation. (Left) Before stimulation, CPN2 and MCN1 were weakly active and there was an ongoing pyloric rhythm (pdn) but no gastric mill rhythm (lgn, dgn). (Middle) After poc stimulation (15 Hz, 30 s), CPN2 and MCN1 were excited and the gastric mill rhythm was triggered. (Right) Expanded section from the middle panel showing that the activity of LG, MCN1 and CPN2 is interrupted in pyloric-time. Note that each such interruption occurs during activity of the pyloric pacemaker PD neuron (grey shading). Most hyperpolarized membrane potential: CPN2, –45 mV.

 
The POC-triggered excitation of MCN1 and CPN2 always coincided with the triggering of the gastric mill rhythm (N=39). After poc stimulation, the firing frequency of MCN1 was consistently higher than pre-stimulation (pre-poc stim.: 4.0±0.5 Hz; post-poc stim.: protraction phase (LG burst), 14.5±1.2 Hz, retraction phase (LG inter-burst), 14.7±1.2 Hz, N=10; protraction and retraction significantly different from control, P<0.05, protraction not significantly different from retraction, P>0.05, RM one-way ANOVA and Student–Newman–Keuls test of multiple comparisons). Similarly, CPN2 firing frequency was consistently increased after POC stimulation (pre-POC: 2.8±1.1 Hz; post-POC: protraction, 18.2±3.3 Hz; retraction, 15.6±3.3 Hz, N=4; protraction and retraction significantly different from control, P<0.05, protraction not significantly different from retraction, P>0.05, RM one-way ANOVA and Student–Newman–Keuls test of multiple comparisons).

A key feature of the MCN1 and CPN2 activity pattern is that their activity was terminated for a portion of each pyloric cycle during both protraction and retraction (Fig. 5). We therefore determined the percentage of the pyloric cycle period during which the projection neurons were active (see Materials and methods). MCN1 and CPN2 were always silent during the pyloric pacemaker neuron burst, which extended from the onset of each pyloric cycle (0%) until approximately the 20% point of each cycle (protraction: 0–20.1±0.4%; retraction: 0–20.2±0.6%; N=6). During the POC-triggered gastric mill rhythms, activity in these two projection neurons generally commenced with a delay after each pyloric pacemaker neuron burst. For example, MCN1 was active for ~65% of each pyloric cycle during protraction (onset: 34.9±2.9%; offset: 100.0±2.5%) and for ~58% of each pyloric cycle during retraction (onset: 39.4±3.1%; offset: 98.3±0.6%; N=6). Comparably, CPN2 was active for ~72% of each pyloric cycle during protraction (onset: 30.3±0.9%; offset: 102.4±0.8%) and was active for ~47% of each pyloric cycle during retraction (onset: 42.3±1.3%; offset: 88.4±7.2%; N=3). MCN1 and CPN2 were presumably silent during the pacemaker burst due to feedback inhibition in the CoGs from the anterior burster, the pyloric pacemaker interneuron (Coleman and Nusbaum, 1994Go; Norris et al., 1994Go; Wood et al., 2004Go).


Figure 6
View larger version (29K):
[in this window]
[in a new window]

  		Fig. 6. The pyloric rhythm in the STG is responsible for the pyloric-timed activity of the CoG projection neuron MCN1 and the gastric mill protractor neuron LG during the POC-triggered gastric mill rhythm. (Left) During the POC-triggered gastric mill rhythm, MCN1 and LG exhibited pyloric-timed activity. (Middle) When the pyloric rhythm was suppressed, by hyperpolarization of the pyloric pacemaker neurons, the POC-triggered gastric mill rhythm persisted but the activity of MCN1 and LG changed from pyloric-timed to tonic. (Right) After releasing the pyloric pacemaker neurons from hyperpolarization, the pyloric rhythm resumed and MCN1 and LG returned to exhibiting pyloric-timed activity.

 
We tested the hypothesis that the anterior burster neuron feedback to MCN1 and CPN2 in the CoGs was responsible for the pyloric-timed activity pattern of these projection neurons during the POC-triggered gastric mill rhythm. Specifically, we used hyperpolarizing current injection into the pyloric pacemaker neurons to suppress their activity and, consequently, that of the pyloric rhythm. The pyloric pacemaker neurons are a group of electrically coupled neurons that include the single anterior burster neuron plus the paired PD and lateral posterior gastric (LPG) neurons (Weimann et al., 1991Go; Weimann and Marder, 1994Go). When the pyloric rhythm was suppressed during the POC-triggered gastric mill rhythm, MCN1 and CPN2 activity switched from pyloric-timed to tonic (N=4; Fig. 6). At these times, the LG neuron activity pattern also switched from pyloric-timed to tonic, presumably because its activity was driven by these projection neurons (Beenhakker and Nusbaum, 2004Go). There is no direct synapse from the pyloric pacemaker neurons to LG (Bartos et al., 1999Go). In contrast to our findings, in the European lobster Homarus gammarus the pyloric-like activity of some CoG projection neurons can persist when the pyloric feedback is eliminated (Cardi and Nagy, 1994Go).

In previously studied gastric mill rhythms (Bartos et al., 1999Go; Wood et al., 2004Go), the gastric mill cycle period was regulated by the pyloric rhythm. Specifically, suppressing the pyloric rhythm increased the gastric mill cycle period. This was due to both inter-circuit interactions within the STG and to the pyloric-timing of MCN1 activity. Thus, we tested whether the cycle period of the POC-triggered gastric mill rhythm was also regulated by the pyloric rhythm. We found that the POC-triggered gastric mill cycle period was indeed increased when the pyloric rhythm was suppressed, from 12.3±1.8 s to 19.4±2.7 s (N=4; P<0.05, paired t-test).

