Interaction of norepinephrine with Purkinje cell responses to cerebellar afferent inputs in aged rats.

Age-related differences in the modulatory actions of NE on the evoked activity of cerebellar Purkinje cells were examined in young (3 month) and old (18-20 month) Fischer 344 rats. We have previously shown that NE is more potent in young than in old rats, in terms of its ability to inhibit spontaneous activity. In this investigation complex spike excitation, simple spike excitation, and inhibition of Purkinje cell discharge were elicited by stimulation of climbing fibers, mossy fibers, and cerebellar parallel fibers, and quantified by computing post-stimulus time histograms of the neuronal response, recorded extracellularly. Histograms were compared before, during and after local ejection of NE from multibarreled micropipettes. In young rats NE preferentially inhibits spontaneous discharge more than evoked excitations. The inhibitory response of the Purkinje cell to activation of basket and stellate cell afferents is potentiated by NE with respect to the inhibition of spontaneous discharge. In old rats the NE-induced potentiation of both excitatory and inhibitory responses was significantly diminished. The loss of noradrenergic enhancement of the relative responsiveness of Purkinje neurons to afferent inputs in senescent animals may relate to behavioral deficits seen in aging.