Two dopamine receptors: biochemistry, physiology and pharmacology

In 1979, two categories of dopamine (DA) receptors (designated as D-1 and D-2) were identified on the basis of the ability of a limited number of agonists and antagonists to discriminate between these two entities. In the past 5 years agonists and antagonists selective for each category of receptor have been identified. Using these selective drugs it has been possible to attribute the effects of DA upon physiological and biochemical processes to the stimulation of either a D-1 or a D-2 receptor. Thus, DA-induced enhancement of both hormone release from bovine parathyroid gland and firing of neurosecretory cells in the CNS of Lymnaea stagnalis has been attributed to stimulation of a D-1 receptor. Likewise, the DA-induced inhibition of the release of prolactin and alpha-MSH from the pituitary gland, as well as of acetylcholine, DA and beta-endorphin from brain, the DA-induced inhibition of chemo-sensory discharge in rabbit carotid body and the DA-induced hyperpolarization of neurosecretory cells in the CNS of Lymnaea stagnalis have been attributed to stimulation of a D-2 receptor. Independently two categories of DA receptors (designated as DA-1 and DA-2) were identified in the cardiovascular system. Stimulation of a DA-1 receptor increases the vascular cyclic AMP content and causes a relaxation of vascular smooth muscle in renal blood vessels, whereas stimulation of a DA-2 receptor inhibits the release of norepinephrine from certain postganglionic sympathetic neurons. Recent studies with the newly developed drugs discriminating between D-1 and D-2 receptors suggest however that the independently developed schemata for classification of dopamine receptors in either the central nervous and endocrine systems or the cardiovascular system are similar although maybe not completely identical.     

Dopamine and dopamine receptors as target sites for cardiovascular drug action

It is controversial whether dopamine (DA) is a peripheral neurotransmitter in the cardiovascular/renal system. The endogenous concentration of DA in the heart and blood vessels is generally only a fraction (5%) of that of norepinephrine (NE). With perhaps the exception of the kidney, the majority of the evidence suggests a precursor role for this amine rather than that of a neurotransmitter. The main weakness of arguments favoring DA as a vascular neurotransmitter is relative lack of data showing selective DA release and lack of effects of selective DA antagonists on neural stimulation. However, DA receptors have been characterized in cardiovascular tissues and are of two types: DA1 receptors located on vascular smooth muscle (postjunctional), which appear to mediate relaxation of the muscle, and DA2 receptors located on sympathetic nerves (pejunctional), which inhibit NE release. These receptors are interesting and potential target sites for novel cardiovascular drug action for the treatment of hypertension and renal ischemia. Moreover, selective DA receptor agonists will be important tools in understanding the role of DA receptors in normal and disease states.     

Modulation of [3H]dopamine release from rabbit retina

The calcium-dependent release of [3H]dopamine ([3H]DA) elicited by field stimulation or potassium is modulated through activation of stereoselective inhibitory DA autoreceptors of the D-2 subtype that are pharmacologically different from the D-1 DA receptor subtype linked to the stimulation of adenylate cyclase (EC 4.6.1.1). The D-2 DA autoreceptors appear to be endogenously activated by DA because DA receptor antagonists such as S-sulpiride increased the stimulation-evoked release of [3H]DA. Nanomolar concentrations of norepinephrine (NE) and epinephrine (E) inhibited in a concentration-dependent manner the electrical stimulation-evoked release of [3H]DA. The inhibitory effect of these catecholamines was not modified by S-sulpiride, which, on the contrary, selectively antagonized the inhibition of [3H]DA release elicited by exogenous DA. Phentolamine or (+/-)-propranolol did not affect the release of [3H]DA from rabbit retina. The alpha antagonist phentolamine competitively antagonized the inhibitory effect of both NE and E, which suggests that these catecholamines activate alpha receptors in retina. The decrease by catecholamines of the calcium-dependent release of [3H]DA appears not to involve beta adrenoceptors because their inhibitory effect was not modified by propranolol. Under identical experimental conditions (i.e., nomifensine, 30 microM), serotonin did not modify the stimulated release of [3H]DA. In conclusion, in the rabbit retina, DA autoreceptors of the D-2 subtype appear to modulate endogenously released DA whereas inhibitory presynaptic alpha receptors might be of pharmacological importance as sites of action for retinal or blood-borne catecholamines.     

