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First published online August 22, 2008
Journal of Experimental Biology 211, 2876-2888 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018515
Hydrozoan nematocytes send and receive synaptic signals induced by mechano-chemical stimuli
Institute for Neurobiology and Behavioral Biology, University of Münster, Badestr. 9, D-48149 Münster, Germany
Author for correspondence (e-mail:
thurmu{at}uni-muenster.de)
Accepted 30 June 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXReferences |
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Key words: cnidocyte, hair cell, mechanoreception, postsynaptic potential
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXReferences |
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). However, more
recent studies show that they may provide informative paradigms for a number
of basic neurobiological mechanisms. The mechanosensory organelle of hydrozoan
nematocytes is the cnidocil apparatus, a concentric hair bundle composed of a
cilium and stereovilli originating from the same cell (in contrast to
multicellular hair bundles of sea anemones). The cnidocil apparatus can be
considered a prototype, as similar concentric hair bundles are found in
mechanosensory hair cells of most invertebrate phyla, from cnidarians to lower
chordates (Slautterback, 1967;
Golz and Thurm, 1991;
Golz, 1994;
Thurm et al., 1998a;
Holtmann and Thurm, 2001b).
Sensory transduction in hydrozoan nematocytes shows mechanically controlled
conductance changes similar to those of vertebrate hair cells but without
directional selectivity (Brinkmann et al.,
1996). Nematocyte receptor potentials drive the discharge of the
cnidocyst, exhibiting a specialized form of exocytosis
(Brinkmann et al., 1996;
Gitter and Thurm, 1996;
Nüchter et al., 2006). In
hydrozoan stenotele nematocytes, the probability of cyst discharge is
increased by contact-chemical stimuli acting at the shaft of the cnidocil thus
providing a paradigm for the modulation of exocytotic processes
(Thurm and Lawonn, 1990;
Brinkmann et al., 1995;
Thurm et al., 1998b;
Thurm et al., 2004).
Originally, nematocytes were thought to be independent senso-effectors
(Parker, 1916) with the
cnidocil acting as the sensory organelle controlling the cnidocyst, the
intracellular effector organelle. However, chemical stimuli have been known to
change the sensitivity of nematocytes via distinct chemoreceptor
cells in anthozoans (Pantin,
1942; Watson and Hessinger,
1989), suggesting intercellular communication. More recently,
electrical responses have been recorded in hydrozoan nematocytes in response
to water-soluble chemical stimuli and these responses were proposed to be
postsynaptic (Purcell and Anderson,
1995; Price and Anderson,
2006). Similarly, we have observed that mechanical stimulation of
cnidocils induces electrical responses in neighbouring nematocytes
(Brinkmann, 1994;
Brinkmann et al., 1995;
Thurm et al., 1998b;
Thurm et al., 2004),
suggestive of synaptic signalling between nematocytes. This observation thus
strongly challenges the concept of nematocytes as independent functional
units. In fact, electron microscopy demonstrated efferent and afferent
synaptic contacts at nematocytes
(Westfall, 1996;
Holtmann and Thurm, 2001a).
Here we provide a detailed electrophysiological analysis of the afferent and
efferent signalling pathways activating hydrozoan nematocytes.
We investigated Stauridiosarsia producta and Dipurena
reesi, both stolonial capitate Hydrozoa with large nematocytes.
Additionally, we chose to study Coryne tubulosa as the smaller size
of their tentacular spheres allowed for an electron-microscopical
characterization of all cells and their connectivity within complete spheres
(Holtmann and Thurm, 2001a;
Holtmann and Thurm, 2001b). We
show that hydrozoan stenotele nematocytes receive sensory signals from other
nematocytes as well as from sensory cells. We demonstrate two types of
stimulus-induced electrical events, both of which are generated via
chemical synapses. These types differ in the origin and route of the
presynaptic signals. The signals conveyed from other nematocytes disclose
nematocytes to play a unique role as bimodal sensory cells detecting
mechano-chemical stimuli indicative of prey. Consequently, nematocytes
contribute substantially to the sensory input of hydrozoan tentacles. Together
with sensory hair cells, they present an evolutionary primordial paradigm for
afferent synaptic signal transmission.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXReferences |
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).
All species were raised at our institute in artificial seawater at
14–16°C and fed with Artemia nauplia every 2–3 days.
Data presented are from Stauridiosarsia producta unless indicated
otherwise.
The tentacles of capitate Hydrozoa concentrate all nematocytes within their
non-contractile terminal spheres (Fig.
1). Besides the nematocytes, which are exclusively of the
stenotele type, the spheres comprise four types of sensory cells:
mechanosensory hair cells with a long or short cilium, rootlet cells and
vesicle-rich, probably chemosensory, cells
(Holtmann and Thurm, 2001b).
Somata of ganglion cells are absent in the spheres.
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).
Briefly, tentacles were cut from relaxed polyps and held at the sphere by a
suction capillary (Fig. 1).
Since the residual shaft at the sphere moved spontaneously and in response to
stimuli applied to the sphere, absence of contact with the capillary was
essential to keep the cells to be studied at a stable position and allowed the
observation of shaft movements. Sphere and shaft were continuously superfused
through one of three capillaries (inner diameter 
1mm) arranged
concentrically such that the tentacular sphere was located at the intersection
of their streams. The fluid flow (2–3 mm s–1) was
controlled by electromagnetic valves, yielding complete solution exchange
within <1 s. Reference solution was artificial seawater (ASW; in
mmoll–1: NaCl 414, MgCl2 24, MgSO4 24,
KCl 9, CaCl2 9, Hepes 1). To enable and accelerate penetration of
agents into the tissue, DMSO was added in two experiments. The concentrations
used (0.025–5%; v/v) had no effect on the sensitivity of nematocytes or
on the parameters studied. All measurements were done at
20–24°C.
During measurements, specimens were observed under a microscope with a x40 or x50 water immersion objective (NA 0.75 or 1.0) and Nomarski interference contrast optics. A video camera, video mixer and video recorder were used to synchronize the electrical recording with the microscopic image for on- and off-line evaluation.
For electrical measurements conventional microcapillaries filled with 1 mol
l–1 KCl or LiCl (for combination with Lucifer Yellow) were
used (20–60 M
). Intracellular voltage measurements were performed
in a current clamp circuit, current measurements in a single-electrode voltage
clamp circuit (switching between voltage measurement and current injection at
6–7.5 kHz; Sylgard-coated microcapillaries of reduced resistance:
8–15 M). Errors in space clamping can be estimated to be <5%
for the penetrated spherical soma and <20% for the stalk and neurites of
the nematocytes (estimated length constants 700µm and 200µm,
respectively (Fig. 1)
(Holtmann and Thurm, 2001a).
In some experiments penetrated cells were visualized by iontophoretic
injection of Lucifer Yellow through the recording electrode [10 nA
hyperpolarizing for 30 s (see Brinkmann et
al., 1996)].
