Differential expression of correlates of classical conditioning in identified medial and lateral type A photoreceptors of Hermissenda.

Classical conditioning of Hermissenda produces neurophysiological correlates in the primary sensory neurons of the pathway mediating the conditioned stimulus (CS), the type B and type A photoreceptors. Biophysical and biochemical changes intrinsic to the type B photoreceptors have been studied extensively in conditioned animals. A second site for intrinsic modification with learning has been recently identified in type A photoreceptors. We have recorded from identified medial and lateral type A photoreceptors in conditioned animals and animals that received pseudorandom presentations of light (CS) and rotation (unconditioned stimulus). Here we report that conditioning produces differential effects upon CS-elicited spike frequency and intrinsic excitability detected in identified lateral and medial type A photoreceptors. Lateral type A photoreceptors from conditioned animals exhibited significant increases in spike frequency elicited by the CS as compared to pseudorandom controls. In contrast, CS-elicited spike frequency recorded in medial type A photoreceptors was not significantly different from random controls. The amplitude of the peak and plateau phases of the generator potential of medial and lateral type A photoreceptors was decreased in conditioned animals as compared to random controls. In addition, only lateral type A photoreceptors exhibited enhanced cellular excitability as expressed by increased spike discharges produced by the injection of extrinsic depolarizing current pulses. Conditioning also decreased spike frequency accommodation in lateral type A photoreceptors. In normal controls, medial type B photoreceptors produced stronger synaptic inhibition of medial type A photoreceptors than the lateral type B photoreceptors inhibition of lateral type A photoreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)