A cerebral central pattern generator in Aplysia and its connections with buccal feeding circuitry

Feedback to the future: motor neuron contributions to central pattern generator function

Motor behaviors depend on neural signals in the brain. Regardless of where in the brain behavior patterns arise, the central nervous system sends projections to motor neurons, which in turn project to and control temporally appropriate muscle contractions; thus, motor neurons are traditionally considered the last relay from the central nervous system to muscles. However, in an array of species and motor systems, an accumulating body of evidence supports a more complex role of motor neurons in pattern generation. These studies suggest that motor neurons not only relay motor patterns to the periphery, but directly contribute to pattern generation by providing feedback to upstream circuitry. In spinal and hindbrain circuits in a variety of animals - including flies, worms, leeches, crustaceans, rodents, birds, fish, amphibians and mammals - studies have indicated a crucial role for motor neuron feedback in maintaining normal behavior patterns dictated by the activity of a central pattern generator. Hence, in this Review, we discuss literature examining the role of motor neuron feedback across many taxa and behaviors, and set out to determine the prevalence of motor neuron participation in motor circuits.

Sensory-evoked perturbations of locomotor activity by sparse sensory input: a computational study

Sensory inputs from muscle, cutaneous, and joint afferents project to the spinal cord, where they are able to affect ongoing locomotor activity. Activation of sensory input can initiate or prolong bouts of locomotor activity depending on the identity of the sensory afferent activated and the timing of the activation within the locomotor cycle. However, the mechanisms by which afferent activity modifies locomotor rhythm and the distribution of sensory afferents to the spinal locomotor networks have not been determined. Considering the many sources of sensory inputs to the spinal cord, determining this distribution would provide insights into how sensory inputs are integrated to adjust ongoing locomotor activity. We asked whether a sparsely distributed set of sensory inputs could modify ongoing locomotor activity. To address this question, several computational models of locomotor central pattern generators (CPGs) that were mechanistically diverse and generated locomotor-like rhythmic activity were developed. We show that sensory inputs restricted to a small subset of the network neurons can perturb locomotor activity in the same manner as seen experimentally. Furthermore, we show that an architecture with sparse sensory input improves the capacity to gate sensory information by selectively modulating sensory channels. These data demonstrate that sensory input to rhythm-generating networks need not be extensively distributed.

Age-associated bidirectional modulation of gene expression in single identified R15 neuron of Aplysia

Background

Despite the advances in our understanding of aging-associated behavioral decline, relatively little is known about how aging affects neural circuits that regulate specific behaviors, particularly the expression of genes in specific neural circuits during aging. We have addressed this by exploring a peptidergic neuron R15, an identified neuron of the marine snail Aplysia californica. R15 is implicated in reproduction and osmoregulation and responds to neurotransmitters such as acetylcholine, serotonin and glutamate and is characterized by its action potential bursts.

Results

We examined changes in gene expression in R15 neurons during aging by microarray analyses of RNAs from two different age groups, mature and old animals. Specifically we find that 1083 ESTs are differentially regulated in mature and old R15 neurons. Bioinformatics analyses of these genes have identified specific biological pathways that are up or downregulated in mature and old neurons. Comparison with human signaling networks using pathway analyses have identified three major networks [(1) cell signaling, cell morphology, and skeletal muscular system development (2) cell death and survival, cellular function maintenance and embryonic development and (3) neurological diseases, developmental and hereditary disorders] altered in old R15 neurons. Furthermore, qPCR analysis of single R15 neurons to quantify expression levels of candidate regulators involved in transcription (CREB1) and translation (S6K) showed that aging is associated with a decrease in expression of these regulators, and similar analysis in three other neurons (L7, L11 and R2) showed that gene expression change during aging could be bidirectional.

Conclusions

We find that aging is associated with bidirectional changes in gene expression. Detailed bioinformatics analyses and human homolog searches have identified specific biological processes and human-relevant signaling pathways in R15 that are affected during aging. Evaluation of gene expression changes in different neurons suggests specific transcriptomic signature of single neurons during aging.

