Comparative pharmacological effects on visual cortical neurons in monocularly deprived cats

Monocularly deprived (MD) cats show a loss of responsiveness to visual stimulation of the deprived eye among visual cortical neurons. Several lines of evidence suggest that this effect involves, at least in part, a suppression of deprived eye input, possibly mediated by GABA inhibition. In order to better understand the nature of this suppression we have evaluated the effectiveness of different types of disinhibitory and excitatory agents to reverse the effects of MD. We investigated bicuculline (a GABA antagonist); picrotoxin (a GABA antagonist with a different mechanism of action from bicuculline); strychnine (a glycine antagonist); ammonium ion (a blocker of membrane chloride channels); physostigmine (a cholinesterase inhibitor); and naloxone (an opiate antagonist and also a GABA antagonist). All drugs were given intravenously. Bicuculline restored binocularity to 50% of the visual cortical neurons tested and naloxone to 36%. With both drugs, receptive fields of the normal eye tended to lose specificity. The emergent deprived eye receptive fields were usually similar to those of the normal eye after drug administration. Ammonium ion produced binocular responses in 27% of neurons tested, but receptive fields were grossly abnormal; moreover, ammonium infusion tended to depress neuronal responsiveness. All other drugs tested failed to restore binocularity. These experiments lend further credence to the hypothesis that GABA inhibition contributes to the cortical effects of MD, since only drugs with GABA antagonistic action were effective in restoring neuronal responsiveness to the deprived eye.