S. Yazulla et Km. Studholme, LIGHT ADAPTATION AFFECTS SYNAPTIC VESICLE DENSITY BUT NOT THE DISTRIBUTION OF GABA(A) RECEPTORS IN GOLDFISH PHOTORECEPTOR TERMINALS, Microscopy research and technique, 36(1), 1997, pp. 43-56
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.