Neuroanatomical, immunocytochemical, and physiological studies of the pharyngeal retractor muscle and its putative regulatory neurons playing a role in withdrawal and feeding in the snail, Helix pomatia

We describe the neurons regulating two separate functions of the pharyngeal retractor muscle (PRM), namely sustained contraction during body withdrawal and rhythmic phasic contractions during feeding, in the snail, Helix pomatia. The distribution of central neurons innervating the PRM is organized into two main units; one in the buccal-cerebral ganglion complex, the other in the subesophageal ganglion complex. Serotonin- (5-HT-), FMRFamide- (FMRFa-), and tyrosine-hydroxylase-immunostained neurons are present among the PRM neurons that densely innervate the PRM. 5HT both decreases and increases the amplitude of the electrically evoked contraction between concentrations of 0.1 microM and 1 microM. Dopamine (DA) only decreases the amplitude of contraction at a 1-microM threshold concentration. In contrast, FMRFa increases the amplitude of the contraction and slightly elevates the tone of the PRM but requires a higher threshold (10 microM). Assay by high-performance liquid chromatography of 5HT and DA in the PRM has shown that the 5HT level decreases during locomotion but increases during feeding, whereas the DA level increases during locomotion but slightly decreases during feeding. Thus, different segments of the PRM are innervated by neurons from different loci within the central nervous system. The segments of the PRM distal to the pharynx are innervated from loci of the subesophageal ganglion complex suggesting that they mediate withdrawal. The proximal segment of the PRM is innervated from cerebral and buccal loci indicating that these neurons mediate the feeding rhythm produced by buccal and cerebral feeding central pattern generators to induce rhythmic phasic contractions in the PRM during feeding.

Extending in vitro conditioning in Aplysia to analyze operant and classical processes in the same preparation

Operant and classical conditioning are major processes shaping behavioral responses in all animals. Although the understanding of the mechanisms of classical conditioning has expanded significantly, the understanding of the mechanisms of operant conditioning is more limited. Recent developments in Aplysia are helping to narrow the gap in the level of understanding between operant and classical conditioning, and have raised the possibility of studying the neuronal processes underlying the interaction of operant and classical components in a relatively complex learning task. In the present study, we describe a first step toward realizing this goal, by developing a single in vitro preparation in which both operant and classical conditioning can be studied concurrently. The new paradigm reproduced previously published results, even under more conservative and homogenous selection criteria and tonic stimulation regime. Moreover, the observed learning was resistant to delay, shortening, and signaling of reinforcement.

Thee effect of food intake on the central monoaminergic system in the snail, Lymnaea stagnalis

We investigated the effect of food intake on the serotonin and dopamine levels of the CNS as well as on the spontaneous firing activity of the CGC in isolated preparations from starved, feeding and satiated animals. Furthermore we investigated the effects of 1 microM serotonin and/or dopamine and their mixture on the firing activity of the CGC. The HPLC assay of serotonin and dopamine showed that during food intake both the serotonin and dopamine levels of the CNS increased whereas in satiated animals their levels were not significantly more than the control levels. Recording from the CGC in isolated CNS preparation from starved, feeding or satiated animals showed that feeding increased the firing frequency of the CGC compared to the starved control. The application of 1 microM dopamine decreased the firing frequency whereas the application of 1 microM serotonin increased the firing frequency of the CGC. We conclude that during food intake the external and internal food stimuli increase the activity of the central monoaminergic system and also increase the levels of monoamines in the CNS. Furthermore, we also suggest that the increased dopamine and serotonin levels both affect the activity of the serotonergic neurons during the different phases of feeding.

In vitro analog of classical conditioning of feeding behavior in aplysia

The feeding behavior of Aplysia californica can be classically conditioned using tactile stimulation of the lips as a conditioned stimulus (CS) and food as an unconditioned stimulus (US). Moreover, several neural correlates of classical conditioning have been identified. The present study extended previous work by developing an in vitro analog of classical conditioning and by investigating pairing-specific changes in neuronal and synaptic properties. The preparation consisted of the isolated cerebral and buccal ganglia. Electrical stimulation of a lip nerve (AT4) and a branch of the esophageal nerve (En2) served as the CS and US, respectively. Three protocols were used: paired, unpaired, and US alone. Only the paired protocol produced a significant increase in CS-evoked fictive feeding. At the cellular level, classical conditioning enhanced the magnitude of the CS-evoked synaptic input to pattern-initiating neuron B31/32. In addition, paired training enhanced both the magnitude of the CS-evoked synaptic input and the CS-evoked spike activity in command-like neuron CBI-2. The in vitro analog of classical conditioning reproduced all of the cellular changes that previously were identified following behavioral conditioning and has led to the identification of several new learning-related neural changes. In addition, the pairing-specific enhancement of the CS response in CBI-2 indicates that some aspects of associative plasticity may occur at the level of the cerebral sensory neurons.