For mechanical stimulation of individual cnidocils and sensory hair bundles
a glass probe (tip diameter 1–4 µm) was mounted onto a
two-dimensional piezo-electric driver. Its stimulating movements were
performed in the x–y plane and were
opto-electronically controlled in two feedback loops. The tip of the probe,
30–100 µm long, was bent at a right angle to be aligned parallel with
the optical axis. Thus, movements along the longitudinal axis of the probe in
the x–y plane could be used, providing high stiffness and
allowing for recording of tip movements with a time resolution of 10 µs
[for time course of excursions see Brinkmann
(Brinkmann et al., 1996); for
influence of steepness of stimulation see Appendix]. The opto-electronic
records obtained in the feedback control were calibrated and used in the
figures as stimulus trace. In the resting state, the tip of the probe was
usually not in contact with the cnidocil or hair bundle. Thus, the excursion
of the latter was smaller than the recorded tip movement.
For contact-chemical stimulation of individual cnidocils, the tip of a
glass probe (as above) was coated with
L-
-phosphatidylcholine, dissolved in 1:1 chloroform:methanol
(2mg10ml–1). The probe was dipped into this solution, then
air-dried for 
30 s. During experiments, coating was repeated at least
every 30 min.
For electrical stimulation of nematocytes, voltage impulses were applied by an extracellular capillary (5–15 µm diameter) filled with ASW. Its polished tip enclosed the apical area of a nematocyte without touching the cnidocil apparatus, thus avoiding mechanical stimulation. The capillary was pushed against the cell surface and negative pressure was applied to ensure close contact. Current was thus fed transepithelially and focused at a single nematocyte. An apical anodic voltage impulse thus hyperpolarized the apical surface membrane and depolarized the basolateral membrane, the site of putative afferent synapses, and vice versa for a cathodic impulse. In some experiments the tip of the capillary was filled with a suspension of lecithin micelles.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXReferences |
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). Mechanical or
electrical stimulation of the recorded cell itself did not induce similar
voltage changes. Corresponding to their different time courses, the voltage
changes were classified as L (long)-potentials or T
(transient)-potentials.
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The amplitudes of L-potentials that occurred without discharge of the stimulated cell, in contrast, varied between <1 mV and 24 mV (N=164) even for saturating stimuli, depending on the pre-stimulus history (see below). The latencies of these potentials varied between 2.6 ms and 45 ms (N=164) (Fig. 3A–C). A strict negative correlation between these latencies and amplitudes became evident when the responses from individual pairs of cells were compared, yielding correlation coefficients around –0.95 (Fig. 3C). Also the discharge-associated maximal amplitudes and minimal latencies fit this correlation in the individual pairs of cells. By contrast, when comparing discharge-associated responses of different pairs of cells, no correlation between amplitude and latency was found (coefficient 0.05; Fig. 3B).
When a given nematocyte was subjected to repeated stimulation at intervals
<1 min, without inducing a discharge, the amplitude and latency of
L-potentials recorded in a distant nematocyte usually decayed with repetitions
(Fig. 3A). This decay was most
prominent with purely mechanical stimulation. The number of responses that
could be elicited within a minute was 
5. Responsiveness recovered within
several minutes. While the responsiveness to stimulation of a given nematocyte
was still reduced, stimulation of another nematocyte could induce a maximal
response in the recorded cell (Fig.
3A), similar to that described above for sensory hair cell
stimulation. However, when the probe was lecithin-coated and stimulus
repetition rates were near or above 1 Hz, consecutive response amplitudes
usually increased and latencies decreased. This increase of amplitudes usually
culminated and coincided with the discharge of the stimulated cell
(Fig. 2C,D). The spatial
distance between the stimulated and the responding nematocyte, measured as the
divergence angle (Fig.
1), had no detectable influence on the amplitude or latency of
L-potentials (Fig. 3D).
(3) Pulling at the everted tubule (thread) of a discharged nematocyte
induced L-potentials with a latency of about 100 ms and reproducible amplitude
and time course at repetition rates of stimuli below 1 Hz. The probe was
hooked behind one of the stylets of the tubule and displaced the tubule by
about 1 µm. We assume that in this setting a sensory rootlet cell close to
the discharged nematocyte was stimulated
(Holtmann and Thurm, 2001b)
instead of the injured cell itself.
T-potentials
T-potentials appeared as discrete `transient' voltage changes, more uniform
in time course than L-potentials (Fig.
4) and somewhat reminiscent of action potentials. Compared to
L-potentials, their kinetics was faster, yet variable
(Fig. 4C,D; supplementary
material Table S1). Overlap of T-potentials caused only little summation (3 in
Fig. 4C). T-potentials often
occurred repetitively in a regular pattern of up to 3s–1. At
frequencies of 1s–1 amplitudes decreased with the number
of potentials (Fig. 4F,G). The
maximal amplitude was 40 mV in Stauridiosarsia and 55 mV in
Coryne.
In a subset of nematocytes (8% in Stauridiosarsia, 100% in Coryne and Dipurena) the depolarization was followed by a hyperpolarizing phase (maximal amplitude –3 mV in Stauridiosarsia, –11 mV in Coryne; Fig. 4D,E). A steep repolarization preceded the hyperpolarization. Thus, the duration of the depolarizing phase varied considerably, corresponding to the amplitude of the hyperpolarizing phase and yielding a duration at half amplitude between 5 and 32 ms in Stauridiosarsia. The amplitude of the actual hyperpolarization decreased with frequency (Fig. 4H) and number of repetitions (not shown) and was independent of the depolarizing phase (Fig. 4D 1).
T-potentials were generally accompanied by contractions of the epitheliomuscular cells of the tentacular shaft. Besides occurring spontaneously, i.e. without intended stimulation, T-potential and contraction were induced by the following mechanical stimulations: (i) a movement of a few micrometres of the probe against the tentacular shaft (`impact'; Fig. 4A); (ii) a forced bending of the shaft, (iii) contact of the glass probe with the cut surface of the shaft, (iv) combined mechanical and contact-chemical stimulation of a cnidocil (Fig. 3A and Fig. 5A), (v) a discharge of a nematocyte (Fig. 2D and Fig. 4F), (vi) pulling at the everted tubule (thread) of a discharged nematocyte (Fig. 4E 2 and Fig. 7C). In this case it was most obvious that the contractions occurred asymmetrically, leading to a bend of the tentacular shaft towards the site of stimulation.
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Signals are not propagated via the environmental space
The observed inter-cellular communication could be mediated by a chemical
or mechanical effect emitted by the stimulated cell into the surrounding sea
water, which in turn may trigger the electrical events observed in other
nematocytes. In particular, it is conceivable that the discharge of a
nematocyte emits some stimulatory substance or pressure wave. To test this
hypothesis, we stimulated nematocytes in one tentacle, inducing discharge
(N=6), while recording from nematocytes in a second, separate
tentacle about 80 µm apart. No response occurred under these conditions,
whereas responses could be evoked when two cells a similar distance apart were
part of the same tentacle.
