Characterization of a radula opener neuromuscular system in Aplysia

Morphology of the buccal apparatus of Dendronotus frondosus (Gastropoda: Nudibranchia)

Nudibranchia is the order of Gastropoda, which is known for its high diversity of dietary specializations and a diversified morphology of the buccal armature. The characters of the buccal armature are adaptive to the feeding mechanisms and diet preferences, but they also are phylogenetically informative. The morphological differences in the buccal armature imply different mechanisms of the synthesis and morphology of the formation zones. Here, we report new data on the morphology of the buccal complex in a cladobranch species with broad radula, Dendronotus frondosus, using light microscopy, transmission and scanning elctron microscopy and computer-based three- dimensional reconstruction and compare them with those on other nudibranchs. We report the presence of protective chitinous spindles in the epithelia of the oral tube and esophagus, which suggests the presence of Cnidaria in the diet of the studied mollusc species. Similar to other Cladobranchia, the jaw plate of D. frondosus is synthesized by the layer of gnathoblasts located posteriorly in the epithelial fold. Near the zone of jaw synthesis, the gnathoblasts with concentric granular endoplasmic reticulum structures were found. This may indicate the turnover of gnathoblasts during the life of a specimen. Despite that D. frondosus has a broad radula, the zone of its synthesis does not split into two parts, unlike other molluscs with broad radulae. A single radular tooth is formed by a few cells, such as in other Nudibranchia. The number of odontoblasts per tooth in D. frondosus is not defined by a tooth size. At the same time, the linear dimensions of the cells secreting radular teeth depend on the size of the teeth synthesized by them.

Soft-surface grasping: radular opening in Aplysia californica

Grasping soft, irregular material is challenging both for animals and robots. The feeding systems of many animals have adapted to this challenge. In particular, the feeding system of the marine mollusk Aplysia californica, a generalist herbivore, allows it to grasp and ingest seaweeds of varying shape, texture and toughness. On the surface of the grasper of A. californica is a structure known as the radula, a thin flexible cartilaginous sheet with fine teeth. Previous in vitro studies suggested that intrinsic muscles, I7, are responsible for opening the radula. Lesioning I7 in vivo does not prevent animals from grasping and ingesting food. New in vitro studies demonstrate that a set of fine muscle fibers on the ventral surface of the radula - the sub-radular fibers (SRFs) - mediate opening movements even if the I7 muscles are absent. Both in vitro and in vivo lesions demonstrate that removing the SRFs leads to profound deficits in radular opening, and significantly reduces feeding efficiency. A theoretical biomechanical analysis of the actions of the SRFs suggests that they induce the radular surface to open around a central crease in the radular surface and to arch the radular surface, allowing it to softly conform to irregular material. A three-dimensional model of the radular surface, based on in vivo observations and magnetic resonance imaging of intact animals, provides support for the biomechanical analysis. These results suggest how a soft grasper can work during feeding, and suggest novel designs for artificial soft graspers.

Cellular Effects of Repetition Priming in the Aplysia Feeding Network Are Suppressed during a Task-Switch But Persist and Facilitate a Return to the Primed State

Many neural networks are multitasking and receive modulatory input, which configures activity. As a result, these networks can enter a relatively persistent state in which they are biased to generate one type of output as opposed to another. A question we address is as follows: what happens to this type of state when the network is forced to task-switch? We address this question in the feeding system of the mollusc Aplysia This network generates ingestive and egestive motor programs. We focus on an identified neuron that is selectively active when programs are ingestive. Previous work has established that the increase in firing frequency observed during ingestive programs is at least partially mediated by an excitability increase. Here we identify the underlying cellular mechanism as the induction of a cAMP-dependent inward current. We ask how this current is impacted by the subsequent induction of egestive activity. Interestingly, we demonstrate that this task-switch does not eliminate the inward current but instead activates an outward current. The induction of the outward current obviously reduces the net inward current in the cell. This produces the decrease in excitability and firing frequency required for the task-switch. Importantly, however, the persistence of the inward current is not impacted. It remains present and coexists with the outward current. Consequently, when effects of egestive priming and the outward current dissipate, firing frequency and excitability remain above baseline levels. This presumably has important functional implications in that it will facilitate a return to ingestive activity.SIGNIFICANCE STATEMENT Under physiological conditions, an animal generating a particular type of motor activity can be forced to at least briefly task-switch. In some circumstances, this involves the temporary induction of an "antagonistic" or incompatible motor program. For example, ingestion can be interrupted by a brief period of egestive activity. In this type of situation, it is often desirable for behavioral switching to occur rapidly and efficiently. In this report, we focus on a particular aspect of this type of task-switch. We determine how the priming that occurs when a multitasking network repeatedly generates one type of motor activity can be retained during the execution of an incompatible motor program.

Discovery of leucokinin-like neuropeptides that modulate a specific parameter of feeding motor programs in the molluscan model, Aplysia

A better understanding of neuromodulation in a behavioral system requires identification of active modulatory transmitters. Here, we used identifiable neurons in a neurobiological model system, the mollusc Aplysia, to study neuropeptides, a diverse class of neuromodulators. We took advantage of two types of feeding neurons, B48 and B1/B2, in the Aplysia buccal ganglion that might contain different neuropeptides. We performed a representational difference analysis (RDA) by subtraction of mRNAs in B48 versus mRNAs in B1/B2. The RDA identified an unusually long (2025 amino acids) peptide precursor encoding Aplysia leucokinin-like peptides (ALKs; e.g. ALK-1 and ALK-2). Northern blot analysis revealed that, compared with other ganglia (e.g. the pedal-pleural ganglion), ALK mRNA is predominantly present in the buccal ganglion, which controls feeding behavior. We then used in situ hybridization and immunohistochemistry to localize ALKs to specific neurons, including B48. MALDI-TOF MS on single buccal neurons revealed expression of 40 ALK precursor-derived peptides. Among these, ALK-1 and ALK-2 are active in the feeding network; they shortened the radula protraction phase of feeding motor programs triggered by a command-like neuron. We also found that this effect may be mediated by the ALK-stimulated enhancement of activity of an interneuron, which has previously been shown to terminate protraction. We conclude that our multipronged approach is effective for determining the structure and defining the diverse functions of leucokinin-like peptides. Notably, the ALK precursor is the first verified nonarthropod precursor for leucokinin-like peptides with a novel, marked modulatory effect on a specific parameter (protraction duration) of feeding motor programs.