Search for cerebral G cluster neurons responding to taste stimulation with seaweed in Aplysia kurodai by the use of calcium imaging

The calcium imaging method can detect the spike activities of many neurons simultaneously. In the present experiments, this method was used to search for unique neurons contributing to feeding behavior in the cerebral ganglia of Aplysia kurodai. We mainly explored the neurons whose cell bodies were located in the G cluster and the neuropile region posterior to this cluster on the ventral surface of the cerebral ganglia. When the extract of the food seaweed Ulva was applied to the tentacle-lip region, many neurons stained with a calcium-sensitive dye, Calcium Green-1, showed changes in fluorescence. Some neurons showed rhythmic responses and others showed transient responses, suggesting that these neurons may be partly involved in the feeding circuits. We also identified three motor neurons among these neurons that showed rhythmic fluorescence responses to the taste stimulation. One of them was a motor neuron shortening the anterior tentacle (ATS), and the other two were motor neurons producing lip opening-like (LO(G)) and closing-like (LC(G)) movements, respectively. Application of the Ulva extract to the tentacle-lip region induced phase-locked rhythmic firing activity in these motor neurons, suggesting that these neurons may contribute to the rhythmic patterned movements of the anterior tentacles and lips during the ingestion of seaweed.

Identification and characterization of the feeding circuit-activating peptides, a novel neuropeptide family of aplysia

We use a multidisciplinary approach to identify, map, and characterize the bioactivity of modulatory neuropeptides in the circuitry that generates feeding behavior in Aplysia. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of the cerebral-buccal connective (CBC), a nerve containing axons of many interneurons that control feeding behavior of Aplysia, was used to identify neuropeptides that may participate in generation and shaping of feeding motor programs. Using this functionally oriented search, we identified a novel family of peptides that we call the feeding circuit-activating peptides (FCAPs). Two peptides with masses identical to those observed in the CBCs (molecular weight 1387 and 1433) were purified from buccal ganglia and partially sequenced using mass spectrometry. The amino acid sequence was then used to clone the FCAP precursor, which encodes multiple copies of eight different FCAPs. The two FCAPs present in highest copy number correspond to those observed in the CBC. The distribution of FCAP expression was mapped using Northern analysis, whole-mount in situ hybridization, and immunocytochemistry. Consistent with our initial findings, FCAP-immunopositive axons were observed in the CBC. Furthermore, we found that FCAP was present in some cerebral-buccal and buccal-cerebral interneurons. As their name suggests, FCAPs are capable of initiating rhythmic feeding motor programs and are the first neuropeptides with such activity in this circuit. The actions of FCAPs suggest that these peptides may contribute to the induction and maintenance of food-induced arousal. FCAPs were also localized to several other neuronal systems, suggesting that FCAPs may play a role in the regulation of multiple behaviors.

Colocalization of gamma-aminobutyric acid-like immunoreactivity and catecholamines in the feeding network of Aplysia californica

