Intracortical pathway involving dysgranular cortex conveys hindlimb inputsto S-I forelimb-stump representation of neonatally amputated rats

Reorganization of the primary somatosensory cortex (S-I) forelimb-stump rep resentation of rats that sustained neonatal forelimb removal is characteriz ed by the expression of hindlimb inputs that are revealed when cortical GAB A receptors are pharmacologically blocked. Recent work has shown that the m ajority of these inputs are transmitted from the S-I hindlimb representatio n to the forelimb-stump field via an, as yet, unidentified pathway between these regions. In this study, we tested the possibility that hindlimb input s to the S-I forelimb-stump representation of neonatally amputated rats are conveyed through an intracortical pathway between the S-I hindlimb and for elimb-stump representations that involves the intervening dysgranular corte x by transiently inactivating this area and evaluating the effect on hindli mb expression in the S-I forelimb-stump representation during GABA receptor blockade. Of 332 S-I forelimb-stump recording sites from six neonatally am putated rats, 68.3% expressed hindlimb inputs during GABA receptor blockade . Inactivation of dysgranular cortex with cobalt chloride (CoCl2) resulted in a significant decrease in the number of hindlimb responsive sites (9.5%, P< 0.001 vs. cortex during GABA receptor blockade before CoCl2 treatment). Results were also compiled from S-I forelimb recording sites from three no rmal rats: 14.1% of 136 sites were responsive to the hindlimb during GABA r eceptor blockade, and all of these responses were abolished during inactiva tion of dysgranular cortex with CoCl2 (P< 0.05). These results indicate tha t the S-I hindlimb representation transmits inputs to the forelimb-stump fi eld of neonatally amputated rats through a polysynaptic intracortical pathw ay involving dysgranular cortex. Furthermore the findings from normal rats suggest that this pathway might reflect the amplification of a neuronal cir cuit normally present between the two representations.