Long-lasting depolarization and hyperpolarization in eye of Hermissenda

The five photoreceptors in each eye of the nudibranch Hermissenda were analyzed in three kinds of preparations: intact, 'synaptic lesion', and 'isolated generator potential'. A razor cut approximately 90 μm from the cell soma and rhabdome eliminated synaptic interactions (synaptic lesion). A cut approximately 80 μm 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. cm² x s^-1). This long-lasting depolarization (LLD) was associated with a decreased membrane conductance (1.5 - 3.0 x) 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, which was followed by an LLD. The lateral type A photoreceptor (intact) gave a light response which was 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 x cm² x s^-1 , was associated with only slight changes in membrane conductance. Impulses of type B photoreceptors caused IPSPs one for one 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. This evidence indicated that type B photoreceptors during their LLD contribute to the LLH of the lateral type A cell by both 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.