Feeding motor program in Limax. I. Neuromuscular correlates and control by chemosensory input

Comparative neurobiology of feeding in the opisthobranch sea slug, Aplysia, and the pulmonate snail, Helisoma: evolutionary considerations

The motor systems that generate feeding-related behaviors of gastropod mollusks provide exceptional opportunities for increasing our understanding of neural homologies and the evolution of neural networks. This report examines the neural control of feeding in Helisoma trivolvis, a pulmonate snail that ingests food by rasping or scraping material from the substrate, and Aplysia californica, an opisthobranch sea slug that feeds by using a grasping or seizing motion. Two classes of neurons that are present in the buccal ganglia of both species are considered: (1) clusters of peptidergic mechanoafferent cells that transmit sensory information from the tongue-like radula/odontophore complex to the central motor circuit; and (2) sets of octopamine-immunoreactive interneurons that are intrinsic to the feeding network. We review evidence that suggests homology of these cell types and propose that their roles have been largely conserved in the control of food-scraping and food-grasping consummatory behaviors. We also consider significant differences in the feeding systems of Aplysia and Helisoma that are associated with the existence of radular closure in Aplysia, an action that does not occur in Helisoma. It is hypothesized that a major adaptation in the innervation patterns of analogous, possibly homologous muscles could distinguish the food-scraping versus food-grasping species. It appears that although core CPG elements have been largely conserved in this system, the neuromuscular elements that they regulate have been more evolutionarily labile.

Neurophysiological correlates of unconditioned and conditioned feeding behavior in the pond snail Lymnaea stagnalis

We used a behavioral appetitive learning paradigm followed by electrophysiological analysis to investigate the neuronal expression of appetitive conditioning in Lymnaea. We first established the levels of unconditioned and conditioned feeding responses in intact animals. We then demonstrated that neuronal correlates of both unconditioned responses to touch and food and a conditioned response to touch could be found in semi-intact preparations of the same animals that had been subjected to behavioral tests and conditioning trials. In the conditioning experiments, the experimental animals received 15 trials in which touch to the lips, the conditioned stimulus (CS), was paired with sucrose, the unconditioned food stimulus (US). Control animals received 15 presentations of either CS or US, or both, applied in a random manner. After training, a strong conditioned response to touch was established in the experimental but not in the control groups. For subsequent electrophysiological analysis of posttraining neuronal responses to the touch CS, semi-intact preparations were set up from the same animals that had been behaviorally conditioned or subjected to control procedures. Intracellular recordings, made from previously identified motoneurons of the feeding system, allowed the fictive feeding response to the CS to be monitored. In experimental preparations, touch applied to the lips evoked significantly more fictive feeding cycles than in controls, and this demonstrated the existence of a neurophysiological correlate of the appetitively conditioned response observed in the whole animals.

In vivo neuropharmacological and in vitro laser ablation techniques as tools in the analysis of neuronal circuits underlying behavior in a molluscan model system

1. This paper reviews the selective lesioning techniques employed to elucidate the role of the neurotransmitters dopamine and serotonin and single, identified interneurons in the feeding system of the pond snail Lymnaea stagnalis. 2. The pathway lesioning work reviewed in this paper showed that dopamine is necessary for the feeding response to occur and serotonin has a mainly modulatory role in the feeding system of Lymnaea. 3. The photoinactivation results reviewed here assist in the elucidation of the different roles that different types of interneurons play in the initiation and modulation of patterned neuronal activity underlying feeding.

Vital staining from dye-coated microprobes identifies new olfactory interneurons for optical and electrical recording

A versatile technique for dye application in living tissue is described, which results in labeling of viable cells from which electrophysiological or optical recordings can be obtained. The dye-coated surface of a glass microelectrode tip is used to apply anatomical tracers or calcium sensitive probes with spatial precision. A total of three types of dyes have been applied in this way to find and record from olfactory interneurons in the terrestrial mollusc Limax maximus. Crystals of 1,1'-didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) formed on the tips of glass microelectrodes were placed in the procerebral lobe, the major olfactory processing center of Limax. Somata in buccal and pedal ganglia with processes extending several 100 microm to the procerebral lobe were stained within 4-6 h. Intracellular recordings from DiI stained buccal (B(PC)) and pedal (P(PC)) cells were obtained. Cross correlograms of the oscillatory field potential in the procerebral lobe and spontaneous action potentials in P(PC) or B(PC) show that P(PC) activity is weakly coupled to the oscillation in the procerebral lobe, while B(PC) activity is clearly coupled to the oscillation. Stimulation of the procerebral lobe with nitric oxide activated P(PC) cells but suppressed activity in B(PC) cells. Calcium green-10Kdextran coated electrodes were used to place calcium green in the cell body layer of the procerebral lobe. Bursting and nonbursting procerebral neurons incorporated and transported the calcium green-dextran. Optical recordings of changes in fluorescence signals from several bursting cells recorded simultaneously were used to test alternative mechanisms of bursting cell coupling. Application of biotin 3Kdextran to the midline of the cerebral ganglion revealed a group of cells in each procerebral lobe with processes crossing the midline of the cerebral ganglion. These cells may couple right and left procerebral lobe activity during odor processing.

