John W. Kebabian

Multiple receptors for dopamine

Pharmacological and biochemical criteria can be used to separate those dopamine receptors which are linked to the enzyme adenylyl cyclase and those which are not.

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.

Biochemical and Physiological Studies of the Beta-Adrenoceptor and the D-2 Dopamine Receptor in the Intermediate Lobe of the Rat Pituitary Gland: A Review

The intermediate lobe (IL) of the rat pituitary gland responds to catecholamines. Catecholamines interacting with the beta-adrenoceptor stimulate adenylate cyclase activity, enhance cyclic AMP formation and thereby trigger the release of alpha-melanocyte-stimulating hormone (alpha-MSH). Catecholamines interacting with a D-2 dopamine receptor (in the classification schema of Kebabian and Calne) diminish adenylate cyclase activity and thereby decrease the capacity of IL cells to synthesize cyclic AMP. Dopaminergic agonists also inhibit the release of alpha-MSH from IL cells. The homogeneity of the IL facilitates biochemical investigations of this tissue.

Multiple classes of dopamine receptors in mammalian central nervous system: The involvement of dopamine-sensitive adenylyl cyclase

Abstract Two classes of dopamine receptor mechanism are defined according to their association with, or independence from, a dopamine-sensitive adenylyl cyclase. Dopamine receptors unrelated to adenylyl cyclase are designated type alpha. Dopamine receptors linked to adenylyl cyclase are designated type beta. Drugs discriminate between the two receptor mechanisms. The dopaminergic ergots (lisuride, lergotrile and CB-154) and their antagonists (such as metoclopramide) are relatively specific for the alpha-dopaminergic receptor in the anterior pituitary. Other agonists (e.g. apomorphine and dopamine) and antagonists (e.g. antipsychotic phenothiazines and butyrophenones) affect both classes of receptor.

A68930: a potent agonist selective for the dopamine D1 receptor

A68930, (1R,3S)-1-aminomethyl-5,6-dihydroxy-3-phenylisochroman HCl, is a potent (EC50 = 2.5 nM), partial (intrinsic activity = 66% of dopamine) agonist in the fish retina dopamine-sensitive adenylate cyclase model of the D1 dopamine receptor. In the rat caudate-putamen model of the D1 dopamine receptor, A68930 is a potent (EC50 = 2.1 nM) full agonist. In contrast, A68930 is a much weaker (EC50 = 3920 nM) full agonist in a biochemical model of the dopamine D2 receptor. The orientation of the 3-phenyl substituent in the molecule is critical for the affinity and selectivity of the molecule towards the dopamine D1 receptor. A68930 also displays weak alpha 2-agonist activity but the molecule is virtually inactive at the alpha 1- and beta-adrenoceptors. When tested in rats bearing a unilateral 6-OHDA lesion of the nigro-neostriatal neurons, A68930 elicits prolonged (greater than 20 h) contralateral turning that is antagonized by dopamine D1 receptor selective doses of SCH 23390 but not by D2 receptor selective doses of haloperidol. In this lesioned rat model, A68930 increases 2-deoxyglucose accumulation in the lesioned substantia nigra, pars reticulata. When tested in normal rats, A68930 elicits hyperactivity and, at higher doses, produces a forelimb clonus.

An iodinated ligand identifying the D-1 dopamine receptor

Iodination (with 125I) of SKF 83692 (2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol) produces a ligand of high specific activity. In the caudate nucleus of rat brain, this ligand binds to a site which discriminates between the active and inactive enantiomers of drugs selective for the D-1 receptor but which does not recognize drugs selective for the D-2 dopamine receptor or a serotonin receptor. Non-specific binding of this ligand is less than 5% of the total binding.