The POC neurons project through the medial aspect of the cocTG to innervate the CoGs
As a step towards localizing the POC neurons, we determined whether their axons preferentially projected through the lateral or medial aspect of the cocTG. We anticipated that the POC neurons projected through the medial cocTG, by analogy with the fact that most projections through the cocB that innervate the CoG do so via the medial cocB (Kirby and Nusbaum, 2007Go). To determine if this was indeed the case for the POC neurons, we first stimulated the poc with the entire cocTG intact, to ensure the ability of this input to trigger the gastric mill rhythm in these preparations (Fig. 7). We then selectively transected either the lateral (N=3) or medial (N=3) aspect of the cocTG, after which we again assessed the ability of poc stimulation to trigger the gastric mill rhythm (Fig. 7). There were no landmarks to enable precise transection of exactly one half of each cocTG. Therefore, these transections were done in a fashion to ensure the retention of the lateral-most or medial-most cocTG, with a variable degree of transection of the central aspect of this nerve from preparation to preparation.


Figure 7
View larger version (39K):
[in this window]
[in a new window]

  		Fig. 7. The POC neurons project through the medial aspect of the cocTG to influence MCN1 and CPN2 in the CoG. (A) STNS schematic indicating the location and extent of the cocTG transections (grey boxes), the results of which are shown in B and C. (B) Transecting the medial aspect of the cocTG eliminated the ability of poc stimulation to trigger the gastric mill rhythm. (Left) Before medial cocTG transection, poc stimulation triggered the gastric mill rhythm. (Right) After medial cocTG transection, poc stimulation did not trigger the gastric mill rhythm. (C) Transecting the lateral aspect of the cocTG did not alter the ability of poc stimulation to trigger the gastric mill rhythm. The gastric mill rhythm was triggered both (left) before, and (right) after lateral cocTG transection by poc stimulation. B and C are from different preparations.

 
The gastric mill rhythm was never triggered by poc stimulation after medial cocTG transection (N=3) (Fig. 7B). By contrast, poc stimulation consistently triggered the gastric mill rhythm in every preparation after the lateral cocTG was transected (N=3). The resulting motor pattern retained its characteristic pyloric-timed activity pattern during the protractor phase (Fig. 7C). In these latter experiments, the resulting gastric mill rhythm continued to persist for a long duration, ranging from 8 to 24 min (N=3).

To ensure that the CoG projection neurons and STG circuit neurons were still capable of generating the gastric mill rhythm after medial cocTG transection, we stimulated the VCN neurons (Beenhakker et al., 2004Go; Beenhakker and Nusbaum, 2004Go). The VCN-triggered gastric mill rhythm was readily elicited in each of the three preparations after the medial cocTG was transected (not shown).

The POC neurons appear to contain the peptide transmitter CabTRP Ia
There is a dense CabTRP Ia-immunoreactive (CabTRP Ia-IR) arborization within the anterior CoG neuropil, called the anterior commissural organ (ACO; Fig. 8A) (Messinger et al., 2005Go). The ACO innervates each CoG via a population of small diameter axons that project as a bundle through the medial aspect of the cocTG (Goldberg et al., 1988Go; Messinger et al., 2005Go). This CabTRP Ia-IR bundle does not project through the cocB (Fig. 8A) (Goldberg et al., 1988Go; Messinger et al., 2005Go). Based on the results of the cocTG transection experiments reported above, and the fact that MCN1 and CPN2 arborize in the anterior CoG neuropil (Coleman and Nusbaum, 1994Go; Norris et al., 1994Go), we examined whether the ACO axons projected through the poc and therefore might be the axons of the POC neurons.


Figure 8
View larger version (36K):
[in this window]
[in a new window]

  		Fig. 8. A CabTRP Ia-immunoreactive axon bundle projects through the poc and medial aspect of the anterior cocTG to form terminal arborizations in the CoG. (A) CabTRP Ia immunoreactivity (CabTRP Ia-IR) occurred in a tightly associated axon bundle in the medial aspect of the cocTG (arrowhead) that terminated as a dense arborization in the antero-medial CoG. There was also more diffuse CabTRP Ia-IR throughout the CoG neuropil and in a subset of CoG neuronal somata. Asterisk indicates area examined to determine the number of CabTRP Ia-IR fibers present in the cocTG (see text). (B) The CabTRP Ia-lR axon bundle in the medial aspect of the cocTG (filled arrowhead) projected past the poc towards the TG, and also projected through the poc (open arrowhead). Asterisk indicates area examined to determine the number of CabTRP Ia-IR fibers present in the poc (see text). (C) CabTRP Ia-IR bundle was transected in a preparation in which the medial cocTG was transected (arrowhead). (D) CabTRP Ia-IR bundle was not transected in a preparation in which the lateral cocTG was transected (arrowhead). Spatial axes in C are for A–C. All scale bars, 150 µm.

 
Whole-mount immunocytochemistry revealed that the ACO axon population did indeed project through the poc (Fig. 8B). Specifically, at the junction between the cocTG and poc, a fraction of the CabTRP Ia-IR axon bundle in the medial cocTG separated and projected through the poc, whereas the remainder projected posteriorly past the poc as a tight bundle along the medial cocTG and terminated as the ACO in the ipsilateral CoG (N=16; Fig. 8B). This CabTRP Ia-IR fiber bundle projection continued in the medial cocTG, past the poc, towards the TG (Fig. 8B; N=16).

As further support that the POC neurons were likely to be the source of the ACO, we determined whether the CabTRP-IR bundle in the medial cocTG was transected or retained in each of the above cocTG transection experiments. We found that, in each experiment in which the medial cocTG was transected and poc stimulation no longer triggered the gastric mill rhythm, the CabTRP Ia-IR bundle had been transected (Fig. 8C; N=3). Conversely, the CabTRP Ia-IR bundle remained intact in preparations in which the lateral cocTG was transected and poc stimulation still triggered the gastric mill rhythm (Fig. 8D; N=3).

