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First published online November 14, 2008
Journal of Experimental Biology 211, 3703-3711 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.023606
Episodic swimming behavior in the nematode C. elegans
Department of Molecular Genetics, Albert Einstein College of Medicine, Bronx, NY 10461, USA
Author for correspondence (e-mail:
emmons{at}aecom.yu.edu)
Accepted 2 October 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: locomotory behavior, episodic behavior, acetylcholine, command interneuron, behavioral quiescence
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Schall, 2001). With less
complex invertebrates, selection among stereotyped behavioral patterns may be
regarded as switches between well-defined behavioral states
(Calabrese, 2003;
Garcia et al., 2001).
Rhythm-generating circuits are examples of neural networks with simple
behavioral output that undergoes well-defined state transitions, for example,
between episodes of activity and inactivity. These behavioral transitions may
be governed by sensory input, internal conditions via
neuromodulators, or interactions with other oscillating circuits or with
command interneurons (Briggman et al.,
2005; Calabrese,
1998; Marder et al.,
2005; Staras et al.,
2003).
Study of the regulation of behavior in the nematode C. elegans
takes advantage of the fact that its behaviors are relatively simple and its
nervous system contains a constant number of neurons whose pattern of synaptic
connectivity is known (Chen et al.,
2006; White et al.,
1986). During navigation on a solid surface, switches between
forward and backward locomotion are controlled by command interneurons that
synapse onto excitatory motor neurons, but can also occur within the motor
neuron circuitry itself (Brockie et al.,
2001a; Chalfie et al.,
1985; Chalfie and White,
1988; Von Stetina et al.,
2006; Zheng et al.,
1999b). In a non-uniform environment, switching between forward
and backward locomotion and a sharp turn is influenced by sensory input acting
through interneuron circuitry situated upstream of the command interneurons
(Gray et al., 2005a;
Tsalik and Hobert, 2003;
Wakabayashi et al., 2004).
Here we document a novel behavioral transition in C. elegans locomotion. We show that when C. elegans and several additional nematode species swim in liquid, their locomotion is episodic: periods of active swimming and extended periods of inactivity spontaneously alternate with great regularity. We show that quiescence appears to be induced by a mechanism acting at the level or downstream of the motor neurons, and is induced by high levels of acetylcholine (ACh). The timing of swimming–quiescence cycling may be controlled in part by the command interneurons.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). For the
swimming assay, adults were transferred to a fresh unseeded plate at 25°C
and allowed to forage for 2 min, after which they were picked singly into
wells of a Costar microtiter plate containing 200 µl M9 buffer at 25°C
(Brenner, 1974). The assays
were carried out in a room where the temperature was maintained between
24°C and 25°C. To score swimming versus quiescent behavior, worms were observed visually for 5 s every minute with a dissecting microscope (Wild Heerbrugg, Gais, Switzerland). The assay duration was at least 3 h except where noted otherwise. The cutoff for the transition from swimming to quiescence was less than or equal to 2 body bends per 5 s. Transitions in both directions were distinct. During the quiescent interval, worms remained motionless except for a slow relaxation towards a straight posture. More than 2 but less than 5 body bends per 5 s was scored as slow swimming. Greater than 5 body bends per 5 s was considered as swimming. Muscle contraction resulting in a bend on one side (dorsal or ventral) was considered as 1 body bend. During swimming, the wild-type worm generally made 3.5–4 body bends s–1, hence swimming was clearly distinguishable from quiescence. In certain mutants the swimming was poor, for example worms made fewer body bends per second or they swam with a different posture. In scoring and analyzing the data for such worms, a swimming worm was one that was moving but did not satisfy the criteria of quiescence as defined above. In prodding assays, quiescent worms were prodded 2 times at either the posterior or the anterior end with a platinum wire.
Data analysis
Histograms of swimming and quiescent bout durations include all swimming
bouts excluding the first swimming bout and all quiescent bouts for at least 3
h of assay (unless mentioned otherwise) plotted with 2 min bins. In the text,
average and standard error of the mean are given unless noted otherwise. The
D'Agostino and Pearson omnibus normality test with 
=0.05 was used to
decide the subsequent statistical analysis to be performed on log-transformed
data sets. In cases where the data sets did not pass the normality test, the
Kruskal–Wallis test followed by Dunn's multiple comparison
post-hoc test was performed to determine the two-tailed
P-value. In cases where two groups were compared, a
Mann–Whitney U-test was performed. These analyses were done
using the Graphpad Prism version 4.00 software for Macintosh. If a data set
passed the normality test, ANOVA with Bonferroni–Dunn post-hoc
test was used to determine significance. These analyses were performed with
Statview 5.0 software.
