GABA increases stimulus selectivity of neurons in primary visual cortices of cats chronically treated with morphine.

Chronic exposure to opiates leads to maladaptive changes in various functions of the mammalian brain, including properties of neuronal response in the visual pathway. In the present study, we used multibarreled microelectrodes to study the effects of electrophoretic application of GABA or the GABAA receptor antagonist bicuculline on the properties of individual V1 neurons in cats which were chronically treated with morphine (MTCs) or saline (STCs). The results showed that the application of either GABA or bicuculline significantly altered spontaneous activity as well as orientation selectivity and signal-to-noise ratios of visually evoked responses in both MTCs and STCs. While administration of bicuculline exerted a much stronger effect on neuronal responses of V1 neurons of the STCs, administration of GABA resulted in improved visual function mainly in MTCs. Most importantly, GABA-treated cells in area V1 of the MTCs displayed similar responses to those in STCs. These results are consistent with the idea that: (1) there is a decrease in GABA-mediated inhibition in area V1 of cats exposed chronically to morphine, and (2) this decrease contributes strongly to the apparent degradation of neuronal function observed in animals exposed chronically to morphine.