Processing of mechano- and chemosensory information in the lip nerve and cerebral ganglia of the snail Helix pomatia L

Neurophysiologists have long been seeking simple model systems in which to analyze the neuronal mechanisms underlying the organization of behavior. The feeding behavior of molluscs has proved to be one of the most useful simple systems for the analysis of cyclical motor patterns, the interactions of central pattern generating interneurons, and the role of sensory inputs in the initiation and maintenance of the behavior. Considerable progress has been made in one or both of the first two aspects of this research in Lymnaea, Helisoma, Limax, Planorbarius, Pleurobranchaea, and Tritonia (for reviews see [3, 7, 8, 15]), and more recently, in Aplysia [39] and Planorbis [1]. The role of mechano- and chemosensory inputs in the organization of the feeding behavior was studied in at least twenty molluscan species (for a review see [3]). However, in only less than half of them was the analysis extended to the effect of tactile and chemical inputs on identified neurons in the buccal and cerebral ganglia which contain the feeding circuitry (Aplysia: [12, 22, 36, 41]; Pleurobranchaea: [9, 16, 17]; Tritonia: [2]; Helisona: [21]; Limax: [11, 14, 35]: Helix: [6, 19, 24-26, 32, 38]). In the present work I would like to review our earlier findings on the processing of mechano and chemosensory information in the lip nerves and cerebral ganglia of Helix pomatia L. These findings were published in a series of papers between 1982 and 1987 [19, 20, 24-26]. The results reviewed here prepared the way for the development of new lines of research in our laboratory on the plasticity and serotonergic modulation of feeding in this widely used experimental animal [27, 40].

The timing of activity in motor neurons that produce radula movements distinguishes ingestion from rejection in Aplysia

1. We have studied the neural circuitry mediating ingestion and rejection in Aplysia using a reduced preparation that produces ingestion-like and rejection-like motor patterns in response to physiological stimuli. 2. We have characterized 3 buccal ganglion motor neurons that produce specific movements of the radula and buccal mass. B8a and B8b act to close the radula. B10 acts to close the jaws and retract the radula. 3. The patterns of activity in these neurons can be used to distinguish the ingestion-like and rejection-like motor patterns. B8a, B8b and B10 are active together during the ingestion-like pattern. Activity in B8a and B8b ends prior to the onset of activity in B10 during the rejection-like pattern. 4. Our data suggest that these neurons undergo similar patterns of activity in vivo. During both feeding-like patterns, the activity and peripheral actions of B8a, B8b, and B10 are consistent with radula movements observed during ingestion and rejection. In addition, the extracellular activity produced by these neurons is consistent with neural activity observed in vivo during ingestion and rejection. 5. Our data suggest that the different activity patterns observed in these motor neurons contribute to the different radula movements that distinguish ingestion from rejection.

In vivo buccal nerve activity that distinguishes ingestion from rejection can be used to predict behavioral transitions in Aplysia

1. We are studying the neural basis of consummatory feeding behavior in Aplysia using intact, freely moving animals. 2. Video records show that the timing of radula closure during the radula protraction-retraction cycle constitutes a major difference between ingestion (biting or swallowing) and rejection. During ingestion, the radula is closed as it retracts. During rejection, the radula is closed as it protracts. 3. We observed two patterns of activity in nerves which are likely to mediate these radula movements. Patterns I and II are associated with ingestion and rejection, respectively, and are distinguished by the timing of radula nerve activity with respect to the onset of buccal nerve 2 activity. 4. The association of ingestion with pattern I is maintained when the animal feeds on a polyethylene tube, the same food substrate used to elicit rejection responses. Under these conditions, pattern I is associated with either swallowing or no net tube movement. 5. Most transitions from swallowing to rejection were preceded by one or more occurrences of pattern I in which there was no net tube movement, suggesting that these transitions can be predicted. 6. Our data suggest that these two patterns can be used to distinguish ingestion from rejection.

