Serotonergic and peptidergic modulation of the buccal mass protractor muscle (I2) in aplysia

Multifaceted Expression of Peptidergic Modulation in the Feeding System of Aplysia

Neuropeptides are present in species throughout the animal kingdom and generally exert actions that are distinct from those of small molecule transmitters. It has, therefore, been of interest to define the unique behavioral role of this class of substances. Progress in this regard has been made in experimentally advantageous invertebrate preparations. We focus on one such system, the feeding circuit in the mollusc Aplysia. We review research conducted over several decades that played an important role in establishing that peptide cotransmitters are released under behaviorally relevant conditions. We describe how this was accomplished. For example, we describe techniques developed to purify novel peptides, localize them to identified neurons, and detect endogenous peptide release. We also describe physiological experiments that demonstrated that peptides are bioactive under behaviorally relevant conditions. The feeding system is like others in that peptides exert effects that are both convergent and divergent. Work in the feeding system clearly illustrates how this creates potential for behavioral flexibility. Finally, we discuss experiments that determined physiological consequences of one of the hallmark features of peptidergic modulation, its persistence. Research in the feeding system demonstrated that this persistence can change network state and play an important role in determining network output.

Motor neuronal activity varies least among individuals when it matters most for behavior

How does motor neuronal variability affect behavior? To explore this question, we quantified activity of multiple individual identified motor neurons mediating biting and swallowing in intact, behaving Aplysia californica by recording from the protractor muscle and the three nerves containing the majority of motor neurons controlling the feeding musculature. We measured multiple motor components: duration of the activity of identified motor neurons as well as their relative timing. At the same time, we measured behavioral efficacy: amplitude of grasping movement during biting and amplitude of net inward food movement during swallowing. We observed that the total duration of the behaviors varied: Within animals, biting duration shortened from the first to the second and third bites; between animals, biting and swallowing durations varied. To study other sources of variation, motor components were divided by behavior duration (i.e., normalized). Even after normalization, distributions of motor component durations could distinguish animals as unique individuals. However, the degree to which a motor component varied among individuals depended on the role of that motor component in a behavior. Motor neuronal activity that was essential for the expression of biting or swallowing was similar among animals, whereas motor neuronal activity that was not essential for that behavior varied more from individual to individual. These results suggest that motor neuronal activity that matters most for the expression of a particular behavior may vary least from individual to individual. Shaping individual variability to ensure behavioral efficacy may be a general principle for the operation of motor systems.

Excitatory neurotransmitters in the tentacle flexor muscles responsible for space positioning of the snail olfactory organ

Recently, three novel flexor muscles (M1, M2 and M3) in the posterior tentacles of the snail have been described, which are responsible for the patterned movements of the tentacles of the snail, Helix pomatia. In this study, we have demonstrated that the muscles received a complex innervation pattern via the peritentacular and olfactory nerves originating from different clusters of motoneurons of the cerebral ganglia. The innervating axons displayed a number of varicosities and established neuromuscular contacts of different ultrastructural forms. Contractions evoked by nerve stimulation could be mimicked by external acetylcholine (ACh) and glutamate (Glu), suggesting that ACh and Glu are excitatory transmitters at the neuromuscular contacts. Choline acetyltransferase and vesicular glutamate transporter immunolabeled axons innervating flexor muscles were demonstrated by immunohistochemistry and in Western blot experiments. Nerve- and transmitter-evoked contractions were similarly attenuated by cholinergic and glutamatergic antagonists supporting the dual excitatory innervation. Dopamine (DA, 10⁻⁵ M) oppositely modulated thin (M1/M2) and thick (M3) muscle responses evoked by stimulation of the olfactory nerve, decreasing the contractions of the M1/M2 and increasing those of M3. In both cases, the modulation site was presynaptic. Serotonin (5-HT) at high concentration (10⁻⁵ M) increased the amplitude of both the nerve- and the ACh-evoked contractions in all muscles. The relaxation rate was facilitated suggesting pre- and postsynaptic site of action. Our data provided evidence for a DAergic and 5-HTergic modulation of cholinergic nerves innervating flexor muscles of the tentacles as well as the muscles itself. These effects of DA and 5-HT may contribute to the regulation of sophisticated movements of tentacle muscles lacking inhibitory innervation.

Autaptic muscarinic self-excitation and nitrergic self-inhibition in neurons initiating Aplysia feeding are revealed when the neurons are cultured in isolation

Properties of a neuron may arise via endogenous mechanisms, or via interactions with other neurons. Culturing a neuron in isolation is a useful tool to distinguish between endogenous and circuit-derived properties. We identified two remarkable functional features of pattern initiator neurons B31/B32 in Aplysia when these neurons were cultured in isolation. These features were also present in situ, but were less prominent, and would have been missed had they not been observed first in the isolated cultured neurons. The properties are likely to be present in neurons of higher animals, but have not yet been observed. One feature was autaptic muscarinic self-excitation that contributes to the neuron's plateau potential, by which it initiates behavior. The other feature was the release of nitric oxide (NO) in the absence of spiking, which causes self-inhibition at rest. The nitrergic modulation of B31/B32 is likely to contribute to the control of feeding by dietary changes in the concentration of L: -arginine, the precursor from which NO is synthesized.

