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.     

Gustatory feedback affects feeding related motor pattern generation in starved 3rd instar larvae of Calliphora vicina

Gustatory feedback allows animals to distinguish between edible and noxious food and adapts centrally generated feeding motor patterns to environmental demands. In reduced preparations obtained from starved Calliphora larvae, putatively appetitive (ethanol), aversive (sodium acetate) and neutral (glucose) gustatory stimuli were applied to the anterior sense organs. The resulting sensory response was recorded from the maxillary- and antennal nerves. All three stimuli increased the neural activity in both nerves. Recordings obtained from the antennal nerve to monitor the activation pattern of the cibarial dilator muscles, demonstrated an effect of gustatory input on the central pattern generator for feeding. Ethanol consistently enhanced the rhythmic activity of the CDM motor neurons either by speeding up the rhythm or by increasing the burst duration. Ethanol also had an enhancing effect on the motor patterns of a protractor muscle which moves the cephalopharyngeal skeleton relative to the body. Sodium acetate showed a state dependent effect: in preparations without spontaneous CDM activity it initiated rhythmic motor patterns, while an ongoing CDM rhythm was inhibited. Surprisingly glucose had an enhancing effect which was less pronounced than that of ethanol. Gustatory feedback therefore can modify and adapt the motor output of the multifunctional central pattern generator for feeding.     

The Role of the Escape Swim Motor Network in the Organization of Behavioral Hierarchy and Arousal in Pleurobranchaea

The escape swimming pattern generator of the notaspid opisthobranchPleurobranchaea drives a high threshold, override behavior.The pattern generator is integrated with neural networks ofother behaviors so as to coordinate unitary behavioral expressionand to promote general behavioral arousal. These functions areseparately produced by different swim network elements. Oneset of swim premotor neurons, the A1/A10 ensemble, A3 and I_VS,generate the swim pattern and, through corollary activity, suppresspotentially conflicting feeding behavior by exerting broad inhibitionat major feeding network interneurons. A second set of swimneurons, the serotonergic As1–4 neurons, provides intrinsicneuromodulatory excitation to the swim pattern generator thatsustains the escape swim episode through multiple cycles. TheAs1–4 also provide neuromodulatory excitation to importantmodulatory, serotonergic cells in the feeding motor networkand locomotor network, and may have a general regulatory rolein the distributed serotonergic arousal network of the mollusk.The As1–4 appear to be also necessary to both avoidanceand orienting turning, and are therefore likely to be critical,multi-functional components upon which much of the organizationof the animal's behavior rests.     

Prey capture phase of feeding behavior in the pteropod mollusc,clione limacina: neuronal mechanisms

The prey capture phase of feeding behavior in the pteropod mollusc Clione limacina consists of an explosive extrusion of buccal cones, specialized structures which are used to catch the prey, and acceleration of swimming with frequent turning and looping produced by tail bend. A system of neurons which control different components of prey capture behavior in Clione has been identified in the cerebral ganglia. Cerebral B and L neurons produce retraction of buccal cones and tightening of the lips over them — their spontaneous spike activities maintain buccal cones in the withdrawn position. Cerebral A neurons inhibit B and L cells and produce opening of the lips and extrusion of buccal cones. A pair of cerebral interneurons C-BM activates cerebral A neurons and synchronously initiates the feeding motor program in the buccal ganglia. Cerebral T neurons initiate acceleration of swimming and produce tail bending which underlies turning and looping during the prey capture. Both tactile and chemical inputs from the prey produce activation of cerebral A and T neurons. This reaction appears to be specific, since objects other than alive Limacina or Limacina juice do not initiate activities of A and T neurons.     

