The contribution of GABA-mediated inhibitory mechanisms to visual response properties of neurons in the kitten's striate cortex.

The effect of the microiontophoretic administration of the GABA antagonist bicuculline methiodide (BMI) on the responses of striate cortical neurons to light stimulation was investigated in kittens ranging in age between 11 and 28 d. The orientation sensitivity of the majority of the 88 neurons tested for this parameter decreased (40%) or was eliminated (18%) following the administration of BMI. Changes were seen in all layers and in all neuronal types, and the proportions of neurons that changed their orientation specificity were about the same during the second, third, and fourth postnatal weeks. Twenty-eight percent of the neurons were not affected in their orientation sensitivity by BMI. These neurons were recorded at all postnatal ages; they were located preferentially in layers IV and VI; and they had unimodal, bimodal, or ON-OFF mixed receptive fields. The remaining 14% of the neurons were initially unresponsive or responded in an erratic way to visual stimulation. These neurons became responsive and even exhibited orientation-specific responses during administration of BMI. The majority (56%) of direction-specific neurons became direction-nonspecific after the administration of BMI. Seventeen percent preserved some direction specificity, whereas 27% did not show any change at all. The effects of BMI on direction sensitivity were seen at all postnatal ages and on all neuronal types throughout layers III-VI. The majority of neurons unaffected by BMI in their direction sensitivity resided in layer VI. In those cases where orientation sensitivity was reduced, direction sensitivity (if present) was usually diminished as well. However, some neurons were found in which only 1 of the 2 parameters was significantly changed by BMI. The spatial structure of the receptive field, as revealed by stationary stimulation, was changed significantly by BMI in about half the neurons tested. A straightforward correlation between the alteration of orientation and/or direction sensitivity and changes in receptive-field structure was not found. The results demonstrate that, in the immature striate cortex, receptive-field properties of many neurons are determined by inhibitory processes mediated by GABA, which may also dictate the actual visual responsiveness of the neurons. The dissociations in the effects of BMI on orientation sensitivity, direction sensitivity, and receptive-field substructure indicate that the synaptic organization responsible for the various functional parameters is unlikely to be the same.