We also combined CabTRP Ia immunocytochemistry and confocal microscopy to determine the number and distribution of axon diameters for the CabTRP Ia-IR axons in the poc and medial cocTG bundle. In the poc, as well as in the cocTG adjacent to the CoG, the CabTRP Ia-IR axons were of small diameter (<1 µm) and often tightly fasciculated. Their relatively small diameter and tight fasciculation made it difficult to unambiguously determine the number of individual axons present. However, we counted the fibers to the best of our ability in order to obtain an estimate of the population size. We obtained a distribution of CabTRP Ia-IR axon counts from the left cocTG (88±5, N=5) and right cocTG (83±6, N=5). In the same five preparations, the distribution of axon counts in the poc suggested a smaller number of CabTRP Ia-IR axons (66±4), supporting our observation that only a subset of the CabTRP Ia-IR bundle in each cocTG projected through the poc. In no preparation was branching from the axon bundles seen within the cocTG or poc.


Figure 9
View larger version (24K):
[in this window]
[in a new window]

  		Fig. 9. Exogenous CabTRP Ia mimics the POC activation of MCN1 and CPN2. A brief (500 ms) puff of CabTRP Ia (10–4 mol l–1) into the antero-medial aspect of the CoG neuropil excited MCN1 and CPN2 (monitored as excitatory postsynaptic potentials in GM; see text), and subsequently activated LG, GM and DG. Note that CabTRP Ia triggered pyloric-timed activity in MCN1, CPN2 and LG. Insets, showing an expanded time scale, indicate that the GM membrane potential was not pyloric-timed before CabTRP Ia application but exhibited barrages of excitatory postsynaptic potentials that were interrupted in pyloric-time after this application. Most hyperpolarized membrane potential: GM, –67 mV.

 
In all preparations examined, the CabTRP Ia-IR bundle in the medial cocTG was traced to the junction of the cocTG with the TG (N=5). At this location, the POC axon bundle was less tightly fasciculated, often fanning out and covering a large portion of the nerve (not shown). In five separate preparations, we obtained similar axon counts to those from the cocTG near the CoG (left cocTG: 78±4; right cocTG: 73±9). Owing to the density and intensity of CabTRP Ia-IR within the TG, it was not possible to identify the destination of the POC axons within this ganglion. Although CabTRP Ia-IR somata within the TG may well be the origin of the POC axons, no discrete clusters of 50-100 CabTRP Ia-IR somata were identified within this ganglion to support that possibility (data not shown).

The POC neurons appear to use the peptide transmitter CabTRP Ia
To determine whether ACO-released CabTRP Ia mediated the long-term actions of the POC neurons on MCN1 and/or CPN2, we examined whether focal application of CabTRP Ia mimicked the POC excitation of these projection neurons. In some of these experiments (e.g. Fig. 9), CPN2 activity was monitored via intracellular GM neuron recordings. CPN2 is the sole source of discrete excitatory postsynaptic potentials in the GM neuron (Norris et al., 1994Go).

Brief, focal application of CabTRP Ia (10–4 mol l–1: 500 msec) into the anterior CoG neuropil triggered increased activity in MCN1 and CPN2 (N=4; Fig. 9). This increased activity was consistently pyloric-timed. In some preparations, the CabTRP Ia-triggered excitation of MCN1 and CPN2 led to the equivalent of a single gastric mill cycle, including an action potential burst in the retractor DG neuron preceding a burst in the protractor LG and GM neurons (Fig. 9).


Figure 10
View larger version (36K):
[in this window]
[in a new window]

  		Fig. 10. Blocking extracellular peptidase-mediated degradation of CabTRP Ia prolongs the actions of the POC neurons. (A) Before, during and after superfusion of the endopeptidase inhibitor phosphoramidon (10–5 mol l–1) to the CoGs, CPN2 was weakly active before poc stimulation and LG was silent (left panel: top, middle, bottom). CPN2 activity was monitored with an intra-axonal recording near the entrance to the STG (Beenhakker and Nusbaum, 2004Go). 30 s after poc stimulation (15 Hz, 15 s), the gastric mill rhythm was triggered (as indicated by the rhythmic LG bursting) and CPN2 activity was strengthened (middle panel: top, middle, bottom). 90 s after poc stimulation, the gastric mill rhythm had terminated and CPN2 activity had subsided during saline superfusion, both before and after phosphoramidon application (right panel: top, bottom). By contrast, 90 s after poc stimulation during phosphoramidon superfusion, CPN2 activity remained strong and the gastric mill rhythm persisted. (B) (Left) There was a significant increase in the duration of LG bursting after poc stimulation in the presence of phoshoramidon (Phos., 10–5 mol l–1; P<0.05, N=5), compared with its bursting duration in saline before phosphoramidon application (Ctl.). (Right) By contrast, phosphoramidon (10–5 mol l–1) did not alter the duration of LG bursting after stimulation of the proprioceptor sensory GPR neuron. Most hyperpolarized membrane potentials: CPN2stn, –73 mV; LG, –63 mV.

 
To further assay whether CabTRP Ia mediated the actions of the POC neurons on MCN1 and/or CPN2, we determined whether suppressing the extracellular peptidase-mediated degradation of this peptide would prolong the POC influence on these projection neurons. To this end, we applied the endopeptidase inhibitor phosphoramidon (10–5 mol l–1), which effectively prolongs the actions of both focally applied and neuronally released CabTRP Ia (Wood et al., 2000Go; Stein et al., 2007Go). Because the poc stimulus protocol used to trigger the gastric mill rhythm had such a long-lasting effect, we used briefer poc stimulations (15 Hz, 15 s) to achieve relatively brief control responses. These control stimulations triggered increased activity in MCN1 and CPN2 as well as a relatively short-lasting gastric mill rhythm (duration: 0.5–13 min, N=5; Fig. 10A). For example, in Fig. 10A the projection neuron activity was subsiding and the gastric mill rhythm had terminated by 90 s post-POC stimulation. Although phosphoramidon alone did not alter CPN2 or LG activity prior to poc stimulation (e.g. Fig. 10A, middle left panel), the POC-triggered rhythm during phosphoramidon superfusion persisted for more than 90 s post-stimulation. After washout of the phosphoramidon, the POC action on CPN2 and the gastric mill rhythm returned to pre-application levels (Fig. 10A). In all cases, when phosphoramidon (10–5 mol l–1) was superfused selectively to the CoGs, poc stimulation triggered a more prolonged excitation of MCN1 (not shown) and CPN2 and triggered a longer-lasting gastric mill rhythm (N=5).