Specificity of repetition priming: the role of chemical coding

We investigate stimulus specificity of repetition priming in a tractable model system; the feeding network of Aplysia. Previous studies primarily focused on an aspect of behavior that is altered during ingestive priming, radula opening. Priming of radula opening occurs when two modulatory peptides [feeding circuit activating peptide (FCAP) and cerebral peptide-2 (CP-2)] are released from the cholinergic command-like neuron cerebral buccal interneuron 2. Effects of FCAP/CP-2 on radula opening motor neurons are cAMP mediated. The present experiments sought to determine whether FCAP/CP-2 and cAMP are also involved in the priming of radula opening during an incompatible activity, i.e., during egestive motor programs. Egestive priming is induced when motor programs are triggered by afferents with processes in the esophageal nerve. We demonstrate that egestive priming is not FCAP/CP-2 mediated. Instead, it is induced by an unrelated peptide (small cardioactive peptide), which exerts PKC-mediated effects. Our data, therefore, suggest that different feeding motor programs are primed via actions of different sets of intercellular and intracellular substances. We suggest that this accounts for the stimulus specificity that can be characteristic of repetition priming. Different stimuli activate different central pattern generator inputs. These inputs release different modulators, which induce functionally distinct motor programs.

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.

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.

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.

Distinct inhibitory neurons exert temporally specific control over activity of a motoneuron receiving concurrent excitation and inhibition

Recent work suggests that concurrent excitation and inhibition originating in central pattern generators (CPGs) may be used to control rhythmic motoneuronal activity. The specific roles that the inhibition plays in such cases are not well understood, however, in part because of the lack of identification of presynaptic inhibitory neurons. Here we demonstrate that, in the Aplysia feeding CPG, inhibitory inputs may be critical for flexible control of the activity of motoneurons in different forms of behavior. The feeding CPG generates ingestive and egestive motor programs, differing in the high and low activity, respectively, of the motoneuron B8 during the retraction phase of the programs. We show that, during retraction, B8 receives concurrent excitation and inhibition that produces a high-conductance state. The inhibition originates in two types of CPG neurons, B4/5 and B70, that are more active in egestion than ingestion and play a role in suppressing B8 activity during egestion. In turn, the activities of both B4/5 and B70 are suppressed by the ingestion-promoting descending interneuron CBI-3 (for cerebral-buccal interneuron 3). Thus, concurrent excitation and inhibition may be an effective means of controlling motoneuronal activity in a behavior-dependent manner. More detailed analyses reveal, furthermore, that B4/5 and B70 exert complementary actions by acting preferentially in the early and late part of retraction, respectively. Thus, the use of multiple neurons to generate inhibitory inputs to motoneurons that receive concurrent excitation and inhibition brings an additional level of flexibility that allows a temporally specific control of motoneuronal activity within a single phase of motor programs.

Feeding CPG in Aplysia directly controls two distinct outputs of a compartmentalized interneuron that functions as a CPG element

In the context of motor program generation in Aplysia, we characterize several functional aspects of intraneuronal compartmentalization in an interganglionic interneuron, CBI-5/6. CBI-5/6 was shown previously to have a cerebral compartment (CC) that includes a soma that does not generate full-size action potentials and a buccal compartment (BC) that does. We find that the synaptic connections made by the BC of CBI-5/6 in the buccal ganglion counter the activity of protraction-phase neurons and reinforce the activity of retraction-phase neurons. In buccal motor programs, the BC of CBI-5/6 fires phasically, and its premature activation can phase advance protraction termination and retraction initiation. Thus the BC of CBI-5/6 can act as an element of the central pattern generator (CPG). During protraction, the CC of CBI-5/6 receives direct excitatory inputs from the CPG elements, B34 and B63, and during retraction, it receives antidromically propagating action potentials that originate in the BC of CBI-5/6. Consequently, in its CC, CBI-5/6 receives depolarizing inputs during both protraction and retraction, and these depolarizations can be transmitted via electrical coupling to other neurons. In contrast, in its BC, CBI-5/6 uses spike-dependent synaptic transmission. Thus the CPG directly and differentially controls the program phases in which the two compartments of CBI-5/6 may transmit information to its targets.

Endogenous motor neuron properties contribute to a program-specific phase of activity in the multifunctional feeding central pattern generator of Aplysia

Multifunctional central pattern generators (CPGs) are circuits of neurons that can generate manifold actions from a single effector system. This study examined a bilateral pair of pharyngeal motor neurons, designated B67, that participate in the multifunctional feeding network of Aplysia californica. Fictive buccal motor programs (BMPs) were elicited with four distinct stimulus paradigms to assess the activity of B67 during ingestive versus egestive patterns. In both classes of programs, B67 fired during the phase of radula protraction and received a potent inhibitory postsynaptic potential (IPSP) during fictive radula retraction. When programs were ingestive, the retraction phase IPSP exhibited a depolarizing sag and was followed by a postinhibitory rebound (PIR) that could generate a postretraction phase of impulse activity. When programs were egestive, the depolarizing sag potential and PIR were both diminished or were not present. Examination of the membrane properties of B67 disclosed a cesium-sensitive depolarizing sag, a corresponding I(h)-like current, and PIR in its responses to hyperpolarizing pulses. Direct IPSPs originating from the influential CPG retraction phase interneuron B64 were also found to activate the sag potential and PIR of B67. Dopamine, a modulator that can promote ingestive behavior in this system, enhanced the sag potential, I(h)-like current, and PIR of B67. Finally, a pharyngeal muscle contraction followed the radula retraction phase of ingestive, but not egestive motor patterns. It is proposed that regulation of the intrinsic properties of this motor neuron can contribute to generating a program-specific phase of motor activity.

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.

Neuromechanics of coordination during swallowing in Aplysia californica

Bernstein (1967) hypothesized that preparation of the periphery was crucial for correct responses to motor output. To test this hypothesis in a behaving animal, we examined the roles of two identified motor neurons, B7 and B8, which contribute to feeding behavior in the marine mollusk Aplysia californica. Neuron B7 innervates a hinge muscle and has no overt behavioral effect during smaller-amplitude (type A) swallows, because the hinge muscle is too short to exert force. Neuron B8 activates a muscle (I4) that acts solely to grasp material during type A swallows. During larger-amplitude (type B) swallows, the behavioral actions of both motor neurons change, because the larger-amplitude anterior movement of the grasper sets up the periphery to respond differently to motor outputs. The larger anterior movement stretches the hinge muscle, so that activating neuron B7 mediates the initial retraction phase of swallowing. The changed position of the I4 muscle allows neuron B8 not only to induce grasping but also to pull material into the buccal cavity, contributing to retraction. Thus, larger-amplitude swallows are associated with the expression of two new degrees of freedom (use of the hinge to retract and use of the grasper to retract) that are essential for mediating type B swallows. These results provide a direct demonstration of Bernstein's hypothesis that properly positioning the periphery can be crucial for its ability to correctly respond to motor output and also demonstrate that biomechanical context can alter the functions of identified motor neurons.

