Multiple receptors for dopamine

Inhibition of neurotransmitter receptor binding by ergot derivatives

Bromocriptine, lergotrile, lisuride, metergoline, and the Sandoz ergot derivatives 25-397, 29-712, and 29-717 have been tested for their ability to inhibit the synaptic receptor binding of spiroperidol, 5-hydroxytryptamine (5-HT), d-lysergic acid diethylamide (LSD), quinuclidinyl benzilate (QNB), WB.4101, and gamma-aminobutyric acid (GABA). Only GABA binding was not affected, and QNB binding was decreased only by lergotrile and metergoline at high concentrations. The most potent inhibitors of the other ligands were bromocriptine and lisuride for spiroperidol (1-2 nMM), metergoline for 5-HT (29 nM), lisuride for LSD (15 nM), and lergotrile for WB.4101 (17 nM). The direct receptor effects of the ergot derivatives in vitro may contribute to understanding their in vivo effects on behavior and in predicting their therapeutic potential in neurological and neuroendocrine disorders.

3H-Flupenthixol binding in post-mortem brains of schizophrenics: evidence for a selective increase in dopamine D2 receptors

The binding of 3H-cis flupenthixol (3H-FPT) to dopamine receptors in membrane preparations from control subjects and schizophrenics was studied. Using a fixed concentration of 3H-FPT, no differences were observed between controls and all schizophrenics, although 3H-FPT binding was increased in schizophrenics who apparently were drug-free at the time of death. Scatchard analysis of 3H-FPT binding revealed that in drug-treated schizophrenics both the number of binding sites (BM) and the dissociation constant (KD) were increased, whilst in drug-free schizophrenics only the BM was increased. Using domperidone to differentiate 3H-FPT binding to D1 and D2 dopamine sites, it was found that only D2 sites were increased in drug-free schizophrenics. The results are discussed with reference to previous studies on dopamine receptors in schizophrenia, and the effects of neuroleptic treatment. It is suggested that a selective increase in D2 receptors may be associated with the disease process in schizophrenia.

The beta-adrenergic receptor and its mode of coupling to adenylate cyclase

The article first includes a discussion on the classification of catecholamine receptors followed by a discussion on the binding studies of beta-receptors and their affinity labeling. Next a brief discussion on the solubilization and the current attempts to purify the receptor is presented. A large section is then devoted to the mode of coupling between beta-receptors and cyclase where much space is devoted to the role of GTP and of the membrane matrix. The review ends with a discussion on beta-receptor desensitization, supersensitivity, and the "spare receptor" concept.

Effects of acute or chronic administration of low doses of a dopamine agonist on drinking and locomotor activity in the rat

Low doses of piribedil (0.25-5.0 mg/kg) administered acutely produced reliable decrements in locomotor activity in thirsty and non-thirsty animals, the greatest effect occurring at the highest dose. A sequence of ten daily injections of piribedil produced indications of the development of tolerance, at the two highest doses (2.5 and 5.0 mg/kg) in thirsty animals. The smallest doses used, given either acutely or chronically, produced a weak enhancement of drinking behaviour within the first 15 min of a drinking test, as shown by a reduction in latency to drink and an increase in the amount of water consumption. Tolerance did develop with respect to drinking behaviour; animals treated chronically with piribedil displayed higher levels of drinking at several dose levels when compared with acutely treated subjects. The tolerance displayed at the two highest doses could have a close affinity with that shown with regard to locomotor activity.

A clinical and pharmacodynamic evaluation of sulpiride

In a double-blind study the therapeutic efficacy of sulpiride was compared to that of haloperidol--an established neuroleptic agent. A total of 30 female patients with a diagnosis of chronic schizophrenia were initially stabilised on the dosage of haloperidol which produced optimum therapeutic response when given once or twice daily. The patients were then randomly allocated to receive either sulpiride or haloperidol over a period of 12 weeks. Plasma drug concentrations and prolactin levels were determined. Clinical effects were evaluated by Brief Psychiatric Rating Scale (BPRS), Wing Rating Scale, and Extrapyramidal Symptoms Rating Scale (EPS). A standardised side-effects checklist was used. Treatment with sulpiride was associated with a significant rise in plasma prolactin level, but paradoxically these patients had significantly reduced extrapyramidal symptoms. No significant correlation was found between plasma sulpiride concentration and prolactin level or any of the clinical variables. The study confirms the antipsychotic activity of sulpiride.

