Effects of repeated tiapride administration on anterior pituitary dopamine receptors and prolactin release in the rat

The substituted benzamides tiapride and sulpiride, and the classic neuroleptic haloperidol, were studied in the rat to assess their interaction with the anterior pituitary (AP) dopamine (DA) receptors both in vitro ([3H]spiperone binding) and in vivo prolactin-PRL-release). Tiapride weakly inhibited [3H]spiperone binding in both pituitary and striatal membranes with affinity 5-7 times lower than sulpiride and 400-300 times lower than haloperidol. All three drugs were more potent in displacing [3H]spiperone from striatum than from AP. In vivo, tiapride produced weak and transient stimulation of PRL release reaching a full effect at 2 mg/kg i.p. Similar doses of sulpiride produced longer-lasting effects. Haloperidol was more potent than both benzamides. In prolonged treatments (15 or 60 days), tiapride, given twice daily at 0.5 mg/kg i.p., did not modify [3H]spiperone binding in either AP or striatum, nor did it induce significant changes of basal PRL levels. The challenge with a low threshold dose of TIA (0.2 mg/kg ip) produced similar increases of PRL release in the group either treated with TIA or saline. The data indicate that the benzamides examined have low potency for interaction with DA receptors in pituitary and striatum. In particular, tiapride displayed weaker affinity for AP-DA receptors than the other drugs and induced only slight stimulation of PRL levels. Results from repeated tiapride administration indicate that the drug, at a clinically relevant dose, is unable to modify either kinetic characteristics of DA receptors in the pituitary or plasma PRL levels.     

Enhancing effect of dopamine blockers on evoked acetylcholine release in rat striatal slices: a classical D-2 antagonist response?

The effects of chlorpromazine, pipotiazine, haloperidol, domperidone, sulpiride and SCH 23390 on the potassium-evoked release of [3H]acetylcholine [( 3H]ACh) were studied in rat striatal slices. All 5 dopamine (DA) antagonists with D-2 blockade efficacy induced an increase of [3H]ACh release whereas the specific D-1 antagonist SCH 23390 was devoid of significant effects. The maximal effect (about 100% increase) was obtained with haloperidol, pipotiazine and sulpiride but not with domperidone and chlorpromazine. Interestingly, sulpiride was found to exert an unexpected marked potency. The comparison of the activities of the 6 compounds on evoked ACh release to their affinities for D-2 receptors [( 3H]N-propylnorapomorphine binding sites) indicates that the pharmacological profile of the dopamine receptor implicated in the regulation of ACh release cannot be superimposed on that of the classical D-2 receptor. Participation of DA presynaptic receptors could however explain the differences in efficacy observed with the compounds studied.     

Repeated administration of sulpiride for three weeks produces behavioural and biochemical evidence for cerebral dopamine receptor supersensitivity

Administration of sulpiride (2 × 100 mg/kg i.p.) or haloperidol (5 mg/kg i.p.) to rats for 3 weeks with subsequent withdrawal for 3 or 4 days induced cerebral dopamine receptor supersensitivity. Apomorphine-induced stereotyped behaviour after drug withdrawal was enhanced by pretreatment with either haloperidol or sulpiride both of which increased the number of specific striatal binding sites (B_max) for [³H]spiperone, [³H]N,n-propylnorapomorphine and [³H]sulpiride. Neither drug altered the dissociation constant (K_D) for the ligand binding assays. Striatal dopamine sensitive adenylate cyclase activity was unaltered by such a pretreatment with either haloperidol or sulpiride. The data show that sulpiride, like haloperidol, is capable of inducing behavioural and biochemical supersensitivity of cerebral dopamine receptors.     

3H] (-)sulpiride binding in rat striatum, cortex and anterior pituitary: an improved assay

The interaction of [3H] (-)sulpiride with D2 dopamine (DA) receptors in the striatum, anterior pituitary and medial-prefrontal cortex was studied in rats, using an improved [3H] (-)sulpiride radioreceptor binding technique. Incubation on ice and a fast filtration resulted in higher specific binding (approximately 85% of total binding) a lower affinity constant (about 3 nM) and higher Bmax than reported with previous procedures. Pharmacological interactions confirmed the high selectivity of [3H] (-)sulpiride for D2 DA receptors.     

