Associative learning changes intrinsic to Hermissenda type A photoreceptors

The eyes of the nudibranch mollusc Hermissenda have previously been shown to contain 2 classes of photoreceptors. Type B photoreceptors exhibit increased light responses and membrane excitability after repeated pairings of light and rotation and play an important role in the mediation of associatively produced reductions in phototactic behavior. Type A photoreceptors have also been shown to change with associative training. In previous research, Type A photoreceptors from trained animals were found to have reduced light responses. Because these recordings were obtained from synaptically intact cells, it was not possible to determine whether the effects of associative training reflected changes in synaptic input to Type A photoreceptors or intrinsic changes in somatic conductances. In the present study, intracellular recordings from synaptically isolated Type A photoreceptors were obtained on retention days after training, and pairing-specific decreases in light-induced generator potentials and decreases in resting input resistance were observed. Current- and voltage-clamp analysis of Type A photoreceptors from untrained animals revealed that an important determinant of the steady-state light response was a calcium-activated K+ current (IK-Ca). Thus, Type A photoreceptors also appear to be a primary site for associative information storage in Hermissenda. It is suggested that enhancement of IK-Ca by associative training may contribute to the diminished light response of Type A photoreceptors.