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

Dopamine receptor agonistic and antagonistic effects of 3-PPP enantiomers

The pharmacological profile of the enantiomers of the proposed selective dopamine (DA) autoreceptor agonist 3-PPP [3-(3-hydroxyphenyl)-N-n-propylpiperidine] has been studied. In vitro both enantiomers showed weak DA agonistic activity, and (--)-3-PPP some DA antagonistic effect on DA-stimulated adenylate cyclase activity. Both enantiomers in low doses had a similar profile in vivo: Inhibition of locomotor activity of mice and rats, induction of contralateral circling behaviour in 6-hydroxy-DA-lesioned rats and an emetic effect in dogs. At higher doses, differential effects of the enantiomers were found: (+)-3-PPP induced hyperactivity, weak stereotyped behaviour and ipsilateral circling in hemitransected rats. (--)-3-PPP had depressant effects in high doses, inhibited d-amphetamine-induced hyperactivity and d-amphetamine-, methylphenidate- and apomorphine-induced stereotyped licking/biting in rats and antagonized apomorphine-induced emesis in dogs. However, (+)-3-PPP also showed a weak antagonistic activity against d-amphetamine-induced hyperactivity and d-amphetamine- and apomorphine-induced stereotypy in rats and inhibited apomorphine-induced emesis in dogs. It is suggested that both enantiomers have significant effects on postsynaptic DA receptors in high doses: (--)-3-PPP with weak antagonistic activity in some test models and (+)-3-PPP with agonistic and antagonistic effect. Since these effects of (+)-3-PPP were of low intensity at high doses, (+)-3-PPP may be a partial DA agonist at postsynaptic receptors in high doses.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Is TL-99 a selective presynaptic dopamine receptor agonist?

The claim that the 2-aminotetralin analogue TL-99 is a selective presynaptic dopamine (DA) receptor agonist has been investigated both in vivo and in vitro in the rat. The pharmacological specificity of the hypomotility caused by TL-99 has been examined using various selective antagonists. In addition its effects on DA metabolism and noradrenaline (NA) and DA turnover (alpha-MT method) as well as its distribution in the brain have been studied. These in vivo studies provide evidence that although TL-99 is able to activate presynaptic DA receptors it is also a potent agonist of NA receptors as shown by the fact that the hypomotility could be partly reversed by the selective alpha 2-adrenoceptor antagonists yohimbine and piperoxan. Further supporting evidence for these findings was provided by in vitro studies on the inhibition of K+-induced [3H]dopamine, [14C]acetylcholine and [3H]noradrenaline release from striatal and cortical slices where it was shown that TL-99 is not only active at both pre- and postsynaptic DA receptors but also at alpha 2-NA receptors. For the latter receptor it had a potency comparable to that of the potent alpha 2-agonist clonidine and this may explain, to some extent, the hypomotility caused by TL-99. Thus, ascribing this hypomotility solely to an interaction with presynaptic DA receptors may be an oversimplification. It is therefore concluded that TL-99 should not be considered as a selective presynaptic DA receptor agonist.

Potent dopamine agonist activity of a novel ergoline, 6-ethyl-9-oxaergoline (EOE)

6-Ethyl-9-oxaergoline (EOE) and its enantiomers were compared with apomorphine in a number of tests designed to measure dopamine (DA) agonist activity within the central nervous system. In rats, the tests were: interaction with DA receptors labeled with 3H-apomorphine or 3H-spiroperidol; the effects on DA synthesis as assessed by the gamma-butyrolactone procedure; turning in 6-OHDA lesioned animals; stereotypy; and, slowing of DA cell firing rates. In the mouse, locomotor activity, hypothermia and postural asymmetry in caudectomized animals were studied. Emesis in the beagle was also examined. The (-)-enantiomer of EOE was more potent than either the (+)-enantiomer or the racemate in all tests. With the exception of inducing stereotypy and the displacement of 3H-apomorphine from rat striatal membranes, (-)-EOE was equi- or more potent than apomorphine in all test procedures. (-)-EOE was effective following oral administration and exhibited a longer duration of action than apomorphine. The results indicate EOE is a potent DA agonist.

Interaction of the putative dopamine autoreceptor agonists, 3-PPP and TL-99, with the dopamine-sensitive adenylate cyclase of carp retina

The putative dopamine autoreceptor agonists, 3-PPP and TL-99 were examined for their ability to stimulate postsynaptic dopamine receptors associated with adenylate cyclase (D1-receptors) in the carp retina. In intact pieces of retina, 3-PPP had no significant effect on cyclic AMP production at concentrations up to 100 microM, whereas the aminotetralin, TL-99, caused a concentration-dependent increase in cyclic AMP levels with an approximate EC50 of 3.6 microM. Dopamine and ADTN had EC50 values of 3.5 and 3.1 microM respectively. Furthermore, in homogenates of the retina, 100 microM ADTN and 100 microM TL-99 stimulated adenylate cyclase activity 92 and 79% respectively as compared to the stimulation evoked by 100 microM DA. In contrast, 100 microM 3-PPP was essentially inactive at stimulating adenylate cyclase in carp retinal homogenates. These findings suggest that TL-99 can interact with postsynaptic D1-receptors and is not as selective a dopamine autoreceptor agonist as 3-PPP, which has no apparent activity at the D1-receptor.

Dopamine autoreceptor stimulation: clinical significance

Recent studies of the effects of low doses of dopamine agonists, designed to stimulate dopamine autoreceptors and hence diminish the synthesis and release of dopamine, were based on a series of basic research studies which demonstrated the existence of autoreceptors on dopamine neurones of the nigrostriatal, mesolimbic and mesocortical dopaminergic neurones. Evidence for autoreceptors on the tuberoinfundibular dopamine neurones which participate in the regulation of prolactin and growth hormone secretion is lacking. Some recent reports have questioned the existence of dopamine autoreceptors on the mesolimbic and nigrostriatal dopamine neurones. Specificity of various dopamine agonists and antagonists for the dopamine autoreceptor will be reviewed. The sedative, anxiolytic, antipsychotic, antidyskinetic and neuroendocrine effects of low dose dopamine agonists in man will be described. Low dose apomorphine, N-propylapomorphine and bromocriptine have been reported to have antipsychotic effects in the major psychoses, to diminish tardive dyskinesia and to enhance extrapyramidal insufficiency. A unique depressive state which developed in a small proportion of psychiatric patients after low dose apomorphine will be described. Further evidence for the lack of dopamine autoreceptors on the tuberoinfundibular dopamine neurones in man will be presented. Strategies for further study of the dopamine autoreceptor concept in man will be discussed.

Structure activity relationships of presynaptic dopamine receptor agonists

Structure activity relationship (SAR) studies have identified many structural entities that interact with dopamine receptors. The aminotetralin structure may be regarded as an active moiety of apomorphine. An unanswered question concerns the SAR of the 4,7-dimethoxy indane derivatives. These agents do not appear to match well with models of dopamine receptors. At least there can be little doubt that SAR research has been a powerful stimulus during the past decade for understanding the function, distribution, and spatial aspects of dopamine receptors.