The actions of (-)N-n-propylnorapomorphine and selective dopamine D1 and D2 receptor agonists to modify the release of [3H]dopamine from the rat nucleus accumbens

The ability of (-)N-n-propylnorapomorphine and selective D1 and D2 dopamine receptor agonists and antagonists to modify the release of [3H]dopamine, induced by potassium from the nucleus accumbens, was studied using an in vitro superfusion technique. (-)N-n-Propylnorapomorphine, in picomolar concentrations, inhibited the release of [3H]dopamine, the inhibition being antagonised by fluphenazine and the selective D2 receptor antagonist sulpiride; the selective D1 receptor antagonist SCH 23390 was ineffective. The selective D1 receptor agonist SKF 38393 and the selective D2 agonist quinpirole, both inhibited the potassium-induced release of [3H]dopamine; no synergistic effect was observed to a combined treatment with SKF 38393 and quinpirole. The effects of SKF 38393 and quinpirole were selectively antagonised by SCH 23390 and sulpiride, respectively, although both antagonists failed to modify the release of [3H]dopamine when administered alone. Receptor antagonists for other transmitter sites, e.g. noradrenaline, 5-hydroxytryptamine and acetylcholine, failed to modify potassium-induced release of [3H]dopamine, when administered alone or to prevent the inhibition of the release caused by (-)N-n-propylnorapomorphine. It is concluded that the action of dopamine agonists on both dopamine D1 and D2 receptors in the nucleus accumbens can reduce the release of [3H]dopamine in the in vitro system. Comparable actions in vivo may contribute to the ability of dopamine agonists to moderate locomotor responding.     

The characterization of [3H]sulpiride binding sites in pig striatal membranes

Pig striatal membranes have [3H]sulpiride-binding sites similar to those identified in rat striatal membranes. The pharmacological profile indicates that this binding is to dopamine receptors. Agonist displacement of [3H]sulpiride binding in pig striatal membranes is subject to guanine nucleotide regulation. This effect is mimicked by heat treatment. N-ethyl maleamide (20 microM) and dithioerythritol (3 mM) decrease agonist affinity for the [3H]sulpiride-binding site in pig striatal membranes without significantly affecting maximal displacement.     

Dopamine receptors in pancreatic acinar cells from dog

Dispersed acini from dog pancreas were used to characterize dopamine receptors in a study on the stimulation of cellular cyclic AMP formation and the binding of [3H]dopamine. Dopamine elicited concentration-dependent stimulation of cellular cyclic AMP with a maximal increase occurring at a concentration of 0.1 mM (EC50 = 1 microM). Epinine produced a potent stimulation similar to that by dopamine; the alpha-adrenoceptor agonists (amidephrine and noradrenaline) were less potent. Isoproterenol and apomorphine were ineffective. The effect of dopamine was potently blocked by dopamine receptor antagonists such as cis-(Z)-flupenthixol, haloperidol, fluphenazine, whereas spiperone, sulpiride and domperidone were weakly active. Apomorphine acted as an antagonist to inhibit dopamine-stimulated cellular cyclic AMP formation as well as phenoxybenzamine. Yohimbine, prazosin or clonidine were poorly or not active. The affinity of agents to stimulate cellular cyclic AMP formation or to inhibit dopamine-stimulated cellular cyclic AMP formation was closely related to their affinity to inhibit the binding of [3H]dopamine to pancreatic acini, providing evidence that dopamine binding sites are receptors that mediate the action of dopamine on cAMP accumulation. This was further substantiated by the demonstration of specific binding sites for [3H]cis-(Z)-flupenthixol.     

Chronic haloperidol does not alter agonist affinity for dopamine receptors in vitro

Agonist competition for [3H]spiperone binding to striatal dopamine D2 receptors was studied in rats rendered supersensitive by chronic treatment with haloperidol. The classical dopamine agonist (-)-N-n-propylnorapomorphine displaced [3H]spiperone biphasically, with IC50 values of 0.5 and 140 nM for the high and low affinity components, respectively. Neither the relative density nor the affinity of either site for (-)-N-propylnorapomorphine was affected by chronic haloperidol treatment. On the other hand, the novel agonist EMD 23 448 displaced [3H]spiperone monophasically. Although this agent only displays potent dopaminergic agonism in supersensitive animals, chronic treatment with haloperidol likewise did not alter the affinity of this drug for [3H]spiperone binding sites. The results suggest that the enhanced in vivo potency of certain agonists in supersensitive animals is probably not mediated by changes in D2 receptor affinity.     

