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 effects of classical conditioning. Lateral type A photoreceptors exhibit an increase in excitability to both the conditioned stimulus (CS; light) and extrinsic current. In contrast, medial type A photoreceptors do not express enhanced excitability, but do show enhancement of the medial B to medial A synaptic connection. Therefore, both enhanced excitability and changes in synaptic strength may contribute to long-term plasticity underlying classical conditioning. The activation of protein kinase C (PKC) is involved in the induction of enhanced excitability of identified type B photoreceptors produced by one-trial conditioning and the expression of enhanced excitability in B photoreceptors after multitrial classical conditioning. We have examined a possible role for persistent kinase activity in the expression of enhanced excitability in lateral type A photoreceptors and enhancement of the medial B to medial type A synaptic connection after classical conditioning. Injection of the PKC inhibitor peptide PKC(19-36) into medial type B photoreceptors of conditioned animals did not significantly change the amplitude of medial A IPSPs elicited by single spikes in the medial B photoreceptor. Injections of PKC(19-36) into medial B photoreceptors of pseudorandom controls also did not significantly change the amplitude of IPSPs recorded from the medial A photoreceptor. In contrast, spikes elicited by extrinsic current in lateral type A photoreceptors of conditioned animals were significantly reduced in frequency after intracellular injection of PKC(19-36) as compared with pseudorandom controls. Injection of the noninhibitory analog peptide [glu27]PKC(19-36) did not affect excitability. Thus, enhanced excitability in the lateral A photoreceptor of conditioned animals seems to be influenced, in part, by a constitutively active kinase or a persistent kinase activator, whereas synaptic enhancement of the connection between the medial B and medial A photoreceptors of conditioned animals may involve a different mechanism.