Mechanisms of signal transduction at the dopamine D2 receptor

Both A1 and A2a purine receptors regulate striatal acetylcholine release

The receptors responsible for the adenosine-mediated control of acetylcholine release from immunoaffinity-purified rat striatal cholinergic nerve terminals have been characterized. The relative affinities of three analogues for the inhibitory receptor were (R)-phenylisopropyladenosine greater than cyclohexyladenosine greater than N-ethylcarboxamidoadenosine (NECA), with binding being dependent of the presence of Mg2+ and inhibited by 5'-guanylylimidodiphosphate [Gpp(NH)p] and adenosine receptor antagonists. Adenosine A1 receptor agonists inhibited forskolin-stimulated cholinergic adenylate cyclase activity, with an IC50 of 0.5 nM for (R)-phenylisopropyladenosine and 500 nM for (S)-phenylisopropyladenosine. A1 agonists inhibited acetylcholine release at concentrations approximately 10% of those required to inhibit the cholinergic adenylate cyclase. High concentrations (1 microM) of adenosine A1 agonists were less effective in inhibiting both adenylate cyclase and acetylcholine release, due to the presence of a lower affinity stimulatory A2 receptor. Blockade of the A1 receptor with 8-cyclopentyl-1,3-dipropylxanthine revealed a half-maximal stimulation by NECA of the adenylate cyclase at 10 nM, and of acetylcholine release at approximately 100 nM. NECA-stimulated adenylate cyclase activity copurified with choline acetyltransferase in the preparation of the cholinergic nerve terminals, suggesting that the striatal A2 receptor is localized to cholinergic neurones. The possible role of feedback inhibitory and stimulatory receptors on cholinergic nerve terminals is discussed.

Further evidence against the coupling of dopamine receptors to phosphoinositide hydrolysis in rat striatum

The effects of D1 and D2 dopamine receptor agonists on phosphoinositide hydrolysis were studied by measuring the accumulation of radioactive inositol phosphates in slices of rat corpus striatum prelabelled with [3H]inositol. All assays were performed in the presence of lithium. Neither the D1 receptor agonist SKF 38393 nor the D2 receptor agonist quinpirole, alone or in combination, had an effect on basal accumulation of inositol phosphates. The muscarinic receptor agonist carbachol produced a robust increase in the accumulation of inositol monophosphate and a smaller increase in the accumulation of inositol bisphosphate. These effects were not altered by the presence of quinpirole. Additionally, quinpirole also had no effect when assays were conducted in the presence of the muscarinic receptor antagonist scopolamine, the glutamic acid receptor antagonist kynurenic acid, and the antioxidant glutathione. These results are discussed in relation to recent contradictory reports and lend support to the position that D2 dopamine receptors are not coupled to phosphoinositide hydrolysis in rat striatum.

Evidence that a guanine nucleotide-binding protein linked to a muscarinic receptor inhibits directly phospholipase C

The mechanism of phospholipase C regulation by inhibitory receptors was analyzed both in intact and in permeabilized rat thyroid cells (FRTL5). In this system, the muscarinic agonist carbachol inhibited phospholipase C, as indicated by the decrease in the basal levels of inositol 1,4,5-trisphosphate as well as by the reduced adrenergic stimulation of phosphoinositol accumulation, which was paralleled by a fall in the cytosolic Ca2+ levels. This inhibition involved an M2 muscarinic receptor because it was abolished by atropine but not by the M1 antagonist pirenzepine. Cells pretreated with pertussis toxin were not responsive to carbachol, indicating the involvement of a guanine nucleotide-binding protein in this inhibitory process. This possibility was further evaluated in permeabilized cells, where the carbachol inhibition was shown to be completely dependent on GTP. Known second messengers were not involved in this inhibitory process since Ca2+, cAMP, and activators of protein kinases were not able to mimic or prevent the carbachol effect either in intact or in permeabilized FRTL5 cells. In this system, the phospholipases C and A2 are coupled to two classes of muscarinic receptors that display a different sensitivity to pertussis toxin. The carbachol inhibitory effect occurred under conditions that prevented activation of phospholipase A2, excluding a role of the arachidonic acid metabolism in this process. Taken together these data provide the strongest support to date that an inhibitory guanine nucleotide-binding protein sensitive to pertussis toxin can directly mediate receptor-induced inhibition of phospholipase C.

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.

Cloning of two additional catecholamine receptors from rat brain

An approach based on the polymerase chain reaction (PCR) was used to isolate additional members of the G-linked receptor family from a rat striatal lambda gtII cDNA library. Priming with one degenerate probe corresponding to highly conserved consensus sequences in the third transmembrane (TM) domain of 15 G-linked receptors and sequences in the phage vector resulted in one clone (G-13) encoding a dopamine D2 receptor variant with a 29 amino acid insert in the third cytoplasmic loop. In addition, the amino acid sequence encoded by clone G-36 contained conserved sequences characteristic of the G-linked class of receptors and displayed sequence homology in TM domains with the beta 2-adrenergic receptor (48%). Two conserved serine residues in TM5 postulated to be part of a ligand binding site in the adrenergic receptor, suggests that G-36 encodes a catecholaminergic receptor. Northern blot analysis confirmed the expression of G-36 in rat brain, but not in kidney, heart and lung. Several strong hybridizing bands to G-36 were obtained in both human and rat genomic DNA. The general PCR strategy employed here should prove to be extremely useful for the isolation of other members of the G-linked receptor family.

