Premotor neurons in the feeding system of Aplysia californica

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.

Fast synaptic connections from CBIs to pattern-generating neurons in Aplysia: initiation and modification of motor programs

Consummatory feeding movements in Aplysia californica are organized by a central pattern generator (CPG) in the buccal ganglia. Buccal motor programs similar to those organized by the CPG are also initiated and controlled by the cerebro-buccal interneurons (CBIs), interneurons projecting from the cerebral to the buccal ganglia. To examine the mechanisms by which CBIs affect buccal motor programs, we have explored systematically the synaptic connections from three of the CBIs (CBI-1, CBI-2, CBI-3) to key buccal ganglia CPG neurons (B31/B32, B34, and B63). The CBIs were found to produce monosynaptic excitatory postsynaptic potentials (EPSPs) with both fast and slow components. In this report, we have characterized only the fast component. CBI-2 monosynaptically excites neurons B31/B32, B34, and B63, all of which can initiate motor programs when they are sufficiently stimulated. However, the ability of CBI-2 to initiate a program stems primarily from the excitation of B63. In B31/B32, the size of the EPSPs was relatively small and the threshold for excitation was very high. In addition, preventing firing in either B34 or B63 showed that only a block in B63 firing prevented CBI-2 from initiating programs in response to a brief stimulus. The connections from CBI-2 to the buccal ganglia neurons showed a prominent facilitation. The facilitation contributed to the ability of CBI-2 to initiate a BMP and also led to a change in the form of the BMP. The cholinergic blocker hexamethonium blocked the fast EPSPs induced by CBI-2 in buccal ganglia neurons and also blocked the EPSPs between a number of key CPG neurons within the buccal ganglia. CBI-2 and B63 were able to initiate motor patterns in hexamethonium, although the form of a motor pattern was changed, indicating that non-hexamethonium-sensitive receptors contribute to the ability of these cells to initiate bursts. By contrast to CBI-2, CBI-1 excited B63 but inhibited B34. CBI-3 excited B34 and not B63. The data indicate that CBI-1, -2, and -3 are components of a system that initiates and selects between buccal motor programs. Their behavioral function is likely to depend on which combination of CBIs and CPG elements are activated.

A population of pedal-buccal projection neurons associated with appetitive components of Aplysia feeding behavior

Backfills of the cerebral-buccal connective (CBC) of Aplysia californica revealed a cluster of five to seven pedal-buccal projection neurons in the anterolateral quadrant of the ventral surface of each pedal ganglion. Intra- and extracellular recordings showed that the pedal-buccal projection neurons shared common electrophysiological properties and synaptic inputs. However, they exhibited considerable heterogeneity with respect to their projection patterns. All pedal-buccal projection neurons that were tested received a slow excitatory postsynaptic potential from the ipsi- and contralateral cerebral-pedal regulator (C-PR) neuron, a cell that is thought to play a key role in the generation of a food-induced arousal state. Tests were conducted to identify potential synaptic follower neurons of the pedal-buccal projection neurons in the cerebral and buccal ganglia, but none were detected. Finally, nerve recordings revealed projections from the pedal-buccal projection neurons in the nerves associated with the buccal ganglion. In tests designed to determine the functional properties of these peripheral projections, no evidence was obtained supporting a mechanosensory or proprioceptive role and no movements were observed when they were fired. It is proposed that peripheral elements utilized in consummatory phases of Aplysia feeding may be directly influenced by a neuronal pathway that is activated during the food-induced arousal state.

