Pharmacological profiles of the putative dopamine autoreceptor agonists 3-PPP and TL-99

The putative dopamine (DA) autoreceptor agonists, N-n-propyl-3-(3-hydroxyphenyl)-piperidine (3-PPP) and 6, 7-dihydroxy-2-dimethylaminotetralin (TL-99) were compared with apomorphine in a series of tests indicative of DA receptor activation. All three agents displaced [3H] apomorphine and [3H] spiroperidol from DA recognition sites in rat brain and caused contralateral turning in the 6-hydroxydopamine lesioned rat. Apomorphine and TL-99 were generally more potent than 3-PPP. All three agents were also active at the DA autoreceptor that controls the synthesis of dopamine as indicated in vivo using the gamma-butyrolactone (GBL) procedure and in vitro using a synaptosomal preparation. In addition, all agents produced emesis in beagles. clear differences in the drugs' actions were observed in other test procedures. In the rat, apomorphine was the only compound which caused stereotypy or rotation following a reversible KCI-induced lesion of the striatum. Conversely, TL-99 and 3-PPP lacked activity in these procedures. Presumably, activity in these two tests indicates postsynaptic DA receptor activation. Each of the putative autoreceptor agonists produced a monotonic dose-related decrease in the mouse locomotor activity as opposed to the biphasic effect exerted by apomorphine. This action on the mouse locomotor activity, coupled with the results for the GBL test, provides an index of autoreceptor activation. In contrast to 3-PPP, both apomorphine and TL-99 increased locomotor activity in animals pretreated with reserpine and alpha-methyl-p-tyrosine and caused rotation in unilaterally caudectomized mice. In these test procedures thought to reflect activity at the postsynaptic DA receptor, TL-99 differed in its action from 3-PPP in a manner which suggests 3-PPP may be a more selective DA autoreceptor agonist.