Errata

Due to a printer's error, Fig. 10 on p. 33 was reversed. A loose-leaf insertion of the corrected page can be found in this volume.

SCP-containing R20 neurons modulate respiratory pumping in Aplysia

Respiratory pumping in Aplysia consists of transient, synchronous pumping actions of the gill, siphon, mantle shelf, and parapodia. This behavior has previously been shown to be driven by a network of coupled interneurons in the abdominal ganglion, the R25 and the L25 cells. We describe here a pair of electrically coupled cells, the R20 cells, which when active can initiate respiratory pumping or increase its spontaneous rate of occurrence. This action is mediated by a slow, long-lasting excitation of the endogenous burst mechanism of the cells in the R25/L25 network. The R20 cells, which are located in the abdominal ganglion, also make slow inhibitory connections to the RB cells and to the RG cells in that ganglion, and to the gill motoneurons in the branchial ganglion. The R20 cells are immunoreactive to SCPB, a molluscan neuropeptide. Biochemical purification studies demonstrate that each of the R20 cells synthesizes not only SCPB, but also SCPA, a closely related molecule known to be encoded by the same gene as SCPB. The R20 cells also synthesize in abundance several other low-molecular-weight, methionine-containing peptides. The excitatory actions of the R20 cells on the R25/L25 network are mimicked by SCPA and SCPB. However, the inhibitory actions of the R20 cells on the RB cells, the RG cells, and on the cells of the branchial ganglion are not mimicked by the SCPs. Thus, the data support the hypothesis that the R20 cells release SCPA and SCPB and at least one other unidentified transmitter.

The role of a modulatory neuron in feeding and satiation in Aplysia: effects of lesioning of the serotonergic metacerebral cells

Food-induced arousal in Aplysia is characterized by a progressive increase in the speed and strength of biting responses elicited by a seaweed stimulus. Data from semi-intact and dissected preparations suggest that the identified, serotonergic, metacerebral cells (MCCs) of the cerebral ganglion contribute to food-induced arousal by enhancing the strength of buccal muscle contractions, and by modulating the output of the central pattern generator for biting movements. In order to test this hypothesis in intact, free-moving animals and to determine if the MCCs play a role in satiation of feeding, the behavior of animals that had their MCCs destroyed by intracellular injection of proteases was compared with that of B Cell-Lesion and Dye injection control animals (Experiment 1) or surgical control animals (Experiment 2). Nonfeeding behaviors such as defensive withdrawal responses, locomotion, and righting reflexes were unaffected by MCC lesioning. Also unaffected by MCC lesioning were appetitive feeding behaviors and the amount of food needed to satiate animals. Significant behavioral deficits in consummatory feeding behaviors, which remained stable for periods exceeding 10 d, were observed in the MCC-lesioned animals but not in controls. Lesioned animals exhibited a slowing of rate of repetitive biting responses by 40% of controls and had reduced magnitudes of repetitive bites, particularly at the end of a testing run of 10 consecutive bites. The deficit in bite magnitude was minimally evident in food-deprived animals (Experiment 1) but became more pronounced as animals were fed to satiation (Experiment 2). MCC-lesioned animals still exhibited a residual build-up of the rate and magnitude of biting responses at the onset of feeding behavior. This suggests that, in addition to the MCCs, there are other sources of modulation that contribute to plasticity of consummatory responses during the food-induced arousal state.

Purification and sequencing of neuropeptides contained in neuron R15 of Aplysia californica

R15 is a large identified neuron present in the abdominal ganglion of the mollusc Aplysia. Previous studies have indicated that this neuron may play a role in water balance and possibly renovascular functions. A peptidic factor contained in the neuron R15 has been shown to increase the water content of Aplysia. To determine the structure of the peptides contained in R15, we purified the extracts of 820 R15 cells by means of two steps of reverse-phase HPLC. The purification yielded a number of peptides, only one of which, R15 alpha 1, resulted in water uptake when injected into animals. Determination of the amino acid content and sequence analysis of the R15 alpha 1 peptide demonstrated that this peptide contains 38 residues, including two cysteines. The peptide failed to react with iodoacetate, indicating that the two cysteines are connected by a disulfide bridge. To confirm the assigned structure, the peptide was synthesized with a disulfide bridge. The chromatographic properties and bioactivity of the synthetic material were identical to those of the native peptide. Several other R15 peptides were inactive in the bioassay for water uptake. The sequence of one of these peptides (R15 beta) was determined, and it was established that the peptide contains 28 residues. Amino acid analysis of three other peaks was performed. One of these peaks contained a peptide (R15 beta f) whose amino acid composition suggests that it is a fragment of the R15 beta peptide. The other two peaks contained peptides with identical amino acid compositions, suggesting that they are variants of a single peptide (R15 gamma). The amino acid sequences of all the peptides identified in neuron R15 correspond to stretches of a polyprotein encoded by a recently sequenced R15 cDNA.

