Sensitized by a sea slug: Site-specific short-term and general long-term sensitization in Aplysia following Navanax attack.

Neurobiological studies of the model species, Aplysia californica (Mollusca, Gastropoda, Euopisthobranchia), have helped advance our knowledge of the neural bases of different forms of learning, including sensitization, a non-associative increase in withdrawal behaviors in response to mild innocuous stimuli. However, our understanding of the natural context for this learning has lagged behind the mechanistic studies. Previous studies, which exclusively used artificial stimuli, such as electric shock, to produce sensitization, left open the question of which stimuli might cause sensitization in nature. Our laboratory first addressed this question by testing for short and long-term sensitization after predatory attack by a natural predator, the spiny lobster. In the present study, we tested for sensitization after attack by a very different predator, the predacious sea-slug, Navanax inermis (Mollusca, Gastropoda, Euopisthobranchia). Unlike the biting and prodding action of lobster attack, Navanax uses a rapid strike that sucks and squeezes its prey in an attempt to swallow it whole. We found that Navanax attack to the head of Aplysia caused strong immediate sensitization of head withdrawal, and weaker, delayed, sensitization of tail-mantle withdrawal. By contrast, attack to the tail of Aplysia resulted in no sensitization of either reflex. We also developed an artificial attack stimulus that allowed us to mimick a more consistently strong attack. This artificial attack produced stronger but qualitatively similar sensitization: Strong immediate sensitization of head withdrawal and weaker sensitization of tail-mantle withdrawal after head attack, immediate sensitization in tail-mantle withdrawal, but no sensitization of head withdrawal after tail attack. We conclude that Navanax attack causes robust site-specific sensitization (enhanced sensitization near the site of attack), and weaker general sensitization (sensitization of responses to stimuli distal to the attack site). We also tested for long-term sensitization (lasting longer than 24 h) after temporally-spaced delivery of four natural Navanax attacks to the head of subject Aplysia. Surprisingly, these head attacks, any one of which strongly sensitizes head withdrawal in the short term, failed to sensitize head-withdrawal in the long term. Paradoxically, these repeated head attacks produced long-term sensitization in tail-mantle withdrawal. These experiments and observations confirm that Navanax attack causes short, and long-term sensitization of withdrawal reflexes of Aplysia. They add site-specific sensitization as well as paradoxical long-term sensitization of tail-mantle withdrawal to a short list of naturally induced learning phenotypes in this model species. Together with previous observations of sensitization after lobster attack, these data strongly support the premise that sensitization in Aplysia is an adaptive response to sub-lethal predator attack.

Rare-Earth Magnets Influence Movement Patterns of the Magnetically Sensitive Nudibranch Tritonia exsulans in Its Natural Habitat.

AbstractThe nudibranch Tritonia exsulans (previously Tritonia diomedea) is known to have behaviors and neurons that can be modified by perturbations of the Earth's magnetic field. There is no definitive evidence for how this magnetic sense is used in nature. Using an exploratory approach, we tested for possible effects of magnetic perturbations based on underwater video of crawling patterns in the slugs' natural habitat, with magnets of varying strength deployed on the substrate. For analysis, we used a paired comparison of tracks of animals between segments 25-50 cm distant from the magnets and segments of the same tracks 0-25 cm from the magnets, to determine whether any differences depended on the strength of the magnet. Most track measurements (length, displacement, velocity, and tortuosity) showed no such differences. However, effects were observed for the changes in track headings between successive points. These results showed that tracks had relatively higher heading variability when they moved closer to stronger magnets. We suggest that this supports a hypothesis that T. exsulans continuously uses a magnetic sense to help maintain straight-line navigation. Further specific testing of the hypothesis is now needed to verify this new possibility for how animals can benefit from a compass sense.

Consolidated data on the phylogeny and evolution of the family Tritoniidae (Gastropoda: Nudibranchia) contribute to genera reassessment and clarify the taxonomic status of the neuroscience models Tritonia and Tochuina.

