Paradoxical actions of the serotonin precursor 5-hydroxytryptophan on the activity of identified serotonergic neurons in a simple motor circuit

Continuous Real-Time Detection of Serotonin Using an Aptamer-Based Electrochemical Biosensor

Serotonin (5-HT) is a critical neurotransmitter involved in many neuronal functions, and 5-HT depletion has been linked to several mental diseases. The fast release and clearance of serotonin in the extracellular space, low analyte concentrations, and a multitude of interfering species make the detection of serotonin challenging. This work presents an electrochemical aptamer-based biosensing platform that can monitor 5-HT continuously with high sensitivity and selectivity. Our electrochemical sensor showed a response time of approximately 1 min to a step change in the serotonin concentration in continuous monitoring using a single-frequency EIS (electrochemical impedance spectroscopy) technique. The developed sensing platform was able to detect 5-HT in the range of 25-150 nM in the continuous sample fluid flow with a detection limit (LOD) of 5.6 nM. The electrochemical sensor showed promising selectivity against other species with similar chemical structures and redox potentials, including dopamine (DA), norepinephrine (NE), L-tryptophan (L-TP), 5-hydroxyindoleacetic acid (5-HIAA), and 5-hydroxytryptophan (5-HTP). The proposed sensing platform is able to achieve high selectivity in the nanomolar range continuously in real-time, demonstrating the potential for monitoring serotonin from neurons in organ-on-a-chip or brain-on-a-chip-based platforms.

Storage and erasure of behavioural experiences at the single neuron level

Although predictions from the past about the future have been of major interest to current neuroscience, how past and present behavioral experience interacts at the level of a single neuron remains largely unknown. Using the pond snail Lymnaea stagnalis we found that recent experience of terrestrial locomotion (exercise) results in a long-term increase in the firing rate of serotonergic pedal (PeA) neurons. Isolation from the CNS preserved the "memory" about previous motor activity in the neurons even after the animals rested for two hours in deep water after the exercise. In contrast, in the CNS, no difference in the firing rate between the control and "exercise-rested" (ER) neurons was seen. ER snails, when placed again on a surface to exercise, nevertheless showed faster locomotor arousal. The difference in the firing rate between the control and ER isolated neurons disappeared when the neurons were placed in the microenvironment of their home ganglia. It is likely that an increased content of dopamine in the CNS masks an increased excitation of PeA neurons after rest: the dopamine receptor antagonist sulpiride produced sustained excitation in PeA neurons from ER snails but not in the control. Therefore, our data suggest the involvement of two mechanisms in the interplay of past and present experiences at the cellular level: intrinsic neuronal changes in the biophysical properties of the cell membrane and extrinsic modulatory environment of the ganglia.

Zebrafish behavioural profiling identifies GABA and serotonin receptor ligands related to sedation and paradoxical excitation

Anesthetics are generally associated with sedation, but some anesthetics can also increase brain and motor activity—a phenomenon known as paradoxical excitation. Previous studies have identified GABA_A receptors as the primary targets of most anesthetic drugs, but how these compounds produce paradoxical excitation is poorly understood. To identify and understand such compounds, we applied a behavior-based drug profiling approach. Here, we show that a subset of central nervous system depressants cause paradoxical excitation in zebrafish. Using this behavior as a readout, we screened thousands of compounds and identified dozens of hits that caused paradoxical excitation. Many hit compounds modulated human GABA_A receptors, while others appeared to modulate different neuronal targets, including the human serotonin-6 receptor. Ligands at these receptors generally decreased neuronal activity, but paradoxically increased activity in the caudal hindbrain. Together, these studies identify ligands, targets, and neurons affecting sedation and paradoxical excitation in vivo in zebrafish.

Some anesthetics despite being generally associated with sedation, can also increase brain activity—a phenomenon called paradoxical excitation. The authors identified dozens of compounds that generally decrease neuronal activity, but increase activity in the caudal hindbrain of zebrafish.

