GABA Improve Response Modulation Of Primary Visual Cortex In Chronic Morphine Exposed Cats

Receptive field properties of neuron is the structural basis of all physiological functions in visual cortex. Response modulation to sinusoidal grating stimuli is considered to represent the spatiotemporal stucture of the receptive field. The role of inhibition in establishing receptive field's spatiotemporal stucture is still debated. In these study, by electrophoretically applying the inhibitory transmitter GABA and the GABAA antagonist bicuculline on the same individual neurons in area 17, we studied the effect of inhibition on response modulation to the saline-treated cats and chronic morphine exposure cats.The study on saline-treated cats showed that applying GABA increased the response modulation ratio of primary visual cortex neurons. It suggested that the GABA could decrease the overlap or increase the separation between ON and OFF subregion. Meanwhile, bicuculline didn't show remarkable effection to the response modulation on the overall level. Also, the study found that the bimodal distribution of response modulation ratio disappeared after applying drugs.Some investigation in our lab and others have found decreases of the direction and orientation selectivity, response modulation properties and signal-noise ratio, etc, of neurons in chronic morphine exposure cats. Morphology experiments also found that GABAergic neurons reduced in the morphine-treated rats. Therefore, we have reason to speculate that the getting worse of response modulation in morphine cats related to the impairment on the inhibitory system. To test the hypothesis, we studied the effect of GABA and bicuculline on the area 17 cells in morphine-treated cats. The results showed that the application of GABA could improve the response modulation of area 17 cells in morphine-treated cats. Especially, the improvement in morphine-treated cats is greater than in saline-treated group. This demonstrated that morphine exposure definitely caused the degradation of GABAergic inhibition in visual system. As GABA transmitters showed a more remarkable compensate. We speculate the impairment of morphine-treated cat is related to the decrease of inhibitory transmitter GABA or the increase of endocytosis. Moreover, we compared F1 response components and F0 components of control with that after GABA applying, it suggested that the increase of modulation ratio after application of GABA was due to the decrease of F0 components.