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The actions in vitro of SCH 23390, YM 09151-2 and both enantiomers of 3-PPP on D-1 and D-2 dopamine receptors were investigated in superfused rat neostriatal slices. For comparison the following neuroleptics of different chemical classes were incorporated in our investigations: (+)-bulbocapnine, clozapine, chlorpromazine, cis-flupenthixol, (-)-sulpiride and haloperidol. The increase in the efflux of cyclic AMP was used as a measure for D-1 receptor stimulation. The decrease in the K+-evoked release of [3H]acetylcholine was used as measure of D-2 receptor stimulation. None of the drugs stimulated the D-1 receptor. Only (+)-3-PPP stimulated the D-2 receptor. All other drugs, including (-)-3-PPP, behaved as antagonists on the D-2 receptor, YM 09151-2 being the most potent. SCH 23390 was the most potent antagonist on the D-1 receptor. Haloperidol, cis-flupenthixol and (+)-bulbocapnine showed an appreciable D-1 receptor blocking potency in our model, whereas the other drugs were inactive. We found SCH 23390 to be the most D-1 selective antagonist although the drug still displayed considerable potency on the D-2 receptor. YM 09151-2 was the most D-2 selective antagonist.

The agonistic and antagonistic effects of the new compound SCH 23390 were tested in functional model systems for the D-1 dopamine receptor and for the D-2 dopamine receptor in vitro. In superfused rat neostriatal slices the increase in the efflux of cyclic AMP was used as a parameter for D-1 receptor stimulation. D-2 receptor stimulation was measured as the decrease in the K+-evoked release of [3H]-acetylcholine. SCH 23390 had no agonistic activity in these two models. SCH 23390 was a potent antagonist of the stimulating effect of dopamine in the D-1 receptor model (apparent pA2 = 7.28). SCH 23390 also antagonized the effect of the D-2 receptor agonist LY 141865 in the D-2 receptor model (apparent pA2 = 6.34). This D-2 receptor antagonism proved to be of a competitive nature.

Using an antibody raised against dopamine the occurrence of dopamine-containing fibers was demonstrated in the prefrontal cortex, anterior cingulate cortex, parietal neocortex, piriform cortex and entorhinal cortex. In extracts of these cortical regions significant amounts of dopamine, although approximately a 100-fold less than in the neostriatum or nucleus accumbens, were detected with high performance liquid chromatography. The release of [3H]dopamine from slices of all these cortical regions was studied in vitro in a superfusion system and desipramine was used to prevent the uptake of [3H]dopamine in noradrenergic nerve terminals. It appeared that the electrically evoked release of radioactivity was inhibited by drugs stimulating D-2 dopamine-receptors in all the regions studied. Cation-exchange column chromatography revealed that the radioactivity released consisted predominantly of [3H]dopamine, indicating that D-2 receptors mediate the inhibition of the release of [3H]dopamine from dopaminergic nerve terminals. Likewise, in the neostriatum as well as in the nucleus accumbens D-2 receptor stimulation inhibits the release of [3H]dopamine. Therefore it is our conclusion that D-2 receptors regulate the release of dopamine from dopaminergic neurons originating in the ventral tegmental area as well as in the substantia nigra.

Homogenates of the rat basal hypothalamus and the neurointermediate lobe of the pituitary gland contained relatively high levels of dopamine as was estimated by high-performance liquid chromatography (HPLC). The release of [3H]dopamine was studied in these regions and desipramine was used to prevent uptake of [3H]dopamine in noradrenergic nerve terminals. The release of radioactivity could be stimulated electrically and was calcium-dependent. It appeared that the release of radioactivity could be inhibited by drugs stimulating D2-dopamine receptors in both regions. The radioactivity released during electrical stimulation was analysed by cation exchange chromatography and appeared to consist predominantly of [3H]dopamine. It is our conclusion that D2-receptors mediate the inhibition of the release of [3H]dopamine from dopaminergic nerve terminals in the basal hypothalamus and in the neurointermediate lobe.

3-PPP has been reported to be a selective dopamine autoreceptor agonist, a conclusion based on in vivo studies. In rat neostriatum in vitro however, 3-PPP antagonized concentration-dependently the stimulation of cyclic AMP efflux caused by the D-1 receptor agonist SKF 38393 and the inhibition of K+-induced [3H]acetylcholine release caused by the D-2 receptor agonist LY 141865. 3-PPP alone did not affect [3H]acetylcholine release or cyclic AMP efflux. When interpreting the effects of 3-PPP in vivo dopamine receptor antagonism should be considered.