Bicuculline-induced alterations in neuronal responses to controlled tactile stimuli in the second somatosensory cortex of the cat: a microiontophoretic study.

Single-neuron activity (n = 29) was recorded from the second somatosensory cortex of cats, and the effect of glutamate, gamma-amino-butyric acid (GABA), and bicuculline methiodide (BMI) on spontaneous and stimulus-induced responses were analyzed. Iontophoresis of glutamate produced dose-dependent increases in spontaneous activity, whereas GABA suppressed both spontaneous and glutamate-induced activity. Neuronal responses elicited by cutaneous stimuli were also inhibited by GABA in a dose-dependent fashion; current levels needed to produce at least a 25-50% decrease in stimulus-evoked activity ranged from 5 to 100 nA, with a mean of about 45 nA. Iontophoresis of BMI (10-75 nA) effectively antagonized GABA-induced inhibition of stimulus-evoked responses without altering spontaneous activity. Furthermore, BMI increased the magnitude of responses produced by ramp stimuli and caused a several-fold increase in receptive field size. For neurons responsive to sinusoidal stimulation, BMI caused an increase in the frequency-following probability at preferred frequencies, but failed to alter the response to nonpreferred frequencies. These results suggest that GABA-ergic circuits may limit response magnitude but not the submodality properties of somatosensory cortical neurons.