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 inappropria te hindlimb inputs in the forelimb-stump representation of primary somatose nsory cortex (S-I) that are revealed when GABA(A) and GABA(B) receptor acti vity 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 suppre ss hindlimb expression in the S-I stump representation were evaluated: sile ncing S-I neurons with dual stump and hindlimb receptive fields, silencing neurons with receptive fields restricted to the hindlimb alone, and/or sele ctive silencing of hindlimb inputs to neurons that normally express a stump receptive field only. These possibilities were tested using single-unit re cording techniques to evaluate the receptive fields of S-I forelimb-stump n eurons before, during, and after blockade of GABA receptors with bicucullin e 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 G ABA receptor blockade, 54.3% of these cells became responsive to the hindli mb, and in all but two cases, these same neurons also expressed a stump rec eptive field. Most of these cells (82.0%) expressed only stump receptive fi elds prior to GABA receptor blockade. In 71 neurons recorded from normal ra ts, 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 u nresponsive. GABA receptor blockade also resulted in increases in both the spontaneous activity and response magnitudes of these neurons. These data s upport the conclusion that GABA mechanisms generally act to specifically su ppress hindlimb inputs to S-I forelimb-stump neurons that normally express a receptive field on the forelimb stump only.