Neurochemical and behavioral evidence for a selective presynaptic dopamine receptor agonist

A new dopamine analog, 6,7-dihydroxy-2-dimethylaminotetralin (TL-99), was compared to apomorphine in three tests of dopaminergic function in the central nervous system. The tests, performed on rats, included production of changes in locomotor activity (involving both presynaptic and postsynaptic receptors), inhibition of dopa accumulation (quantifying presynaptic receptor activity), and the rotation model (quantifying postsynaptic receptor activation). Apomorphine was efficacious at both presynaptic and postsynaptic receptors, whereas TL-99 was much more efficacious at the presynaptic receptor. This result indicates not only that differences exist between presynaptic and postsynaptic dopamine receptors, but also that these differences may be exploited in the design of selective dopamine agonists.

Catecholamine-sensitive guanylate cyclase from human caudate nucleus

Partial purification of soluble guanylate cyclase on DEAE-Sephacel yields two separate peaks of guanylate cyclase activity. After 10-fold purification of the soluble enzyme, guanylate cyclase is markedly inhibited by micromolar concentrations of dopamine (I50 = 0.2 microM). Dopamine inhibition is observed whether the reaction is conducted with Mn2+ or with Mg2+, under atmosphere or N2(g), and using enzyme from either peak from the DEAE-Sephacel column. Other catecholamines also inhibit partially purified guanylate cyclase with an order of potency at 1 microM of: dopamine = L-DOPA > norepinephrine = isoproterenol = adrenochrome > epinephrine. The structural requirements for inhibition are two free hydroxyl groups on the phenyl ring and an ethylamine side chain. Dopamine also inhibits the Triton X-100-solubilized microsomal guanylate cyclase after partial purification on DEAE-Sephacel. Neither chlorpromazine, propranolol, nor phentolamine at 20 microM effectively block the dopamine inhibition of partially purified soluble guanylate cyclase. Micromolar concentrations of the reducing agents dithiothreitol and glutathione also inhibit partially purified guanylate cyclase, but unlike these agents, catecholamines can inhibit whether added in the reduced or the oxidized forms. Inhibition of enzyme activity by micromolar concentrations of dopamine, adrenochrome, or dithiothreitol is rapidly reversed by dilution and the dopamine inhibition is competitive with MgGTP. Inhibition does not appear to involve covalent binding or to result from the ability of catecholamines to reduce the concentrations of oxygen or free radicals in solution.

Controlled trial of sulpiride in chronic schizophrenic patients

In a double-blind comparative trial of sulpiride (600-1,800 mg/day) and trifluoperazine (15-45 mg/day) in 38 chronic schizophrenic patients, ratings of therapeutic results and unwanted effects were made at two-weekly interviews during the six weeks treatment period. The results show that sulpiride has neuroleptic properties and a spectrum of therapeutic activity similar to that of trifluoperazine. There was an association between plasma sulpiride levels and therapeutic response after four weeks' treatment.

Dopamine receptor blockade by imidoline and its proposed active conformation

Imidoline, 1-[2-(N,N-dimethylamino)ethyl]-3-m-chlorophenyl-2-imidazolidinone, has been found to be as potent as chlorpromazine in increasing striatal DOPA accumulation and prolactin secretion in vivo. In contrast, imidoline exhibited only weak inhibitory activity towards dopamine-sensitive adenylate cyclase and 3H-spiroperidol binding to striatal membranes in vitro. These neuroleptic effects in vivo are probably caused by blockade of dopamine receptors since imidoline did not deplete the striatum of dopamine. Imidoline is of interest because its structure is distinct from those of other neuroleptics. A proposed active conformation involves intramolecular hydrogen bonding between the protonated dimethylamino group and the oxygen of the imidazolidinone ring. The spatial relationship between the amine nitrogen and phenyl ring in this conformation allows proper fit of imidoline with key dimensions described for the dopamine receptor.

