Identification and characterization of cerebral-to-buccal interneurons implicated in the control of motor programs associated with feeding in Aplysia

We identified candidate neurons in the cerebral ganglion that regulate feeding responses mediated by the buccal ganglion. Backfilling the cerebral-buccal connectives revealed that each cerebral hemi-ganglion contains approximately 20 neurons that project axons to the buccal ganglion. Three M-cluster neurons (CBI-1, CBI-2, CBI-3) and one E-cluster neuron (CBI-4) were identified as cerebral-to-buccal interneurons (CBIs) based on position, morphology, synaptic connections, and ability to drive buccal motor programs (BMPs). CBI-1 responds to touch of the tentacles, lips, and buccal mass. It receives monosynaptic EPSPs from interganglionic, cerebral-to-buccal mechanoafferent (ICBM) neurons and monosynaptically excites buccal cells, some of which are also excited by the ICBMs. Tonic firing of CBI-1 usually evokes a single cycle of BMP activity. CBI-1 phase-shifts the rhythmic BMP driven by firing a dopaminergic neuron in the buccal ganglion. CBI-1 itself exhibits dopamine-like histofluorescence following formaldehyde-glutaraldehyde fixation. CBI-2 is excited by food stimuli applied to the lips. Constant-current intracellular stimulation of CBI-2 produces phasic firing of the cell that reliably evokes a rhythmic BMP that incorporates buccal and cerebral motor neurons, putative pattern-generating and pattern-initiating neurons, and neuromodulatory cells (metacerebral cells). CBI-4 also evokes a rhythmic BMP, but the details of its actions and synaptic effects differ from that of CBI-2. CBI-3 does not evoke a BMP, even though it is excited by food stimuli applied to the lips, and it makes monosynaptic connections (both excitatory and inhibitory) to many follower cells of the other CBIs. Firing of CBI-3 phase-delays the BMP driven by CBI-2. Since its activity is incorporated into BMPs and it provides direct inputs to elements of the feeding circuitry, it may play a role in pattern generation. The distinctive features of the CBIs suggest that the consummatory phase of feeding may be controlled by a population of interneurons that subserve different roles.     

Coordination of distinct motor structures through remote axonal coupling of projection interneurons

Complex behaviors often require coordinated movements of dissimilar motor structures. The underlying neural mechanisms are poorly understood. We investigated cycle-by-cycle coordination of two dissimilar feeding structures in Aplysia californica: the external lips and the internal radula. During feeding, the lips open while the radula protracts. Lip and radula motoneurons are located in the cerebral and buccal ganglia, respectively, and radula motoneurons are controlled by a well characterized buccal central pattern generator (CPG). Here, we examined whether the three electrically coupled lip motoneurons C15/16/17 are controlled by the buccal CPG or by a previously postulated cerebral CPG. Two buccal-cerebral projection interneurons, B34 and B63, which are part of the buccal CPG and mediate radula protraction, monosynaptically excite C15/16/17. Recordings from the B34 axon in the cerebral ganglion demonstrate its direct electrical coupling with C15/16/17, eliminating the need for a cerebral CPG. Moreover, when the multifunctional buccal CPG generates multiple forms of motor programs due to the activation of two inputs, the command-like neuron CBI-2 and the esophageal nerve (EN), C15/16 exhibit activity patterns that are distinct from C17. These distinct activity patterns result from combined activity of B34 and B63 and their differential excitation of C15/16 versus C17. In more general terms, we identified neuronal mechanisms that allow a single CPG to coordinate the phasing and activity of remotely located motoneurons innervating distinct structures that participate in the production of different motor outputs. We also demonstrated that axodendritic electrical coupling by projection interneurons plays a pivotal role in coordinating activity of these remotely located neurons.     

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.     

