Electrophysiological characteristics and pharmacological responsiveness of midbrain dopaminergic neurons of the aged rat.

The extracellular electrophysiological activities of A9 and A10 dopaminergic (DA) neurons were examined in chloral hydrate-anesthetized young (3 months) and aged (24-28 months) Fischer 344 male rats. Average firing rate, firing rate distribution and the incidence of spontaneous activity (cells per electrode track) did not change in either region as a function of age. Next, the inhibitory effects of several DA agonists on the firing rates of antidromically identified nigrostriatal DA cells were compared in young and old rats. The inhibitory potencies of i.v. apomorphine (D1/D2 agonist), quinpirole (D2 agonist) and d-amphetamine (indirect agonist) did not change with age. Similarly, the inhibitory effects of microiontophoretically applied dopamine were unaltered in aged rats. Stimulation of the sciatic nerve resulted in brief periods of inhibition of the activity of nigrostriatal DA neurons in both the young and the old age groups. The duration of this inhibitory response was, however, enhanced markedly in the old rats. The D1 agonist SKF 38393 enhanced the duration of the inhibitory effect of sciatic nerve stimulation in both young and old rats. These results suggest that, although no gross differences in the activity of DA cells in young and old rats were evident, subtle changes in the regulation of the activity of these neurons may occur with age.