Role of electrical coupling?
The contribution of electrical coupling of cells via gap junctions
was tested by applying octanol (1 mmol l–1; DMSO added, see
Methods). Octanol did not change the sensitivity of nematocytes to discharge
at adequate chemo-mechanical or local electrical stimulation. In the presence
of octanol, L-potentials occurred in nematocytes as usual when a distant
nematocyte discharged (N=4) or was stimulated at sub-threshold
strength. However, T-responses were completely absent in the presence of
octanol, even when the stimulated nematocyte discharged. In the absence of
octanol, in contrast, 90% of all distant discharge events were associated with
a series of T-potentials (Fig.
4F). Moreover, no spontaneous T-potentials occurred in the
presence of octanol.
Voltage dependence and reversal potentials
In order to study the voltage dependence of the conductances underlying T-
and L-responses, nematocytes of Stauridiosarsia were voltage-clamped
and responses were evoked by mechanical stimulation as shown in
Fig. 2C,D and
Fig. 3A. At clamped membrane
voltages near the resting potential of nematocytes (–50 to –70
mV), we recorded current responses with time courses similar to those of L-
and T-potentials (Fig. 5A,B and
Fig. 5C, respectively).
For a closer examination of T-responses, the tentacular shaft was stimulated repetitively. Current responses evoked at potentials between –70 and +130 mV (Fig. 5D,E) showed a near-linear dependence on membrane voltage (Fig. 5E), with a reversal potential between –5 mV and +29 mV (mean=+11±14 mV, N=4). At resting potential (–50 to –60 mV) the peak current was up to 1.5 nA. According to these results, the conductance that produces depolarizing T-type responses is voltage insensitive and amounts to about 20 nS.
At reduced negative membrane voltages, the initial inward current was often followed by an outward current. This delayed current occurred even in cells lacking a hyperpolarizing phase at unclamped resting potential (Fig. 5D, see also Fig. 4B). Its voltage dependence indicates a negative reversal potential down to –70 mV in Coryne (c.f. Fig. 4D). Under voltage clamp, the rise of the delayed outward current curtails the decay of the initial inward current (at –10 mV; e.g. Fig. 5D), indicating that the onset of this hyperpolarizing component occurs during the depolarizing current.
Since the spike-like appearance of the T-potentials initially suggested
some relation to action potentials, we further characterized the conductances
underlying regenerative spiking in nematocytes
(Brinkmann et al., 1996).
Action potentials, in contrast to T-potentials, could be elicited in
nematocytes by depolarizing the cell to above –15 mV (threshold in
Stauridiosarsia: –7.7±2.6 mV, N=30;
Fig. 6A–C). In voltage
clamp, a corresponding voltage-dependent conductance was activated above
–10 mV (Fig. 6D,E).
Additionally, in many cells a non-inactivating voltage-sensitive conductance
was activated at about –40 mV (Fig.
6C). The resulting regenerative voltage step could boost a
depolarization to elicit an action potential. Each suprathreshold current
injection elicited only one action potential, independent of the duration of
depolarization.
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Ionic dependence of L- and T-potentials
Dependence on sodium
Substitution of sodium by choline reversibly abolished T-potentials in
Stauridiosarsia within about 4min; recovery after return to ASW also
required about 4min (Fig. 7A).
Substitution of sodium by choline also blocked action potentials of
nematocytes (not shown). Na+ dependence of L-potentials was not
systematically tested, as the nematocyte discharge that induces L-potentials
also required Na+ (data not shown). However, L-potentials seem to
have a different Na+ dependence than T-potentials, as L-potentials
could be induced without amplitude change only 35 s after sodium re-addition
(following 4min of sodium removal), while T-potentials were still
abolished.
Dependence on calcium
In `Ca2+-free' sea water (ASW without Ca2+ added)
T-potentials were reversibly reduced to nearly 0 mV within 1 min
(Fig. 7B). By contrast, action
potentials remained unchanged by a reduction of [Ca2+] to as low as
0.1 mmol l–1 (not shown). L-potentials have not been tested
in Ca2+-free seawater, since removal of Ca2+ impairs
their stimulatory input, i.e. the stability of hair bundles
(Brinkmann et al., 1996). At a
tenfold reduction of the Ca2+ concentration to 0.9 mmol
l–1 L-potentials remained constant in amplitude while the
amplitude of T-potentials was reduced to about 5 mV.
Dependence on Mg2+/Ca2+ ratio
When we reduced Ca2+ concentration as in the previous experiment
(1mmoll–1) but increased Mg2+ concentration
twofold (96mmoll–1; i.e. Mg2+/Ca2+
ratio increased 18-fold) both L- and T-responses were completely abolished
within 5 min (Fig. 7C).
Restitution of normal ASW reversed the effect within 3–5 min. The
reverse, a reduction of [Mg2+] to about 10% (5 mmol
l–1) with an unchanged [Ca2+], changed the
spontaneous occurrence of T-potentials within 1 min to irregularly
long-lasting (up to >1 s) and partially multi-phasic potentials
(Fig. 7D).
Involvement of neurotransmitter receptors
We studied the effects of modulators of synaptic transmission in
Dipurena. The responses of a given nematocyte were highly
reproducible when a distant nematocyte was stimulated repetitively at
intervals of approximately 1 min, using purely mechanical stimuli of constant
strength that did not induce discharge. Each agent was tested on at least five
recorded cells of at least two polyps.
Superfusion with 50 mmol l–1 sodium glutamate in ASW caused a reversible tonic depolarization of the nematocytes of 9±3 mV within 5 min (Fig. 8A). Simultaneously, the amplitudes of superimposed L- and T-potentials decreased from 42±5 mV to 32±5 mV (Fig. 8Ab), i.e. the absolute peak voltage remained about constant.
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The cholinergic agonists acetylcholine (0.1–5 mmol
l–1) and carbachol (0.1 pmol l–1 to 1 mmol
l–1) had no detectable influence on L- and T-potentials.
Acetylcholine (5 mmol l–1), however, reversibly paralyzed the
contractility of the whole superfused polyps within 5–10 s. Carbachol
efficiently sensitized nematocytes, as already described
(Sieger and Thurm, 1997).
The cholinergic antagonist mecamylamine blocked the hyperpolarizing component of T-potentials reversibly at a concentration of 10 µmol l–1 within 5 min (Fig. 8C). The opposite effect was exerted by physostigmine, a blocker of choline esterase: 100 µmol l–1 physostigmine doubled the hyperpolarizing amplitude and increased its duration at half amplitude from 210±63 ms to 418±85 ms (N=5; Fig. 8D).
Reserpine, applied in order to test for an involvement of catecholamines or serotonin, had no noticeable effect on L- and T-potentials in concentrations of 1–100 µmol l–1.