Measurement of body length
Worm images on a dissecting microscope were recorded with a Videolabs
analogue camera attached to a Toshiba VHS recorder. Recorded movies were
converted into ImageJ-compatible format using iMovie (5.0.2, Apple Computers,
www.apple.com/ilife/imovie/).
Animals were measured while swimming and after entering quiescence and the two
measured lengths compared.
Laser ablations
Laser ablations followed published procedures
(Bargmann and Avery, 1995).
Pharmacological analysis
The GABA agonist muscimol (Sigma) and the Ach agonist levamisole (Sigma) in
M9 buffer were added directly to the liquid in which the worm was swimming to
achieve a final concentration or included in the assay buffer initially. To
determine the effect of the drug on the length of quiescence, worms were
allowed to go into quiescence in 100 µl M9 buffer for 1 min and drugs were
added in a volume of 100 µl. To control for the possible effect of
disturbance of the liquid, experiments are compared with control data in which
100 µl of buffer without drug was added.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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; Gray and Lissmann,
1964; Korta et al.,
2007; Tsechpenakis et al.,
2008). By observing worms in liquid over longer periods, we found
that after transfer from a solid surface into liquid, a worm initially swam
continuously under our assay conditions for a period of 92.6±2.5 min
(s.e.m., N=109), after which it stopped and lay quiescent
(Fig. 1B). The transition from
swimming to quiescence was distinct and abrupt, lasting an average of
6.9±1.3 s (N=7). During quiescence, a worm maintained a
straight rather than sinusoidal posture, often with a slight bend that
gradually straightened (Fig.
1B). The quiescent interval lasted for several minutes, after
which the worm spontaneously and equally abruptly started swimming again
(supplementary material Movie 1). After the initial longer swimming bout and
first quiescent bout, worms entered a period of alternating swimming and
quiescence, a pattern that has been observed to go on for over 7 h. Over
longer durations (>4 h), periods of swimming gradually declined and periods
of quiescence became longer (Fig.
1C). We focused our study on the initial 4 h interval after
transfer into liquid, during which time, after the first swimming bout, there
was little change. During this interval, the periods of swimming and
quiescence were highly regular (Fig.
2A). Excluding the first swimming bout, intervals of swimming were
on average 14.0±0.5 min and quiescent periods were 4.8±0.1 min
(Table 1). Three additional
C. elegans strains and five nematode species tested all entered and
exited a quiescent state in liquid, although none of the other species had the
regularity of C. elegans (Fig.
1D). We conclude that, in a liquid environment, swimming and
quiescence are alternative behavioral states of the nematode motor system.
Similar extended periods of quiescence were not seen for adult worms crawling
on an agar surface (data not shown).
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Quiescence is a regulated, neural state
Touching worms on the tail with a wire 1 min after entering quiescence, but
not on the head, caused them to commence swimming (supplementary material
Movie 2). Thus worms in quiescence are capable of swimming. Moreover, this
observation implicates specific neuronal circuitry in the induction of
swimming from the quiescent state. After being `awakened' by prodding
following 1 min of the first quiescent bout, worms swam for the same amount of
time as if they had resumed swimming spontaneously after 4.8 min (prodded,
16.1±1.5 min, N=64; spontaneous, 17.1±1.4 min,
N=42; P=0.3). There was no correlation between the length of
the quiescent period and the length of the preceding swimming period in C.
elegans or the other species (data not shown). The timing of the
transition between swimming and quiescence was strongly influenced by
environmental variables and sensory input (data not shown). Notably, inclusion
of food in the swimming buffer (E. coli OD=0.6) decreased the initial
swimming period (69±5 min, 32 bouts) and increased the average length
of quiescent periods (9.3±1.1 min, 167 bouts). All these observations
argue against simple fatigue and recovery as the reason for quiescence.
During quiescence muscles are initially in a state of equal tension requiring ACh
As an approach to understand the nature of the quiescent state, we
investigated the status of the motor system during quiescence in order to gain
insight into why periodic muscle contractions stopped. We examined three of
the principal elements of the motor system: the excitatory neurotransmitter
ACh, the inhibitory neurotransmitter GABA and the muscles
(Chalfie and White, 1988;
Von Stetina et al., 2006).