Changes of internal state are expressed in coherent shifts of neuromuscular activity in Aplysia feeding behavior

The multitasking central pattern generator (CPG) that drives consummatory feeding behaviors of Aplysia can produce ingestive, egestive, and intermediate motor programs. External stimuli trigger the programs but, remarkably, do not directly specify which type of program is produced. Rather, recent work has proposed, the type of program is determined by the internal network state of the CPG that has developed in response to the previous history of the stimulation. Here we have tested a key prediction of this network-state hypothesis. If the network state has a real existence and governs real functional behavior, changes in the state should be seen as coherent, coordinated changes along many dimensions of interneuron and motor neuron activity, muscle contraction, and ultimately movement, that underlie functional behavior. In reduced neuromuscular preparations, we elicited repetitive motor programs by continued stimulation of the esophageal nerve while recording the firing of motor neurons B8, B15, B16, B4/5, and B48, and contractions of the accessory radula closer and I7-I10 muscles that respectively close and open the animal's food-grasping organ, the radula. Using sonomicrometric techniques, we similarly recorded the movement of the radula in the complete buccal mass. Successive esophageal nerve programs indeed exhibited clear progressive changes in motor neuron firing, muscle contractions, and the phasing of radula movements within each cycle, from an initially intermediate or even ingestive character to a strongly egestive character. We conclude that the Aplysia feeding CPG really has a coherent internal network state whose dynamics are likely to be reflected in the real behavior of the animal.

Tight or loose coupling between components of the feeding neuromusculature of Aplysia?

Like other complex behaviors, the cyclical, rhythmic consummatory feeding behaviors of Aplysia-biting, swallowing, and rejection of unsuitable food-are produced by a complex neuromuscular system: the animal's buccal mass, with numerous pairs of antagonistic muscles, controlled by the firing of numerous motor neurons, all driven by the motor programs of a central pattern generator (CPG) in the buccal ganglia. In such a complex neuromuscular system, it has always been assumed that the activities of the various components must necessarily be tightly coupled and coordinated if successful functional behavior is to be produced. However, we have recently found that the CPG generates extremely variable motor programs from one cycle to the next, and so very variable motor neuron firing patterns and contractions of individual muscles. Here we show that this variability extends even to higher-level parameters of the operation of the neuromuscular system such as the coordination between entire antagonistic subsystems within the buccal neuromusculature. In motor programs elicited by stimulation of the esophageal nerve, we have studied the relationship between the contractions of the accessory radula closer (ARC) muscle, and the firing patterns of its motor neurons B15 and B16, with those of its antagonist, the radula opener (I7) muscle, and its motor neuron B48. There are two separate B15/B16-ARC subsystems, one on each side of the animal, and these are indeed very tightly coupled. Tight coupling can, therefore, be achieved in this neuromuscular system where required. Yet there is essentially no coupling at all between the contractions of the ARC muscles and those of the antagonistic radula opener muscle. We interpret this result in terms of a hypothesis that ascribes a higher-order benefit to such loose coupling in the neuromusculature. The variability, emerging in the successive feeding movements made by the animal, diversifies the range of movements and thereby implements a trial-and-error search through the space of movements that might be successful, an optimal strategy for the animal in an unknown, rapidly changing feeding environment.

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.

Cycle-to-Cycle Variability of Neuromuscular Activity inAplysiaFeeding Behavior

Aplysia consummatory feeding behavior, a rhythmic cycling of biting, swallowing, and rejection movements, is often said to be stereotyped. Yet closer examination shows that cycles of the behavior are very variable. Here we have quantified and analyzed the variability at several complementary levels in the neuromuscular system. In reduced preparations, we recorded the motor programs produced by the central pattern generator, firing of the motor neurons B15 and B16, and contractions of the accessory radula closer (ARC) muscle while repetitive programs were elicited by stimulation of the esophageal nerve. In other similar experiments, we recorded firing of motor neuron B48 and contractions of the radula opener muscle. In intact animals, we implanted electrodes to record nerve or ARC muscle activity while the animals swallowed controlled strips of seaweed or fed freely. In all cases, we found large variability in all parameters examined. Some of this variability reflected systematic, slow, history-dependent changes in the character of the central motor programs. Even when these trends were factored out, however, by focusing only on the differences between successive cycles, considerable variability remained. This variability was apparently random. Nevertheless, it too was the product of central history dependency because regularizing merely the high-level timing of the programs also regularized many of the downstream neuromuscular parameters. Central motor program variability thus appears directly in the behavior. With regard to the production of functional behavior in any one cycle, the large variability may indicate broad tolerances in the operation of the neuromuscular system. Alternatively, some cycles of the behavior may be dysfunctional. Overall, the variability may be part of an optimal strategy of trial, error, and stabilization that the CNS adopts in an uncertain environment.

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.

Ionic currents underlying fast action potentials in the obliquely striated muscle cells of the octopus arm

The octopus arm provides a unique model for neuromuscular systems of flexible appendages. We previously reported the electrical compactness of the arm muscle cells and their rich excitable properties ranging from fast oscillations to overshooting action potentials. Here we characterize the voltage-activated ionic currents in the muscle cell membrane. We found three depolarization-activated ionic currents: 1) a high-voltage-activated L-type Ca(2+) current, which began activating at approximately -35 mV, was eliminated when Ca(2+) was substituted by Mg(2+), was blocked by nifedipine, and showed Ca(2+)-dependent inactivation. This current had very rapid activation kinetics (peaked within milliseconds) and slow inactivation kinetics (tau in the order of 50 ms). 2) A delayed rectifier K(+) current that was totally blocked by 10 mM TEA and partially blocked by 10 mM 4-aminopyridine (4AP). This current exhibited relatively slow activation kinetics (tau in the order of 15 ms) and inactivated only partially with a time constant of ~150 ms. And 3) a transient A-type K(+) current that was totally blocked by 10 mM 4AP and was partially blocked by 10 mM TEA. This current exhibited very fast activation kinetics (peaked within milliseconds) and inactivated with a time constant in the order of 60 ms. Inactivation of the A-type current was almost complete at -40 mV. No voltage-dependent Na(+) current was found in these cells. The octopus arm muscle cells generate fast (~3 ms) overshooting spikes in physiological conditions that are carried by a slowly inactivating L-type Ca(2+) current.

