Comparison of receptive field expansion produced by GABA(B) and GABA(A) receptor antagonists in raccoon primary somatosensory cortex.

Recordings were made from 62 neurons in the forepaw representation of primary somatosensory cortex in anesthetized raccoons. Microiontophoretic administration of a specific GABA(B) receptor antagonist, CGP 55845, produced receptive field expansion in 74% of 46 neurons, in which it was tested first. The mean receptive field area was approximately doubled, with increases ranging from 12 to 500%. The GABA(B) receptor agonist baclofen reduced the receptive field in most (11 of 16) neurons, but increased the size in 4 neurons. Comparison of the effects of GABA(B) and GABA(A) antagonists in the same cells showed that GABA(A) receptor blockade produced greater expansion than GABA(B) blockade (144% vs 114%, respectively). Simultaneous administration of the two antagonists produced additional expansion in 16 of 25 neurons. There was no evidence of separate skin regions being masked by the two GABA receptor subtypes, as the larger expansion usually included the skin that was unmasked by the less effective drug. These results indicate that both GABA(B) and GABA(A) receptors play a role in shaping the normal receptive fields in somatosensory cortex.