The hypotheses of different dopamine receptor mechanisms

Receptors: perspectives in pathology and clinical medicine

Life implies organization and thus regulation. This requires intra- and intercellular communication and communication with the environment. The exchange of information is to a large extent based on messenger molecules with the capacity to interact selectively with and to activate particular molecular receivers, the receptors. Receptor dysfunction may be a factor in the aetiology of various diseases. Receptor research has evolved to a separate discipline "receptorology". This should not hamper the development of applied or clinical receptorology. Various aspects thereof are discussed such as: inborn errors of receptor function, receptor-autoimmune diseases, receptors and cancer, iatrogenic receptor imbalance, receptor histochemistry, receptors and targeting of drugs, and radio-receptor assay. Attention is paid to receptor differentiation and particularly to receptor denomination. Here for practical reasons a self-evident, rational, and simple system is required. Receptorology may, even more than enzymology, contribute to a reintegration of various physiological disciplines, including biochemistry, endocrinology, immunology, neurology and particularly also pathology, leading to molecular physiology. The adaptation of methods and techniques common in receptor research to the needs of clinical sciences is an important step in that direction.

The purported dopamine agonist DPI inhibits [3H]noradrenaline release from rat cortical slices but not [3H]dopamine and [14C]acetylcholine release from rat striatal slices in-vitro

The effects of the purported dopamine (DA) receptor agonist (3,4-dihydroxyphenylimino)-2-imidazolidine (DPI) upon the in-vitro K+-induced release of [3H]DA and [14C]acetylcholine from rat neostriatal slices, and of [3H]noradrenaline from rat neocortical slices have been investigated and compared with those of the DA receptor agonist TL-99 and the alpha-adrenoceptor agonist clonidine, respectively. The rapid decomposition of the catechol compounds DPI and TL-99 in the Krebs-Ringer bicarbonate superfusion medium was shown to be inhibited by both the chelating agent EDTA and the reducing agent ascorbic acid. The results suggest that in-vitro DPI is unable to stimulate striatal DA receptors, whereas it is effective in stimulating cortical alpha 2-adrenoceptors (EC50 = 61 nM). It is concluded that DPI should be considered as a mixed alpha 1/alpha 2-adrenoceptor agonist and that the designation of DPI as a DA receptor agonist should be abandoned.

Studies of dopamine pharmacology in molluscs

Dopamine has been established as a putative neurotransmitter in several species of molluscs. Biochemical and neurophysiological studies of the cellular pharmacology of dopamine have revealed several properties of molluscan dopamine receptors. The biochemical synthesis and degredation of dopamine in molluscs follows the same pathways that have been described in mammals. Adenylate cyclase is present, and the receptor mediating CAMP production is blocked by neuroleptics and certain ergot alkaloids. Studies of this enzyme and of radioligand binding indicate that molluscan dopamine receptors and serotonin receptors share certain characteristics. Neurophysiological studies have shown that dopamine induces several forms of ionic conductance changes in molluscan neurons. The receptors mediating these conductance changes may be differentiated pharmacologically. Neuroleptics are antagonists at certain receptors and ergot alkaloids have been shown to be either partial agonists or antagonists. Present evidence indicates that molluscan and mammalian CNS dopamine receptors have some similarities. However, further biochemical and neurophysiological investigations will be necessary to fully characterize molluscan dopamine receptors.

A comparison of the potencies of various dopamine receptor agonists in models for pre- and postsynaptic receptor activity

