Effects of dopamine receptor agonists and antagonists at peripheral neuronal and vascular dopamine receptors in the anaesthetised cat

The potencies of several putative dopamine receptor agonists and antagonists have been determined and compared at neuronal and vascular dopamine receptors in anaesthetised cats. N,N-diethyldopamine and the N,N-di-n-propyl derivatives of 5,6-ADTN (6-amino-5,6,7,8-tetrahydro-1,2-napthalenediol) and 6,7-ADTN (6-amino-5,6,7,8-tetrahydro-2,3-napthalenediol) were, relatively, much more potent agonists than dopamine at neuronal dopamine receptors on the terminals of sympathetic nerves supplying the heart and nictitating membrane than at the vascular dopamine receptors in the mesenteric vascular bed. SK&F 82526 [6-chloro-2,3,4,5-tetrahydro-1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-di ol] showed the reverse profile. Haloperidol, (+/-) sulpiride, cis alpha-flupenthixol, and fluphenazine were much more potent antagonists at neuronal dopamine receptors than at vascular dopamine receptors. The profiles of activity of the agonists and antagonists show that neuronal and vascular dopamine receptors in the cat are different; in addition, the neuronal dopamine receptors in the cat heart and nictitating membrane closely resemble those in the dog femoral vasculature, whereas the vascular dopamine receptors in the cat mesenteric vascular bed are very similar to those in the dog mesenteric circulation.     

Alterations in peripheral and central dopamine receptor sensitivity after subchronic treatment with fluphenazine and sulpiride

The effects of subchronic treatment with the antipsychotic drugs fluphenazine and sulpiride on the sensitivity of peripheral neuronal dopamine receptors and central dopamine autoreceptors were evaluated. The ability of apomorphine, a dopamine agonist, to inhibit electrically induced sympathetic vasoconstriction in pithed rats, and apomorphine-induced inhibition of spontaneous locomotor activity in awake rats were used as indices of peripheral and central dopamine receptor sensitivity, respectively. A single injection of fluphenazine decanoate, a long-acting preparation of fluphenazine, enhanced the central locomotor inhibitory effect of low doses of apomorphine 4 and 6 weeks after drug administration, whereas the antidopaminergic effect on peripheral dopamine receptors was prolonged and persisted at least up to 6 weeks. In another set of experiments rats were treated with fluphenazine hydrochloride and sulpiride for 10 days and subsequently challenged with apomorphine after various withdrawal times. Both antipsychotic drugs augmented the inhibitory effect of apomorphine in the periphery, although the time courses of the potentiation were different. Both treatments also enhanced the locomotor inhibitory effect of apomorphine. These results are in line with our previous finding that long-term treatment with dopamine antagonists can induce neuronal dopamine receptor up-regulation also outside the central nervous system. Peripheral neuronal dopamine receptors thus show similar adaptive responses to long-term blockade as central dopamine autoreceptors, and may serve as a useful experimental model in studies concerned with mechanisms of dopaminergic autoregulation in the central nervous system.     

The yawning-penile erection syndrome as a model for putative dopamine autoreceptor activity

The efficacy of several drugs to elicit yawning and penile erections were determined in rats. The dopamine agonists, N-propylnorapomorphine, apomorphine, pergolide, (+/-)-3-PPP, TL-99 and N,N-dipropylamino-5,6-dihydroxy-1,2,3,4-tetrahydronaphthalene (N,N-dipropyl A-5,6-DTN) all elicited yawning accompanied by an increase in spontaneous penile erections. The potencies of these drugs in causing yawning closely resemble published data concerning their actions in biochemical tests reputedly indicative of autoreceptor activity. In contrast, SK&F 38393, A-5,6-DTN and clonidine produced no yawning and few or no penile erections. Although physostigmine also caused yawning, the effect was not accompanied by penile erections. Studies with the optical isomers of 3-PPP showed that (+)-3-PPP was considerably more potent than (-)-3-PPP. Haloperidol antagonised dopamine agonist-induced yawning and penile erections. Apomorphine-induced yawning and penile erections were also antagonised by sulpiride and atropine but not by domperidone. The suitability of elicitation of the combined syndrome of yawning plus penile erections as useful behavioural model for dopamine autoreceptor agonists is discussed.     

Prejunctional dopamine receptors modulate twitch responses to parasympathetic nerve stimulation in the rabbit isolated rectococcygeus muscle.