Kinematics of the buccal mass during swallowing based on magnetic resonance imaging in intact, behaving Aplysia californica

A novel magnetic resonance imaging interface has been developed that makes it possible to image movements in intact, freely moving subjects. We have used this interface to image the internal structures of the feeding apparatus (i.e. the buccal mass) of the marine mollusc Aplysia californica. The temporal and spatial resolution of the resulting images is sufficient to describe the kinematics of specific muscles of the buccal mass and the internal movements of the main structures responsible for grasping food, the radula and the odontophore. These observations suggest that a previously undescribed feature on the anterior margin of the odontophore, a fluid-filled structure that we term the prow, may aid in opening the jaw lumen early in protraction. Radular closing during swallowing occurs near the peak of protraction as the radular stalk is pushed rapidly out of the odontophore. Retraction of the odontophore is enhanced by the closure of the lumen of the jaws on the elongated odontophore, causing the odontophore to rotate rapidly towards the esophagus. Radular opening occurs after the peak of retraction and without the active contraction of the protractor muscle 12 and is due, in part, to the movement of the radular stalk into the odontophore. The large variability between responses also suggests that the great flexibility of swallowing responses may be due to variability in neural control and in the biomechanics of the ingested food and to the inherent flexibility of the buccal mass.

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.

Multiple types of control by identified interneurons in a sensory-activated rhythmic motor pattern

Modulatory interneurons that can drive central pattern generators (CPGs) are considered as good candidates for decision-making roles in rhythmic behaviors. Although the mechanisms by which such neurons activate their target CPGs are known in detail in many systems, their role in the sensory activation of CPG-driven behaviors is poorly understood. In the feeding system of the mollusc Lymnaea, one of the best-studied rhythmical networks, intracellular stimulation of either of two types of neuron, the cerebral ventral 1a (CV1a) and the slow oscillator (SO) cells, leads to robust CPG-driven fictive feeding patterns, suggesting that they might make an important contribution to natural food-activated behavior. In this paper we investigated this contribution using a lip-CNS preparation in which feeding was elicited with a natural chemostimulant rather than intracellular stimulation. We found that despite their CPG-driving capabilities, neither CV1a nor SO were involved in the initial activation of sucrose-evoked fictive feeding, whereas a CPG interneuron, N1M, was active first in almost all preparations. Instead, the two interneurons play important and distinct roles in determining the characteristics of the rhythmic motor output; CV1a by modulating motoneuron burst duration and SO by setting the frequency of the ongoing rhythm. This is an example of a distributed system in which (1) interneurons that drive similar motor patterns when activated artificially contribute differently to the shaping of the motor output when it is evoked by the relevant sensory input, and (2) a CPG rather than a modulatory interneuron type plays the most critical role in initiation of sensory-evoked rhythmic activity.

Glutamate is the fast excitatory neurotransmitter of small cardioactive peptide-containing Aplysia radula mechanoafferent neuron B21

B21 is a radula mechanoafferent neuron in the mollusc Aplysia which likely plays a crucial role in integrating environmental cues into the feeding motor program. To facilitate understanding B21's interactions with its postsynaptic followers, we sought to identify its neurotransmitter. We find that B21 makes a chemical synapse onto the follower neuron B8. Although B21-induced excitatory postsynaptic potentials (EPSPs) in B8 paradoxically diminish in amplitude with B8 hyperpolarization, we show that an inwardly rectifying current is responsible. We conclude that these B21-induced EPSPs are likely glutamatergic as they are blocked by the glutamate antagonist DNQX. Furthermore, B8 exhibits a depolarizing response to exogenous glutamate, which is antagonized by DNQX. Finally, exogenous glutamate occludes B21-evoked EPSPs in B8.

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.

Synaptic modulation contributes to firing pattern generation in jaw motor neurons during rejection of seaweed in Aplysia kurodai

