Stereoselectivity of presynaptic autoreceptors modulating dopamine release.

The effects of the (R)- and (S)-enantiomers of sulpiride and butaclamol were studied on the spontaneous and field stimulation-evoked release of total radioactivity from slices of rabbit caudate nucleus prelabelled with [3H]dopamine. (S)-Sulpiride in concentrations ranging from 0.01--1 microM enhanced the electrically evoked release of [3H]dopamine while (R)-sulpiride was 10 times less potent than (S)-sulpiride. Exposure to (S)-butaclamol (0.01--1 microM) but not to (R)-butaclamol (0.1--10 microM) enhanced the field-stimulated release of [3H]dopamine. The facilitatory effects of (S)- and (R)-sulpiride and (S)-butaclamol on the stimulated release of the labelled neurotransmitter were observed under conditions in which these drugs did not modify the spontaneous outflow of radioactivity. Only the active enantiomers of sulpiride and butaclamol antagonized the inhibition by apomorphine (1 microM) of the stimulated release of [3H]dopamine. Our results indicate that the presynaptic inhibitory dopamine autoreceptors modulating the stimulation-evoked release of [3H]dopamine in the caudate nucleus are, like the classical postsynaptic dopamine receptors, chemically stereoselective.