Cerebellar noradrenergic systems in aging: studies in situ and in in oculo grafts

Age-related changes in noradrenergic function in the rat cerebellum were examined using electrophysiological and electrochemical techniques. Sprague-Dawley and Fischer 344 rats showed subsensitivity to norepinephrine (NE) locally applied onto cerebellar Purkinje neurons. The modulatory actions of NE on Purkinje cell-evoked activity was also examined. In young rats NE preferentially inhibits spontaneous activity more than evoked excitations when compared to control. These modulatory actions of NE are not seen in senescent Fischer 344 rats. The intrinsic vs. extrinsic influences determining the loss of efficacy to NE were examined using three groups of rats with in oculo cerebellar grafts. The first group had young grafts grown in young hosts and these grafts showed a potent response to perfused NE. The second group, old grafts in old hosts, showed a diminished responsiveness to NE with respect to the first group. The third group consisted of young grafts in old hosts. These grafts demonstrated a responsiveness to NE that was indistinguishable from those in the first group. The integrity of the presynaptic NE fibers was examined in the grafts using electrochemical techniques. No difference in the release of NE was observed in the old grafts. Taken together, these results suggest a loss of postsynaptic NE function that is intrinsically determined. The change in NE modulation could influence information processing within the aged cerebellar cortex. This deficit could underlie behavioral changes seen in senescence.