Tactile impoverishment and sensorimotor restriction deteriorate the forepaw cutaneous map in the primary somatosensory cortex of adult rats

We investigated the effects of sensory deprivation on the forepaw represent ation in the primary somatosensory cortex (SI) in the adult rat. Cortical m aps were constructed from high-resolution multiunit recordings of the respo nse of layer IV neurons to somatosensory stimuli. The main features of the forepaw representation were described in terms of areal extent and topograp hy of the cortical map, and sensory submodality, size, and location of the receptive field (RF) of small clusters of the cortical neurons. After being weaned, two groups of Long-Evans rats were housed in a standard (SE) or im poverished (IE) environment for 65-115 days. A third group of SE rats was s ubjected to severe sensorimotor restriction ISR) of one forepaw for 7 days or 14 days, by using a one-sleeved cast. A concomitant effect of unilateral forelimb immobilization was a forced use of the nonrestricted forelimb in postural balance. The maps of both forepaws were derived 24 h after the cas t was removed and the animal was allowed normal limb use. In a fourth group , SE rats experienced a 7-day immobilization followed by symmetrical limb u se for 7 days before we mapped the hemisphere contralateral to the casted l imb. For the SE and IE rats, the total areal extent of the cutaneous forepa w representation was similar, but IE rats exhibited a significant expansion of cortical islets serving high-threshold, presumably noncutaneous inputs, which were included in the cutaneous maps. In addition, SI neurons of IE r ats had greatly enlarged glabrous, but not hairy, skin RFs. For the SR rats , the areal extent of the cutaneous map of the casted forepaw decreased by about 50%. after both 7- and 14-day forelimb immobilization. Large cortical sectors presumed to be formerly activated by cutaneous inputs were driven by high-threshold inputs that disrupted the somatotopic representation of t he forepaw skin surfaces. These "emergent" representational sectors were to pographically organized. By contrast, the areal extent and topography of th e noncasted forepaw representation did not differ from those of SE rats. Th e size of glabrous RFs on the casted forepaw was similar to that of SE rats . On the contrary, glabrous RFs on the noncasted forepaw of SR rats were la rger than those on their casted forepaw The size of hairy RFs was not alter ed by the forelimb restriction. Interestingly, alteration of the somatotopi c features of the casted forepaw map persisted after 7 days of symmetric us e of the forelimbs. The present study demonstrates that continuous sensory experience is needed for the organizational features of SI maps to be maint ained.