Corticocortical inhibition of peripheral inputs within primary somatosensory cortex: the role of GABA(A) and GABA(B) receptors.

A conditioning-test pulse paradigm was used in combination with microiontophoresis to examine the corticocortical modulation of somatosensory processing. Single-cell recordings were made in the glabrous digit representation of primary somatosensory (S1) cortex in anesthetized raccoons. Test stimulation of the periphery (the on-focus digit) was preceded by conditioning stimulation of the cortical area that represents an adjacent digit at interstimulus intervals ranging from 5 to 200 ms. An early and prolonged inhibitory modulation was produced in most of the 61 neurons examined, and an early facilitation followed by inhibition was produced in about one-third of the cells. Microiontophoretic administration of a potent GABA(B) receptor antagonist, CGP 55845, blocked the inhibition and in many cases revealed a facilitation of the sensory response. Microiontophoretic administration of a GABA(A) receptor antagonist, gabazine, blocked inhibition at short interstimulus intervals and reduced the longer inhibition by half. These results indicate that connections between glabrous digit representations within S1 cortex produce predominantly inhibitory modulation of sensory input and that both GABA(A) and GABA(B) receptors contribute to this modulation. The relevance of these connections to the effects of peripheral nerve injury and subsequent reorganization is discussed.