To determine whether muscle contractions stopped because motor neurons
ceased to secrete ACh, we compared animals lacking this neurotransmitter with
animals in quiescence. In liquid, animals lacking ACh due to mutations in the
ACh transporter unc-17 or the biosynthetic enzyme choline
acetyltransferase cha-1 (Alfonso
et al., 1993; Rand,
1989; Rand and Russell,
1984; Rand and Russell,
1985) assumed and maintained a ventrally coiled posture
(Fig. 1E). This posture was
different from the straight, slightly kinked posture of wild-type worms in
quiescence (Fig. 1B). To
eliminate the possibility that coiling was due to a developmental defect, we
examined a temperature-sensitive allele of cha-1. cha-1(y226ts) worms
move apparently normally at permissive temperature on plates, but when
transferred to liquid at 25°C they curl up and cease movement (98% after
10 min, 30 worms). Because of internal hydrostatic pressure, in the absence of
muscle contraction worms assume a straight posture. A coiled posture indicates
that, in the absence of ACh, there is some degree of spontaneous muscle
contraction with differential tension between the ventral and dorsal sides of
the animal. From these observations we conclude that, while ACh is necessary
for swimming as expected, it is also necessary for the straightened posture
maintained by wild-type worms in quiescence.
Since ACh released at the neuromuscular junctions between the excitatory
motor neurons and the body wall muscles stimulates the inhibitory GABAergic
motor neurons (Chalfie and White,
1988; Von Stetina et al.,
2006), it was possible that the maintenance by ACh of a straight
posture during quiescence occurred through the stimulation of GABA release and
relaxation of muscle. To assess this possibility, we examined the behavior of
three GABA-deficient mutants: unc-25(sa94ts), unc-25(e156) and
unc-30(e318). unc-25 encodes the GABA biosynthetic enzyme glutamate
decarboxylase and unc-30 encodes a homeodomain transcription factor
necessary for expression of unc-25 as well as a GABA vesicular
transporter (McIntire et al.,
1993). In liquid, animals homozygous for GABA mutations underwent
swimming–quiescence cycling. In quiescence, they maintained a posture
similar to that of wild-type animals. Thus GABA is not necessary for
quiescence or for the maintenance of straight posture during quiescence.
However, absence of GABA did have an effect on the timing of transitions between swimming and quiescence. For unc-25(sa94ts) and unc-30(e318) worms, the average length of swimming bouts was shorter than that of wild-type worms, while for unc-25(e156) there was no significant difference in the length of swimming bouts (Fig. 2A; Table 1). For all three mutations the average length of quiescent bouts was increased (Fig. 2A; Table 1). Thus GABA promoted swimming – it maintained worms in a swimming state for a longer time and caused worms in quiescence to begin swimming again sooner. The reason for the difference between unc-25(e156) and the other GABA-deficient mutants is unknown but might be because of developmental compensation that does not occur for the temperature-sensitive allele raised at 20°C or for the transcription factor mutation.
We further tested the effect of GABAergic signaling on the reinitiation of swimming by adding the GABA agonist muscimol to the liquid surrounding a worm that had entered quiescence. Addition of muscimol (2 mmol l–1) 1 min after a worm entered quiescence caused reinitiation of swimming significantly sooner than if buffer alone was added (after addition of drug, 1.1±0.2 min, N=30; after addition of buffer, 2.4±0.3 min, N=12; P<0.001). Thus pharmacological treatment also suggested that GABAergic signaling promoted the termination of quiescence and the reinitiation of swimming.
We next examined the status of the muscles during quiescence. Although the straight posture of worms in quiescence could not be due to relaxation of the muscles by GABA, it could be caused by another muscle relaxant. Alternatively, the cessation of movement and straight posture might come about because all the body wall muscles were exerting equal tension. To discriminate between these two possibilities, we measured the lengths of worms during swimming and quiescence. We found that quiescent worms were initially contracted to 95±1% of their length while swimming before entering quiescence. This initial shortening attenuated during the quiescent period. To gain an indication of the change in body length that would occur if there were different degrees of muscle relaxation or contraction, we measured worms treated with the GABA agonist muscimol or the ACh agonist levamisole. Muscimol (20 mmol l–1) caused worms to relax but produced no measurable change in length, while levamisole (2 mmol l–1) caused a contraction to 77±1% of untreated length. We interpret these results to indicate that at the start of quiescence the body wall muscles are under some degree of contraction. Hence, the straight posture of worms is not caused by a complete relaxation of the body wall muscles but apparently by a uniform degree of tension all along the body wall.
