Neuropharmacology and stereochemistry of dopamine receptor agonist and antagonist enantiomeric pairs

Altered spontaneous behavior and sensitivity to apomorphine in rats following pretreatment with S(+)-aporphines or fluphenazine

Rats were pretreated for 2 weeks with similarly effective doses of the typical neuroleptic fluphenazine (FPZ) or the experimental weak partial D2 agonists S(+)N-n-propylnorapomorphine (NPA) and S(+)11-hydroxy-N-n-propylnoraporphine (11-OH-NPa). Spontaneous and dopamine (DA) agonist (apomorphine; APO) stimulated stereotyped behaviors or locomotion, and interactions with APO were evaluated over the following 2 weeks. While FPZ induced marked supersensitivity in APO stereotype, (+)NPA showed no significant change, and (+)11-OH-NPa produced only a small, transient increase in response; NPA also lacked a supersensitizing effect on locomotor arousal induced by APO. The time-course of stereotyped responses to APO following pretreatment with FLZ included a marked increase following FPZ that became maximal at day 5 and normalized by day 9; there was a parallel reduction of acute antisteotypy efficacy of FPZ. (+)11-OH-NPa had similar, but much lesser and shorter-lived effects. Spontaneous locomotion was markedly depressed following FPZ, recovered in 1 week, exceeded controls at day 9, and returned to baseline by day 11; (+)11-OH-NPa, again, had similar but smaller effects. Acute effects of FPZ to reduce spontaneous or APO-induced locomotion were greater after FPZ pretreatment and normalized within a week; (+)11-OH-NPa had a similar but smaller effect. Locomotor arousal by APO was altered inconsistently in the week after pretreatment with FPZ or (+)11-OH-NPa. Thus, FPZ appeared to induce tolerance and supersensitivity in central DA systems, most clearly seen following a several-day period to eliminate the drug.(ABSTRACT TRUNCATED AT 250 WORDS)

5 Centrally Acting Dopamine D2 Receptor Ligands: Agonists

The collected amount of research on D2 agonists is immense. The D2 research field has been, and still is, very dynamic. Despite this fact, only a few compounds have reached the clinic so far. At present there are clinical trials ongoing with partial D2 agonists possessing a range of intrinsic efficacies. Some of these agonists are tested for their potential effects in Parkinson's disease (high intrinsic efficacy), while others are tested for potential anti-psychotic effects (low intrinsic efficacy). An interesting possibility has arisen through the research on the synergism between D1 and D2 receptors. This could possibly be utilized in the alleviation of Parkinsonian symptomatology. The near future will show whether the compounds under evaluation hold promise for being new, valuable medicines for treating major diseases like Parkinson's disease and schizophrenia.

R(-)2-fluoro-N-n-propylnorapomorphine: a very potent and D2-selective dopamine agonist

A series of 2-substituted N-n-propylnorapomorphine (NPA) derivatives were synthesized and compared with other DA agonists for affinity to D1 and D2 dopamine (DA) receptors in rat brain corpus striatum tissue. The 2-substituents tested reduced D1 affinity similarly, but enhanced D2 affinity in the rank order: F greater than OH greater than Br greater than OCH3 greater than H greater than or equal to NH2. The extraordinarily high D2 affinity (Ki = 12 pM) and D2 vs. D1 selectivity (57,500) of 2-F-NPA far-exceeded that of all other DA agonists tested, and it was about 10-times more potent than NPA in vivo.

Effects of isomers of hydroxyaporphines on dopamine metabolism in rat brain regions

The effects of isomers of di- and monohydroxyaporphines on cerebral dopamine (DA) metabolism were evaluated in representative extrapyramidal (corpus striatum) and limbic (nucleus accumbens septi) tissues of rat brain by three methods: (1) changes in the ratio of homovanillic acid (HVA) to DA, (2) accumulation of L-dihydroxyphenylalanine (DOPA) after inhibiting its decarboxylation to DA under "open-loop" conditions, as well as (3) after gamma-butyrolactone (GBL) pretreatment to provide selective effects at presynaptic DA autoreceptors. The DA-agonist R(-) isomers of the aporphines apomorphine (APO), N-n-propylnorapomorphine (NPA), and 11-hydroxy-N-n-propylnoraporphine (11-OH-NPa) showed consistent dose-dependent inhibition of DA synthesis in both brain regions with all models; the neuroleptic haloperidol had the opposite effect in the first two models only, as expected. The S(+) isomers of NPA and 11-OH-NPa have shown behavioral evidence of antidopaminergic activity, especially in the limbic system. Unlike the neuroleptic, S(+)NPA did not show DA-synthesis enhancing actions in accumbens or striatal tissue but, instead, inhibited DA synthesis like its R(-) antipode in all three test paradigms. S(+)11-OH-NPa given alone produced minor changes in the HVA/DA ratio and did not antagonize R(-)11-OH-NPa, weakly increased accumulation of DOPA in the second model, and had no effect in the third--all without regional selectivity. In the test of autoreceptor functioning, the dihydroxyaporphine S(+)NPA, but not S(+)11-OH-NPa, inhibited DA synthesis and this effect, in turn, was largely reversed by haloperidol, as were the inhibitory effects of the three R(-)aporphines tested. In this model, however, neither S(+)NPA nor S(+)11-OH-NPa antagonized the DA-synthesis inhibiting effect of R(-)APO as haloperidol did. Overall, these results are consistent with evidence that R(-)NPA and 11-OH-NPa have high affinity at D-2 receptor sites in rat brain and show behavioral effects of typical DA agonists. The non-stereoselective inhibitory effects of NPA on DA synthesis may reflect its activity as a weak DA agonist with very low intrinsic activity, but may also include a direct "catechol-effect" on tyrosine hydroxylase. In contrast, R(-)11-OH-NPa appears to be a stereoselective D-2 agonist, active at autoreceptors as well as postsynaptic receptors, that lacks the nonstereospecific effects on DA metabolism of its catechol-aporphine congener. It may be a useful probe for the further characterization of dopamine receptors and autoreceptors.

