Delayed reorganization of the shoulder representation in forepaw barrel subfield (FBS) in first somatosensory cortex (SI) following forelimb deafferentation in adult rats

We previously reported that 6-16 weeks after forelimb amputation in adult rats, neurons in layer IV of rat first somatosensory cortex (SI) in the forepaw barrel subfield (FBS) associated with the representation of the forepaw became responsive to new input from the shoulder (Pearson et al. 1999). These new shoulder-responsive sites in deafferented FBS had longer evoked response latencies than did sites in the shoulder representation located in the posterior part of the trunk subfield, hereafter referred to as the original shoulder representation. Furthermore, projection neurons in the original shoulder representation in both intact and deafferented adults did not extend their axons into the FBS, and ablation of the original shoulder representation cortex and/or the second somatosensory cortex (SII) failed to eliminate new shoulder input in the deafferented FBS (Pearson et al. 2001). These results led us to conclude that large-scale reorganization in FBS quite likely involved a subcortical substrate. In addition, the time course for large-scale cortical reorganization following forelimb amputation was unknown, and this information could shed light on potential mechanisms for large-scale cortical reorganization. In the present study, we extended our previous findings of large-scale cortical reorganization in the FBS by investigating the time course for reorganization following forelimb amputation. The major findings are: a) deafferented forelimb cortex remained unresponsive to shoulder stimulation during the 1st week following forelimb amputation; b) new responses to shoulder stimulation were first observed in deafferented forelimb cortex 2-3 weeks after forelimb amputation; however, the new shoulder input was restricted to locations in the former forearm cortex; c) islet(s) of new shoulder representation were first observed in deafferented FBS 4 weeks after amputation; these islets occupied a larger percentage of FBS in subsequent weeks; d) portions of FBS remained unresponsive as many as 4 months after deafferentation ( maximum time examined between amputation and recording); and e) the increase in total size of the shoulder representation appeared to result from the establishment of new shoulder representations that were often discontinuous from the original shoulder representation. These findings provide evidence that forelimb amputation results in delayed reorganization of the FBS and we describe possible mechanisms and substrates underlying the reorganization.