The enantiomers of the dopamine agonist N-0437: in vivo and in vitro effects on the release of striatal dopamine

The enantiomers of the potent and selective dopamine (DA) D-2 receptor agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin, N-0437, were tested for their pharmacological actions on DA D-2 autoreceptors in vivo, by measuring DA release by microdialysis during local administration of both drugs and in vitro, by measuring their effects on the electrically stimulated release of [³H]DA from striatal slices. In both experimental situations (-)-N-0437, at low doses, acted as an agonist on receptors controlling DA release. However, in vivo at a concentration of 10 μM (-)-N-0437 induced a short-lasting increase in DA release and in vitro the inhibitory effect of (-)-N-0437 was significantly less pronounced at higher concentrations (1-10 μM). (+)-N-0437 (0.1-10 μM) showed an antagonistic action both in vivo and in vitro. Following inhibition of the neuronal impulse flow with tetrodotoxin, (+)-N-0437 failed to increase DA release suggesting the effect of this enantiomer is not associated with an amphetamine-like action on the nerve terminal. The use of the DA re-uptake inhibitor GBR 12909 confirmed the antagonism of (+)-N-0437 towards DA receptors. Since it is known that (+)-N-0437 acts as an agonist on DA D-2 autoreceptors controlling the synthesis of DA, these results provide additional evidence for the existence of distinct DA D-2 autoreceptor populations involved in the release and synthesis of DA. The differential actions of (-)- and (+)-N-0437 gave rise to mutual antagonism of the actions of the enantiomers both in vivo and in vitro, thus providing a strong argument for using the enantiomers instead of the racemate in clinical situations.