Increase in serotonin precursor levels reinstates the context memory during reconsolidation

In the present study, we tested possible ways of modification of the context long-term memory using the reconsolidation as a tool. Recently, using a depletion of the serotonin content, it was shown that the reinforcing neurotransmitter serotonin is necessary for successful repeated reconsolidation of context memory in terrestrial snails Helix lucorum (Balaban et al. in Sci Rep 6:36933, 2016), and in the present study, we investigated effects of serotonin increase in memory maintenance by injection of the serotonin precursor 5-hydroxytryptophan (5-HTP). We studied reinstatement of the context memory after its impairment during reconsolidation with a protein synthesis blocker anisomycin (ANI) or with a specific inhibitor of protein-kinase Mζ (ZIP). It was observed that applications of 5-HTP alone, known to increase the release of serotonin, or reactivation of memory alone did not restore the ZIP- or ANI-impaired context memory, while combination of the 5-HTP + reactivation of memory effectively reinstated the context memory. The data obtained confirmed the assumption that serotonin/reinforcing transmitter is a part of successful reconsolidation necessary for memory maintenance, demonstrated possible ways of long-term memory regulation during the reconsolidation process.

Neuromodulatory control of a goal-directed decision

Many cost-benefit decisions reduce to simple choices between approach or avoidance (or active disregard) to salient stimuli. Physiologically, critical factors in such decisions are modulators of the homeostatic neural networks that bias decision processes from moment to moment. For the predatory sea-slug Pleurobranchaea, serotonin (5-HT) is an intrinsic modulatory promoter of general arousal and feeding. We correlated 5-HT actions on appetitive state with its effects on the approach-avoidance decision in Pleurobranchaea. 5-HT and its precursor 5-hydroxytryptophan (5-HTP) augmented general arousal state and reduced feeding thresholds in intact animals. Moreover, 5-HT switched the turn response to chemosensory stimulation from avoidance to orienting in many animals. In isolated CNSs, bath application of 5-HT both stimulated activity in the feeding motor network and switched the fictive turn response to unilateral sensory nerve stimulation from avoidance to orienting. Previously, it was shown that increasing excitation state of the feeding network reversibly switched the turn motor network response from avoidance to orienting, and that 5-HT levels vary inversely with nutritional state. A simple model posits a critical role for 5-HT in control of the turn network response by corollary output of the feeding network. In it, 5-HT acts as an intrinsic neuromodulatory factor coupled to nutritional status and regulates approach-avoidance via the excitation state of the feeding network. Thus, the neuromodulator is a key organizing element in behavioral choice of approach or avoidance through its actions in promoting appetitive state, in large part via the homeostatic feeding network.

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.

Opposing modulatory effects of D1- and D2-like receptor activation on a spinal central pattern generator

The role of dopamine in regulating spinal cord function is receiving increasing attention, but its actions on spinal motor networks responsible for rhythmic behaviors remain poorly understood. Here, we have explored the modulatory influence of dopamine on locomotory central pattern generator (CPG) circuitry in the spinal cord of premetamorphic Xenopus laevis tadpoles. Bath application of exogenous dopamine to isolated brain stem-spinal cords exerted divergent dose-dependent effects on spontaneous episodic patterns of locomotory-related activity recorded extracellularly from spinal ventral roots. At low concentration (2 μM), dopamine reduced the occurrence of bursts and fictive swim episodes and increased episode cycle periods. In contrast, at high concentration (50 μM) dopamine reversed its actions on fictive swimming, now increasing both burst and swim episode occurrences while reducing episode periods. The low-dopamine effects were mimicked by the D2-like receptor agonists bromocriptine and quinpirole, whereas the D1-like receptor agonist SKF 38393 reproduced the effects of high dopamine. Furthermore, the motor response to the D1-like antagonist SCH 23390 resembled that to the D2 agonists, whereas the D2-like antagonist raclopride mimicked the effects of the D1 agonist. Together, these findings indicate that dopamine plays an important role in modulating spinal locomotor activity. Moreover, the transmitter's opposing influences on the same target CPG are likely to be accomplished by a specific, concentration-dependent recruitment of independent D2- and D1-like receptor signaling pathways that differentially mediate inhibitory and excitatory actions.

Neural mechanisms underlying the evolvability of behaviour

The complexity of nervous systems alters the evolvability of behaviour. Complex nervous systems are phylogenetically constrained; nevertheless particular species-specific behaviours have repeatedly evolved, suggesting a predisposition towards those behaviours. Independently evolved behaviours in animals that share a common neural architecture are generally produced by homologous neural structures, homologous neural pathways and even in the case of some invertebrates, homologous identified neurons. Such parallel evolution has been documented in the chromatic sensitivity of visual systems, motor behaviours and complex social behaviours such as pair-bonding. The appearance of homoplasious behaviours produced by homologous neural substrates suggests that there might be features of these nervous systems that favoured the repeated evolution of particular behaviours. Neuromodulation may be one such feature because it allows anatomically defined neural circuitry to be re-purposed. The developmental, genetic and physiological mechanisms that contribute to nervous system complexity may also bias the evolution of behaviour, thereby affecting the evolvability of species-specific behaviour.

