Suppression of hindlimb inputs to S-I forelimb-stump representation of rats with neonatal forelimb removal: GABA receptor blockade and single-cell responses.

Neonatal forelimb removal in rats results in the development of inappropriate hindlimb inputs in the forelimb-stump representation of primary somatosensory cortex (S-I) that are revealed when GABA(A) and GABA(B) receptor activity are blocked. Experiments carried out to date have not made clear what information is being suppressed at the level of individual neurons. In this study, three potential ways in which GABA-mediated inhibition could suppress hindlimb expression in the S-I stump representation were evaluated: silencing S-I neurons with dual stump and hindlimb receptive fields, silencing neurons with receptive fields restricted to the hindlimb alone, and/or selective silencing of hindlimb inputs to neurons that normally express a stump receptive field only. These possibilities were tested using single-unit recording techniques to evaluate the receptive fields of S-I forelimb-stump neurons before, during, and after blockade of GABA receptors with bicuculline methiodide (for GABA(A)) and saclofen (for GABA(B)). Recordings were also made from normal rats for comparison. Of 92 neurons recorded from the S-I stump representation of neonatally amputated rats, only 2.2% had receptive fields that included the hindlimb prior to GABA receptor blockade. During GABA receptor blockade, 54.3% of these cells became responsive to the hindlimb, and in all but two cases, these same neurons also expressed a stump receptive field. Most of these cells (82.0%) expressed only stump receptive fields prior to GABA receptor blockade. In 71 neurons recorded from normal rats, only 5 became responsive to the hindlimb during GABA receptor blockade. GABA receptor blockade of cortical neurons, in both normal and neonatally amputated rats, resulted in significant enlargements of receptive fields as well as the emergence of receptive fields for neurons that were normally unresponsive. GABA receptor blockade also resulted in increases in both the spontaneous activity and response magnitudes of these neurons. These data support the conclusion that GABA mechanisms generally act to specifically suppress hindlimb inputs to S-I forelimb-stump neurons that normally express a receptive field on the forelimb stump only.