Interaction of the putative dopamine autoreceptor agonists 3-PPP and TL-99 with [3H]apomorphine binding sites in rat striatal membranes.

Examination of the binding of [3H]apomorphine to rat striatal membranes in the presence and absence of the dopamine antagonist, domperidone, confirmed the previously reported presence of two classes of dopamine binding site, those designated D2 which show a high affinity for both agonist and antagonists and those designated D3 which have a high affinity for agonists and a low affinity for antagonists. In contrast to the previously reported single high affinity (KD congruent to 1 nM) D2- and D3-binding sites, two lower affinity sites (D2KD = 7-50 nM; D3KD = 41 nM) were also observed. Examination of the binding characteristics of the putative dopamine autoreceptor agonists, 3-PPP (N,N-propyl-3-(3-hydroxyphenyl)piperdine) and TL-99 (6,7-dihydroxy-2-dimethylaminotetraline) showed that they, like a number of other dopamine agonists including n-propylnorapomorphine, apomorphine and dopamine showed no preferential affinity for the D3, presynaptic binding site. It is concluded that the selectivity of dopamine agonists for the autoreceptor cannot be assessed by the in vitro radioligand binding parameters defined by the use of domperidone.