Feedforward compensation mediated by the central and peripheral actions of a single neuropeptide discovered using representational difference analysis

Compensatory mechanisms are often used to achieve stability by reducing variance, which can be accomplished via negative feedback during homeostatic regulation. In principle, compensation can also be implemented through feedforward mechanisms where a regulator acts to offset the anticipated output variation; however, few such neural mechanisms have been demonstrated. We provide evidence that an Aplysia neuropeptide, identified using an enhanced representational difference analysis procedure, implements feedforward compensation within the feeding network. We named the novel peptide "allatotropin-related peptide" (ATRP) because of its similarity to insect allatotropin. Mass spectrometry confirmed the peptide's identity, and in situ hybridization and immunostaining mapped its distribution in the Aplysia CNS. ATRP is present in the higher-order cerebral-buccal interneuron (CBI) CBI-4, but not in CBI-2. Previous work showed that CBI-4-elicited motor programs have a shorter protraction duration than those elicited by CBI-2. Here we show that ATRP shortens protraction duration of CBI-2-elicited ingestive programs, suggesting a contribution of ATRP to the parametric differences between CBI-4-evoked and CBI-2-evoked programs. Importantly, because Aplysia muscle contractions are a graded function of motoneuronal activity, one consequence of the shortening of protraction is that it can weaken protraction movements. However, this potential weakening is offset by feedforward compensatory actions exerted by ATRP. Centrally, ATRP increases the activity of protraction motoneurons. Moreover, ATRP is present in peripheral varicosities of protraction motoneurons and enhances peripheral motoneuron-elicited protraction muscle contractions. Therefore, feedforward compensatory mechanisms mediated by ATRP make it possible to generate a faster movement with an amplitude that is not greatly reduced, thereby producing stability.

Repetition priming of motoneuronal activity in a small motor network: intercellular and intracellular signaling

The characteristics of central pattern generator (CPG) outputs are subject to extensive modulation. Previous studies of neuromodulation largely focused on immediate actions of neuromodulators, i.e., actions that were exerted at the time when either neuromodulators were present or neuromodulatory inputs to the CPG were active. However, neuromodulatory actions are known to persist when neuromodulators are no longer present. In Aplysia, stimulation of cerebral-buccal interneuron-2 (CBI-2), which activates the feeding CPG, produces a repetition priming of motor programs. This priming is reflected in an increase of firing of motoneurons. As CBI-2 contains two neuromodulatory peptides, FCAP (feeding circuit-activating peptide) and CP2 (cerebral peptide 2), we hypothesized that repetition priming may involve persistent peptidergic neuromodulation. We find that these peptides produce priming-like effects, i.e., they increase the firing of radula-opening (B48) and radula-closing (B8) motoneurons during motor programs. Proekt et al. (2004, 2007) showed that repetition priming of neuron B8 is implemented by modulatory inputs that B8 receives from the CPG. In contrast, our current findings indicate that priming of B48 may be implemented by a direct peptidergic modulation of its intrinsic characteristics via a pathway that activates cAMP. We suggest that the direct versus indirect, i.e., CPG-dependent, repetition priming may be related to the type of input that individual motoneurons receive from the CPG. We suggest that in motoneurons that are driven by concurrent excitation-inhibition, repetition priming is indirect as it is preferentially implemented via modulation of the output of CPGs. In contrast, in motoneurons that are driven by alternating excitation-inhibition, direct modulation of motoneurons may be preferentially used.

The peptide hormone pQDLDHVFLRFamide (crustacean myosuppressin) modulates the Homarus americanus cardiac neuromuscular system at multiple sites

pQDLDHVFLRFamide is a highly conserved crustacean neuropeptide with a structure that places it within the myosuppressin subfamily of the FMRFamide-like peptides. Despite its apparent ubiquitous conservation in decapod crustaceans, the paracrine and/or endocrine roles played by pQDLDHVFLRFamide remain largely unknown. We have examined the actions of this peptide on the cardiac neuromuscular system of the American lobster Homarus americanus using four preparations: the intact animal, the heart in vitro, the isolated cardiac ganglion (CG), and a stimulated heart muscle preparation. In the intact animal, injection of myosuppressin caused a decrease in heartbeat frequency. Perfusion of the in vitro heart with pQDLDHVFLRFamide elicited a decrease in the frequency and an increase in the amplitude of heart contractions. In the isolated CG, myosuppressin induced a hyperpolarization of the resting membrane potential of cardiac motor neurons and a decrease in the cycle frequency of their bursting. In the stimulated heart muscle preparation, pQDLDHVFLRFamide increased the amplitude of the induced contractions, suggesting that myosuppressin modulates not only the CG, but also peripheral sites. For at least the in vitro heart and the isolated CG, the effects of myosuppressin were dose-dependent (10(-9) to 10(-6) mol l(-1) tested), with threshold concentrations (10(-8)-10(-7) mol l(-1)) consistent with the peptide serving as a circulating hormone. Although cycle frequency, a parameter directly determined by the CG, consistently decreased when pQDLDHVFLRFamide was applied to all preparation types, the magnitudes of this decrease differed, suggesting the possibility that, because myosuppressin modulates the CG and the periphery, it also alters peripheral feedback to the CG.

Motor outputs in a multitasking network: relative contributions of inputs and experience-dependent network states

Network outputs elicited by a specific stimulus may differ radically depending on the momentary network state. One class of networks states-experience-dependent states-is known to operate in numerous networks, yet the fundamental question concerning the relative role that inputs and states play in determining the network outputs remains to be investigated in a behaviorally relevant manner. Because previous work indicated that in the isolated nervous system the motor outputs of the Aplysia feeding network are affected by experience-dependent states, we sought to establish the behavioral relevance of these outputs. We analyzed the phasing of firing of radula opening motoneurons (B44 and B48) relative to other previously characterized motoneurons. We found that the overall pattern of motoneuronal firing corresponds to the phasing of movements during feeding behavior, thus indicating a behavioral relevance of network outputs. Previous studies suggested that network inputs act to trigger a response rather than to shape its characteristics, with the latter function being fulfilled by network states. We show this is an oversimplification. In a rested state, different inputs elicited distinct responses, indicating that inputs not only trigger but also shape the responses. However, depending on the combination of inputs and states, responses were either dramatically altered by the network state or were indistinguishable from those observed in the rested state. We suggest that the relative contributions of inputs and states are dynamically regulated and, rather than being fixed, depend on the specifics of states and inputs.