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.

Different functions for homologous serotonergic interneurons and serotonin in species-specific rhythmic behaviours

Closely related species can exhibit different behaviours despite homologous neural substrates. The nudibranch molluscs Tritonia diomedea and Melibe leonina swim differently, yet their nervous systems contain homologous serotonergic neurons. In Tritonia, the dorsal swim interneurons (DSIs) are members of the swim central pattern generator (CPG) and their neurotransmitter serotonin is both necessary and sufficient to elicit a swim motor pattern. Here it is shown that the DSI homologues in Melibe, the cerebral serotonergic posterior-A neurons (CeSP-As), are extrinsic to the swim CPG, and that neither the CeSP-As nor their neurotransmitter serotonin is necessary for swim motor pattern initiation, which occurred when the CeSP-As were inactive. Furthermore, the serotonin antagonist methysergide blocked the effects of both the serotonin and CeSP-As but did not prevent the production of a swim motor pattern. However, the CeSP-As and serotonin could influence the Melibe swim circuit; depolarization of a cerebral serotonergic posterior-A was sufficient to initiate a swim motor pattern and hyperpolarization of a CeSP-A temporarily halted an ongoing swim motor pattern. Serotonin itself was sufficient to initiate a swim motor pattern or make an ongoing swim motor pattern more regular. Thus, evolution of species-specific behaviour involved alterations in the functions of identified homologous neurons and their neurotransmitter.

Tryptophan research in panic disorder

A considerable body of evidence suggests the involvement of serotonin neurotransmission in the pathogenesis of panic disorder. Research on pathways and functions of tryptophan, an essential amino acid converted into serotonin, may advance our understanding of serotonergic actions in panic disorder and related phenomena. The investigative approaches in this field include manipulations of tryptophan availability as well as genetic association and functional brain imaging studies. In this review we examine the principle findings of these studies and propose further research directions.

5-HT and 5-HT-SO4, but not tryptophan or 5-HIAA levels in single feeding neurons track animal hunger state

Serotonin (5-HT) is an intrinsic modulator of neural network excitation states in gastropod molluscs. 5-HT and related indole metabolites were measured in single, well-characterized serotonergic neurons of the feeding motor network of the predatory sea-slug Pleurobranchaea californica. Indole amounts were compared between paired hungry and satiated animals. Levels of 5-HT and its metabolite 5-HT-SO4 in the metacerebral giant neurons were observed in amounts approximately four-fold and two-fold, respectively, below unfed partners 24 h after a satiating meal. Intracellular levels of 5-hydroxyindole acetic acid and of free tryptophan did not differ significantly with hunger state. These data demonstrate that neurotransmitter levels and their metabolites can vary in goal-directed neural networks in a manner that follows internal state.

Parameter space analysis suggests multi-site plasticity contributes to motor pattern initiation in Tritonia

This research examines the mechanisms that initiate rhythmic activity in the episodic central pattern generator (CPG) underlying escape swimming in the gastropod mollusk Tritonia diomedea. Activation of the network is triggered by extrinsic excitatory input but also accompanied by intrinsic neuromodulation and the recruitment of additional excitation into the circuit. To examine how these factors influence circuit activation, a detailed simulation of the unmodulated CPG network was constructed from an extensive set of physiological measurements. In this model, extrinsic input alone is insufficient to initiate rhythmic activity, confirming that additional processes are involved in circuit activation. However, incorporating known neuromodulatory and polysynaptic effects into the model still failed to enable rhythmic activity, suggesting that additional circuit features are also required. To delineate the additional activation requirements, a large-scale parameter-space analysis was conducted (~2 x 10(6) configurations). The results suggest that initiation of the swim motor pattern requires substantial reconfiguration at multiple sites within the network, especially to recruit ventral swim interneuron-B (VSI) activity and increase coupling between the dorsal swim interneurons (DSIs) and cerebral neuron 2 (C2) coupling. Within the parameter space examined, we observed a tendency for rhythmic activity to be spontaneous and self-sustaining. This suggests that initiation of episodic rhythmic activity may involve temporarily restructuring a nonrhythmic network into a persistent oscillator. In particular, the time course of neuromodulatory effects may control both activation and termination of rhythmic bursting.

