A SIMPLIFIED PREPARATION FOR RELATING CELLULAR EVENTS TO BEHAVIOR - MECHANISMS CONTRIBUTING TO HABITUATION, DISHABITUATION, AND SENSITIZATION OF THE APLYSIA GILL-WITHDRAWAL REFLEX

To relate cellular events to behavior in a more rigorous fashion, we h ave developed a simplified preparation for studying the gill-withdrawa l reflex of Aplysia, in which it is relatively easy to record the acti vity of individual neurons during simple forms of learning. Approximat ely 84% of the reflex in this preparation is mediated through the sing le motor neuron LDG1, so that changes in the firing of LDG1 can accoun t for most of the changes in behavior. We have used this preparation t o investigate cellular mechanisms contributing to habituation, dishabi tuation, and sensitization by recording evoked firing, the complex pos tsynaptic potential (PSP), and the monosynaptic component of the compl ex PSP in LDG1. Our results suggest that habituation is largely attrib utable to depression at sensory neuron synapses. By contrast, dishabit uation and sensitization involve several mechanisms at different loci, including facilitation at sensory neuron synapses, enhancement in the periphery (perhaps attributable to post-tetanic potentiation at the n euromuscular junction), and both facilitation and inhibition of excita tory and inhibitory interneurons. Moreover, these different mechanisms contribute preferentially at different times after training, so that information processing in the neuronal circuit for the reflex is distr ibuted not only in space but also in time. Nonetheless, our results al so suggest that the neuronal circuit is not a highly distributed neura l network. Rather, plasticity of the reflex can evidently be accounted for by several specific mechanisms and loci of plasticity in a define d neural circuit, including a limited number of neurons, some of which make a large contribution to the behavior.