Field stimulation of the parasympathetic nerves supplying the rabbit isolated rectococcygeus muscle produced individual twitch responses that were inhibited by dopamine (1 x 10(-8)-1 x 10(-5)M). The twitch-inhibitory effect of dopamine was reversed by haloperidol or sulpiride (1 x 10(-8)-1 x 10(-5)M for either) but not by phentolamine or yohimbine (up to 1 x 10(-4)M). Haloperidol and sulpiride were potent, specific, competitive antagonists of the twitch-inhibitory effect of dopamine; their pA2 values were 8.39 and 7.75, respectively. In contrast, cis alpha-flupenthixol, fluphenazine, bulbocapnine and thioridazine were weak or inactive against dopamine. Concentrations of dopamine that inhibited the twitch response to parasympathetic nerve stimulation had little or no effect on contractions elicited by carbachol or by direct muscle stimulation after abolition of neuronal conduction by tetrodotoxin. Thus, the effects of dopamine on responses elicited by parasympathetic nerve stimulation seem to be exerted at a prejunctional level rather than directly on the smooth muscle. The twitch-inhibitory effect of dopamine was mimicked by epinine, N,N-diethyldopamine, N,N-di-n-propyldopamine, 5,6-ADTN, N,N-di-n-propyl 5,6-ADTN, 6,7-ADTN, apomorphine and Sandoz 27-403. Sulpiride reversed the effects of all these agonists. N,N-di-n-propyl-6,7-ADTN and SK & F 82526 also inhibited the twitch response but their effects were not reversed by sulpiride. SK & F 38393 and DPI had little effect on the twitch response. The pharmacological characteristics of the presynaptic dopamine receptors in the rabbit rectococcygeus muscle show that they resemble those in the cat heart and rabbit ear artery in some respects but there are differences that suggest that the presynaptic dopamine receptors in the rabbit rectococcygeus muscle constitute a specific subgroup of receptors.     

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.     

Comparison of the cardiovascular actions of N,N-di-n-propyl dopamine and sodium nitroprusside in conscious and chloralose-anaesthetised dogs

The cardiovascular actions of N,N-Di-n-propyldopamine (DPDA), a dopamine receptor agonist, and the directly acting vasodilator sodium nitroprusside have been compared in both conscious and chloralose-anaesthetised dogs. Bolus intravenous doses of DPDA (6.5-32 micrograms/kg) and sodium nitroprusside (3-10 micrograms/kg) induced dose-related falls in blood pressure associated with reflex tachycardia in both preparations. The dopamine receptor antagonist sulpiride (0.5 mg/kg) eliminated these effects of DPDA and its ability to reduce renal and femoral vascular resistances in the chloralose-anaesthetised dog, but did not influence the actions of sodium nitroprusside. Ganglion blockade (hexamethonium, 5-10 mg/kg alone or in a combination with atropine, 1 mg/kg) abolished the reflex tachycardia to both compounds and potentiated the depressor effects of sodium nitroprusside but either reduced or did not alter the hypotension produced by DPDA. The renal dilator response to DPDA was also reduced, while femoral vasodilation was abolished. This study illustrates that DPDA can reduce blood pressure in conscious and chloralose-anaesthetised dogs but that this effect is accompanied by reflex tachycardia. Inhibitory dopamine receptors on sympathetic nerves, and to a lesser extent postjunctional (vascular) dopamine receptors, are involved in these actions of DPDA.     

The action of a dopamine (DA1) receptor agonist, fenoldopam in human vasculature in vivo and in vitro.

This study was designed to investigate dopaminergic mechanisms in human vasculature using the selective vascular dopamine receptor agonist fenoldopam in vivo and in vitro. In vivo, forearm blood flow was measured plethysmographically and in vitro isolated rings of human blood vessels from a variety of sites were used for tissue bath studies. Intra-arterial fenoldopam markedly increased forearm blood flow, this effect was antagonised by (R) sulpiride, a vascular dopamine (DA1) antagonist, but not by metoclopramide, a neuronal (DA2) antagonist, or by guanethidine, an adrenergic neurone blocking agent. In vitro, fenoldopam relaxed preconstricted human renal, mesenteric and lumbar arteries, but not saphenous vein in a concentration dependent manner. (RS) sulpiride and SCH 23390 competitively antagonised this effect. These studies demonstrate the presence of a vasodilatory vascular dopamine receptor in man both in vivo and in vitro.     

Conformational analysis of dopamine by the INDO molecular orbital method.