Nudibranch molluscs of the family Tritoniidae are widely used neuroscience model systems for understand the behavioural and genetic bases of learning and memory. However species identity and genus-level taxonomic assignment of the tritoniids remain contested. Herein we present a taxonomic review of the family Tritoniidae using integration of molecular phylogenetic analysis, morphological and biogeographical data. For the first time the identity of the model species Tritonia tetraquetra (Pallas, 1788) and Tritonia exsulans Bergh, 1894 is confirmed. T. tetraquetra distributes across the large geographic and bathymetric distances in the North-Eastern (NE) and North-Western (NW) Pacific. In turn, at NE Pacific coasts the separate species T. exsulans is commonly occured. Thus, it reveals a misidentification of T. tetraquetra and T. exsulans species in neuroscience applications. Presence of more hidden lineages within NW Pacific T. tetraquetra is suggested. The long lasting confusion over identity of the species from the genera Tritonia and Tochuina is resolved using molecular and morphological data. We also disprove a common indication about "edible T. tetraquetra" at the Kuril Islands. It is shown that Tochuina possesses specialized tritoniid features and also some characters of "arminacean nudibranchs", such as Doridoxa and Heterodoris. Diagnoses for the families Doridoxidae and Heterodorididae are provided. Taxonomy of the genus Doridoxa is clarified and molecular data for the genus Heterodoris presented for the first time. A taxonomic synopsis for the family Tritoniidae is provided. A new genus among tritoniid taxa is proposed. Importance of the ontogeny-based taxonomy is highlighted. The cases when apomorphic characters considerably modified in a crown group due to the paedomorphosis are revealed. Tracing of the character evolution is presented for secondary gills-a key external feature of the family Tritoniidae and traditional dendronotacean nudibranchs.

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.

Serial-section atlas of the Tritonia pedal ganglion.

The pedal ganglion of the nudibranch gastropod Tritonia diomedea has been the focus of neurophysiological studies for more than 50 yr. These investigations have examined the neural basis of behaviors as diverse as swimming, crawling, reflex withdrawals, orientation to water flow, orientation to the earth's magnetic field, and learning. Despite this sustained research focus, most studies have confined themselves to the layer of neurons that are visible on the ganglion surface, leaving many neurons, which reside in deeper layers, largely unknown and thus unstudied. To facilitate work on such neurons, the present study used serial-section light microscopy to generate a detailed pictorial atlas of the pedal ganglion. One pedal ganglion was sectioned horizontally at 2-µm intervals and another vertically at 5-µm intervals. The resulting images were examined separately or combined into stacks to generate movie tours through the ganglion. These were also used to generate 3D reconstructions of individual neurons and rotating movies of digitally desheathed whole ganglia to reveal all surface neurons. A complete neuron count of the horizontally sectioned ganglion yielded 1,885 neurons. Real and virtual sections from the image stacks were used to reveal the morphology of individual neurons, as well as the major axon bundles traveling within the ganglion to and between its several nerves and connectives. Extensive supplemental data are provided, as well as a link to the Dryad Data Repository site, where the complete sets of high-resolution serial-section images can be downloaded. NEW & NOTEWORTHY Because of the large size and relatively low numbers of their neurons, gastropod mollusks are widely used for investigations of the neural basis of behavior. Most studies, however, focus on the neurons visible on the ganglion surface, leaving the majority, located out of sight below the surface, unexamined. The present light microscopy study generates the first detailed visual atlas of all neurons of the highly studied Tritonia pedal ganglion.

The Potential of Indonesian Heterobranchs Found around Bunaken Island for the Production of Bioactive Compounds.

The species diversity of marine heterobranch sea slugs found on field trips around Bunaken Island (North Sulawesi, Indonesia) and adjacent islands of the Bunaken National Marine Park forms the basis of this review. In a survey performed in 2015, 80 species from 23 families were collected, including 17 new species. Only three of these have been investigated previously in studies from Indonesia. Combining species diversity with a former study from 2003 reveals in total 140 species from this locality. The diversity of bioactive compounds known and yet to be discovered from these organisms is summarized and related to the producer if known or suspected (might it be down the food chain, de novo synthesised from the slug or an associated bacterium). Additionally, the collection of microorganisms for the discovery of natural products of pharmacological interest from this hotspot of biodiversity that is presented here contains more than 50 species that have never been investigated before in regard to bioactive secondary metabolites. This highlights the great potential of the sea slugs and the associated microorganisms for the discovery of natural products of pharmacological interest from this hotspot of biodiversity.

