Control of feeding rhythm in the terrestrial slug Incilaria bilineata.

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

Activation and reconfiguration of fictive feeding by the octopamine-containing modulatory OC interneurons in the snail Lymnaea

We describe the role of the octopamine-containing OC interneurons in the buccal feeding system of Lymnaea stagnalis. OC neurons are swallowing phase interneurons receiving inhibitory inputs in the N1 and N2 phases, and excitatory inputs in the N3 phase of fictive feeding. Although the OC neurons do not always fire during feeding, the feeding rate is significantly (P < 0.001) higher when both SO and OC fire in each cycle than when only the SO fires. In 28% of silent preparations, a single stimulation of an OC interneuron evokes the feeding pattern. Repetitive stimulation of the OC interneuron increases the proportion of responsive preparations to 41%. The OC interneuron not only changes both the feeding rate and reconfigures the pattern. Depolarization of the OC interneurons increases the feeding rate and removes the B3 motor neuron from the firing sequence. Hyperpolarization slows it down (increasing the duration of N1 and N3 phases) and recruits the B3 motor neuron. OC interneurons form synaptic connections onto buccal motor neurons and interneurons but not onto the cerebral (cerebral giant cell) modulatory neurons. OC interneurons are electrically coupled to all N3 phase (B4, B4Cl, B8) feeding motor neurons. They form symmetrical connections with the N3p interneurons having dual electrical (excitatory) and chemical (inhibitory) components. OC interneurons evoke biphasic synaptic inputs on the protraction phase interneurons (SO, N1L, N1M), with a short inhibition followed by a longer lasting depolarization. N2d interneurons are hyperpolarized, while N2v interneurons are slowly depolarized and often fire a burst after OC stimulation. Most motor neurons also receive synaptic responses from the OC interneurons. Although OC and N3p interneurons are both swallowing phase interneurons, their synaptic contacts onto follower neurons are usually different (e.g., the B3 motor neurons are inhibited by OC, but excited by N3p interneurons). Repetitive stimulation of OC interneuron facilitates the excitatory component of the biphasic responses evoked on the SO, N1L, and N1M interneurons, but neither the N2 nor the N3 phase interneurons display a similar longer-lasting excitatory effect. OC interneurons are inhibited by all the buccal feeding interneurons, but excited by the serotonergic modulatory CGC neurons. We conclude that OC interneurons are a new kind of swallowing phase interneurons. Their connections with the buccal feeding interneurons can account for their modulatory effects on the feeding rhythm. As they contain octopamine, this is the first example in Lymnaea that monoaminergic modulation and reconfiguration are provided by an intrinsic member of the buccal feeding network.     

Phase-Dependent Coordination of Two Motor Programs in the Buccal Ganglion of a Pteropod Mollusk

Rhythmic activity in two independent structures of the digestive apparatus of Clione limacina – the radula and the hooks – is coordinated by neural networks in the buccal ganglion during feeding behavior. Optical recording of neuron activity in the buccal ganglion, which allows simultaneous recording of large numbers of neurons, showed that the activity of all neurons producing volley discharges can be assigned to only two phases of a single rhythm. Instead of the four theoretically possible phases of rhythmic neural activity, all experiments yielded recordings of biphasic activity, even in conditions of electrical stimululation of the cerebrobuccal connectives, which triggers rhythmic movements of this apparatus in preparations. These data demonstrate the phase-dependent coordination of two independent rhythmic food-procuring movements in Clione.     