R(-) and S(+) stereoisomers of 11-hydroxy- and 11-methoxy-N-n-propylnoraporphine: central dopaminergic behavioral activity in the rat

R(-)11-Hydroxy-N-n-propylnoraporphine (11-OH-NPa) induced stereotyped behavior in the rat as potently (ED50 = 0.80 mg/kg, i.p.) as R(-)apomorphine (APO) and this effect was blocked by haloperidol; the 11-methoxy congener, R(-)11-MeO-NPa, had a weak effect (ED50 greater than 10 mg/kg) and the S(+) isomers had none. The isomer R(-)11-OH-NPa potentiated locomotion stimulated by apomorphine; S(+)11-OH-NPa inhibited it and the isomers of 11-MeO-NPa were inactive. Catecholaporphines usually are inactive orally, but both R(-) and S(+)11-OH-NPa were similarly potent after oral or parenteral administration. The isomer S(+)11-OH-NPa inhibited spontaneous and apomorphine-induced locomotion (ID50 = 1.8-2.7 mg/kg, p.o. and i.p.) and stereotyped behavior (ID50 = 3 mg/kg, p.o. or i.p.), all without inducing catalepsy. While apomorphine was short-acting (1-2 hr), the effects of R(-)11-OH-NPa persisted up to 6-7 hr and those of the S(+) isomer for at least 2.5 hr; moreover, the efficacy of R(-)11-OH-NPa increased markedly up to 3-4 hr, although its ED50 was independent of time (ED50 = 1.7-1.9 mg/kg, i.p. from 1-3 hr). The total effect of R(-)11-OH-NPa (p.o. or i.p.) over time was more than 10-times greater than that of injected apomorphine. These observations accord with the reported high (nM) affinity of 11-OH-NPa at cerebral DA receptor sites (D2 greater than D1) and weak interactions of the 11-methoxy congener. They support the conclusion that the R(-) and S(+) stereoisomers are neuropharmacologically active, respectively, as DA agonist and apparent antagonist, as was found with the enantiomers of N-n-propylnorapomorphine, perhaps due to the low intrinsic postsynaptic agonist activity of the S(+) isomers. Moreover, 11-OH-NPa was highly bioavailable orally and unusually long-acting; it may be absorbed slowly or have active metabolites. Hydroxy-substitution of aporphines at the 11-position, homologous to the 3-OH of DA, evidently is critical for affinity and activity at the DA receptor. These or other monohydroxyaporphines may represent leads to potentially useful DA agonist or antagonist drugs.

Efficacy and potency comparisons among aporphine enantiomers: effects on dopamine neurons in substantia nigra of rat

Extracellular single unit recording studies were carried out on male rats to determine the responses of dopamine neurons of the substantia nigra to intravenous administration of the enantiomers of the aporphine congeners, apomorphine (APO), N-n-propylnorapomorphine (NPA) and 11-hydroxy-N-n-propylnorapomorphine (11-OH-NPa). The R-(-)-configuration was found to be the most critical determinant of the efficacy and potency of the agonists. All R-(-)-aporphines were full agonists, able to inhibit completely firing of dopamine cells. The order of potencies, defined by the ID50s, was: (-)NPA, 2.0 +/- 0.4 nmol/kg greater than (-)11-OH-NPa, 4.7 +/- 0.7 nmol/kg greater than (-)APO, 18.0 +/- 4.0 nmol/kg. Thus, potency was increased about 9-fold by replacing the 6N methyl of APO with an n-propyl (NPA). Conversely, the 10-hydroxy was not essential for agonist activity (11-OH-NPa) but could increase potency. In the S-(+)-series responses varied. (+)N-n-Propylnorapomorphine exhibited agonist properties and could fully inhibit dopamine cells, but its potency was low (ID50 1550 nmol/kg); (+)APO produced only slight but significant decreases in firing at large (8434 nmol/kg) doses and (+)11-OH-NPa was devoid of efficacy in that it caused no significant changes in firing. Despite their loss of efficacy and potency, the (+)-enantiomers apparently did retain affinity for DA receptors, since they could act as antagonists if given before (-)APO or NPA. These findings suggest that stereochemical conformation and key structural elements of the aporphines are interactive in determining agonist efficacy and potency within this physiological test system.