Japanese species, Aplysia kurodai, feeds well on Ulva but rejects Gelidium (Geli.) or Pachydictyon (Pach.) with different rhythmic patterned movements of the jaws and radula. During ingestion the jaws open at the radula-protraction phase and remain half open at the initial phase of the radula-retraction, whereas during rejection the jaws open similarly but start to close immediately after the onset of the radula-retraction. These can be induced not only by the natural seaweed but by the extract solutions. We previously showed that the change of the patterned jaw movements from the ingestion to the rejection may result from the decrease in the delay of the firing onset of the jaw-closing (JC) motor neurons during their depolarization. This diminished delay produces a phase advance relative to the radula-retraction phase. In that study, we showed that during ingestion the buccal multiaction (MA) neurons may generate the delay of firing onset of the JC motor neurons by producing monosynaptic inhibitory postsynaptic potentials (IPSPs) during the initial portion of their depolarization. In the present experiments, the firing patterns in the MA neurons induced by application of the Geli. or Pach. extract to the lips were explored in the semi-intact preparations. During the Pach. response the duration and the firing frequency of the MA firing at each depolarizing phase were decreased in comparison with the Ulva response. No decreases in the MA firing were observed during the Geli. response, suggesting that the similar patterned jaw movements during rejection of Geli. and Pach. may be generated by different neural mechanisms. Moreover, the size of the MA-induced IPSP in the JC motor neurons was largely decreased by application of the Geli. or Pach. extract to the lips in the reduced preparations consisting of the tentacle-lips and the cerebral-buccal ganglia. Voltage-clamp experiments on the JC motor neurons showed that the size of synaptic current induced by the MA spikes was decreased by application of these solutions to the lips. The decrease was induced when the buccal ganglia were bathed in a solution to block polysynaptic pathways. These results suggest that the advance of the onset of the JC firing at each depolarizing phase during the Geli. or Pach. response may be mainly or partly caused by the decrease in the size of the MA-induced IPSP in the motor neurons. Modulatory action of cerebral neurons or peripheral afferent neurons in the lip region may contribute to this synaptic plasticity.

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.

Axonal regeneration in the central nervous system of aplysia californica determined by anterograde transport of biocytin

Rhythmic biting, a component of consummatory feeding behavior in the sea hare Aplysia californica, is eliminated following bilateral cerebral-buccal connective (CBC) crushes and recovers within 14 days postlesion. To assess axonal regeneration after CBC lesions, we used biocytin backfills of CBCs followed by fluorescence labeling with streptavidin-lissamine rhodamine. Anterograde transport of biocytin showed up to 1 mm of outgrowth by regenerating axons at 3 days postlesion. At 7 days postlesion, the regenerated axons approached or had entered the ipsilateral buccal neuropil and exhibited numerous varicosities; the average rate of axonal growth was 326 microm/day for the longest, most rapidly growing axons labeled in the CBC. The number of varicosities on labeled axons, suggestive of intercellular interactions, was increased dramatically at all times postlesion. At 14 and 20 days postlesion, regenerated axons branched extensively in the ipsilateral buccal neuropil, entered the contralateral buccal neuropil, and entered peripheral nerves on both sides of the midline. At these later times postlesion, some labeled axons encircled unlabeled buccal cell bodies and exhibited branches containing numerous varicosities, indicative of axosomatic contacts. Some regenerating axons were observed in the sheath of the CBC, but the vast majority of labeled axons remained confined to the connective core, as in control preparations. The bilateral projections within the buccal ganglia of labeled cerebral-to-buccal axons and the large number of varicosities present on these processes are indicative of regenerating axons and synapses that likely contribute to the functional recovery of rhythmic biting.