A kinematic model of swallowing inAplysia californicabased on radula/odontophore kinematics andin vivomagnetic resonance images

A kinematic model of the buccal mass of Aplysia californica during swallowing has been developed that incorporates the kinematics of the odontophore, the muscular structure that underlies the pincer-like grasping structure, the radula. The model is based on real-time magnetic resonance images (MRIs) of the mid-sagittal cross section of the buccal mass during swallowing. Using kinematic relationships derived from isolated odontophores induced to perform feeding-like movements, the model generates predictions about movement of the buccal mass in the medio-lateral dimension during the feeding cycle that are well-matched to corresponding coronal MRIs of the buccal mass during swallowing. The model successfully reproduces changes in the lengths of the intrinsic (I) buccal muscles I2 and I3 measured experimentally. The model predicts changes in the length of the radular opener muscle I7 throughout the swallowing cycle, generates hypotheses about the muscular basis of radular opening prior to the onset of forward rotation during swallowing and suggests possible context-dependent functions for the I7 muscle, the radular stalk and the I5 (ARC) muscle during radular opening and closing.

Radula-centric and odontophore-centric kinematic models of swallowing inAplysia californica

Two kinematic models of the radula/odontophore of the marine mollusc Aplysia californica were created to characterize the movement of structures inside the buccal mass during the feeding cycle in vivo. Both models produce a continuous range of three-dimensional shape changes in the radula/odontophore, but they are fundamentally different in construction. The radulacentric model treats the radular halves as rigid bodies that can pitch, yaw and roll relative to a fixed radular stalk, thus creating a three-dimensional shape. The odontophore-centric model creates a globally convex solid representation of the radula/odontophore directly, which then constrains the positions and shapes of internal structures. Both radula/odontophore models are placed into a pre-existing kinematic model of the I1/I3 and I2 muscles to generate three-dimensional representations of the entire buccal mass. High-temporal-resolution, mid-sagittal magnetic resonance(MR) images of swallowing adults in vivo are used to provide non-invasive, artifact-free shape and position parameter inputs for the models. These images allow structures inside the buccal mass to be visualized directly, including the radula, radular stalk and lumen of the I1/I3 cavity. Both radula-centric and odontophore-centric models were able to reproduce two-dimensional, mid-sagittal radula/odontophore and buccal mass kinematics,but the odontophore-centric model's predictions of I1/I3, I2 and I7 muscle dimensions more accurately matched data from MR-imaged adults and transilluminated juveniles.

Kinematics of the buccal mass during swallowing based on magnetic resonance imaging in intact, behaving Aplysia californica

A novel magnetic resonance imaging interface has been developed that makes it possible to image movements in intact, freely moving subjects. We have used this interface to image the internal structures of the feeding apparatus (i.e. the buccal mass) of the marine mollusc Aplysia californica. The temporal and spatial resolution of the resulting images is sufficient to describe the kinematics of specific muscles of the buccal mass and the internal movements of the main structures responsible for grasping food, the radula and the odontophore. These observations suggest that a previously undescribed feature on the anterior margin of the odontophore, a fluid-filled structure that we term the prow, may aid in opening the jaw lumen early in protraction. Radular closing during swallowing occurs near the peak of protraction as the radular stalk is pushed rapidly out of the odontophore. Retraction of the odontophore is enhanced by the closure of the lumen of the jaws on the elongated odontophore, causing the odontophore to rotate rapidly towards the esophagus. Radular opening occurs after the peak of retraction and without the active contraction of the protractor muscle 12 and is due, in part, to the movement of the radular stalk into the odontophore. The large variability between responses also suggests that the great flexibility of swallowing responses may be due to variability in neural control and in the biomechanics of the ingested food and to the inherent flexibility of the buccal mass.

Interneuronal and peptidergic control of motor pattern switching in Aplysia

It has been proposed that a choice of specific behaviors can be mediated either by activation of behavior-specific higher order neurons or by distinct combinations of such neurons in different behaviors. We examined the role that two higher order neurons, CBI-2 and CBI-3, play in the selection of motor programs that correspond to ingestion and egestion, two stimulus-dependent behaviors that are generated by a single central pattern generator (CPG) of Aplysia. We found that CBI-2 could evoke either ingestive, egestive, or ambiguous motor programs depending on the regime of stimulation. When CBI-2 recruited CBI-3 firing via electrical coupling, the motor program tended to be ingestive. In the absence of CBI-3 activation, the program was usually egestive. When CBI-2 was stimulated to produce ingestive programs, hyperpolarization of CBI-3 converted the programs to egestive or ambiguous. When CBI-2 was stimulated to produce egestive or ambiguous programs, co-stimulation of CBI-3 converted them into ingestive. These findings are consistent with the idea that combinatorial commands are responsible for the choice of specific behaviors. Additional support for this view comes from the observations that appropriate stimulus conditions exist both for activation of CBI-2 together with CBI-3, and for activation of CBI-2 without a concomitant activation of CBI-3. The ability of CBI-3 to convert egestive and ambiguous programs into ingestive ones was mimicked by application of APGWamide, a neuropeptide that we have detected in CBI-3 by immunostaining. Thus combinatorial actions of higher order neurons that underlie pattern selection may involve the use of modulators released by specific higher order neurons.

Sonometric measurements of motor-neuron-evoked movements of an internal feeding structure (the radula) in Aplysia

In many systems used to study rhythmic motor programs, the structures that generate behavior are at least partially internal. In these systems, it is often difficult to directly monitor neurally evoked movements. As a consequence, although motor programs are relatively well characterized, it is generally less clear how neural activity is translated into functional movements. This is the case for the feeding system of the mollusk Aplysia. Here we used sonomicrometry to monitor neurally evoked movements of the food-grasping organ in Aplysia, the radula. Movements were evoked by intracellular stimulation of motor neurons that innervate radula muscles that have been extensively studied in reduced preparations. Nevertheless our results indicate that the movements and neural control of the radula are more complex than has been assumed. We demonstrate that motor neurons previously characterized as radula openers (B48) and closers (B8, B15, B16) additionally produce other movements. Moreover, we show that the size of the movement evoked by a motor neuron can depend on the preexisting state of the radula. Specifically, the motor neurons B15 and B16 produce large closing movements when the radula is partially open but produce relatively weak closing movements in a preparation at rest. Thus the efficacy of B15 and B16 as radula closers is context dependent.

