Gamma-aminobutyric acid and afferent inhibition in the cat and rat ventrobasal thalamus.

Extracellular single neuron recordings were made in the ventrobasal thalami of anaesthetized rats and cats. Physiological stimulation of vibrissa and hair follicle afferents was performed with an air jet (10-20 ms duration) directed at a single vibrissa or small area of hairy skin. Paired conditioning and test air jets delivered to the excitatory portion of receptive fields revealed inhibition of the response of ventrobasal thalamic neurons to test stimuli following the excitatory response to the conditioning stimulus. Such inhibitions could last up to 500 ms. An increase in neuronal excitability was sometimes observed following this inhibitory period. In addition, it was possible to produce inhibition without an excitatory response using conditioning stimuli delivered adjacent to the excitatory receptive field. Iontophoretic application of bicuculline methochloride, with currents that were adequate to antagonize iontophoretically applied GABA, was found to reduce the inhibition of test responses evoked by conditioning stimuli in almost all of the neurons studied. In most cases, no excitatory responses to conditioning stimuli directed outside the original excitatory receptive field were revealed by application of the GABA antagonist. In rats, bicuculline also led to a decrease in the post-inhibitory excitation, whereas in cats the converse appeared to be the case. These results suggest that GABAergic transmission may underlie inhibitory responses of cat and rat ventrobasal thalamus neurons to physiological stimulation of somatosensory afferents. Furthermore, removal of such inhibition does not appear to reveal excitatory inputs from outside of the original excitatory receptive field.