Swim Central Pattern Generator Research Articles

Surprising multifunctionality of a Tritonia swim CPG neuron: C2 drives the early phase of postswim crawling despite being silent during the behavior.

In response to a suitably aversive skin stimulus, the marine mollusk Tritonia diomedea launches an escape swim followed by several minutes of high-speed crawling. The two escape behaviors are highly dissimilar: whereas the swim is a muscular behavior involving alternating ventral and dorsal whole body flexions, the crawl is a nonrhythmic gliding behavior mediated by the beating of foot cilia. The serotonergic dorsal swim interneurons (DSIs) are members of the swim central pattern generator (CPG) and also strongly drive crawling. Although the swim network is very well understood, the Tritonia crawling network to date comprises only three neurons: the DSIs and pedal neurons 5 and 21 (Pd5 and Pd21). Since Tritonia's swim network has been suggested to have arisen from a preexisting crawling network, we examined the possible role that another swim CPG neuron, C2, may play in crawling. Because of its complete silence in the postswim crawling period, C2 had not previously been considered to play a role in driving crawling. However, semi-intact preparation experiments demonstrated that a brief C2 spike train surprisingly and strongly drives the foot cilia for ∼30 s, something that cannot be explained by its synaptic connections to Pd5 and Pd21. Voltage-sensitive dye (VSD) imaging in the pedal ganglion identified many candidate crawling motor neurons that fire at an elevated rate after the swim and also revealed several pedal neurons that are strongly excited by C2. It is intriguing that unlike the DSIs, which fire tonically after the swim to drive crawling, C2 does so despite its postswim silence.NEW & NOTEWORTHY Tritonia swim central pattern generator (CPG) neuron C2 surprisingly and strongly drives the early phase of postswim crawling despite being silent during this period. In decades of research, C2 had not been suspected of driving crawling because of its complete silence after the swim. Voltage-sensitive dye imaging revealed that the Tritonia crawling motor network may be much larger than previously known and also revealed that many candidate crawling neurons are excited by C2.

Error Function Optimization to Compare Neural Activity and Train Blended Rhythmic Networks.

We present a novel set of quantitative measures for "likeness" (error function) designed to alleviate the time-consuming and subjective nature of manually comparing biological recordings from electrophysiological experiments with the outcomes of their mathematical models. Our innovative "blended" system approach offers an objective, high-throughput, and computationally efficient method for comparing biological and mathematical models. This approach involves using voltage recordings of biological neurons to drive and train mathematical models, facilitating the derivation of the error function for further parameter optimization. Our calibration process incorporates measurements such as action potential (AP) frequency, voltage moving average, voltage envelopes, and the probability of post-synaptic channels. To assess the effectiveness of our method, we utilized the sea slug Melibe leonina swim central pattern generator (CPG) as our model circuit and conducted electrophysiological experiments with TTX to isolate CPG interneurons. During the comparison of biological recordings and mathematically simulated neurons, we performed a grid search of inhibitory and excitatory synapse conductance. Our findings indicate that a weighted sum of simple functions is essential for comprehensively capturing a neuron's rhythmic activity. Overall, our study suggests that our blended system approach holds promise for enabling objective and high-throughput comparisons between biological and mathematical models, offering significant potential for advancing research in neural circuitry and related fields.

Bursting emerges from the complementary roles of neurons in a four-cell network.

