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

The cortical forepaw representation of adult rats was mapped by using multi unit recordings of layer IV neurons within the primary somatosensory cortex . Electrophysiological maps were based on somatosensory "submodality" (cuta neous vs noncutaneous), location and size of the receptive fields. Age-rela ted changes in the organizational features of the forepaw representation in the primary somatosensory cortex were analysed in adult rats whose ages ra nged from 3.5 to five months (young), about eight months (mature), 15-17 mo nths (presencscent) to 24-28 months (senescent), Rats were housed from wean ing (30 days postnatal) in a standard laboratory environment. The organizat ion of the forepaw map was not gradually altered with advancing age, but st riking changes occurred in early adulthood (before eight months) and did no t progress with further aging. The main alterations consisted of a prominen t decrease in, and a fragmentation of, the cutaneous area of the forepaw re presentation. Representational zones formerly serving cutaneous surfaces be came predominantly activated by high-threshold, presumably non-cutaneous, i nputs which appeared somatotopically organized. These emergent non-cutaneou s zones were interspersed with cutaneous sectors, thereby disrupting the so matotopic organization of the map of the forepaw skin. No significant modif ication in the size of glabrous or hairy cutaneous receptive fields accompa nied these changes, Subjective evaluation of the responses evoked by tactil e stimulation suggests that neuronal responsiveness was increased in the ei ght- 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. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.