Amino acids and serotonin in Limax maximum after a tryptophan devoid diet

1. Animals avoid diets lacking an essential amino acid, such as tryptophan (TRP), the precursor for serotonin (5-HT). 5-HT is important in the control of feeding. 2. To study the effects of TRP deprivation, slugs were fed TRP-devoid (DEV) or control (COR) diets. 3. Food intake was depressed in DEV, as expected, but after 2 weeks, the serontonergic metacerebral giant cell in DEV was still functional. 4. Neither brain 5-HT nor plasma TRP concentration was affected. 5. Compared with food-restricted animals that had reductions in most amino acids, the DEV group sustained a marked plasma amino acid imbalance.

Premotor neurons in the feeding system of Aplysia californica

Central pattern generator (CPG) circuits control cyclic motor output underlying rhythmic behaviors. Although there have been extensive behavioral and cellular studies of food-induced feeding arousal as well as satiation in Aplysia, very little is known about the neuronal circuits controlling rhythmic consummatory feeding behavior. However, recent studies have identified premotor neurons that initiate and maintain buccal motor programs underlying ingestion and egestion in Aplysia. Other newly identified neurons receive synaptic input from feeding CPGs and in turn synapse with and control the output of buccal motor neurons. Some of these neurons and their effects within the buccal system are modulated by endogenous neuropeptides. With this information we can begin to understand how neuronal networks control buccal motor output and how their activity is modulated to produce flexibility in observed feeding behavior.

Distribution of FMRFamide-like immunoreactivity in the nervous system of the slug Limax maximus

The distribution of FMRFamide-like immunoreactive neurons in the nervous system of the slug Limax maximus was studied using immunohistochemical methods. Approximately one thousand FMRFamide-like immunoreactive cell bodies were found in the central nervous system. Ranging between 15 micron and 200 micron in diameter, they were found in all 11 ganglia of the central nervous system. FMRFamide-like immunoreactive cell bodies were also found at peripheral locations on buccal nerve roots. FMRFamide-like immunoreactive nerve fibres were present in peripheral nerve roots and were distributed extensively throughout the neuropil and cell body regions of the central ganglia. They were also present in the connective tissue of the perineurium, forming an extensive network of varicose fibres. The large number, extensive distribution and great range in size of FMRFamide-like immunoreactive cell bodies and the wide distribution of immunoreactive fibres suggest that FMRFamide-like peptides might serve several different functions in the nervous system of the slug.

Control of feeding movements in the freshwater snail Planorbis corneus. I. Rhythmical neurons of buccal ganglia

(1) The buccal mass of the freshwater snail Planorbis corneus, dissected together with the buccal ganglia, performs rhythmic feeding movements. Radula movements and the electrical activity in various nerves of buccal ganglia were recorded in such a preparation. The cycle of radula movements consisted of three phases: quiescence (Q), protraction (P) and retraction (R). The activity in the radular nerve was observed mainly in the P-phase and that in the dorsobuccal nerve, largely in the R-phase. (2) Isolated buccal ganglia were capable of generating a feeding rhythm, the activity in buccal nerves being similar to that observed in the buccal mass-buccal ganglion preparation, i.e., a burst in the radular nerve preceded a burst in the dorsobuccal nerve. The activity of neurons in isolated buccal ganglia during generation of the feeding rhythm has been studied with intracellular microelectrodes. About 10% of ganglion neurons exhibited periodic activity related to the feeding rhythm ("rhythmic" neurons). (3) Rhythmic neurons have been divided into 7 groups according to the phase of their activity and to the characteristics of slow oscillations of the membrane potential during the feeding cycle. Group 1 neurons revealed a gradual increase of depolarization during the Q- and P-phases. In subgroup 1e neurons, spike discharges began in the Q-phase, while in subgroup 1d neurons activity started in the P-phase. During the R-phase, group 1 neurons were strongly hyperpolarized, and their discharges terminated. In group 2 neurons, small depolarization gradually increased during the Q- and P-phases. Then, in the R-phase, a large (20-50 mV) rectangular wave of depolarization arose with superimposed high-frequency oscillations. Group 3 neurons exhibited an excitatory postsynaptic potential (EPSP) in the P-phase and inhibitory postsynaptic potential (IPSP) in the R-phase. The neurons of group 4 revealed two EPSPs: a small one in the P-phase and a larger one in the R-phase. Group 5 neurons exhibited an EPSP in the P-phase, those of group 7 - an IPSP in the R-phase, and those of group 9 - IPSPs in the P- and R-phases. Neurons within each of the groups 1, 2 and 4 were electrically coupled, and in addition, there were also electrical connections between neurons of groups 2 and 4. (4) Data are presented showing that neurons of groups 1 and 2 are the main source of postsynaptic potentials in rhythmic neurons in the P-phase and in the R-phase of the cycle, respectively.

