LIGHT ADAPTATION AFFECTS SYNAPTIC VESICLE DENSITY BUT NOT THE DISTRIBUTION OF GABA(A) RECEPTORS IN GOLDFISH PHOTORECEPTOR TERMINALS

GABA is a likely feedback transmitter from H1 horizontal cells to cone photoreceptors in fish retinas. Spinules arise from H1 cell dendrites in light-adapted retinas, are correlated with responses attributed to feedback, and have been proposed to be the GABA release sites. We use d mAb 62-3G1, an antibody against the beta(2)/beta(3) subunits of the GABA(A) receptor complex, to visualize GABA(A) receptor immunoreactivi ty (GABAr-IR) in photoreceptors as a function of light and dark adapta tion at the electron microscopical level. Regardless of adaptation, GA BAr-IR was restricted to the synaptic terminals of all cones and most rods; synaptic vesicular membrane and plasma membrane exhibited GABAr- IR. Contrary to expectations, the density of GABAr-IR was least on the plasma membrane within the invagination, regardless of the presence o r absence of spinules. Dense GABAr-IR was observed on the lateral surf ace of cone pedicles, on cone processes proximal to the invagination, and on presumed telodendria from nearby cones. There was no difference in GABAr-IR of rod plasma membranes within or outside of the invagina tion or with adaptation. The only novel effect of adaptation was in re gards to the density of synaptic vesicles. Cones showed a 29% increase in vesicle density with dark adaptation, whereas rods showed a 17% de crease. We conclude that all goldfish photoreceptors will be GABA-sens itive and that the sensitivity is distributed over the surface of the synaptic terminal rather than localized to within the invagination. Th e role of spinules in GABA release remains to be determined, but we co nclude that spinules are not related to the GABA sensitivity of goldfi sh photoreceptors. (C) 1997 Wiley-Liss, Inc.