FUNCTIONAL-ROLE OF GABA IN CAT RETINA .2. EFFECTS OF GABA(A) ANTAGONISTS

Putative GABAergic mechanisms were studied in the cat retina by exogen ous application of the GABA, antagonists picrotoxin (PTX), native bicu culline (BCC), and bicuculline methyl bromide (BCC MeBr). When recordi ng intracellular responses from horizontal cells (HCs) and amacrine ce lls as well as electroretinograms (ERGs), drugs were added to the perf usate used to maintain the isolated eyecup; when recording extracellul ar spikes from ganglion cells of anesthetized cats, drugs were introdu ced by iontophoretic injection. Both PTX and BCC MeBr had relatively l ittle influence upon the response properties of HCs. In contrast, nati ve BCC tended to decrease the amplitude of and to slow the photic resp onse to light onset and both to quicken and to increase the amplitude of response to light offset; in the presence of native BCC, HC respons es were dominated by a prominent spike-like ''Off-overshoot.'' The inf luence of GABA(A) agonists upon HC responses was not blocked by GABA(A ) antagonists. ERG b-wave amplitude was reduced both by PTX and by nat ive BCC, but was not influenced by BCC MeBr. Latency (time to half-pea k) was increased by low doses of native BCC, and to a lesser extent PT X but not BCC MeBr. Rod-amacrine On-transient responses were increased in amplitude by PTX. Extracellular recordings from On- and Off-X and Y ganglion cell types became considerably more transient with applicat ion of either PTX, native BCC, or BCC MeBr this tendency was greater i n Off-type ganglion cells. Collectively, these results strengthen conc lusions from the previous paper suggesting that GABA serves to stow on set and offset kinetics of retinal neurons, making them more sustained and less phasic. They also suggest that in mammalian retina heterogen eous types of GABA, receptors exist, segregated into different zones: a distal zone, sensitive only to native BCC, a central zone sensitive to both native BCC and PTX, and a proximal zone sensitive to native BC C, BCC methyl halides (BCC MeH), and PTX. Only the proximal zone obeys conventional GABA(A) pharmacology.