Recruitment of Polysynaptic Connections Underlies Functional Recovery of a Neural Circuit after Lesion.

The recruitment of additional neurons to neural circuits often occurs in accordance with changing functional demands. Here we found that synaptic recruitment plays a key role in functional recovery after neural injury. Disconnection of a brain commissure in the nudibranch mollusc, Tritonia diomedea, impairs swimming behavior by eliminating particular synapses in the central pattern generator (CPG) underlying the rhythmic swim motor pattern. However, the CPG functionally recovers within a day after the lesion. The strength of a spared inhibitory synapse within the CPG from Cerebral Neuron 2 (C2) to Ventral Swim Interneuron B (VSI) determines the level of impairment caused by the lesion, which varies among individuals. In addition to this direct synaptic connection, there are polysynaptic connections from C2 and Dorsal Swim Interneurons to VSI that provide indirect excitatory drive but play only minor roles under normal conditions. After disconnecting the pedal commissure (Pedal Nerve 6), the recruitment of polysynaptic excitation became a major source of the excitatory drive to VSI. Moreover, the amount of polysynaptic recruitment, which changed over time, differed among individuals and correlated with the degree of recovery of the swim motor pattern. Thus, functional recovery was mediated by an increase in the magnitude of polysynaptic excitatory drive, compensating for the loss of direct excitation. Since the degree of susceptibility to injury corresponds to existing individual variation in the C2 to VSI synapse, the recovery relied upon the extent to which the network reorganized to incorporate additional synapses.

Memory Formation in Tritonia via Recruitment of Variably Committed Neurons.

Prior studies have found that functional networks can rapidly add neurons as they build short-term memories, yet little is known about the principles underlying this process. Using voltage-sensitive dye imaging, we found that short-term sensitization of Tritonia's swim motor program involves rapid expansion of the number of participating neurons. Tracking neurons across trials revealed that this involves the conversion of recently discovered variably participating neurons to reliable status. Further, we identify a candidate serotonergic cellular mechanism mediating this process. Our findings reveal a new mechanism for memory formation, involving recruitment of pre-positioned, variably committed neurons into memory networks. This represents a shift from the field's long-term focus on synaptic plasticity, toward a view that certain neurons have characteristics that predispose them to join networks with learning.

Monitoring Spiking Activity of Many Individual Neurons in Invertebrate Ganglia.

Optical recording with fast voltage sensitive dyes makes it possible, in suitable preparations, to simultaneously monitor the action potentials of large numbers of individual neurons. Here we describe methods for doing this, including considerations of different dyes and imaging systems, methods for correlating the optical signals with their source neurons, procedures for getting good signals, and the use of Independent Component Analysis for spike-sorting raw optical data into single neuron traces. These combined tools represent a powerful approach for large-scale recording of neural networks with high temporal and spatial resolution.

Photoprotection in sequestered plastids of sea slugs and respective algal sources.

Some sea slugs are capable of retaining functional sequestered chloroplasts (kleptoplasts) for variable periods of time. The mechanisms supporting the maintenance of these organelles in animal hosts are still largely unknown. Non-photochemical quenching (NPQ) and the occurrence of a xanthophyll cycle were investigated in the sea slugs Elysia viridis and E. chlorotica using chlorophyll fluorescence measurements and pigment analysis. The photoprotective capacity of kleptoplasts was compared to that observed in their respective algal source, Codium tomentosum and Vaucheria litorea. A functional xanthophyll cycle and a rapidly reversible NPQ component were found in V. litorea and E. chlorotica but not in C. tomentosum and E. viridis. To our knowledge, this is the first report of the absence of a functional xanthophyll cycle in a green macroalgae. The absence of a functional xanthophyll cycle in C. tomentosum could contribute to the premature loss of photosynthetic activity and relatively short-term retention of kleptoplasts in E. viridis. On the contrary, E. chlorotica displays one of the longest functional examples of kleptoplasty known so far. We speculate that different efficiencies of photoprotection and repair mechanisms of algal food sources play a role in the longevity of photosynthetic activity in kleptoplasts retained by sea slugs.