Reciprocally inhibitory modules that form half-center oscillators require mechanisms for escaping or being released from inhibition. The central pattern generator underlying swimming by the nudibranch mollusc, Dendronotus iris, is composed of only four neurons that are organized into two competing modules of a half-center oscillator. In this system, bursting activity in left-right alternation is an emergent property of the network as a whole; none of the neurons produces bursts on its own. We found that the unique synaptic actions and membrane properties of the two neurons in each module (Si2 and the contralateral Si3) play complementary roles in generating stable bursting in this network oscillator. Although Si2 and Si3 each inhibits its contralateral counterpart, Si2 plays a dominant role in evoking fast and strong inhibition of the other module, the termination of which initiates postinhibitory rebound in the Si3 of that module by activating a hyperpolarization-activated inward current. Within each module, the synaptic actions and membrane properties of the two neurons complement each other: Si3 excites Si2, which then feeds back slow inhibition to Si3, terminating the burst. Using dynamic clamp, we showed that the magnitude of the slow inhibition sets the period of the oscillator. Thus, the synaptic actions of Si2 provide the hyperpolarization needed for the other module to rebound stably, whereas the membrane properties of Si3 in each module cause it to rebound first and excite Si2 to maintain the burst until terminated by the slow inhibition from Si2, which releases the other module to become active.NEW & NOTEWORTHY Half-center oscillators composed of reciprocally inhibitory neurons have been posited for over a century to underlie the production of rhythmic movements. The Dendronotus swim central pattern generator may be the simplest such circuit with only two pairs of bilaterally represented neurons. This study completes the description of the mechanism by which this network oscillator functions, showing how stable rhythmic activity arises from the complementary membrane and synaptic properties of the two neurons in the competing modules.

Analyzing synchronized clusters in neuron networks

The presence of synchronized clusters in neuron networks is a hallmark of information transmission and processing. Common approaches to study cluster synchronization in networks of coupled oscillators ground on simplifying assumptions, which often neglect key biological features of neuron networks. Here we propose a general framework to study presence and stability of synchronous clusters in more realistic models of neuron networks, characterized by the presence of delays, different kinds of neurons and synapses. Application of this framework to two examples with different size and features (the directed network of the macaque cerebral cortex and the swim central pattern generator of a mollusc) provides an interpretation key to explain known functional mechanisms emerging from the combination of anatomy and neuron dynamics. The cluster synchronization analysis is carried out also by changing parameters and studying bifurcations. Despite some modeling simplifications in one of the examples, the obtained results are in good agreement with previously reported biological data.

Single neuron serotonin receptor subtype gene expression correlates with behaviour within and across three molluscan species.

The marine mollusc, Pleurobranchaea californica varies daily in whether it swims and this correlates with whether serotonin (5-HT) enhances the strength of synapses made by the swim central pattern generator neuron, A1/C2. Another species, Tritonia diomedea, reliably swims and does not vary in serotonergic neuromodulation. A third species, Hermissenda crassicornis, never produces this behaviour and lacks the neuromodulation. We found that expression of particular 5-HT receptor subtype (5-HTR) genes in single neurons correlates with swimming. Orthologues to seven 5-HTR genes were identified from whole-brain transcriptomes. We isolated individual A1/C2 neurons and sequenced their RNA or measured 5-HTR gene expression using absolute quantitative PCR. A1/C2 neurons isolated from Pleurobranchaea that produced a swim motor pattern just prior to isolation expressed 5-HT2a and 5-HT7 receptor genes, as did all Tritonia samples. These subtypes were absent from A1/C2 isolated from Pleurobranchaea that did not swim on that day and from Hermissenda A1/C2 neurons. Expression of other receptors was not correlated with swimming. This suggests that these 5-HTRs may mediate the modulation of A1/C2 synaptic strength and play an important role in swimming. Furthermore, it suggests that regulation of receptor expression could underlie daily changes in behaviour as well as evolution of behaviour.

The central pattern generator underlying swimming in Dendronotus iris: a simple half-center network oscillator with a twist.