Functional consequences of neurotransmitter coexistence and cotransmission can be readily studied in certain experimentally favorable invertebrate motor systems. In this study, whole-mount histochemical methods were used to identify neurons in which gamma-aminobutyric acid (GABA)-like immunoreactivity (GABAli) was colocalized with catecholamine histofluorescence (CAh; FaGlu method) and tyrosine hydroxylase (TH)-like immunoreactivity (THli) in the feeding motor circuitry (buccal and cerebral ganglia) of the marine mollusc Aplysia californica. In agreement with previous reports, five neurons in the buccal ganglia were found to exhibit CAh. These included the paired B20 buccal-cerebral interneurons (BCIs), the paired B65 buccal interneurons, and an unpaired cell with projections to both cerebral-buccal connectives (CBCs). Experiments in which the FaGlu method was combined with the immunohistochemical detection of GABA revealed double labeling of all five of these neurons. An antibody generated against TH, the rate-limiting enzyme in the biosynthesis of catecholamines, was used to obtain an independent determination of GABA-CA colocalization. Biocytin backfills of the CBC performed in conjunction with TH immunohistochemistry revealed labeling of the rostral B20 cell pair and the unpaired CBI near the caudal surface of the right hemiganglion. THli was also present in a prominent bilateral pair of caudal neurons that were not stained with CBC backfills. On the basis of their position, size, shape, and lack of CBC projections, the lateral THli neurons were identified as B65. Double-labeling immunohistochemical experiments revealed GABAli in all five buccal THli neurons. Finally, GABAli was observed in individual B20 and B65 neurons that were identified using electrophysiological criteria and injected with a marker (neurobiotin). Similar methods were used to demonstrate that a previously identified catecholaminergic cerebral-buccal interneuron (CBI) designated CBI-1 contained THli but did not contain GABAli. Although numerous THli and GABAli neurons and fibers were present in the cerebral and buccal ganglia, additional instances of their colocalization were not observed. These findings indicate that GABA and a catecholamine (probably dopamine) are colocalized in a limited number of interneurons within the central pattern generator circuits that control feeding-related behaviors in Aplysia.

Voltage-gated ionic currents in an identified modulatory cell type controlling molluscan feeding

An important modulatory cell type, found in all molluscan feeding networks, was investigated using two-electrode voltage- and current-clamp methods. In the cerebral giant cells of Lymnaea, a transient inward Na+ current was identified with activation at -58 +/- 2 mV. It was sensitive to tetrodotoxin only in high concentrations (approximately 50% block at 100 microm), a characteristic of Na+ channels in many molluscan neurons. A much smaller low-threshold persistent Na+ current (activation at < -90 mV) was also identified. Two purely voltage-sensitive outward K+ currents were also found: (i) a transient A-current type which was activated at -59 +/- 4 mV and blocked by 4-aminopyridine; (ii) a sustained tetraethylammonium-sensitive delayed rectifier current which was activated at -47 +/- 2 mV. There was also evidence that a third, Ca2+-activated, K+ channel made a contribution to the total outward current. No inwardly rectifying currents were found. Two Ca2+ currents were characterized: (i) a transient low-voltage (-65 +/- 2 mV) activated T-type current, which was blocked in NiCl2 (2 mm) and was completely inactivated at approximately -50 mV; (ii) A sustained high voltage (-40 +/- 1 mV) activated current, which was blocked in CdCl2 (100 microm) but not in omega-conotoxin GVIA (10 microm), omega-agatoxin IVA (500 nm) or nifedipine (10 microm). This current was enhanced in Ba2+ saline. Current-clamp experiments revealed how these different current types could define the membrane potential and firing properties of the cerebral giant cells, which are important in shaping the wide-acting modulatory influence of this neuron on the rest of the feeding network.

RCPH modulation of a multi-oscillator network: effects on the pyloric network of the spiny lobster

The neuropeptide red pigment concentrating hormone (RPCH), which we have previously shown to activate the cardiac sac motor pattern and lead to a conjoint gastric mill-cardiac sac pattern in the spiny lobster Panulirus, also activates and modulates the pyloric pattern. Like the activity of gastric mill neurons in RPCH, the pattern of activity in the pyloric neurons is considerably more complex than that seen in control saline. This reflects the influence of the cardiac sac motor pattern, and particularly the upstream inferior ventricular (IV) neurons, on many of the pyloric neurons. RPCH intensifies this interaction by increasing the strength of the synaptic connections between the IV neurons and their targets in the stomatogastric ganglion. At the same time, RPCH enhances postinhibitory rebound in the lateral pyloric (LP) neuron. Taken together, these factors largely explain the complex pyloric pattern recorded in RPCH in Panulirus.