Octopamine: a new feeding modulator in Lymnaea

The role of octopamine (OA) in the feeding system of the pond snail,Lymnaea stagnalis, was studied by applying behavioural tests on intact animals, and a combination of electrophysiological analysis and morphological labelling in the isolated central nervous system. OA antagonists phentolamine, demethylchlordimeform (DCDM) and 2–chloro–4–methyl–2–(phenylimino)–imidazolidine (NC–7) were injected into intact snails and the sucrose–induced feeding response of animals was monitored. Snails that received 25–50 mg kg-1phentolamine did not start feeding in sucrose, and the same dose of NC–7 reduced the number of feeding animals by 80–90% 1–3 hours after injection. DCDM treatment reduced feeding by 20–60%. In addition, both phentolamine and NC–7 significantly decreased the feeding rate of those animals that still accepted food after 1–6 hours of injection. In the central nervous system a pair of buccal neurons was identified by electrophysiological and morphological criteria. After double labelling (intracellular staining with Lucifer yellow followed by OA–immunocytochemistry) these neurons were shown to be OA immunoreactive, and electrophysiological experiments confirmed that they are members of the buccal feeding system. Therefore the newly identified buccal neurons were called OC neurons (putative OA containing neurons or OAergic cells). Synchronous intracellular recordings demonstrated that the OC neurons share a common rhythm with feeding neurons either appearing spontaneously or evoked by intracellularly stimulated feeding interneurons. OC neurons also have synaptic connections with identified members of the feeding network: electrical coupling was demonstrated between OC neurons and members of the B4 cluster motoneurons, furthermore, chemically transmitted synaptic responses were recorded both on feeding motoneurons (B1, B2 cells) and the SO modulatory interneuron after the stimulation of OC neurons. However, elementary synaptic potentials could not be recorded on the follower cells of OC neurons. Prolonged (20 to 30 s) intracellular stimulation of OC cells activated the buccal feeding neurons leading to rhythmic activity pattern (fictive feeding) in a way similar to OA applied by perfusion onto isolated central nervous system (CNS) preparations. Our results suggest that OA acts as a modulatory substance in the feeding system ofLymnaea stagnalisand the newly identified pair of OC neurons belongs to the buccal feeding network.

Identification and characterization of catecholaminergic neuron B65, which initiates and modifies patterned activity in the buccal ganglia of Aplysia

Catecholamines are believed to play an important role in regulating the properties and functional organization of the neural circuitry mediating consummatory feeding behaviors in Aplysia. In the present study, we morphologically and electrophysiologically identified a pair of catecholaminergic interneurons, referred to as B65, in the buccal ganglia. Their processes innervate both the ipsi- and contralateral neuropil, and separate branches of B65 appeared to innervate the somata of both ipsi- and contralateral B4/5 neurons. B65 exhibited patterned burst(s) of activity during spontaneous cycles of fictive feeding. Patterned activity in B65 also was elicited by stimulation of the radula nerve, by depolarization of the pattern initiating neurons B31/32 or B63, and by bath application of -3,4-dihydroxyphenylalanine (DOPA). B65 appeared to be a member of the protraction group of neurons. Action potentials in B65 elicited fast one-for-one excitatory postsynaptic potentials (EPSPs) in neurons B4/5, B8A/B, B31/32, B63, and B64. In turn, B31/32 and B63 excited B65 and B64 inhibited B65. Some of the synaptic connections of B65 were plastic. For example, the fast EPSPs elicited in B4/5 and B64 decremented, whereas those in B31/32 andB8A/B facilitated. In addition to fast EPSPs, B65 elicited slow postsynaptic potentials in some of its follower cells. Depolarization of B65 elicited cycles of patterned activity indicative of fictive feeding in buccal neurons, including B65 itself. During series of B65-induced patterns, the properties of the buccal motor programs appeared to change. In particular, the activity of radula closure motor neurons B8A/B, which initially coincided mainly with the protraction phase of a cycle, gradually extended to overlap mostly with the retraction phase. This observation suggests that prolonged activity in B65 may play a role in transitioning from rejection-like to ingestion-like fictive feeding. The phase shift of the activity of B8A/B appears due, at least in part, to a decrease in activity of B4/5, and thus a reduction in inhibition from B4/5 onto B8A/B, during the retraction phase. The functional properties and synaptic connections of B65 suggest that it may play an important role in determining features of patterned neural activity in the buccal ganglia.

