Synaptic facilitation at connections of Hermissenda type B photoreceptors.
The enhancement of excitability in type B photoreceptors is an important neural mechanism underlying classically conditioned suppression of phototaxis in the marine mollusk Hermissenda crassicornis. However, the possibility that type B photoreceptors also exhibit synaptic plasticity has not previously been explored. We now report that connections of type B photoreceptors onto type A photoreceptors exhibit synaptic facilitation, and that this facilitation involves the same first messenger (5-HT) and second messenger (protein kinase C) previously implicated in the learning-produced excitability changes. In brief, we found that application of 5-HT dramatically facilitates synaptic potentials evoked by type B cells in type A cell cells, and that this facilitation is blocked by preincubation with staurosporine, a protein kinase inhibitor. Furthermore, activation of protein kinase C also induces synaptic facilitation, whereas activation of the cAMP-dependent protein kinase has no effect. Changes in synaptic strength produced by these manipulations are paralleled by changes in type B cell input resistance (a simple index of cellular excitability), whereas type A cell input resistance is unaffected. These findings indicate a previously unrecognized form of neuronal plasticity in Hermissenda that may contribute importantly to the learned changes in behavior, and thereby highlight general principles of learning-related neuronal plasticity shared by different preparations and species.
Schuman
E M
EM
Psychology Department, Princeton University, New Jersey 08544.
Clark
G A
GA
D000470:Alkaloids / Q000494:pharmacology; D000818:Animals; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004789:Enzyme Activation; D008974:Mollusca / Q000502:physiology*; D009434:Neural Pathways / Q000166:cytology / Q000502:physiology; D017956:Photoreceptor Cells, Invertebrate / Q000502:physiology*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism; D012701:Serotonin / Q000494:pharmacology; D019311:Staurosporine; D013569:Synapses / Q000187:drug effects / Q000502:physiology*
D000470:Alkaloids; D012701:Serotonin; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D019311:Staurosporine