Factors Influencing the Expression and Suppression of Hindlimb Receptive Fields in the Forelimb-stump Representation of Primary Somatosensory Cortex (SI) in Neonatally Amputated Rats
Previous studies from our laboratory have demonstrated that, in adult rats following forelimb amputation on the day of birth, approximately one-third of multi-unit recording sites in the forelimb-stump representation of primary somatosensory cortex (SI)also respond to cutaneous stimulation of the hindlimb (on the amputated side) when cortical GABAA+B receptors are pharmacologically blocked (GRB). These normally suppressed hindlimb inputs originate in the SI hindlimb representation and synapse in the dysgranular cortex before reaching the SI forelimb-stump representation. The experiments in this thesis tested three hypotheses based on these findings. The first hypothesis is that the development of SI contributes to the deferential “invasion” of the deafferented SI forelimb-stump region by hindlimb rather than face inputs. This was tested by comparing SI maps in fetal, neonatal, and adult amputees. There was significant expression of face receptive fields in fetal amputees during GRB, compared to hindlimb responses in neonatal amputees, and none in adult amputees. These findings demonstrate that the developmental state of SI at the time of amputation does influence whether face or hindlimb inputs are expressed in the deafferented zone during GRB. The second hypothesis is that the suppression of hindlimb responses in neonatal amputees is locally mediated by GABAergic synapses within the SI forelimb-stump zone. This was tested by making separate targeted GRB injections at individual recording sites, in the SI hindlimb region, and in the dysgranular cortex, followed by continuous monitoring for the presence of hindlimb responses. Hindlimb responses were revealed primarily by the local recording site 211 injections, indicating that the GABAergic suppression of hindlimb responses is indeed locally mediated. This is consistent with the morphology of inhibitory SI nterneurons. The third hypothesis is that hindlimb responses can be revealed by downregulating one known source of SI input, the contralateral SI representation. This was tested by inactivating the contralateral SI forelimb region via lidocaine injections, and monitoring SI forelimb-stump sites for hindlimb responses. This manipulation did reveal hindlimb responses, although not to the same extent as that found in the same animals during GRB. This demonstrates that hindlimb responses can be revealed independent of GRB, by reducing one source of excitatory drive to SI GABAergic neurons.
School:University of Toledo Health Science Campus
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:reorganization amputation somatosensory cortex electrophysiology
Date of Publication:01/01/2006