We quantified the influence of phosphoramidon on the duration of POC actions by measuring the time during which the LG neuron generated rhythmic bursts after poc stimulation. Specifically, phosphoramidon application reversibly increased the duration of LG bursting by approximately fourfold (Fig. 10B; saline: 6.1±1.9 min, phosphoramidon: 22.5±6.7 min, wash: 12.5±4.8 min; N=5; P<0.05, RM one-way ANOVA and Student–Newman–Keuls test of multiple comparisons).

To control for the specificity of phosphoramidon action, we examined the influence of phosphoramidon on the duration of LG bursting after stimulating the gastro-pyloric receptor neuron (GPR) (Katz et al., 1989Go; Katz and Harris-Warrick, 1990Go). GPR stimulation excites MCN1 and CPN2 and thereby elicits the gastric mill rhythm (Blitz et al., 2004Go). GPR does not, however, contain CabTRP Ia but instead contains the co-transmitters acetylcholine, serotonin and allatostatin (Katz and Harris-Warrick, 1990Go; Skiebe and Schneider, 1994Go). Phosphoramidon (10–5 mol l–1) superfusion did not change the duration of LG bursting after GPR stimulation (Fig. 10B; N=4, P>0.5 RM one-way ANOVA).


   DISCUSSION
TOP
 Summary
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
We have identified an extrinsic input, the POC neurons, that triggers a long-lasting activation of identified CoG projection neurons and thereby initiates a distinct version of the gastric mill rhythm in the C. borealis STG. The POC axons project as a tightly associated bundle through the medial aspect of each cocTG, from the direction of the TG, to innervate the ipsilateral CoG. A subset of these axons also project through the poc, enabling them to innervate the contralateral CoG. The long-term activation of the CoG projection neurons MCN1 and CPN2 by POC stimulation is probably mediated by the peptide transmitter CabTRP Ia.

The POC neurons appear to be the source of the extensive CabTRP Ia-IR arborization in the anterior CoG neuropil (Goldberg et al., 1988Go). This arborization was recently characterized as a neurohemal organ, the ACO, which is well-situated to release CabTRP Ia into the hemolymph as a circulating hormone in the related species Cancer productus (Messinger et al., 2005Go). In that study, the ACO was also studied extensively for the presence of co-transmitters but none were identified. One function of circulating hormones, including CabTRP Ia, is to modulate the properties of muscles that mediate movements of the foregut (Jorge-Rivera and Marder, 1996Go; Messinger et al., 2005Go). Therefore, POC-mediated release of CabTRP Ia may well coordinately trigger the gastric mill rhythm and modulate the response of gastric mill muscles to the incoming motor pattern. Recently, a second isoform of CabTRP (CabTRP II) was isolated from the STNS, including the CoGs (Stemmler et al., 2007Go). Both CabTRP isoforms are recognized by the same antibody and have similar actions on the pyloric rhythm (Stemmler et al., 2007Go). Thus, either or both CabTRP peptides may mediate the POC actions in this system.

The likelihood that the CabTRP Ia released from the ACO terminals locally excites MCN1 and CPN2 supports the hypothesis that this neuronal population has both paracrine and endocrine functions. Given the sensitivity of MCN1 and CPN2 to relatively brief POC stimulation, there may well be times when this input acts largely or exclusively as a local modulator of neuronal activity, whereas at other times its activation results in both paracrine and endocrine actions. Previous studies in other systems have established the ability of the same neurons to release signaling molecules that act both locally, in a paracrine fashion, and as circulating hormones (Mayeri, 1979Go; Sigvardt et al., 1986Go; Jung and Scheller, 1991Go; Loechner and Kaczmarek, 1994Go; Ludwig and Pittman, 2003Go; Fort et al., 2004Go; Oliet et al., 2007Go).

We have not yet identified the location of the POC neuronal somata. These somata may be located within the TG, in which the cocTG terminates. In C. borealis, the entire ventral nerve cord is compressed into the single TG (Horridge, 1965Go). However, the POC somata may instead be located within one or more peripheral nerves or related structures, as is common for muscle- and abdominal-stretch sensitive sensory neuron populations in decapod crustaceans (Alexandrowicz, 1951Go; Cattaert et al., 2002Go; Katz et al., 1989Go; Beenhakker et al., 2004Go). Whether these neurons originate in the TG or a peripheral structure, their point of origin appears likely to be outside the STNS. Thus, the POC neurons may help to coordinate the chewing of food with other behaviors, perhaps acting as a trigger for chewing in response to cues from other regions of the animal. In addition, these neurons may well contribute to the long-term maintenance of chewing in the intact crab and lobster insofar as the gastric mill rhythm can persist for hours after food is ingested (Fleischer, 1981Go; Turrigiano and Selverston, 1990Go). Similarly, there are long-lasting actions of the vertebrate tachykinin peptide, substance P, on rhythmic locomotor activity in the vertebrate CNS (Treptow et al., 1983Go; Parker and Grillner, 1999Go). Further, short-duration sensory stimuli can trigger long-term activation of descending reticulospinal neurons that drive locomotion in lamprey (Di Prisco et al., 1997Go).

The POC-elicited gastric mill rhythm is qualitatively different from gastric mill rhythms elicited by other extrinsic inputs in C. borealis. Specifically, the protraction phase activity pattern of MCN1, CPN2 and LG is pyloric-timed during the POC-triggered rhythm whereas these neurons exhibit tonic protraction phase activity during other gastric mill rhythms (Beenhakker and Nusbaum, 2004Go; Blitz et al., 2004Go; Christie et al., 2004Go; Saideman et al., 2007bGo). The LG-innervated muscles mediate protraction of the lateral teeth within the gastric mill. Thus, the distinct LG neuron activity pattern during the POC-triggered gastric mill rhythm could result in a different mode of chewing relative to the previously characterized gastric mill rhythms. In fact, both smooth protraction and pyloric-timed movements of the lateral teeth occur during in vivo endoscopic recordings of these teeth movements in Cancer crabs (Heinzel et al., 1993Go). Future work will be needed to establish whether the pyloric-timed LG neuron pattern is retained at the level of the LG-innervated muscles during the POC-triggered rhythm.