Effect of putative neuromodulators on rhythmic buccal motor output in Lymnaea stagnalis

The effects of a variety of neuromodulator substances on rhythmic motor output and activity in neurons in the feeding circuitry of Lymnaea stagnalis were examined. Each neuromodulator produced a unique combination of effects at different levels in the network: i.e., pattern-generating interneurons (N1, N2, and N3), an identified higher-order interneuron (cerebral giant cell, CGC), and buccal motoneurons. 5-Hydroxytryptamine, acetylcholine, and FMRFamide all inhibited rhythmic motor activity. However, this was achieved in different ways. Dopamine changed the nature of rhythmic activity from one in which N2 interneuronal activity was predominant ("N2 rhythm") to a feeding rhythm. Dopamine was the only substance capable of activating the feeding rhythm. Activity in the CGC was increased by 5-hydroxytryptamine, dopamine, and acetylcholine and reduced by FMRFamide. Differential responses in buccal motoneurons were also observed. The results are discussed in relation to previous work on other species and also in terms of the selection of different patterns of motor output by neuromodulators.     

Prey Capture in Actinopterygian Fishes: A Review of Suction Feeding Motor Patterns with New Evidence from an Elopomorph Fish, Megalops atlanticus

Suction feeding is recognized as the dominant mode of aquaticprey capture in fishes. While much work has been done identifyingmotor pattern variations of this behavior among diverse groupsof actinopterygian fishes, many ray-finned groups are stillnot represented. Further, the substantial amount of inherentvariation in electromyography makes much of the pioneering workof suction feeding motor patterns in several basal groups insufficientfor evolutionary comparisons. Robust evolutionary comparisonshave identified conserved qualitative traits in the order ofmuscle activation during suction feeding (jaw opening > buccalcavity expansion > jaw closing). However, quantitative traitsof suction motor patterns (i.e., burst durations and relativeonset times) have changed over evolutionary time among actinopterygianfishes. Finally, new motor pattern evidence is presented froma previously neglected group, the Elopomorpha. The results suggestthat future investigations of the muscles influencing lateralexpansion of the mouth cavity and head anatomy may provide valuablenew insights into the evolution of suction feeding motor patternsin ray-finned fishes. In addition, the evidence illustratesthe value of comprehensive EMG surveys of cranial muscle activitiesduring suction feeding behavior.     

The brain can eat: Establishing the existence of a central pattern generator for feeding in third instar larvae of Drosophila virilis and Drosophila melanogaster

To establish the existence of a central pattern generator for feeding in the larval central nervous system of two Drosophila species, the gross anatomy of feeding related muscles and their innervation is described, the motor units of the muscles identified and rhythmic motor output recorded from the isolated CNS. The cibarial dilator muscles that mediate food ingestion are innervated by the frontal nerve. Their motor pathway projects from the brain through the antennal nerves, the frontal connectives and the frontal nerve junction. The mouth hook elevator and depressor system is innervated by side branches of the maxillary nerve. The motor units of the two muscle groups differ in amplitude: the elevator is always activated by a small unit, the depressor by a large one. The dorsal protractors span the cephalopharyngeal skeleton and the body wall hence mediating an extension of the CPS. These muscles are innervated by the prothoracic accessory nerve. Rhythmic motor output produced by the isolated central nervous system can simultaneously be recorded from all three nerves. The temporal pattern of the identified motor units resembles the sequence of muscle contractions deduced from natural feeding behavior and is therefore considered as fictive feeding. Phase diagrams show an almost identical fictive feeding pattern is in both species.     

The feeding strategies of two large marine copepods

We compared the feeding behaviour of the two copepods Paraeuchaetanorvegica and Chiridius armatus, allowing them to prey on othercalanoids in small-scale laboratory experiments. Several differenceswere found. When fed either live, free-swimming or dead, non-movingprey, P.norvegica seemed unable to locate the dead prey itemswhile C.armatus foraged heavily on them. When starved, P.norvegicaincreased its feeding rate while C.armatus reduced its feedingafter an initial increase. None of the predators changed theirfeeding rates when exposed to light. Nighttime versus daytimefeeding was tested only with P.norvegcia, which seemed to possessan endogenous feeding rhythm with increased rates at night.Chiridius armatus infected with epizooic ciliates appeared tohave increased feeding rates.     