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.

Structure and action of buccalin: a modulatory neuropeptide localized to an identified small cardioactive peptide-containing cholinergic motor neuron of Aplysia californica

A model system that consists of a muscle utilized in biting, the accessory radula closer (ARC), and the two cholinergic motor neurons innervating this muscle, neurons B15 and B16, has been used to study the expression of food-induced arousal in the marine mollusk Aplysia. The ARC muscle receives modulatory input from an extrinsic source, the serotonergic metacerebral cells, which partially accounts for the progressive increase in the strength of biting seen in aroused animals. Another source of modulation may arise from the ARC motor neurons themselves, which synthesize neuropeptides that can potentiate ARC contractions. Neuron B15 synthesizes the two homologous peptides, small cardioactive peptides A and B, whereas neuron B16 synthesizes the structurally unrelated peptide myomodulin. Here we report the purification and sequencing of a neuropeptide termed buccalin and show that it is colocalized with the small cardioactive peptides to neuron B15. Buccalin is also bioactive at the ARC neuromuscular junction but, in contrast to the small cardioactive peptides, when exogenously applied, it decreases rather than increases the size of muscle contractions elicited by firing of the motor neurons. Also unlike the small cardioactive peptides, which exert postsynaptic actions, buccalin seems to act only presynaptically. It has no effect on muscle relaxation rate and decreases motor neuron-elicited excitatory junction potentials in the ARC without affecting contractions produced by direct application of acetylcholine to the muscle. Neuron B15, therefore, appears to contain three modulatory neurotransmitters, two of which may act postsynaptically on the muscle to potentiate the action of the primary neurotransmitter acetylcholine and one of which may act presynaptically on nerve terminals to inhibit acetylcholine release.

Association of neuroactive peptides with the protein secretory pathway in identified neurons of Aplysia californica: immunolocalization of SCPA and SCPB to the contents of dense-core vesicles and the trans face of the Golgi apparatus

The subcellular distribution of two molluscan neuropeptides, the small cardioactive peptides A and B (SCPA and SCPB), has been determined in two identified Aplysia buccal ganglion neurons, B1 and B2. These neurons were previously shown to synthesize and release these neuropeptides. B1 and B2, identified by their size and location within the ganglion, were labeled by intrasomatic injection of an electron-dense particulate marker (ferritin or Imposil) permitting the unequivocal identification of their somata and proximal processes in thin sections. The somatic cytoplasm of both neurons had a conspicuous population of large dense-core vesicles along with a smaller number of compound vesicles and small lucent vesicles. All three vesicle types are found in the neurites within the neuropil and proximal axon in the esophageal nerve. Immunoreactivity was localized on the surface of thin sections by the indirect immunogold method. The primary antiserum was shown to recognize both SCPA and SCPB after the neuropeptides had been immobilized on protein-coated nitrocellulose membranes by means of glutaraldehyde, the primary fixative used to immobilize SCPA and SCPB in situ. SCP immunoreactivity was present in the lumens of the dense-core vesicles distributed throughout the cytoplasm of B1 and B2 and in dense-core regions of the Golgi apparatus in the somatic cytoplasm. Taken together with biochemical evidence that B1 and B2 synthesize and release SCPs, these data suggest that the neuropeptides are sequestered into the protein secretory pathway of B1 and B2, a distribution that supports the notion that the SCPs function physiologically as neurotransmitters or neuromodulators.

Central peptidergic neurons regulate gut motility in Aplysia

1. The small cardioactive peptides (SCPs) are potent modulatory neuropeptides in Aplysia. Buccal ganglia neurons B1 and B2 are the largest neurons that exhibit SCP-like immunoreactivity. High-pressure liquid chromatography (HPLC)-bioassay and in vivo radiolabeling procedures confirm that these neurons contain and synthesize very large quantities of SCPA and SCPB. 2. Both B1 and B2 innervate the gut. HPLC-bioassay measurements indicate that the SCPs are present throughout the anterior sections of the gut. SCP-like immunoreactivity was largely confined to fibers and varicosities in the gut, although occasional immunoreactive enteric neurons were also observed. The purpose of this study was to determine the physiological roles of B1 and B2 and to what extent these roles are mediated by release of the SCPs. 3. Low-frequency tonic stimulation of B1 led to an increase in peristaltic contractions in a relatively distal portion of the gut. This action could be mimicked by superfusion of the same portion of the gut with very low concentrations of the SCPs. 4. B2 produced discrete contractions of the anterior portions of the gut only when fired in bursts. These actions could not be reproduced by superfusion with the SCPs and may be mediated by ACh. 5. B1 and/or B2 are active during the swallowing cycle of each feeding movement, which suggests that these effects on the gut are likely to occur during feeding. Thus the SCPs play a major role in the central regulation of gut motility.