Differential synapse formation and neurite outgrowth at two branches of the metacerebral cell of Aplysia in dissociated cell culture

The metacerebral cell (MCC) of Aplysia californica was isolated with its bifurcate axon from the cerebral ganglion and maintained in vitro under three conditions: (a) with no targets, (b) with identified buccal ganglion neurons B1 or B2 placed near the stump of the large diameter cerebral-buccal connective (CBC) branch, and (c) with B1 or B2 placed near the stump of the small diameter posterior lip nerve (PLN) branch. After 5 days in culture, the two branches differed significantly in the formation of chemical connections and in the extent of neurite outgrowth. Chemical connections characteristic of MCC-B1(B2) connections in vivo were observed in more than 90% of the cultures in which the buccal neuron was contacted by neurites emerging from the CBC branch, but in only 20% of the cultures in which the buccal neuron was contacted by neurites extending from the PLN branch. Neurite outgrowth from the CBC stump was always greater than growth from the PLN and was not affected significantly by the presence of a buccal neuron target at either branch. In contrast, neurite outgrowth from the PLN decreased significantly when the target was contacted by neurites from the CBC branch. These results suggest that two branches of a single neuron can differ in their capacities to form chemical connections. In addition, the two branches show differential growth as a result of target interaction at one of the branches. This simple in vitro system may therefore be useful in exploring the ways in which individual neurons control neurite extension from different branches as they seek to form chemical connections with their targets.     

An identified pleural ganglion interneuron inhibits patterned motor activity in the buccal ganglia of the snail, Helisoma

Neuron, Pl1, an interneuron that inhibits patterned motor output underlying feeding in the snail, Helisoma, is identified. The soma of neuron Pl1 is in the pleural ganglion and its axon projects through the pedal and cerebral ganglia to the buccal ganglia. A train of action potentials in neuron Pl1 suppresses rhythmic activity in the buccal pattern generator even in the presence of strong pharmacological stimulation with serotonin.     

Enhanced synaptic transmission at identified synaptic connections in the cerebral ganglion of Aplysia.

The identified A-B neuron synaptic connections in the cerebral ganglion of Aplysia exhibited a novel form of enhanced synaptic transmission. A brief high-frequency train of action potentials (2 s, 10-30 Hz) in the presynaptic A neurons produced a long-lasting increase in the amplitude of excitatory postsynaptic potentials (EPSPs) in B neurons. The increase in synaptic efficacy was termed slow developing potentiation (SDP) since the EPSP amplitude increased slowly with the peak occurring 5 min after the tetanizing train. Peak EPSP amplitudes increased relative to the initial EPSP by an average of greater than 250%. SDP decayed as a single exponential with a time constant of tau = 24 min. The enhanced transmission was neuron specific. Only the connections made by the tetanized A neuron were potentiated. However, potentiation apparently occurred at all the synapses made by the tetanized A neuron. Tetanizing the postsynaptic B neurons neither induced, nor when paired with A neuron tetanization, increased SDP. SDP appears to be primarily due to increased transmitter release by the presynaptic neuron.     

Selective modulation of spike duration by serotonin and the neuropeptides, FMRFamide, SCPB, buccalin and myomodulin in different classes of mechanoafferent neurons in the cerebral ganglion of Aplysia

An examination of the cellular properties and synaptic outputs of mechanoafferent neurons found on the ventrocaudal surface of the cerebral ganglion of Aplysia indicated that the cerebral mechanoafferent (CM) neurons are a heterogeneous population of cells. Based on changes in action potential duration in response to bath applications of 5-HT in the presence of TEA, CM neurons could be divided into 2 broad classes: mechanoafferents whose spikes broaden in response to 5-HT (CM-SB neurons) and mechanoafferents whose spikes narrow in response to 5-HT (CM-SN neurons). Morphological and electrophysiological studies of the CM-SN neurons indicated that they were comprised of previously identified interganglionic cerebral-buccal mechanoafferent (ICBM) neurons and a novel set of sensory neurons that send an axon into the LLAB cerebral nerve and have perioral zone receptive fields that are similar to those of ICBM neurons. Changes in spike width due to 5-HT were correlated with changes in synaptic output as indicated by the magnitudes of EPSPs evoked in postsynaptic neurons. Electrical stimulation of cerebral nerves and connectives also produced spike narrowing or broadening, and the sign of the effect was a function of the parameters of stimulation. Both heterosynaptic facilitation and heterosynaptic depression of EPSPs evoked in follower cells could be demonstrated. A variety of putative neuromodulators other than 5-HT were also found to affect the duration of action potentials in both classes of CM neurons. FMRFamide had effects opposite to that of 5-HT. SCPB and a recently characterized Aplysia neuropeptide, buccalin, broadened the spikes of both CM classes. Another neuropeptide, myomodulin, decreased the duration of CM-SB neuron spikes but had no effect on CM-SN spikes. Since the CM neurons appear to mediate a variety of competing behaviors, including feeding, locomotion, and defensive withdrawal, the various neuromodulator actions may contribute to the mechanisms whereby behaviors are selected and modified.     