Role of dopamine receptors in the mechanism of formation of stress-induced stomach lesions

Experiments on rats with the use of different exposures to stress (generalized electrization and "social stress") have demonstrated that stimulation of dopamine receptors localized in the central nervous system is one of the reasons for stress-induced gastric lesions, particularly for massive hemorrhages. Stimulation of peripheral dopamine receptors seems to have a gastroprotective action. As judged from the intensity of the effects of the dopamine agonists, apomorphine and L-DOPA, on stress-induced lesions of the gastric mucosa, stimulation of D2- rather than of D1-dopamine receptors is of greater importance in stress.     

Analysis of the effect of (-)-BPAP,a selective enhancer of the impulse propagation mediated release of catecholamines and serotonin in the brain

Endogenous and synthetic enhancer substances enhance in low concentration the impulse propagation mediated release of transmitters from the catecholaminergic and serotonergic neurons in the brain. The purpose of this study was to see whether uptake or MAO inhibition or agonists have similar enhancing prospectives as the enhancer substances. We measured the electrical stimulation induced release of [3H]-norepinephrine or [3H]-dopamine or [3H]-serotonin from the isolated brain stem of rats. (-)-1-Benzofuran-2-yl)-2-propylaminopentane HCl [(-)-BPAP] was used as a prototype of the enhancer compounds. 50 ng/ml (-)-BPAP was the most effective concentration in enhancing the nerve stimulation induced release of [3H]-norepinephrine and [3H]-dopamine, 10 ng/ml (-)-BPAP was highly effective in enhancing the release of [3H]-serotonin. In contrast, 250 ng/ml desmethylimipramine (DMI), a selective inhibitor of the uptake of norepinephrine, did not change significantly the nerve stimulation induced release of [3H]-norepinephrine and 50 ng/ml fluoxetine, a selective inhibitor of the uptake of serotonin, did not change the release of [3H]-serotonin. Neither 250 ng/ml clorgyline, a selective inhibitor of MAO-A, nor 250 ng/ml lazabemide, a selective inhibitor MAO-B, was capable to significantly increase the nerve stimulation induced release of either [3H]-serotonin or [3H]-norepinephrine. The potent dopamine receptor agonists, pergolide and bromocriptine did not change significantly the release of [3H]-dopamine in 50 ng/ml concentration, which is sufficient to stimulate the dopamine receptors. The results prove that stimulation of catecholaminergic and serotonergic neurons in the brain via the enhancing mechanism is clearly different from influencing uptake or MAO.     

Cutting Edge: Stimulation of dopamine D4 receptors induce T cell quiescence by up-regulating Kruppel-like factor-2 expression through inhibition of ERK1/ERK2 phosphorylation

The neurotransmitter dopamine (DA) is an important regulator of human T cell functions. Although it has been observed that DA, by acting through the D1/D5, D2, and D3 receptors, can activate resting T cells by stimulating the release of cytokines and the expression of surface integrins and also inhibit the proliferation of activated T cells by down-regulating nonreceptor tyrosine kinases, there is not yet a report indicating the functional significance of the D4 DA receptors present in these cells. The present work, for the first time, demonstrates that the stimulation of D4 DA receptors in human T cells induces T cell quiescence by up-regulating lung Krüppel-like factor-2 expression through the inhibition of ERK1/ERK2 phosphorylation. These results reveal a new link between the nervous system and T cell quiescence and indicate that D4 DA receptor agonists may have a therapeutic value in diseases with uncontrolled T cell proliferation.     