An identified histaminergic neuron can modulate the outputs of buccal-cerebral interneurons in Aplysia via presynaptic inhibition

We have identified 2 buccal-cerebral interneurons (BCIs), B17 and B18, that appear to be involved in the coordination of feeding behavior in Aplysia. The BCIs have their cell bodies in the buccal ganglion, but send axons to the cerebral ganglion via the cerebral-buccal connectives. The BCIs appear to make monosynaptic connections with neurons in the cerebral ganglion that modulate extrinsic muscles involved in feeding behavior. B17 and B18 are activated antiphasically during a motor program induced by stimulating the esophageal nerve and appear to "read out" different phases of the buccal program to different cells in the cerebral ganglion. B17 and B18 are not necessary, and probably not sufficient, to generate the buccal program. These BCIs, and other cells like them in the buccal ganglion, may be capable of coordinating the activity of the intrinsic muscles of the buccal mass with the activity of its extrinsic muscles, and perhaps with those of the lips, mouth, and tentacles. Identified histaminergic neuron, C2, can modulate the outputs of the BCIs onto their synaptic followers in the cerebral ganglion. Firing of C2 inhibits spiking of the BCIs, probably via cerebral-buccal interneurons. C2 also decreases the size of the EPSP that B17 and B18 evoke in cerebral neuron C4. C2 appears to do so monosynaptically, and it decreases the conductance of C4, ruling out one possible postsynaptic mechanism of action. Variance analysis of the EPSPs evoked by B18 supports the hypothesis that C2 acts presynaptically to decrease the release of transmitter. Applications of histamine to the solution bathing the neuron mimic the effect of firing C2 and reduce the size of the EPSPs B18 induces in C4. The bath-applied histamine appears to act directly on B18, since it elicits a voltage-dependent increased conductance hyperpolarization recorded in the soma of B18, and the hyperpolarization persists in a solution in which synaptic transmission has been blocked. Histamine did not produce any marked changes of the duration of a TEA-broadened somatic action potential of B18. To the extent that the soma of B18 reflects the membrane properties of its synaptic terminal region, the data suggest that histamine may produce presynaptic inhibition by hyperpolarizing the synaptic terminal region.

Motor control of the appetitive phase of feeding behavior in Aplysia

The appetitive phase of feeding behavior, in the gastropod, Aplysia, consists of head lifting, head waving, orientation of the head to food, and locomotion. We have initiated studies of the neural control of head waving using three methods: (i) anatomical description of the nerves innervating muscles that are involved in head movement, (ii) electrical stimulation of nerves in a semi-intact preparation, and (iii) recording from nerves in free-moving animals. The muscles controlling head movements, located in the dorsal and lateral neck region, are innervated primarily by pleural nerve 1 and pedal nerves 2, 3, and 5. Electrical stimulation of these nerves caused both longitudinal and lateral contractions of the neck muscles, the largest contractions being in the area where the nerve first enters the muscle. Extracellular recordings from pleural nerve 1 and pedal nerves, in free-moving animals, showed an increase in extracellular activity during head lifting, at the onset of appetitive feeding behavior. Directionally specific inhibition and excitation in neural activity occurred in pleural nerve 1 and pedal nerve 5 during leftward and rightward movements of the head (head waving). Cobalt and nickel backfills of pleural nerve 1 and pedal nerve 5 revealed cell bodies in the cerebral, pedal, and pleural ganglia. The neurons are therefore putative motor neurons for the neck muscles involved in appetitive behavior. This evidence suggests that appetitive control of feeding may involve the coordinated activity of several different ganglia.

Mechanisms underlying satiation of feeding behavior of the mollusc Aplysia

Animals filled almost to satiation by nonnutritive bulk do not satiate when they ingest a small amount of seaweed. This suggests that satiation is not triggered by chemostimulation of an anteriorly located "hot spot." Inflation of a balloon placed in the gut of the animal results in satiation as reflected in a number of different parameters of feeding behavior. The suppressive effect of a relatively brief inflation is rapidly and fully reversible, although repeated inflation and deflation appeared to produce slowly reversible or irreversible effects. The parameters of the changes in feeding during gut inflation are comparable to those of normal animals that are slowly fed individual pieces of food. The inflation volume needed to satiate the animal is a function of the rate of inflation--more rapid inflations requiring larger volumes. Cutting of the esophageal nerves results in a significant increase in the volume needed to satiate the animals, but nevertheless they eventually cease feeding and generally do not show a burst gut. The evidence indicates that the satiation that eventually occurs in nerve-sectioned animals, at least in part, is due to depression of feeding following very prolonged sensory stimulation. The data suggest that for a rapidly consumed meal, satiation results primarily due to distension-related gut signals conveyed by the esophageal nerves, whereas for very slowly consumed meals, the former factor interacts with a process associated with sensory stimulation, such as receptor adaptation. The current results indicate that balloon distension can serve as a reasonable stimulus in experiments in simplified preparations in which the nervous system can be studied.