Slow oscillations of membrane potential in interneurons that control heartbeat in the medicinal leech

In the preceding paper (Arbas and Calabrese, 1987), we identified several properties that contribute to the activity of neurons (HN cells) that control heartbeat in the medicinal leech. Premotor HN (7) interneurons, which do not generate the heartbeat rhythm, exhibit Na+-dependent fast action potentials, Ca2+-mediated plateau potentials in the absence of Na+, and hyperpolarization-activated "restorative" changes in membrane potential that depolarize the membrane potential on hyperpolarization due to injected currents or synaptic inhibition. HN interneurons of ganglia 3 and 4 (i.e., timing oscillator interneurons) exhibit all of the properties described for HN (7) interneurons and have the additional characteristic that they are connected in oscillatory circuits. Reciprocal oscillations in membrane potential occurred in the bilateral HN interneurons (3) and (4) in the presence of elevated Ca2+ that were independent of Na+ -mediated action potentials. Their ability to oscillate in this way is based on 3 parameters: (1) production of a regenerative plateau potential by one of the pair of HN neurons in either ganglion, (2) inhibition of the contralateral HN neuron by the HN neuron in plateau, and (3) a phase transition mediated by escape from inhibition by the hyperpolarized HN neuron. The conductances responsible for restorative membrane potential shifts activated by hyperpolarization during synaptic inhibition may mediate the escape from inhibition that times the phase transition of the 2 HN neurons.     

An identified histaminergic neuron modulates feeding motor circuitry in Aplysia

An identified histaminergic neuron, C2, in the marine mollusk Aplysia is a complex mechanoafferent which appears to contribute to the maintenance of food arousal by means of its synaptic connections to the metacerebral cell (MCC). Because C2 also has extensive synaptic outputs to neurons other than the MCC, we studied its possible motor functions. We identified several synaptic followers of C2 and found that some were excitatory motor neurons for extrinsic muscles of the buccal mass, while others were modulatory motor neurons that inhibited contractions. In addition, we found that these neurons and other synaptic followers of C2 received powerful inputs during feeding motor programs. In order to determine the functional significance of the synaptic outputs of C2, we studied extrinsic buccal muscles (E4 and E5) whose motor neuron (C6) is excited by C2. Extracellular recordings from these muscles indicated that they receive input during swallowing and rejection, but not during biting movements. Lesions of these muscles, or of all extrinsic muscles, did not prevent animals from feeding, but decreased feeding efficiency, that is, the amount of seaweed an animal could ingest with each swallow. The data suggest that C2 is an integrative proprioceptive cell that functions as a premotor neuron. The non-MCC synaptic outputs of C2 may reinforce the actions of the central feeding motor program. Specifically, C2 appears to aid the functioning of muscles that produce fine adjustments of the buccal mass and contribute to the efficiency of feeding behavior, rather than in producing gross movements.     

Cellular and synaptic morphology of a feeding motor circuit in Aplysia californica

The cellular and synaptic morphology of a component of the feeding motor circuit in Aplysia californica was examined with light and electron microscopic techniques. The circuit consists of a pair of inhibitory premotor interneurons, B4 and B5, as well as two motoneurons, B15 and B16, which innervate the accessory radula closer muscle. The neurons have wide, varicose arborizations in the buccal ganglion neuropil. All four of these neurons are cholinergic, and in addition, B15 contains immunoreactivity to sera raised against small cardioactive peptide B. Varicose processes in the accessory radula closer muscle are immunoreactive with antisera against several neuropeptides. We identified specific neuromuscular junctions by visualizing horseradish peroxidase uptake in recycled synaptic vesicles. Direct innervation of the accessory radula closer muscle by B15 and B16 is demonstrated by experiments in which horseradish peroxidase is transported from motoneuronal soma to the terminals on muscle fibers. In addition, specific synaptic contacts between B4 and B5 and each of the motoneurons are observed in the buccal ganglion neuropil. Finally, multiple contacts consistent with peptidergic, serotoninergic, and cholinergic synapses are made onto the neurons, suggesting that a variety of transmitters modulate motor output at each level of the hierarchical circuit. These results support the physiological evidence suggesting the involvement of neuropeptides as well as "classical" transmitters in the modulation of circuitry governing feeding behavior in Aplysia.     