The Occurrence of Endogenous Rhythms in the Coleoptiles in Various Cereal Genera

The rate of growth of the coleoptiles was determined from photographstaken by infra-red radiation. CO₂ output was measured by meansof an infra-red gas analyser. The rhythm of CO₂ output from the coleoptile of Avena was inducedby a change from red light to darkness. It has a period of about24 hours and agrees in timing with the growth-rate rhythm previouslyrecorded. Some degree of rhythmicity in the growth-rate was found in Triticumvulgare (var. ‘Eclipse’) and in Secale cereale (var.Petkus). Very slight indications of rhythmicity were found inTriticum spelta and in Hordeum vulgare. Negative results wereobtained with Oryza sativa and with Zea mays. Where rhythmicityin the coleoptile is less strongly developed, the peaks comecloser together, the interval being about 18–20 hours.Cereals cannot be sharply separated into two groups accordingto the presence or absence of rhythmicity in the coleoptile.Of the genera examined, the most marked endogenous rhythms occurin Avena. It is doubtful if the ability of the coleoptile toexhibit an endogenous rhythm has any beneficial effect on thedevelopment of the seedling. Under normal conditions of germinationinduction of the rhythm would not occur.     

Models for unambiguousE-vector navigation in the bee

Summary The metacerebral giant (MCG) neurons of the molluskPleurobranchaea have been analyzed using a wide range of methods (cobalt staining, histochemical, biophysical and electrophysiological) on several types of preparations (isolated nervous systems, semi-intact preparations, and behaving whole-animal preparations). The MCG is serotonergic. The bilaterally-symmetrical neurons have somata in the anterior brain. Each MCG neuron sends an axon out the ipsilateral mouth nerve of the brain and also into the ipsilateral cerebrobuccal connective which descends to the buccal ganglion. The descending axon sends one or more branches out most buccal nerves.The MCG makes mono- and polysynaptic chemical excitatory and inhibitory connections with identified feeding motoneurons in the buccal ganglion. In quiescent preparations (isolated CNS or semi-intact), MCG stimulation caused coordinated eversion activity followed immediately by withdrawal activity. During an ongoing feeding rhythm (spontaneous output or induced by stimulation of the stomatogastric nerve), tonic stimulation of one or both MCG's at physiological discharge frequencies typically caused a significant increase in the frequency of the rhythm, and usually emphasized the eversion component at the expense of the withdrawal component. Phasic stimulation of one or both MCG's at physiological discharge frequencies and in normal discharge patterns (bursts; see below) accelerated and phaselocked the feeding rhythm.The MCG neurons receive synaptic feedback from identified neurons in the feeding network. Brain motoneurons are reciprocally coupled with the MCG by non-rectifying electrical synapses, while buccal ganglion neurons (the previously identified corollary discharge neurons) inhibit the MCG. Recordings from the MCG during cyclic feeding show that it discharges cyclically and that its membrane potential oscillates in phase with the feeding rhythm, presumably reflecting the above synaptic feedback. Two biophysical properties of the MCG membrane, namely anomalous rectification and postspike conductance increase, are presumed to contribute to the MCG's oscillatory activity.Chemosensory (food stimuli) and mechanosensory inputs from the oral veil excite the MCG's. In whole-animal preparations, these sensory inputs typically cause discharge in the MCG's and other descending neurons, accompanied by feeding motor output.The data collectively suggest that the MCG's ofPleurobranchaea are members of a population of neurons that normally function to command (i.e., arouse, initiate and sustain) the rhythmic feeding behavior. The demonstrated central feedback to the MCG is presumed to amplify these command functions.Supported by an NIH Postdoctoral Fellowship (1 F22 NS00511) to R.G. and NIH Research Grants NS 09050 and MH 23254 to W.J.D. We thank Kathryn H. Britton for histological assistance. We also thank Mark P. Kovac, who produced the records of Figures 8 and 18, for permission to reproduce them here.     