Myomodulin: a bioactive neuropeptide present in an identified cholinergic buccal motor neuron of Aplysia

When Aplysia are initially exposed to food stimuli, their biting responses show progressive increases in speed and strength. The accessory radula closer (ARC) buccal muscles have been used to study this phenomenon, and it has been shown that changes in ARC muscle contraction are partially due to activity of a serotonergic neuron that modulates this muscle, by both a direct action and an action on two ARC motor neurons (B15 and B16). The motor neurons use acetylcholine as their excitatory transmitter, but they also contain bioactive peptides that can potentiate muscle contractions when they are exogenously applied. Motor neuron B15 contains the structurally related small cardioactive peptides A and B, whereas motor neuron B16 contains a different peptide--termed myomodulin. In the present study we determined the full amino acid sequence of myomodulin. Myomodulin is present in the ARC muscle, and exogenous application of the peptide potentiates ARC muscle contractions in a manner similar to the potentiation by small cardioactive peptides A and B. The structure of myomodulin, however, bears little resemblance to the small cardioactive peptides. Thus it appears that ARC muscle contractions may be regulated by at least three distinct classes of neuromodulators: serotonin, the small cardioactive peptides, and myomodulin.

Multiple neuropeptides in cholinergic motor neurons of Aplysia: evidence for modulation intrinsic to the motor circuit

Changes in Aplysia biting responses during food arousal are partially mediated by the serotonergic metacerebral cells (MCCs). The MCCs potentiate contractions of a muscle utilized in biting, the accessory radula closer (ARCM), when contractions are elicited by stimulation of either of the two cholinergic motor neurons B15 or B16 that innervate the muscle. We have now shown that ARCM contractions may also be potentiated by peptide cotransmitters in the ARCM motor neurons. We found that motor neuron B15 contains small cardioactive peptides A and B (SCPA and SCPB)--i.e., whole B15 neurons were bioactive on the SCP-sensitive Helix heart, as were reverse-phase HPLC fractions of B15 neurons that eluted like synthetic SCPA and SCPB. Furthermore, [35S]methionine-labeled B15 peptides precisely coeluted with synthetic SCPA and SCPB. SCPB-like immunoreactivity was associated with dense-core vesicles in the soma of B15 and in neuritic varicosities and terminals in the ARCM. B16 motor neurons did not contain SCPA or SCPB but contained an unidentified bioactive peptide. RP-HPLC of [35S]methionine-labeled B16s resulted in one major peak of radioactivity that did not coelute with either SCP and which, when subject to Edman degradation, yielded [35S]methionine in positions where there is no methionine in the SCPs. Exogenously applied B16 peptide potentiated ARCM contractions elicited by stimulation of B15 or B16 neurons. Thus, in this system there appear to be two types of modulation; one type arises from the MCCs and is extrinsic to the motor system, whereas the second type arises from the motor neurons themselves and hence is intrinsic.

Biochemical and immunocytological localization of the neuropeptides FMRFamide, SCPA, SCPB, to neurons involved in the regulation of feeding in Aplysia

The localization of the neuropeptide FMRFamide in the buccal ganglia and buccal muscles of Aplysia was studied by immunocytology and high-pressure liquid chromatography (HPLC) combined with either a sensitive bioassay or 35S-methionine labeling. Immunocytology with an antiserum directed to FMRFamide stained a large number of fibers, varicosities, and neuronal somata. Two groups of stained neurons were of particular interest. One was the S cells, a group comprised of many small neurons, the majority of which were stained. HPLC of pooled labeled S cells confirmed that at least some of these neurons synthesize FMRFamide. The other group of stained neurons were in the ventral cluster, a group comprised of a small number of large neurons, many of which are motor neurons that innervate the buccal muscles involved in producing biting and swallowing movements. Several of the ventral neurons were previously shown to contain 2 other neuropeptides, the small cardioactive peptides SCPA and SCPB. These neurons are sufficiently large to permit HPLC analyses of the neuropeptides synthesized by individual neurons. This procedure confirmed that individual ventral neurons synthesized FMRFamide, or the SCPs, or all 3 peptides. The coexistence of FMRFamide and the SCPs in the same neuron was confirmed by simultaneous staining of sections from the buccal ganglia with a monoclonal antibody to the SCPs and an antiserum to FMRFamide. The coexistence of the 3 peptides in the same neuron was surprising in light of the observations that these peptides often have opposite biological activity. The ventral neurons are large and potentially identifiable as individuals. Thus, these neurons may be particularly useful for studying the physiological and behavioral roles of neuropeptides in generating complex behaviors.