Synaptic transmission: excitatory autapses find a function?

An autapse is a synapse between a neuron and itself, a peculiar structure with an unclear function. A new study suggests that excitatory autapses contribute to a positive-feedback loop that maintains persistent electrical activity in neurons.

Identification, molecular structure and expression of two cloned serotonin receptors from the pond snail, Helisoma trivolvis

Helisoma trivolvis has served as a model system to study the functions of serotonin (5-HT) from cellular, developmental, physiological and behavioural perspectives. To further explore the serotonin system at the molecular level, and to provide experimental knockout tools for future studies, in this study we identified serotonin receptor genes from the H. trivolvis genome, and characterized the molecular structure and expression profile of the serotonin receptor gene products. Degenerate oligonucleotide primers, based on conserved regions of the Lymnaea stagnalis 5-HT(1Lym) receptor, were used to amplify G protein-coupled biogenic amine receptor sequences from H. trivolvis genomic cDNA, resulting in the cloning of two putative serotonin receptors. The deduced gene products both appear to be G protein-coupled serotonin receptors, with well-conserved structure in the functional domains and high variability in the vestibule entrance of the receptor protein. Phylogenetic analysis placed these receptors in the 5-HT(1) and 5-HT(7) families of serotonin receptors. They are thus named the 5-HT(1Hel) and 5-HT(7Hel) receptors, respectively. In situ hybridization and immunofluorescence studies revealed that these genes and gene products are expressed most heavily in the ciliated pedal and mantle epithelia of H. trivolvis embryos. In adults, widespread expression occurred in all ganglia and connectives of the central nervous system. Expression of both receptor proteins was localized exclusively to neurites when examined in situ. In contrast, when isolated neurons were grown in culture, 5-HT(1Hel) and 5-HT(7Hel) immunoreactivity were located primarily in the cell body. This is the first study to reveal a 5-HT(7) receptor in a molluscan species.

State dependence of spike timing and neuronal function in a motor pattern generating network

When sustained firing of a neuron is similar in different types of motor programs, its role in the generation of these programs is often similar. We investigated whether this is also the case for neurons involved in phase transition. In the Aplysia feeding central pattern generator (CPG), identified interneuron B64 starts firing at the transition between the protraction and the retraction phases of all types of motor programs, and its firing is sustained during the retraction phase. It was thought that B64 functions as a protraction terminator as it provides strong inhibitory input to protraction interneurons and motoneurons. Furthermore, premature activation of B64 can lead to premature termination of the protraction phase. Indeed, as we show here, B64 can terminate the protraction phase regardless of the type of motor program. However, B64 actually only functions as a protraction terminator in ingestive-like but not in egestive-like programs. This differential role of B64 results from a differential timing of the initiation of B64 spiking in the two types of programs. In turn, this differential timing of the initiation of B64 firing is determined by the internal state of the CPG. Thus, this study indicates the importance of the timing of initiation of firing in determining the functional role of a neuron and demonstrates that this role depends on the activity-dependent state of the network.

Currents contributing to decision making in neurons B31/B32 of Aplysia

Biophysical properties of neurons contributing to the ability of an animal to decide whether or not to respond were examined. B31/B32, two pairs of bilaterally symmetrical Aplysia neurons, are major participants in deciding to initiate a buccal motor program, the neural correlate of a consummatory feeding response. B31/B32 respond to an adequate stimulus after a delay, during which time additional stimuli influence the decision to respond. B31/B32 then respond with a ramp depolarization followed by a sustained soma depolarization and axon spiking that is the expression of a commitment to respond to food. Four currents contributing to decision making in B31/B32 were characterized, and their functional effects were determined, in current- and voltage-clamp experiments and with simulations. Inward currents arising from slow muscarinic transmission were characterized. These currents contribute to the B31/B32 depolarization. Their slow activation kinetics contribute to the delay preceding B31/B32 activity. After the delay, inward currents affect B31/B32 in the context of two endogenous inactivating outward currents: a delayed rectifier K+ current (I(K-V)) and an A-type K+ current (I(K-A)), as well as a high-threshold noninactivating outward current (I(maintained)). Hodgkin-Huxley kinetic analyses were performed on the outward currents. Simulations using equations from these analyses showed that I(K-V) and I(K-A) slow the ramp depolarization preceding the sustained depolarization. The three outward currents contribute to braking the B31/B32 depolarization and keeping the sustained depolarization at a constant voltage. The currents identified are sufficient to explain the properties of B31/B32 that play a role in generating the decision to feed.