Increased cholinergic activity promotes quiescence
To test further the role of ACh in swimming–quiescence cycling, we
examined the swimming behavior of animals with increased ACh signaling. The
inhibitor aldicarb increases the level of ACh at the neuromuscular junction by
blocking the activity of the degradative enzyme acetylcholine esterase
(Nguyen et al., 1995). We
found that, by expression of a cell-death caspase, aldicarb decreased swimming
and increased quiescence (Fig.
2A,B; Table 1). The
posture of quiescent worms in aldicarb was similar to that of worms in the
absence of the drug.
We also assayed several mutants with increased ACh signaling. Three
acetylcholine esterase-coding genes have been identified in C.
elegans: ace-1, ace-2 and ace-3. A mutant of all three
is lethal (Johnson et al.,
1988). Mutation of two of the three in the
ace-1(g72);ace-2(p1000) double mutant reduced the average duration of
swimming bouts and increased the average duration of quiescent bouts
(Fig. 2A;
Table 1). In a second test, we
examined a mutation in a diacylglycerol kinase, DGK-1. The mutation
dgk-1(nu62) enhances the release of ACh from motor neurons in the
ventral cord by elevation of DAG levels
(Nurrish et al., 1999). Like
ace mutants, dgk-1(nu62) mutants also exhibited shortened
swimming bouts and longer quiescent bouts
(Fig. 2A;
Table 1).
Thus genetic and pharmacological experiments gave similar results and suggested that elevated levels of ACh signaling both promoted the transition to quiescence, shortening the average length of swimming bouts, and prolonged the average duration of quiescence.
An initial prodding-insensitive phase of quiescence is prolonged by ACh
The above observations are consistent with the hypothesis that high ACh
signaling inhibits swimming and causes the worms to go into quiescence. The
origin of this ACh is not known but a likely source is the excitatory motor
neurons of the ventral cord. Once a worm enters quiescence, release of ACh by
the excitatory motor neurons might stop or decrease. ACh concentration at
neuromuscular junctions should then fall due to the action of acetylcholine
esterase. The level of ACh must fall to a level permissible for swimming
within 1 min of entering quiescence, because worms are capable of swimming at
this time if prodded (100%, N=42).
To test for an initial refractory period during which worms could not swim if prodded, we prodded worms 10 s after they entered quiescence. Only 40% of the worms resumed swimming (N=38), demonstrating an initial refractory period as predicted. If this refractory period was due to the continuing presence of ACh, it should be extended by addition of aldicarb. Indeed we found that when worms in 0.01 mmol l–1 aldicarb were prodded after 1 min of quiescence, only 70% (N=27) and 58% (N=26) resumed swimming at the first and second quiescent bouts, respectively. When they were prodded similarly after 5 min in quiescence, 100% (N=18) and 90% (N=28) resumed swimming. Therefore, blocking acetylcholine esterase activity prolonged a refractory period during which the worms were not responsive to prodding. ACh signaling appeared to prevent quiescent worms from undergoing the transition from a prodding-insensitive to a prodding-sensitive state.
Command interneurons are dispensable for cycling
Initially after entering quiescence a worm is in a state insensitive to
prodding that appears to be induced by ACh signaling. However, the state of
the worm soon changes and after 1 min it can swim if prodded, yet it remains
in quiescence for several minutes longer. Thus a secondary mechanism, possibly
independent of ACh signaling, extends the quiescent period. Command
interneurons are logical candidates for providing this secondary mechanism. To
assess the role of the command interneurons in both swimming and quiescent
intervals, we examined the behavior of animals (glr-1::ICE) in which
all five pairs of command interneurons are killed along with 12 other classes
of neurons (Zheng et al.,
1999a). In spite of this drastic loss of upstream circuitry,
glr-1::ICE worms continued to undergo swimming–quiescence
cycling. Their posture during quiescence resembled that of non-transgenic
wild-type worms rather than the coiled posture of ACh-deficient mutants. Thus
command interneurons are not necessary for swimming–quiescence
cycling.
