Experience – dependent plasticity reorganizes connections between neurons according to importance of incoming sensory information. In mice, during the exploratory behaviour, one the most important instruments are facial vibrissae that function as a tactile sensors. Their representation in the cerebral cortex can be modified by sensory experience and learning. The results of the present study demonstrate differences in the effect of age on the plasticity of vibrissal representations in the somatosensory cortex.
Two methods of inducing cortical plasticity were employed – associative learning (when the mice learnt that stimulation of whiskers signals arrival of an unpleasant stimulus) and sensory deprivation (where increased functional activation of a row of whiskers is enforced by clipping the remaining whiskers). In young animals, both treatments cause a plastic change in the cortical representations of whiskers. Functional changes in the barrel cortex were visualized by [14C]2-deoxyglucose uptake autoradiography, a method that reflects global neuronal activity.
Learning-related changes in the cortical representation of vibrissae, found in young mice, were vulnerable to aging and could not be detected already in mature mice. Deprivation-induced plasticity was also found to decline with age, but the plastic effect, albeit decreased, was still observed in mature and old mice. We found that during the aging process, modifiability of layer IV of the sensory cortex was more affected than in other cortical layers, so that in old animals the effect of deprivation was no longer visible in that layer. Layer II/III and layer V/VI was less affected by aging in a sense that its plasticity occurred even in very old animals and the functional remodeling of cortical connections was still observed.
The presented data show that the effects of ageing depend not only on the brain region examined but also on the mechanism of the plastic change, since in the same cortical area distinct forms of experience-dependent plasticity are differentially affected by the aging process.