Modification of the effects of glutamate by nitric oxide (NO) in a pattern-generating network

Previous studies have shown that nitric oxide (NO) transforms the responses of various neurons to glutamate, though it remained unclear whether this mechanism is involved in the formation of behavior. We therefore studied the buccal generator of the feeding rhythm of the mollusk Lymnaea stagnalis (pond snail). In this organism, glutamate and NO are synthesized by defined neurons; glutamate is the neurotransmitter for the second phase of the standard triphasic feeding rhythm. Motoneuron B4 was used for monitoring. Studies using isolated CNS preparations showed that in some cases glutamate evoked hyperpolarization of B4 and terminated rhythmic network activity (n = 17; group 1), while in other cases glutamate evoked depolarization of B4 and activation of a non-standard biphasic rhythm (n = 12; group 2). In group 1, the NO donor nitroprusside lifted the inhibitory effect of glutamate (n = 13), with transformation into an excitatory effect in nine cases. In group 2, the NO acceptor PTIO transformed the excitatory effect of glutamate into an inhibitory effect (n = 7). These results provide evidence that: 1) the responses of the central generator of the buccal motor rhythm to glutamate depend on the NO level, and 2) this regulatory mechanism can modify feeding behavior. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 93, No. 3, pp. 236–247, March, 2007.     

Crustacean cardioactive peptide (CCAP)-related molluscan peptides (M-CCAPs) are potential extrinsic modulators of the buccal feeding network in the pond snail Lymnaea stagnalis

We combine electrophysiological and immunocytochemical analyses in the snail Lymnaea stagnalis of M-CCAP1 and M-CCAP2, two molluscan peptides with structure similar to crustacean cardioactive peptide CCAP, originally isolated from the snail Helix pomatia. Both M-CCAP peptides (M-CCAP1 and M-CCAP2, 1 microM) had an excitatory effect, depolarizing all the identified neurons of the buccal feeding network (including motoneurons: B1, B2, B4 and modulatory interneurons SO, OC: 62 neurons in 33 preparations). Additionally, in 67% of preparations, rhythmic activity (fictive feeding) was recorded with a mean rate of 7 cycles/min. No significant difference in the proportion of preparations showing fictive feeding or mean feeding rate was found between M-CCAP1 and M-CCAP2. The extrinsic feeding modulator, the serotonergic CGC neuron, responds by increase of the spontaneous activity after M-CCAP application (9 of 18 preparations). Crustacean CCAP (1 microM) evokes a slight membrane depolarization in 3 out of 8 preparations but never evokes fictive feeding. Immunostaining revealed no cell bodies in the buccal ganglia, but a dense network of CCAP immunopositive fibers arborizing in the buccal neuropil. Many of these fibers originate from a symmetrical pair of CCAP-immunoreactive cerebro-buccal interneurons, which are the most likely candidates for extrinsic modulatory interneurons in the buccal feeding network. Our data are the first results suggesting that M-CCAP-peptides exist as effective modulators in mollusc.     

Control of feeding movements in the pteropod mollusc,Clione limacina

Summary (1) The buccal apparatus of the pteropod marine mollusc Clione limacina, isolated together with buccal ganglia, could perform rhythmic feeding movements. Movements of the radula and the hooks (which the Clione inserts into the body of its prey) as well as the electroneurogram of the radular nerve were recorded. Usually one could observe rhythmic radula movements alone, while the hooks were motionless. But sometimes the hooks also performed rhythmic movements which were more or less synchronous with those of the radula. The radula movement cycle consisted of the protraction and the retraction phases, which were occasionally followed by a quiescent phase. Corresponding to each radula movement was a burst of activity in the radular nerve, consisting of the protractor and the retractor components. (2) Isolated buccal ganglia were capable of feeding rhythm generation. Most of the buccal neurons exhibited rhythmic activity correlating with the activity in the radular nerve. According to the phase of activity in the feeding cycle, rhythmic neurons were divided into two groups — the protractor and the retractor ones. The neurons within each of the groups were electrically coupled with each other. The protractor and retractor neurons inhibited each other. (3) Protractor and retractor neurons were extracted from buccal ganglia by means of a microelectrode. Many isolated cells generated slow oscillations of membrane potential and bursts of spikes, the pattern of this activity being similar to that before isolation. (4) A model of the feeding rhythm generator is discussed. It consists of two (protractor and retractor) groups of neurons with mutual inhibitory connections, neurons of each group being endogenous bursters.     