Role of membrane potential in calcium signaling during rhythmic bursting in tritonia swim interneurons

Rhythmic bursting in neurons is accompanied by dynamic changes in intracellular Ca(2+) concentration. These Ca(2+) signals may be caused by membrane potential changes during bursting and/or by synaptic inputs. We determined that membrane potential is responsible for most, if not all, of the cytoplasmic Ca(2+) signal recorded during rhythmic bursting in two neurons of the escape swim central pattern generator (CPG) of the mollusk, Tritonia diomedea: ventral swim interneuron B (VSI) and cerebral neuron 2 (C2). Ca(2+) signals were imaged with a confocal laser scanning microscope while the membrane potential was recorded at the soma. During the swim motor pattern (SMP), Ca(2+) signals in both neurons transiently increased during each burst of action potentials with a more rapid decay in secondary than in primary neurites. VSI and C2 were then voltage-clamped at the soma, and each neuron's own membrane potential waveform recorded during the SMP was played back as the voltage command. In all regions of VSI, this completely reproduced the amplitude and time course of Ca(2+) signals observed during the SMP, but in C2, the amplitude was lower in the playback experiments than during the SMP, possibly due to space clamp problems. Therefore in VSI, the cytoplasmic Ca(2+) signal during the SMP can be accounted for by its membrane potential excursions, whereas in C2 the membrane potential excursions can account for most of the SMP Ca(2+) signal.

Homologues of serotonergic central pattern generator neurons in related nudibranch molluscs with divergent behaviors

Homologues of a neuron that contributes to a species-specific behavior were identified and characterized in species lacking that behavior. The nudibranch Tritonia diomedea swims by flexing its body dorsally and ventrally. The dorsal swim interneurons (DSIs) are components of the central pattern generator (CPG) underlying this rhythmic motor pattern and also activate crawling. Homologues of the DSIs were identified in six nudibranchs that do not exhibit dorsal-ventral swimming: Tochuina tetraquetra, Melibe leonina, Dendronotus iris, D. frondosus, Armina californica, and Triopha catalinae. Homology was based upon shared features that distinguish the DSIs from all other neurons: (1) serotonin immunoreactivity, (2) location in the Cerebral serotonergic posterior (CeSP) cluster, and (3) axon projection to the contralateral pedal ganglion. The DSI homologues, named CeSP-A neurons, share additional features with the DSIs: irregular basal firing, synchronous inputs, electrical coupling, and reciprocal inhibition. Unlike the DSIs, the CeSP-A neurons were not rhythmically active in response to nerve stimulation. The CeSP-A neurons in Tochuina and Triopha also excited homologues of the Tritonia Pd5 neuron, a crawling efferent. Thus, the CeSP-A neurons and the DSIs may be part of a conserved network related to crawling that may have been co-opted into a rhythmic swim CPG in Tritonia.

Comparative mapping of serotonin-immunoreactive neurons in the central nervous systems of nudibranch molluscs

The serotonergic systems in nudibranch molluscs were compared by mapping the locations of serotonin-immunoreactive (5-HT-ir) neurons in 11 species representing all four suborders of the nudibranch clade: Dendronotoidea (Tritonia diomedea, Tochuina tetraquetra, Dendronotus iris, Dendronotus frondosus, and Melibe leonina), Aeolidoidea (Hermissenda crassicornis and Flabellina trophina), Arminoidea (Dirona albolineata, Janolus fuscus, and Armina californica), and Doridoidea (Triopha catalinae). A nomenclature is proposed to standardize reports of cell location in species with differing brain morphologies. Certain patterns of 5-HT immunoreactivity were found to be consistent for all species, such as the presence of 5-HT-ir neurons in the pedal and cerebral ganglia. Also, particular clusters of 5-HT-ir neurons in the anterior and posterior regions of the dorsal surface of the cerebral ganglion were always present. However, there were interspecies differences in the number of 5-HT-ir neurons in each cluster, and some clusters even exhibited strong intraspecies variability that was only weakly correlated with brain size. Phylogenetic analysis suggests that the presence of particular classes of 5-HT-ir neurons exhibits a great deal of homoplasy. The conserved features of the nudibranch serotonergic system presumably represent the shared ancestral structure, whereas the derived characters suggest substantial independent evolutionary changes in the number and presence of serotonergic neurons. Although a number of studies have demonstrated phylogenetic variability of peptidergic systems, this study suggests that serotonergic systems may also exhibit a high degree of homoplasy in some groups of organisms.