The dopamine of the rat mammotroph in cell culture as a model for drug action

Dopamine can act directly on pituitary cells to inhibit prolactin release. This action can be blocked by dopamine receptor blocking drugs such as haloperidol, sulpiride and other neuroleptic agents. Comparison of the properties of the mammotroph dopamine receptor with the adenylate cyclase linked dopamine receptor of the limbic forebrain reveals some obvious differences. For example, dopamine receptor stimulants such as S-584 and lergotrile mesylate are inactive in stimulating the adenylate cyclase preparations but are potent in inhibiting pituitary prolactin secretion. Such inhibition of prolactin secretion can be reversed by haloperidol or sulpiride. In contrast to these observations, sulpiride does not block dopamine stimulation of cAMP formation. In addition, dopamine, apomorphine or lergotrile mesylate have no effect on a pituitary adenylate cyclase preparation and dopamine fails to elevate cAMP in the intact cells in culture. Despite the similarity between these two dopamine sensitive systems with respect to a number of agonists and antagonists, the exceptions described suggest that the pituitary system with further study may offer some greater reliability as a predictive test for clinically useful agents. These results also suggest that the receptors for dopamine, like that for norepinephrine, are of two types, only one of which is coupled to adenylate cyclase.     

Prevention of stress-induced gastric ulcers by dopamine agonists in the rat

Dopamine (DA) and DA agonists have been shown to exert a protective role against the formation of duodenal ulcers. The effect of stimulation of DA receptors on the development of stress-induced gastric ulcers is currently unknown. Accordingly, we evaluated the effect of several DA agonists on the development of gastric ulcers induced by 3 h of cold + restraint stress (CRS) in rats. Apomorphine, d-amphetamine, methylphenidate, and threo-dl-p-hydroxymethylphenidate (an hydroxylated analog of methylphenidate), significantly reduced both the incidence and severity of CRS-induced gastric ulcers. The gastric cytoprotection afforded by these agents was dose-related, and completely antagonized by pretreatment with the peripheally acting DA antagonist domperidone. Because domperidone blocks peripheral, but not central, DA receptors, and since the entry of threo-dl-p-hydroxymethylphenidate across the blood-brain barrier into the brain is restricted to a great extent, we conclude that stimulation of peripheral DA receptors is primarily involved in the gastric cytoprotection induced by dopamimetics.The pathogenesis of stress-induced gastric ulcers remains largely unknown, and significant efforts have been made over the last decade to functionally characterize some of the factors involved in the etiology of this disease. Considerable attention has been focused on gastric acid secretion, but its primary role in stress-induced gastric ulcer disease remains uncertain. In fact, agents which effectively inhibit or neutralize gastric acid secretion such as cimetidine or antacids do not necessarily exert protection against stress-induced gastric ulcers (1,2). Moreover, in our original studies with neurotensin, a brain and gastrointestinal peptide, we have found that central administration of this neuropeptide, which completely prevents the development of cold + restraint stress (CRS)-induced gastric ulcers, does not appreciably alter gastric acid secretion (2). These findings support the contention that gastric acid secretion may not be an important factor in the development of this type of gastric ulcer.There is, however, considerable evidence that the automatic nervous system plays an intermediary role in the development of these ulcers (3,4). In this regard, surgical or pharmacological blockade of the vagal (cholinergic) division of the autonomic nervous system prevents the appearance of stress-associated gastric ulcers (5,6). Direct stimulation of catecholamine receptors, or indirect activation via increased sympathetic outflow to the periphery (7,4,8–11) appears to produce a salutary effect of stress-induced gastric ulcers.Szabo and his associates (12, 13, 14) have extensively studied the anti ulcer effects of dopamine (DA) in duodenal ulcer formation. Whether DA also modifies the development of stress-induced gastric ulcers is currently unknown.We have therefore evaluated the effect of selected DA receptor agonists and antagonists on CRS-induced gastric ulcer formation in rats.     

Vascular dopamine receptors: Demonstration and characterization by in vitro studies

