EXPRESSION OF DIFFERENT TYPES OF INWARD RECTIFIER CURRENTS CONFERS SPECIFICITY OF LIGHT AND DARK RESPONSES IN TYPE-A AND TYPE-B PHOTORECEPTORS OF HERMISSENDA

Each eye of the mollusc Hermissenda consists of five photoreceptors, t wo type A and three type B cells. Type A cells are quiescent, whereas B cells are spontaneously active in the dark. Differences in the intri nsic membrane properties of type A and B photoreceptors were studied u sing voltage- and current-clamp techniques. The current density of a N i2+-sensitive, low-voltage activated Ca2+ current was similar in the t wo cell types. However, type B cells express an inward rectifier curre nt (I-n) that has different permeation and pharmacological properties from the inward rectifier current in type A cells. The current in the B cells was time-dependent and was blocked by Cs+. Na+ and K+ were the charge carriers for I-h. The inward rectifier current in A cells (I-K 1) was time-independent, was selectively permeable to K+, and was bloc ked by Ba2+. Ni2+ reduced the spontaneous spike activities of type A a nd B cells, whereas Cs+ produced membrane hyperpolarization and reduce d the spike activities of dark-adapted B cells. The application of bot h Cs+ and Ni2+ completely blocked dark-adapted spontaneous activities of B cells. Moreover, Ba2+ increased the excitability of type A cells but not B cells. Hence, differential expression of the two distinct in ward rectifiers found in type A and B cells contributes to differences in their intrinsic membrane properties. Because changes in the excita bility of the two cell types are correlates of conditioning in Hermiss enda, modulation of these underlying currents may play a major role du ring conditioning-induced plasticity.