Agonist and antagonist effects of 3-PPP enantiomers on functional dopamine autoreceptors and postsynaptic dopamine receptors in vitro

In contrast to racemic 3-PPP (3-(3-hydroxyphenyl)-N-n-propylpiperidine), (+)-3-PPP appeared to inhibit the electrically evoked release of both [3H]dopamine (DA) and [14C]acetylcholine (ACh) from superfused rat neostriatal slices, although it was considerably less potent in this respect that the DA receptor agonists apomorphine, TL-99 (6,7-dihydroxy-N,N-dimethyl-2-aminotetralin) and LY 141865. At concentrations higher than 1 microM both of the 3-PPP enantiomers increased the spontaneous efflux of 3H but not that of 14C. (+)3-PPP also inhibited the cholera toxin-stimulated release of immunoreactive alpha-MSH from dispersed intermediate lobe cells of the rat pituitary gland. The inhibitory effects of (+)3-PPP on both transmitter and alpha-MSH release were antagonized by the selective D-2 receptor antagonist (-)-sulpiride. Neither [3H]DA nor [14C]ACh release were inhibited by (-)3-PPP but, in contrast, the release-inhibiting effect of the selective D-2 receptor agonist LY 141865 as well as that of (+)3-PPP were antagonized by (-)3-PPP, although less effectively than by (-)sulpiride. The inhibitory effect of LY 141865 on alpha-MSH release from intermediate lobe cells was also antagonized by (-)3-PPP. The data indicate that (+)3-PPP is a weak agonist and (-)3-PPP a weak antagonist at D-2 receptors and that neither of the 3-PPP enantiomers interacts selectively with DA autoreceptors mediating presynaptic modulation of striatal DA release.     

Characterization of dopamine autoreceptor and [3H]spiperone binding sites in vitro with classical and novel dopamine receptor agonists

The specific D2 receptor agonist, LY 141865, but not the specific D1-receptor agonist, SK&F 38393, potently inhibited electrically evoked [3H]dopamine release from slices of the cat caudate. Similarly, LY 141865, but not SK&F 38393, inhibited [3H]spiperone binding to membranes of the cat caudate. The inhibition by dopamine receptor agonists of electrically evoked [3H]dopamine release was antagonized by the specific D2-receptor antagonist S-sulpiride. The inhibition of the electrically evoked release of [3H]dopamine by apomorphine was not, however, antagonized by the specific D1-receptor antagonist, bulbocapnine. Similarly, S-sulpiride but not bulbocapnine potently inhibited [3H]spiperone binding to membranes of the cat caudate. These results suggest that the dopamine autoreceptor modulating the depolarization-evoked release of [3H]dopamine, and the binding site of [3H]spiperone, are valid in vitro models for D2-dopamine receptors. Contrary to some previous reports, DPI was inactive in both in vitro dopamine receptor models. The IC50 values of a series of dopamine receptor agonists correlated very well in the two in vitro dopamine receptor models. One exception to this correlation was bromocriptine, which was more potent at [3H]spiperone binding sites than at the dopamine autoreceptor. With the exception of bromocriptine, all dopamine receptor agonists had one-hundred fold higher potency at the dopamine autoreceptor than at [3H]spiperone binding sites. [3H]Spiperone binding sites are localized primarily postsynaptic to dopamine terminals. Possible differences between the pharmacological properties of pre- and postsynaptic dopamine receptors should become apparent in the comparison of the two in vitro dopamine receptor models. However, the order of potency of dopamine receptor agonists with both in vitro models, dopamine autoreceptor and [3H]spiperone binding, was the same: N-n-propylnorapomorphine greater than TL-99 = 7-HAT greater than M-7 greater than Apomorphine greater than LY 141865.     

In vivo binding of N-n-propylnorapomorphine in the rat brain: regional localization, quantification in striatum and lack of correlation with dopamine metabolism

The accumulation and retention of radioactivity in rat brain were studied after intravenous injection of the dopamine (DA) agonist [3H]N-n-propylnorapomorphine ( [3H]NPA). Dose-dependent saturable accumulation of label was found in the striatum, nucleus accumbens and olfactory tubercle. DA agonists (apomorphine, N,N-dipropyl-5,6-ADTN) and antagonists (haloperidol, cis-flupenthixol) prevented this accumulation. Enhanced accumulation of radioactivity in the striatum was found after 6-OHDA lesions and short- and long-term treatment with reserpine. These results are an indication of specific NPA binding to presumably postsynaptically situated DA receptors. One hour after administration of the drug, the effect of NPA on striatal DA metabolism was not correlated with receptor saturation. Maximal numbers of in vivo NPA binding sites (about 30 and 22 pmol . g-1) in striatal tissue were calculated from independent measurements at 15 and 60 min after NPA injection. Regional distribution of radioactivity after a tracer dose of [3H]NPA was assessed in 35 brain areas and parts of the spinal cord. In addition to the already mentioned DA-rich areas receptor-specific NPA binding was also found in several other brain parts.     