Substantial evidence has accumulated that in certain vascular beds dopamine produces its relaxant effect through stimulation of specific dopamine receptors. The goal of this review is to describe several in vitro models (perfused mesenteric vessels of the dog; renal, mesenteric, splenic, coronary and cerebral arterial strips of rabbits, dogs and cats; perfused kidney of the rat) recently developed to demonstrate such specific relaxations induced by dopamine and dopaminomimetics. On these models studies on structure-activity relationship for activation of the dopamine receptor resulted in the following order of potency for agonists: SK&F 38393 (partial agonist) greater than epinine greater than A-6, 7-DTN greater than or equal to dopamine greater than N, N-di-n-propyl-dopamine (partial agonist) greater than apomorphine (partial agonist). The dopamine receptor antagonists (+)-butaclamol, cis-alpha-flupenthixol, metoclopramide, droperidol and bulbocapnine were found to competitively antagonize dopamine induced relaxation. In addition, in two isolated organ systems (rabbit mesenteric artery, rat perfused kidney) stereospecificity of the vascular dopamine receptor was demonstrated with the isomers of butaclamol. With the development of several in vitro models demonstrating a specific antagonism against dopamine induced relaxation an important requirement for definition of a specific dopamine receptor if fulfilled according to classical pharmacological criteria. Thus, there can be do doubt on the existence of post-synaptic dopamine receptors mediating vasodilation in certain vascular tissues.     

Dihydroergotoxine decreases blood pressure in spontaneously hypertensive rats by interacting with peripheral dopamine receptors

Dihydroergotoxine (10 micrograms/kg s.c.) decreased mean carotid blood pressure in urethane-anaesthetized spontaneously hypertensive rats but failed to modify the same parameter in normotensive rats. The effect was statistically significant 20 min after the injection and relatively long lasting (up to 90 min). Pharmacological characterization of the phenomenon indicated that it is mediated by stimulation of dopamine receptors, since pretreatment with haloperidol, cis-flupentixol but not with trans-flupentixol, completely prevent the reduction in blood pressure induced by dihydroergotoxine. Moreover, a challenge dose of dihydroergotoxine did not reduce mean blood pressure values in spontaneously hypertensive rats pretreated with domperidone or (-)sulpiride, but not with (+)sulpiride. These results suggest that the ergot derivative modifies the cardiovascular system by interaction with peripheral dopamine receptors of the DA2 type.     

Receptor mechanisms of prenodal lymphatic constriction by dopamine

It has been proposed that alterations in lymphatic smooth muscle activity significantly impact lymphatic function. Numerous endogenous vasoactive agents are known to constrict prenodal lymph vessels. In this study, we assessed the ability of dopamine to alter lymphatic smooth muscle tone in perfused prenodal lymph vessels. Additionally, the receptor mechanisms of dopamine's actions were elucidated. Both intralymphatic (i.l.) and intra-arterial (i.a.) dopamine significantly increased lymphatic perfusion pressure. The increase in lymphatic pressure was completely blocked by i.a. phentolamine, suggesting involvement of alpha(1)- and/or alpha(2)-adrenoreceptors. Intra-arterial infusion of the specific alpha(1)-receptor antagonist prazosin completely abolished the constriction seen during i.l. phenylephrine but only attenuated that produced by dopamine. Intralymphatic infusion of the DA(1)-receptor agonist SKF 82526-J and the DA(2)-receptor agonist LY 171555 caused significant relaxation of lymph vessels that had been previously constricted by i.a. norepinephrine infusion. These data indicate that the constriction produced by dopamine, in the concentrations employed in this study, is mediated by both alpha(1)- and alpha(2)-adrenoreceptors. These lymph vessels do contain both DA(1)- and DA(2)-receptors but stimulation of these receptors results in lymphatic smooth muscle relaxation.     

Allosteric modulation of dopamine D2 receptors by homocysteine

It has been suggested that L-DOPA-induced hyperhomocysteinemia can increase the risk of stroke, heart disease, and dementia and is an additional pathogenetic factor involved in the progression of Parkinson's disease. In Chinese hamster ovary (CHO) cells stably cotransfected with adenosine A(2A) and dopamine D2 receptors, homocysteine selectively decreased the ability of D2 receptor stimulation to internalize adenosine A(2A)-dopamine D2 receptor complexes. Radioligand-binding experiments in the same cell line demonstrated that homocysteine acts as an allosteric D2 receptor antagonist, by selectively reducing the affinity of D2 receptors for agonists but not for antagonists. Mass spectrometric analysis showed that, by means of an arginine (Arg)-thiol electrostatic interaction, homocysteine forms noncovalent complexes with the two Arg-rich epitopes of the third intracellular loop of the D2 receptor, one of them involved in A(2A)-D2 receptor heteromerization. However, homocysteine was unable to prevent or disrupt A(2A)-D2 receptor heteromerization, as demonstrated with Fluorescence Resonance Energy Transfer (FRET) experiments in stably cotransfected HEK cells. The present results could have implications for Parkinson's disease.     