An Endogenous Rhythm in the Rate of Carbon Dioxide Output of Bryophyllum: I. SOME PRELIMINARY EXPERIMENTS

A techinique is described for recording automatically, withthe aid of an infrared gas analyzer, the rate CO₂ output orabsorption by plant material under controlled conditions. An examination of the rate of CO₂ output by excised leaves of16 species of succulent plants in darkness and in a CO_-2-freeatmosphere revealed clearly defined rhythms in only Bryophyllumfedtschenkoi, B. daigremontianum and B. calycinum (pinnatum). Further investigation of the rhythm in leaves of B. fedtschenkoirevealed that: (1) daylength has no effect upon the period ofthe rhythm in subsequent darkness, the phase being set at thetime the lights are extinguished; (2) normal air suppressesthe rhythm; (3) removel of the epidermis and cutting the mesophyllinto pieces 1 cm² does not effect either the phase or periodof the rhythm; (4) continuous illumination at an intensity of3,000 lux inhibits the rhythm which restarts when the lightsare extinguished. The phase of the rhythm can be set at anytime of day according to the time at which the lights are extinguished.The time which elapses between the onset of darkness and thefirst peak decreases as the length of the light treatment isincreased. The endogenuous nature of the rhythm is fully established. Theresults are compared with of other researches.     

Nucleus raphe obscurus modulates hypoglossal output of neonatal rat in vitro transverse brain stem slices

Nucleus raphéobscurus (NRo) modulates hypoglossal (XII) nerve motor output in the invitro transverse brain stem slice of neonatal rats (1-5 days old);chemical ablation of NRo and its focal CO₂ acidificationmodulated the bursting rhythm of XII nerves. We microinjected a 4.5 mMsolution of kainic acid into the NRo to disrupt cellular activity andobserved that XII nerve activity was temporarily abolished(n = 10). We also microinjected CO₂-acidified (pH = 6.00 ± 0.01) artificialcerebrospinal fluid (aCSF) into the NRo (n = 6), thepre-Bötzinger complex (PBC) (n = 6), as well as acontrol region in the lateral tegmental field equidistant to NRo, PBC,and the XII motor nuclei (n = 12). CO₂acidification of the control region had no effect on XII motor output.CO₂ acidification of the NRo significantly(P < 0.05) increased the burst discharge frequency ofXII nerves by 77%; integrated burst amplitude and burst durationincreased by 64% and 52%, respectively. CO₂ acidificationof the PBC significantly (P < 0.05) increased theburst discharge frequency of XII nerves by 65%, but neither integratedburst amplitude nor burst duration changed. These results demonstratethat chemical ablation of the NRo can abolish XII nerve bursting rhythmand that stimulation of the NRo with CO₂-acidified aCSF canexcite XII nerve bursting activity. From these observations, weconclude that, in transverse brain stem slices, the NRo containspH/CO₂-sensitive cells that modulate XII motor output.

     

Centrally patterned rhythmic activity integrated by a peripheral circuit linking multiple oscillators

The central pattern generator for heartbeat in the medicinal leech, Hirudo generates rhythmic activity conveyed by heart excitor motor neurons in segments 3-18 to coordinate the bilateral tubular hearts and side vessels. We focus on behavior and the influence of previously un-described peripheral nerve circuitry. Extracellular recordings from the valve junction (VJ) where afferent vessels join the heart tube were combined with optical recording of contractions. Action potential bursts at VJs occurred in advance of heart tube and afferent vessel contractions. Transections of nerves were performed to reduce the output of the central pattern generator reaching the heart tube. Muscle contractions persisted but with a less regular rhythm despite normal central pattern generator rhythmicity. With no connections between the central pattern generator and heart tube, a much slower rhythm became manifest. Heart excitor neuron recordings showed that peripheral activity might contribute to the disruption of centrally entrained contractions. In the model presented, peripheral activity would normally modify the activity actually reaching the muscle. We also propose that the fundamental efferent unit is not a single heart excitor neuron, but rather is a functionally defined unit of about three adjacent motor neurons and the peripheral assembly of coupled peripheral oscillators.     