The nudibranch mollusc, Dendronotus iris, swims by rhythmically flexing its body from left to right. We identified a bilaterally represented interneuron, Si3, that provides strong excitatory drive to the previously identified Si2, forming a half-center oscillator, which functions as the central pattern generator (CPG) underlying swimming. As with Si2, Si3 inhibited its contralateral counterpart and exhibited rhythmic bursts in left-right alternation during the swim motor pattern. Si3 burst almost synchronously with the contralateral Si2 and was coactive with the efferent impulse activity in the contralateral body wall nerve. Perturbation of bursting in either Si3 or Si2 by current injection halted or phase-shifted the swim motor pattern, suggesting that they are both critical CPG members. Neither Si2 nor Si3 exhibited endogenous bursting properties when activated alone; activation of all four neurons was necessary to initiate and maintain the swim motor pattern. Si3 made a strong excitatory synapse onto the contralateral Si2 to which it is also electrically coupled. When Si3 was firing tonically but not exhibiting bursting, artificial enhancement of the Si3-to-Si2 synapse using dynamic clamp caused all four neurons to burst. In contrast, negation of the Si3-to-Si2 synapse by dynamic clamp blocked ongoing swim motor patterns. Together, these results suggest that the Dendronotus swim CPG is organized as a "twisted" half-center oscillator in which each "half" is composed of two excitatory-coupled neurons from both sides of the brain, each of which inhibits its contralateral counterpart. Consisting of only four neurons, this is perhaps the simplest known network oscillator for locomotion.

Selective Gating of Neuronal Activity by Intrinsic Properties in Distinct Motor Rhythms.

Many neural circuits show fast reconfiguration following altered sensory or modulatory inputs to generate stereotyped outputs. In the motor circuit of Xenopus tadpoles, I study how certain voltage-dependent ionic currents affect firing thresholds and contribute to circuit reconfiguration to generate two distinct motor patterns, swimming and struggling. Firing thresholds of excitatory interneurons [i.e., descending interneurons (dINs)] in the swimming central pattern generator are raised by depolarization due to the inactivation of Na(+) currents. In contrast, the thresholds of other types of neurons active in swimming or struggling are raised by hyperpolarization from the activation of fast transient K(+) currents. The firing thresholds are then compared with the excitatory synaptic drives, which are revealed by blocking action potentials intracellularly using QX314 during swimming and struggling. During swimming, transient K(+) currents lower neuronal excitability and gate out neurons with weak excitation, whereas their inactivation by strong excitation in other neurons increases excitability and enables fast synaptic potentials to drive reliable firing. During struggling, continuous sensory inputs lead to high levels of network excitation. This allows the inactivation of Na(+) currents and suppression of dIN activity while inactivating transient K(+) currents, recruiting neurons that are not active in swimming. Therefore, differential expression of these currents between neuron types can explain why synaptic strength does not predict firing reliability/intensity during swimming and struggling. These data show that intrinsic properties can override fast synaptic potentials, mediate circuit reconfiguration, and contribute to motor-pattern switching.

Two interconnected kernels of reciprocally inhibitory interneurons underlie alternating left-right swim motor pattern generation in the mollusk Melibe leonina

The central pattern generator (CPG) underlying the rhythmic swimming behavior of the nudibranch Melibe leonina (Mollusca, Gastropoda, Heterobranchia) has been described as a simple half-center oscillator consisting of two reciprocally inhibitory pairs of interneurons called swim interneuron 1 (Si1) and swim interneuron 2 (Si2). In this study, we identified two additional pairs of interneurons that are part of the swim CPG: swim interneuron 3 (Si3) and swim interneuron 4 (Si4). The somata of Si3 and Si4 were both located in the pedal ganglion, near that of Si2, and both had axons that projected through the pedal commissure to the contralateral pedal ganglion. These neurons fulfilled the criteria for inclusion as members of the swim CPG: 1) they fired at a fixed phase in relation to Si1 and Si2, 2) brief changes in their activity reset the motor pattern, 3) prolonged changes in their activity altered the periodicity of the motor pattern, 4) they had monosynaptic connections with each other and with Si1 and Si2, and 5) their synaptic actions helped explain the phasing of the motor pattern. The results of this study show that the motor pattern has more complex internal dynamics than a simple left/right alternation of firing; the CPG circuit appears to be composed of two kernels of reciprocally inhibitory neurons, one consisting of Si1, Si2, and the contralateral Si4 and the other consisting of Si3. These two kernels interact with each other to produce a stable rhythmic motor pattern.