The distinct activity pattern of MCN1 during the POC rhythm also has consequences for motor pattern generation and inter-circuit coordination. For example, the pyloric circuit feedback to MCN1 during the protractor phase of the spontaneous POC-like gastric mill rhythm enables the pyloric rhythm to regulate the speed and pattern of the gastric mill rhythm, as well as its coordination with the pyloric rhythm (Wood et al., 2004Go). This is also evident in the present study from the change in gastric mill cycle period that occurred when the pyloric rhythm was suppressed. This pyloric regulation of the gastric mill rhythm during the protractor phase, via feedback inhibition of MCN1 and CPN2, occurs only during the POC-type of gastric mill rhythm (Beenhakker and Nusbaum, 2004Go; Blitz et al., 2004Go; Christie et al., 2004Go). Previous work documented additional cellular and synaptic mechanisms underlying inter-circuit regulation during other versions of the gastric mill rhythm (Bartos and Nusbaum, 1997Go; Clemens et al., 1998Go; Bartos et al., 1999Go; Wood et al., 2004Go). Although coordination between different behaviors, such as locomotion and respiration, occurs in many animals (Bramble and Carrier, 1983Go; Syed and Winlow, 1991Go; Kawahara et al., 1989Go; Morin and Viala, 2002Go; Saunders et al., 2004Go), the underlying cellular mechanisms remain to be determined in these other systems.

It appears likely that the POC-like gastric mill rhythm previously studied by Wood et al. (Wood et al., 2004Go) does represent POC-triggered rhythms, presumably resulting from POC activation that occurred during the dissection. In both cases there was a prominent activation of MCN1, and they further share the distinct pyloric-timed activity pattern during the protractor phase. CPN2 activity, however, was not studied in the earlier work (Wood et al., 2004Go). Wood et al. (Wood et al., 2004Go) did establish that pyloric-timed MCN1 stimulation elicited a gastric mill rhythm that was comparable to the spontaneous POC-like rhythm.

Given that MCN1 and CPN2 are necessary and sufficient to elicit the VCN- and GPR-elicited gastric mill rhythms (Blitz et al., 2004Go; Beenhakker and Nusbaum, 2004Go), it is likely that they play pivotal roles during the POC-triggered rhythm as well. Addressing this issue will provide insight into the extent to which this system uses convergent activation of the same projection neurons to elicit distinct activity patterns. This would be in contrast to the prevalent hypothesis in other model systems that the generation of distinct but related movements results from the activation of distinct but overlapping sets of projection neurons (Georgopoulos, 1995Go; Kristan and Shaw, 1997Go; Lewis and Kristan, 1998Go; Liu and Fetcho, 1999Go).

LIST OF ABBREVIATIONS

ACO
anterior commissural organ

AGR
anterior gastric receptor

AM
anterior median

CabTRP Ia
Cancer borealis tachykinin-related peptide Ia

CabTRP Ia-IR
Cancer borealis tachykinin-related peptide Ia immunoreactivity/immunoreactive

coc
circumoesophageal connective

cocB
circumoesophageal connective projecting from the CoG to the brain

cocTG
circumoesophageal connective projecting from the CoG to the TG

CoG
commissural ganglion

CPN2
commissural projection neuron 2

DG
dorsal gastric

dgn
dorsal gastric nerve

dpon
dorsal posterior oesophageal nerve

EPSPs
excitatory postsynaptic potentials

GM
gastric mill

GPR
gastro-pyloric receptor

IC
inferior cardiac

Int1
interneuron 1

LG
lateral gastric

lgn
lateral gastric nerve

LPG
lateral posterior gastric

MCN1
modulatory commissural neuron 1

MG
medial gastric

OG
oesophageal ganglion

PD
pyloric dilator

pdn
pyloric dilator nerve

poc/POC
post-oesophageal commissure

STG
stomatogastric

STNS
stomatogastric nervous system

TG
thoracic ganglion

VCNs
ventral cardiac neurons

VD
ventricular dilator


   Acknowledgments
 
This work supported by grant NS 42813 (M.P.N.) from the National Institute of Neurological Disorders and Stroke, and a Mount Desert Island Biological Laboratory New Investigator Award from the Salisbury Cove Research Fund provided through the Thomas H. Maren Foundation (A.E.C.).


   Footnotes
 
* Present address: Department of Biological Sciences, Columbia University, New York, NY 10027, USA Back


   References
TOP
 Summary
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

Alexandrowicz, J. S. (1951). Muscle receptor organs in the abdomen of Homarus vulgaris and Palinurus vulgaris.Q. J. Microsc. Sci. 92,163 -199.

Bartos, M. and Nusbaum, M. P. (1997). Intercircuit control of motor pattern modulation by presynaptic inhibition. J. Neurosci. 17,2247 -2256.[Abstract/Free Full Text]

Bartos, M., Manor, Y., Nadim, F., Marder, E. and Nusbaum, M. P. (1999). Coordination of fast and slow rhythmic neuronal circuits. J. Neurosci. 19,6650 -6660.[Abstract/Free Full Text]

Beenhakker, M. P. and Nusbaum, M. P. (2004). Mechanosensory activation of a motor circuit by coactivation of two projection neurons. J. Neurosci. 24,6741 -6750.[Abstract/Free Full Text]

Beenhakker, M. P., Blitz, D. M. and Nusbaum, M. P. (2004). Long-lasting activation of rhythmic neuronal activity by a novel mechanosensory system in the crustacean stomatogastric nervous system. J. Neurophysiol. 91,78 -91.[Abstract/Free Full Text]

Blitz, D. M. and Nusbaum, M. P. (1997). Motor pattern selection via inhibition of parallel pathways. J. Neurosci. 17,4965 -4975.[Abstract/Free Full Text]

Blitz, D. M. and Nusbaum, M. P. (1999). Distinct functions for cotransmitters mediating motor pattern selection. J. Neurosci. 19,6774 -6783.[Abstract/Free Full Text]