3H-apomorphine labels both dopamine postsynaptic receptors and autoreceptors

Evidence points to the existence of multiple classes of dopamine (DA) receptor in mammalian brain which can be distinguished by their pharmacological specificities and localizations, and by the actions they mediate. Among them, dopaminergic autoreceptors are regulatory receptors presumed to be present in the membrane of the DA neurones themselves, and believed to mediate an inhibition of these neurones' activity, either at nerve endings or on cell bodies. However, the pharmacology of autoreceptors remains to be established because attempts to characterize autoreceptors by 3H-ligand binding techniques have produced controversial data. Thus Seeman and co-workers stated that 3H-apomorphine selectively labels autoreceptors, whereas Creese et al. concluded that this ligand selectively labels postsynaptic DA receptors. In addition, large differences in the capacity and drug specificity of 3H-apomorphine receptor sites in rat striatum have been reported. We demonstrate here that 3H-apomorphine labels two classes of DA receptor, distinguishable using domperidone, a selective DA antagonist. Lesion studies indicate that they correspond to a certain class of postsynaptic receptor and to autoreceptors, respectively. Each of these classes displays a clearly distinct pharmacological specificity for antipsychotics and DA agonists.

Antagonism of the renal vasodilator activity of dopamine by metoclopramide

The interaction of metoclopramide with renal dopamine receptors has been characterized in anesthetized dogs surgically prepared with arterial blood pressure catheters and renal artery blood flowprobes. In normal dogs, i.v. dopamine (3 microgram/kg) produced consistent and selective decrements in renal vascular resistance (RVR) and increments in renal blood flow over a 220 min test period; mean arterial blood pressure and cardiac rate were minimally affected. Pretreatment with metoclopramide, 1 and 10 mg/kg i.v., resulted in dose-related inhibition (maximum inhibition 44% and 94%, respectively) of the renal vasodilator activity of dopamine without altering baseline parameters. The duration of antagonism produced by 1 mg/kg of metoclopramide was approximately 30 min, while 10 mg/kg resulted in significant attenuation for the entire test period. Decreases in RVR produced by prostaglandin A1 (0.03 and 0.3 microgram/kg, i.v.) and bradykinin (3 and 15 microgram/kg, i.v.) that were comparable to those of dopamine were unaltered by metoclopramide. Furthermore, metoclopramide did not affect the diastolic blood pressure responses to noradrenaline (0.2-3 microgram/kg, i.v.) or isoproterenol (0.03-0.3 microgram/kg, i.v.), nor did it alter dopamine-induced vasoconstriction of the iliac vasculature. In phenoxybenzamine (3 mg/kg, i.v.) treated dogs, dopamine (0.3-30 microgram/kg, i.v.) produced dose-related reductions in RVR. Administration of metoclopramide (10 mg/kg, i.v.) resulted in a 10-fold parallel displacement, to the right, of the RVR dose-response curve to dopamine. These findings demonstrate that metoclopramide is an effective antagonist of renal dopamine receptors following systemic administration in the dog. The results are not consistent with the classification of metoclopramide as a selective antagonist of D-2 receptors.

Neuroleptic treatment for a substantial proportion of adult life: behavioural sequelae of 9 months haloperidol administration

Rats were treated chronically with haloperidol (1.5 mg/kg per day in drinking water) for up to 9 months. At 1 week, 3.5 months and 9 months after commencing treatment, spontaneous activity in separate groups was depressed by 30-40%. Apomorphine stereotypy was also attenuated at 9 months. Following a 7-10 day withdrawal period after 9 months of treatment, both measures were elevated. Enhancement of spontaneous activity appeared to be at least in part characterised by a deficit in psychomotor habituation to the test apparatus. DA receptor blockade during haloperidol treatment for a substantial proportion of a rat's adult life was found to be an enduring effect, paralleling its antipsychotic action. Developing neurochemical supersensitivity was only manifested behaviourally after withdrawal of neuroleptic. The implications of such findings for the pathophysiology of schizophrenia and tardive dyskinesia and for the functional significance of dopamine receptor heterogeneity are discussed.