FMRFamide peptides in Helisoma: identification and physiological actions at a peripheral synapse

Previous reports have demonstrated powerful neuromodulatory actions of the molluscan tetrapeptide FMRFamide in both the central and peripheral nervous systems of the freshwater snail Helisoma. The present study was designed to examine both the nature of the FMRFamide-like peptides in Helisoma and to define their physiological actions at a peripheral synapse. We report that, as determined by HPLC/RIA and mass spectrometry, Helisoma contains both FMRFamide and 2 of its analogs, FLRFamide and GDPFLRFamide. Whereas whole animals contain about 100 pmol/gm of these peptides, they were enriched in the nervous system (3000 pmol/gm) and in a peripheral target organ, the salivary glands (500 pmol/gm). For histochemical and physiological studies we examined the salivary glands, which are known to be innervated by neuron 4 of the buccal ganglion. We confirmed the presence of FMRFamide-like fibers on the salivary gland by immunohistochemistry using a polyclonal antiserum. These fibers appear to be largely derived from somata located in the central ring ganglia. For physiological tests we examined the neuron 4-gland synapse, at which presynaptic action potentials normally evoke a suprathreshold EPSP in gland cells. Bath application of FMRFamide, FLRFamide, or GDPFLRFamide at micromolar concentration to a buccal ganglia/salivary gland preparation completely suppressed spontaneous rhythmic activity. The sites of action of these peptides were examined by iontophoretic application of FMRFamide to neuron 4 or the salivary gland. Application of the peptide to the soma of neuron 4 caused a hyperpolarization that suppressed spontaneously generated action potentials. When applied to the salivary gland, FMRFamide caused a hyperpolarization that reduced the EPSPs evoked by neuron 4 to below spike threshold. The latter observation implies a postsynaptic locus of action for FMRFamide, and this possibility was tested by direct depolarization of the gland with iontophoresis of ACh (the putative transmitter of neuron 4). Such depolarizations were also reduced by FMRFamide. We conclude that Helisoma contains FMRFamide and 2 of its analogs, these peptides being enriched in the nervous system and salivary glands. Furthermore, these peptides can suppress activation of the salivary glands by actions both directly on gland cells and on the effector neuron.     

The common inhibitor innervates muscles proximal to the autotomy fracture plane in Carcinus maenas

The neuron common to proximal leg muscles of the crab Carcinus maenas projects to the propus opener nerve. Inhibitory effects include hyperpolarizing postsynaptic potentials, prolonged muscle fiber membrane hyperpolarization upon repetitive stimulation and suppression of force development by excitatory motorneurons of the anterior levator. The soma of the common inhibitor of fifth legs is located ventrally along the ganglion midline, contralateral to its axonal projections.     

Appearance and maturation of synaptic plasticity during juvenile development in aplysia

In the present study we examine the developmental appearance and maturation of synaptic plasticity at the A–B neuron synapse in the cerebral ganglion of Aplysia. In the CNS of juvenile Aplysia 120 days after hatching, the excitatory synaptic connection between A and B cluster neurons is essentially the same as in the adult cerebral ganglion. No differences were observed between the amplitudes of the initial EPSPs in the cerebral ganglia of juveniles and adults. One form of plasticity, low frequency synaptic depression, is also present in juveniles. Another form of activity-dependent plasticity, slow developing potentiation (SDP) appears and matures during the late juvenile stage of development. At 120 days posthatching SDP, evoked by tetanic stimulation, is largely absent. Potentiated EPSPs have a significantly smaller amplitude than in adults. Over the next 80 days SDP undergoes a maturation process. The peak potentiation increases linearly with age from 135±12% at 125 days to 275±20% at 188 days. The duration of the potentiation, as measured by the time-constant its decay, also increases linearly from 12.7±3.4 min to 27.9±3.9 min. From 120–170 days, <50% of the A neurons tested exhibited SDP. After 170 days, >85% exhibited SDP. Changes in the rising phase of the A neuron action potential have been implicated in mediating SDP. At 120 days, the A neuron action potential has a significantly shorter duration (half-width) than in the adult. Between 120 and 200 days, both the duration and rise-time of the A neuron action potential increase linearly. These results confirm findings by other investigators, that different forms of synaptic plasticity develop independently, with depression appearing before potentiation.     

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

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. This 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.     

