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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.

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.

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.

A major goal of neuroscience is to identify the neural and cellular mechanisms of behavior and its plasticity. Progress toward this goal has come particularly from work with a small number of tractable model preparations. One of these is the simple neuromuscular circuit consisting of the accessory radula closer (ARC) muscle of the mollusk Aplysia californica and its innervating motor and modulatory neurons. Contraction of the ARC muscle underlies a component of Aplysia feeding behavior, and plasticity of the behavior is in large part due to modulation of the amplitude and duration of the contractions of the muscle by a variety of modulatory neurotransmitters and peptide cotransmitters, among them the small cardioactive peptides (SCPs), myomodulins (MMs), and serotonin (5-HT). We have studied single dissociated ARC muscle fibers in order to determine whether modulation of membrane ion currents in the muscle might underlie these effects. First, we confirmed that the dissociated fibers were functionally intact: just as with the whole ARC muscle, their contractions were potentiated by 5-HT and SCPB and potentiated as well as depressed by MMA, and their cAMP content was greatly elevated by 5-HT, SCPA and SCPB, and to a lesser extent by MMA and MMB. Next, using voltage-clamp techniques, we found that two ion currents present in the fibers were indeed modulated. The fibers possess a dihydropyridine-sensitive, high-threshold "L"-type Ca current. This current was enhanced by the modulators that potentiate ARC-muscle contractions--5-HT, SCPA and SCPB, and MMA and MMB--but not by buccalinA, a modulator that does not act directly on the ARC muscle. All of the potentiating modulators, as well as elevation of cAMP in the fibers by forskolin or a cAMP analog, maximally enhanced the current about twofold and mutually occluded each other's effects. Since the Ca current supplies Ca2+ necessary for contraction of the muscle, the enhancement of the current is a good candidate to be a major mechanism of the potentiation of the contractions. In the following article we report that the modulators also, to different degrees, activate a distinctive K current and thereby depress the contractions. Net potentiation or depression then depends on the balance between the relative strengths of the modulation of the two ion currents.

The neural and cellular mechanisms of plasticity apparent in the feeding behavior of the mollusk Aplysia californica have been extensively studied in a simple neuromuscular circuit consisting of the accessory radula closer (ARC) muscle and its innervating motor and modulatory neurons. In this circuit, the plasticity is largely due to modulation of the amplitude and duration of the contractions of the muscle by a variety of modulatory neurotransmitters and peptide cotransmitters, among them the small cardioactive peptides (SCPs), myomodulins (MMs), and serotonin (5-HT). We have studied dissociated but functionally intact ARC muscle fibers to determine whether modulation of membrane ion currents in the muscle might underlie these effects. Using voltage-clamp techniques, we found that two currents were indeed modulated. In the preceding article, we proposed that enhancement of "L"-type Ca current is the mechanism by which the modulators potentiate the amplitude of ARC-muscle contractions. Here, we report that the modulators also activate a distinctive K current. Large K currents were activated, in particular, by MMA, while MMB, the SCPs, and 5-HT activated much smaller currents most likely of the same kind. Buccalins, modulators that do not act directly on the ARC muscle, were ineffective. The modulator-induced K current was strongly enhanced by depolarization, but relatively slowly so that its amplitude continued to increase for several hundred milliseconds following a depolarizing voltage step. The current was Ca2+ independent, not readily blocked by extracellular Cs+ or Ba2+ and only by high concentrations of tetraethylammonium. However, it was almost completely blocked by as little as 10 microM 4-aminopyridine. In contrast to the modulator-induced enhancement of Ca current, activation of the K current was not significantly mimicked by elevation of cAMP. In the intact as well as the dissociated ARC muscle, although low levels of all of the modulators potentiate contractions, even moderate levels of MMA strongly depress them, whereas the other modulators depress them weakly only at high concentrations. The modulator-induced K current appears well suited to counteract depolarization of the muscle and thus limit activation of the "L"-type Ca current that provides Ca2+ essential for contraction. We therefore propose that the modulators depress ARC-muscle contractions in large part by activating the K current. This occurs simultaneously with the enhancement of the Ca current; net potentiation or depression then depends on the balance between the relative strengths of the modulation of the two ion currents.