Several dopamine (DA) receptor agonists, notably N,N-dipropyl-2-aminotetralin analogues differing in the number and position of phenolic hydroxyl groups, were evaluated in model systems for pre- and postsynaptic dopaminergic activity. Apomorphine, piribedil and pergolide were included for comparison. All compounds inhibited the gamma-butyrolactone (GBL)-induced increase in DA concentrations in the rat striatum and olfactory tubercle, although a dose-dependency could not be demonstrated for one of the compounds, i.e. N,N-dipropyl-2-amino-5,6-dihydroxy-tetralin. In addition to the reversal of the DA-increase all compounds decreased the HVA and DOPAC levels in a dose-dependent manner, in much the same way as in normal, non GBL-pretreated rats. The potencies of the drugs to decrease HVA in normal rats and to inhibit the DA-increase and to decrease HVA in GBL-pretreated rats, both in the striatum and the olfactory tubercle were compared with each other and with the potencies to induce stereotyped behaviour. It may be concluded that (1) N,N-dipropyl-2-amino-7-hydroxytetralin shows the largest difference in activity in the biochemical and the behavioural models, suggesting a selective presynaptic activity. This was corroborated by the appearance of a marked hypomotility after low doses of this compound; (2) The potencies to decrease striatal HVA concentrations are generally somewhat different from the potencies to inhibit GBL-induced DA-increases, but appear to be comparable to the potencies to inhibit GBL-induced dihydroxyphenylalanine (DOPA)-increases; (3) There is no indication that the DA agonists in general are more potent at presynaptic receptors in the tubercle than in the striatum.

Aldosterone suppression with dopamine infusion in low-renin hypertension

A dopaminergic mechanism has been proposed to suppress aldosterone secretion. To assess the possibility that a defect in the dopaminergic mechanism might enhance aldosterone secretion in hypertensive patients, we determined basal and adrenocorticotropic hormone (ACTH)-stimulated plasma aldosterone (PA), cortisol, renin activity, and potassium concentrations before and during dopamine receptor stimulation with dopamine infusion and bromocriptine administration and dopamine receptor blockade with metoclopramide. The patient study groups included: (a) seven patients with low-renin hypertension and abnormal aldosterone suppression with sodium loading and presumed bilateral zona glomerulosa hyperplasia (ZGHP); (b) two patients with aldosterone-producing adenoma; (c) five patients with low-renin hypertension but normal aldosterone suppression with sodium loading; and (d) six patients with normal-renin hypertension. Dopamine infusion in patients with ZGHP caused PA to fall (P less than 0.01) into the normal range, but did not block the enhanced (P less than 0.05) aldosterone response to ACTH that is characteristic of these patients. Dopamine infusion in patients with low-renin hypertension but normal aldosterone suppression also suppressed PA (P less than 0.01), whereas it had no effect upon PA in patients with normal-renin hypertension or aldosterone-producing adenoma and did not blunt the PA response to ACTH in either group. Bromocriptine administration had no effect upon basal or ACTH-stimulated PA. Dopamine infusion in patients with ZGHP also enhanced (P less than 0.05) diuresis and natriuresis in comparison with normal-renin patients. Metoclopramide administration increased (P less than 0.01) PA in all patients. Thus, a dopaminergic mechanism appears to be important in the regulation of aldosterone secretion in patients with ZGHP and in other low-renin hypertensives with normal aldosterone suppression with sodium loading. In contrast, this latter group does not exhibit an enhanced aldosterone response to ACTH. Both of these groups differ from normal-renin hypertensives, who have no PA suppression with dopamine infusion.

Frequency-dependent release of acetylcholine and dopamine from rabbit striatum: its modulation by dopaminergic receptors

The release of [3H]dopamine (DA) and [14C]acetylcholine (ACh) was monitored from single slices of the rabbit striatum. In all cases, the evoked overflow of ACh showed a higher peak and was of shorter duration than that of 3H products. For ACh, the release per pulse showed a marked decline with increasing frequency of stimulation, whereas flat frequency-release curves were obtained for DA. At 0.1 and 1 Hz the evoked overflows of ACh were 15 and 7 times greater, respectively, than those of DA. Haloperidol (0.03 microM) and sulpiride (1 microM) produced large increases in the evoked overflow of DA and ACh at 3 and 10 Hz; little effect was observed at lower frequencies. These results indicate that the frequency-release curves for DA and ACh are different and that at high frequencies the slope of the curves is modified by activation of pre- and postsynaptic DA receptors. Apomorphine inhibited in a concentration-dependent fashion the evoked overflow of DA and ACh; greater inhibition was obtained at lower frequencies of stimulation. At 0.3 Hz the DA agonist was two times more potent in inhibiting DA than ACh overflow (IC50: 12.0 +/- 2.2 versus 22.0 +/- 2.8 nM; p less than 0.01). The greater sensitivity of pre- than postsynaptic sites to apomorphine was also seen at higher frequencies (3 Hz). Benztropine (1 microM) reduced the evoked overflow of ACh at 10 Hz, and enhanced that of 3H products at all rates of stimulation (0.3-10 Hz). These results suggest that the release of DA and ACh is regulated by dopaminergic receptors. They also indicate that the effects of DA agonists and antagonists and of uptake inhibitors on DA and ACh release are highly dependent on the frequency of stimulation used.