Serotonin function in panic disorder: important, but why?

The essential role of serotonin (5-hydroxytryptamine (5-HT)) system in the neurobiology and pharmacotherapy of panic disorder (PD) continues to be a topic of intensive interdisciplinary research. Interest in the involvement of 5-HT in PD has been fuelled by clinical studies demonstrating that medications increasing the synaptic availability of 5-HT, such as selective 5-HT re-uptake inhibitors, are effective in the treatment of PD. Rival theories of 5-HT deficiency vs excess have attempted to explain the impact of 5-HT function in PD. In the past decade, knowledge of the role of 5-HT in the neurobiology of PD has expanded dramatically due to much new research including experimental, treatment, brain-imaging, and genetic studies. The current review attempts to summarize the new data and their implications. The challenge and treatment studies generally confirm the specific inhibitory influence of 5-HT on panicogenesis. The brain-imaging studies in PD patients demonstrate functional and clinically relevant alterations in various elements of 5-HT system affecting the neurocircuitry of panic. The findings of genetic association studies suggest that certain 5-HT-related genes may contribute to the susceptibility to PD; however, these data are rather limited and inconsistent. It appears that, even if not the primary etiological factor in PD, the 5-HT function conveys important vulnerability, as well as adaptive factors. A better understanding of these processes may be critical in achieving progress in the treatment of patients suffering from PD.

Pharmacological manipulation of serotonin levels in the nervous system of the opisthobranch mollusc Tritonia diomedea

Serotonin-related disorders can be treated by manipulating serotonin synthesis with the serotonin precursor 5-hydroxytryptophan (5-HTP) or other pharmacological agents. The mollusc Tritonia diomedea is a model for investigating the effects of altering serotonin content on the functions of identified neurons. We used high-performance liquid chromatography and immunohistochemistry to examine the amount and localization of 5-HTP, serotonin, and the serotonin breakdown product 5-hydroxyindolacetic acid (5-HIAA) in the Tritonia brain after various pharmacological treatments. Exposure to 5-HTP (2 mM for 30 min-1 h) caused an immediate and massive increase in total 5-HTP content, which lasted more than 20 h, and the widespread appearance of 5-HTP immunoreactivity in neurons. Serotonin levels rose gradually, but only a restricted number of additional neurons displayed serotonin immunoreactivity. 5-HTP treatment also caused an increase in the total amount of 5-HIAA and the appearance of 5-HIAA immunoreactivity throughout the brain. Treatment with the synthesis cofactor tetrahydrobiopterin, the initial precursor tryptophan, or serotonin itself had no persistent effect on total serotonin content. The amino acid decarboxylase inhibitor hydroxybenzylhydrazine (NSD-1015) also had no effect on the total serotonin content, although it caused an accumulation of 5-HTP. Thus, serotonin levels in the brain of T. diomedea appear to be maintained by a homeostatic mechanism that can be disrupted by 5-HTP.

Serotonergic Modulation in Aplysia. II. Cellular and Behavioral Consequences of Increased Serotonergic Tone

