SYNAPTIC ENHANCEMENT AND ENHANCED EXCITABILITY IN PRESYNAPTIC AND POSTSYNAPTIC NEURONS IN THE CONDITIONED-STIMULUS PATHWAY OF HERMISSENDA

Identified type A photoreceptors of Hermissenda express differential e ffects of classical conditioning. Lateral type A photoreceptors exhibi t an increase in excitability to both the conditioned stimulus (CS; li ght) and extrinsic current. In contrast, medial type A photoreceptors do not express enhanced excitability, but do show enhancement of the m edial B to medial A synaptic connection. Therefore, both enhanced exci tability and changes in synaptic strength may contribute to long-term plasticity underlying classical conditioning. The activation of protei n kinase C (PKC) is involved in the induction of enhanced excitability of identified type B photoreceptors produced by one-trial conditionin g and the expression of enhanced excitability in B photoreceptors afte r multitrial classical conditioning. We have examined a possible role for persistent kinase activity in the expression of enhanced excitabil ity in lateral type A photoreceptors and enhancement of the medial B t o medial type A synaptic connection after classical conditioning. Inje ction of the PKC inhibitor peptide PKC(19-36) into medial type B photo receptors of conditioned animals did not significantly change the ampl itude of medial A IPSPs elicited by single spikes in the medial B phot oreceptor. Injections of PKC(19-36) into medial B photoreceptors of ps eudorandom controls also did not significantly change the amplitude of IPSPs recorded from the medial A photoreceptor. In contrast, spikes e licited by extrinsic current in lateral type A photoreceptors of condi tioned animals were significantly reduced in frequency after intracell ular injection of PKC(19-36) as compared with pseudorandom controls. I njection of the noninhibitory analog peptide [glu(27)]PKC(19-36) did n ot affect excitability. Thus, enhanced excitability in the lateral A p hotoreceptor of conditioned animals seems to be influenced, in part, b y a constitutively active kinase or a persistent kinase activator, whe reas synaptic enhancement of the connection between the medial B and m edial A photoreceptors of conditioned animals may involve a different mechanism.