Comparative mapping of GABA‐immunoreactive neurons in the central nervous systems of nudibranch molluscs

The relative simplicity of certain invertebrate nervous systems, such as those of gastropod molluscs, allows behaviors to be dissected at the level of small neural circuits composed of individually identifiable neurons. Elucidating the neurotransmitter phenotype of neurons in neural circuits is important for understanding how those neural circuits function. In this study, we examined the distribution of γ-aminobutyric-acid;-immunoreactive (GABA-ir) neurons in four species of sea slugs (Mollusca, Gastropoda, Opisthobranchia, Nudibranchia): Tritonia diomedea, Melibe leonina, Dendronotus iris, and Hermissenda crassicornis. We found consistent patterns of GABA immunoreactivity in the pedal and cerebral-pleural ganglia across species. In particular, there were bilateral clusters in the lateral and medial regions of the dorsal surface of the cerebral ganglia as well as a cluster on the ventral surface of the pedal ganglia. There were also individual GABA-ir neurons that were recognizable across species. The invariant presence of these individual neurons and clusters suggests that they are homologous, although there were interspecies differences in the numbers of neurons in the clusters. The GABAergic system was largely restricted to the central nervous system, with the majority of axons confined to ganglionic connectives and commissures, suggesting a central, integrative role for GABA. GABA was a candidate inhibitory neurotransmitter for neurons in central pattern generator (CPG) circuits underlying swimming behaviors in these species, however none of the known swim CPG neurons were GABA-ir. Although the functions of these GABA-ir neurons are not known, it is clear that their presence has been strongly conserved across nudibranchs.

Toward robust phase-locking in Melibe swim central pattern generator models

Small groups of interneurons, abbreviated by CPG for central pattern generators, are arranged into neural networks to generate a variety of core bursting rhythms with specific phase-locked states, on distinct time scales, which govern vital motor behaviors in invertebrates such as chewing and swimming. These movements in lower level animals mimic motions of organs in higher animals due to evolutionarily conserved mechanisms. Hence, various neurological diseases can be linked to abnormal movement of body parts that are regulated by a malfunctioning CPG. In this paper, we, being inspired by recent experimental studies of neuronal activity patterns recorded from a swimming motion CPG of the sea slug Melibe leonina, examine a mathematical model of a 4-cell network that can plausibly and stably underlie the observed bursting rhythm. We develop a dynamical systems framework for explaining the existence and robustness of phase-locked states in activity patterns produced by the modeled CPGs. The proposed tools can be used for identifying core components for other CPG networks with reliable bursting outcomes and specific phase relationships between the interneurons. Our findings can be employed for identifying or implementing the conditions for normal and pathological functioning of basic CPGs of animals and artificially intelligent prosthetics that can regulate various movements.

Homology and homoplasy of swimming behaviors and neural circuits in the Nudipleura (Mollusca, Gastropoda, Opisthobranchia).

How neural circuit evolution relates to behavioral evolution is not well understood. Here the relationship between neural circuits and behavior is explored with respect to the swimming behaviors of the Nudipleura (Mollusca, Gastropoda, Opithobranchia). Nudipleura is a diverse monophyletic clade of sea slugs among which only a small percentage of species can swim. Swimming falls into a limited number of categories, the most prevalent of which are rhythmic left-right body flexions (LR) and rhythmic dorsal-ventral body flexions (DV). The phylogenetic distribution of these behaviors suggests a high degree of homoplasy. The central pattern generator (CPG) underlying DV swimming has been well characterized in Tritonia diomedea and in Pleurobranchaea californica. The CPG for LR swimming has been elucidated in Melibe leonina and Dendronotus iris, which are more closely related. The CPGs for the categorically distinct DV and LR swimming behaviors consist of nonoverlapping sets of homologous identified neurons, whereas the categorically similar behaviors share some homologous identified neurons, although the exact composition of neurons and synapses in the neural circuits differ. The roles played by homologous identified neurons in categorically distinct behaviors differ. However, homologous identified neurons also play different roles even in the swim CPGs of the two LR swimming species. Individual neurons can be multifunctional within a species. Some of those functions are shared across species, whereas others are not. The pattern of use and reuse of homologous neurons in various forms of swimming and other behaviors further demonstrates that the composition of neural circuits influences the evolution of behaviors.