Blitz, D. M., Christie, A. E., Coleman, M. J., Norris, B. J., Marder, E. and Nusbaum, M. P. (1999). Different proctolin neurons elicit distinct motor patterns from a multifunctional neuronal network. J. Neurosci. 19,5449 -5463.[Abstract/Free Full Text]

Blitz, D. M., Beenhakker, M. P. and Nusbaum, M. P. (2004). Different sensory systems share projection neurons but elicit distinct motor patterns. J. Neurosci. 24,11381 -11390.[Abstract/Free Full Text]

Bramble, D. M. and Carrier, D. R. (1983). Running and breathing in mammals. Science 219,251 -256.[Abstract/Free Full Text]

Brocard, F., Bardy, C. and Dubuc, R. (2005). Modulatory effect of substance P to the brain stem locomotor command in lampreys. J. Neurophysiol. 93,2127 -2141.[Abstract/Free Full Text]

Cardi, P. and Nagy, F. (1994). A rhythmic modulatory gating system in the stomatogastric nervous system of Homarus gammarus. III. Rhythmic control of the pyloric CPG. J. Neurophysiol. 71,2503 -2516.[Abstract/Free Full Text]

Cattaert, D., Le Bon, M. and Le Ray, D. (2002). Efferent controls in crustacean mechanoreceptors. Microsc. Res. Tech. 58,312 -324.[CrossRef][Medline]

Christie, A. E., Lundquist, C. T., Nassel, D. R. and Nusbaum, M. P. (1997). Two novel tachykinin-related peptides from the nervous system of the crab Cancer borealis. J. Exp. Biol. 200,2279 -2294.[Abstract]

Christie, A. E., Stein, W., Quinlan, J. E., Beenhakker, M. P., Marder, E. and Nusbaum, M. P. (2004). Actions of a histaminergic/peptidergic projection neuron on rhythmic motor patterns in the stomatogastric nervous system of the crab Cancer borealis. J. Comp. Neurol. 469,153 -169.[CrossRef][Medline]

Clemens, S., Combes, D., Meyrand, P. and Simmers, J. (1998). Long-term expression of two interacting motor pattern-generating networks in the stomatogastric system of freely behaving lobster. J. Neurophysiol. 79,1396 -1408.[Abstract/Free Full Text]

Coleman, M. J. and Nusbaum, M. P. (1994). Functional consequences of compartmentalization of synaptic input. J. Neurosci. 11,6544 -6552.

Coleman, M. J., Nusbaum, M. P., Cournil, I. and Claiborne, B. J. (1992). Distribution of modulatory inputs to the stomatogastric ganglion of the crab, Cancer borealis. J. Comp. Neurol. 325,581 -594.[CrossRef][Medline]

Coleman, M. J., Meyrand, P. and Nusbaum, M. P. (1995). A switch between two modes of synaptic transmission mediated by presynaptic inhibition. Nature 378,502 -505.[CrossRef][Medline]

Combes, D., Simmers, A. J. and Moulins, M. (1995). Structural and functional characterization of a muscle tendon proprioceptor in lobster. J. Comp. Neurol. 363,221 -234.[CrossRef][Medline]

Combes, D., Meyrand, P. and Simmers, J. (1999). Dynamic restructuring of a rhythmic motor program by a single mechanoreceptor neuron in lobster. J. Neurosci. 19,3620 -3628.[Abstract/Free Full Text]

Di Prisco, G. V., Pearlstein, E., Robitaille, R. and Dubuc, R. (1997). Role of sensory-evoked NMDA plateau potentials in the initiation of locomotion. Science 278,1122 -1125.[Abstract/Free Full Text]

Di Prisco, G. V., Pearlstein, E., Le Ray, D., Robitaille, R. and Dubuc, R. (2000). A cellular mechanism for the transformation of a sensory input into a motor command. J. Neurosci. 20,8169 -8176.[Abstract/Free Full Text]

Fleischer, A. G. (1981). The effect of eyestalk hormones on the gastric mill in the intact lobster Panulirus interruptus.J. Comp. Physiol. 141,363 -368.[CrossRef]

Fort, T. J., Brezina, V. and Miller, M. W. (2004). Modulation of an integrated central pattern generator-effector system: dopaminergic regulation of cardiac activity in the blue crab Callinectes sapidus. J. Neurophysiol. 92,3455 -3470.[Abstract/Free Full Text]

Georgopoulos, A. P. (1995). Current issues in directional motor control. Trends Neurosci. 18,506 -510.[CrossRef][Medline]

Goldberg, D., Nusbaum, M. P. and Marder, E. (1988). Substance P-like immunoreactivity in the stomatogastric nervous systems of the crab Cancer borealis and the lobsters Panulirus interruptus and Homarus americanus. Cell Tissue Res. 252,515 -522.[CrossRef][Medline]

Gordon, I. T. and Whelan, P. J. (2006). Deciphering the organization and modulation of spinal locomotor central pattern generators. J. Exp. Biol. 209,2007 -2014.[Abstract/Free Full Text]

Heinzel, H. G. (1988). Gastric mill activity in the lobster. I. Spontaneous modes of chewing. J. Neurophysiol. 59,528 -550.[Abstract/Free Full Text]

Heinzel, H. G., Weimann, J. M. and Marder, E. (1993). The behavioral repertoire of the gastric mill in the crab, Cancer pagurus: an in situ endoscopic and electrophysiological examination. J. Neurosci. 13,1793 -1803.[Abstract]

Hooper, S. L. and Moulins, M. (1990). Cellular and synaptic mechanisms responsible for a long-lasting restructuring of the lobster pyloric network. J. Neurophysiol. 64,1574 -1589.[Abstract/Free Full Text]

Horridge, G. A. (1965). Arthropoda: general anatomy. In Structure and Function in the Nervous System of Invertebrates (ed. T. H. Bullock and G. A. Horridge), pp.801 -964. San Francisco: W. H. Freeman.

