Protein phosphorylation and associative learning in Hermissenda.

Phototaxis in the nudibranch mollusc Hermissenda crassicornis can be modified by a conditioning paradigm which consists of paired light and rotation. Changes in K+ currents and protein phosphorylation have been demonstrated in photoreceptors and eyes of conditioned animals. A working hypothesis has been proposed in which stimulation of photoreceptors and hair cells during the presentation of paired light and rotation leads to synaptic and light-induced depolarization which gives rise to an increase in internal free Ca2+ in photoreceptors. The conditioning-induced increase in Ca2+ can in turn activate protein phosphorylation mechanisms which regulate K+ channel activity. In experiments designed to test this hypothesis, we have found that light plus depolarization increases internal Ca2+, as measured by Arsenazo absorption, Ca2+ increases the rate of inactivation of the same K+ currents which are reduced following conditioning, intracellular injection of Ca2+/calmodulin-dependent and cAMP-dependent protein kinases leads to reductions in these K+ currents, and potent Ca2+- and cAMP-dependent protein kinases are present in Hermissenda neural tissue. Further studies are needed to investigate the nature of the proteins whose levels of phosphorylation are altered following training and the nature of the proteins that are affected by the intracellularly injected protein kinases and to determine if the phosphoproteins affected by conditioning are related to the function of K+ channels.