Incremental redistribution of protein kinase C underlies the acquisition curve during in vitro associative conditioning in Hermissenda.

An incremental increase in the excitability (i.e., input resistance, evoked spike frequency) of B photoreceptors in Hermissenda accompanied successive pairings of light and presynaptic stimulation of vestibular hair cells (simulating light-rotation pairings in an intact animal). Analysis of protein kinase C (PKC) in the Hermissenda's photoreceptors indicated a training-induced incremental reduction of PKC in cytosolic compartments, a tendency toward an increase in membrane compartments, and a small decrease in total enzyme activity (possibly owing to a downregulation or conversion of PKC to a calcium-independent state). Neither the biophysical or biochemical effects were observed in Hermissenda exposed to unpaired light and rotation or in those trained in the presence of the selective PKC inhibitor NPC-15437 (which had no effect on synaptic interactions or light-induced generator potentials). These results suggest that the intracellular redistribution of a protein kinase contributes critically to the kinetics of new learning.