Ionic basis of learning-correlated excitability changes in Hermissenda type A photoreceptors. Farley, Joseph and Yunru Han. Programs in Neural Science and Biochemistry, Indiana University, Bloomington, Indiana 47405.
APStracts 4:0002N, 1997.
ABSTRACT
Repeated pairings of light and rotation (conditioning) result in persistent changes in excitability of Hermissenda type B and A photoreceptors, which are correlated with pairing-specific reductions in phototactic behavior. While considerable attention has been devoted to characterization of conditioning- produced neurophysiological changes that occur in type B cells, less information is available concerning the changes produced in type A cells. Here, we recorded from identified, synaptically- isolated lateral and medial type A photoreceptors from conditioned, random-control, or untrained animals on retention days following conditioning. Type A photoreceptors from conditioned animals responded to light with a receptor potential that was significantly smaller than those of random-control or untrained animals, which did not differ. The phototactic suppression and type A cell light response magnitudes were negatively correlated for individual conditioned animals. Animals exhibiting strong phototactic suppression also showed small light responses. Expression of the training-associated light response difference was a calcium-dependent phenomenon: reducing extracellular calcium to less than 1 uM enhanced the generator potential of A cells, regardless of conditioning history, and greatly reduced the differences in generator potential amplitude attributable to training. Voltage-clamp studies revealed that conditioning resulted in a 2-3 fold increase in the amplitude of a voltage-dependent, sustained, outward K+ current (IDelayed). IDelayed magnitudes were positively correlated with phototactic suppression for individual conditioned animals: type A cells of animals exhibiting strong phototactic suppression expressed large IDelayed currents. IDelayed is a composite current, consisting of at least three separable components: 1) residual A current, 2) slow, TEA- sensitive calcium-activated K+ current (IK- Ca), and 3) a delayed-rectifier type, voltage-dependent K+ current (IK,v). Analysis of these currents failed to reveal significant training-associated changes in IA or IK-Ca. But IK,v was enhanced by 60-150 % in both lateral and medial cells and thus contributes to the conditioning-associated increase in IDelayed. 

Received 19 December 1996; accepted in final form 19 December 1996.
APS Manuscript Number J725-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 21 January 1997