To beat or not to beat: a decision taken at the network level

The cells of the inferior olivary nucleus, the sole source of the cerebellar climbing fibers, form a network of electrically coupled neurons. Experimental observations show that these neurons produce a large repertoire of electrical signals, among which sub-threshold oscillations of the membrane potential. Simultaneous recordings from pairs of neurons and optical imaging of voltage sensitive dyes show that sub-threshold activity occurs in synchrony throughout the network. The mechanism underlying the generation of the sub-threshold oscillations is not fully understood. Experimental observations suggest that the electrical coupling is essential but insufficient for their generation. Several theoretical mechanisms have been suggested to explain these observations. Up-to-date, the most realistic model is the heterogeneity model, that assumes a certain degree of heterogeneity among olivary neurons. The heterogeneity model proposes that sub-threshold oscillations are produced by electrical coupling of neurons with the same types of ionic conductances, but with different densities. The variability in channel densities yield neurons of different functional types. The main prediction of the model is that different functional types of neurons should be found in the inferior olive. Dynamic clamp experiments support this prediction.

Topographic organization of efferent neurons with different neurochemical characters in the cerebral ganglia of the snail Helix pomatia

This study provides a description of the organization of neurons efferent to different head areas in the cerebral ganglia of Helix pomatia, revealed by simultaneous Ni-lysine and Co-lysine back-filling of different pairs of cerebral nerves. The backfills show that labeled cerebral neurons that innervate the head areas are concentrated in seven representation foci distributed in different parts of the cerebral ganglia. Almost each head area is represented in each focus. At a gross level, the representation of the different head areas in the representation foci shows a topographic arrangement. Each focus is constituted by neurochemically different groups of neurons. All head areas are innervated by serotonin-containing fibers from a single focus (Focus 2) and by dopamine-containing fibers from Foci 1, 2, and 4. However, they are innervated by CARP and FMRFamide-containing fibers from all of the foci. The combination of retrograde labeling with 5, 6-dihydroxytriptamine induced pigment labeling of serotonin-containing neurons or with fluorescence tyrosinehydroxylase immunocytochemistry to detect dopamine-containing neurons showed that the different head areas are topographycally represented in the clusters of both the serotonin- and dopamine-containing cells. The combination of Ni-lysine backfillings from different cerebral nerves with fluorescence CARP and FMRFamide immunocytochemistry revealed that the head areas are represented also in both the CARP and FMRFamide immunoreactive groups of neurons in the different foci.

Classical conditioning of feeding in Aplysia: II. Neurophysiological correlates

Feeding behavior in Aplysia californica can be classically conditioned using tactile stimulation of the lips as conditional stimulus (CS) and food as unconditional stimulus (US) [ (companion paper)]. Conditioning resulted in an increase in the number of CS-evoked bites that persisted for at least 24 hr after training. In this study, neurophysiological correlates of classical conditioning training were identified and characterized in an in vitro preparation of the cerebral and buccal ganglia. Stimulation of a lip nerve (AT(4)), which mediates mechanosensory information, resulted in a greater number of buccal motor patterns (BMPs) in ganglia isolated from animals that had received paired training than in ganglia from control animals. The majority of the evoked BMPs were classified as ingestion-like patterns. Intracellular recordings from pattern-initiating neuron B31/32 revealed that stimulation of AT(4) evoked greater excitatory input in B31/32 in preparations from animals that had received paired training than from control animals. In contrast, excitatory input to buccal neuron B4/5 in response to stimulation of AT(4) was not significantly increased by paired training. Moreover, correlates of classical conditioning were specific to stimulation of AT(4). The number of spontaneously occurring BMPs and the intrinsic properties of two buccal neurons (B4/5 and B31/32) did not differ between groups. These results suggest that appetitive classical conditioning of feeding resulted in the pairing-specific strengthening of the polysynaptic pathway between afferent fibers and pattern-initiating neurons of the buccal central pattern generator.