The molluscan neuropeptide, SCPB, increases the responsiveness of the feeding motor program of Limax maximus

Small cardioactive peptide B (SCPB) has an excitatory effect on both buccal neurons and musculature in numerous molluscan species. The present study reports the effects of SCPB on the activity of specified buccal neurons and the expression of the feeding motor program of the terrestrial slug, Limax maximus. Superfusion of an isolated CNS preparation with 10(-6)M SCPB results in a 3-4-fold increase in the burst frequency of the fast salivary burster neuron (FSB), while having no effect on the activity of another endogenous burster, the bilateral salivary neuron (BSN). The response of the FSB to SCPB is dose dependent, with a threshold concentration of 2 X 10(-8)M. The response of the FSB to SCPB showed no indication of desensitization, even after long-term exposure (20 min). The feeding motor program (FMP) in Limax is a discrete pattern of cyclical motor activity that can be initiated by lip nerve stimulation. In the presence of SCPB a previously subthreshold stimulus can initiate the full FMP. The pattern of the FMP, once initiated, appears unaffected by SCPB. Thus it is the responsiveness of the initiation process that is enhanced by SCPB. Histochemical studies revealed a number of buccal neuron somata and fibers that stain for SCPB-like immunoreactive material (SLIM).

Acetylcholine activates cerebral interneurons and feeding motor program in Limax maximus

The cellular and network effects of acetylcholine (ACh) on the control system for feeding in Limax maximus were measured by intracellular recordings from feeding command-like interneurons and whole nerve recordings from buccal ganglion motor nerve roots that normally innervate the ingestive feeding muscles. The buccal ganglion motor nerve root discharge pattern that causes rhythmic feeding movements, termed the feeding motor program (FMP), was elicited either by attractive taste solutions applied to the lip chemoreceptors or by ACh applied to the cerebral ganglia. The ability of exogenous ACh applied to the cerebral ganglia to trigger FMP was blocked by the cholinergic antagonists curare and atropine. If the strength of the lip-applied taste stimulus was in the range of 1-2 times threshold, cerebral application of the cholinergic antagonists blocked or greatly decreased the ability of lip-applied taste solutions to trigger FMP (5 of 8 trials). The cerebral feeding interneurons, some of which activate FMP when stimulated intracellularly, are excited by small pulses of ACh applied directly to the cell body from an ACh-filled micropipette. A pulse of ACh that activates several of the feeding interneurons simultaneously triggers FMP. The data suggest that under certain stimulus conditions an obligatory set of cholinergic synapses onto the feedininterneurons must be activated for taste inputs to trigger ingestion. The determination of ACh's action within the feeding control system is necessary for understanding how enhanced cholinergic transmission leads to prolonged associative memory retention (Sahley, et al., 1986).

In vitro expression of in vivo learning by an isolated molluscan CNS

The associative learning functions of the central nervous system of Limax maximus have been established with behavioral experiments on intact animals (Sahley et al., J. comp. Physiol., 144 (1981) 1) and physiological experiments on the isolated CNS (Culligan and Gelperin, Brain Research, 266 (1983) 319). We now report experiments in which memory states were established by behavioral training and read out by a test procedure applied to the isolated brains derived from the trained animals. With a success rate of one-half we found that isolated lip-brain preparations derived from previously conditioned slugs express in vitro the learning acquired in vivo.

One-trial associative learning by an isolated molluscan CNS: use of different chemoreceptors for training and testing

An isolated preparation of the lips, cerebral ganglia and buccal ganglia of the terrestrial slug, Limax maximus, can display one-trial associative learning. The lip-brain preparation can learn to suppress feeding motor program responses to lip stimulation with a standard food extract after a single pairing of food extract and quinidine stimulation to the isolated lips. Learning can occur with training stimuli applied to one lip and testing stimuli applied to the opposite, "naive' lip. Learning can also occur if the food extract and quinidine are applied during training to opposite lips. These results indicate that the synaptic alteration due to learning occurs in the CNS, not at the sensory periphery.