A newly identified buccal interneuron initiates and modulates feeding motor programs in aplysia

Despite considerable progress in characterizing the feeding central pattern generator (CPG) in Aplysia, the full complement of neurons that generate feeding motor programs has not yet been identified. The distribution of neuropeptide-containing neurons in the buccal and cerebral ganglia can be used as a tool to identify additional elements of the feeding circuitry by providing distinctions between otherwise morphologically indistinct neurons. For example, our recent study revealed a unique and potentially interesting unpaired PRQFVamide (PRQFVa)-containing neuron in the buccal ganglion. In this study, we describe the morphological and electrophysiological characterization of this novel neuron, which we designate as B50. We found that activation of B50 is capable of producing organized rhythmic output of the feeding CPG. The motor programs elicited by B50 exhibit some similarities as well as differences to motor programs elicited by the command-like cerebral-to-buccal interneuron CBI-2. In addition to activating the feeding CPG, B50 may act as a program modulator.     

Control of feeding movements in the freshwater snail Planorbis corneus

Isolated buccal ganglia of Planorbis corneus are capable of generating a feeding rhythm. In the present work, "rhythmic" neurons of different groups (see Arshavsky et al. 1988a) have been extracted, by means of an intracellular microelectrode, from the buccal ganglia. (1) After extraction, efferent neurons of groups 3, 5, 7, 9 and most group 4 neurons generated repeated spikes at a frequency controlled by a polarizing current. Any periodic oscillations, similar to those during feeding rhythm generation, were absent in these isolated neurons. It is concluded, therefore, that these neurons are "followers", that is, their rhythmic activity before extraction is determined by synaptic inputs from other neurons of the ganglia. (2) Isolated interneurons of groups 1 and 2 generated slow periodic oscillations similar to those observed in these neurons before their extraction. Subgroup 1e neurons generated smoothly growing depolarization accompanied by increasing spike activity; this depolarization was periodically interrupted by abrupt hyperpolarization, after which a new cycle started. Subgroup 1d neurons periodically generated short series of spikes. Group 2 neurons periodically generated a rectangular wave of depolarization with spike-like oscillations on its top. These results suggest that feeding rhythm generation in Planorbis is based on the endogenous rhythmic activity of group 1 and 2 neurons. (3) A pulse of hyperpolarizing current injected into an isolated neuron of subgroup 1e stopped the growth of depolarization in the neuron and reinitiated the process. This property as well as the character of the synaptic interactions of the interneurons (group 1 neurons excite those of group 2, while those of group 2 inhibit group 1 neurons; Arshavsky et al. 1988b) determine the alternating activity of groups 1 and 2.     

Identified higher-order neurons controlling the feeding motor program of helisoma

A bilaterally symmetrical pair of neurons in the anterior region of the cerebral ganglia of the snail Helisoma trivolvis were found to have excitatory input to the feeding motor program contained in the buccal ganglia. Intracellular microelectrodes were used to stimulate and record from these cells while the motor output of the buccal ganglia was monitored with a combination of intracellular and extracellular recordings. Experimentally evoked tonic activity in an individual cerebral cell could initiate and maintain the patterned motor output from buccal ganglia, characteristic of the activity underlying buccal mass feeding movements. The rate of buccal motor output could be modified directly by varying the firing frequency of the cerebral cell. Cobaltous chloride backfills of cerebrobuccal connectives revealed that these higher-order neurons were the only large cells in the anterior portion of the ganglia to send processes into the connectives. Furthermore, they are the only cells in this region to fluoresce when processed with a sucrose-phosphate-glyoxylic acid solution, indicating that they contain an indoleamine, probably serotonin. Application of low concentrations of serotonin to isolated buccal ganglia or buccal ganglia-buccal mass preparations mimics the effects of the cerebral cells' activity on the buccal motor output, implying that serotonin is a putative transmitter for these cerebral cells.     