Jorge-Rivera, J. C. and Marder, E. (1996). TNRNFLRFamide and SDRNFLRFamide modulate muscles of the stomatogastric system of the crab Cancer borealis. J. Comp. Physiol. A 179,741 -751.[Medline]

Jung, L. J. and Scheller, R. H. (1991). Peptide processing and targeting in the neuronal secretory pathway. Science 251,1330 -1335.[Abstract/Free Full Text]

Katz, P. S. and Harris-Warrick, R. M. (1990). Neuromodulation of the crab pyloric central pattern generator by serotonergic/cholinergic proprioceptive afferents. J. Neurosci. 10,1495 -1512.[Abstract]

Katz, P. S., Eigg, M. H. and Harris-Warrick, R. M. (1989). Serotonergic/cholinergic muscle receptor cells in the crab stomatogastric nervous system. I. Identification and characterization of the gastropyloric receptor cells. J. Neurophysiol. 62,558 -570.[Abstract/Free Full Text]

Kawahara, K., Kumagai, S., Nakazono, Y. and Myamoto, Y. (1989). Coupling between respiratory and stepping rhythms during locomotion in decerebrate cats. J. Appl. Physiol. 67,110 -115.[Abstract/Free Full Text]

Kiehn, O. (2006). Locomotor circuits in the mammalian spinal cord. Annu. Rev. Neurosci. 29,279 -306.[CrossRef][Medline]

Kirby, M. S. and Nusbaum, M. P. (2007). Central nervous system projections to and from the commissural ganglion of the crab Cancer borealis. Cell Tissue Res. 328,625 -637.[CrossRef][Medline]

Kristan, W. B. and Shaw, B. K. (1997). Population coding and behavioral choice. Curr. Opin. Neurobiol. 7,826 -831.[CrossRef][Medline]

LeBeau, F. E., El Manira, A. and Grillner, S. (2005). Tuning the network: modulation of neuronal microcircuits in the spinal cord and hippocampus. Trends Neurosci. 28,552 -561.[CrossRef][Medline]

Lewis, J. E. and Kristan, W. B. (1998). A neuronal network for computing population vectors in the leech. Nature 391,76 -79.[CrossRef][Medline]

Liu, K. S. and Fetcho, J. R. (1999). Laser ablations reveal functional relationships of segmental hindbrain neurons. Neuron 23,325 -335.[CrossRef][Medline]

Loechner, K. J. and Kaczmarek, L. K. (1994). Autoactive peptides act at three distinct receptors to depolarize the bag cell neurons of Aplysia. J. Neurophysiol. 71,195 -203.[Abstract/Free Full Text]

Ludwig, M. and Pittman, Q. J. (2003). Talking back: dendritic neurotransmitter release. Trends Neurosci. 26,255 -261.[CrossRef][Medline]

Marder, E. and Bucher, D. (2007). Understanding circuit dynamics using the stomatogastric nervous system of lobsters and crabs. Annu. Rev. Physiol. 69,291 -316.[CrossRef][Medline]

Marder, E., Bucher, D., Schulz, D. J. and Taylor, A. L. (2005). Invertebrate central pattern generation moves along. Curr. Biol. 15,R685 -R699.[CrossRef][Medline]

Mayeri, E. (1979). Local hormonal modulation of neural activity in Aplysia. Fed. Proc. 38,2103 -2108.[Medline]

McLean, D. L. and Sillar, K. T. (2004). Metamodulation of a spinal locomotor network by nitric oxide. J. Neurosci. 24,9561 -9571.[Abstract/Free Full Text]

Messinger, D. I., Kutz, K. K., Le T., Verley, D. R., Hsu, Y. W., Ngo, C. T., Cain, S. D., Birmingham, J. T., Li, L. and Christie, A. E. (2005). Identification and characterization of a tachykinin-containing neuroendocrine organ in the commissural ganglion of the crab Cancer productus. J. Exp. Biol. 208,3303 -3319.[Abstract/Free Full Text]

Meyrand, P., Simmers, J. and Moulins, M. (1994). Dynamic construction of a neural network from multiple pattern generators in the lobster stomatogastric nervous system. J. Neurosci. 14,630 -644.[Abstract]

Morin, D. and Viala, D. (2002). Coordinations of locomotor and respiratory rhythms in vitro are critically dependent on hindlimb sensory inputs. J. Neurosci. 22,4756 -4765.[Abstract/Free Full Text]

Norris, B. J., Coleman, M. J. and Nusbaum, M. P. (1994). Recruitment of a projection neuron determines gastric mill motor pattern selection in the stomatogastric nervous system of the crab, Cancer borealis. J. Neurophysiol. 72,1451 -1463.[Abstract/Free Full Text]

Nusbaum, M. P. and Beenhakker, M. P. (2002). A small-systems approach to motor pattern generation. Nature 417,343 -350.[CrossRef][Medline]

Nusbaum, M. P., Blitz, D. M., Swensen, A. M., Wood, D. and Marder, E. (2001). The roles of co-transmission in neural network modulation. Trends Neurosci. 24,146 -154.[CrossRef][Medline]

Oliet, S. H., Baimoukhametova, D. V., Piet, R. and Bains, J. S. (2007). Retrograde regulation of GABA transmission by the tonic release of oxytocin and endocannabinoids governs postsynaptic firing. J. Neurosci. 27,1325 -1333.[Abstract/Free Full Text]

Parker, D. and Grillner, S. (1999). Long-lasting substance-P-mediated modulation of NMDA-induced rhythmic activity in the lamprey locomotor network involves separate RNA- and protein-synthesis-dependent stages. Eur. J. Neurosci. 11,1515 -1522.[CrossRef][Medline]

Saideman, S. R., Ma, M., Kutz-Naber, K. K., Cook, A., Torfs, P., Schoofs, L., Li, L. and Nusbaum, M. P. (2007a). Modulation of rhythmic motor activity by pyrokinin peptides. J. Neurophysiol. 97,579 -595.[Abstract/Free Full Text]

Saideman, S. R., Blitz, D. M. and Nusbaum, M. P. (2007b). Convergent motor patterns from divergent circuits. J. Neurosci. 27,6664 -6674.[Abstract/Free Full Text]