Modulation of fictive feeding by dopamine and serotonin in aplysia

The buccal ganglia of Aplysia contain a central pattern generator (CPG) that mediates rhythmic movements of the buccal apparatus during feeding. Activity in this CPG is believed to be regulated, in part, by extrinsic serotonergic inputs and by an intrinsic and extrinsic system of putative dopaminergic cells. The present study investigated the roles of dopamine (DA) and serotonin (5-HT) in regulating feeding movements of the buccal apparatus and properties of the underlying neural circuitry. Perfusing a semi-intact head preparation with DA (50 microM) or the metabolic precursor of catecholamines (L-3-4-dihydroxyphenylalanine, DOPA, 250 microM) induced feeding-like movements of the jaws and radula/odontophore. These DA-induced movements were similar to bites in intact animals. Perfusing with 5-HT (5 microM) also induced feeding-like movements, but the 5-HT-induced movements were similar to swallows. In preparations of isolated buccal ganglia, buccal motor programs (BMPs) that represented at least two different aspects of fictive feeding (i.e., ingestion and rejection) could be recorded. Bath application of DA (50 microM) increased the frequency of BMPs, in part, by increasing the number of ingestion-like BMPs. Bath application of 5-HT (5 microM) did not significantly increase the frequency of BMPs nor did it significantly increase the proportion of ingestion-like BMPs being expressed. Many of the cells and synaptic connections within the CPG appeared to be modulated by DA or 5-HT. For example, bath application of DA decreased the excitability of cells B4/5 and B34, which in turn may have contributed to the DA-induced increase in ingestion-like BMPs. In summary, bite-like movements were induced by DA in the semi-intact preparation, and neural correlates of these DA-induced effects were manifest as an increase in ingestion-like BMPs in the isolated ganglia. Swallow-like movements were induced by 5-HT in the semi-intact preparation. Neural correlates of these 5-HT-induced effects were not evident in isolated buccal ganglia, however.

Localization of GABA-like immunoreactivity in the central nervous system of Aplysia californica

Gamma-aminobutyric acid (GABA) is present in the central nervous system of Aplysia californica (Gastropoda, Opisthobranchia) where its role as a neurotransmitter is supported by pharmacological, biochemical, and anatomical investigations. In this study, the distribution of GABA-immunoreactive (GABAi) neurons and fiber systems in Aplysia was examined by using wholemount immunohistochemistry and nerve backfill methods. GABAi neurons were located in the buccal, cerebral, and pedal ganglia. Major commissural fiber systems were present in each of these ganglia, whereas more limited fiber systems were observed in the ganglionic connectives. Some of the interganglionic fibers were found to originate from two unpaired GABAi neurons, one in the buccal ganglion and one in the right pedal ganglion, each of which exhibited bilateral projections. No GABAi fibers were found in the nerves that innervate peripheral sensory, motor, or visceral organs. Although GABAi cells were not observed in the pleural or abdominal ganglia, these ganglia did receive limited projections of GABAi fibers originating from neurons in the pedal ganglia. The distribution of GABAi neurons suggests that this transmitter system may be primarily involved in coordinating certain bilateral central pattern generator (CPG) systems related to feeding and locomotion. In addition, the presence of specific interganglionic GABAi projections also suggests a role in the regulation or coordination of circuits that produce components of complex behaviors.

Distinct functions for cotransmitters mediating motor pattern selection

Motor patterns are selected from multifunctional networks by selective activation of different projection neurons, many of which contain multiple transmitters. Little is known about how any individual projection neuron uses its cotransmitters to select a motor pattern. We address this issue by using the stomatogastric ganglion (STG) of the crab Cancer borealis, which contains a neuronal network that generates multiple versions of the pyloric and gastric mill motor patterns. The functional flexibility of this network results mainly from modulatory inputs it receives from projection neurons that originate in neighboring ganglia. We demonstrated previously that the STG motor pattern selected by activation of the modulatory proctolin neuron (MPN) results from direct MPN modulation of the pyloric rhythm and indirect MPN inhibition of the gastric mill rhythm. The latter action results from MPN inhibition of projection neurons that excite the gastric mill rhythm. These projection neurons are modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2). MPN excitation of the pyloric rhythm is mimicked by bath application of proctolin, its peptide transmitter. Here, we show that MPN uses only its small molecule transmitter, GABA, to inhibit MCN1 and CPN2 within their ganglion of origin. We also demonstrate that MPN has no proctolin-mediated influence on MCN1 or CPN2, although exogenously applied proctolin directly excites these neurons. Thus, motor pattern selection occurs during MPN activation via proctolin actions on the STG network and GABA-mediated actions on projection neurons in the commissural ganglia, demonstrating a spatial and functional segregation of cotransmitter actions.