Electrical stimulation of presynaptic nematocytes
In order to directly test a relationship between pre- and postsynaptic
electrical events of communicating nematocytes, we bypassed mechanoelectrical
transduction by directly applying square pulses of voltage to single
nematocytes, using a transepithelial configuration of electrodes (see
Materials and methods). As described above for mechanical stimulation, the
membrane voltage of distant nematocytes was recorded simultaneously.
Basolaterally depolarizing (anodic) voltage pulses of 100–200 mV at 20–40 ms duration induced L-potentials in distant nematocytes with characteristics as shown before (Fig. 2E). Similar to the largest L-potentials, the responding cell depolarized up to about –25 mV when the stimulated cell discharged (eight of 12 stimulated cells). Responses that were not associated with a discharge of the stimulated cell had variable sub-maximal amplitudes (Fig. 2E).
Apically depolarizing (cathodic) pulses elicited discharge of the
stimulated nematocyte at considerably smaller amplitude: –25 to
–95 mV [comparable to results obtained with Hydra
(Gitter and Thurm, 1996)]. In
the recorded nematocytes (N=6) these discharges were associated with
a large L-potential of short latency (<10 ms) and by superimposed
T-potentials, as described above for discharges elicited mechanically
(Fig. 2F; c.f.
Fig. 2D and
Fig. 3A, cell II). When a
cathodic pulse did not elicit a discharge it did not induce an L- or
T-potential; i.e. these basolaterally hyperpolarizing impulses did not induce
sub-maximal, graded L-potentials, in contrast to basolaterally depolarizing
impulses.
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXReferences |
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). We classify these signals according to their different
stimulus-dependent properties. (1) L-potentials: these are characterized by a
graded amplitude between a few mV and 40 mV and of variable duration. These
parameters correlate to some degree with parameters of the stimulus, however,
less strictly than found for receptor potentials
(Brinkmann et al., 1996). (2)
T-potentials: the time course and occurrence is comparable with those of
action potentials as they have stereotyped impulse-like waveforms with a
variable number and frequency of repetitions.
Are T-potentials actually action potentials as Price and Anderson
(Price and Anderson, 2006)
proposed for T-potential-like responses of Cladonema, a capitate
hydroid closely related to the species examined in this study? Their `Class I'
and `Class II' potentials were induced in nematocytes by water-borne
chemosensory stimulation of the isolated tentacle. Two of our findings show
that the ion channels which generate T-potentials are electrically
non-excitable: (1) T-potentials cannot be triggered by depolarization of the
recorded nematocyte; (2) the current–voltage relationship of
T-potentials is linear. Genuine action potentials, by contrast, can indeed be
triggered electrically in nematocytes as demonstrated in
Fig. 5 (see also
Brinkmann et al., 1996;
Anderson and McKay, 1987).
However, their threshold is remarkably high (–10 mV) and is usually not
even reached by the peaks of T-potentials. Moreover, T-potentials are
different in their Ca2+ dependence and time course from the action
potentials of the same cells. These findings unequivocally reveal that
T-potentials are not action potentials.
Are T- and/or L-potentials conducted electrotonically from a separate
cellular source into the observed nematocyte, e.g. via gap junctions?
Since Price and Anderson (Price and
Anderson, 2006) proposed that nematocytes in Cladonema
are electrically coupled to one another, implying that `Class I and II
potentials' were conducted from cell to cell, we carefully considered this
possibility. However, several lines of evidence converge to the conclusion
that intact nematocytes are not electrically coupled.
). As a caveat, it cannot be excluded that cell
couplings ofminute size, as described by Germain and Anctil
(Germain and Anctil, 1996),
might have been overlooked in nematocytes. ); Stauridiosarsia
(Brinkmann et al., 1996);
Hydra (Lawonn,
1999)]. However, when the dye was injected into an
epitheliomuscular cell, it spread into neighbouring epitheliomuscular cells,
but not into nematocytes [experiments in Hydra
(Lawonn, 1999)]. Octanol,
known to block gap junctions of hydrozoans as well as of anthozoans
(Dunlap et al., 1987;
Mire et al., 2000), prevented
the spread of Lucifer Yellow between epitheliomuscular cells
(Lawonn, 1999). It should be
mentioned that one study (Price and
Anderson, 2006) observed spreading of Lucifer Yellow from an
injected to neighbouring nematocytes and to other unidentified cells `with
time' and after prolonged injection of stain (5–20 min, unspecified
current strength). It seems possible that this spreading was due to
deterioration of the impaled cell. It was not blocked by heptanol and octanol.
However, electrical coupling of cells can exist even if Lucifer Yellow is not
able to permeate (Murphey et al.,
1983; Shibayama et al.,
2005). ); studying Cladonema: coupling between nematocytes
(Price and Anderson, 2006)].
These measurements may have produced ambiguous results because the reference
electrode for injected current was located outside the epithelial tissue. Thus
this current could lead to voltage drops across cells surrounding the injected
cell even if these cells were not directly coupled to the cell receiving the
current injection (c.f. Fig.
2E,F). ; Thurm et al.,
1998a; Thurm et al.,
1998b; Thurm et al.,
2004). In more than 50 recorded nematocytes the stimulation of
neighbouring or distant nematocytes never led to the occurrence of any
attenuated receptor potential but was exclusively reflected by L-potentials.
Accordingly, nematocytes were neither directly nor indirectly electrically
coupled with each other. The same conclusion holds true for nematocytes from
Cladonema: large L-potential-like potentials (`Class III') induced by
adequate chemical stimuli were not reflected in neighbouring nematocytes
(Price and Anderson, 2006),
arguing against any ohmic coupling between these cells. In principle, one
cannot exclude artificial decoupling of a nematocyte induced by penetration of
the microelectrode. However, all impaled and successfully observed cells in
our study showed T-potentials in the absence of electrical coupling of
receptor potentials, arguing against an electrotonic origin of T-potentials.
This conclusion cannot be disproved either by the synchronicity of
T-potential-like potentials in neighbouring nematocytes as observed in
Cladonema and Physalia
(Purcell and Anderson, 1995;
Price and Anderson, 2006) or
by the block of T-potentials under Octanol that we observed. Both findings
will be discussed below as being due to the presynaptic signal source. ). The
amount of deviation depends on the coupling coefficient. Even a low resistance
coupling, with coupling coefficient 0.5, would increase the difference between
resting and true reversal potential by 100%. The amplitude of regular
T-potentials argues against such an error in the measured reversal
potential.]
