THALAMOCORTICAL AND INTRACORTICAL PROJECTIONS TO THE FORELIMB-STUMP SI REPRESENTATION OF RATS THAT SUSTAINED NEONATAL FORELIMB REMOVAL

We previously reported the abnormal expression of hindlimb receptive f ields in the stump representation of the primary somatosensory cortex (SI) in rats that sustained neonatal forelimb removal when cortical ga mma-aminobutyric acid (GABA) receptors were pharmacologically blocked (Lane et al. [1997] J. Neurophysiol. 77:2723-2735). In this study, we attempted to identify the substrate for this functional modification. Three potential substrates were examined: 1) changes in intracortical connections within SI; 2) alterations in the projection pattern of tha lamocortical afferents from the ventroposterior lateral (VPL) nucleus to SI; and 3) changes in the receptive fields of thalamocortical neuro ns. We used biotinylated dextran amine and Phaseolus vulgaris leucoagg lutinin to examine the intracortical projections associated with the s tump and hindlimb representations of SI. True Blue and Diamidino Yello w were used to study the organization of the VPL projections to SI. Fi nally, single-unit recordings from VPL neurons were made to examine th e functional organization of this nucleus in neonatally amputated adul t rats. Tracer studies demonstrated no significant change in the intra cortical connections or VPL projections associated with the stump and hindlimb SI in neonatally amputated rats. Recordings from VPL of neona tally manipulated rats revealed a small, but significant, population o f cells (19.0%) within the stump representation that had dual stump an d hindlimb receptive fields. Thus, the data suggest that the functiona l reorganization observed in SI of neonatally amputated rats may refle ct functional alterations occurring in its thalamic inputs. J. Comp. N eurol. 401:187-204, 1998. (C) 1998 Wiley-Liss, Inc.