Proteins that bind serotonin in the nervous system of Aplysia californica

The central nervous system of Aplysia has been found to contain two forms of proteins that bind serotonin with high affinity. These proteins share several similarities with the serotonin binding proteins that have been isolated from the vertebrate nervous system. Their ability to bind serotonin is enhanced by Fe2+ and inhibited by Na+,-SH reagents as well as reserpine and N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophanamide.

Sequence of small cardioactive peptide A: a second member of a class of neuropeptides in Aplysia

Previous studies have shown that the nervous system and other tissues of molluscs contain a number of peptides that potently excite molluscan hearts. Two such peptides, termed small cardioactive peptides A and B (SCPA and SCPB) are present in large quantities in the nervous system of Aplysia. These peptides are widely distributed within the CNS and peripheral tissues and have been found to be potent modulators of synaptic transmission in Aplysia. SCPB has previously been purified from nervous tissue and sequenced. In this paper, we report the purification of SCPA and propose its sequence. This sequence was confirmed by comparing the chromatographic properties of native SCPA (labelled in organ culture) with a synthetic peptide that has the proposed sequence. A significant proportion of the sequence of the two SCPs is conserved, indicating that they are members of the same peptide class, a finding that is consistent with the recent observation that the two peptide sequences are present in a single precursor.

Release of neuropeptides during intracellular stimulation of single identified Aplysia neurons in culture

An important criterion for classifying a substance as a neurotransmitter is that it is released in an activity-dependent fashion. We have utilized cell culture of individual neurons of Aplysia to demonstrate the release of the neuropeptides SCPA and SCPB (small cardioactive peptides A and B). Neurons B1 and B2 were isolated from the buccal ganglion of Aplysia and maintained in cell culture. The cells grew new processes, which were immunoreactive to antibodies for the neuropeptide SCPB. These processes contained SCPA and SCPB that were detectable by bioassay on snail heart. The cells synthesized the SCPs from radiolabeled precursors and transported the peptides to their neurites. Single cells released SCPs in a calcium-dependent fashion upon intracellular electrical stimulation. Taken together, these results provide critical evidence that SCPs are neurotransmitters. The results also indicate that the cell culture of individual identified neurons can be used to investigate the release of peptides.

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.

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.

Modulatory synaptic actions of an identified histaminergic neuron on the serotonergic metacerebral cell of Aplysia

Possible sources of excitatory synaptic input to the serotonergic metacerebral cell (MCC) were determined by stimulating various neurons in the cerebral ganglion. Firing of the previously identified histaminergic neuron C2 was found to produce synaptic input to the MCC. The synaptic input consists of fast excitatory-inhibitory synaptic potentials on a background of a slow EPSP. The slow EPSP appears to be monosynaptic and chemically mediated since it persists in a solution of high divalent cations; broadening of the presynaptic spike enhances the EPSP; the size of the EPSP is a function of the Mg2+ and Ca2+ concentrations of the bathing solution; and the EPSP can be mimicked by application of histamine to the MCC. The slow EPSP, in addition to firing the MCC, can increase the excitability of the cell, even under conditions in which C2 is fired at a rate too slow to produce a measurable EPSP when the MCC is at rest potential. This property appears to be due to the fact that the slow EPSP results from an apparent decrease of membrane conductance so that the size of the EPSP increases markedly as the cell is depolarized, and the EPSP appears to be highly voltage-dependent so that it is small or absent close to the rest potential of the MCC. When the MCC is voltage-clamped, application of histamine to the bath results in an inward current that disappears when the MCC is hyperpolarized. The potential at which the histamine-induced current reverses or disappears is dependent on the concentration of external potassium, suggesting that, at least in part, the slow EPSP is due to a decrease of potassium conductance. The data on C2 are consistent with its being an element of the neuronal system that mediates a state of food arousal in Aplysia.

Activity of an identified histaminergic neuron, and its possible role in arousal of feeding behavior in semi-intact Aplysia

The possible functions of histaminergic neuron C2 were studied in an isolated head preparation from which it was possible to obtain intracellular recordings while the buccal mass exhibited feeding-like responses. Application of food to the lips of the isolated head preparation elicited rhythmic buccal movements that appeared to be ingestion responses, since they moved seaweed into the buccal cavity and towards the esophagus, and their frequency and regularity was similar to the ingestion responses studied in a group of intact animals. The ingestion responses of the buccal mass consisted of 2 main phases of movement of the radula from a middle rest position: forward and return to rest, and backward and return to rest. The relative magnitudes and timing of these 2 phases were variable. Intracellular recordings from C2 in the isolated head revealed that C2 is silent when the buccal mass is quiescent, but that it can be excited into spike activity, either by mechanical stimulation of the perioral zone or by chemostimulation that results in rhythmic movement. C2 fires a burst of spikes in phase with each protraction-retraction cycle, and, if the movements continue, C2 fires even when the eliciting stimulus has been removed. Activity of the cell was usually preceded by fast depolarizing responses that appeared to be blocked axon spikes. The evidence suggests that C2 is part of a positive feedback loop that may help maintain the persistence of arousal of feeding behavior beyond the time that food stimuli are removed.