Parallel processing of proprioceptive signals by spiking local interneurons and motor neurons in the locust

The connections made by afferents from a proprioceptor at the femorotibial joint in a hind leg of a locust, the femoral chordotonal organ (FCO), were determined by making intracellular recordings from motor neurons and spiking local interneurons in the central nervous system and from afferent cell bodies in the periphery. Staining the central projections of the afferent neurons with dye introduced into their axons at the receptor, and the intracellular injection of dye into motor neurons and interneurons, shows that the branches of all 3 types of neuron overlap in specific regions of neuropile. Afferents excited by a movement of the receptor apodeme that is equivalent to an imposed extension of the femorotibial joint excite flexor tibiae motor neurons and some spiking local interneurons with cell bodies at the ventral midline of the metathoracic ganglion. The opposite movement excites extensor tibiae motor neurons and a different set of spiking local interneurons. Spikes in afferents that excite flexor motor neurons evoke depolarizing potentials that follow each spike with a consistent central latency of approximately 1.5 msec. The amplitude of the depolarizing potentials is dependent upon the membrane potential of the motor neuron. This evidence points to the connection being direct and to the potentials' being EPSPs. Simultaneous recordings from certain spiking local interneurons and certain flexor motor neurons show that they receive many synaptic potentials in common and are driven in a parallel fashion by movements of the receptor apodeme. Spikes of some afferents evoke EPSPs in both neurons with the same consistency and latency. An afferent can therefore synapse directly upon a motor neuron and a spiking local interneuron. Each afferent synapses on several motor neurons and possibly upon several interneurons. In turn, each motor neuron and each interneuron receives inputs from several afferents.     

Octopamine is the synaptic transmitter between identified neurons in the buccal feeding network of the pond snail lymnaea stagnalis

We report the pharmacological properties of synaptic connections from the three octopamine-containing OC interneurons to identified buccal feeding neurons in the pond snail, Lymnaea stagnalis. Intracellular stimulation of an OC interneuron evokes inhibitory postsynaptic potentials in the B3 motoneurons and N2 (d) interneurons, while the synapse between OC and N3 (phasic) interneurons has two components: an initial electrical excitation followed by chemical inhibition. All synaptic responses persist in a saline with elevated calcium and magnesium suggesting that the connections are monosynaptic. Local perfusion of 10(-4) M octopamine produces the same inhibitory membrane responses from these buccal neurons as OC stimulation. These responses also persist in high Mg(2+)/Ca(2+) saline indicating direct membrane effects. The similarities in reversal potentials for the synaptic hyperpolarization evoked on B3 neurons after OC stimulation (-89.0 mV, S.E.M.=14.1, n=10) and the octopamine response of the B3 neurons (-84.7 mV, S.E.M.=6.6, n=6) indicate that increased K(+)-conductance underlies both responses. Bath application of the octopaminergic drugs phentolamine (10(-6) M), epinastine (10(-6) M) or DCDM (10(-4) M) blocks the inhibitory synapse onto B3 or N2 neurons and the chemical component of the N3 response. They also block the octopamine-evoked inhibition of B3, N2 and N3 neurons. NC-7 (2x10(-5) M) has a hyperpolarizing agonist effect (like octopamine) on these neurons and also blocks their chemical synaptic input from the OC interneurons. These results provide pharmacological evidence that the neurotransmitter between the octopamine-immunopositive OC interneurons and its followers is octopamine. This is the first example of identified octopaminergic synaptic connections within the snail CNS.     

Neurons controlling jumping in froghopper insects

The neurons innervating muscles that deliver the enormous power enabling froghopper insects to excel at jumping were revealed by backfilling the nerves from those muscles. The huge trochanteral depressor muscle (M133) of a hind leg consists of four parts. The two largest parts (M133b,c) occupy most of the metathorax and are innervated by the same two motor neurons that have small, laterally placed somata in the metathoracic ganglion and axons in nerve N3C(2). They are also supplied by three dorsal unpaired median (DUM) neurons with the largest diameter somata in the central nervous system. A small metathoracic part of the muscle (M133d) is supplied by two motor neurons with lateral somata and by common inhibitory motor neuron CI(1), all with axons in nerve N3C(3) The motor neuron with the larger soma has a thick primary neurite that projects across the midline of the ganglion so that its branches overlap those of its symmetrical counterpart,innervating the same muscle of the other hind leg. The fourth coxal part of the muscle (M133a) is innervated by two motor neurons (one with a ventral and the other with a dorsal and lateral soma), by CI(1), and by a DUM neuron with a small soma. All have axons in nerve N5A. The two trochanteral levator muscles of a hind leg are contained within the coxa and are separately innervated by nerves N3B and N4, respectively. The properties of the different motor neurons are discussed in the context of the neural patterns that generate jumping.     

