Sensorimotor experience modulates age-dependent alterations of the forepawrepresentation in the rat primary somatosensory cortex

In a previous study, we round that the forepaw representation in the primar y somatosensory cortex of rats housed in standard laboratory conditions was drastically altered during the aging process. In other studies we reported that exposure to an enriched environment improved the topographical organi zation and increased the spatial resolution of the forepaw cutaneous map in young adult rats, whereas housing in impoverished environment resulted in a loss of somatotopic details in the forepaw map. The main purpose of the p resent study was to investigate the influence or differential sensorimotor experience promoted by exposure to enriched or impoverished environments on the mutability of the cortical forepaw representation during aging. Two gr oups of Long-Evans rats were reared in enriched and impoverished environmen ts from weaning to the age of 3.5-5 months (young adults). 6.5-8 months (ma ture rats). and 23-28 months (senescent rats). The electrophysiological map s of the forepaw representation were based on the somatosensory 'submodalit y' (cutaneous vs, non-cutaneous), size, and location of the receptive field s of small clusters of layer IV neurons. Moreover, the mechanical threshold s of neuronal response to cutaneous stimulation were assessed with calibrat ed von Frey filaments in mature and senescent animals. Age-related alterati ons of the topographic features of the forepaw map were characterized by a decrease in and a fragmentation of the cortical zones serving the glabrous skin of the forepaw. These changes were less pronounced in the enriched rat s than in the impoverished rats. Glabrous skin receptive fields were smalle r in young adult and mature enriched rats than in their impoverished counte rparts. However. during aging glabrous receptive fields increased in the en riched rats, but decreased in the senescent impoverished rats so that old r ats of either groups displayed receptive fields of similar sizes: in contra st, the size of hairy skin receptive fields was not affected by housing con ditions or aging. Measurement of the neuronal responses to calibrated force s applied to the skin indicated that cortical excitability to near-threshol d cutaneous input was lower in senescent rats than in mature rats, regardle ss of environmental conditions. The present study demonstrates that use-dependent remodeling of somatosenso ry maps occurs throughout life and that environmental and social interactio ns can partially offset the age-related breakdown of somatosensory cortical maps. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserve d.