Multiple dopamine receptors and behavior

The therapeutic effects of dopamine (DA) agonists and DA antagonists used in the treatment of schizophrenia (antipsychotics, DA antagonists), Huntington's chorea (DA antagonists) and Parkinson's disease (antiparkinsonian agents, DA agonists) have been thought to result largely from actions on DA receptors located in the striatum (caudate nucleus and putamen). Many of the classical drugs used to treat these disorders are known to have a high incidence of extrapyramidal side effects (EPS). However, a number of drugs, the atypical antipsychotics and antiparkinsonian agents, have been developed which have a low incidence of EPS. It has been of enormous interest to researchers and clinicians alike to determine what characteristics of the atypical antipsychotics and antiparkinsonian agents are responsible for their unique behavioral profile. Because all of the antipsychotics and antiparkinsonian agents act on DA receptors, much attention has focused on potential differences in the interactions of the atypical agents with DA receptors. An hypothesis that has been raised, due to the knowledge that there are multiple subtypes of DA receptors located in the striatum, is that the atypical agents could have their therapeutic actions as a result of an interaction with one specific subtype of DA receptor. This review emphasizes two major points: (1) it is unlikely that the atypical antipsychotics and antiparkinsonian agents interact with only one subtype of DA receptor, or have their therapeutic actions only through that receptor; (2) other pharmacological characteristics of these agents are more critically involved in their unique behavioral effects. The applicability of animal models to assess the pharmacological and behavioral profiles of these agents is discussed, and the relevance to the clinical profiles of these agents is emphasized.

Intestinal vascular responses to dopamine during fentanyl-nitrous oxide anaesthesia, supplemented with dixyrazin

Intestinal haemodynamics in response to continuous i.v. administration of dopamine were investigated in cats anaesthetized with fentanyl-nitrous oxide either with or without supplement of dixyrazin. A dose-dependent vasodilatation was observed in the dopamine dose range 2.5-35 micrograms . kg-1 . min-1 and the subsequent maximal intestinal blood flow increase was 121%. No net intestinal vasoconstriction was evident even at the largest dopamine doses, although the vascular response reached a plateau at 17.5 micrograms . kg-1 . min-1. Control experiments during chloralose anaesthesia gave similar results. Changes in mean arterial pressure and heart rate were small. Renal blood flow was virtually unchanged at dopamine doses below 10 micrograms . kg-1 . min-1, while renal vasoconstriction was evident following dopamine doses above that level. The addition of i.v. dixyrazin (0.15-0.30 mg . kg-1) to the fentanyl-nitrous oxide anaesthesia substantially potentiated the intestinal vasodilator response to i.v. dopamine and the maximal blood flow increase was 183% at 10-15 micrograms . kg-1 . min-1. In vitro experiments using mesenteric resistance vessels from the rat demonstrated a dose-dependent relaxation to dopamine. At very large doses this response was counteracted, but not reversed into vasoconstriction by dopamine-induced alpha-adrenergic stimulation.

Physiological and pharmacological aspects of adrenergic receptor classification

The questions raised are: what is the physiological or pharmacological basis for the differentiation into beta 1- and beta 2-, and alpha 1- and alpha 2-adrenergic receptors?; and do the neurotransmitter norepinephrine and the hormone epinephrine differ in their receptors? On the basis of a preference of beta 2- and alpha 2-adrenergic receptors for epinephrine, the hormone, and of beta 1-and alpha 1-receptors for norepinephrine, the neurotransmitter, it was postulated that the alpha 2- and beta 2-receptors are predominantly epinephrinergic in nature and located extrajunctionally and presynaptically whereas the alpha 1- and beta 1-receptors are predominantly norepinephrinergic in nature and located postsynaptically in the sympathetic terminal junction. The alpha 2- and beta 2-character of the presynaptic receptors matches that of the corresponding extrajunctional receptors. This indicates that a circulating catecholamine, namely epinephrine, is involved in the regulation of adrenergic transmitter release.