D1-D2 dopamine receptor interaction within the nucleus accumbens mediates long-loop negative feedback to the ventral tegmental area (VTA)

The objective of the present study was to examine the effects of perfusion of dopamine (DA) D1- and D2-like receptor agonists in the nucleus accumbens (ACB) on the long-loop negative feedback regulation of mesolimbic somatodendritic DA release in the ventral tegmental area (VTA) of Wistar rats employing ipsilateral dual probe in vivo microdialysis. Perfusion of the ACB for 60 min with the D1-like receptor agonist SKF 38393 (SKF, 1-100 microM) dose-dependently reduced the extracellular levels of DA in the ACB, whereas the extracellular levels of DA in the VTA were not changed. Similarly, application of the D2-like receptor agonist quinpirole (Quin, 1-100 microM) through the microdialysis probe in the ACB reduced the extracellular levels of DA in the ACB in a concentration-dependent manner, whereas extracellular levels of DA in the VTA were not altered. Co-application of SKF (100 microM) and Quin (100 microM) produced concomitant reductions in the extracellular levels of DA in the ACB and VTA. The reduction in extracellular levels of DA in the ACB and VTA produced by co-infusion of SKF and Quin was reversed in the presence of either 100 microM SCH 23390 (D1-like antagonist) or 100 microM sulpiride (D2-like antagonist). Overall, the results suggest that (a) activation of dopamine D1- or D2-like receptors can independently regulate local terminal DA release in the ACB, whereas stimulation of both subtypes is required for activation of the negative feedback pathway to the VTA.     

In vivo activation of midbrain dopamine neurons via sensitized, high-affinity alpha 6 nicotinic acetylcholine receptors

Alpha6-containing (alpha6*) nicotinic ACh receptors (nAChRs) are selectively expressed in dopamine (DA) neurons and participate in cholinergic transmission. We generated and studied mice with gain-of-function alpha6* nAChRs, which isolate and amplify cholinergic control of DA transmission. In contrast to gene knockouts or pharmacological blockers, which show necessity, we show that activating alpha6* nAChRs and DA neurons is sufficient to cause locomotor hyperactivity. alpha6(L9'S) mice are hyperactive in their home cage and fail to habituate to a novel environment. Selective activation of alpha6* nAChRs with low doses of nicotine, by stimulating DA but not GABA neurons, exaggerates these phenotypes and produces a hyperdopaminergic state in vivo. Experiments with additional nicotinic drugs show that altering agonist efficacy at alpha6* provides fine tuning of DA release and locomotor responses. alpha6*-specific agonists or antagonists may, by targeting endogenous cholinergic mechanisms in midbrain or striatum, provide a method for manipulating DA transmission in neural disorders.     

Effect of intracerebral administration of NMDA and AMPA on dopamine and glutamate release in the ventral pallidum and on motor behavior

The present study investigates the modulation of the ventral tegmental area (VTA)-ventral pallidum (VP) dopaminergic system by glutamate agonists in rats. The glutamate receptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were infused via reversed microdialysis into the VTA, and dopamine (DA), glutamate, and aspartate levels in the VTA and ipsilateral VP were monitored together with motor behavior screened in an open field. NMDA (750 microM) infusion, as well as AMPA (50 microM) infusion, induced an increase of DA and glutamate levels in the VTA, followed by an increase of DA levels in the ipsilateral VP and by enhanced locomotor activity. The increase of DA in the VP was similar after administration of these two glutamate agonists, although motor activity was more pronounced and showed an earlier onset after NMDA infusion. Glutamate levels in the VP were not increased by the stimulation of DA release. It is concluded that DA is released from mesencephalic DA neurons projecting to the VP and that these neurons are controlled by glutamatergic systems, via NMDA and AMPA receptors. Thus, DA in the VP has to be considered as a substantial modulator. Dysregulation of the mesopallidal DA neurons, as well as their glutamatergic control, may play an additional or distinct role in disorders like schizophrenia and drug addiction.     