One rhinophore probably provides sufficient sensory input for odour-based navigation by the nudibranch mollusc Tritonia diomedea.

Tritonia diomedea (synonymous with Tritonia tetraquetra) navigates in turbulent odour plumes, crawling upstream towards prey and downstream to avoid predators. This is probably accomplished by odour-gated rheotaxis, but other possibilities have not been excluded. Our goal was to test whether T. diomedea uses odour-gated rheotaxis and to simultaneously determine which of the cephalic sensory organs (rhinophores and oral veil) are required for navigation. In a first experiment, slugs showed no coherent responses to streams of odour directed at single rhinophores. In a second experiment, navigation in prey and predator odour plumes was compared between animals with unilateral rhinophore lesions, denervated oral veils, or combined unilateral rhinophore lesions and denervated oral veils. In all treatments, animals navigated in a similar manner to that of control and sham-operated animals, indicating that a single rhinophore provides sufficient sensory input for navigation (assuming that a distributed flow measurement system would also be affected by the denervations). Amongst various potential navigational strategies, only odour-gated positive rheotaxis can produce the navigation tracks we observed in prey plumes while receiving input from a single sensor. Thus, we provide strong evidence that T. diomedea uses odour-gated rheotaxis in attractive odour plumes, with odours and flow detected by the rhinophores. In predator plumes, slugs turned downstream to varying degrees rather than orienting directly downstream for crawling, resulting in greater dispersion for negative rheotaxis in aversive plumes. These conclusions are the first explicit confirmation of odour-gated rheotaxis as a navigational strategy in gastropods and are also a foundation for exploring the neural circuits that mediate odour-gated rheotaxis.

Hidden synaptic differences in a neural circuit underlie differential behavioral susceptibility to a neural injury.

Individuals vary in their responses to stroke and trauma, hampering predictions of outcomes. One reason might be that neural circuits contain hidden variability that becomes relevant only when those individuals are challenged by injury. We found that in the mollusc, Tritonia diomedea, subtle differences between animals within the neural circuit underlying swimming behavior had no behavioral relevance under normal conditions but caused differential vulnerability of the behavior to a particular brain lesion. The extent of motor impairment correlated with the site of spike initiation in a specific neuron in the neural circuit, which was determined by the strength of an inhibitory synapse onto this neuron. Artificially increasing or decreasing this inhibitory synaptic conductance with dynamic clamp correspondingly altered the extent of motor impairment by the lesion without affecting normal operation. The results suggest that neural circuit differences could serve as hidden phenotypes for predicting the behavioral outcome of neural damage.

Parallel evolution of serotonergic neuromodulation underlies independent evolution of rhythmic motor behavior.

Neuromodulation can dynamically alter neuronal and synaptic properties, thereby changing the behavioral output of a neural circuit. It is therefore conceivable that natural selection might act upon neuromodulation as a mechanism for sculpting the behavioral repertoire of a species. Here we report that the presence of neuromodulation is correlated with the production of a behavior that most likely evolved independently in two species: Tritonia diomedea and Pleurobranchaea californica (Mollusca, Gastropoda, Opisthobranchia, Nudipleura). Individuals of both species exhibit escape swimming behaviors consisting of repeated dorsal-ventral whole-body flexions. The central pattern generator (CPG) circuits underlying these behaviors contain homologous identified neurons: DSI and C2 in Tritonia and As and A1 in Pleurobranchaea. Homologs of these neurons also can be found in Hermissenda crassicornis where they are named CPT and C2, respectively. However, members of this species do not exhibit an analogous swimming behavior. In Tritonia and Pleurobranchaea, but not in Hermissenda, the serotonergic DSI homologs modulated the strength of synapses made by C2 homologs. Furthermore, the serotonin receptor antagonist methysergide blocked this neuromodulation and the swimming behavior. Additionally, in Pleurobranchaea, the robustness of swimming correlated with the extent of the synaptic modulation. Finally, injection of serotonin induced the swimming behavior in Tritonia and Pleurobranchaea, but not in Hermissenda. This suggests that the analogous swimming behaviors of Tritonia and Pleurobranchaea share a common dependence on serotonergic neuromodulation. Thus, neuromodulation may provide a mechanism that enables species to acquire analogous behaviors independently using homologous neural circuit components.