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

Different feeding-related behaviors in Aplysia require substantial variations in the coordination of movements of two separate body parts, the lips and buccal mass. The central pattern generators (CPGs) and motoneurons that control buccal mass movements reside largely in the buccal ganglion. It was previously thought that control of the cerebral neuronal circuitry and motoneurons that generate lip movements was coordinated directly by feedback from buccal interneurons. Here, we describe cerebral lip motoneuron C15, which drives rhythmic activity in the isolated cerebral ganglion. Other lip motoneurons are active during this program, so we define it as a cerebral motor program (CMP). The C15 in each cerebral hemiganglion drives the CMP in ipsilateral neurons only, suggesting there are independent CPGs in each hemiganglion. The cerebral and buccal CPGs interact at several points. For example, cerebral-to-buccal interneurons (CBIs), which can drive the buccal CPG, receive excitatory input when the cerebral CPG is active. Likewise, C15, which can drive the cerebral CPG, is excited when the buccal CPG is active. This excitation is simultaneous in both C15s, coupling the activity in the two hemiganglionic cerebral CPGs. Therefore, there are independent cerebral and buccal CPGs, which can produce distinct rhythms, but which interact at several points. Furthermore, the connections between the cerebral and buccal CPGs alter during different forms of motor program. We suggest that such alterations in the interactions between these CPGs might contribute to the generation of the various forms of coordination of lip and buccal mass movements that are necessary during different feeding-related behaviors.

Structure, localization, and action of buccalin B: a bioactive peptide from Aplysia

The cholinergic motor neurons for the accessory radula closer (ARC) contain several neuropeptides that affect muscle contractions. In the present study, we have purified and sequenced a sixth ARC neuropeptide, using a combination of high pressure liquid chromatography and bioassays. This neuropeptide, Gly-Leu-Asp-Arg-Tyr-Gly-Phe-Val-Gly-Gly-Leu-amide, has been named buccalin B (BUCb) because it is significantly homologous to the previously characterized neuropeptide buccalin A. BUCb was found to be two-three times more potent than buccalin A in depressing motor neuron induced contractions.

Dopaminergic neuron B20 generates rhythmic neuronal activity in the feeding motor circuitry of Aplysia

We have identified a buccal neuron (B20) that exhibits dopamine-like histofluorescence and that can drive a rhythmic motor program of the feeding motor circuitry of Aplysia. The cell fires vigorously during episodes of patterned buccal activity that occur spontaneously, or during buccal programs elicited by stimulation of identified cerebral command-like neurons for feeding motor programs. Preventing B20 from firing, or firing B20 at inappropriate times, can modify the program driven by the cerebral feeding command-like neuron CBI-2. When B20 is activated by means of constant depolarizing current it discharges in phasic bursts, and evokes a sustained coordinated rhythmic buccal motor program. The program incorporates numerous buccal and cerebral neurons associated with aspects of feeding responses. The B20-driven program can be reversibly blocked by the dopamine-antagonist ergonovine, suggesting that dopamine may be causally involved in the generation of the program. Although firing of B20 evokes phasic activity in cerebral command-like neurons, the presence of the cerebral ganglion is not necessary for B20 to drive the program. The data are consistent with the notion that dopaminergic neuron B20 is an element within the central pattern generator for motor programs associated with feeding.

Control of feeding rhythm in the terrestrial slug Incilaria bilineata

A modulatory neuron of feeding rhythm was newly identified in the buccal ganglia of the isolated central nervous system (CNS) of the terrestrial slug Incilaria bilineata. This neuron was termed the "feeding rhythm modulator" (FRM). Its morphological and electrical properties were compared with those of the MGC (metacerebral giant cell, a cerebral modulatory neuron of feeding rhythm). There was no direct connection between FRM and MGC. In order to investigate the control mechanism of the buccal central pattern generator, feeding rhythm was observed by varying the activities of MGC and FRM simultaneously. At a lower level of activity of MGC, feeding rhythm was not only sensitive to the activity of MGC but also to that of FRM. As the level of activity of MGC increased, feeding rhythm was exclusively controlled by the activity of MGC, and became unaffected by the activity of FRM. This indicates that cerebral neurons such as MGC primarily control feeding rhythm and modulate the contribution of FRM in a hierarchical manner.