Biochemical and immunocytological localization of molluscan small cardioactive peptides in the nervous system of Aplysia californica

High pressure liquid chromatography (HPLC) followed by bioassay on isolated snail hearts were used to locate two related peptides, termed small cardioactive peptides A and B (SCPA and SCPB) in each of the central ganglia of Aplysia. The peptides are most concentrated in the buccal ganglia, the ganglia involved in the control of feeding movements. Immunocytology with antisera raised to conjugated SCPB stained three groups of neurons in the buccal ganglia. One group consisted of relatively small neurons that were tightly clustered. The second group was comprised of larger neurons that were more scattered. The third group was made up of several neurons including the two largest in the ganglia, identified cells B1 and B2. B1 and B2 and other neurons in this group innervate the gut by way of the esophageal nerve. HPLC-bioassay of single, individually dissected B1 or B2 neurons demonstrated that the two peptides are present in a single cell. For B2, but not B1, choline injected into the cell body was converted to the conventional transmitter, acetylcholine. This indicates that, in addition to the two peptides, B2 also contains choline acetyltransferase, and raises the possibility that acetylcholine and the SCPs may act as co-transmitters in B2. Strong immunocytological staining of fibers and varicosities was observed in the neuropilar region of the cerebral, pleural, pedal, and abdominal ganglia. In addition to the buccal ganglia, immunoreactive neurons were observed in all of the other central ganglia. The high concentration of the SCPs and the relatively large number of immunoreactive neurons in the buccal ganglion suggest a particularly important role of these peptides specifically in feeding behavior. However, the widespread occurrence of the SCPs in fibers and neuronal cell bodies throughout the nervous system suggests that these peptides also may have additional behavioral functions in Aplysia.

The small cardioactive peptides A and B of Aplysia are derived from a common precursor molecule

We have identified cells in the central nervous system of the marine mollusc Aplysia that react with antibody raised against the small cardioactive peptide B (SCPB). Antisera to this neuropeptide stained a subset of central neurons that include the large identified buccal neurons, B1 and B2. The distribution of SCP-containing neurons was used in a strategy to isolate a cDNA clone encoding the precursor protein for the peptide. RNA from neurons B1 and B2 and from cells that did not stain with SCPB antisera was used to direct the synthesis of radiolabeled cDNA probes. A cDNA clone complimentary to mRNA specifically expressed in the B1 and B2 cells was isolated by differentially screening a buccal cDNA library with these probes. The cloned cDNA segment is 1394 nucleotides in length and contains a 408-base-pair open reading frame. The predicted precursor protein is composed of 136 amino acids and has a characteristic hydrophobic leader sequence. The sizes of the precursor protein with and without this leader sequence agree with in vivo and in vitro labeling studies. The amino acid sequences for SCPB and a related peptide, SCPA, are present and are flanked by known proteolytic processing sites.

Atherogenic diet-induced modification of the subfraction distribution of high density lipoproteins in monkeys

High density lipoproteins (HDL) were isolated from the plasma of adult male African green monkeys fed control or atherogenic diets and were then subfractionated in a density gradient by centrifugation at 50,000 rpm, 20 degrees C for 18 hours in the Beckman VTi50 vertical rotor. Material from the gradient was pooled into six subfractions for further analysis. With increasing density across the gradient, the subfractions had progressively decreasing flotation rates, molecular weights, apo A-I/apo A-II mass ratios, lipid percentages of mass, and average sizes. Apoprotein content per particle was reasonably constant compared to lipid content. The physical and chemical properties of the HDL subfractions suggested that the gradient separated three subclasses of HDL equivalent to HDL 2b, HDL 2a, and HDL3 of human beings. When values among animals were compared, the atherogenic diet shifted the relationship between total plasma cholesterol concentration and total HDL concentration from a positive to a negative correlation. The subfractions most affected were those of d less than 1.10 g/ml (HDL2b), while the 1.10 less than d less than 1.13 g/ml (HDL2a) subfractions were affected to a lesser degree; the more responsive the animal to dietary cholesterol, the lower was the mass concentrations of these subfractions. This pattern suggested that diet-induced changes in HDL2 subfraction particle concentrations may be related to the development of atherosclerosis in African green monkeys; parallel changes in particle composition were not identified.

Internal stimuli enhance feeding behavior in the mollusc Aplysia

The hypothesis that subsatiating levels of internal food stimuli can arouse and potentiate feeding behavior was examined in the mollusc Aplysia californica. Animals were fed a small quantity of seaweed and their latencies to show biting responses were determined after food arousal was permitted to partially decay. Control animals were stimulated with food, but were not permitted to ingest it, or were fed nonnutritive glass-fiber filter paper. Compared to controls, animals that were fed showed significantly shorter latencies to respond when tested up to 80 min after previous exposure to food. These results indicate that internal stimuli can function like external stimuli to enhance responsiveness to food and suggest the hypothesis that satiation may be viewed as an interactive process involving the interplay of excitatory and inhibitory influences arising from the alimentary system.