Neural control of the velum in larvae of the gastropod, Ilyanassa obsoleta

Larval molluscs commonly use ciliated vela to swim and feed. In this study we used immunohistochemistry to demonstrate innervation of velar cilia and muscles by monoaminergic and peptidergic fibres in the caenogastropod, Ilyanassa obsoleta. Photoelectric recordings from pre-oral cilia on isolated pieces of velum revealed that serotonin increased, whereas catecholamines (dopamine and norepinephrine) decreased beat frequency at concentrations of 10(-6) to 10(-9) mol l(-1). Catecholamines also increased the frequency of momentary, isolated arrests of pre-oral cilia, but failed to suppress beating of the post-oral cilia at these concentrations. The neuropeptides, FMRFamide and Leu-enkephalin, did not affect the frequency of ciliary beating or of isolated ciliary arrests, but did induce numerous muscular contractions, which were accompanied by sustained ciliary arrests. In terms of whole animal behaviour, serotonin caused larvae to concentrate toward the top of a water column and to increase feeding, whereas catecholamines caused larvae to concentrate toward the bottom of a water column and decrease feeding. Monoamine analogues which facilitated or opposed the effects of synthetic transmitters on larval behaviour, further suggested that these transmitters are released endogenously to control velar function. Finally, applications of peptides to whole larvae caused increased frequency of locomotory arrests. Together these findings demonstrate several potential roles for the nervous system in controlling larval behaviour in gastropods.

The kinematics of multifunctionality: comparisons of biting and swallowing in Aplysia californica

What are the mechanisms of multifunctionality, i.e. the use of the same peripheral structures for multiple behaviors? We studied this question using the multifunctional feeding apparatus of the marine mollusk Aplysia californica, in which the same muscles mediate biting (an attempt to grasp food) and swallowing (ingestion of food). Biting and swallowing responses were compared using magnetic resonance imaging of intact, behaving animals and a three-dimensional kinematic model. Biting is associated with larger amplitude protractions of the grasper (radula/odontophore) than swallowing, and smaller retractions. Larger biting protractions than in swallowing appear to be due to a more anterior position of the grasper as the behavior begins, a larger amplitude contraction of protractor muscle I2, and contraction of the posterior portion of the I1/I3/jaw complex. The posterior I1/I3/jaw complex may be context-dependent, i.e. its mechanical context changes the direction of the force it exerts. Thus, the posterior of I1/I3 may aid protraction near the peak of biting, whereas the entire I1/I3/jaw complex acts as a retractor during swallowing. In addition, larger amplitude closure of the grasper during swallowing allows an animal to exert more force as it ingests food. These results demonstrate that differential deployment of the periphery can mediate multifunctionality.

Variability of motor neuron spike timing maintains and shapes contractions of the accessory radula closer muscle of Aplysia

The accessory radula closer (ARC) muscle of Aplysia has long been studied as a typical "slow" muscle, one that would be assumed to respond only to the overall, integrated spike rate of its motor neurons, B15 and B16. The precise timing of the individual spikes should not much matter. However, but real B15 and B16 spike patterns recorded in vivo show great variability that extends down to the timing of individual spikes. By replaying these real as well as artificially constructed spike patterns into ARC muscles in vitro, we examined the consequences of this spike-level variability for contraction. Replaying the same pattern several times reproduces precisely the same contraction shape: the B15/B16-ARC neuromuscular transform is deterministic. However, varying the timing of the spikes produces very different contraction shapes and amplitudes. The transform in fact operates at an interface between "fast" and "slow" regimens. It is fast enough that the timing of individual spikes greatly influences the detailed contraction shape. At the same time, slow integration of the spike pattern through the nonlinear transform allows the variable spike timing to determine also the overall contraction amplitude. Indeed, the variability appears to be necessary to maintain the contraction amplitude at a robust level. This phenomenon is tuned by neuromodulators that tune the speed and nonlinearity of the transform. Thus, the variable timing of individual spikes does matter, in at least two, functionally significant ways, in this "slow" neuromuscular system.

Nitric oxide signals that aplysia have attempted to eat, a necessary component of memory formation after learning that food is inedible

Inhibiting nitric oxide (NO) synthesis during learning that food is inedible in Aplysia blocks subsequent memory formation. To gain insight into the function of NO transmission during learning we tested whether blocking NO synthesis affects aspects of feeding that are expressed both in a nonlearning context and during learning. Inhibiting NO synthesis with L-NAME and blocking guanylyl cyclase with methylene blue decreased the efficacy of ad libitum feeding. D-NAME had no effect. L-NAME also decreased rejection responses frequency, but did not affect rejection amplitude. The effect of L-NAME was explained by a decreased signaling that efforts to swallow are not successful, leading to a decreased rejection rate, and a decreased ability to reposition and subsequently consume food in ad libitum feeding. Signaling that animals have made an effort to swallow is a critical component of learning that food is inedible. Stimulation of the lips with food alone did not produce memory, but stimulation combined with the NO donor SNAP did produce memory. Exogenous NO at a concentration causing memory also excited a key neuron responding to NO, the MCC. Block of the cGMP second-messenger cascade during training by methylene blue also blocked memory formation after learning. Our data indicate that memory arises from the contingency of three events during learning that food is inedible. One of the events is efforts to swallow, which are signaled by NO by cGMP.