Stimulatory action of lisuride on dopamine-sensitive adenylate cyclase in the rat striatal homogenate

Effect of lisuride, an ergot derivative of isolysergic structure, on dopamine-sensitive adenylate cyclase was studied in the homogenate of rat corpus striatum. Stimulatory action of lisuride, similar to the actions of dopamine and apomorphine, on striatal adenylate cyclase was potentiated significantly by guanosine triphosphate (GTP) and by guanyl-5'-yl imidodiphosphate (GMP-PNP), although with lisuride alone, there was only a slight stimulation. The maximal stimulation attained in the presence of GTP corresponded to about 1.4 times the basal rate of cyclic AMP formation in the homogenate and was abolished by an addition of haloperidol. Lisuride at a concentration about 3 microM inhibited stimulation of cyclic AMP formation by dopamine. The effect of lisuride and the extent of potentiation by the guanyl nucleotides were almost comparable to the effects of apomorphine, under corresponding conditions. Thus, lisuride, like apomorphine, acts as a partial agonist-antagonist, and has the ability to stimulate the dopamine-sensitive adenylate cyclase in the rat corpus striatum.

In vivo 3H-d-LSD binding in small punched out rat brain regions

We have developed an in vivo 3H-d-LSD-binding method in the rat brain to measure the 3H-d-LSD concentration in specific brain regions and nuclei. In the cortex, d-LSD binding sites increase from the occipital to the frontal part without a change in their affinity. Compared to the hippocampus, striatum, cortex and raphe nuclei, the substantia nigra contains a d-LSD binding site of lower affinity.

Effects of dobutamine on cyclic AMP accumulation induced by the stimulation of dopamine receptors in rabbit retina in vitro

Intact rabbit retinae were used for testing in vitro the potential activation of dopamine receptors by a new cardioactive sympathetic amine dobutamine. It was found that despite the structure relationship of dobutamine with other dopamine-analog, the pharmacological action of this compound is not comparable to that of apomorphine, N-methyl-dopamine and/or ADTN.

Metergoline and cyproheptadine suppress prolactin release by a non-5-hydroxytryptaminergic, non-dopaminergic mechanism

A dispersed rat anterior pituitary cell system has been used to investigate the effects of cyproheptadine and metergoline on prolactin secretion. Both drugs were potent inhibitors of prolactin secretion. However, the inhibition was not antagonized by either 5-hydroxytryptamine or a variety of dopamine antagonists. We conclude that both drugs act through mechanisms that are neither 5-hydroxytryptaminergic nor dopaminergic.

Convenient synthesis of (S)-(+)-apomorphine from (R)-(-)-apomorphine

A method was devised for preparing (S)-(+)-apomorphine from (R)-(-)-apomorphine. Dehydrogenation of the dimethyl ether of (R)-(-)-apomorphine with 10% palladium-on-carbon followed by reduction with sodium cyanoborohydride under acidic conditions resulted in quantitative racemization to give (R,S)-apomorphine dimethyl ether, which then was resolved with (-)-tartaric acid. Ether cleavage of (S)-(+)-apomorphine dimethyl ether (-)-tartrate with hydriodic acid in acetic anhydride yielded (S)-(+)-apomorphine, which was isolated as the hydrochloride salt in 99% enantiomeric excess.

Topographical model of the renal vascular dopamine receptor

Based on review and examination of recent additions to dopaminergic structure--activity relationships, a topographical model of the renal vascular dopamine receptor is presented. The implications of this model for the design of new dopaminergic drug candidates are discussed briefly.