Interaction of Neurons at the Level of Cell Bodies in the Snail CNS. Heterogeneity of the Neuroactive Environment

Experiments on the CNS of snail Lymnaea stagnalis in which a cell isolated from the serotonin cluster PeA was used as a mobile sensor neuron demonstrated the presence of neuroactive factors at the surface of the cellular “cortex” of the pedal ganglion. Apart from the previously known factor serotonin, effective concentrations of a factor suppressing the electrical activity of PeA were found at this site, along with a depolarizing factor which, unlike serotonin, narrowed PeA action potentials. The ability of these factors to control the electrical activity of the sensor neuron demonstrates the possible involvement of chemical agents in the intercellular space of the “cortex” in neuronal signaling.__________Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 90, No. 3, pp. 345–350, March, 2004.     

C329X in KRIT1 is a founder mutation among CCM patients in Sardinia

Cerebral cavernous malformations (CCMs) are CNS vascular anomalies associated with seizures, headaches and hemorrhagic strokes and represent 10–20% of cerebral lesions. CCM is present in 0.1–0.5 of the population. This disorder most often occurs sporadically but may also be familial. Familial cases are inherited as a dominant trait with incomplete penetrance and are estimated to account for KRIT1 10–40% of the patients. The identification of the genes involved in such disorders allows to characterize carriers of the mutations without clear symptoms. The first gene involved in CCM1 is KRIT1. In addition to two other genes have been described: MGC4607 (CCM2) and PDCD10 (CCM3). We selected 13 patients belonging to seven Sardinian families on the basis of clinical symptoms and Magnetic Resonance results. In MGC4607 gene an undescribed exon five deletion likely producing a truncated protein was identified in one family. In two patients with clear phenotype and in three asymptomatic relatives a 4 bp deletion in exon 9 of KRIT1 gene, leading to a premature stop codon, was detected. A unique nonsense mutation (C329X) has been found in seven patients and two asymptomatic subjects belonging to four unrelated families. Haplotype analysis revealed a common origin of this mutation. These data suggest a “founder effect” in Sardinia for the C329X mutation, similar to other mutations described in different populations.     

Interactions of the slow oscillator interneuron with feeding pattern-generating interneurons in Lymnaea stagnalis

We have used intracellular recording from groups of interneurons in the feeding system of the pond snail, Lymnaea stagnalis, to examine the connections of a modulatory interneuron, the slow oscillator (SO), with the network of pattern-generating interneurons (N1, N2, and N3). The SO is an interneuron whose axon branches solely within the buccal ganglia. There is only one such cell in each snail. In half the snails the cell body is in the right buccal ganglion and in the other half in the left buccal ganglion. Stimulation of either the SO or one of the N1 pattern-generating interneurons elicits the feeding rhythm, but of all the buccal neurons, only the SO can drive the feeding rhythm at the frequency seen in the intact snail. The SO makes reciprocal excitatory synapses with the N1 interneurons that drive the protraction of the radula. This ensures strong activation of the feeding system. The SO inhibits the N2 interneurons. Postsynaptic potentials evoked by stimulation of the SO facilitate without spike broadening in the SO. The SO is strongly inhibited by N2 and N3 interneurons, which are active during the retraction phase. This gates any excitatory inputs to the SO, probably preventing protraction of the radula while retraction is underway. The results support the idea of a single interneuron capable of driving a hierarchically organized motor system.     

Pattern generation in the buccal system of freely behaving Lymnaea stagnalis

Central pattern generators (CPGs) are neuronal circuits that drive active repeated movements such as walking or swimming. Although CPGs are, by definition, active in isolated central nervous systems, sensory input is thought play an important role in adjusting the output of the CPGs to meet specific behavioral requirements of intact animals. We investigated, in freely behaving snails (Lymnaea stagnalis), how the buccal CPG is used during two different behaviors, feeding and egg laying. Analysis of the relationship between unit activity recorded from buccal nerves and the movements of the buccal mass showed that electrical activity in laterobuccal/ventrobuccal (LB/VB) nerves was as predicted from in vitro data, but electrical activity in the posterior jugalis nerve was not. Autodensity and interval histograms showed that during feeding the CPG produces a much stronger rhythm than during egg laying. The phase relationship between electrical activity and buccal movement changed little between the two behaviors. Fitting the spike trains recorded during the two behaviors with a simple model revealed differences in the patterns of electrical activity produced by the buccal system during the two behaviors investigated. During egg laying the bursts contained less spikes, and the number of spikes per burst was significantly more variable than during feeding. The time between two bursts of in a spike train was longer during egg laying than during feeding. The data show what the qualitative and quantitative differences are between two motor patterns produced by the buccal system of freely behaving Lymnaea stagnalis.     