Different proctolin neurons elicit distinct motor patterns from a multifunctional neuronal network

Distinct motor patterns are selected from a multifunctional neuronal network by activation of different modulatory projection neurons. Subsets of these projection neurons can contain the same neuromodulator(s), yet little is known about the relative influence of such neurons on network activity. We have addressed this issue in the stomatogastric nervous system of the crab Cancer borealis. Within this system, there is a neuronal network in the stomatogastric ganglion (STG) that produces many versions of the pyloric and gastric mill rhythms. These different rhythms result from activation of different projection neurons that innervate the STG from neighboring ganglia and modulate STG network activity. Three pairs of these projection neurons contain the neuropeptide proctolin. These include the previously identified modulatory proctolin neuron and modulatory commissural neuron 1 (MCN1) and the newly identified modulatory commissural neuron 7 (MCN7). We document here that each of these neurons contains a unique complement of cotransmitters and that each of these neurons elicits a distinct version of the pyloric motor pattern. Moreover, only one of them (MCN1) also elicits a gastric mill rhythm. The MCN7-elicited pyloric rhythm includes a pivotal switch by one STG network neuron from playing a minor to a major role in motor pattern generation. Therefore, modulatory neurons that share a peptide transmitter can elicit distinct motor patterns from a common target network.

Cellular, synaptic, network, and modulatory mechanisms involved in rhythm generation

The membrane properties and the synaptic interactions of individual neurons, as well as the interactions between neuronal networks, all contribute to the formation of the complex patterns of activity that underlie rhythmic motor patterns and slow-wave sleep rhythms. These properties and interactions are potential points of modulation for further refining network output. Recent work illustrates the range of these properties and interactions and suggests how they may be modulated.

Multiple memory processes following training that a food is inedible in Aplysia

In many organisms, memory after training can be separated into a number of processes. We now report that separable memory processes are also initiated by a training procedure affecting Aplysia feeding behavior, a model system for examining the neural mechanisms underlying the regulation of a complex behavior. Four distinct memory process were identified: (1) a very short-term memory that declines within 15 min, (2) a short-term memory that persists for 0.5-1.0 hr, (3) an intermediate-term memory, observed 4 hr after training, and (4) a long-term memory that is seen only after a 12- to 24-hr delay. The four memory processes can be distinguished by the different training procedures that are required to elicit them. A single 5-min training session is sufficient to elicit the very short-term memory. However, a longer training session that continues until the animal stops responding to food is needed to elicit short-term memory. Intermediate-term memory is observed only after a spaced training procedure (three 5-min training sessions separated by 30-min intervals). A single 5-min training session that does not cause either short-term or intermediate-term memory is sufficient to induce long-term memory, indicating that short- and long-term memory are independent, parallel processes. Short- and long-term memory can also be separated by the effects of a post-training experience. Long-term, but not short-term, memory can be attenuated by cooling animals immediately after training. Cooling before the training does not affect either the training or the subsequent short- or long-term memory.

Multiple Memory Processes Following Training That a Food Is Inedible in Aplysia

In many organisms, memory after training can be separated into a number of processes. We now report that separable memory processes are also initiated by a training procedure affectingAplysia feeding behavior, a model system for examining the neural mechanisms underlying the regulation of a complex behavior. Four distinct memory process were identified: (1) a very short-term memory that declines within 15 min, (2) a short-term memory that persists for 0.5–1.0 hr, (3) an intermediate-term memory, observed 4 hr after training, and (4) a long-term memory that is seen only after a 12- to 24-hr delay. The four memory processes can be distinguished by the different training procedures that are required to elicit them. A single 5-min training session is sufficient to elicit the very short-term memory. However, a longer training session that continues until the animal stops responding to food is needed to elicit short-term memory. Intermediate-term memory is observed only after a spaced training procedure (three 5-min training sessions separated by 30-min intervals). A single 5-min training session that does not cause either short-term or intermediate-term memory is sufficient to induce long-term memory, indicating that short- and long-term memory are independent, parallel processes. Short- and long-term memory can also be separated by the effects of a post-training experience. Long-term, but not short-term, memory can be attenuated by cooling animals immediately after training. Cooling before the training does not affect either the training or the subsequent short- or long-term memory.