Axonal conduction block as a novel mechanism of prepulse inhibition.

In prepulse inhibition (PPI), the startle response to a strong, unexpected stimulus is diminished if shortly preceded by the onset of a different stimulus. Because deficits in this inhibitory gating process are a hallmark feature of schizophrenia and certain other psychiatric disorders, the mechanisms underlying PPI are of significant interest. We previously used the invertebrate model system Tritonia diomedea to identify the first cellular mechanism for PPI--presynaptic inhibition of transmitter release from the afferent neurons (S-cells) mediating the startle response. Here, we report the involvement of a second, more powerful PPI mechanism in Tritonia: prepulse-elicited conduction block of action potentials traveling in the startle pathway caused by identified inhibitory interneurons activated by the prepulse. This example of axo-axonic conduction block--neurons in one pathway inhibiting the propagation of action potentials in another--represents a novel and potent mechanism of sensory gating in prepulse inhibition.

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.

Responses to magnetic stimuli recorded in peripheral nerves in the marine nudibranch mollusk Tritonia diomedea.

Prior behavioral and neurophysiological studies provide evidence that the nudibranch mollusk Tritonia orients to the earth's magnetic field. Earlier studies of electrophysiological responses in certain neurons of the brain to changing ambient magnetic fields suggest that although certain identified brain cells fire impulses when the ambient field is changed, these neuron somata and their central dentritic and axonal processes are themselves not primary magnetic receptors. Here, using semi-intact animal preparations from which the brain was removed, we recorded from peripheral nerve trunks. Using techniques to count spikes in individual nerves and separately also to identify, then count individual axonal spikes in extracellular records, we found both excitatory and inhibitory axonal responses elicited by changes in the direction of ambient earth strength magnetic fields. We found responses in nerves from many locations throughout the body and in axons innervating the body wall and rhinophores. Our results indicate that primary receptors for geomagnetism in Tritonia are not focally concentrated in any particular organ, but appear to be widely dispersed in the peripheral body tissues.

Validation of independent component analysis for rapid spike sorting of optical recording data.

Independent component analysis (ICA) is a technique that can be used to extract the source signals from sets of signal mixtures where the sources themselves are unknown. The analysis of optical recordings of invertebrate neuronal networks with fast voltage-sensitive dyes could benefit greatly from ICA. These experiments can generate hundreds of voltage traces containing both redundant and mixed recordings of action potentials originating from unknown numbers of neurons. ICA can be used as a method for converting such complex data sets into single-neuron traces, but its accuracy for doing so has never been empirically evaluated. Here, we tested the accuracy of ICA for such blind source separation by simultaneously performing sharp electrode intracellular recording and fast voltage-sensitive dye imaging of neurons located in the central ganglia of Tritonia diomedea and Aplysia californica, using a 464-element photodiode array. After running ICA on the optical data sets, we found that in 34 of 34 cases the intracellularly recorded action potentials corresponded 100% to the spiking activity of one of the independent components returned by ICA. We also show that ICA can accurately sort action potentials into single neuron traces from a series of optical data files obtained at different times from the same preparation, allowing one to monitor the network participation of large numbers of individually identifiable neurons over several recording episodes. Our validation of the accuracy of ICA for extracting the neural activity of many individual neurons from noisy, mixed, and redundant optical recording data sets should enable the use of this powerful large-scale imaging approach for studies of invertebrate and suitable vertebrate neuronal networks.