Coordinated excitatory effect of GABAergic interneurons on three feeding motor programs in the mollusk Clione limacina

Coordination between different motor centers is essential for the orderly production of all complex behaviors. Understanding the mechanisms of such coordination during feeding behavior in the carnivorous mollusk Clione limacina is the main goal of the current study. A bilaterally symmetrical interneuron identified in the cerebral ganglia and designated Cr-BM neuron produced coordinated activation of neural networks controlling three main feeding structures: prey capture appendages called buccal cones, chitinous hooks used for prey extraction from the shell, and the toothed radula. The Cr-BM neuron produced strong excitatory inputs to motoneurons controlling buccal cone protraction. It also induced a prominent activation of the neural networks controlling radula and hook rhythmic movements. In addition to the overall activation, Cr-BM neuron synaptic inputs to individual motoneurons coordinated their activity in a phase-dependent manner. The Cr-BM neuron produced depolarizing inputs to the radula protractor and hook retractor motoneurons, which are active in one phase, and hyperpolarizing inputs to the radula retractor and hook protractor motoneurons, which are active in the opposite phase. The Cr-BM neuron used GABA as its neurotransmitter. It was found to be GABA-immunoreactive in the double-labeling experiments. Exogenous GABA mimicked the effects produced by Cr-BM neuron on the postsynaptic neurons. The GABA antagonists bicuculline and picrotoxin blocked Cr-BM neuron-induced PSPs. The prominent coordinating effect produced by the Cr-BM neuron on the neural networks controlling three major elements of the feeding behavior in Clione suggests that this interneuron is an important part of the higher-order system for the feeding behavior.     

Actions of a pair of identified cerebral-buccal interneurons (CBI-8/9) in Aplysia that contain the peptide myomodulin

A combination of biocytin back-fills of the cerebral-buccal connectives and immunocytochemistry of the cerebral ganglion demonstrated that of the 13 bilateral pairs of cerebral-buccal interneurons in the cerebral ganglion, a subpopulation of 3 are immunopositive for the peptide myomodulin. The present paper describes the properties of two of these cells, which we have termed CBI-8 and CBI-9. CBI-8 and CBI-9 were found to be dye coupled and electrically coupled. The cells have virtually identical properties, and consequently we consider them to be "twin" pairs and refer to them as CBI-8/9. CBI-8/9 were identified by electrophysiological criteria and then labeled with dye. Labeled cells were found to be immunopositive for myomodulin, and, using high pressure liquid chromatography, the cells were shown to contain authentic myomodulin. CBI-8/9 were found to receive synaptic input after mechanical stimulation of the tentacles. They also received excitatory input from C-PR, a neuron involved in neck lengthening, and received a slow inhibitory input from CC5, a cell involved in neck shortening, suggesting that CBI-8/9 may be active during forward movements of the head or buccal mass. Firing of CBI-8 or CBI-9 resulted in the activation of a relatively small number of buccal neurons as evidenced by extracellular recordings from buccal nerves. Firing also produced local movements of the buccal mass, in particular a strong contraction of the I7 muscle, which mediates radula opening. CBI-8/9 were found to produce a slow depolarization and rhythmic activity of B48, the motor neuron for the I7 muscle. The data provide continuing evidence that the small population of cerebral buccal interneurons is composed of neurons that are highly diverse in their functional roles. CBI-8/9 may function as a type of premotor neuron, or perhaps as a peptidergic modulatory neuron, the functions of which are dependent on the coactivity of other neurons.     