Serotonin (5-HT) plays an important role in sensitization of defensive reflexes in Aplysia and is also involved in several aspects of arousal, such as the control of locomotion and of cardiovascular tone. In the preceding paper, we showed that tail-nerve shock, a noxious stimulus that readily induces sensitization, increases the firing rate of a large number of serotonergic neurons throughout the CNS. However, the functional consequences of such an increase in serotonergic tone are still poorly understood. In this study, we examined this question by using the 5-HT precursor 5-hydroxytryptophan (5-HTP) to specifically increase 5-HT release in the CNS. Increased tonic 5-HT release after 5-HTP treatment was manifested by facilitation of sensorimotor (SN-MN) synapses, increased firing rate of serotonergic neurons in the pedal and abdominal ganglia, and enhanced 5-HT release evoked by tail-nerve shock. When 5-HTP was administered to freely moving animals, it produced a strong arousal response characterized by increased locomotion and heart rate, which was reminiscent of the defensive arousal reaction triggered by noxious stimulation such as tail-shock. In contrast, 5-HTP actually inhibited the tail-induced siphon-withdrawal reflex. It is possible that 5-HT-induced facilitation of SN-MN synapses was counteracted by inhibition of polysynaptic reflex pathways between SNs and MNs, resulting in transient behavioral inhibition of the reflex, which could favor escape locomotion and/or respiration shortly after an aversive stimulus. We conclude that a major function associated with the activation of the Aplysia serotonergic system evoked by noxious stimuli is the triggering of a defensive arousal response. It is known that tail-shock-induced serotonergic activation contributes to memory encoding at least in part by facilitating SN-MN synapses. However, this effect in isolation might not be sufficient for the behavioral expression of sensitization.

Apical sensory neurones mediate developmental retardation induced by conspecific environmental stimuli in freshwater pulmonate snails

Freshwater pond snails Helisoma trivolvis and Lymnaea stagnalis undergo larval development and metamorphosis inside egg capsules. We report that their development is permanently under slight tonic inhibitory influence of the anterior sensory monoaminergic neurones, which are the remnants of the apical sensory organ. Conspecific juvenile snails, when reared under conditions of starvation and crowding, release chemical signals that are detected by these neurones in encapsulated larvae and reversibly suppress larval development, thus providing a link between environmental signals and developmental regulation. Induced retardation starts from the trochophore stage and results in up to twofold prolongation of the larval lifespan. Upon stimulation with the signal, the neurones increase synthesis and release of monoamines [serotonin (5-HT) in Helisoma and dopamine in Lymnaea] that inhibit larval development acting via ergometrine-sensitive internal receptors. Thus, the novel regulatory mechanism in larval development of molluscs is suggested and compared with the phenomenon of dauer larvae formation in the nematode Caenorhabditis elegans.

Cycle period of a network oscillator is independent of membrane potential and spiking activity in individual central pattern generator neurons

Rhythmic motor patterns are thought to arise through the cellular properties and synaptic interactions of neurons in central pattern generator (CPG) circuits. Yet, when examining the CPG underlying the rhythmic escape response of the opisthobranch mollusc, Tritonia diomedea, we found that the cycle period of the fictive swim motor pattern recorded from the isolated nervous system was not altered by changing the resting membrane potential or the level of spiking activity of any of the 3 known CPG cell types: ventral swim interneuron-B (VSI-B), the dorsal swim interneurons (DSIs), and cerebral neuron 2 (C2). Furthermore, tonic firing in one or more DSIs or C2 evoked rhythmic bursting that did not differ from the cycle period of the motor pattern evoked by nerve stimulation, regardless of the firing frequency. In contrast, the CPG produced a large range of cycle periods as a function of temperature. The temperature sensitivity of the fictive motor pattern produced by the isolated nervous system was similar to the temperature sensitivity of the swimming behavior produced by the intact animal. Thus, although the CPG is capable of producing a wide range of cycle periods under the influence of temperature, the membrane potentials and spiking activity of the identified CPG neurons do not determine the periodicity of the motor pattern. This suggests that the timing of activity in this network oscillator may be determined by a mechanism that is independent of the membrane potentials and spike rate of its constituent neurons.

Spike timing-dependent serotonergic neuromodulation of synaptic strength intrinsic to a central pattern generator circuit

Neuromodulation is often thought to have a static, gain-setting function in neural circuits. Here we report a counter example: the neuromodulatory effect of a serotonergic neuron is dependent on the interval between its spikes and those of the neuron being modulated. The serotonergic dorsal swim interneurons (DSIs) are members of the escape swim central pattern generator (CPG) in the mollusk Tritonia diomedea. DSI spike trains heterosynaptically enhanced synaptic potentials evoked by another CPG neuron, ventral swim interneuron B (VSI-B), when VSI-B action potentials occurred within 10 sec of a DSI spike train; however, if VSI-B was stimulated 20-120 sec after DSI, then the amplitude of VSI-B synaptic potentials decreased. Consistent with this, VSI-B-evoked synaptic currents exhibited a temporally biphasic and bidirectional change in amplitude after DSI stimulation. Both the DSI-evoked enhancement and decrement were occluded by serotonin and blocked by the serotonin receptor antagonist methysergide, suggesting that both phases are mediated by serotonin. In most preparations, however, bath-applied serotonin caused only a sustained enhancement of VSI-B synaptic strength. The heterosynaptic modulation interacted with short-term homosynaptic plasticity: DSI-evoked depression was offset by VSI-B homosynaptic facilitation. This caused a complicated temporal pattern of neuromodulation when DSI and VSI-B were stimulated to fire in alternating bursts to mimic the natural motor pattern: DSI strongly enhanced summated VSI-B synaptic potentials and suppressed single synaptic potentials after the cessation of the artificial motor pattern. Thus, spike timing-dependent serotonergic neuromodulatory actions can impart temporal information that may be relevant to the operation of the CPG.