Hypophysectomy does not prevent development of striatal dopamine receptor supersensitivity induced by repeated neuroleptic treatment

Hruska et al. (1980) reported that hypophysectomy prevented the onset of dopamine receptor supersensitivity. We have repeated this investigation administering haloperidol (0.75 mg/day) or sulpiride (2 X 15 mg/day) or saline for 17 days, followed by a 3 day drug washout period, to sham-operated or hypophysectomised rats. Haloperidol or sulpiride pretreatment caused an enhancement of apomorphine-induced stereotyped behaviour and increased the number of specific striatal [3H]spiperone binding sites (Bmax) in both hypophysectomised and sham-operated animals compared to their respective saline controls. We conclude that hypophysectomy does not prevent the onset of striatal dopamine receptor supersensitivity induced by repeated neuroleptic treatment in the rat.     

The thermodynamics of agonist and antagonist binding to dopamine D-2 receptors

The ability of dopamine agonists and antagonists to compete with [3H]spiperone binding to rat striatal membrane preparations at 4, 15, 26, and 37 degrees varied markedly with temperature. Dopamine and the dopamine agonist 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (ADTN) were more potent at lower temperatures. The ability of the dopamine antagonists, haloperidol, cis-flupenthixol, cis-N-(1-benzyl-1-methypyrrolidin-3-yl)-5-chloro-2-methoxy-9- methylaminobenzamide (YM 09151-2), raclopride, and clozapine, and of the agonists apomorphine and pergolide, to compete with [3H]spiperone binding was little altered by temperature. (+)-Butaclamol was more potent at higher temperatures. In contrast, the antagonists sulpiride, metoclopramide, clebopride, sultopride, tiapride, piquindone, and zetidoline were more potent at lower temperatures. The interaction of the agonists dopamine and ADTN was driven by a decrease in enthalpy, allowing an energetically unfavorable decrease in entropy. The binding of the antagonists, haloperidol, cis-flupenthixol, YM 09151-2, raclopride, (+)-butaclamol, and clozapine, and also of the agonists, apomorphine and pergolide, was entropy driven. The interaction of the antagonists sulpiride, metoclopramide, clebopride, alizapride, sultopride, tiapride, piquindone, and zetidoline differed from that of other antagonists in being enthalpy driven. The observed entropy changes correlated with the lipophilicity of the displacing drugs and not with their intrinsic activity.     

Binding of [3H]apomorphine to an aporphine binding site as well as to dopamine sites in tissue from bovine caudate nucleus

Binding of the tritiated dopamine (DA) agonists, apomorphine (APO) and a dihydroxyaminotetralin (ADTN) to a membrane preparation from the caudate nucleus of calf brain was compared. Binding of [3H]dihydroxyaminotetralin at small (nM) concentrations followed simple, monophasic inhibition (over 80% at less than 500 nM) by concentrations of apomorphine between 50 pM and 1 mM. Inhibition of the binding of [3H]apomorphine by dihydroxyaminotetralin was more complex, and included in component with a low (microM) affinity for dihydroxyaminotetralin accounting for approx. 20% of total binding. The kinetics of binding of the ligands to high-affinity sites were virtually identical (apparent Kd = 0.81 nM; Bmax = 211 fmol/mg protein) and could not be distinguished by curve-fitting techniques adapted to analysis by microcomputer. In contrast, the binding of [3H]apomorphine with a "blank" defined by excess (10 microM) dihydroxyaminotetralin could be resolved into the same high-affinity component and a lower-affinity site (Kd = 124 nM; Bmax = 5740 fmol/mg). The pharmacology of the lower-affinity binding of [3H]apomorphine was evaluated by coincubating with 0.5 microM dihydroxyaminotetralin to "mask" high-affinity sites, and was compared to high-affinity binding of [3H]apomorphine and [3H]dihydroxyaminotetralin. The high-affinity binding was stereoselective for DA receptor agonists and antagonists. The pharmacology of the lower-affinity site resembled no known DA receptor type and showed highest affinities for aporphines but was not stereoselective and reacted weakly and nonspecifically with dihydroxyaminotetralin, DA, other catecholamines and neuroleptics. Thus, [3H]apomorphine, under certain conditions, may detect an aporphine binding site of uncertain pharmacological significance, as well as high-affinity DA agonist (D-3) sites.     