Cerebral CBM1 neuron contributes to synaptic modulation appearing during rejection of seaweed in Aplysia kurodai

The Japanese species Aplysia kurodai feeds well on Ulva but rejects Gelidium with distinctive rhythmic patterned movements of the jaws and radula. We have previously shown that the patterned jaw movements during the rejection of Gelidium might be caused by long-lasting suppression of the monosynaptic transmission from the multiaction MA neurons to the jaw-closing (JC) motor neurons in the buccal ganglia and that the modulation might be directly produced by some cerebral neurons. In the present paper, we have identified a pair of catecholaminergic neurons (CBM1) in bilateral cerebral M clusters. The CBM1, probably equivalent to CBI-1 in A. californica, simultaneously produced monosynaptic excitatory postsynaptic potentials (EPSPs) in the MA and JC neurons. Firing of the CBM1 reduced the size of the inhibitory postsynaptic currents (IPSCs) in the JC neuron, evoked by the MA spikes, for >100 s. Moreover, the application of dopamine mimicked the CBM1 modulatory effects and pretreatment with a D1 antagonist, SCH23390, blocked the modulatory effects induced by dopamine. It could also largely block the modulatory effects induced by the CBM1 firing. These results suggest that the CBM1 may directly modulate the synaptic transmission by releasing dopamine. Moreover, we explored the CBM1 spike activity induced by taste stimulation of the animal lips with seaweed extracts by the use of calcium imaging. The calcium-sensitive dye, Calcium Green-1, was iontophoretically loaded into a cell body of the CBM1 using a microelectrode. Application of either Ulva or Gelidium extract to the lips increased the fluorescence intensity, but the Gelidium extract always induced a larger change in fluorescence compared with the Ulva extract, although the solution used induced the maximum spike responses of the CBM1 for each of the seaweed extracts. When the firing frequency of the CBM1 activity after taste stimulation was estimated, the Gelidium extract induced a spike activity of ~30 spikes/s while the Ulva extract induced an activity of ~20 spikes/s, consistent with the effective firing frequency (>25 spikes/s) for the synaptic modulation. These results suggest that the CBM1 may be one of the cerebral neurons contributing to the modulation of the basic feeding circuits for rejection induced by the taste of seaweeds such as Gelidium.     

Gastric and pyloric motor pattern control by a modulatory projection neuron in the intact crab Cancer pagurus

Neuronal release of modulatory substances provides motor pattern generating circuits with a high degree of flexibility. In vitro studies have characterized the actions of modulatory projection neurons in great detail in the stomatogastric nervous system, a model system for neuromodulatory influences on central pattern generators. Less is known about the activities and actions of modulatory neurons in fully functional and richly modulated network settings, i.e., in intact animals. It is also unknown whether their activities contribute to the motor patterns in different behavioral conditions. Here, we show for the first time the activity and effects of the well-characterized modulatory projection neuron 1 (MCN1) in vivo and compare them to in vitro conditions. MCN1 was always spontaneously active, typically in a rhythmic fashion with its firing being interrupted by ascending inhibitions from the pyloric motor circuit. Its activity contributed to pyloric motor activity, because 1) the cycle period of the motor pattern correlated with MCN1 firing frequency and 2) stimulating MCN1 shortened the cycle period while 3) lesioning of the MCN1 axon reduced motor activity. In addition, gastric mill motor activity was elicited for the duration of the stimulation. Chemosensory stimulation of the antennae moved MCN1 away from baseline activity by increasing its firing frequency. Following this increase, a gastric mill rhythm was elicited and the pyloric cycle period decreased. Lesioning the MCN1 axon prevented these effects. Thus modulatory projection neurons such as MCN1 can control the motor output in vivo, and they participate in the processing of exteroceptive sensory information in behaviorally relevant conditions.     

