Long-lasting depolarization and hyperpolarization in eye of Hermissenda.

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Abstract
The 5 photoreceptors in each eye of the nudibranch H. crassicornis were analyzed in 3 kinds of preparations: intact, synaptic lesion, and isolated generator potential. A razor cut above 90 .mu.m from the cell soma and rhabdome eliminated synaptic interactions (synaptic lesion). A cut approximately 80 .mu.m from the cell soma eliminated both synaptic interactions and impulses (isolated generator potential). All photoreceptors in both synaptic lesion and isolated generator potential preparations depolarized for 2-3 min following a 30-s light step (102-103 ergs .cntdot. cm2 .cntdot. s-1). This long-lasting depolarization (LLD) was associated with a decreased membrane conductance (1.5-3.0 .times.) and was eliminated by sufficient hyperpolarization of the photoreceptor during the light step. The LLD did not follow large depolarizations produced by injection of positive current. With intact synapses and impulses all photoreceptors but the lateral type-A gave a light response followed by an LLD. The lateral type-A photoreceptor (intact) gave a light response followed by almost no LLD and in most cases by a long-lasting hyperpolarization (LLH). The LLH, elicited by light steps of 102-103 ergs .cntdot. cm2 .cntdot. s-1, was associated with only slight changes in membrane conductance. Impulses of type-B photoreceptors caused IPSP [inhibitory postsynaptic potential] 1 for 1 during the LLH of the type-A photoreceptor. Inhibition of the lateral type-A photoreceptor by type-B impulse trains (elicited by positive-current injection) showed rapid fatigue. The LLH was eliminated by synaptic blockade effected by Co2+. Ipsilateral statocyst removal markedly reduced the LLH. Type-B photoreceptors during their LLD probably contributed to the LLH of the lateral type-A cell by direct (monosynaptic) and indirect (through optic ganglion and then statocyst hair cells) inhibition. Both impulse activity of type-B cells during their LLD and the number of inhibitory postsynaptic potentials of the lateral type-A cell during its LLH increased following light steps paired with rotation (compared to light steps alone). These findings may help explain previously observed associative training and correlated neural modification.

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