The action of RFamide neuropeptides on molluscs, with special reference to the gastropods Buccinum undatum and Busycon canaliculatum

The ever-growing RFamide neuropeptide superfamily has members in all animal phyla. Their effects in molluscs, on both smooth and cardiac muscle as well as on neurons, has been studied in detail. These neuropeptides exert a variety of functions: excitatory, inhibitory or even biphasic. Firstly, the literature on the excitatory effect of the RFamide neuropeptides on molluscan muscle and neurons has been reviewed, with greater emphasis and examples from the gastropods Buccinum undatum and Busycon canaliculatum. The peptides seem to be potent activators of contraction, sometimes generating slow tonic force and other times twitch activity. Secondly, the literature on the inhibitory effect of the superfamily has been reviewed. These peptides can exert an inhibitory effect, hyperpolarizing the cells rather than depolarizing them. Thirdly, the neuropeptides may play a variety of other roles, such as contributing to the regulation or maturation process of the animals. There have been cases recorded of RFamide neuropeptides acting as potent venoms in members of the Conus sp. The pathway of action of these multiple roles, their interaction with the parent neurotransmitters acetylcholine and serotonin, as well as the calcium dependency of the RFamide neuropeptides has been discussed, again with special reference to the above mentioned gastropods. A better understanding of the mode of action, the effects, and the importance of the RFamide neuropeptides on molluscan physiology and pharmacology has been attempted by reviewing the existing literature, recognizing the importance of the RFamide neuropeptide actions on molluscs.

Mechanical reconfiguration mediates swallowing and rejection in Aplysia californica

Muscular hydrostats, such as tongues, trunks or tentacles, have fewer constraints on their degrees of freedom than musculoskeletal systems, so changes in a structure's shape may alter the positions and lengths of other components (i.e., induce mechanical reconfiguration). We studied mechanical reconfiguration during rejection and swallowing in the marine mollusk Aplysia californica. During rejection, inedible material is pushed out of an animal's buccal cavity. The grasper (radula/odontophore) closes on inedible material, and then a posterior muscle, I2, pushes the grasper toward the jaws (protracts it). After the material is released, an anterior muscle complex (the I1/I3/jaw complex) pushes the grasper toward the esophagus (retracts it). During swallowing, the grasper is protracted open, and then retracts closed, pulling in food. Grasper closure changes its shape. Magnetic resonance images show that grasper closure lengthens I2. A kinetic model quantified the changes in the ability of I2 and I1/I3 to exert force as grasper shape changed. Grasper closure increases I2's ability to protract during rejection, and increases I1/I3's ability to retract during swallowing. Motor neurons controlling radular closure may therefore affect the behavioral outputs of I2's and I1/I3's motor neurons. Thus, motor neurons may modulate the outputs of other motor neurons through mechanical reconfiguration.

Identification of a new neuropeptide precursor reveals a novel source of extrinsic modulation in the feeding system of Aplysia

The Aplysia feeding system is advantageous for investigating the role of neuropeptides in behavioral plasticity. One family of Aplysia neuropeptides is the myomodulins (MMs), originally purified from one of the feeding muscles, the accessory radula closer (ARC). However, two MMs, MMc and MMe, are not encoded on the only known MM gene. Here, we identify MM gene 2 (MMG2), which encodes MMc and MMe and four new neuropeptides. We use matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to verify that these novel MMG2-derived peptides (MMG2-DPs), as well as MMc and MMe, are synthesized from the precursor. Using antibodies against the MMG2-DPs, we demonstrate that neuronal processes that stain for MMG2-DPs are found in the buccal ganglion, which contains the feeding network, and in the buccal musculature including the ARC muscle. Surprisingly, however, no immunostaining is observed in buccal neurons including the ARC motoneurons. In situ hybridization reveals only few MMG2-expressing neurons that are mostly located in the pedal ganglion. Using immunohistochemical and electrophysiological techniques, we demonstrate that some of these pedal neurons project to the buccal ganglion and are the likely source of the MMG2-DP innervation of the feeding network and musculature. We show that the MMG2-DPs are bioactive both centrally and peripherally: they bias egestive feeding programs toward ingestive ones, and they modulate ARC muscle contractions. The multiple actions of the MMG2-DPs suggest that these peptides play a broad role in behavioral plasticity and that the pedal-buccal projection neurons that express them are a novel source of extrinsic modulation of the feeding system of Aplysia.

Temperature compensation of neuromuscular modulation in aplysia

Physiological systems that must operate over a range of temperatures often incorporate temperature-compensatory mechanisms to maintain their output within a relatively narrow, functional range of values. We analyze here an example in the accessory radula closer (ARC) neuromuscular system, a representative part of the feeding neuromusculature of the sea slug Aplysia. The ARC muscle's two motor neurons, B15 and B16, release, in addition to ACh that contracts the muscle, modulatory peptide cotransmitters that, through a complex network of effects in the muscle, shape the ACh-induced contractions. It is believed that this modulation is critical in optimizing the performance of the muscle for successful, efficient feeding behavior. However, previous work has shown that the release of the modulatory peptides from the motor neurons decreases dramatically with increasing temperature. From 15 to 25 degrees C, for example, release decreases 20-fold. Yet Aplysia live and feed successfully not only at 15 degrees C, but at 25 degrees C and probably at higher temperatures. Here, working with reduced B15/B16-ARC preparations in vitro as well as a mathematical model of the system, we have found a resolution of this apparent paradox. Although modulator release decreases 20-fold when the temperature is raised from 15 to 25 degrees C, the observed modulation of contraction shape does not decrease at all. Two mechanisms are responsible. First, further downstream within the modulatory network, the modulatory effects themselves-experimentally dissected by exogenous modulator application-have temperature dependencies opposite to that of modulator release, increasing with temperature. Second, the saturating curvature of the dose-response relations within the network diminishes the downstream impact of the decrease of modulator release. Thus two quite distinct mechanisms, one depending on the characteristics of the individual components of the network and the other emerging from the network's structure, combine to compensate for temperature changes to maintain the output of this physiological system.

