Enhancement of type B and A photoreceptor inhibitory synaptic connections in conditioned Hermissenda.

Intrinsic changes have been identified in isolated and intact type A and type B photoreceptors following classical conditioning of the nudibranch mollusk Hermissenda. Aspects of various intrinsic, nonsynaptic modifications are expressed by alterations in the excitability of identified photoreceptors in response to the conditioned stimulus. In addition to changes in cellular excitability, changes in synaptic strength between identified neurons have been proposed as a possible mechanism of associative learning in several invertebrate preparations. Here we report that classical conditioning produces differential effects upon the strength of inhibitory monosynaptic connections between identified pairs of type B and type A photoreceptors. The amplitude of IPSPs elicited by an action potential in the medial type B photoreceptor and recorded from medical type A photoreceptors was significantly enhanced in conditioned animals as compared to pseudorandom controls. In contrast, the amplitude of IPSPs elicited by an action potential in the lateral type B photoreceptor and recorded from lateral type A photoreceptors did not show significant synaptic enhancement following conditioning. These results provide additional evidence for differential effects of conditioning upon cellular modifications in identified type A and type B photoreceptors, and further indicate that multiple sites of cellular plasticity exist in the visual system of conditioned Hermissenda.