Optical methods can be utilized to map the location and activity of putative motor neurons and interneurons during rhythmic patterns of activity in the buccal ganglion of Aplysia

We sought to develop a map of the locations of neurons that are active during patterned activity in the buccal ganglion of Aplysia using optical techniques. Staining ganglia with a voltage-sensitive absorbance dye (JPW 1124) did not prevent them from generating patterned activity similar to that observed before staining, in response to shock of the esophageal nerve. Absorbance changes were monitored with a 124-element photodiode array, while extracellular electrodes monitored activity of the 6 buccal nerves. Optical and extracellular spikes were grouped with the aid of a template matching program; a total of 120 distinct units were detected in one 15 s recording. Optical signals (83 units) were found in the region of the ganglion containing mainly large neurons. Of these, 13 were detected on both optical and extracellular electrode recordings, suggesting that they might be motor neurons, while 25 of the optically detected neurons appeared not to be correlated with extracellular activity, suggesting that they might be interneurons. It was not possible to determine whether the remaining 45 optically identified units did or did not have correlated nerve activity. The ganglionic locations of putative motor neurons corresponded to the locations of large neurons identified by backfilling nerves of other buccal ganglia, and were consistent with the locations of putative motor neurons found in two other ganglia studied using optical methods. Thus, optical methods have generated a map of the locations and activity patterns of putative motor neurons and interneurons in the buccal ganglion that may be involved in the generation of rhythmic patterns.

Neuromuscular organization of the buccal system in Aplysia californica

The intrinsic muscles and peripheral nerves in the buccal system of the sea hare Aplysia californica were studied to build a foundation on which to base future investigations of feeding in intact animals. A detailed description of the bilaterally paired intrinsic muscles is given identifying previously unreported muscles. Each of the six buccal nerves (n1-n6) and the cerebrobuccal connective (CBC) have been characterized in several respects. Cell bodies in the buccal ganglion with projections into each of the buccal nerves have been identified via the cobalt backfilling technique. All nerves contain axons of cell bodies in the ipsilateral as well as the contralateral ganglia. For each nerve, there is a consistent pattern in the distribution of cell bodies in the paired ganglia with the number of cell bodies in the contralateral ganglion being less than or equal to the number in the ipsilateral ganglion. Although the total number of backfilled cell bodies varies among the nerves, their size ranges are similar with the majority being small. Nerves 1, 2, 4, 5, and 6 provide motor innervation to the intrinsic buccal muscles in varying degrees with nerve 4 supplying all the intrinsic muscles; nerve 2 supplies only one. The axon composition of each nerve was scrutinized and revealed large numbers of axon profiles, the majority of which were less than 2 microns in diameter. The present study provides a framework for analysis of feeding behavior in Aplysia californica.

Peptidergic motoneurons in the buccal ganglia of Aplysia californica: immunocytochemical, morphological, and physiological characterizations

We used physiological recordings, intracellular dye injections and immunocytochemistry to further identify and characterize neurons in the buccal ganglia of Aplysia californica expressing Small Cardioactive Peptide-like immunoreactivity (SCP-LI). Neurons were identified based upon soma size and position, input from premotor cells B4 and B5, axonal projections, muscle innervation patterns, and neuromuscular synaptic properties. SCP-LI was observed in several large ventral neurons including B6, B7, B9, B10, and B11, groups of s1 and s2 cluster cells, at least one cell located at a branch point of buccal nerve n2, and the previously characterized neurons B1, B2 and B15. B6, B7, B9, B10 and B11 are motoneurons to intrinsic muscles of the buccal mass, each displaying a unique innervation pattern and neuromuscular plasticity. Combined, these motoneurons innervate all major intrinsic buccal muscles (I1/I3, I2, I4, I5, I6). Correspondingly, SCP-LI processes were observed on all of these muscles. Innervation of multiple nonhomologous buccal muscles by individual motoneurons having extremely plastic neuromuscular synapses, represents a unique form of neuromuscular organization which is prevalent in this system. Our results show numerous SCPergic buccal motoneurons with widespread ganglionic processes and buccal muscle innervation, and support extensive use of SCPs in the control of feeding musculature.