Age-related changes in cerebellar noradrenergic pre- and postsynaptic mechanisms: intrinsic vs extrinsic determinants evaluated with brain grafts in oculo

Intrinsic versus extrinsic determinants of changes in cerebellar noradrenergic transmission during senescence in the rat were measured using homologous cerebellar grafts in oculo. Postsynaptic sensitivity of Purkinje neurons to catecholamines was determined by perfusing the anterior eye chamber with known concentrations of norepinephrine (NE) dissolved in a balanced salt solution. NE elicited a dose-dependent slowing of spontaneous Purkinje neuron discharge in both young (3-6 months) and aged (20-22 months) cerebellar grafts. Hill plots demonstrated that the dose-response relationships in both age-groups were linear and parallel to one another. Aged transplant Purkinje neurons manifested a marked and highly significant subsensitivity to NE with an EC₅₀ of 583 μM, as compared with an EC₅₀ of only 15.9 μM in the young grafts. Young grafts in 15-21-month-old hosts manifested an EC₅₀ of 20 μM for the depressant actions of NE. Collaterals of host iris sympathetic fibers innervate the grafts. Activity of these fibers can be reflexly altered by changing illumination of the retina. The dynamics of presynaptic NE release from these fibers was evaluated using in vivo electrochemistry with Nafion-coated graphite epoxy capillary electrodes, which are highly selective for the monoamine neurotransmitters. As illumination of the ipsilateral retina is increased, the release of catecholamine in the cerebellar graft decreases. A mean change in the extracellular electroactive species of 4.2 ± 0.6 μM was found in young cerebellar grafts. Equivalent stimuli induced a mean change of 2.3 ± 0.8 μM in aged grafts. However, this diminished release was not statistically significant. In addition, whole tissue levels of NE in the 3 groups of grafts, measured by high-performance liquid chromatography, were not statistically different. Taken together, these results suggest that the marked age-related postsynaptic subsensitivity to NE in cerebellar Purkinje cells, seen here and previously, is intrinsically determined. Moreover, in this model system, little age-related alteration is seen in presynaptic noradrenergic mechanisms.