The neuronal basis of feeding in the snail, Helisoma, with comparisons to selected gastropods

Research on identified neurons during the last quarter century was forecast at a conference in 1973 that discussed "neuronal mechanisms of coordination in simple systems." The focus of the conference was on the neuronal control of simple stereotyped behavioral acts. Participants discussing the future of such research called for a comparative approach; emphasis on structure-function interactions; attention to environmental and behavioral context; and the development of new techniques. Significantly, in some cases amazing progress has been made in these areas. Major conclusions of the last quarter century are that so-called simple behaviors and the neural circuitry underlying them tend to be less simple, more flexible, and more highly modulated than originally imagined. However, the comparative approach has, as yet, failed to reach its potential. Molluscan preparations, along with arthropods and annelids, have always been at the forefront of neuroethological studies. Circuitry underlying feeding has been studied in a handful of species of gastropod molluscs. These studies have contributed substantially to our understanding of sensorimotor organization, the hierarchical control of behavior and coordination of multiple behaviors, and the organization and modulation of central pattern generators. However, direct interspecific comparisons of feeding circuitry and potentially homologous neurons have been lacking. This is unfortunate because much of the vast radiation of the class Gastropoda is associated with variations in feeding behaviors and feeding apparatuses, providing ample substrates for comparative studies including the evolution of defined circuitry. Here, the neural organization of feeding in the snail, Helisoma, is examined critically. Possible direct interspecific comparisons of neural circuitry and potentially homologous neurons are made. A universal model for central pattern generators underlying rasping feeding is proposed. Future comparative studies can be expected to combine behavioral, morphological, electrophysiological, molecular and genetic techniques to identify neurons and define neural circuitry. Digital resources will undoubtedly be exploited to organize and interface databases allowing illumination of the evolution of homologous identified neurons and defined neural circuitry in the context of behavioral change.

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

Plasticity of Aplysia feeding has largely been measured by noting changes in radula protraction. On the basis of previous work, it has been suggested that peripheral modulation may contribute to behavioral plasticity. However, peripheral plasticity has not been demonstrated in the neuromuscular systems that participate in radula protraction. Therefore in this study we investigated whether contractions of a major radula protraction muscle (I2) are subject to modulation. We demonstrate, first, that an increase in the firing frequency of the cholinergic I2 motoneurons will increase the amplitude of the resulting muscle contraction but will not modulate its relaxation rate. We show, second, that neuronal processes on the I2 muscle are immunoreactive to myomodulin (MM), RFamide, and serotonin (5-HT), but not to small cardioactive peptide (SCP) or buccalin. The I2 motoneurons B31, B32, B61, and B62 are not immunoreactive to RFamide, 5-HT, SCP, or buccalin. However, all four cells are MM immunoreactive and are capable of synthesizing MMa. Third, we show that the bioactivity of the different modulators is somewhat different; while the MMs (i.e., MMa and MMb) and 5-HT increase I2 muscle relaxation rate, and potentiate muscle contraction amplitude, MMa, at high concentrations, depresses muscle contractions. Fourth, our data suggest that cAMP at least partially mediates effects of modulators on contraction amplitude and relaxation rate.

Classical conditioning of feeding in Aplysia: II. Neurophysiological correlates

Feeding behavior in Aplysia californica can be classically conditioned using tactile stimulation of the lips as conditional stimulus (CS) and food as unconditional stimulus (US) [ (companion paper)]. Conditioning resulted in an increase in the number of CS-evoked bites that persisted for at least 24 hr after training. In this study, neurophysiological correlates of classical conditioning training were identified and characterized in an in vitro preparation of the cerebral and buccal ganglia. Stimulation of a lip nerve (AT(4)), which mediates mechanosensory information, resulted in a greater number of buccal motor patterns (BMPs) in ganglia isolated from animals that had received paired training than in ganglia from control animals. The majority of the evoked BMPs were classified as ingestion-like patterns. Intracellular recordings from pattern-initiating neuron B31/32 revealed that stimulation of AT(4) evoked greater excitatory input in B31/32 in preparations from animals that had received paired training than from control animals. In contrast, excitatory input to buccal neuron B4/5 in response to stimulation of AT(4) was not significantly increased by paired training. Moreover, correlates of classical conditioning were specific to stimulation of AT(4). The number of spontaneously occurring BMPs and the intrinsic properties of two buccal neurons (B4/5 and B31/32) did not differ between groups. These results suggest that appetitive classical conditioning of feeding resulted in the pairing-specific strengthening of the polysynaptic pathway between afferent fibers and pattern-initiating neurons of the buccal central pattern generator.

A proprioceptive role for an exteroceptive mechanoafferent neuron in Aplysia

Afferent regulation of centrally generated activity is likely to be more complex than has been established. We show that a neuron that is an exteroceptor can also function as a proprioceptor. We study the Aplysia neuron B21. Previous data suggest that B21 functions as an exteroceptor during the radula closing/retraction phase of ingestive feeding. We show that the tissue innervated by B21, the subradula tissue (SRT), is innervated by a motor neuron (B66) and that B66-induced SRT contractions trigger centripetal spikes in B21. Thus, B21 is also a proprioceptor. To determine whether exteroceptive and proprioceptive activities occur during the same phase of ingestive feeding, we further characterize B66. We show that B66 stimulation does not close or retract the radula. Instead it opens it. Moreover, B66 is electrically coupled to other opening/protraction neurons. Finally, we elicit motor programs in semi-intact preparations and show that during radula opening/protraction we observe B66 activity, SRT contractions, and spikes in B21 that can be eliminated if B66 is indirectly hyperpolarized. B21 is, therefore, likely to act as an exteroceptor during one phase of ingestive feeding and as a proprioceptor during the antagonistic phase. Previous experiments have shown that centripetal spikes in B21 are only transmitted to one follower if they are "gated in" by depolarization. During ingestive programs B21 is centrally depolarized during closing/retraction, but it is not depolarized during opening/protraction. We sought to determine whether there are other followers that receive B21 input when it is not centrally depolarized. We found one such cell. Moreover, we found that stimulation of B21 during radula opening/protraction significantly decreases the duration of this phase of behavior. Thus, proprioceptive activity in B21 is likely to have an impact on motor programs.

Diverse synaptic connections between peptidergic radula mechanoafferent neurons and neurons in the feeding system of Aplysia

The buccal ganglion of Aplysia contains a heterogeneous population of peptidergic, radula mechanoafferent (RM) neurons. To investigate their function, two of the larger RM cells (B21, B22) were identified by morphological and electrophysiological criteria. Both are low-threshold, rapidly adapting, mechanoafferent neurons that responded to touch of the radula, the structure that grasps food during ingestive and egestive feeding movements. Sensory responses of the cells consisted of spike bursts at frequencies of 8-35 Hz. Each cell was found to make chemical, electrical, or combined synapses with other sensory neurons, motor neurons and interneurons involved in radula closure and/or protraction-retraction movements of the odontophore. Motor neurons receiving input included the following: B8a/b, B15, and B16, which innervate muscles contributing to radula closing; and B82, a newly identified neuron that innervates the anterodorsal region of the I1/I3 muscles of the buccal mass. B21 and B22 can affect buccal motor programs by way of their connections to interneurons such as B19 and B64. Fast, chemical, excitatory postsynaptic potentials (EPSPs) produced by RM neurons, such as B21, exhibited strong, frequency-dependent facilitation, a form of homosynaptic plasticity. Firing B21 also produced a slow EPSP in B15 that increased the excitability of the cell. Thus a sensory neuron mediating a behavioral response may have modulatory effects. The data suggest multiple functions for RM neurons including 1) triggering of phase transitions in rhythmic motor programs, 2) adjusting the force of radula closure, 3) switching from biting to swallowing or swallowing to rejection, and 4) enhancing food-induced arousal.

