SYNAPTIC FACILITATION AT CONNECTIONS OF HERMISSENDA TYPE-B PHOTORECEPTORS

The enhancement of excitability in type B photoreceptors is an importa nt neural mechanism underlying classically conditioned suppression of phototaxis in the marine mollusk Hermissenda crassicornis. However, th e possibility that type B photoreceptors also exhibit synaptic plastic ity has not previously been explored. We now report that connections o f type B photoreceptors onto type A photoreceptors exhibit synaptic fa cilitation, and that this facilitation involves the same first messeng er (5-HT) and second messenger (protein kinase C) previously implicate d in the learning-produced excitability changes. In brief, we found th at application of 5-HT dramatically facilitates synaptic potentials ev oked by type B cells in type A cell cells, and that this facilitation is blocked by preincubation with staurosporine, a protein kinase inhib itor. Furthermore, activation of protein kinase C also induces synapti c facilitation, whereas activation of the cAMP-dependent protein kinas e has no effect. Changes in synaptic strength produced by these manipu lations are paralleled by changes in type B cell input resistance (a s imple index of cellular excitability), whereas type A cell input resis tance is unaffected. These findings indicate a previously unrecognized form of neuronal plasticity in Hermissenda that may contribute import antly to the learned changes in behavior, and thereby highlight genera l principles of learning-related neuronal plasticity shared by differe nt preparations and species.