Blockade of GABAergic inhibition reveals reordered cortical somatotopic maps in rats that sustained neonatal forelimb removal

A previous study from this laboratory demonstrated that forelimb removal at birth results in invasion of the cuneate nucleus (CN) by sciatic nerve axons and the development of CN cells including thalamic projection neurons with receptive fields that include both the forelimb stump and the hindlimb. However, recordings from unit clusters in lamina IV of the primary somatosensory cortex (SI) of these animals revealed the presence of only a very few sites in the forelimb stump representation where responses to hindlimb stimulation could also be recorded. In the present study we tested the possibility that input from the hindlimb was suppressed in lamina IV of the cortical stump representation via GABAergic inhibitory mechanisms by mapping this cortical region, applying the gamma-aminobutyric acid-A (GABA(A)) and GABA(B) receptor antagonists bicuculline and phaclofen (50 mu M each), and then remapping the same sites. In six neonatally manipulated rats, 15 of 242 sites (6.2%) in the stump representation responded to hindlimb stimulation before GABA receptor blockade and 107 (44.2%) of the same sites responded to stimulation of the hindlimb during blockade (P < 0.05). In six normal adult rats, 7 of 264 sites (2.7%) in the forelimb representation responded to hindlimb stimulation before the application of bicuculline and phaclofen. During GABA receptor blockage, 31 of these sites (11.7%) responded to such stimulation (P < 0.02 us. the untreated normal cortex and P < 0.01 vs. the neonatally manipulated rats treated with GABA blockers). To specifically test the role of GABA(A) versus GABA(B) receptors in the inhibition of hindlimb input to the SI stump representation in rats that sustained neonatal forelimb removal, either bicuculline or phaclofen alone was applied to SI in nine neonatally manipulated animals. In four rats treated with bicuculline, 12 of 184 sites (6.5%) in the stump representation responded to hindlimb stimulation before treatment and 61 of 184 sites (33.2%) responded to such stimulation during application (P < 0.01). In animals (n = 5) treated with phaclofen, 18 of 251 sites (7.2%) responded to hindlimb stimulation before treatment and 64 of these sites (25.5%) responded to such stimulation during application (P < 0.05). There was no significant difference between the results obtained with bicuculline alone, phaclofen alone, or the two GABA blockers delivered together(P > 0.05). These results indicate that hindlimb input to the portion of SI representing the forelimb stump is functionally suppressed in rats that have sustained neonatal forelimb removal and that GABAergic inhibition, mediated by both GABA(A) and GABA(B) receptors, is involved in this process.