Sensorimotor experience modulates age-dependent alterations of the forepaw representation in the rat primary somatosensory cortex.

ABSTRACT

In a previous study, we found that the forepaw representation in the primary 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 organization 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 present study was to investigate the influence of differential sensorimotor experience promoted by exposure to enriched or impoverished environments on the mutability of the cortical forepaw representation during aging. Two groups of Long-Evans rats were reared in enriched and impoverished environments from weaning to the age of 3.5-5 months (young adults), 6.5-8 months (mature rats), and 23-28 months (senescent rats). The electrophysiological maps of the forepaw representation were based on the somatosensory 'submodality' (cutaneous vs. non-cutaneous), size, and location of the receptive fields of small clusters of layer IV neurons. Moreover, the mechanical thresholds of neuronal response to cutaneous stimulation were assessed with calibrated von Frey filaments in mature and senescent animals. Age-related alterations 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 rats than in the impoverished rats. Glabrous skin receptive fields were smaller in young adult and mature enriched rats than in their impoverished counterparts. However, during aging glabrous receptive fields increased in the enriched rats, but decreased in the senescent impoverished rats so that old rats of either groups displayed receptive fields of similar sizes; in contrast, the size of hairy skin receptive fields was not affected by housing conditions or aging. Measurement of the neuronal responses to calibrated forces applied to the skin indicated that cortical excitability to near-threshold cutaneous input was lower in senescent rats than in mature rats, regardless of environmental conditions. The present study demonstrates that use-dependent remodeling of somatosensory maps occurs throughout life and that environmental and social interactions can partially offset the age-related breakdown of somatosensory cortical maps.