However, the timing of swimming and quiescent intervals was affected in this strain. glr-1::ICE worms began cycling immediately upon transfer to liquid (Fig. 2B). Thus one or more of the neurons killed in this strain were necessary to maintain swimming during the first swimming bout. During cycling, the average length of swimming bouts was decreased while the average length of quiescent bouts was increased (Fig. 3A; Table 1). However, the distribution of both swimming and quiescent intervals was more highly skewed due to a relative increase in the frequency of short bouts. For quiescent intervals, although the average length was increased, the shortest bouts were the most frequent class. Hence one or more glr-1-expressing neurons inhibit the reinitiation of swimming during the first few minutes of quiescence and evidence for a timing mechanism is lost. glr-1::ICE worms were sensitive to aldicarb, similar to non-transgenic animals (Fig. 2B). Along with the straight posture of the glr-1::ICE worms in quiescence, this observation is consistent with the hypothesis that induction of quiescence in worms lacking command interneurons occurs by the same ACh-mediated mechanism as in non-transgenic worms.
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). In this
strain there was no significant difference from non-transgenic worms in the
average duration of swimming or quiescent periods
(Fig. 3A;
Table 1). However, the
distributions of both swimming and quiescent intervals were broadened compared
with wild-type, indicating that the neurons killed in the nmr-1::ICE
strain contributed to the accuracy of the timing mechanism. Comparison of quiescent bouts in glr-1::ICE and nmr-1::ICE worms suggested that AVB might be important in suppressing short quiescent bouts. To test the function of the AVB neurons, we killed the AVB neurons with a laser microbeam. Elimination of AVBL and AVBR decreased the average length of swimming bouts, while there was no effect on quiescent bouts (Table 1). Therefore, while AVB may be important in sustaining swimming, any role that it plays in promoting quiescence overlaps that of other neurons. We searched for other neurons that might be responsible for timing by ablating them individually. The neurons we tested included AVA, AVE, PVC, AVD and RIM. Ablation of AVA, AVD and RIM resulted in broadened distributions of quiescent bouts, but loss of any single neuron did not affect timing drastically (supplementary material Fig. S1 and Table S1).
Constitutive depolarization of upstream interneurons prevents worms from recovering from quiescence
Since ACh signaling appeared to promote quiescence over swimming,
depolarization of the command interneurons, which is expected to depolarize
the excitatory motor neurons and hence increase their release of ACh, might
also promote quiescence over swimming. To test this prediction, we examined a
series of strains in which the command interneurons along with additional
neurons are constitutively depolarized by expression of a leaky form of the
glutamate channel GLR-1, denoted GLR-1(A/T)
(Zheng et al., 1999b). As
predicted, in strains where the command interneurons are depolarized
[(glr-1::GLR-1(A/T), nmr-1::GLR-1(A/T)], the length of the swimming
bouts decreased and the length of the quiescent bouts increased
(Fig. 3B;
Table 1). These effects were
progressively more severe through a series of three strains – all
command interneurons but AVB plus RIML, RIMR and AVG depolarized
[nmr-1::GLR-1(A/T)], all command interneurons plus 12 classes of
additional neurons depolarized [glr-1::GLR-1(A/T)], AVB depolarized
and the remaining command interneurons plus 12 classes of additional neurons
killed [glr-1::GLR-1(A/T);nmr-1(ICE)] – such that in
the last of these strains, worms went into a quiescent state from which they
rarely or never emerged (Fig.
3C). Although they can crawl on plates
(Zheng et al., 1999b), these
worms cannot swim.
The cGMP-dependent protein kinase EGL-4 prolongs both swimming and quiescent states
Two previously studied quiescent states of C. elegans are promoted
by activity of the cGMP-dependent protein kinase EGL-4
(Avery, 1993;
Raizen et al., 2006;
Raizen et al., 2008;
Van Buskirk and Sternberg,
2007). We examined the effect of mutation in egl-4 on
swimming–quiescence cycling by scoring worms carrying four different
loss-of-function or null alleles of egl-4, with similar results. All
four mutants underwent swimming–quiescence cycling, indicating that
egl-4 function is not necessary for spontaneous quiescence during
swimming. However, the average length of both swimming and quiescent bouts was
shorter than in wild-type (Fig.
2A; Table 1). In
quiescence, egl-4 worms had a different posture from wild-type worms;
rather than a straightened posture, egl-4 worms stopped moving while
maintaining a deeply bent posture (Fig.
1F; supplementary material Movie 3). Further, while the body was
immobilized in this bent posture, the nose of the animal continued to move
rapidly back and forth. Thus while egl-4 function was not necessary
for cycling between swimming and quiescent states, it was necessary for the
straightened posture during quiescence and for quiescence of the head
muscles.
