The identification and some functions of GABAergic neurons in the distal catfish retina

Putative GABAergic neurons in the outer retina of the Texas channel catfish, Ictalurus punctatus, were studied using autoradiographic, biochemical and electrophysiological techniques. A red cone horizontal cell was found to accumulate exogenous GABA in the presence of red light. GABA could be released from these cells with high K+ Ringers solution. The release was only partially blocked by Co2+ and therefore may be only partially Ca2+ dependent. The red cone horizontal cells were found to contain significant activities of L-glutamic acid decarboxylase and GABA transaminase, the enzymes responsible for GABA synthesis and degradation respectively. These data suggest that catfish red cone horizontal cells are GABAergic. To substantiate this, recordings were made from photoreceptors and horizontal cells during the superfusion of the GABA blocking agents bicuculline methochloride or picrotoxin. These agents modified the cone responses in the manner specified if they were blocking the feedback pathway from horizontal cells to cones. Thus it is likely that the horizontal cells are using GABA as the transmitter in the feedback pathway. In addition, the GABA blocking agents were found to interfere with changes in horizontal cell responses which occur during light adaptation.