[3H]-2-deoxyglucose autoradiography in a molluscan nervous system

We have used [3H]2-deoxyglucose autoradiography to correlate the labeling of individual neurons with electrical activity within the central nervous system of a terrestrial mollusc, Limax maximus. In an electrically quiescent control preparation where a single neuron is impaled with a glass microelectrode but not stimulated, several somata are uniformly labeled at 3-5 times background. In preparations where a single cell is impaled and stimulated, one or more somata are heavily labeled with [3H]2-deoxyglucose at 10-50 times tissue background. This technique may be useful for surveying metabolically active neurons during spontaneous and driven electrical activity.

Localization of[3H]-2-deoxyglucose in single molluscan neurones

The glucose analogue 2-deoxyglucose (2-DG) can be used quantitatively to measure metabolic activity and is widely used qualitatively for mapping functional activity in the brain. The resolution (meaning the full width at half maximum of the grain density distribution around a line source) of the technique using [14C]-2-DG and X-ray film is limited to about 100 micrometers. Attempts have been made to improve the resolution using [3H]-2-DG (ref. 6) and cellular resolution has been achieved in the goldfish retina and with cultured mouse neurones. An anatomical technique for mapping the metabolic activity of individual neurones would be useful for studying invertebrate central nervous systems, which are relatively simple and stereotyped compared to vertebrate brains. The [3H]-2-DG technique was applied to an invertebrate in a study of the Drosophila visual system, though without cellular resolution. We present here modifications of the [3H]-2-DG technique to demonstrate localization of 2-DG in single neurones of Limax maximus, a gastropod mollusc, with a resolution of less than 1 micrometer.

Rapid taste-aversion learning by an isolated molluscan central nervous system

The isolated lips and nervous system of the terrestrial slug Limax maximus will produce some of the feeding behavior of the intact animal; i.e., they generate the rhythmic neural activity characteristic of ingestion in response to food extracts applied to the lips. This preparation will respond to a variety of food extracts that elicit feeding in the whole animal. This provides the opportunity for aversive conditioning experiments involving taste discrimination. Pairing lip chemostimulation by attractive food extracts with lip chemostimulation by using bitte plant secondary substances can cause the isolatd brain to selectively suppress its neural response to one food extract while remaining responsive to another. Such isolated brains can learn after one or two trials and retain the learning for more than 8 hr.

Synaptic organization of expansion motoneurons of Navanax inermis

The opisthobranch mollusc, Navanax, feeds by rapid pharyngeal expansion that sucks in prey followed by peristaltic swallowing that moves prey into the esophagus. Several identifiable neurons on the ventral surface of the buccal ganglia control radial musculature within the pharyngeal wall, contraction of which leads to pharyngeal expansion. These are considered expansion motoneurons because their axons run into the muscle and twitches and EMGs occur one for one with action potentials. The motoneurons are electrotonically coupled. Electrotonic PSPs, the components of spread associated with impulses, can summate with subthreshold DC depolarizations to yield synchronous impulses in coupled cells. During a train of responses the later electrotonic PSPs can be facilitated because of increase in amplitude and duration of the presynaptic impulses. Expansion motoneurons are synaptically connected by two apparently interneuronal pathways: a low threshold pathway activated by subthreshold depolarization of the two largest expansion motoneurons (the G-cells) that inhibits the entire population, and a high threshold pathway that is activated by a train of G-cell impulses and produces largely excitatory PSPs in the smaller expansion motoneurons and an EPSP--IPSP sequence in the G-cells. Coupling among expansion motoneurons can be abolished by chemical inhibitory synaptic inputs that are activated by electrical stimulation of the pharyngeal nerve or tactile stimulation of the pharyngeal wall. This uncoupling phenomenon can be explained by a simple equivalent circuit in which inhibitory synapses along the coupling pathway short circuit electrotonic spread. Uncoupling can outlast the evoking stimulus by several seconds. During uncoupling the smaller expansion motoneurones can fire independently while the G-cell is inhibited, and impulses still propagate from somata to the periphery. The expansion motoneuron population receives excitatory input from the mechanoreceptors in protractor muscles. Mechanical stimulation of the pharyngeal wall activates primary sensory neurons in the buccal ganglia that fire during excitation and during inhibition and uncoupling of expansion motoneurons.