Neural control exploits changing mechanical advantage and context dependence to generate different feeding responses in Aplysia

How does neural control reflect changes in mechanical advantage and muscle function? In the Aplysia feeding system a protractor muscle's mechanical advantage decreases as it moves the structure that grasps food (the radula/odontophore) in an anterior direction. In contrast, as the radula/odontophore is moved forward, the jaw musculature's mechanical advantage shifts so that it may act to assist forward movement of the radula/odontophore instead of pushing it posteriorly. To test whether the jaw musculature's context-dependent function can compensate for the falling mechanical advantage of the protractor muscle, we created a kinetic model of Aplysia's feeding apparatus. During biting, the model predicts that the reduction of the force in the protractor muscle I2 will prevent it from overcoming passive forces that resist the large anterior radula/odontophore displacements observed during biting. To produce protractions of the magnitude observed during biting behaviors, the nervous system could increase I2's contractile strength by neuromodulating I2, or it could recruit the I1/I3 jaw muscle complex. Driving the kinetic model with in vivo EMG and ENG predicts that, during biting, early activation of the context-dependent jaw muscle I1/I3 may assist in moving the radula/odontophore anteriorly during the final phase of protraction. In contrast, during swallowing, later activation of I1/I3 causes it to act purely as a retractor. Shifting the timing of onset of I1/I3 activation allows the nervous system to use a mechanical equilibrium point that allows I1/I3 to act as a protractor rather than an equilibrium point that allows I1/I3 to act as a retractor. This use of equilibrium points may be similar to that proposed for vertebrate control of movement.

Modeling neuromuscular modulation in Aplysia. III. Interaction of central motor commands and peripheral modulatory state for optimal behavior

Recent work in computational neuroethology has emphasized that "the brain has a body": successful adaptive behavior is not simply commanded by the nervous system, but emerges from interactions of nervous system, body, and environment. Here we continue our study of these issues in the accessory radula closer (ARC) neuromuscular system of Aplysia. The ARC muscle participates in the animal's feeding behaviors, a set of cyclical, rhythmic behaviors driven by a central pattern generator (CPG). Patterned firing of the ARC muscle's two motor neurons, B15 and B16, releases not only ACh to elicit the muscle's contractions but also peptide neuromodulators that then shape the contractions through a complex network of actions on the muscle. These actions are dynamically complex: some are fast, but some are slow, so that they are temporally uncoupled from the motor neuron firing pattern in the current cycle. Under these circumstances, how can the nervous system, through just the narrow channel of the firing patterns of the motor neurons, control the contractions, movements, and behavior in the periphery? In two earlier papers, we developed a realistic mathematical model of the B15/B16-ARC neuromuscular system and its modulation. Here we use this model to study the functional performance of the system in a realistic behavioral task. We run the model with two kinds of inputs: a simple set of regular motor neuron firing patterns that allows us to examine the entire space of patterns, and the real firing patterns of B15 and B16 previously recorded in a 2 1/2-h-long meal of 749 cycles in an intact feeding animal. These real patterns are extremely irregular. Our main conclusions are the following. 1) The modulation in the periphery is necessary for superior functional performance. 2) The components of the modulatory network interact in nonlinear, context- and task-dependent combinations for best performance overall, although not necessarily in any particular cycle. 3) Both the fast and the slow dynamics of the modulatory state make important contributions. 4) The nervous system controls different components of the periphery to different degrees. To some extent the periphery operates semiautonomously. However, the structure of the peripheral modulatory network ensures robust performance under all circumstances, even with the irregular motor neuron firing patterns and even when the parameters of the functional task are randomly varied from cycle to cycle to simulate a variable feeding environment. In the variable environment, regular firing patterns, which are fine-tuned to one particular task, fail to provide robust performance. We propose that the CPG generates the irregular firing patterns, which nevertheless are guaranteed to give robust performance overall through the actions of the peripheral modulatory network, as part of a trial-and-error feeding strategy in a variable, uncertain environment.

Transforming tonic firing into a rhythmic output in the Aplysia feeding system: presynaptic inhibition of a command-like neuron by a CpG element

Tonic stimuli can elicit rhythmic responses. The neural circuit underlying Aplysia californica consummatory feeding was used to examine how a maintained stimulus elicits repetitive, rhythmic movements. The command-like cerebral-buccal interneuron 2 (CBI-2) is excited by tonic food stimuli but initiates rhythmic consummatory responses by exciting only protraction-phase neurons, which then excite retraction-phase neurons after a delay. CBI-2 is inhibited during retraction, generally preventing it from exciting protraction-phase neurons during retraction. We have found that depolarizing CBI-2 during retraction overcomes the inhibition and causes CBI-2 to fire, potentially leading CBI-2 to excite protraction-phase neurons during retraction. However, CBI-2 synaptic outputs to protraction-phase neurons were blocked during retraction, thereby preventing excitation during retraction. The block was caused by presynaptic inhibition of CBI-2 by a key buccal ganglion retraction-phase interneuron, B64, which also causes postsynaptic inhibition of protraction-phase neurons. Pre- and postsynaptic inhibition could be separated. First, only presynaptic inhibition affected facilitation of excitatory postsynaptic potentials (EPSPs) from CBI-2 to its followers. Second, a newly identified neuron, B54, produced postsynaptic inhibition similar to that of B64 but did not cause presynaptic inhibition. Third, in some target neurons B64 produced only presynaptic but not postsynaptic inhibition. Blocking CBI-2 transmitter release in the buccal ganglia during retraction functions to prevent CBI-2 from driving protraction-phase neurons during retraction and regulates the facilitation of the CBI-2 induced EPSPs in protraction-phase neurons.