The neuromuscular transform: the dynamic, nonlinear link between motor neuron firing patterns and muscle contraction in rhythmic behaviors

The nervous system issues motor commands to muscles to generate behavior. All such commands must, however, pass through a filter that we call here the neuromuscular transform (NMT). The NMT transforms patterns of motor neuron firing to muscle contractions. This work is motivated by the fact that the NMT is far from being a straightforward, transparent link between motor neuron and muscle. The NMT is a dynamic, nonlinear, and modifiable filter. Consequently motor neuron firing translates to muscle contraction in a complex way. This complexity must be taken into account by the nervous system when issuing its motor commands, as well as by us when assessing their significance. This is the first of three papers in which we consider the properties and the functional role of the NMT. Physiologically, the motor neuron-muscle link comprises multiple steps of presynaptic and postsynaptic Ca(2+) elevation, transmitter release, and activation of the contractile machinery. The NMT formalizes all these into an overall input-output relation between patterns of motor neuron firing and shapes of muscle contractions. We develop here an analytic framework, essentially an elementary dynamical systems approach, with which we can study the global properties of the transformation. We analyze the principles that determine how different firing patterns are transformed to contractions, and different parameters of the former to parameters of the latter. The key properties of the NMT are its nonlinearity and its time dependence, relative to the time scale of the firing pattern. We then discuss issues of neuromuscular prediction, control, and coding. Does the firing pattern contain a code by means of which particular parameters of motor neuron firing control particular parameters of muscle contraction? What information must the motor neuron, and the nervous system generally, have about the periphery to be able to control it effectively? We focus here particularly on cyclical, rhythmic contractions which reveal the principles particularly clearly. Where possible, we illustrate the principles in an experimentally advantageous model system, the accessory radula closer (ARC)-opener neuromuscular system of Aplysia. In the following papers, we use the framework developed here to examine how the properties of the NMT govern functional performance in different rhythmic behaviors that the nervous system may command.

Modulation of fictive feeding by dopamine and serotonin in aplysia

The buccal ganglia of Aplysia contain a central pattern generator (CPG) that mediates rhythmic movements of the buccal apparatus during feeding. Activity in this CPG is believed to be regulated, in part, by extrinsic serotonergic inputs and by an intrinsic and extrinsic system of putative dopaminergic cells. The present study investigated the roles of dopamine (DA) and serotonin (5-HT) in regulating feeding movements of the buccal apparatus and properties of the underlying neural circuitry. Perfusing a semi-intact head preparation with DA (50 microM) or the metabolic precursor of catecholamines (L-3-4-dihydroxyphenylalanine, DOPA, 250 microM) induced feeding-like movements of the jaws and radula/odontophore. These DA-induced movements were similar to bites in intact animals. Perfusing with 5-HT (5 microM) also induced feeding-like movements, but the 5-HT-induced movements were similar to swallows. In preparations of isolated buccal ganglia, buccal motor programs (BMPs) that represented at least two different aspects of fictive feeding (i.e., ingestion and rejection) could be recorded. Bath application of DA (50 microM) increased the frequency of BMPs, in part, by increasing the number of ingestion-like BMPs. Bath application of 5-HT (5 microM) did not significantly increase the frequency of BMPs nor did it significantly increase the proportion of ingestion-like BMPs being expressed. Many of the cells and synaptic connections within the CPG appeared to be modulated by DA or 5-HT. For example, bath application of DA decreased the excitability of cells B4/5 and B34, which in turn may have contributed to the DA-induced increase in ingestion-like BMPs. In summary, bite-like movements were induced by DA in the semi-intact preparation, and neural correlates of these DA-induced effects were manifest as an increase in ingestion-like BMPs in the isolated ganglia. Swallow-like movements were induced by 5-HT in the semi-intact preparation. Neural correlates of these 5-HT-induced effects were not evident in isolated buccal ganglia, however.

The neuromuscular transform constrains the production of functional rhythmic behaviors

We continue our study of the properties and the functional role of the neuromuscular transform (NMT). The NMT is an input-output relation that formalizes the processes by which patterns of motor neuron firing are transformed to muscle contractions. Because the NMT acts as a dynamic, nonlinear, and modifiable filter, the transformation is complex. In the preceding paper we developed a framework for analysis of the NMT and identified with it principles by which the NMT transforms different firing patterns to contractions. The ultimate question is functional, however. In sending different firing patterns through the NMT, the nervous system is seeking to command different functional behaviors, with specific contraction requirements. To what extent do the contractions that emerge from the NMT actually satisfy those requirements? In this paper we extend our analysis to address this issue. We define representative behavioral tasks and corresponding measures of performance, for a single neuromuscular unit, for two antagonistic units, and, in a real illustration, for the accessory radula closer (ARC)-opener neuromuscular system of Aplysia. We focus on cyclical, rhythmic behaviors which reveal the underlying principles particularly clearly. We find that, although every pattern of motor neuron firing produces some state of muscle contraction, only a few patterns produce functional behavior, and even fewer produce efficient functional behavior. The functional requirements thus dictate certain patterns to the nervous system. But many desirable functional behaviors are not possible with any pattern. We examine, in particular, how rhythmic behaviors degrade and disintegrate as the nervous system attempts to speed up their cycle frequency. This happens because, with fixed properties, the NMT produces only a limited range of contraction shapes that are kinetically well matched to the firing pattern only on certain time scales. Thus the properties of the NMT constrain and restrict the production of functional behaviors. In the following paper, we see how the constraint may be alleviated and the range of functional behaviors expanded by appropriately tuning the properties of the NMT through neuromuscular plasticity and modulation.