Altogether, our findings indicate that T- or L-potentials do not enter
nematocytes by passive propagation via gap junctions. Thus, the
electrical sources of L- and T-potentials must be located in the recorded
nematocyte itself, suggesting a postsynaptic mechanism. In fact, we found
features of chemical synapses. L- and T-potentials were blocked by an increase
in [Mg2+] at reduced Ca2+-concentration, typical for an
exocytotic mechanism (Katz and Miledi,
1967). Furthermore, L- and T-potentials were abolished by
antagonists of the neurotransmitters glutamate and acetylcholine,
respectively, while glutamate depolarized the cell, and blocking of
acetylcholine-degradation increased hyperpolarizing responses. Long delays of
all these actions (5–30 min) indicate low accessibility of the site of
action and suggest considerable difference between applied and effective
concentrations. Electron microscopy revealed afferent and efferent synaptic
structures located basally at the nematocytes and separated from the surface
medium by septate junctions, known as diffusion barriers
(Holtmann and Thurm, 2001a;
Holtmann and Thurm, 2001b).
Histochemical identification of glutamate receptors at nematocytes has been
reported (Kass-Simon and Scappaticci,
2004; Kass-Simon and Pierobon,
2007).
From this set of results we conclude that both, L- and T-potentials are
postsynaptic potentials (PSPs), probably induced by glutamate; both are
excitatory (EPSP), with the exception of the hyperpolarizing second component
of T-potentials. According to the nicotinic pharmacology and delayed time
course, this latter component is probably a separate, though tightly
EPSP-coupled, synaptic event. Its hyperpolarization caused by a negative
reversal potential characterizes it as an inhibitory PSP (IPSP). Purely
hyperpolarizing T-PSP-like potentials that occur independently of a
depolarizing component have been found previously in epitheliomuscular cells
of Hydra (Lawonn,
1999).
The magnitude of the T-IPSP phase differed between different species and
individual cells and thus contributed to the considerable variability of the
time course and duration of T-potentials, since the inhibitory component
curtailed the excitatory to varying degrees, depending on their amplitude
ratio and phase relation. When T-potentials occurred repetitively, the
amplitudes of T-EPSPs and T-IPSPs decreased independently of each other with
the number and frequency of repetitions (c.f.
Fig. 4G and
Fig. 3H). These reductions may
be interpreted as synaptic depression at two independent synaptic sites that
are driven from a common source of excitation. The same processes are likely
to explain the differences between `Class I' and `Class II' potentials and
their variability and transitions, which Price and Anderson
(Price and Anderson, 2006)
found to be induced by chemosensory stimulation in the nematocytes of
Cladonema.
Neuronal connectivity underlying the generation of synaptic signals in nematocytes
L- and T-potentials largely differ in their induction, time course and
susceptibility to the gap-junction blocker octanol. These differences provide
important insights into their presynaptic input, electrical signalling and
neuronal connectivity.
L-potentials were induced by mechanical stimulation of single sensory hair
cells and mechanical or electrical stimulation of other nematocytes. Hair
cells are in somato-somatic synaptic contact with nematocytes and nematocytes
are in somato-neuritic contact with other nematocytes
(Holtmann and Thurm, 2001a).
Taken together, our results and previous ultrastructural findings suggest that
L-potentials should arise from monosynaptic transmission and that presynaptic
receptor potentials as well as action potentials
(Brinkmann et al., 1996) can
induce postsynaptic L-potentials. However, with monosynaptic transmission, why
do L-potentials show such variable amplitudes and latencies, in particular the
conspicuous differences between those L-responses associated with discharge of
the stimulated cyst and those not discharge-associated
(Fig. 3B,C)? Considering the
putative presynaptic potentials sheds some light on the mechanisms underlying
these differences. The discharge of a cyst is associated with an action
potential triggered by a receptor potential or by injected current
(Brinkmann, 1994;
Brinkmann et al., 1996). The
presynaptic event of discharge-associated L-potentials is a `standardized'
signal with an amplitude of about 80 mV, consistent with the relatively
uniform amplitudes of these L-potentials of 25±6 mV (up to 44 mV) and
with the uniform latencies of 2.1±1.2ms. However, L-potentials that
were not discharge-accompanied reached amplitudes of 24 mV at most, despite
being recorded from the same pairs of nematocytes and induced by equally
saturating mechanical stimuli as the discharge-associated L-potentials.
Receptor potentials, measured under these conditions, had amplitudes of
40–50 mV and mostly did not elicit an action potential
(Brinkmann et al., 1996). This
indicates that the postsynaptic voltage signal is a function of the
presynaptic signal amplitude, i.e. the synapse generating L-potentials works
in a graded manner.
However, in the absence of a discharge, L-EPSPs showed a correlated
variation in amplitude and in latency. Typically, such variation occurred as a
gradual decline of L-potential amplitude with concomitant increase in latency
upon repeated mechanical stimulation of the presynaptic cell of a given pair
of nematocytes. This finding suggests that the synaptic process that generates
L-potentials is subject to strong synaptic depression. Several lines of
evidence suggest that this alteration targets synaptic mechanisms in the
presynaptic, but not the postsynaptic cell. First, L-potentials depended on
the pre-stimulus history of the stimulated cell, including repeated mechanical
stimulation and chemical sensitization. Second, amplitude reduction induced by
stimulation of one presynaptic cell had no impact on the size of L-potentials
induced by stimulation of a different presynaptic nematocyte. Third,
presynaptic receptor potentials do not show comparable adaptation in response
to repeated stimulation (Brinkmann et al.,
1996; Thurm et al.,
2004), indicating modulation of downstream events. Finally, it is
quite unlikely that the observed concomitant reduction of amplitude and
increase of latencies results from a reversible and gradual change in synaptic
connectivity from a monosynaptic contact to a chain of serial synapses.
In fact it has been shown that short-term plasticity, like depression,
facilitation and synaptic modulation in various model synapses may involve
latency increases with amplitude depression
(Waldeck et al., 2000;
Boudkkazi et al., 2006) and
latency decreases with facilitation or positive modulation
(Vyshedskiy and Lin, 1998;
Vyshedskiy et al., 1998). The
surprisingly large extent of use-dependent latency increase in nematocytes is
associated with a unique organization of these synapses (c.f.
Holtmann and Thurm,
2001a).
For comparison with delay times of established monosynaptic transmission,
we only considered L-potentials that were not affected by synaptic depression,
i.e. L-potentials associated with cyst-discharge. Their latencies of
2.1±1.2 ms are well within the range of delays for neuromuscular
synapses of other hydrozoans, e.g. 0.9–7.0 ms in Polyorchis
(mean 3.2 ms) (Spencer, 1982)
and 0.7±0.1 ms in Aglantha
(Kerfoot et al., 1985). It
should be noted, that for meaningful comparison one has to add at least 1 ms
to the motoneuronal delays since these have been measured from the peak of the
presynaptic action potentials whereas L-potential latencies were measured from
the onset of the mechanical stimulus [delay and rise time of the receptor
potential 1 ms (Brinkmann et al.,
1996)]. Thus, the induction of L-potentials is fully consistent
with monosynaptic transmission between nematocytes.