Evidence for multiple receptors mediating fluid secretion in salivary glands of ticks

Using isolated salivary glands of the ixodid tick Amblyomma hebraeum Koch, we tested the effectiveness of butaclamol and sulpiride in blocking fluid secretion stimulated by a number of agonists. (+)-Butaclamol was a potent inhibitor of dopamine, N-methyldopamine and noradrenaline (Ki congruent to 30-60 nM), but was less effective on ergometrine (Ki congruent to 310 nM). Tranylcypromine-stimulated fluid secretion in the absence and presence of (+)-butaclamol and (+/-)-sulpiride suggested that tranylcypromine's action is mediated through two receptors. (+/-)-Sulpiride, though a rather weak antagonist of ergometrine (Ki congruent to 6150 nM), was ineffectual as a dopamine blocker, indicating distinct receptor sites on this epithelium for dopamine and ergometrine. Both (+)-butaclamol and sulpiride reversed the autoinhibition associated with supramaximal levels of dopamine. Sulpiride also abolished spiperone's potentiation of dopamine. Butaclamol, on the other hand, had no such effect on spiperone's potentiation of dopamine. Finally, although the CNS of ticks contains both dopamine and noradrenaline in quantity (congruent to 650 and congruent to 370 ng . g-1 res respectively), the salivary glands contain far more dopamine than noradrenaline (congruent to 85 and congruent to 6 ng . g-1 respectively). The data support the hypothesis that dopamine is a natural transmitter substance in the tick salivary gland, and that there are distinct receptor sites in the epithelium mediating the actions of catecholamines, ergot alkaloids and butyrophenones. The physiological significance of the ergot alkaloid and butyrophenone sites is not clear.

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.

The purported dopamine agonist (3,4-dihydroxyphenylimino)-2-imidazolidine (DPI) acts as a nonselective alpha-adrenoceptor agonist in inducing hypertension, hypomotility and hypothermia in the rat

Following peripheral administration the purported dopamine (DA) agonist (3,4-dihydroxyphenylimino)-2-imidazolidine (DPI) was shown to increase the diastolic blood pressure of pithed rats and to decrease rat motility and rectal temperature. Dose-effect relationships were established and half-maximal effective doses for the hypertensive and hypothermic response to DPI were calculated to be 4.4 nmol/kg i.v. and 2.0 mumol/kg i.p., respectively. Pretreatment with various antagonists revealed that both alpha 1- and alpha 2-adrenergic mechanisms were responsible for the DPI-induced hypertension, hypomotility and hypothermia, indicating that DPI acts as a non-selective alpha-adrenoceptor agonist. Qualitatively the DPI-induced effects were found to correlate well with those reported for its structural analogue clonidine, thus suggesting a similar mechanism of action for these agents. DA receptor antagonists appeared to lack inhibitory potency towards any of DPI-elicited responses. The results do not therefore support the designation of DPI as a DA receptor agonist.

Altered characteristics of striatal [3H]ADTN binding following substantia nigra lesions

The present experiments examined some characteristics of the enhanced striatal [3H]ADTN binding which occurs following substantia nigra lesion. The amount of binding of a fixed concentration of [3H]ADTN was greatly increased. The amount of guanine nucleotide sensitivity increased to an even greater extent than total [3H]ADTN binding. Saturation experiments showed one component of [3H]ADTN binding (KD 7 nM) in intact striata whereas there were two components of binding (KD 9 nM and KD 1 nM) in substantia nigra-lesioned striata. Most of the enhanced binding in substantia nigra-lesioned striata was the higher affinity component. Kainic acid lesion of substantia nigra-lesioned striata caused loss of the enhanced [3H]ADTN binding and the high affinity component of [3H]ADTN binding. These results indicate that following dopaminergic denervation, either the binding characteristics of a single type of dopamine receptor are altered or there is a specific increase in a distinct subtype of dopamine receptor with high agonist affinity and sensitivity to guanine nucleotides.