Episodic swimming in the larval zebrafish is generated by a spatially distributed spinal network with modular functional organization

Despite the diverse methods vertebrates use for locomotion, there is evidence that components of the locomotor central pattern generator (CPG) are conserved across species. When zebrafish begin swimming early in development, they perform short episodes of activity separated by periods of inactivity. Within these episodes, the trunk flexes with side-to-side alternation and the traveling body wave progresses rostrocaudally. To characterize the distribution of the swimming CPG along the rostrocaudal axis, we performed transections of the larval zebrafish spinal cord and induced fictive swimming using N-methyl-d-aspartate (NMDA). In both intact and spinalized larvae, bursting is found throughout the rostrocaudal extent of the spinal cord, and the properties of fictive swimming observed were dependent on the concentration of NMDA. We isolated series of contiguous spinal segments by performing multiple spinal transections on the same larvae. Although series from all regions of the spinal cord have the capacity to produce bursts, the capacity to produce organized episodes of fictive swimming has a rostral bias: in the rostral spinal cord, only 12 contiguous body segments are necessary, whereas 23 contiguous body segments are necessary in the caudal spinal cord. Shorter series of segments were often active but produced either continuous rhythmic bursting or sporadic, nonrhythmic bursting. Both episodic and continuous bursting alternated between the left and right sides of the body and showed rostrocaudal progression, demonstrating the functional dissociation of the circuits responsible for episodic structure and fine burst timing. These findings parallel results in mammalian locomotion, and we propose a hierarchical model of the larval zebrafish swimming CPG.

The hemisegmental locomotor network revisited

The organization of the minimal neuronal substrate capable of generating locomotor rhythmicity in vertebrates is investigated in several species, with an emphasis on identifying evolutionary-conserved features. In lamprey, an eel-like lower vertebrate that swims by undulatory movements of the body, the network has been identified as a recurrent network of excitatory interneurons localized in each spinal hemisegment. This conclusion rested upon the observation that each side of the spinal cord is able to express rhythmic locomotor-related bursting after being surgically separated along the midline, even in the absence of inhibition. An important caveat, however, is that this rhythmicity must be an intrinsic capability of the hemisegmental networks and not a newly acquired property as a result of a plastic remodeling of the network occurring after the lesion. Here we examine this issue by recording the motor output expressed by the electrically activated hemicord in the first minutes after hemisection. We observed clear rhythmic bursting in the frequency range previously linked to the operation of the central pattern generator for swimming. Moreover, we recorded the output of the unilateral networks in the intact spinal cord (i.e. no midline section performed) by activating them with asymmetrical stimulation. We thus conclude that the lamprey hemicord does possess the intrinsic capability of generating the basic rhythmic drive of locomotion. The wider significance of these data stems from the lamprey being a model of axial locomotion, and from the many lesion studies previously performed in other animals.

Neurochemical and Neuroanatomical Identification of Central Pattern Generator Neuron Homologues in Nudipleura Molluscs