A series of two or more nematocyte–nematocyte synapses also are conceivable, since discharge-associated L-potentials approach the amplitude of large receptor potentials; thus the L-potential itself may trigger transmission onto another nematocyte. L-potentials that result from transmission through two or three synapses sequentially may be expected to show an increased latency and a smaller amplitude. Yet, when different pairs of cells were compared, no correlation between latency and amplitude of discharge-associated L-potentials was found (Fig. 3B). However, di- or polysynaptic L-responses may instead contribute to the delayed components of large multiphasic L-potentials (Fig. 2D and Fig. 3A).
In conclusion, our data support the idea that L-potentials are predominantly transmitted by a single synaptic step and are the postsynaptic response to presynaptic receptor and action potentials of nematocytes and hair cells. This is in agreement with the ineffectiveness of the gap junction blocker octanol and previous ultrastructural data.
T-potentials, in contrast to L-potentials, were not found to have any
presynaptic electrical correlate in directly stimulated and discharging
nematocytes (Brinkmann, 1994;
Brinkmann et al., 1996).
Instead, T-potentials were evoked by mechanical stimulation at any distance
from the recorded nematocyte, e.g. by minute deformations of the tentacular
sphere or shaft. This suggests that the tentacular signals that ultimately
induce T-potentials spread over long distances without any appreciable
amplitude decay, pointing to the involvement of action potential conduction.
If action potentials are the presynaptic events of T-potentials this explains
the uniform amplitude and time course of T-potentials. The fact that
T-potentials are mostly or always associated with contractions of
epitheliomuscular cells suggests that the ectoderm is a pathway for this
presynaptic conduction. T-potential-like depolarisations were indeed recorded
in epitheliomuscular cells of Hydra
(Lawonn, 1999). Gap
junction-connected epitheliomusclular cells are an essential pathway of
electrical signal conduction in Cnidaria (reviewed by
Mackie, 2004). Their
involvement explains the blockage of T-potentials by octanol. This more
complex kind of signal transmission preceding T-potentials, compared with
L-potentials, is well consistent with the differences between their shortest
latencies (T-potentials: 30 ms; L-potentials: 0.6 ms) and the different delays
of signal recovery after Na+ removal.
Even when the site of stimulation inducing T-potentials was the tentacular
sphere, epitheliomuscular cells of the tentacular shaft contracted. Thus,
epithelial conduction was involved, although in the sphere non-contractile
supporting cells take the place of epitheliomuscular cells. The supporting
cells enclose each nematocyte and sensory cell and are interconnected
via gap junctions like the contractile cells
(Holtmann and Thurm, 2001b).
As a consequence, all or many nematocytes of a tentacular sphere should be
surrounded by essentially the same flow of action potentials, leading to
similar patterns of T-PSPs in all of these nematocytes. In accordance with
this conclusion, Purcell and Anderson
(Purcell and Anderson, 1995)
and Price and Anderson (Price and
Anderson, 2006) found T-potential-like EPSPs (`Class I and II
potentials') that were synchronous in all recorded pairs of nematocytes within
a tentacular battery of Physalia or a tentacle of Cladonema,
whereas L-potential-like slow postsynaptic potentials in the same records did
not exhibit coincidence (Price and
Anderson, 2006), indicating that the synchrony did not result from
direct electrical coupling between nematocytes.
In conclusion, our data demonstrate that T-potentials result from chemical
transmission and are induced by signals conducted through the tentacle, most
likely epithelial action potentials. The observation that T-potentials are
composed of phase-coupled excitatory and inhibitory PSPs generated by
different transmitters, points to a complex synaptic organization. More
functional data are required for a mechanistic interpretation of the available
ultrastructural results (Holtmann and
Thurm, 2001a).
Nematocytes as sensory cells
L- and T-potentials induced by stimulation of distant nematocytes reveal
that stenotele nematocytes produce afferent synaptic signals, i.e. they act as
sensory cells. Evidently, the discharge of the cnidocyst is not the sole
action of these cells, as often assumed, but the very last and most striking.
Other types of cells, besides nematocytes, also receive afferent signals from
nematocytes, as is revealed by the occurrence of T-potentials: stimulated
nematocytes induce T-potentials in other nematocytes, but they do not directly
produce the presynaptic potentials that elicit these T-potentials.
Probability and amplitude of L-responses were dependent on the chemical
properties of the surface of the stimulation probe. Thus, nematocytes function
as mechanoreceptor cells with substance-dependent mechanical sensitivity,
adding a vital sensory quality to the spectrum of mechanosensation that is
provided by the sensory cells (Holtmann
and Thurm, 2001b). Although having no effect on the
mechanoreceptor potential of nematocytes
(Brinkmann et al., 1996),
chemical stimuli strongly affect their exocytotic outputs, i.e. the cnidocyst
discharge and presynaptic activity. Therefore, we propose that sensory
bi-modality results from chemosensory modulation of the exocytotic processes
[see Thurm et al. (Thurm et al.,
1998b) and Appendix]. It is conceivable that the same chemosensory
intracellular signalling pathway modulates both the apical cnidocyst discharge
and the basolateral exocytotic release of synaptic transmitter.
Functional role of efferent signals in nematocytes
Our recordings from nematocytes revealed that these cells receive signals
reflecting a considerable number of different mechanical events acting on the
tentacle. Besides the mechano-chemical stimulation of other nematocytes and
their cyst discharge, these stimuli are: hydrodynamic movements, touch of the
tentacular surface, deformations and contractions of the tentacular shaft, and
forces pulling at discharged cnidocysts. Similar responses can be induced by
water-soluble chemical stimuli, as reported from other hydrozoan species
(Purcell and Anderson, 1995;
Price and Anderson, 2006).
Both sensory modalities are reflected in the nematocytes by graded EPSPs,
induced by presynaptic receptor potentials, and by more stereotyped PSPs,
which we attributed to presynaptic epithelial action potentials. In contrast
to Hydra (Thurm et al., 1998), feeding or starvation of the Corynidae
did not change the behaviour of the nematocytes (D.O., M.B., T.S. and U.T.,
unpublished observation), which may be related to the nutritional buffer
capacity of the stolonial configuration of Corynidae as opposed to
non-stolonial Hydra.
What is the impact of L- and T-potentials on nematocyte function? So far,
we can infer that synaptic depolarization adds to any receptor potential
resulting from direct stimulation of the nematocyte. Synaptic potentials may
thus contribute to reaching the threshold for discharge of the cnidocyst
(Brinkmann et al., 1996) (c.f.
Kass-Simon and Scappaticci,
2004). An opposite effect may result from the inhibitory component
of T-potentials. However, T-potentials may be too brief to have substantial
influence in this direct way. Instead, one may speculate that postsynaptic
second messenger pathways may modulate the sensitivity or readiness of a
response, comparable to the modulation exerted by contact-chemical stimulation
of the cnidocil (Thurm et al.,
1998b). While our study revealed the integrative signal flow to
and from nematocytes, it remains a challenge for future studies to work out
the cellular effects of the synaptic inputs found.