Passive hinge forces in the feeding apparatus of Aplysia aid retraction during biting but not during swallowing

Swallowing and biting responses in the marine mollusk Aplysia are both mediated by a cyclical alternation of protraction and retraction movements of the grasping structure, the radula and underlying odontophore, within the feeding apparatus of the animal, the buccal mass. In vivo observations demonstrate that Aplysia biting is associated with strong protractions and rapid initial retractions, whereas Aplysia swallowing is associated with weaker protractions and slower initial retractions. During biting, the musculature joining the radula/odontophore to the buccal mass (termed the "hinge") is stretched more than in swallowing. To test the hypothesis that stretch of the hinge might contribute to rapid retractions observed in biting, we analyzed the hinge's passive properties. During biting, the hinge is stretched sufficiently to assist retraction. In contrast, during swallowing, the hinge is not stretched sufficiently for its passive forces to assist retraction, because the odontophore's anterior movement is smaller than during biting. A quantitative model demonstrated that steady-state passive forces were sufficient to generate the retraction movements observed during biting. Experimental measures of the relative magnitude of the hinge's active and passive forces at the protraction displacements of biting suggest that passive forces are at least a third of the total force.

Neuromuscular modulation in Aplysia. II. Modulation of the neuromuscular transform in behavior

In this work we use mathematical modeling and complementary experiments to study the dynamics of modulation in the accessory radula closer (ARC) neuromuscular system of Aplysia. Here we join a dynamic model of the modulation from the preceding paper to a model of the basal neuromuscular transform (NMT). The resulting complete model of the NMT allows us to predict, test, and analyze the actual modulated contraction shapes in different types of feeding behavior, through entire quasi-realistic meals. The model reproduces a variety of published and new experimental observations. We find that components of the modulatory network act in interdependency and mutual complementarity, one or another playing a key role depending on the behavior and its past history. The history is remembered by slow dynamical components whose persistence prepares the system for future behavior of the same kind. The persistence becomes counterproductive, however, when the behavior suddenly changes. Superposition of fast dynamical components alleviates the problem under most, but not all, circumstances. In the quasi-realistic meals, the modulation improves functional performance on average, but degrades it after certain behavioral switches, when the model predicts sharp contraction transients. These are indeed seen in the real muscle. We propose that the real system does not switch the underlying motor neuron firing patterns abruptly, but relaxes them gradually, matching the relaxation of the peripheral modulatory state, through such behavioral transitions. We model food-induced arousal, a known phenomenon of this kind. The peripheral dynamics of the modulated NMT thus constrain the motor commands of the CNS.

PRQFVamide, a novel pentapeptide identified from the CNS and gut of Aplysia

We have purified a novel pentapeptide from the Aplysia nervous system using bioassay on gut contractions. The structure of the peptide is Pro-Arg-Gln-Phe-Val-amide (PRQFVa). The precursor for PRQFVa was found to code for 33 copies of PRQFVamide and four related pentapeptides. Peaks corresponding to the predicted masses of all five pentapeptides were detected in Aplysia neurons by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Northern analysis revealed that expression of the precursor is abundant in the abdominal ganglion, much less in the pedal and cerebral ganglia, and rarely seen in the buccal and pleural ganglia. PRQFVa-positive neurons, mapped by immunohistochemistry and in situ hybridization, were present in all the central ganglia. PRQFVa immunopositive processes were observed in the gut, particularly in association with the vasculature. Some arteries and other highly vascularized tissues, such as the gill and the kidney, also contain numerous PRQFVa immunopositive processes. Application of synthetic PRQFVa suppresses not only contractions of the gut but also contractions of vasculature. PRQFVa is expressed in some of the neurons within the feeding circuitry and application of synthetic PRQFVa was found to decrease the excitability of some (B4/5 and B31/32) but not all (B8) neurons of the buccal feeding circuit. Our findings suggest that PRQFVa may act as a modulator within the feeding system as well as in other systems of Aplysia.

Fast synaptic connections from CBIs to pattern-generating neurons in Aplysia: initiation and modification of motor programs

Consummatory feeding movements in Aplysia californica are organized by a central pattern generator (CPG) in the buccal ganglia. Buccal motor programs similar to those organized by the CPG are also initiated and controlled by the cerebro-buccal interneurons (CBIs), interneurons projecting from the cerebral to the buccal ganglia. To examine the mechanisms by which CBIs affect buccal motor programs, we have explored systematically the synaptic connections from three of the CBIs (CBI-1, CBI-2, CBI-3) to key buccal ganglia CPG neurons (B31/B32, B34, and B63). The CBIs were found to produce monosynaptic excitatory postsynaptic potentials (EPSPs) with both fast and slow components. In this report, we have characterized only the fast component. CBI-2 monosynaptically excites neurons B31/B32, B34, and B63, all of which can initiate motor programs when they are sufficiently stimulated. However, the ability of CBI-2 to initiate a program stems primarily from the excitation of B63. In B31/B32, the size of the EPSPs was relatively small and the threshold for excitation was very high. In addition, preventing firing in either B34 or B63 showed that only a block in B63 firing prevented CBI-2 from initiating programs in response to a brief stimulus. The connections from CBI-2 to the buccal ganglia neurons showed a prominent facilitation. The facilitation contributed to the ability of CBI-2 to initiate a BMP and also led to a change in the form of the BMP. The cholinergic blocker hexamethonium blocked the fast EPSPs induced by CBI-2 in buccal ganglia neurons and also blocked the EPSPs between a number of key CPG neurons within the buccal ganglia. CBI-2 and B63 were able to initiate motor patterns in hexamethonium, although the form of a motor pattern was changed, indicating that non-hexamethonium-sensitive receptors contribute to the ability of these cells to initiate bursts. By contrast to CBI-2, CBI-1 excited B63 but inhibited B34. CBI-3 excited B34 and not B63. The data indicate that CBI-1, -2, and -3 are components of a system that initiates and selects between buccal motor programs. Their behavioral function is likely to depend on which combination of CBIs and CPG elements are activated.

