Aging affects all levels of neural processing including changes of intracortical inhibition and cortical excitability. The paired-pulse stimulation protocol, the application of 2 stimuli in close succession, is used to investigate cortical excitability. The paired-pulse behavior is characterized by the fact that the second response is significantly suppressed at short interstimulus intervals (ISIs) but approaches the first response with increasing ISIs. However, there are controversial reports about the influence of age on paired-pulse behavior. We therefore used pairs of tactile stimuli (ISIs from tens to hundreds of milliseconds) to record extracellular responses of somatosensory cortical neurons of young and aged rats. Paired-pulse behavior was quantified as the ratio of the amplitude of the second response divided by the first. For all ISIs, we found significantly higher ratios in the old animals indicating reduced paired-pulse suppression (PPS). Evaluation of the single response components revealed a significant reduction of the response to the first stimulus for old animals but no age-dependent decrement to the second. Changes in PPS are usually mediated by modulating the second response characteristics. Thus, our data demonstrate reduced PPS due to an overall reduction of the first response as a form of modified PPS developing at old age.
Getting older is associated with a decline of cognitive and sensorimotor abilities, but it remains elusive whether age-related changes are due to accumulating degenerational processes, rendering them largely irreversible, or whether they reflect plastic, adaptational and presumably compensatory changes. Using aged rats as a model we studied how aging affects neural processing in somatosensory cortex. By multi-unit recordings in the fore- and hindpaw cortical maps we compared the effects of aging on receptive field size and response latencies. While in aged animals response latencies of neurons of both cortical representations were lengthened by approximately the same amount, only RFs of hindpaw neurons showed severe expansion with only little changes of forepaw RFs. To obtain insight into parallel changes of walking behavior, we recorded footprints in young and old animals which revealed a general age-related impairment of walking. In addition we found evidence for a limb-specific deterioration of the hindlimbs that was not observed in the forelimbs. Our results show that age-related changes of somatosensory cortical neurons display a complex pattern of regional specificity and parameter-dependence indicating that aging acts rather selectively on cortical processing of sensory information. The fact that RFs of the fore- and hindpaws do not co-vary in aged animals argues against degenerational processes on a global scale. We therefore conclude that age-related alterations are composed of plastic-adaptive alterations in response to modified use and degenerational changes developing with age. As a consequence, age-related changes need not be irreversible but can be subject to amelioration through training and stimulation.
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