Synaptic modulation contributes to firing pattern generation in jaw motor neurons during rejection of seaweed in Aplysia kurodai

Japanese species, Aplysia kurodai, feeds well on Ulva but rejects Gelidium (Geli.) or Pachydictyon (Pach.) with different rhythmic patterned movements of the jaws and radula. During ingestion the jaws open at the radula-protraction phase and remain half open at the initial phase of the radula-retraction, whereas during rejection the jaws open similarly but start to close immediately after the onset of the radula-retraction. These can be induced not only by the natural seaweed but by the extract solutions. We previously showed that the change of the patterned jaw movements from the ingestion to the rejection may result from the decrease in the delay of the firing onset of the jaw-closing (JC) motor neurons during their depolarization. This diminished delay produces a phase advance relative to the radula-retraction phase. In that study, we showed that during ingestion the buccal multiaction (MA) neurons may generate the delay of firing onset of the JC motor neurons by producing monosynaptic inhibitory postsynaptic potentials (IPSPs) during the initial portion of their depolarization. In the present experiments, the firing patterns in the MA neurons induced by application of the Geli. or Pach. extract to the lips were explored in the semi-intact preparations. During the Pach. response the duration and the firing frequency of the MA firing at each depolarizing phase were decreased in comparison with the Ulva response. No decreases in the MA firing were observed during the Geli. response, suggesting that the similar patterned jaw movements during rejection of Geli. and Pach. may be generated by different neural mechanisms. Moreover, the size of the MA-induced IPSP in the JC motor neurons was largely decreased by application of the Geli. or Pach. extract to the lips in the reduced preparations consisting of the tentacle-lips and the cerebral-buccal ganglia. Voltage-clamp experiments on the JC motor neurons showed that the size of synaptic current induced by the MA spikes was decreased by application of these solutions to the lips. The decrease was induced when the buccal ganglia were bathed in a solution to block polysynaptic pathways. These results suggest that the advance of the onset of the JC firing at each depolarizing phase during the Geli. or Pach. response may be mainly or partly caused by the decrease in the size of the MA-induced IPSP in the motor neurons. Modulatory action of cerebral neurons or peripheral afferent neurons in the lip region may contribute to this synaptic plasticity.

Differential modulation of motor neurons that innervate the same muscle but use different excitatory transmitters in aplysia

The medial portion of intrinsic buccal muscle 3 (I3m) is innervated by two excitatory motor neurons, B3 and B9. B3 uses glutamate as its fast transmitter and expresses the neuropeptide FMRFamide, whereas B9 uses acetylcholine as its fast transmitter and expresses the neuropeptide SCP. This preparation was used to study peptidergic modulation of muscles innervated by neurons that use different fast excitatory transmitters. First, we determined the effects of the application of the neuropeptides expressed in these neurons on excitatory junction potentials (EJPs) and contractions. FMRFamide increased the amplitude of EJPs and contractions evoked by B3 while decreasing those evoked by B9. This is the first observation in buccal muscle of a substance that modulates two excitatory neurons innervating the same muscle in opposite directions. SCP increased EJPs contraction amplitude, and the rate of muscle relaxation for both motor neurons. We determined that SCP potently increased cAMP levels in I3m as it does in other buccal muscles. Stimulation of B9 also caused increased cAMP levels in I3m providing independent evidence for SCP release. Finally, stimulation of B9 increased both the contraction amplitude and relaxation rate of B3-evoked I3m contractions in a manner similar to that observed using exogenous SCP. By inhibiting B9's cholinergic transmission with an antagonist, we were able to observe modulatory effects of B9 in the absence of fast excitatory effects. We found that the magnitude of the modulation was dependent on the firing frequency and did occur at frequencies and patterns of firing recorded previously for B9 during ingestive-like motor programs.

Modulation of radula opener muscles in Aplysia

We observed fibers immunoreactive (IR) to serotonin (5-HT), the myomodulins (MMs), and FMRFamide on the I7-I10 complex in the marine mollusk Aplysia californica. The I7-I10 muscle complex, which produces radula opening, is innervated primarily by one motor neuron, B48. B48 is MM-IR and synthesizes authentic MM(A). When B48 is stimulated in a physiological manner, cAMP levels are increased in opener muscles. cAMP increases also are seen when the MMs are applied to opener muscles but are not seen with application of the B48 primary neurotransmitter acetylcholine (ACh). Possible physiological sources of 5-HT and FMRFamide are discussed. When modulators are applied to resting opener muscles, changes in membrane potential are observed. Specifically, 5-HT, MM(B), and low concentrations of MM(A) all depolarize muscle fibers. This depolarization is generally not sufficient to elicit myogenic activity in the absence of neural activity under "rest" conditions. However, if opener muscles are stretched beyond rest length, stretch- and modulator-induced depolarizations can summate and elicit contractions. This only occurs, however, if "depolarizing" modulators are applied alone. Thus other modulators (i.e., FMRFamide and high concentrations of MM(A)) hyperpolarize opener muscle fibers and can prevent depolarizing modulators from eliciting myogenic activity. All modulators tested affected parameters of motor neuron-elicited contractions of opener muscles. MM(B) and 5-HT increased contraction size over the range of concentrations tested, whereas MM(A) potentiated contractions when it was applied at lower concentrations but decreased contraction size at higher concentrations. FMRFamide decreased contraction size at all concentrations and did not affect relaxation rate. Additionally, the MMs and 5-HT increased muscle relaxation rate, decreased contraction latency, and decreased the rate at which tension was developed during motor neuron-elicited muscle contractions. Thus these modulators dramatically affect the ability of opener muscles to follow activity in the opener motor neuron B48. The possible physiological significance of these findings is discussed.

Glutamate is a fast excitatory transmitter at some buccal neuromuscular synapses in Aplysia

Studies of the modulation of synaptic transmission in buccal muscle of Aplysia were limited because the conventional fast transmitter used by a number of large buccal motor neurons was unknown. Most of the identified buccal motor neurons are cholinergic because they synthesize acetylcholine (ACh) and their excitatory junction potentials (EJPs) are blocked by the cholinergic antagonist hexamethonium. However, three large identified motor neurons (B3, B6, and B38) do not synthesize ACh and their EJPs are not inhibited by hexamethonium. To identify the fast excitatory transmitter used by these noncholinergic motor neurons, we surveyed putative transmitters for their ability to evoke contractions. Of the noncholinergic transmitters tested, glutamate was the most effective at evoking contractions. The pharmacology of the putative glutamate receptor is different from previously characterized glutamate receptors in that glutamate agonists and antagonists previously used to classify glutamate receptors had little effect in this system. In addition, glutamate itself was the most effective agent tested at reducing EJPs evoked by the noncholinergic motor neurons presumably by desensitizing glutamate receptors. Finally, immunocytology using an antiserum raised to conjugated glutamate in parallel with intracellular fills indicated that the varicose axons of these motor neurons were glutamate-immunoreactive. Taken together, these results indicate that the fast transmitter used by the noncholinergic neurons is almost certainly glutamate itself. This information should help us understand the role of transmitters and cotransmitters in the generation of feeding behaviors in Aplysia.