FOOD-RELATED CONDITIONING AND NEURONAL CORRELATES IN THE FRESHWATER SNAIL LYMNAEA STAGNALIS

The feeding response of Lymnaea stagnalis shows robust appetitiveconditioning to a novel chemostim-ulus (amyl acetate), whichis retained for more than 4 days. In scrru-isolated centralnervous system (CNS)-hp preparations taken from conditionedsnails, application of amyl acetate to lip tissue led to theonset of fictive feeding in 5/17 individuals, and excitationof an identified cerebral-buccal feeding intemeuron, CV1_a. ina further 8. Control group snails showed no response to amylacetate. Attempts were made to aversively condition L. stagnalis usinga food stimulus (sucrose) as the conditioned stimulus and eitheran electric shock or mechanical stimulus as the unconditionedstimulus. No conditioned response to sucrose (i.e. withdrawal)was seen following training. However, trained snails exhibiteda reduction in responsiveness which was manifest as inhibitionof the feeding response to sucrose and an increase in the timetaken to emerge from the shell following handling. This reducedresponsiveness was seen at 1 hr but was lost by 24 hr aftertraining, and was not observed in control group snails. Semi-isolatedCNS-hp preparations from experimental snails recorded 1-3 hrafter training exhibited either inhibition of fictive feedingor no response following application of sucrose. Control groupsnails showed the normal excitatory response to sucrose includinginduction of feeding motor output Differences in food-related learning between Lymnaea and othergastropod species are discussed in relation to lifestyle andfeeding strategies (Received 14 January 1996; accepted 3 May 1996)     

Experiments on the central pattern generator for swimming in amphibian embryos

The central nervous system of paralysed Xenopus laevis embryos can generate a motor output pattern suitable for swimming locomotion. By recording motor root activity in paralysed embryos with transected nervous systems we have shown that: (a) the spinal cord is capable of swimming pattern generation; (b) swimming pattern generator capability in the hindbrain and spinal cord is distributed; (c) caudal hindbrain is necessary for sustained swimming output after discrete stimulation. By recording similarly from embryos whose central nervous system was divided longitudinally into left and right sides, we have shown that: (a) each side can generate rhythmic motor output with cycle periods like those in swimming; (b) during this activity cycle period increases within an episode, and there is the usual rostrocaudal delay found in swimming; (c) this activity is influenced by sensory stimuli in the same way as swimming activity; (d) normal phase coupling of the left and right sides can be established by the ventral commissure in the spinal cord. We conclude that interactions between the antagonistic (left and right) motor systems are not necessary for swimming rhythm generation and present a model for swimming pattern generation where autonomous rhythm generators on each side of the nervous system drive the motoneurons. Alternation is achieved by reciprocal inhibition, and activity is initiated and maintained by tonic excitation from the hindbrain.     

Octopamine-containing (OC) interneurons enhance central pattern generator activity in sucrose-induced feeding in the snail <Emphasis Type="Italic">Lymnaea</Emphasis>

In the pond snail Lymnaea stagnalis octopamine-containing (OC) interneurons trigger and reconfigure the feeding pattern in isolated CNS by excitation of the central pattern generator. In semi-intact (lip–mouth—CNS) preparations, this central pattern generator is activated by chemosensory inputs. We now test if sucrose application to the lips activates the OC neurons independently of the rest of the feeding central pattern generator, or if the OC interneuron is activated by inputs from the feeding network. In 66% of experiments, sucrose stimulated feeding rhythms and OC interneurons received regular synaptic inputs. Only rarely (14%) did the OC interneuron fire action potentials, proving that firing of OC interneurons is not necessary for the sucrose-induced feeding. Prestimulation of OC neurons increased the intensity and duration of the feeding rhythm evoked by subsequent sucrose presentations. One micromolar octopamine in the CNS bath mimicked the effect of OC interneuron stimulation, enhancing the feeding response when sucrose is applied to the lips. We conclude that the modulatory OC neurons are not independently excited by chemosensory inputs to the lips, but rather from the buccal central pattern generator network. However, when OC neurons fire, they release modulatory octopamine, which provides a positive feedback to the network to enhance the sucrose-activated central pattern generator rhythm.     

