Facilitation of monosynaptic and complex PSPs in type I interneurons of conditioned Hermissenda.

Synaptic plasticity and intrinsic changes in neuronal excitability are two mechanisms for Pavlovian conditioning. Pavlovian conditioning of Hermissenda produces synaptic facilitation of monosynaptic medial B-medial A IPSPs and intrinsic changes in excitability of type A and B cells in isolated and intact sensory neurons of the conditioned stimulus (CS) pathway. Recently two types of interneurons that receive either excitatory or inhibitory monosynaptic or polysynaptic input from photoreceptors have been identified. On the basis of morphological and electrophysiological criteria, the interneurons have been classified as type I(e), I(i) (direct), and type II(e), II(i) (indirect). We have now examined synaptic facilitation of monosynaptic PSPs in type I(e) and I(i) interneurons after conditioning and pseudorandom control procedures. Here we report that CS-elicited spike activity is increased in type I(e) interneurons and decreased in type I(i) interneurons of conditioned animals relative to their respective baseline activity and pseudorandom control groups. Classical conditioning resulted in synaptic facilitation of type I(e) and I(i) monosynaptic PSPs elicited by lateral B spikes and enhancement of the amplitude of complex PSPs elicited by the CS. These results provide additional sites of plasticity in the neural circuit involved with the expression of learned behavior produced by Pavlovian conditioning of Hermissenda.