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Fujiwara et al., 2002;
Gray et al., 2005b). Our
results indicate that during swimming, transitions into and out of quiescence
are governed by a deterministic timing mechanism. The origin or target of the ACh signaling that induces quiescence is not known. Among the several cholinergic neurons in the nervous system, the excitatory motor neurons are obvious candidates for the source of ACh. Possibly quiescence is induced during swimming because of a higher rate of release of ACh by the excitatory motor neurons accompanying the higher rate of motion of the worm in liquid compared with its motion on a solid surface. Such greater release of ACh might have its effect directly on muscle, or the released ACh might act on receptors in neurons, stimulating a secondary process involved in stopping muscle contractions. A higher rate of ACh release could lead to ACh levels rising above a threshold level or to activation of remote receptors by spillover within the ventral cord or elsewhere in the nervous system. The equal state of body wall muscle tension might result from a mechanism within the muscles themselves, such as an increase in their electrical coupling. Alternatively, it could be due to a mechanism that causes equal release of ACh by all the excitatory motor neurons. One or more additional neurotransmitters, for example peptide neurotransmitters, could be triggered and act on either the motor neurons or the muscles, or on both, to bring about the state of inactivity and equal tension.
While they were not necessary for swimming–quiescence cycling,
upstream neurons including the command interneurons were necessary for the
regularity of cycling. We could identify no single interneuron responsible for
this effect, consistent with the distributed nature of the control circuitry
observed by others (Brockie et al.,
2001b). The upstream neurons served to stabilize the behavioral
choice. Command interneurons serve a similar stabilizing function in
transitions between forward and backward locomotion during crawling
(Zheng et al., 1999b).
Upstream interneurons impose regularity by prolonging quiescence beyond the initial prodding-insensitive period. Thus they provide a latch that prevents a change in behavioral state for a specified period. One possibility consistent with activation by prodding is that during the latch phase of quiescence the command interneurons are hyperpolarized and thereby inhibit the excitatory motor neurons.
The nature of the mechanism that `times out' after about 5 min, lifting the
latch and allowing spontaneous reinitiation of swimming, remains to be
determined. It is also unclear how the spontaneous mechanism relates to the
mechanism that operates on prodding. There is a high degree of
interconnectivity among the interneurons upstream of the motor neurons which
stands in the way of any obvious explanation for why posterior but not
anterior prodding reawakens the system
(White et al., 1986). We
examined the swimming behavior of worms lacking functional touch neurons
[mec-3(e1338) and mec-4(e1611)
(Chalfie and Sulston, 1981)]
and found they have quiescent bouts of normal length but somewhat shortened
swimming bouts (data not shown). Normal quiescent bout length in these mutants
suggests the spontaneous mechanism does not function through activity of the
touch neurons.
The straight body posture and the ability to be awakened by prodding are
similar to the characteristics of other described quiescent states of C.
elegans, raising the possibility that spontaneous quiescence during
swimming might be the same behavioral state as quiescence during lethargus or
quiescence induced by EGF (Avery,
1993; Raizen et al.,
2006; Van Buskirk and
Sternberg, 2007). Indeed, our observation that egl-4
activity promotes the quiescent state in liquid strengthens this conjecture. A
significant difference between quiescence during swimming and that during
lethargus or induced by EGF is that prodding in the last two cases induces
only transient locomotion, whereas in the first it induces a normal swimming
period before quiescence is resumed. It is possible that a humoral factor
inducing the quiescent state in lethargus or EGF-induced quiescence, but not
during swimming, persists and soon reinitiates the quiescent state. This
supposition is consistent with our suggestion that ACh is the inducer of
quiescence during swimming and is quickly lost once a worm enters quiescence,
with quiescence being maintained thereafter by a neural latch.
We have shown that the function of the C. elegans motor circuit is inherently unstable when a worm swims in liquid. It spontaneously switches between two behavioral states, a swimming state and a quiescent state. While C. elegans is thought to normally inhabit two-dimensional liquid films on solid surfaces in soil or on an invertebrate host, it is likely that from time to time it may encounter a flooded environment in which it is obliged to swim. Hence the behavior we have observed is likely to be adaptive. Possibly the quiescent state is a reset state that allows the system to recover after running too fast. Or perhaps periods of no swimming aid a worm in escaping the liquid environment. An intrinsic ability to change behavioral state in the absence of a change in sensory input might be important to the animal in adaptively coping with an unchanging environment.
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Acknowledgments |
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Footnotes |
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* Present address: 110 Carl Icahn Laboratory, Princeton University,
Princeton, NJ 08544, USA 
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References |
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