Certain invertebrate neurons can be identified by their behavioral functions. However, evolutionary divergence can cause some species to not display particular behaviors, thereby making it impossible to use physiological characteristics related to those behaviors for identifying homologous neurons across species. Therefore, to understand the neural basis of species-specific behavior, it is necessary to identify homologues using characteristics that are independent of physiology. In the Nudipleura mollusc Tritonia diomedea, Cerebral Neuron 2 (C2) was first described as being a member of the swim central pattern generator (CPG). Here we demonstrate that neurochemical markers, in conjunction with previously known neuroanatomical characteristics, allow C2 to be uniquely identified without the aid of electrophysiological measures. Specifically, C2 had three characteristics that, taken together, identified the neuron: 1) a white cell on the dorsal surface of the cerebral ganglion, 2) an axon that projected to the contralateral pedal ganglion and through the pedal commissure, and 3) immunoreactivity for the peptides FMRFamide and Small Cardioactive Peptide B. These same anatomical and neurochemical characteristics also uniquely identified the C2 homologue in Pleurobranchaea californica (called A1), which was previously identified by its analogous role in the Pleurobranchaea swim CPG. Furthermore, these characteristics were used to identify C2 homologues in Melibe leonina, Hermissenda crassicornis, and Flabellina iodinea, species that are phylogenetically closer to Tritonia than Pleurobranchaea, but do not display the same swimming behavior as Tritonia or Pleurobranchaea. These identifications will allow future studies comparing and contrasting the physiological properties of C2 across species that can and cannot produce the type of swimming behavior exhibited by Tritonia.

Identification and characterization of homologous neurons in sea slugs exhibiting analogous and non-analogous behaviors.

Identification and characterization of homologous neurons in sea slugs exhibiting analogous and non-analogous behaviors.

Different Roles for Homologous Interneurons in Species Exhibiting Similar Rhythmic Behaviors

It is often assumed that similar behaviors in related species are produced by similar neural mechanisms. To test this, we examined the neuronal basis of a simple swimming behavior in two nudibranchs (Mollusca, Opisthobranchia), Melibe leonina and Dendronotus iris. The side-to-side swimming movements of Dendronotus [1] strongly resemble those of Melibe [2, 3]. In Melibe, it was previously shown that the central pattern generator (CPG) for swimming is composed of two bilaterally symmetric pairs of identified interneurons, swim interneuron 1 (Si1) and swim interneuron 2 (Si2), which are electrically coupled ipsilaterally and mutually inhibit both contralateral counterparts [2, 4]. We identified homologs of Si1 and Si2 in Dendronotus. (Henceforth, homologous neurons in each species will be distinguished by the subscripts (Den) and (Mel).) We found that Si2(Den) and Si2(Mel) play similar roles in generating the swim motor pattern. However, unlike Si1(Mel), Si1(Den) was not part of the swim CPG, was not strongly coupled to the ipsilateral Si2(Den), and did not inhibit the contralateral neurons. Thus, species differences exist in the neuronal organization of the swim CPGs despite the similarity of the behaviors. Therefore, similarity in species-typical behavior is not necessarily predictive of common neural mechanisms, even for homologous neurons in closely related species.

A hyperpolarization-activated inward current alters swim frequency of the pteropod mollusk Clione limacina

The pteropod mollusk, Clione limacina, exhibits behaviorally relevant swim speed changes that occur within the context of the animal's ecology. Modulation of C. limacina swimming speed involves changes that occur at the network and cellular levels. Intracellular recordings from interneurons of the swim central pattern generator show the presence of a sag potential that is indicative of the hyperpolarization-activated inward current (I h). Here we provide evidence that I h in primary swim interneurons plays a role in C. limacina swimming speed control and may be a modulatory target. Recordings from central pattern generator swim interneurons show that hyperpolarizing current injection produces a sag potential that lasts for the duration of the hyperpolarization, a characteristic of cells possessing I h. Following the hyperpolarizing current injection, swim interneurons also exhibit postinhibitory rebound (PIR). Serotonin enhances the sag potential of C. limacina swim interneurons while the I h blocker, ZD7288, reduces the sag potential. Furthermore, a negative correlation was found between the amplitude of the sag potential and latency to PIR. Because latency to PIR was previously shown to influence swimming speed, we hypothesize that I h has an effect on swimming speed. The I h blocker, ZD7288, suppresses swimming in C. limacina and inhibits serotonin-induced acceleration, evidence that supports our hypothesis.

