The scotopic threshold response of the cat ERG is suppressed selectively by GABA and glycine

Corneal electroretinograms (ERGs) were recorded from anesthetized cats under scotopic conditions. We examined whether the scotopic threshold response (STR) of the ERG could be functionally distinguished from scotopic PII and a-wave using intravitreal application of neuroactive agents. We found that neurotransmitters with active sites on third order neurons had several different effects. Results were: (1) glycine and gamma-amino butyric acid (GABA) selectivity suppressed the STR but had relatively small and/or opposite effects on PII; (2) serotonin, acetylcholine and dopamine were nonselective and suppressed both STR and PII; (3) strychnine blocked the suppression of the STR by glycine. GABA-a antagonists alone only partially blocked GABA effects on the STR, and GABA-b antagonists were ineffective; (4) strychnine enhanced the STR. Bicuculline also increased STR amplitudes, but only in the presence of haloperidol. Our results suggest that the retinal pathway that contributes to the rod-driven STR is strongly influenced by cells that release glycine or GABA in the dark. These cells are possibly third order neurons in the retina. Our results also suggest that picrotoxin and bicuculline can facilitate the release of dopamine in the cat retina. Furthermore, the data indicate a light evoked release of dopamine which was first noticeable at about two log units above ERG threshold.