Interactions of novel dopaminergic ligands with D-1 and D-2 dopamine receptors

The interactions of three novel dopaminergic ligands, SKF38393, SKF82526 and SKF83742, with D-1 and D-2 dopamine (DA) receptors have been investigated using radioligand binding techniques and computer modeling procedures. Using the bovine anterior pituitary D-2 DA receptor system, SKF38393 and SKF82526 behave as agonists demonstrating biphasic agonist/3H-antagonist competition curves. For both drugs, the high affinity phase comprised 30% of the total displacement curve. Such findings are atypical as previously tested classical dopamine agonists demonstrated high and low affinity displacement phases in equal proportions. Such behavior exhibited by the SKF agonists may be related to their activity as partial agonists. In contrast, SKF83742 behaves as an antagonist exhibiting homogeneous monophasic competition curves. Similar results are obtained in the rat striatal membrane D-2 DA receptor system. Both SKF38393 and SKF82526 also demonstrate shallow biphasic displacement curves on rat striatal D-1 receptors labeled with 3H-flupentixol whereas SKF83742/3H-flupentixol curves are uniphasic. Of all the ligands, only SKF38393 clearly demonstrates higher affinity for 3H-flupentixol labeled D-1 receptors.     

Anomalously High Concentrations of Brain Extracellular Uric Acid Detected with Chronically Implanted Probes: Implications for In Vivo Sampling Techniques

The concentration of dopamine (DA) in the synaptic cleft in the mouse striatum in vivo was estimated from the competition between the synaptic DA and the 3H-labelled DA D2 receptor agonists N-n-propylnorapomorphine (NPA) or N,N-diethyl-N'-[(3 alpha, 4a alpha, 10 beta)-1,2,3,4,4a,5,10,10a-octahydro- 7-hydroxyl-1-propyl-3-benzo (g) quinolinyl]sulfamide (Sandoz 205-501) injected intravenously in tracer doses. Knowing the inhibitor constant for DA in inhibiting the binding of these receptor agonists in vitro, attempts were made to calculate the changes in the synaptic DA concentration from the changes in the in vivo binding of the receptor agonists evoked by various pharmacological agents. Inhibiting the firing of the dopaminergic neurons by gamma-butyrolactone (GBL) increased the binding of the receptor agonists corresponding to a decrease in the synaptic DA concentration of 55 +/- 2 nM in the experiments with [3H]Sandoz 205-501 and 48 +/- 3 nM in the experiments with tracer doses of [3H]NPA. These values may therefore approximate the normal DA concentration in the synaptic cleft in the mouse striatum. With this technique it was also possible to determine the synaptic concentration of NPA by its competition with [3H]Sandoz 205-501 for the DA D2 receptors in the striatum of GBL-treated mice in vivo. To compare the estimated synaptic concentration of DA with the affinity of DA to D1 and D2 receptors and to the DA transporter in the mouse striatum the kinetic parameters were determined at 37 degrees C in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)     

On the mechanism by which dopamine inhibits prolactin release in the anterior pituitary

An $\text{in}$$\text{vitro}$ perfusion system was used to assess the effects of chloride channel blockers, dopamine (DA) receptor agonists and antagonists, and GABA receptor agonists and antagonists on prolactin release from the mouse anterior pituitary. Dopamine and muscimol inhibited prolactin release ($\text{IC}{}_{50}{}^1\text{= 6 × 10}{}^{\mathrm{?8}}\text{M and}\text{10}{}^{\mathrm{?5}}\text{M}$ respectively). The GABA receptor antagonist bicuculline blocked the inhibition of prolactin release by muscimol but not dopamine. The dopamine receptor antagonist chlorpromazine blocked the dopamine- but not muscimol-induced inhibition of prolactin release. Haloperidol, however, reversed both the muscimol and dopamine induced inhibition of prolactin release. Furthermore, the chloride channel blocker picrotoxinin blocked the inhibition of prolactin release elicited by both dopamine and muscimol. These later results suggest that the anterior pituitary dopamine receptor which mediates the inhibition of prolactin release may be coupled to a picrotoxinin sensitive chloride ionophore and that haloperidol may affect the function of both DA and GABA receptors in the anterior pituitary.