5-HT2 antagonists and minaprine block the 5-HT-induced inhibition of dopamine release from rat brain striatal slices

5-Hydroxytryptamine (5-HT) inhibited the K+-induced [3H]dopamine [( 3H]DA) release from slices of rat striatum. Minaprine (3-(2-morpholinoethylamino)-4-methyl-6-phenylpyridazine) attenuated the inhibitory effect of 5-HT in a dose-dependent manner. 5-HT2 receptor antagonists, ketanserin and mianserin, prevented the effect of 5-HT as well as minaprine did. The inhibitory effect of 5-HT was not mimicked by a 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and was not prevented by a 5-HT1A and 5-HT1B mixed receptor antagonist, propranolol. Minaprine was a potent inhibitor of the binding of [3H]ketanserin to binding sites in the striatum over the concentration range 10(-6)-10(-4) M. Lesion of the medial forebrain bundle with 6-hydroxydopamine (6-OHDA) significantly reduced the K+-induced [3H]DA release from the striatum and release was no longer inhibited by 5-HT. Lesioning, however, did not change significantly the [3H]ketanserin binding in the striatum. These results suggest that minaprine suppresses the inhibitory effect of 5-HT on DA release in the striatum via the inhibition of 5-HT binding at the 5-HT2 receptor on the nerve terminal of the DA-ergic neuron and, further, that the proportion of the 5-HT2 receptor site which is located on the nerve terminal of the DA-ergic neuron is small in the striatum.     

Electroconvulsive treatment and haloperidol: Effects on pre- and postsynaptic dopamine receptors in rat brain

Electroconvulsive treatment (ECT) has a transitory benefical effect on patients with Parkinson's disease (PD). The possibility that this effect is mediated by dopamine (DA) receptors was investigated in the rat brain. Repeated ECT or chronic haloperidol treatment induced supersensitivity of putative autoreceptors in the nigrostrital and mesolimbic DA pathways as reflected by enhanced apomorphine-induced inhibition of DA synthesis. Effect of simultaneous administration of ECT plus haloperidol on DA receptor sensitivity were not additive. Chronic haloperidol treatment induced significant elevations in the density of ³[H]-spiperone striatal binding sites. Concurrent administration of ECT had no effect on the neuroleptic-induced supersensitivity. ECT alone was also without effect on ³[H]-spiperone binding. Thus, ECT-induced increases in the sensitivity of presynaptic autoinhibition of DA release was not reflected by changes in the striatal ³[H]-spiperone binding sites. This suggests that effects of ECT on the DA system are not mediated by dopamine D2 receptors.     

Sulpiride isomers exhibit reversed stereospecificity for D-1 and D-2 dopamine receptors in the CNS

We have investigated the stereospecificity of the interaction of (?) and (+)sulpiride with [³H]cis-flupentixol and [³H]spiperone binding to D-1 and D-2 dopamine receptors respectively in rat strialum. Both isomers of sulpiride compete more potently at D-2 vs. D-1 dopamine receptors. (?)Sulpiride is 50-fold more potent than (+)sulpiride in blocking D-2 receptors, while (+)sulpiride is 3-fold more potent than (?)sulpiride at D-1 receptors. This reversed stereospecificity of sulpiride interactions with CNS D-1 and D-2 dopamine receptors is similar to the stereospecificity of sulpiride interactions at DA₁ and DA₂ dopamine receptors in peripheral vascular beds.Biochemical and radioligand binding studies indicate the presence of D-1 and D-2 dopamine receptor subtypes in the central nervous system (CNS) (Creese, Sibley, Hamblin and Leff, 1983). Pharmacological studies suggest that a similar subclassiflcation may be made for dopamine receptors found in peripheral vascular beds based on their postsynaptic (DA₁) or presynaptic (DA₂) localization (for review see Goldberg and Kohli, 1982). Some pharmacological data suggest that similarities may exist between D-1 vs. DA₁ and D-2 vs. DA dopamine receptor subtypes (Goldberg and Kohli, 1982; Creese, 1983; Shepperson, Duval, Massingham and Langer, 1982; Schmidt and Imbs, 1980). Interestingly, all investigations indicate that DA₁ and DA₂ receptors show an opposite stereoselectivity for the (?)(S) and (+)(R) stereoisomers of sulpiride whereby DA₂ receptors are more potently blocked by (?)sulpiride vs. (+)sulpiride and DA₁ receptors are slightly more sensitive to (+)sulpiride vs. (?)sulpiride. The present study reports a similar stereoselectivity for CNS D-2 and D-1 dopamine receptors, respectively, measured by radioligand binding techniques.     