The results of INDO calculations on dopamine are reported. A conformational energy map and an isodistance map for the key distances N-OH1, N-OH2 in dopamine as functions of the two main torsion angles tau1 and tau2 were constructed. In addition to the three known minima of dopamine corresponding to the trans and gauche forms, two new minima were found. The key distances of the rigid analogues of dopamine, apomorphine, isoapomorphine, 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene and isoquinoline were plotted on the isodistance map of dopamine. By taking the corresponding tau values as coordinates on the energy map, conformations of dopamine, resembling the rigid analogues, could be found. When a conformation is close to a local minimum it is assumed that this conformation is energetically favourable. The possible relation between the energy minima and the biological action of dopamine is discussed. An explanation is suggested for the lack of dopaminergic activity of isoapomorphine.     

Identification of adrenoceptors and dopamine receptors mediating vascular responses in the superior mesenteric arterial bed of the rat

The nature of the adrenoceptors and dopamine receptors mediating vascular responses in the in-situ blood perfused superior mesenteric arterial bed of the rat have been studied. alpha 1-Adrenoceptor agonists produced vasoconstriction but alpha 2-agonists had no significant effect on vascular resistance. The vasoconstrictor effects of noradrenaline were antagonized by low doses of prazosin (26 nmol kg-1 i.v.). Isoprenaline and salbutamol produced vasodilation when the vasculature was preconstricted with arginine vasopressin. The responses to isoprenaline were potently antagonized by propranolol (1.69 mumol kg-1 i.v.) and weakly but significantly reduced by practolol (3.75 mumol kg-1 i.v.) whereas the responses to salbutamol were unaffected by the same dose of practolol. After preconstriction of the vasculature and alpha-adrenoceptor blockade, dopamine and apomorphine produced dilator responses with both compounds producing the same maximal response and apomorphine being 1.8 times more potent than dopamine. The dopamine responses were present after the animals had been pithed and were resistant to spiperone (506 nmol kg-1 i.v.) but were antagonized by cis-alpha-flupenthixol (460 nmol kg-1 i.v.). These results suggest that this vascular bed possesses vasoconstrictor alpha 1- but not alpha 2-adrenoceptors, vasodilator beta 1- and beta 2-adrenoceptors and vasodilator dopamine receptors which appear similar to the D1-type found centrally.     

Effects of the enantiomers of 3-PPP on DA1 and DA2 dopamine receptors in the dog

The (+)- and (−)-enantiomers of 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) were studied for their effects on DA₁ and DA₂ dopamine receptors in pentobarbital anesthetized dogs. 3-PPP enantiomers were administered into the renal artery after phenoxybenzamine pretreatment to determine possible DA₁ activity; dopamine was also injected for comparison. DA₂ activity was determined by injection of the enantiomers into the femoral vascular bed with intact nerve supply and without phenoxybenzamine; dipropyl dopamine (DPDA) or apomorphine were used as standard DA₂ agonists. Antagonists activity of the enantiomers on DA₁ or DA₂ receptors was determined by simultaneous administration of the enantiomer with DA in the renal vascular bed and with DPDA or apomorphine in the femoral vascular bed. Neither enantiomer was active as a DA₁ agonist, but both exhibited antagonist activity. Both enantiomers were found to be agonists of the DA₂ receptor; in addition, both showed DA₂ antagonist activity. In all actions the (−)-enantiomer was approximately 4 times more potent than the (+)-enantiomer.     

Characterization of vascular dopamine receptors in the gastric circulation of the rabbit

An in vivo model is presented for studying the vasodilator effect of dopamine on the gastric vascular bed of the anesthetized rabbit. Dopamine was injected into the common hepatic artery simultaneously measuring the blood flow through the left gastric artery at constant perfusion pressure. The vasoconstrictor response to dopamine was blocked by pretreatment with phenoxybenzamine (0.5 mg/kg i.a.). Dopamine dilated the gastric vascular bed dose-dependently with an average ED50 of 15 nmol. Apomorphine caused vasodilation with an ED50 of 38.8 nmol; the maximum response was significantly lower than dopamine. (-)Isoproterenol was ineffective. cis-Flupentixol (3.5 and 7.0 mumol/kg i.v.) and (+)butaclamol (3.0 mumol/kg i.v.), but not trans-flupentixol (7.0 mumol/kg i.v.) and (-)butaclamol (6.0 mumol/kg i.v.) shifted the dopamine dose-response curve to the right in a parallel fashion, indicating competitive antagonism. Haloperidol, bulbocapnine, and sulpiride also acted as competitive inhibitors of dopamine-induced vasodilation. The antagonists inhibited the dopamine-induced vasodilation in the following order of potency: (+)butaclamol greater than cis-flupentixol greater than haloperidol greater than bulbocapnine greater than sulpiride. The results suggest that the vascular dopamine receptors in the stomach of the rabbit resemble the DA1-receptors in the mesenteric vascular bed of dogs.     