Two forms of the rat D2 dopamine receptor as revealed by the polymerase chain reaction

We have used the polymerase chain reaction technique (PCR) to clone the cDNA of the D2 dopamine receptor from rat striatal mRNA. Two major PCR products were produced; one product was identical to a previously published rat cDNA, while the other, more abundant product differed only by an 87‐nucleotide insert located in the region of the putative third cytoplasmic loop of the D2 receptor. A PCR approach for determining message abundance was used to determine the relative message abundance of the two forms of the D2 receptor in a variety of tissues. Possible implications of the two forms of the D2 receptor for dopamine‐mediated signal transduction are discussed.

Lergotrile and lisuride: In vivo dopaminergic agonists which do not stimulate the presynaptic dopamine autoreceptor

Abstract The presynaptic dopaminergic autoreceptor which regulates tyrosine hydroxylase is unaffected by the dopaminergic ergots and their specific antagonists. Apomorphine inhibits the conversion of tyrosine to catechols by striatal synaptosomes; neither lergotrile nor lisuride mimic this action of apomorphine. Furthermore metoclopramide, a potent dopaminergic antagonist in the anterior pituitary, does not block the inhibitory effect of apomorphine. The data demonstrate that the several categories pharmacologically distinct dopamine receptors exist.

YM-09151-2: A potent antagonist for a peripheral D2-dopamine receptor

YM-09151-2 (cis-N-(1-benzyl-2-methylpyrrolidin-3-yl)-5-chloro-2-methyoxy-4-methylaminobenazamide) is a potent antagonist of the dopamine-induced inhibition of adenylate cyclase in the cholera toxin-treated intermediate lobe of the rat pituitary gland. In this system, YN-09151-2 (calculated Ki of 10.5 nM) is equipotent with fluphenazine as a dopamine antagonist. In contrast, YM-09151-2 is not a potent antagonist of the dopamine-induced stimulation of adenylate cyclase activity of fish retina or the dopamine-stimulated accumulation of cAMP in a preparation of dispersed fish retinal tissue. These observations suggest that YM-09151-2 is a potent antagonist of the D2-dopamine receptor (in the classification schema of Kebabian and Calne) and that YM-09151-2 may be a useful tool in pharmacological investigations of the D1- and the D2-receptor.

A-69024: a non-benzazepine antagonist with selectivity for the dopamine D-1 receptor

A-69024 HBr, 1-(2-bromo-4,5-dimethoxybenzyl)-7-hydroxy-6-methoxy-2-methyl-1,2,3,4- tetrahydroisoquinoline hydrobromide, is a selective antagonist of the dopamine D-1 receptor. A-69024 HBr shows an apparent affinity toward the D-1 receptor (identified using [125I]SCH 23390) of 12.6 (4.15-38.3) nM (mean (90% CL), n = 3); the apparent affinity toward the D-2 receptor (identified using [3H]spiroperidol is 1 290 (1,200-1,380) nM (n = 3); using [125I]lysergic acid diethylamine to identify the 5-HT1C receptor gives apparent affinity of 17,800 (9,700-32,600) nM (n = 3). In assays of adenylate cyclase activity, A-69024 HBr antagonizes the D-1 receptor with a calculated affinity of 43.9 (17.5-110) nM (n = 5), while the molecule antagonizes the D-2 receptor with a calculated affinity greater than 400 nM. Behavioral studies demonstrate that A-69024 HBr (5 mg/kg s.c.) is able to block both amphetamine-induced locomotor activity and apomorphine-induced stereotypy. Furthermore, A-69024 HBr blocks SF&F 38393-, but not quinpirole-, induced rotation in rats having unilateral 6-hydroxydopamine lesions of the substantia nigra. When administered at behaviorally effective doses. A-69024 HBr neither increases the concentration of serum prolactin nor potentiates dihydroxyphenylalanine (DOPA) accumulation in the caudate-putamen of rats pretreated with the DOPA decarboxylase inhibitor NSD 1015. Because A-69024 is a dopamine receptor antagonist discriminating between the D-1 and D-2 receptors, it may be a useful research tool.