Are presynaptic dopamine autoreceptors and postsynaptic dopamine receptors in the rabbit caudate nucleus pharmacologically different?

Slices of the rabbit caudate nucleus were preincubated with [3H]dopamine or [3H]choline and then superfused and stimulated electrically. Apomorphine reduced the stimulation-evoked overflow of tritium over the same concentration range, independently of whether slices had been pre-incubated with [3H]dopamine or with [3H]choline. Each of three antagonists--molindone, sulpiride and metoclopramide--increased the evoked overflow of tritium over the same concentration range in experiments with [3H]dopamine and those with [3H]choline. For each antagonist, the pA2 values against apomorphine obtained in [3H]dopamine experiments and in [3H]choline experiments were very similar. This study is a functional in vitro approach to receptor characterization, as opposed to radioligand binding studies or in vivo investigations. The results show that the dopamine receptor agonist apomorphine and three antagonists are unable to distinguish between the presynaptic, release-inhibiting dopamine autoreceptors and those postsynaptic dopamine receptors which, when activated, depress the release of acetylcholine. Although there are certainly more dopamine receptors in the caudate nucleus, these two physiologically important groups seem to be closely related.