Dopamine receptors in human basilar arteries

After phenoxybenzamine (10(-5) M), pretreatment, and in the presence of propranolol (10(-6) M) and indomethacin (2.8 X 10(-6) M), dopamine caused a marked concentration-dependent relaxation of isolated strips of human basilar artery contracted with PGF2 alpha. This effect was mimicked by apomorphine, 6,7-ADTN and SK&F 38393, but N,N-diethyl dopamine, N,N-di-n-propyl-dopamine and 5,6-ADTN caused only slight relaxation. (+)-Butaclamol, cis-alpha-flupenthixol, fluphenazine and haloperidol competitively antagonised the relaxant effects of dopamine, but sulpiride was ineffective in concentrations as high as 1.3 X 10(-4) M. These findings show that the dopamine receptors in the human basilar artery closely resemble those in the smooth muscle of the rabbit isolated mesenteric and splenic arteries, and the dog renal and mesenteric arteries in vivo, but differ from those located presynaptically on sympathetic nerve terminals.     

Neurotransmission in pig renal artery: the actions of angiotensin II and dopamine.

1. Electrical stimulation of pig renal arteries causes contractions which are potentiated in the presence of angiotensin II (5 x 10(-8) M). 2. The potentiation is followed by a long-lasting inhibitory phase which is mimicked by dopamine and 2-amino-6,7,dihydroxy-1,2,3,4,-tetrahydronaphthalene (ADTN), but blocked by sulpiride (5 x 10(-8) M). 3. Release of noradrenaline from renal artery rings by 25 mM K+ was measured by h.p.l.c. and found to be increased by 5 x 10(-8) M angiotensin II. As this was done in the presence of 1 x 10(-6) M desmethylimipramine it was concluded that the increase was due to increase neuronal release of noradrenaline, not inhibition of neuronal uptake. 4. The experiments indicate that dopamine is present in the artery wall and produces its effects through DA2-receptors.     

PHARMACOLOGICAL CHARACTERIZATION OF NEURONAL DOPAMINE RECEPTORS IN THE RAT HINDQUARTERS,RENAL AND SUPERIOR MESENTERIC VASCULAR BEDS

• 1 The present study was designed to characterize the neuronal dopamine receptors involved in the inhibitory effect of apomorphine on neurogenic vasoconstriction in the isolated autoperfused hindquarters, renal and superior mesenteric vascular beds of the rat.
• 2 In the three vascular beds, the inhibitory effect of apomorphine on neurogenic vasoconstriction was antagonized by the selective DA₂-receptor antagonist domperidone, but not by the selective DA₁-receptor antagonist SCH 23390. Local administration of these antagonists had no effect on the perfusion pressure per se or on the pressure response to electrical stimulation of the sympathetic innervation.
• 3 These results show that the neuronal dopamine receptors present in the rat hindquarters, renal and superior mesenteric vascular beds can be classified as DA₂-receptors.

     

Stereospecific antagonism by d-butaclamol of dopamine-induced relaxation of the isolated rabbit mesenteric artery

We characterized the properties of vascular dopamine receptors on isolated rabbit mesenteric arteries preincubated with phenoxybenzamine (10(-5) M) and contracted with prostaglandin F2 alpha (PGF2 alpha). The dose-response curve for dopamine-induced relaxation was shifted to the right by the dopamine receptor antagonist d-butaclamol (10(-7)--3 X 10(-6) M) in a concentration-dependent manner. The pA2 value for d-butaclamol was calculated as 6.77. In contrast, even a very high concentration (3 X 10(-6) M) of l-butaclamol had no effect, indicating that vascular dopamine receptors require stereospecificity of antagonists. In the same preparation the mechanism of relaxation by 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (A-6,7-DTN; 3 X 10(-7)--10(-4) M) and bromocriptine (10(-6)--3 X 10(-4) M) was found to be dopaminomimetic, since only the dopamine receptor antagonists droperidol (10(-5) M) and metoclopramide (5 X 10(-5) M) could inhibit relaxations, whereas the beta-adrenoceptor antagonists pindolol (10(-7) M) and propranolol (10(-6) M) were without effect. It is concluded that receptors specific for dopamine exist on the rabbit mesenteric artery, which may tentatively be classified as belonging to the D1-type.     