Saunders, S. W., Rath, D. and Hodges, P. W. (2004). Postural and respiratory activation of the trunk muscles changes with mode and speed of locomotion. Gait Posture 20,280 -290.[CrossRef][Medline]

Sigvardt, K. A., Rothman, B. S., Brown, R. O. and Mayeri, E. (1986). The bag cells of Aplysia as a multitransmitter system: identification of alpha bag cell peptide as a second neurotransmitter. J. Neurosci. 6,803 -813.[Abstract]

Skiebe, P. and Schneider, H. (1994). Allatostatin peptides in the crab stomatogastric nervous system: inhibition of the pyloric motor pattern and distribution of allatostatin-like immunoreactivity. J. Exp. Biol. 194,195 -208.[Abstract]

Smarandache, C. R. and Stein, W. (2007). Sensory-induced modification of two motor patterns in the crab, Cancer pagurus. J. Exp. Biol. 210,2912 -2922.[Abstract/Free Full Text]

Smetana, R. W., Alford, S. and Dubuc, R. (2007). Muscarinic receptor activation elicits sustained, recurring depolarizations in reticulospinal neurons. J. Neurophysiol. 97,3181 -3192.[Abstract/Free Full Text]

Stein, W., DeLong, N. D., Wood, D. E. and Nusbaum, M. P. (2007). Divergent co-transmitter actions underlie motor pattern activation by a modulatory projection neuron. Eur. J. Neurosci. 26,1148 -1165.[CrossRef][Medline]

Stemmler, E. A., Peguero, B., Bruns, E. A., Dickinson, P. S. and Christie, A. E. (2007). Identification, physiological actions, and distribution of TPSGFLGMRamide: a novel tachykinin-related peptide from the midgut and stomatogastric nervous system of Cancer crabs.J. Neurochem. 101,1351 -1366.[CrossRef][Medline]

Syed, N. I. and Winlow, W. (1991). Coordination of locomotor and cardiorespiratory networks of Lymnaea stagnalis by a pair of identified interneurones. J. Exp. Biol. 158, 37-62.[Abstract/Free Full Text]

Treptow, K., Oehme, P., Gäbler, E. and Bienert, M. (1983). Modulation of locomotor activity by substance P in rats. Regul. Pept. 5,343 -351.[CrossRef][Medline]

Tryba, A. K., Pena, F. and Ramirez, J. M. (2006). Gasping activity in vitro: a rhythm dependent on 5-HT2A receptors. J. Neurosci. 26,2623 -2634.[Abstract/Free Full Text]

Turrigiano, G. G. and Selverston, A. I. (1990). A cholecystokinin-like hormone activates a feeding-related neural circuit in lobster. Nature 344,866 -868.[CrossRef][Medline]

Weimann, J. M. and Marder, E. (1994). Switching neurons are integral members of multiple oscillatory networks. Curr. Biol. 4,896 -902.[CrossRef][Medline]

Weimann, J. M., Meyrand, P. and Marder, E. (1991). Neurons that form multiple pattern generators: identification and multiple activity patterns of gastric/pyloric neurons in the crab stomatogastric system. J. Neurophysiol. 65,111 -122.[Abstract/Free Full Text]

White, R. S., Nadim, F. and Nusbaum, M. P. (2005). Activation of a peripheral modulatory system elicits a distinct gastric mill rhythm. Abstr. Soc. Neurosci. 2005,722 .52.

Wood, D. E., Stein, W. and Nusbaum, M. P. (2000). Projection neurons with shared cotransmitters elicit different motor patterns from the same neural circuit. J. Neurosci. 20,8943 -8953.[Abstract/Free Full Text]

Wood, D. E., Manor, Y., Nadim, F. and Nusbaum, M. P. (2004). Intercircuit control via rhythmic regulation of projection neuron activity. J. Neurosci. 24,7455 -7563.[Abstract/Free Full Text]


Add to CiteULike CiteULike   Add to Complore Complore   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us   Add to Digg Digg   Add to Reddit Reddit   Add to Technorati Technorati   Add to Twitter Twitter    What's this?


This article has been cited by other articles:


Home page
 J. Neurophysiol.Home page
A. K. Friedman, Y. Zhurov, B. Ch. Ludwar, and K. R. Weiss
Motor Outputs in a Multitasking Network: Relative Contributions of Inputs and Experience-Dependent Network States
J Neurophysiol, December 1, 2009; 102(6): 3711 - 3727.
[Abstract] [Full Text] [PDF]

Home page
 J. Neurosci.Home page
N. D. DeLong, M. S. Kirby, D. M. Blitz, and M. P. Nusbaum
Parallel Regulation of a Modulator-Activated Current via Distinct Dynamics Underlies Comodulation of Motor Circuit Output
J. Neurosci., September 30, 2009; 29(39): 12355 - 12367.
[Abstract] [Full Text] [PDF]

Home page
 J. Neurosci.Home page
F. Nadim, V. Brezina, A. Destexhe, and C. Linster
State Dependence of Network Output: Modeling and Experiments
J. Neurosci., November 12, 2008; 28(46): 11806 - 11813.
[Abstract] [Full Text] [PDF]

Home page
 J. Neurosci.Home page
D. M. Blitz and M. P. Nusbaum
State-Dependent Presynaptic Inhibition Regulates Central Pattern Generator Feedback to Descending Inputs
J. Neurosci., September 17, 2008; 28(38): 9564 - 9574.
[Abstract] [Full Text] [PDF]

Home page
 J. Neurophysiol.Home page
N. Spitzer, G. Cymbalyuk, H. Zhang, D. H. Edwards, and D. J. Baro
Serotonin Transduction Cascades Mediate Variable Changes in Pyloric Network Cycle Frequency in Response to the Same Modulatory Challenge
J Neurophysiol, June 1, 2008; 99(6): 2844 - 2863.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Summary Freely available
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Blitz, D. M.
Right arrow Articles by Nusbaum, M. P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Blitz, D. M.
Right arrow Articles by Nusbaum, M. P.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati   Add to Twitter  
What's this?