Dopamine D-2 auto- and postsynaptic receptors in the nigrostriatal system of the rat brain: localization by quantitative autoradiography with [3H]sulpiride

In vitro receptor autoradiography with [3H]sulpiride (a selective D-2 antagonist) was used to assess the effect of 6-hydroxydopamine and ibotenic acid lesions of the caudate-putamen and substantia nigra pars compacta on D-2 dopamine receptors in rat brain. A marked reduction in [3H]sulpiride binding within the pars compacta of the substantia nigra resulted from lesions of the substantia nigra compacta with either toxin, while substantial reduction in binding within the caudate-putamen followed only ibotenate lesions of that structure. Since (-)sulpiride is a selective D-2 antagonist, these data confirm that autoreceptors on nigral DA neurons are of the D-2 type, while a portion of D-2 receptors in the caudate-putamen are postsynaptic on striatal neurons.     

Striatal synaptosomal tyrosine hydroxylase activity. A model system for study of presynaptic dopamine receptors

Tyrosine hydroxylase activity determined in striatal synaptosomes by the formation of [3H]H2O from [3,5-3H]tyrosine was used as a system for determining receptor affinities of neuroleptic drugs for the dopamine autoreceptor. Four agonists were tested for their abilities to inhibit tyrosine hydroxylation, and the order of potencies was apomorphine greater than dopamine greater than norepinephrine and buspirone was inactive. The abilities of different neuroleptic drugs to shift the IC50 of apomorphine (500 nM without additional drugs) were used to calculate KB values for each drug. The butyrophenones and (+)butaclamol were the most potent of the drugs tested, while (-)sulpiride and thioridazine were very weak inhibitors of apomorphine. Clozapine, promethazine, (-)butaclamol and (+)sulpiride were all inactive. The order of potencies of antipsychotic drugs at the dopamine nerve-ending autoreceptor correlated closely with clinical dose, but it did not correlate with previous reports of displacement of radiolabeled dopamine or apomorphine striatal membrane binding. This observation combined with the fact that the IC50 of apomorphine at this functional receptor was 100 times the concentration needed to saturate its binding site(s) in radioligand receptor assays suggests that the dopamine autoreceptor is not labeled by nanomolar concentrations of apomorphine or dopamine.     

trans-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine: a highly potent selective dopamine D1 full agonist

trans-10,11-Dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenan thridine (4a, dihydrexidine) has been found to be a highly potent and selective agonist of the dopamine D1 receptor in rat brain. Dihydrexidine had an EC50 of approximately 70 nM in activating dopamine-sensitive rat striatal adenylate cyclase and a maximal stimulation equal to or slightly greater than that produced by dopamine. Dihydrexidine had an IC50 of 12 nM in competing for [3H]SCH23390 (1a) binding sites in rat striatal homogenate, and of 120 nM versus [3H]spiperone. These data demonstrate that dihydroxidine has about ten-fold selectivity for D1/D2 receptors. More importantly, however, is the fact that dihydrexidine is a full agonist. Previously available agents, such as SKF38393 (1b), while being somewhat more selective for the D1 receptor, are only partial agonists. The isomeric cis-dihydroxybenzo[a]-phenanthridine neither stimulated cAMP synthesis nor inhibited the cAMP synthesis induced by dopamine. The cis isomer also lacked appreciable affinity for [3H]-1a binding sites. N-Methylation of the title compound decreased affinity for D1 sites about 7-8-fold and markedly decreased ability to stimulate adenylate cyclase. Addition of an N-n-propyl group reduced affinity for D1 sites by about 50-fold and essentially abolished the ability to stimulate adenylate cyclase. However, this latter derivative had twice the affinity of the D2-selective agonist quinpirole for the D2 receptor. The results are discussed in the context of a conceptual model for the agonist state of the D1 receptor.