We studied the GABA sensitivity of horizontal cells
in the isolated goldfish retina. After the glutamatergic
input to the horizontal cells was blocked with DNQX, GABA
depolarized the monophasic and biphasic horizontal cells.
The pharmacology of these GABA-induced depolarizations
was tested with the GABA receptor antagonists bicuculline-methiodide
and picrotoxin, the GABA transporter agonist nipecotic
acid, and the GABA transporter antagonist SKF 89976-A.
The GABA-induced responses of monophasic horizontal cells
consisted of two components; one with characteristics of
GABA-gated chloride channels, and one with characteristics
of GABA transporters. In biphasic horizontal cells, we
only found evidence for GABA-gated chloride channels. The
results show that monophasic horizontal cells in goldfish
contain the two components of a positive feedback loop
(GABA transporters and GABA-gated chloride channels), as
described in salamander. Furthermore, our results indicate
that the monophasic horizontal cells may project directly
to the biphasic horizontal cells, via an excitatory
GABAergic pathway. We propose that the function of these
GABAergic systems in horizontal cells is to abolish cone
dominance in bipolar cells surround in the dark-adapted
retina.