Electrophysiological study of serotoninergic neuron Cl in the pteropod mollusk Clione

Functional characteristics of cerebral serotoninergic neuron Cl, axons of which terminate at the buccal ganglia [7], were investigated in the pteropod molluskClione. Stimulating neuron Cl induced activation of the feeding rhythm generator located in the buccal ganglia — an effect arising after a long latency and persisting for some tens of seconds once stimulation had ended. Neuron Cl receives feedback from buccal ganglion cells and this brings about periodic modulation in ganglia activity during the generation of feeding rhythm. Activity of neuron Cl is correlated with operation of the locomotor rhythm generator located in the pedal ganglia. The firing rate of Cl neurons increased upon activation of the locomotor generator (whether spontaneous or induced by stimulating certain command neurons). The correlation found between workings of the locomotor generator and activity of Cl neurons is thought to be one of the manifestations of feeding synergy involving simultaneous activation of the locomotor and buccal apparatus.Institute for Research on Information Transmission, Academy of Sciences of the USSR, Moscow. M. V. Lomonosov State University, Moscow. Translated from Neirofiziologiya, Vol. 23, No. 1, pp. 18–25, January–February, 1991.     

Evidence that the Central Pattern Generator for Swimming in Tritonia Arose from a Non-Rhythmic Neuromodulatory Arousal System: Implications for the Evolution of Specialized Behavior

Comparisons of the nervous systems of closely related invertebratespecies show that identified neurons tend to be highly conservedeven though the behaviors in which they participate vary. Allopisthobranch molluscs examined have a similar set of serotonin-immunoreactiveneurons located medially in the cerebral ganglion. In a smallnumber of species, these neurons have been physiologically andmorphologically identified. In the nudibranch, Tritonia diomedea,three of the neurons (the dorsal swim interneurons, DSIs) havebeen shown to be members of the central pattern generator (CPG)underlying dorsal/ventral swimming. The DSIs act as intrinsicneuromodulators, altering cellular and synaptic properties withinthe swim CPG circuit. Putative homologues of the DSIs have beenidentified in a number of other opisthobranchs. In the notaspid,Pleurobranchaea californica, the apparent DSI homologues (As1–3)play a similar role in the escape swim and they also have widespreadactions on other systems such as feeding and ciliary locomotion.In the gymnosomatid, Clione limacina, the presumed homologousneurons (Cr-SP) are not part of the swimming pattern generator,which is located in the pedal ganglia, but act as extrinsicmodulators, responding to noxious stimuli and increasing thefrequency of the swim motor program. Putative homologous neuronsare also present in non-swimming species such as the anaspid,Aplysia californica, where at least one of the cerebral serotonergicneurons, CC3 (CB-1), evokes neuromodulatory actions in responseto noxious stimuli. Thus, the CPG circuit in Tritonia appearsto have evolved from the interconnections of neurons that arecommon to other opisthobranchs where they participate in arousalto noxious stimuli but are not rhythmically active.     

Damped oscillation of the lateral hypothalamic multineuronal activity synchronized to daily feeding schedules in rats with suprachiasmatic nucleus lesions.

In male Wistar rats with chronically implanted electrodes, multiple-unit activity (MUA) was recorded from the lateral hypothalamus (LH) and ventromedial hypothalamus (VMH). Blinded rats with bilateral suprachiasmatic nucleus (SCN) lesions showed no circadian rhythm in MUA or motor activity when food was available ad libitum. However, under a restricted-feeding schedule (food was available from 1400 to 1600 hr; water was always available) lasting for 10 days, a gradual increase of MUA of the LH developed, starting 3-4 hr prior to the feeding time. The elevated MUA lasted up to 6-7 hr after feeding and subsequently returned to the baseline level. This circadian rhythm of MUA of the LH persisted up to 4 days under total food deprivation, with quickly decreasing amplitude after termination of the schedule. MUA rhythm in VMH was less obvious than that in LH. Also, general motor activity showed a rhythm comparable to that of MUA, but it was less prominent. The elevated MUA in the LH prior to the feeding time may have been neural substrate of anticipatory activity appearing under the restricted-feeding schedule. These findings may suggest the existence of a quickly damping oscillator mechanism in the brain, presumably in the LH, which can be induced by daily feeding cues in the absence of the SCN.