A population of pedal-buccal projection neurons associated with appetitive components of Aplysia feeding behavior

Backfills of the cerebral-buccal connective (CBC) of Aplysia californica revealed a cluster of five to seven pedal-buccal projection neurons in the anterolateral quadrant of the ventral surface of each pedal ganglion. Intra- and extracellular recordings showed that the pedal-buccal projection neurons shared common electrophysiological properties and synaptic inputs. However, they exhibited considerable heterogeneity with respect to their projection patterns. All pedal-buccal projection neurons that were tested received a slow excitatory postsynaptic potential from the ipsi- and contralateral cerebral-pedal regulator (C-PR) neuron, a cell that is thought to play a key role in the generation of a food-induced arousal state. Tests were conducted to identify potential synaptic follower neurons of the pedal-buccal projection neurons in the cerebral and buccal ganglia, but none were detected. Finally, nerve recordings revealed projections from the pedal-buccal projection neurons in the nerves associated with the buccal ganglion. In tests designed to determine the functional properties of these peripheral projections, no evidence was obtained supporting a mechanosensory or proprioceptive role and no movements were observed when they were fired. It is proposed that peripheral elements utilized in consummatory phases of Aplysia feeding may be directly influenced by a neuronal pathway that is activated during the food-induced arousal state.

Multiple presynaptic and postsynaptic sites of inhibitory modulation by myomodulin at ARC neuromuscular junctions of Aplysia

The functional activity of even simple cellular ensembles is often controlled by surprisingly complex networks of neuromodulators. One such network has been extensively studied in the accessory radula closer (ARC) neuromuscular system of Aplysia. The ARC muscle is innervated by two motor neurons, B15 and B16, which release modulatory peptide cotransmitters to shape ACh-mediated contractions of the muscle. Previous analysis has shown that key to the combinatorial ability of B15 and B16 to control multiple parameters of the contraction is an asymmetry in their peptide modulatory actions. B16, but not B15, releases myomodulin, which, among other actions, inhibits the contraction. Work in single ARC muscle fibers has identified a distinctive myomodulin-activated K current as a candidate postsynaptic mechanism of the inhibition. However, definitive evidence for this mechanism has been lacking. Here, working with the single fibers and then motor neuron-elicited excitatory junction potentials (EJPs) and contractions of the intact ARC muscle, we have confirmed two central predictions of the K-current hypothesis: the myomodulin inhibition of contraction is associated with a correspondingly large inhibition of the underlying depolarization, and the inhibition of both contraction and depolarization is blocked by 4-aminopyridine (4-AP), a potent and selective blocker of the myomodulin-activated K current. However, in the intact muscle, the experiments revealed a second, 4-AP-resistant component of myomodulin inhibition of both B15- and B16-elicited EJPs. This component resembles, and mutually occludes with, inhibition of the EJPs by another peptide modulator released from both B15 and B16, buccalin, which acts by a presynaptic mechanism, inhibition of ACh release from the motor neuron terminals. Direct measurements of peptide release showed that myomodulin also inhibits buccalin release from B15 terminals. At the level of contractions, nevertheless, the postsynaptic K-current mechanism is responsible for much of the myomodulin inhibition of peak contraction amplitude. The presynaptic mechanism, which is most evident during the initial build-up of the EJP waveform, underlies instead an increase of contraction latency.

Heterosynaptic modulation by the octopaminergic OC interneurons increases the synaptic outputs of protraction phase interneurons (SO, N1L) in the feeding system of Lymnaea stagnalis

We examined the cholinergic synapses between protraction phase interneurons (SO or N1L) and their targets (N1M interneuron, B1 motoneuron) in the buccal ganglia of the pond snail Lymnaea stagnalis. We have tested the hypothesis that the OC (octopamine-containing) interneuron, an intrinsic modulator of the feeding network, can increase the synaptic efficacy from the SO or N1L to their targets. Prestimulation of the OC interneuron, 4 s before the activation of the SO or N1L increases the strength of their output synapses by 75% (SO)-110% (N1L). The individual excitatory postsynaptic potentials evoked by SO or N1L stimulation increase in size. OC prestimulation also produces an increase in the firing rate of these presynaptic interneurons: SO 40%; N1L 33%. The facilitation lasts up to 6 s after the end of the OC burst. The enhancement of PSPs is seen at all the output synapses (both excitatory and inhibitory) of the SO and N1L interneurons. The output synapses of the non-cholinergic swallowing phase N3p interneuron are not affected, even when the same postsynaptic target is selected. The SO-->N1M, SO-->B1 and N1L-->N1M synapses are also strengthened by bath application of 1-5 microM octopamine (average increase 60%). The major effect is an increased excitability of the SO; the B1 motoneuron response to the main transmitter of the SO, acetylcholine, is unaffected. Increased synaptic outputs of the protraction phase SO and N1L interneurons is functionally significant for generation of feeding pattern in the Lymnaea CNS. Strengthening the connections of SO and N1L to the central pattern generator (N1M) interneurons enhances their ability to drive fictive feeding. Thus heterosynaptic facilitation by the octopaminergic OC interneurons in the central pattern generator network may contribute to the behavioral plasticity of feeding in the intact animal.