The role of multixenobiotic transporters in predatory marine molluscs as counter-defense mechanisms against dietary allelochemicals.

Multixenobiotic transporters have been extensively studied for their ability to modulate the disposition and toxicity of pharmacological agents, yet their influence in regulating the levels of dietary toxins within marine consumers has only recently been explored. This study presents functional and molecular evidence for multixenobiotic transporter-mediated efflux activity and expression in the generalist gastropod Cyphoma gibbosum, and the specialist nudibranch Tritonia hamnerorum, obligate predators of chemically defended gorgonian corals. Immunochemical analysis revealed that proteins with homology to permeability glycoprotein (P-gp) were highly expressed in T. hamnerorum whole animal homogenates and localized to the apical tips of the gut epithelium, a location consistent with a role in protection against ingested prey toxins. In vivo dye assays with specific inhibitors of efflux transporters demonstrated the activity of P-gp and multidrug resistance-associated protein (MRP) families of ABC transporters in T. hamnerorum. In addition, we identified eight partial cDNA sequences encoding two ABCB and two ABCC proteins from each molluscan species. Digestive gland transcripts of C. gibbosum MRP-1, which have homology to vertebrate glutathione-conjugate transporters, were constitutively expressed regardless of gorgonian diet. This constitutive expression may reflect the ubiquitous presence of high affinity substrates for C. gibbosum glutathione transferases in gorgonian tissues likely necessitating export by MRPs. Our results suggest that differences in multixenobiotic transporter expression patterns and activity in molluscan predators may stem from the divergent foraging strategies of each consumer.

Functional recovery after lesion of a central pattern generator.

In cases of neuronal injury when regeneration is restricted, functional recovery can occur through reorganization of the remaining neural circuitry. We found an example of such recovery in the central pattern generator (CPG) for the escape swim of the mollusc Tritonia diomedea. The CPG neurons are bilaterally represented and each neuron projects an axon through one of two pedal commissures. Cutting the posterior pedal commissure [pedal nerve 6 (PdN6)] in the animal or in the isolated brain caused a deficit in the swim behavior and in the fictive motor pattern, respectively, each of which recovered over the course of 20 h. Locally blocking spiking activity in PdN6 with sodium-free saline and/or tetrodotoxin disrupted the motor pattern in a reversible manner. Maintained blockade of PdN6 led to a functional recovery of the swim motor pattern similar to that observed in response to cutting the commissure. Among the CPG neurons, cerebral neuron 2 (C2) makes functional connection onto the ventral swim interneuron-B (VSI) in both pedal ganglia. Cutting or blocking PdN6 eliminated C2-evoked excitation of VSI in the pedal ganglion distal to the lesion. Associated with the recovery of the swim motor pattern, the synaptic action of C2 onto VSI in the proximal pedal ganglion changed from being predominantly inhibitory to being predominantly excitatory. These results show that the Tritonia swim CPG undergoes adaptive plasticity in response to the loss of critical synaptic connections; reversal of synaptic action in the CPG may be at least partially responsible for this functional recovery.

Evidence that the swim afferent neurons of tritonia diomedea are glutamatergic.

The escape swim response of the marine mollusc Tritonia diomedea is a well-established model system for studies of the neural basis of behavior. Although the swim neural network is reasonably well understood, little is known about the transmitters used by its constituent neurons. In the present study, we provide immunocytochemical and electrophysiological evidence that the S-cells, the afferent neurons that detect aversive skin stimuli and in turn trigger Tritonia's escape swim response, use glutamate as their transmitter. First, immunolabeling revealed that S-cell somata contain elevated levels of glutamate compared to most other neurons in the Tritonia brain, consistent with findings from glutamatergic neurons in many species. Second, pressure-applied puffs of glutamate produced the same excitatory response in the target neurons of the S-cells as the naturally released S-cell transmitter itself. Third, the glutamate receptor antagonist CNQX completely blocked S-cell synaptic connections. These findings support glutamate as a transmitter used by the S-cells, and will facilitate studies using this model system to explore a variety of issues related to the neural basis of behavior.