Source of inappropriate receptive fields in cortical somatotopic maps from rats that sustained neonatal forelimb removal.

Previously this laboratory demonstrated that forelimb removal at birth in rats results in the invasion of the cuneate nucleus by sciatic nerve axons and the development of cuneothalamic cells with receptive fields that include both the forelimb-stump and the hindlimb. However, unit-cluster recordings from primary somatosensory cortex (SI) of these animals revealed few sites in the forelimb-stump representation where responses to hindlimb stimulation also could be recorded. Recently we reported that hindlimb inputs to the SI forelimb-stump representation are suppressed functionally in neonatally amputated rats and that GABAergic inhibition is involved in this process. The present study was undertaken to assess the role that intracortical projections from the SI hindlimb representation may play in the functional reorganization of the SI forelimb-stump field in these animals. The SI forelimb-stump representation was mapped during gamma-aminobutyric acid (GABA)-receptor blockade, both before and after electrolytic destruction of the SI hindlimb representation. Analysis of eight amputated rats showed that 75.8% of 264 stump recording sites possessed hindlimb receptive fields before destruction of the SI hindlimb. After the lesions, significantly fewer sites (13.2% of 197) were responsive to hindlimb stimulation (P < 0.0001). Electrolytic destruction of the SI lower-jaw representation in four additional control rats with neonatal forelimb amputation did not significantly reduce the percentage of hindlimb-responsive sites in the SI stump field during GABA-receptor blockade (P = 0.98). Similar results were obtained from three manipulated rats in which the SI hindlimb representation was silenced temporarily with a local cobalt chloride injection. Analysis of response latencies to sciatic nerve stimulation in the hindlimb and forelimb-stump representations suggested that the intracortical pathway(s) mediating the hindlimb responses in the forelimb-stump field may be polysynaptic. The mean latency to sciatic nerve stimulation at responsive sites in the GABA-receptor blocked SI stump representation of neonatally amputated rats was significantly longer than that for recording sites in the hindlimb representation [26.3 +/- 8.1 (SD) ms vs. 10.8 +/- 2.4 ms, respectively, P < 0.0001]. These results suggest that hindlimb input to the SI forelimb-stump representation detected in GABA-blocked cortices of neonatally forelimb amputated rats originates primarily from the SI hindlimb representation.