Sensory function and gating of histaminergic neuron C2 in Aplysia

This paper explores the possible sensory function of the identified histaminergic neuron C2. Mechanical stimulation of a narrow region around the mouth of the animal (perioral zone) elicits brief depolarizing potentials in C2. Extracellular recordings from the peripheral axons of C2 indicate that the depolarizing potentials are due to action potentials that are conveyed from the periphery but do not invade the cell body, since they fail at a region with a low safety factor within the cerebral ganglion. These blocked axonal spikes (A-spikes) function as if they were excitatory synaptic inputs to C2, since the synaptic output of C2 does not occur unless the A-spikes succeed in evoking full action potentials in the soma (or an electrically close initial segment) of C2. Furthermore, like synaptic potentials, the A-spikes exhibit temporal and spatial summation, and facilitation. C2 receives both tonic and phasic inhibitory synaptic potentials, which can decrease the summation of A-spikes and thereby alter the frequency-filtering properties of C2 or block its synaptic output. Thus, C2 appears to be an unusual proprioceptive afferent that has a high degree of integrative function and may provide critical gating that is dependent on a variety of external and internal conditions.     

Multiple axonal spike initiation zones in a motor neuron: serotonin activation.

The lateral gastric (LG) motor neuron of the stomatogastric nervous system of the crab Cancer borealis has a large soma in the stomatogastric ganglion (STG). The LG motor neuron makes inhibitory synaptic connections within the neuropil of the STG, and also projects to the periphery, where it innervates a series of muscles that control the movements of the lateral teeth of the gastric mill. The LG motor neuron has a spike initiation zone close to its neuropilar integrative regions, from which spikes propagate orthodromically to the muscles. Additionally, under certain conditions, the LG neuron can initiate spikes at peripheral axonal sites that can be 0.5-2.0 cm from the STG. Peripherally initiated spikes propagate antidromically into the STG and also propagate to the muscle. The peripheral spike initiation zones are often active in combined preparations in which the muscles are left attached. When the muscles are removed, depolarization of the LG soma together with 5-HT applied to the motor nerve also evokes peripheral spike initiation. At a given 5-HT concentration, the duration of the trains of antidromic spikes can be controlled by current injection into the soma, suggesting the presence of a slow voltage-dependent conductance in the LG axon. The antidromic spikes contribute to lengthening of the duration of contraction in some of the muscles innervated by the LG, but do not evoke IPSPs onto LG follower neurons. Thus, the LG neuron can send different signals to its peripheral and central targets.     

Input-output relationships of identified buccal neurones involved in feeding control in Aplysia

A group of about 28 neurones located in the lateral portion of the caudal face of Aplysia buccal ganglion and projecting into the cerebro-buccal connective were identified by retrograde cobalt staining, and designated as L neurones. It was found that the L neurones did not establish synaptic relations with the known buccal neurones, which are mainly involved in the production of the consummatory phase of feeding, nor with several cerebral neurones tested, including the well-known serotonin giant cell. Neither did they show responses to stimulation of the nerves directed to the buccal mass. On the other hand, the L neurones showed depolarizing responses, with the possible addition of a weak, slower hyperpolarizing phase, to stimulation of the ipsi- and contralateral oesophageal nerves, which innervate the portion of the gut posterior to the buccal mass. These findings, together with several properties of the oesophageal nerve input, suggest that one function of the L cells is to transmit information about gut regions posterior to the buccal mass towards the cerebral ganglia, and that they may mediate the inhibitory influence which in Aplysia is known to be exerted upon feeding by the presence of bulk in the anterior gut. The L neurones showed synaptic responses - consisting mainly or exclusively of depolarizations - to stimulation of the cerebro-buccal connectives. Besides this, large, tonic EPSPs, which often occurred in the 'spontaneous' activity of the L neurones, were found to be generated by spikes that travelled in the cerebro-buccal connective towards the buccal ganglion.(ABSTRACT TRUNCATED AT 250 WORDS)