Comparative aspects of neurotransmission at nematocytes
The organization of synaptic communication of nematocytes and hair cells of
hydrozoans shows an intriguing analogy to that of vertebrate hair cells. The
mechanosensory cell types from both phyla are characterized by the axon-less
(`secondary') type of sensory cell configuration. Graded receptor potentials
are the presynaptic afferent signals in both cases, instead of action
potentials as in axon-bearing sensory cells. Afferent synapses of both cell
types present highly specialized, but different presynaptic structures, which
may be adaptations to the specific presynaptic electrical signals. The
ribbon-type afferent synapse of vertebrate hair cells uses a large number of
small vesicles (Fuchs et al.,
2003) whereas in nematocytes, a magno-vesicular structure
comprises only one or a few unusually large vesicles (up to 1µm in
diameter) supposed to release transmitter in transient fusion and partial
depletion (Neher, 1993;
Holtmann and Thurm, 2001a).
Both presynaptic structures are considered to be optimized for sustained
transmission of the graded presynaptic signals. Similarities extend to the
sensory cell's transmitters. The excitatory afference supplied by nematocytes,
generating L-potentials, appears to be glutamatergic, using non-adapting
kainate-like receptors. Indeed, glutamate receptors have been identified at
nematocytes in Hydra (Kass-Simon
and Scappaticci, 2004;
Kass-Simon and Pierobon,
2007). Similarly, glutamate is the transmitter of the afferent
synapses of vertebrate hair cells (Fuchs et
al., 2003). Moreover, both nematocytes and vertebrate hair cells
receive an inhibitory efference, operated by presynaptic action potentials. In
nematocytes, inhibition appears to be mediated by acetylcholine via
neuronal nicotinic-like receptors, as inhibitory T-potentials were blocked by
mecamylamine. Similarly, the efferent synapses at vertebrate hair cells are
cholinergic with postsynaptic nicotinic receptors
(Fuchs, 1996). Their inhibitory
PSPs show a biphasic behaviour somewhat reminiscent of the T-potentials
characterized here, but evoked by the single transmitter which in that case
activates a depolarizing Ca2+ influx that subsequently activates
the hyperpolarizing K+ efflux.
|
APPENDIX |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXReferences |
|---|
Square pulses, apically depolarizing single nematocytes at 100 mV
transepithelial amplitude, triggered cnidocyst discharges with a probability
of 78% (N=143), a probability much higher than that produced by
purely mechanical stimulation. When, at unchanged stimulus amplitude, the
steepness of apical depolarization was reduced (ramp 9 ms/90% rise, in
contrast to 33 µs/90% rise of previous pulses) in order to mimic the
maximal steepness of receptor potentials of nematocytes [2–4 ms/90% rise
(Brinkmann et al., 1996)], the
discharge probability was reduced to 10% (N=31), approaching the
probability of 8% obtained by purely mechanical stimuli (N=123).
Chemical sensitization of the cells by phosphatidylcholine, which increased
the probability of discharge at mechanical stimulation from 8% to 67%,
increased the probability of discharge for the steep electrical pulses to
nearly 100%. (Phosphatidylcholine applied within the electrode capillary: see
Materials and methods.)
The results reveal that the voltage-dependent process, which controls the
exocytotic cyst discharge, includes a step that differentiates the time course
of depolarization with a rate in the range of the steepness of receptor
potentials. Consequently, changes of this rate must be crucial for changing
the probability of inducing cyst discharges. As a working hypothesis we
propose that reducing this crucial rate is the process of sensitization that
is induced by the contact-chemoreception of the cnidocil. Earlier results
suggested that the cyst discharge is initiated by voltage-controlled
Ca2+ influx (Gitter et al.,
1994; Gitter and Thurm,
1996; Thurm et al.,
1998b; Nüchter et al.,
2006). We suggest time-dependent inactivation of this Ca influx as
a reasonable candidate for the depolarization-differentiating process that may
be rate-controlled by the chemosensory modulation. We invite experiments
testing this hypothesis.
|
Acknowledgments |
|---|
|
Footnotes |
|---|
* Present address: Institute for Physiology and Pathophysiology, Department
of Neurophysiology, University of Marburg, Deutschhausstrasse 2, D-35037
Marburg, Germany 
Present address: Smiths Medical International, Nottulnerstrasse 25, D-48249
Buldern, Germany
Present address: Tyska Skolan, Karlavägen 25, S-11431 Stockholm,
Sweden
|
References |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXReferences |
|---|
Anderson, P. A. V. and McKay, M. C. (1987). The
electrophysiology of cnidocytes. J. Exp. Biol.
133,215
-230.
Boudkkazi, S., Carlier, E., Ankri, N., Fronzaroli, L., Caillard, O. and Debanne, D. (2006). Synaptic timing at unitary synaptic contacts between layer 5 pyramidal neurons. FENS Abstr. vol. 3,A151.4 .
Brinkmann, A. and Petersen, K. W. (1960). On some distinguishing characters of Dipurena reesi Vanucci 1956 and Cladonema radiatum Dujardin 1843. Publ. Stat. Napoli 31,386 -392.
Brinkmann, M. (1994). Das Rezeptor-Effektor-System der Nesselzellen eines Hydropolypen (Corynidae). Inaugural-Dissertation der Naturwissenschaftlichen Fakultät der Universität Münster,1 -115.
Brinkmann, M., Oliver, D. and Thurm, U. (1995). Interaction of mechano- and chemosensory signals within the same sensory cell. Pflügers Arch. Eur. J. Physiol. 429 (Suppl),R153 .[CrossRef]
Brinkmann, M., Oliver, D. and Thurm, U. (1996). Mechanoelectric transduction in nematocytes of a hydropolyp (Corynidae). J. Comp. Physiol. A 178,125 -138.
Dunlap, K., Takeda, K. and Brehm, P. (1987). Activation of a calcium-dependent photoprotein by chemical signalling through gap junctions. Nature 325, 60-62.[CrossRef][Medline]
Fuchs, P. A. (1996). Synaptic transmission at vertebrate hair cells. Curr. Opin. Neurobiol. 13,514 -519.
Fuchs, P. A., Glowatzki, E. and Moser, T. (2003). The afferent synapse of cochlear hair cells. Curr. Opin. Neurobiol. 13,452 -458.[CrossRef][Medline]
Germain, G. and Anctil, M. (1996). Evidence for intercellular coupling and connexin-like protein in the luminescent endoderm of Renilla koellikeri (Cnidaria, Anthozoa). Biol. Bull. 191,353 -366.[Abstract]
Gitter, A. H. and Thurm, U. (1996). Rapid exocytosis of stenotele nematocysts in Hydra vulgaris. J. Comp. Physiol. A 178,117 -124.
Gitter, A. H., Oliver, D. and Thurm, U. (1994). Calcium- and voltage-dependence of nematocyst discharge in Hydra vulgaris.J. Comp. Physiol. A 175,115 -122.