Pattern-generating role for motoneurons in a rhythmically active neuronal network

The role of motoneurons in central motor pattern generation was investigated in the feeding system of the pond snail Lymnaea stagnalis, an important invertebrate model of behavioral rhythm generation. The neuronal network responsible for the three-phase feeding motor program (fictive feeding) has been characterized extensively and divided into populations of central pattern generator (CPG) interneurons, modulatory interneurons, and motoneurons. A previous model of the feeding system considered that the motoneurons were passive followers of CPG interneuronal activity. Here we present new, detailed physiological evidence that motoneurons that innervate the musculature of the feeding apparatus have significant electrotonic motoneuron-->interneuron connections, mainly confined to cells active in the same phase of the feeding cycle (protraction, rasp, or swallow). This suggested that the motoneurons participate in rhythm generation. This was assessed by manipulating firing activity in the motoneurons during maintained fictive feeding rhythms. Experiments showed that motoneurons contribute to the maintenance and phase setting of the feeding rhythm and provide an efficient system for phase-locking muscle activity with central neural activity. These data indicate that the distinction between motoneurons and interneurons in a complex CNS network like that involved in snail feeding is no longer justified and that both cell types are important in motor pattern generation. This is a distributed type of organization likely to be a general characteristic of CNS circuitries that produce rhythmic motor behavior.

Compartmentalization of information processing in an aplysia feeding circuit interneuron through membrane properties and synaptic interactions

We describe a pair of cerebral-to-buccal interneurons, CBI-5/6, which have outputs and inputs in two ganglia. The soma in the cerebral ganglion received synaptic inputs during buccal motor programs (BMPs) and after mechanical stimulation of the lips. During BMPs the soma received antidromic spikes generated in processes in the buccal ganglion. The soma was driven into a plateau potential by each of these inputs, during which it fired orthodromically at 0-5 Hz. The soma had outputs in the cerebral ganglion consisting of electrical coupling to the adjacent CBI-5/6 and to a cerebral-to-pedal neuron (CPN1). The buccal terminals of CBI-5/6 received inputs that generated rhythmic barrages (up to 25 Hz) of antidromic spikes during BMPs. The buccal terminals had chemical and electrical outputs to motor and premotor elements of feeding circuitry. This combination of synaptic interactions and endogenous properties mean that CBI-5/6 can process information in a number of ways. During the barrage of antidromic spikes, high-frequency firing will produce strong inputs to buccal followers and on their arrival at the soma will transfer excitation electrotonically to CPN1. Subthreshold input to the soma will be transferred electrotonically to cerebral followers but will not be relayed to postsynaptic buccal neurons. Plateau potentials after the antidromic spikes or local cerebral inputs will locally excite CPN1 via electrical coupling but will have little influence on buccal events because of the low orthodromic firing rate. Thus, CBI-5/6 may transmit information locally within the cerebral ganglion or more extensively in both buccal and cerebral ganglia simultaneously.

Intercircuit control of motor pattern modulation by presynaptic inhibition

Rhythmically active neural networks can control the modulatory input that they receive via their synaptic effects onto modulatory neurons. This synaptic control of network modulation can occur presynaptically, at the axon terminals of the modulatory neuron. For example, in the crab stomatogastric ganglion (STG), a gastric mill network neuron presynaptically inhibits transmitter release from a modulatory projection neuron called modulatory commissural neuron 1. We showed previously that the gastric mill rhythm-timed presynaptic inhibition of the STG terminals of MCN1 is pivotal for enabling MCN1 to activate this rhythm. We also showed that MCN1 excites the pyloric rhythm within the STG. Here we show that, because MCN1 stimulation conjointly excites the gastric mill and pyloric rhythms, the gastric mill rhythm-timed presynaptic inhibition of MCN1 causes a rhythmic interruption in the MCN1-mediated excitation of the pyloric rhythm. Consequently, during each protraction phase of the gastric mill rhythm, presynaptic inhibition suppresses MCN1 excitation of the pyloric rhythm, thereby weakening the pyloric rhythm. During the retraction phase, presynaptic inhibition is absent and MCN1 elicits a faster, stronger, and modified pyloric rhythm. Thus, in addition to its role in enabling a neural circuit to regulate the modulatory transmission that it receives, presynaptic inhibition is also used effectively to rhythmically control the activity level of a distinct, but behaviorally related, neural circuit.