Peripheral dopamine receptors, potential targets for a new class of antihypertensive agents. Part I: Subclassification and functional description

The dopamine receptors of the peripheral cardiovascular system are not a pharmacologically uniform population. A number of studies indicate that they belong to at least two distinct subtypes for which it is proposed to adopt the name DA1- and DA2-dopamine receptors in an attempt to follow the nomenclature presently in fashion for several vascular receptors. Typical DA1-dopamine receptors are those occurring postjunctionally in the renal and mesenteric arterial beds where their stimulation mediates direct smooth muscle relaxation. Typical DA2-dopamine receptors are those present on postganglionic sympathetic neurons (axonal varicosities and perhaps ganglionic cell bodies) where their excitation leads, under appropriate physiological conditions, to a reduction of the neural release of norepinephrine. The latter effect can manifest itself by a passive fall in vascular resistance and heart rate. Other populations of dopamine receptors not yet well characterized pharmacologically but of theoretical interest as additional potential target sites for cardiovascular drugs might be present on nephrons and in the adrenal cortex. Their stimulation can mediate a natriuretic effect and a reduction of aldosterone release, respectively. The pharmacological evidence favoring the subclassification of cardiovascular dopamine receptors into two distinct subtypes is reviewed. Furthermore, the main agonists and antagonists of these receptors and the complexity of their pharmacological profile are mentioned. Part II of this minireview will be dedicated to the description of the sites and mechanisms of the antihypertensive action of dopamine receptor agonists.

Comparison of the vasodilator action of dopamine and dopamine agonists in the renal and coronary beds of the dog

1 The effects of dopamine and the dopamine receptor agonists, SK&F 38393 and bromocriptine, on renal and coronary blood flow in the anaesthetized dog were examined. Dopamine was found to dilate both vascular beds, whereas SK&F 38393 increased renal blood flow but did not have any dilator activity in the coronary vasculature. Bromocriptine did not cause vasodilatation in either vascular bed. 2 The vasodilator responses to dopamine and SK&F 38393 were significantly reduced by the dopamine receptor antagonists, ergometrine or metoclopramide. 3 It is proposed that the selective action of SK&F 38393 on the renal vasculature suggests that the dopamine receptors of the renal and coronary vascular beds may be of different types.

Selective labelling of dopamine (D2) receptors in rat striatum by [3H]domperidone but not by [3H]spiperone

Specific binding of [3H]spiperone and [3H]domperidone, displaceable by 1 microM d-butaclamol, was examined in rat striatal membranes. Initial saturation and displacement experiments indicated that [3H]spiperone bound to more sites than [3H]domperidone and that, whilst all displacing drugs were more potent against [3H]domperidone, this difference in potency was greatest for dopamine agonists and specific antagonists and least for 5HT-related drugs. Sulpiride displaced [3H]spiperone biphasically, and was used at a concentration of 50 microM to examine two classes of [3H]spiperone binding: site 1 displaceable by sulpiride, and site 2 displaceable by butaclamol but not by sulpiride. Site 1 had twice the capacity of site 2 and ten times the affinity for [3H]spiperone. Dopaminergic drugs displaced preferentially from site 1, whilst 5HT-related drugs were more potent against site 2. GTP reduced the potency of dopamine, noradrenaline and, to a lesser extent, 5HT at site 1, but had no effect at site 2. [3H]Domperidone sites had the same capacity as [3H]spiperone site 1, and dopamine, noradrenaline and 5HT, in the absence or presence of GTP, and sulpiride had essentially identical affinities for [3H]domperidone sites and [3H]spiperone site 1. It is concluded that [3H]domperidone and [3H]spiperone label an identical population of dopamine (D2) receptors, whilst [3H]spiperone also labels a substantial number of non-dopamine sites, at least some of which are 5HT-related. [3H]spiperone also labels a substantial number of non-dopamine sites, at least some of which are 5HT-related. [3H]Domperidone is the better radioligand for dopamine receptors.

