Age-related alteration of the forepaw representation in the rat primary somatosensory cortex

The cortical forepaw representation of adult rats was mapped by using multiunit recordings of layer IV neurons within the primary somatosensory cortex. Electrophysiological maps were based on somatosensory "submodality" (cutaneous vs non-cutaneous), location and size of the receptive fields. Age-related changes in the organizational features of the forepaw representation in the primary somatosensory cortex were analysed in adult rats whose ages ranged from 3.5 to five months (young), about eight months (mature), 15-17months (presenescent) to 24-28months (senescent). Rats were housed from weaning (30days postnatal) in a standard laboratory environment. The organization of the forepaw map was not gradually altered with advancing age, but striking changes occurred in early adulthood (before eight months) and did not progress with further aging. The main alterations consisted of a prominent decrease in, and a fragmentation of, the cutaneous area of the forepaw representation. Representational zones formerly serving cutaneous surfaces became predominantly activated by high-threshold, presumably non-cutaneous, inputs which appeared somatotopically organized. These emergent non-cutaneous zones were interspersed with cutaneous sectors, thereby disrupting the somatotopic organization of the map of the forepaw skin. No significant modification in the size of glabrous or hairy cutaneous receptive fields accompanied these changes. Subjective evaluation of the responses evoked by tactile stimulation suggests that neuronal responsiveness was increased in the eight- to 17-month-old rats, but less so in the 24- to 28-month-old animals. These results indicate that degradation of the somatotopic organization of the cutaneous representation of the forepaw in the rat somatosensory cortex occurs early during the course of adult life.