Evidence for parallel actions of a molluscan neuropeptide and serotonin in mediating arousal in Aplysia

The neuropeptide designated SCPB (small cardioactive peptide B), the sequence of which has recently been determined, was found in the accessory radula closer muscle, a muscle involved in biting movements. The ganglia and nerves that innervate the accessory radula closer muscle also contain SCPB. At nanomolar concentrations, it enhances the contractions of the muscle. The effect of SCPB on the muscle resembles the effect of an identified serotonergic neuron that previously was shown to mediate behavioral effects that reflect a food arousal state in Aplysia. Like serotonin, SCPB enhances contractions by a postsynaptic action, which appears to involve an increase in cAMP levels in the muscle. Our findings suggest that parallel peptidergic and serotonergic pathways may mediate similar aspects of arousal in Aplysia.

Neuronal mediation of cardiovascular effects of food arousal in aplysia

Previous studies have demonstrated that heart rate and blood pressure increase in Aplysia during an arousal state elicited by food stimuli. In addition, during biting, blood flow is routed alternately to the head (during protraction of the buccal mass) and to the digestive system (during retraction). In this study, cutting the pleuroabdominal connectives eliminated 75% of the heart rate response during food arousal, and cutting the pleuroabdominal connectives eliminated 20-50% of the pressor response. Recording in the abdominal ganglion from the RBHE heart excitor and the three LBVC vasoconstrictor motor neurons in a reduced preparation showed that activity in these neurons was increased 50 and 100%, respectively, during food arousal. Activity of the LBVC cells was maximal during the protraction phase of biting. The LBVC vasoconstrictor motor neurons, when fired at the rates recorded during food arousal, can occlude the abdominal aorta completely. We conclude that the RBHE and LBVC neurons mediate, in part, the heart rate and pressor responses recorded during food arousal and that cyclic activity in LBVC contributes significantly to the cyclic alternation of blood flow between the head and the gut during rhythmic biting behavior.

Lesion of a serotonergic modulatory neuron in Aplysia produces a specific defect in feeding behavior

The serotonergic metacerebral cells (MCCs) of Aplysia were destroyed by intracellular injection of proteolytic enzyme. MCC-lesioned animals showed alterations of biting responses compared to MCC-sham and B-cell-lesioned control animals, as well as to their own preoperative behavior. The alterations of biting responses included a prolongation of the duration of radula protraction and a lengthening of interbite interval. No changes were observed in non-biting feeding responses and in behaviors unrelated to feeding.

Intraneuronal injection of horseradish peroxidase labels glial cells associated with the axons of the giant metacerebral neuron of Aplysia

After injecting the cell body of the giant serotonergic neuron (GCN) in the cerebral ganglion of Aplysia with horseradish peroxidase (HRP), we found that some glial cells were labeled along the cerebrobuccal connective and posterior lip nerve, the two nerves that contain the major axon branches of the injected neuron. Starting at a distance of about 3 mm from the site of the injection, only glial cells in the nerve that are close to an axon branch of the GCN contained HRP. Labeling appears to be selective because, at most, only one out of three glial cells was labeled within an area of 20 micrometer of the axon. Moreover, no HRP was detected in any of the many glial cells within the cerebral ganglion. Reaction product diffusely labeled the cytoplasm of glial cell bodies and processes after intraneuronal injection of HRP. This distribution was markedly different from that observed in glial cells after the uptake of HRP from the extracellular space; HRP, presumed to be taken up by endocytosis, was found to be localized to vesicles, tubules, and multivesicular bodies.

Activity of an identified serotonergic neuron in free moving Aplysia correlates with behavioral arousal

Extracellular recordings of the metacerebral cell (MCC), a serotonergic neuron in Aplysia, were obtained in free moving, undrugged animals. MCC activity was evoked by exposure to food. Arousal level was manipulated by satiating the animals or exposing them to a noxious stimulus. We found that the amount of evoked MCC activity correlated with the level of arousal of the animal.

Ultrastructure of a histaminergic synapses in Aplysia

The ultrastructure of histaminergic synaptic terminals was studied by the means of intrasomatic injection of horseradish peroxidase into the identified histaminergic neuron C2 of Aplysia. The axonal tree of C2 was found to consist, in part, of varicosities that display putative release sites similar in morphology to those described in other neurons in Aplysia. The varicosities contain at least two populations of vesicles: a conspicuous class of of large vesicles with an electron-dense core that almost fills the entire vesicle and a heterogeneous class of large and small electron-lucent vesicles. The small lucent vesicles preferentially cluster near active zones.