Dopamine involvement in ACTH-induced grooming behavior

Low doses of dopamine agonists and antagonists were tested for their effects on the excessive grooming behavior induced by intracerebroventricular (ICV) injections of ACTH1-24. Grooming scores were significantly depressed at doses of haloperidol, metoclopramide, pimozide, and butaclamol that did not decrease locomotor activity. In fact at two doses of haloperidol (0.067 and 0.10 mg/kg), grooming scores were decreased while locomotor activity was increased significantly. Metoclopramide increased grooming scores at a dose reported to block presynaptic dopamine receptors. Apomorphine potentiated the grooming induced by low doses of ACTH. These data support the hypothesis that dopaminergic neurotransmission is necessary for the display of ACTH-induced grooming behavior.

On the preferred rotameric conformation for dopamine agonist action: an illusory quest

Putative dopamine agonists from the 2-aminotetrahydronaphthalene and trans-octahydrobenzo (f) and (g) quinoline series were shown to inhibit the spontaneous locomotor activity of mice. Marked potency differences were observed between the alpha-and beta-rotameric conformations, compounds having the alpha-rotameric conformation having the greater potency. Thus, 2-di-n-propylmino-5,6-dihydroxytetrahydronaphthalene was 339 times more potent than the 6, 7-dihydroxy isomer, and 2-n-propylamino-5,6-dihydroxy-compound was respectively 79 times and 179 times more than 6,7-hydroxy-and 7,8-dihydroxycompounds. trans-7,8-dihydroxy-1-n-propyl-1,2,3,4,a,9,10,10b-octahydrobenzo(f)quinoline was 11 times more potent than the beta-rotamer, the 6,7-dihydroxy compound, and within the trans-octahydrobenzo(g)quinoline series the alpha-rotameric N-propyl derivative was 467 times more potent than the beta-rotamer, and the alpha-rotameric greater than N-H analogue was 46 fold more potent than the beta-rotamer. Thus, the alpha-rotamer appears the more potent in causing the present functional dopaminergic change. The dopaminergic nature of the response was indicated by its sensitivity to spiroperidol but not to yohimbine or prazosin. The possibility that a difference in behavioural potency between the alpha- and beta-rotamers may reflect a differential metabolism by catechol-O-methyl transferase was assessed by administration of different agonists after pyrogallol pretreatment. This potentiated the activity of 2-di-n-propylamino-6,7-dihydroxytetrahydro-naphthalene but not that of the 5,6-dihydroxy analogue. However, changes in the effects of N-propyl derivatives of trans-octahydrobenzo (f) and (g)quinoline were not marked and, in all experiments, pyrogallol treatment failed by orders of magnitude to shift the dose-response curves of the beta-rotamers to indicate a comparable potency to the rotameric forms.

Dopamine receptors in canine caudate nucleus

Physiological, pharmacological, histochemical and biochemical studies indicate that dopamine receptors are heterogenous in the central nervous system with each individual functions. This review describes pharmacological and biochemical characteristics of dopamine receptors, particularly in canine caudate nucleus, which have been studied in our laboratory with a brief comparison to the current studies by other workers in similar research fields. Two distinct dopamine receptors have been characterized by means of [3H]dopamine binding to the synaptic membranes from canine caudate nucleus. One of the receptors with a Kd of about 3 muM for dopamine may be associated with adenylate cyclase and referred to as D2 receptor. The other receptor with a Kd of about 10 nM for dopamine is independent of adenylate cyclase and referred to as D2. A photochemical irreversible association of [3H]dopamine with the membraneous receptors makes it possible to separate D1 and D2 receptors from one another by gel filtration on a Sephadex G-200 column after solubilization with Lubrol PX. On the basis of selective inhibition of [3H]dopamine binding to D1 and D2 receptors, dopamine antagonists can be classified into three classes: D1-selective (YM-09151-2), D2-selective (sulpiride) and nonselective (haloperidol, chlorpromazine). Effects of these typical antagonists on the metabolism of rat brain dopamine suggest that D1 receptor is more closely associated with the neuroleptic-induced increase in dopamine turnover. Studies with 28 benzamide derivatives and some classical neuroleptics reveal that apomorphine-induced stereotypy displays a greater association with D1 than with D2 receptors. Dopamine-sensitive adenylate cyclase in canine caudate nucleus can be solubilized with Lubrol PX in a sensitive form to either dopamine. Gpp(NH)p or fluoride Sephadex G-200 gel filtration separates adenylate cyclase from D1 receptors with a concomitant loss of dopamine sensitivity. Addition of the D1 receptor fraction to the adenylate cyclase restores the responsiveness to dopamine. The solubilized dopamine-unresponsive adenylate cyclase can be further separated into two distinct fractions by a batch-wise treatment with GTP-sepharose: a catalytic unit which does not respond to fluoride, and a guanine nucleotide regulatory protein. The regulatory protein confers distinct responsiveness to Gpp(NH)p and fluoride upon adenylate cyclase. These results indicate that dopamine-sensitive adenylate cyclase is composed of at least three distinct units; D1 receptor, guanine nucleotide regulatory protein and adenylate cyclase.