Dopamine receptors in rat striatum and nucleus accumbens; conformational studies using rigid analogues of dopamine

A study was made of the actions of dopamine and of some 2-amino-1,2,3,4-tetrahydronaphthalenes on dopamine-sensitive adenylate cyclase in homogenates of rat striatum and nucleus accumbens. The compounds were also tested for their ability to stimulate motor activity following bilateral injection into the nucleus accumbens of conscious rats. The most active compounds on adenylate cyclase from both striatum and nucleus accumbens were dopamine and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene(6,7-diOHATN). The 5,6-dihydroxy analogue (5,6-diOHATN) was 50 times less active than 6,7-diOHATN in striatal homogenates and 350 times less active in homogenates of nucleus accumbens. All dihydroxy compounds tested were active in causing stimulation of motor activity, the most active compounds being 6,7- and 5,6-diOHATN. Both dimethoxy derivatives tested were inactive on the adenylate cyclase and as locomotor stimulants.     

The rabbit isolated arterially perfused intestinal segment preparation: A model for vascular dopamine receptors

A simple and inexpensive method for the evaluation in vitro of the effects of compounds on postjunctional vascular dopamine (DA) receptors is described. The rabbit isolated perfused K⁺ -contracted mesenteric and ileal vasculature dilates in response to DA, DA receptor agonists, and papaverine. Responses to DA receptor agonists, but not those to papaverine, can be selectively antagonised by cis-(Z)-flupenthixol (pA₂ 8.5), yet remain unaffected by sulpiride. Comparisons with other models of vascular DA receptors have been made.     

Conformational analysis of dopamine by the INDO molecular orbital method

The results of INDO calculations on dopamine are reported. A conformational energy map and an isodistance map for the key distances N—OH₁, N—OH₂ in dopamine as functions of the two main torsion angles τ₁ and τ₂ were constructed. In addition to the three known minima of dopamine corresponding to the trans and gauche forms, two new minima were found. The key distances of the rigid analogues of dopamine, apomorphine, isoapomorphine, 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene and isoquinoline were plotted on the isodistance map of dopamine. By taking the corresponding τ values as coordinates on the energy map, conformations of dopamine, resembling the rigid analogues, could be found. When a conformation is close to a local minimum it is assumed that this conformation is energetically favourable. The possible relation between the energy minima and the biological action of dopamine is discussed. An explanation is suggested for the lack of dopaminergic activity of isoapomorphine.     

Dopamine receptors in rat striatum and nucleus accumbens; conformational studies using rigid analogues of dopamine.

A study was made of the actions of dopamine and of some 2-amino-1,2,3,4-tetrahydronaphthalenes on dopamine-sensitive adenylate cyclase in homogenates of rat striatum and nucleus accumbens. The compounds were also tested for their ability to stimulate motor activity following bilateral injection into the nucleus accumbens of conscious rats. The most active compounds on adenylate cyclase from both striatum and nucleus accumbens were dopamine and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene(6,7-diOHATN). The 5,6-dihydroxy analogue (5,6-diOHATN) was 50 times less active than 6,7-diOHATN in striatal homogenates and 350 times less active in homogenates of nucleus accumbens. All dihydroxy compounds tested were active in causing stimulation of motor activity, the most active compounds being 6,7-and5,6diOHATN. Both dimethoxy derivatives tested were inactive on the adenylate cyclase and as locomotor stimulants.     

Studies on the behavioural pharmacology of a cyclic analogue of dopamine following its injection into the brains of conscious rats.

1. The cyclic analogue of dopamine, 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (ADTN) was injected into the lateral ventricle or bilaterally into the nucleus accumbens or caudate nucleus of conscious rats and its effect on locomotor activity was investigated. 2. When given intraventricularly, ADTN produced some stereotyped responses which were followed by a strong and long lasting stimulation of locomotor activity. When administered bilaterally into the nucleus accumbens a similar stimulation of locomotor activity was observed. ADTN had no effect on locomotor activity when injected bilaterally into the caudate nucleus. 3. The ADTN-induced locomotor stimulation following its intraventricular injection was completely abolished by a low dose of pimozide (0.01 mg/kg, i.p.) or haloperidol (0.5 mg/kg, i.p.). Pimozide (1 mg/kg, i.p.) given 30 min before ADTN injected bilaterally into the nucleus accumbens completely blocked locomotor stimulation. 4. Unilateral injections of ADTN (5 mug) into the nucleus accumbens caused locomotor stimulation but no turning. 5. Bilateral injections into the nucleus accumbens of 2-amino-6,7-dimethoxy-1,2,3,4-tetrahydronaphthalene or 0.9 percent w/v NaCl solution had no effect on locomotor activity. 6. It is concluded that the central stimulant action of ADTN is due to an effect on the dopamine receptors in the nucleus accumbens.