Non-synaptic integration of the cell bodies of neurons into the central nervous system of the snail

Single neurons (soma and proximal process) were isolated from the serotonergic (5-HT-ergic) PedA cluster in experiments on the pond snail Lymnaea stagnalis, and changes in the electrical activity of isolated neurons were observed during repeated movement of these cells towards and away from the surface of the CNS. The position of cell bodies of 5-HT-ergic neurons had excitatory effects on the isolated neuron. This effect was maximal (at 10(-8)-10(-7) M 5-HT) when neurons were brought close to the PedA cluster and were further enhanced by addition of the 5-HT precursor 5-hydroxytryptophan at concentrations of (1-2) x 10(-4) M. The results obtained here provide evidence 5-HT-ergic neurons cooperate during 5-HT-dependent behaviour, this being based on excitatory interactions at the level of cell bodies.

G protein signaling in a neuronal network is necessary for rhythmic motor pattern production

G protein-coupled receptors are widely recognized as playing important roles in mediating the actions of extrinsic neuromodulatory inputs to motor networks. However, the potential for their direct involvement in rhythmic motor pattern generation has received considerably less attention. Results from this study indicate that G protein signaling appears to be integral to the operation of the central pattern generator (CPG) underlying the escape swim of the mollusk Tritonia diomedea. Blocking G protein signaling in a single CPG neuron, cerebral neuron C2, with intracellular iontophoresis of the guanine nucleotide analogue guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S), prevented the production of the swim motor program. Moreover, tonic activation of G protein signaling in this neuron by iontophoresis of the GTP analogues guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and 5'-guanylyl-imidodiphosphate also inhibited motor pattern production. The possible sites of action of these guanine nucleotide analogues were examined to assess potential mechanisms by which they interfered with motor pattern production. Intracellular iontophoresis of GDP-beta-S into C2 did not affect C2 basal synaptic strength. However, it did reduce heterosynaptic facilitation of C2 synapses caused by the dorsal swim interneurons (DSIs), a set of serotonergic swim CPG neurons. In contrast, GTP-gamma-S directly enhanced C2 synaptic strength onto DFN, mimicking the neuromodulatory effect of the DSIs. GDP-beta-S, but not the GTP analogues, decreased C2 excitability, whereas both GTP analogues, but not GDP-beta-S, blocked the ability of DSI stimulation to increase C2 excitability. The decrease in C2 excitability caused by GDP-beta-S is not likely to be responsible for the inhibition of the swim motor pattern because decreasing C2 firing rate, by injecting hyperpolarizing current, did not prevent the production of the rhythmic motor pattern. Taken together, these data suggest that G protein signaling is a necessary and integral component of the escape swim CPG in Tritonia and that G protein signaling mediates DSI heterosynaptic facilitation of C2 but may not mediate the DSI-evoked enhancement of C2 excitability.

Serotonin release evoked by tail nerve stimulation in the CNS of aplysia: characterization and relationship to heterosynaptic plasticity