Progressive changes in the acute dyskinetic syndrome as a function of repeated elicitation in squirrel monkeys

Various neuroleptic-induced motor disorders that appear in primates previously treated with neuroleptics are collectively designated the acute dyskinetic syndrome. The relative incidence of these motor disorders was examined as the syndrome was repeatedly elicited by haloperidol and other dopamine antagonists in individual monkeys. After several weekly or biweekly treatments with haloperidol (1.25 mg/kg orally), catalepsy began to appear, which was then accompanied by athetoid movements (writhing and limb extensions) as intermittent neuroleptic treatment continued. Other dyskinetic movements ('duck walk', oral dyskinesias, pushing of the head into a cage corner, and perseverative circling) that were suggestive of hyperkinesia subsequently began to be elicited by haloperidol and other neuroleptics after additional treatments with these drugs had intervened. As intermittent treatments continued, tolerance to the athetoid movements gradually developed and, eventually, only circling and pushing could be consistently elicited by haloperidol. In monkeys that had reached this phase, the athetoid movements were not again induced by higher doses of haloperidol (up to 5 mg/kg), chlorpromazine (3 mg/kg), or metoclopramide (3 mg/kg). In these tolerant monkeys, haloperidol impaired Sidman avoidance performance less and benztropine more than in drug-naive monkeys. Neither pharmacokinetic changes nor behavioral tolerance could readily account for these results. It is hypothesized that they reflect progressive functional alterations in dopaminergic or cholinergic neurotransmission.

The pharmacology of sulpiride--a dopamine receptor antagonist

1. The clinical and experimental pharmacology of sulpiride, its effects on the CNS, gastrointestinal tract and cardiovascular system have been reviewed. 2. The majority of its actions are attributable to blockade of dopamine receptors. 3. Although sulpiride has a high affinity for dopamine receptors involved in emesis and prolactin secretion, it lacks part of the behavioural and biochemical profiles of the classical dopamine receptor antagonist neuroleptics. 4. In the cardiovascular system, sulpiride is a potent prejunctional dopamine receptor antagonist but has variable effectiveness in postjunctional dopamine receptor models. 5. These properties are discussed with reference to the mechanisms of action of sulpiride and the classification of dopamine receptors.

Agonistic actions of DPI (2-(3,4-dihydroxyphenylimino)-imidazolidine) on alpha-adrenoceptors and dopamine receptors

Some of the pharmacological actions of 2-(3,4-dihydroxyphenylimino)-imidazolidine (DPI) were studied in vivo and vitro. DPI (1 nM-100 micro M) had a similar affinity but a lower intrinsic activity on alpha-adrenoceptors in rabbit aortic strips to noradrenaline (1 nM-10 micro M). DPI did not affect the uptake of [3H]noradrenaline in guinea-pig isolated atria. The effects of DPI on dopamine receptors were examined in the dog coronary and renal vasculature and in the rat isolated, perfused kidney. In the dog coronary vasculature DPI (0.005-10 micrograms, i.a.), like dopamine (5-50 micrograms, i.a.), caused an increase in coronary blood flow which was antagonized by the dopamine receptor antagonist, ergometrine. In the renal vasculature of both the rat and the dog, dopamine (5-50 micrograms, i.a.) caused vasodilatation but there was no evidence of an effect of DPI on dopamine receptors. It appears that DPI is a selective agonist for dopamine receptors in the coronary vasculature.