Electrophysiological and behavioral analysis of lip touch as a component of the food stimulus in the snail Lymnaea

Electrophysiological and video recording methods were used to investigate the function of lip touch in feeding ingestion behavior of the pond snail Lymnaea stagnalis. Although this stimulus was used successfully as a conditioning stimulus (CS) in appetitive learning experiments, the detailed role of lip touch as a component of the sensory stimulus provided by food in unconditioned feeding behavior was never ascertained. Synaptic responses to lip touch in identified feeding motoneurons, central pattern generator interneurons, and modulatory interneurons were recorded by intracellular electrodes in a semi-intact preparation. We showed that touch evoked a complex but characteristic sequence of synaptic inputs on each neuron type. Touch never simply activated feeding cycles but provided different types of synaptic input, determined by the feeding phase in which the neuron was normally active in the rhythmic feeding cycle. The tactile stimulus evoked mainly inhibitory synaptic inputs in protraction-phase neurons and excitation in rasp-phase neurons. Swallow-phase neurons were also excited after some delay, suggesting that touch first reinforces the rasp then swallow phase. Video analysis of freely feeding animals demonstrated that during normal ingestion of a solid food flake the food is drawn across the lips throughout the rasp phase and swallow phase and therefore provides a tactile stimulus during both these retraction phases of the feeding cycle. The tactile component of the food stimulus is strongest during the rasp phase when the lips are actively pressed onto the substrate that is being moved across them by the radula. By using a semi-intact preparation we demonstrated that application of touch to the lips during the rasp phase of a sucrose-driven fictive feeding rhythm increases both the regularity and frequency of rasp-phase motoneuron firing compared with sucrose applied alone.     

Control of feeding movements in the freshwater snail Planorbis corneus

Summary (1) Neurons of different groups (for group classification, see Arshavsky et al. 1988a) have been polarized through an intracellular recording microelectrode in Planorbis corneus buccal ganglia during feeding rhythm generation. Group 1 neurons, active in the quiescence (Q) and in the protractor (P) phases of the cycle, and also group 2 and 4 neurons, active in the retractor (R) phase, have proved to be influential, i.e., altering the rhythm generator operation. (2) Injection of a depolarizing current into group 1 neurons caused an increase of the rate of depolarization that neurons of this group exhibit in the Q- and P-phases of the feeding cycle. As a result, Q-phase shortened, the P-phase became longer, and the feeding rhythm accelerated. Opposite effects occured when a hyperpolarizing current was injected into group 1 neurons. In some of the experiments, the hyperpolarization of group 1 neurons resulted in cessation of both their activity and the activity of all other protractor neurons. As a result, the P-phase of the cycle disappeared, i.e., the rhythm generator transited from A mode of operation to B mode. (3) With hyperpolarization of individual group 2 or 4 neurons, excitation of the R-phase neurons was delayed and the feeding rhythm phase shifted. This delay was accompanied by the enhanced activity of protractor neurons. (4) A generator model is considered in which two groups (1 and 2) of endogeneously active neurons are coordinated by the excitatory effect of group 1 on group 2 and the inhibitory action of group 2 on group 1. (5) Evidence is given that the different modes of rhythm generator operation (A, B and C, see Arshavsky et al. 1988a) are determined by different tonic inflow to group 1 neurons.     

Esophageal mechanoreceptors in the feeding system of the pond snail, Lymnaea stagnalis

1. We identify esophageal mechanoreceptor (OM) neurons of Lymnaea with cell bodies in the buccal ganglia and axons that branch repeatedly to terminate in the esophageal wall. 2. The OM cells respond phasically to gut distension. Experiments with a high magnesium/low calcium solution suggest that the OM neurons are primary mechanoreceptors. 3. In the isolated CNS preparation, the OM cells receive little synaptic input during the feeding cycle. 4. The OM cells excite the motoneurons active in the rasp phase of the feeding cycle. 5. The OM cells inhibit each of the identified pattern-generating and modulatory interneurons in the buccal ganglia. Experiments with a saline rich in magnesium and calcium suggest that the connections are monosynaptic. 6. Stimulation of a single OM cell to fire at 5-15 Hz is sufficient to terminate the feeding rhythm in the isolated CNS preparation. 7. We conclude that these neurons play a role in terminating feeding behavior.