Considerable experimental evidence suggests that serotonin (5-HT) at sensory neuron-->motor neuron (SN-->MN) synapses, as well as other neuronal sites, contributes importantly to simple forms of learning such as sensitization and classical conditioning in Aplysia. However, the actual release of 5-HT in the CNS induced by sensitizing stimuli such as tail shock has not been directly demonstrated. In this study, we addressed this question by (1) immunohistochemically labeling central 5-HT processes and (2) directly measuring with chronoamperometry the release of 5-HT induced by pedal tail nerve (P9) shock onto tail SNs in the pleural ganglion and their synapses onto tail MNs in the pedal ganglion. We found that numerous 5-HT-immunoreactive fibers surround both the SN cell bodies in the pleural ganglion and SN axons in the pedal ganglion. Chronoamperometric detection of 5-HT performed with carbon fiber electrodes implanted in the vicinity of tail SN somata and synapses revealed an electrochemical 5-HT signal lasting approximately 40 sec after a brief shock of P9. 5-HT release was restricted to discrete subregions (modulatory fields) of the CNS, including the vicinity of tail SN soma and synapses ipsilateral to the stimulation. Increasing P9 shock frequency augmented the amplitude of the 5-HT signal and, in parallel, increased SN excitability and SN synaptic transmission onto tail MNs. However, the relationship between the amount of 5-HT release and the two forms of SN plasticity was not uniform: SN excitability increased in a graded manner with increased 5-HT release, whereas synaptic facilitation exhibited a highly nonlinear relationship. The development of chronoamperometric techniques in Aplysia now paves the way for a more complete understanding of the contribution of the serotonergic modulatory pathway to memory processing in this system.

Highly dissimilar behaviors mediated by a multifunctional network in the marine mollusk Tritonia diomedea

Several motor networks have now been found to be multifunctional, in which one group of neurons participates in the generation of multiple behavioral motor programs. Not surprisingly, the behaviors involved are frequently closely related, often using the same or similar muscle groups. Here we describe an interneuronal network in the marine mollusk Tritonia diomedea that is involved in producing two highly dissimilar behaviors, rhythmic, muscle-based escape swimming and nonrhythmic, cilia-mediated crawling. Several observations support this conclusion. First, the dorsal swim interneurons (DSIs) of the swim central pattern generator (CPG) directly excite Pedal neuron 21 (Pd21) and Pd5, the only identified cilia-activating efferent neurons in Tritonia. Second, stimulation of a single DSI elicits beating of the foot cilia in semi-intact preparations and crawling in intact animal treadmill preparations. Third, the DSIs fire at an elevated rate for nearly 1 hr after a swim motor program, which correlates reasonably well with the period freely behaving animals were found to crawl after they swam. Fourth, silencing the tonically active DSIs after a swim motor program substantially reduces or eliminates ongoing cilia neuron firing, indicating that the DSIs are major contributors to the synaptic input driving these cells. Finally, all of the other swim CPG neurons also connect to the cilia neurons, most monosynaptically. Taken together, these observations indicate that the Tritonia swim CPG network participates in producing both escape swimming and crawling. Given the extreme differences between these behaviors---rhythmic versus tonic, muscular versus ciliary, and brief versus prolonged--these findings reveal a striking versatility for a small multifunctional network.

Identified serotonergic neurons in the Tritonia swim CPG activate both ionotropic and metabotropic receptors

Although G-protein-coupled (metabotropic) receptors are known to modulate the production of motor patterns, evidence from the escape swim central pattern generator (CPG) of the nudibranch mollusk, Tritonia diomedea, suggests that they might also participate in the generation of the motor pattern itself. The dorsal swim interneurons (DSIs), identified serotonergic neurons intrinsic to the Tritonia swim CPG, evoke dual component synaptic potentials onto other CPG neurons and premotor interneurons. Both the fast and slow components were previously shown to be due to serotonin (5-HT) acting at distinct postsynaptic receptors. We find that blocking or facilitating metabotropic receptors in a postsynaptic premotor interneuron differentially affects the fast and slow synaptic responses to DSI stimulation. Blocking G-protein activation by iontophoretically injecting the GDP-analogue guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) did not significantly affect the DSI-evoked fast excitatory postsynaptic potential (EPSP) but decreased the amplitude of the slow component more than 50%. Injection of the GTP analogues guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and 5'-guanylyl-imidodiphosphate, to prolong G-protein activation, had mixed effects on the fast component but increased the amplitude and duration of the slow component of the DSI-evoked response and, with repeated DSI stimulation, led to a persistent depolarization. These results indicate that the fast component of the biphasic synaptic potential evoked by a serotonergic CPG neuron onto premotor interneurons is mediated by ionotropic receptors (5-HT-gated ion channels), whereas the slow component is mediated by G-protein-coupled receptors. A similar synaptic activation of metabotropic receptors might also be found within the CPG itself, where it could exert a direct influence onto motor pattern generation.