A pair of reciprocally inhibitory histaminergic sensory neurons are activated within the same phase of ingestive motor programs in Aplysia

Previous studies have shown that each buccal ganglion in Aplysia contains two B52 neurons, one in each hemiganglion. We now show that there are two B52 neurons in a single buccal hemiganglion and four cells in an animal. We also show that the B52 neurons are histamine-immunoreactive and use reverse phase HPLC to show that the histamine-immunoreactive substance is authentic histamine. Previous studies have shown that the B52 neurons make numerous inhibitory synaptic connections with neurons active during the radula closing/retraction phase of ingestive motor programs. A computational model of the Aplysia feeding central pattern generator has, therefore, suggested that the B52 neurons play a role in terminating closing/retraction. Consistent with this idea we show that both B52 neurons fire at the beginning of radula opening/protraction. We also show that both B52 neurons are sensory neurons. They are depolarized when a flap of connective tissue adjacent to the buccal commissural arch is stretched. During ingestive feeding this is likely to occur at the peak of closing/retraction as opening/protraction begins. In the course of this study we compare the two ipsilateral B52 neurons and show that these cells are virtually indistinguishable; e.g., they use a common neurotransmitter, make the same synaptic connections, and are both sensory as well as premotor neurons. Nevertheless we show that the B52 neurons are reciprocally inhibitory. Our results, therefore, strikingly confirm theoretical predictions made by others that neurons that inhibit each other will not necessarily participate in antagonistic phases of 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.

Proprioceptive input to feeding motor programs in Aplysia

Although central pattern generators (CPGs) can produce rhythmic activity in isolation, it is now generally accepted that under physiological conditions information from the external and internal environment is incorporated into CPG-induced motor programs. Experimentally advantageous invertebrate preparations may be particularly useful for studies that seek to characterize the cellular mechanisms that make this possible. In these experiments, we study sensorimotor integration in the feeding circuitry of the mollusc Aplysia. We show that a premotor neuron with plateau properties, B51, is important for generating the radula closing/retraction phase of ingestive motor programs. When B51 is depolarized in semi-intact preparations, radula closing/retractions are enhanced. When B51 is hyperpolarized, radula closing/retractions are reduced in size. In addition to being important as a premotor interneuron, B51 is also a sensory neuron that is activated when the feeding apparatus, the radula, rotates backward. The number of centripetal spikes in B51 is increased if the resistance to backward rotation is increased. Thus, B51 is a proprioceptor that is likely to be part of a feedback loop that insures that food will be moved into the buccal cavity when difficulty is encountered. Our data suggest, therefore, that Aplysia are able to adjust feeding motor programs to accommodate the specific qualities of the food ingested because at least one of the neurons that generates the basic ingestive motor program also serves as an on-line monitor of the success of radula movements.

Serotonergic neurons differentially modulate the efficacy of two motor neurons innervating the same muscle fibers in Aplysia

Feeding behavior in Aplysia shows substantial plasticity. An important site for the generation of this plasticity is the modulation of synaptic transmission between motor neurons and the buccal muscles that generate feeding movements. We have been studying this modulation in the anterior portion of intrinsic buccal muscle 3 (I3a), which is innervated by two excitatory motor neurons and identified serotonergic modulatory neurons, the metacerebral cells (MCCs). We have shown previously that serotonin (5-HT) applied selectively to the muscle potently modulates excitatory junction potentials (EJPs) and contractions. All the effects of 5-HT were persistent, lasting many hours after wash out. We examined whether the release of endogenous 5-HT from the MCC could produce effects similar to the application of 5-HT. Stimulation of the MCCs did produce similar short-term effects to the application of 5-HT. MCC stimulation facilitates EJPs, potentiates contractions, and decreases the latency between the onset of a motor neuron burst and the onset of the evoked contraction. The effects of MCC stimulation reached a maximum at quite low firing frequencies, which were in the range of those previously recorded during feeding behavior. The maximal effects were similar to those produced by superfusion with approximately 0.1 microM 5-HT. Although the effects of MCC stimulation on EJPs were persistent, they were less persistent than the effects of 0.1 microM 5-HT. Mechanisms that may account for differences in the persistence between released and superfused 5-HT are discussed. Thus activity in the MCCs has dramatic short-term effects on the behavioral output of motor neurons, increasing the amplitude and relaxation rate of contractions evoked by both B3 and B38 and shifting the temporal relationship between B38 bursts and evoked contractions.

Ion currents and mechanisms of modulation in the radula opener muscles of Aplysia

Ion currents and mechanisms of modulation in the radula opener muscles of Aplysia. J. Neurophysiol. 78: 2372-2387, 1997. Numerous studies of plasticity in the feeding behavior of Aplysia have shown that substantial plasticity is due to peripheral neuromodulation of the feeding musculature. Extensive previous work focusing on the accessory radula closer (ARC) muscle has led to the realization that a major function of the modulation in that muscle may be to ensure efficient coordination between its contractions and those of its antagonist muscles. For a more complete understanding, therefore, we must study these muscles also. Here we have studied the radula opener muscles I7-I10. Using single isolated muscle fibers under voltage clamp, we have characterized ion currents gated by voltage and by the physiological contraction-inducing neurotransmitter acetylcholine (ACh) and the effects of the physiological modulators serotonin, myomodulins A and B, and FMRFamide. Our results explain significant aspects of the electrophysiological behavior of the whole opener muscles, as well as why the opener and ARC muscles behave similarly in many ways yet differently in some key respects. Opener muscles express four types of K currents: inward rectifier, A-type [IK(A)], delayed rectifier [IK(V)], and Ca2+-activated [IK(Ca)]. They also express an L-type Ca current [ICa] and a leakage current. ACh activates a positive-reversing cationic current [IACh(cat)] and a negative-reversing Cl current [IACh(Cl)]. The opener muscles differ from the ARC in that, in the openers, activation of IK(A) occurs approximately 9 mV more positive and there is much less IACh(Cl). In both muscles, IACh(cat) most likely serves to depolarize the muscle until ICa activates to supply Ca2+ for contraction, but further depolarization and spiking is opposed by coactivation of IK(A), IK(V), IK(Ca), and IACh(Cl). Thus the differences in IK(A) and IACh(Cl) may well be key factors that prevent spikes in the ARC but often allow them in the opener muscles. As in the ARC, the modulators enhance ICa and so potentiate contractions. They also activate a modulator-specific K current, which causes hyperpolarization and depression of contractions. Finally, in the opener muscles but not in the ARC, the modulators activate a depolarizing cationic current that may help phase-advance the contractions. Each modulator exerts these effects to different degrees and thus has a distinct effect on voltage and contraction size and shape. The overall effect then will depend on the specific combinations of modulators released in different behaviors. By understanding the modulation in the opener muscles, as well as in the ARC, we are now in a position to understand how the behavior of the two muscles is coordinated under a variety of circumstances.