Circling behavior in normoxic or hypoxic rats with unilateral 6-OHDA nigrostriatal lesion. Part 1. Effects of apomorphine and L-dopa.

Unilateral administration of 6-hydroxydopamine (6-OHDA) into the rat nigrostriatal system provokes circling behavior in response to apomorphine and L-dopa. This behavior appears to be mediated by the development in the lesioned side of a postsynaptic dopamine receptors supersensitivity. Acute hypobaric hypoxia does not modify the apomorphine-induced stimulant effects, but does oppose the L-dopa-induced circling behavior. Our data suggest that effects of hypoxia are mediated by a presynaptic mechanism. The antagonism of the effects of L-dopa cannot be explained by the inhibition of activity of the oxygen-dependent enzyme tyrosine hydroxylase, but seems to be related to an impairment of the release of newly synthesized dopamine from exogenous L-dopa.