Functional organization of V2a‐related locomotor circuits in the rodent spinal cord

Studies of mammalian locomotion have been greatly facilitated by the use of the isolated rodent spinal cord preparation that retains the locomotor circuits needed to execute the movement. Physiological and molecular genetic experiments in this preparation have started to unravel the basic circuit organization responsible for walking in mammals. Here, we review these experiments with a focus on the functional role of excitatory V2a interneurons in the mammalian locomotor network. With regard to these neurons and other network structures we also discuss similarities and differences between the mammalian walking central pattern generator (CPG) and the fish swimming CPG.

The Nitric Oxide/cGMP Pathway Tunes the Thermosensitivity of Swimming Motor Patterns inXenopus laevisTadpoles

We investigated the role of the nitric oxide (NO)/cGMP pathway in setting thresholds for failure and recovery during hyperthermic stress of the swimming central pattern generator of immobilized Xenopus tadpoles (stage 42). We recorded swimming motor patterns induced by tail skin stimulation (TS) (1 ms current pulse) or by bath application of 50 microm NMDA. Swimming rhythm frequency increased in a linear manner with increasing temperature. In the presence of the NO donor S-nitroso-N-acetylpenicillamine (SNAP), recovery from hyperthermic failure was greatly slowed, often taking longer than the duration of the experiment. Pharmacological activation of the NO/cGMP pathway using SNAP or 8-bromo-cGMP (1) decreased the duration of TS-evoked swim episodes; (2) decreased the temperature threshold for hyperthermic circuit failure; (3) decreased the temperature at which the circuit recovered; and (4) increased the time taken to recover. Pharmacological inhibition of the NO/cGMP pathway using the NO scavenger CPTIO, the nitric oxide synthase (NOS) inhibitor L-NAME or the guanylyl cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) had the opposite effects. NMDA rhythms were more resistant to hyperthermic failure than TS-evoked swim episodes, but the effects of SNAP on the temperature sensitivity of swimming evoked by NMDA were similar to those on TS-evoked swimming, suggesting that drug effects occur on central pattern-generating networks rather than sensory pathways. We conclude that the NO/cGMP pathway is involved in setting the threshold temperatures for hyperthermic failure and subsequent recovery of fictive swimming in tadpoles, and we suggest that this is part of a variable response to prevent overexcitation during abiotic stress under different environmental conditions.

Locomotor rhythm maintenance: electrical coupling among premotor excitatory interneurons in the brainstem and spinal cord of young Xenopus tadpoles

Electrical coupling is important in rhythm generating systems. We examine its role in circuits controlling locomotion in a simple vertebrate model, the young Xenopus tadpole, where the hindbrain and spinal cord excitatory descending interneurons (dINs) that drive and maintain swimming have been characterised. Using simultaneous paired recordings, we show that most dINs are electrically coupled exclusively to other dINs (DC coupling coefficients ∼8.5%). The coupling shows typical low-pass filtering. We found no evidence that other swimming central pattern generator (CPG) interneurons are coupled to dINs or to each other. Electrical coupling potentials between dINs appear to contribute to their unusually reliable firing during swimming. To investigate the role of electrical coupling in swimming, we evaluated the specificity of gap junction blockers (18-β-GA, carbenoxolone, flufenamic acid and heptanol) in paired recordings. 18-β-GA at 40–60 μm produced substantial (84%) coupling block but few effects on cellular properties. Swimming episodes in 18-β-GA were significantly shortened (to ∼2% of control durations). At the same time, dIN firing reliability fell from nearly 100% to 62% of swimming cycles and spike synchronization weakened. Because dINs drive CPG neuron firing and are critical in maintaining swimming, the weakening of dIN activity could account for the effects of 18-β-GA on swimming. We conclude that electrical coupling among pre motor reticulospinal and spinal dINs, the excitatory interneurons that drive the swimming CPG in the hatchling Xenopus tadpole, may contribute to the maintenance of swimming as well as synchronization of activity.