Response properties and synaptic connections of mechanoafferent neurons in cerebral ganglion of Aplysia

1. The cells of two clusters of small neurons on the ventrocaudal surface of each hemicerebral ganglion of Aplysia were found to exhibit action potentials following tactile stimuli applied to the skin of the head. These neurons appear to be mechanosensory afferents since they possess axons in the nerves innervating the skin and tactile stimulation evokes spikes with no prepotentials, even when the cell bodies are sufficiently hyperpolarized to block some spikes. The mechanosensory afferents may be primary afferents since the sensory response persists after chemical synaptic transmission is blocked by bathing the ganglion and peripheral structures in seawater with a high-Mg2+ and low-Ca2+ content. 2. The mechanosensory afferents are normally silent and are insensitive to photic, thermal, and chemical stimuli. A punctate tactile stimulus applied to a circumscribed region of skin can evoke a burst of spikes. If the stimulus is maintained at a constant forces, the mechanosensory response slowly adapts over a period of seconds. Repeated brief stimuli have little or no effect on spike frequency within a burst. 3. Approximately 81% of the mechanoafferent neurons have a single ipsilateral receptive field. The fields are located on the lips, the anterior tentacles, the dorsal portion of the head, the neck, or the perioral zone. Because many cells have collateral axons in the cerebral connectives, receptive fields elsewhere on the body are a possibility. The highest receptive-field density was associated with the lips. Within each area, receptive fields vary in size and shape. Adjacent fields overlap and larger fields frequently encompass several smaller ones. The features of some fields appear invariant from one animal to the next. A loose form of topographic organization of the mechanoafferent cells was observed. For example, cells located in the medial cluster have lip receptive fields, and most cells in the posterolateral portion of the lateral clusters have tentacle receptive fields. 4. Intracellular stimulation of individual mechanoafferents evokes short and constant-latency EPSPs in putative motor neurons comprising the identified B-cell clusters of the cerebral ganglion. On the basis of several criteria, these EPSPs appear to be several criteria, these EPSPs appear to be chemically mediated and are monosynaptic. 5. Repetitive intracellular stimulation of individual mechanoafferent neurons at low rates results in a gradual decrement in the amplitude of the EPSPs evoked in B cluster neurons. EPSP amplitude can be restored following brief periods of rest, but subsequent stimulation leads to further diminution of the response. 6. A decremented response cannot be restored by strong mechanical stimulation outside the receptive field of the mechanoafferent or by electrical stimulation of the cerebral nerves or connectives...

Functional role of serotonergic neuromodulation in Aplysia

The serotonergic metacerebral cell (MCC) of the mollusk Aplysia produces slow synaptic potentials in motor neurons of the buccal muscle, and increases the rate of ongoing rhythmic burst output of the buccal ganglion. In addition, the MCC acts peripherally to enhance the strength of buccal muscle contractions that are produced by firing of motor neurons. The potentiation of contraction is not associated with any detectable changes of resting membrane potential of muscle cells. Although MCC activity produces a small enhancement of excitatory junctional potentials, several experiments clearly indicate that the MCC has a direct potentiating effect on excitation-contraction coupling. The data suggest that potentiation of contraction might be mediated by cAMP. For example, activity of the MCC enchances the rate of accumulation of cAMP in buccal muscle, application of phosphodiesterase resistant analogs of cAMP potentiates muscle contraction, and a phosphodiesterase inhibitor enhances the effect of MCC stimulation. Recordings from free-moving animals indicate that the MCC becomes activated by exposure of the animal to food stimuli, and that the activation parallels the presence of a food-arousal state. Food-arousal is characterized by enhanced strength and increased frequency of biting responses. Both these effects can result from activity of the MCC. Thus, in this system, modulatory synaptic actions function to provide the substrate for a type behavioral modulation.

Homology of the giant serotonergic neurons (metacerebral cells) in Aplysia and pulmonate molluscs

The properties of the giant cerebral serotonin-containing neurons of the opisthobranch mollusc Aplysia californica were studied and were compared to the existing data on the giant serotonin-containing neurons (metacerebral cells) of pulmonate mulluscs. Among the properties examined were: axonal distribution, synaptic input and output, pharmacological responses, biophysical characteristics, and plasticity. With only minor exceptions, the properties of the serotonin-containing neurons of Aplysia and of pulmonate molluscs were remarkably similar, and it was concluded that these identified neurons are true homologues. The establishment of the homology of the metacerebral cells of Aplysia to the metacerebral cells of pulmonate molluscs extends the known distribution of these neurons to a second major subclass (Opisthobranchiata) of molluscs. Since pulmonate and opisthobranch molluscs differ substantially in behavioral and anatomical features, the study of the metacerebral cells of these two groups may promote the understanding of the evolutionary adaptation of the nervous system to different environmental pressures.