Golz, R. (1994). The apical surface of hydrozoan nematocytes: Structural adaptations to mechanosensory and exocytotic functions. J. Morphol. 222, 49-59.[CrossRef]
Golz, R. and Thurm, U. (1991). Cytoskeleton-membrane interactions in the cnidocil complex of hydrozoan nematocytes. Cell Tissue Res. 263,573 -583.[CrossRef]
Holtmann, M. and Thurm, U. (2001a). Mono- and oligo-vesicular synapses and their connectivity in a cnidarian sensory epithelium (Coryne tubulosa). J. Comp. Neurol. 432,537 -549.[CrossRef][Medline]
Holtmann, M. and Thurm, U. (2001b). Variations of concentric hair cells in a cnidarian sensory epithelium (Coryne tubulosa). J. Comp. Neurol. 432,550 -563.[CrossRef][Medline]
Johnston, D. and Wu, S. M.-S. (1995). Foundations of Cellular Neurophysiology, pp.369 , 392-395. London: MIT Press.
Kass-Simon, G. and Pierobon, P. (2007). Cnidarian chemical neurotransmission, an updated overview. Comp. Biochem. Physiol. A 146,9 -25.[CrossRef][Medline]
Kass-Simon, G. and Scappaticci, A. A., Jr (2004). Glutamatergic and GABAnergic control in the tentacle effector systems of Hydra vulgaris. Hydrobiologia 530/531,67 -71.
Katz, B. and Miledi, R. (1967). The timing of
calcium action during neuromuscular transmission. J.
Physiol. 189,535
-544.
Kerfoot, P. A. H., Mackie, G. O., Meech, R. W., Roberts, A. and
Singla, C. L. (1985). Neuromuscular transmission in the
jellyfish Aglantha digitale. J. Exp. Biol.
116, 1-25.
Lawonn, P. (1999). Die rezeptorischen und effektorischen Funktionen der Nesselzellen von Hydra vulgaris und ihre sensorische und synaptische Modulation. Inaugural-Dissertation, Naturwissenschaftliche Fakultät der Universität Münster1 -187.
Mackie, G. O. (2004). Epithelial conduction: recent findings, old questions, and where do we go from here? Hydrobiologia 530/531,73 -80.
Mire, P., Nasse, J. and Venable-Thibodeaux, S. (2000). Gap junctional communication in the vibration-sensitive response of sea anemones. Hear. Res. 144,109 -123.[CrossRef][Medline]
Murphey, A. D., Hadley, R. D. and Kater, S. B. (1983). Axotomy-induced parallel increases in electrical and dye coupling between identified neurons of Heliosoma. J. Neurosci. 3,1422 -1429.[Abstract]
Neher, E. (1993). Secretion without full fusion. Nature 363,497 -498.[CrossRef][Medline]
Nüchter, T., Benoit, M., Engel, U., Özbek, S. and Holstein, T. W. (2006). Nanosecond-scale kinetics of nematocyst discharge. Curr. Biol. 16,R316 -R318.[CrossRef][Medline]
Pantin, C. F. A. (1942). The excitation of nematocysts. J. Exp. Biol. 19,294 -310.[Abstract]
Parker, G. H. (1916). The effector systems of actinians. J. Exp. Zool. 21,461 -484.[CrossRef]
Price, R. B. and Anderson, P. A. V. (2006). Chemosensory pathways in the capitate tentacles of the hydroid Cladonema. Invert. Neurosci. 6, 23-32.[CrossRef]
Purcell, J. E. and Anderson, P. A. V. (1995). Electrical responses to water-soluble components of fish mucus recorded from the nematocytes of a fish predator, Physalia physalis. Mar. Fresh. Behav. Physiol. 26,149 -162.
Shibayama, J., Paznekas, W., Seki, A., Taffet, S., Jabs, E. W.,
Delmar, M. and Musa, H. (2005). Functional characterization
of connexin43 mutations found in patients with oculodentodigital dysplasia.
Circ. Res. 96,e83
-e91.
Sieger, T. and Thurm, U. (1997). Glutamatergic properties of synaptic transmission of the hair-cell-analog nematocytes of Corynidae polyps and their modulation by choline-derivatives. Göttingen Neurobiol. Report 1997, 736.
Slautterback, D. B. (1967). The cnidoblast-musculoepithelial cell complex in the tentacles of Hydra. Z. Zellforschg. 79,296 -318.[CrossRef]
Spencer, A. N. (1982). The physiology of a coelenterate neuromuscular synapse. J. Comp. Physiol. 148,353 -363.[CrossRef]
Tardent, P. (1995). The cnidarian cnidocyte, a high-tech cellular weaponry. BioEssays 17,351 -362.[CrossRef]
Thurm, U. and Lawonn, P. (1990). Die sensorischen Eigenschaften des Cnidocil-Apparates als Grundlage des Beutefangs der Hydropolypen. Verh. Dtsch. Zool. Ges. 83, 431.
Thurm, U., Brinkmann, M., Golz, R., Lawonn, P. and Oliver, D. (1998a). The supramolecular basis of mechanoelectric transduction studied in concentric hair bundles of invertebrates. In From Structure to Information in Sensory Systems (ed. C. Taddei-Ferretti, and C. Musio), pp. 228-236. Singapore, New Jersey, London, Hong Kong: World Scientific Publishing.
Thurm, U., Brinkmann, M., Holtmann, M., Lawonn, P., Oliver, D. and Sieger, T. (1998b). Modulation of the output of a mechanosensory cell by chemosensory and synaptic inputs. In From Structure to Information in Sensory Systems (ed. C. Taddei-Ferretti and C. Musio), pp. 237-253. London: World Scientific Publishing.
Thurm, U., Brinkmann, M., Golz, R., Holtmann, M., Oliver, D. and Sieger, T. (2004). Mechanoreception and synaptic transmission of hydrozoan nematocytes. Hydrobiologia 530/531,97 -105.
Vyshedskiy, A. and Lin, J.-W. (1998). Change of treansmitter release kinetics during facilitation revealed by prolonged test pulses at the inhibitor of the crayfish opener muscle. J. Neurophysiol. 78,1791 -1799.
Vyshedskiy, A., Delaney, K. R. and Lin, J.-W.
(1998). Neuromodulators enhance transmitter release by two
separate mechanisms at the inhibitor of crayfish opener muscle. J.
Neurosci. 18,5160
-5169.
Waldeck, R. F., Pereda, A. and Faber, D. S.
(2000). Properties and plasticity of paired-pulse depression at a
central synapse. J. Neurosci.
20,5312
-5320.
Watson, G. M. and Hessinger, D. A. (1989). Cnidocytes and adjacent supporting cells form receptor-effector complexes in anemone tentacles. Tissue Cell 21, 17-24.[CrossRef][Medline]
Westfall, J. A. (1996). Synapses in the first-evolved nervous systems. J. Neurocytol. 25,735 -746.[CrossRef][Medline]
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