Expression of striatal D1 dopamine receptors coupled to inositol phosphate production and Ca2+ mobilization in Xenopus oocytes

Expression of central nervous system receptors for dopamine was examined by injection of poly(A)+ RNA (mRNA) from rat striatum into oocytes from Xenopus laevis. Electrophysiological measurements in mRNA-injected oocytes indicated that addition of 100 microM dopamine induced an inward current (40-100 nA) that was consistent with the activation of endogenous Ca2(+)-dependent Cl- channels. This current was also elicited by addition of the selective D1 agonist SKF 38393 but not by the selective D2 agonist quinpirole. Prior addition of the dopaminergic antagonist cis-piflutixol completely abolished dopamine-induced currents but had no effect on currents produced by serotonin. Using 45Ca2+ efflux assays, addition of 100 microM dopamine to injected oocytes stimulated efflux 2- to 3-fold. This increase was mimicked by SKF 38393 and was blocked by the D1-selective antagonist (+)SCH 23390 but not by the D2-selective antagonist domperidone. No increase in 45Ca2+ efflux was seen with 100 microM quinpirole. Size fractionation of striatal mRNA yielded a single peak (2.5-3.0 kilobases) of D1 receptor-mediated 45Ca2+ efflux activity in injected oocytes. In addition, dopamine stimulation of oocytes injected with peak fractions and prelabeled with myo-[3H]inositol caused a 3-fold increase in [3H]inositol 1,4,5-triphosphate [( 3H]InsP3) formation. No effect on [3H]InsP3 production or 45Ca2+ efflux was observed, however, in injected oocytes incubated with 1 mM N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate. Thus, in addition to D1 receptors that stimulate adenylyl cyclase, rat striatum contains D1 receptors that can couple to InsP3 formation and mobilization of intracellular Ca2+.

mu-Opioid receptors and not kappa-opioid receptors are coupled to the adenylate cyclase in the cerebellum

The putative regulatory effect of opioids on adenylate cyclase was investigated in two different preparations containing, respectively, two different populations of opioid receptors: the rabbit cerebellum (greater than 75% mu-opioid receptors) and the guinea pig cerebellum (greater than 80% kappa-opioid receptors). In the mu-preparation, but not in the kappa-preparation, opioids inhibited the basal and the forskolin-stimulated adenylate cyclase activity in a dose-dependent manner and stereospecifically. The inhibition was in the 20-30% range, required the presence in the assay medium of Mg2+ and of GTP, but was independent of the presence of Na+. Pharmacological characterization of the inhibitory response in the rabbit cerebellum clearly showed that it was under the control of a mu-opioid binding site, with the effect being elicited by non-selective (etorphine and morphine) and mu-selective (Tyr-D-Ala-Gly-Me-Phe-Gly-ol) agonists, whereas delta- and kappa-selective agonists were almost totally ineffective. ADP ribosylation of inhibitory GTP-binding protein by pertussis toxin failed to block the inhibitory effect of opioids, and data presented suggest that this failure is likely to be the consequence of a limited access of the toxin to its substrate in rabbit cerebellum membranes.

Muscarinic receptor activation attenuates D2 dopamine receptor mediated inhibition of acetylcholine release in rat striatum: indications for a common signal transduction pathway

In the present investigations, we used a superfusion system to study the effect of simultaneous activation of D2 dopamine receptors and so-called muscarinic "autoreceptors" on the K(+)-evoked in vitro release of [3H]acetylcholine from rat striatal tissue slices. Activation of D2 receptors with the selective agonist LY 171555 (0.01-1 microM) clearly decreased the evoked release of [3H]acetylcholine. This effect was markedly attenuated in the presence of either the selective muscarinic receptor agonist oxotremorine (3 microM) or the cholinesterase inhibitor physostigmine (1 microM). Conversely, D2 receptor activation with LY 171555 (1 microM) completely abolished the muscarinic receptor mediated inhibition of evoked [3H]acetylcholine release induced by oxotremorine (0.03-10 microM). These results show that the inhibitory effects of D2 dopamine receptor and muscarinic receptor activation on striatal acetylcholine release are non-additive and therefore are interdependent processes. In addition, we investigated some aspects of the signal transduction mechanism by which the muscarinic receptor mediates inhibition of K(+)-evoked in vitro release of [3H]acetylcholine from rat striatal tissue slices. It appeared that the effect of muscarinic receptor activation was not significantly influenced either by a lowering of the extracellular Ca2+ concentration from the usual 1.2-0.12 mM or by an increase of the intracellular cyclic adenosine-3',5'-monophosphate content. However, increasing extracellular K+ strongly decreased the inhibition of evoked [3H]acetylcholine release mediated by activation of muscarinic receptors. This set of results indicates that the muscarinic "autoreceptor" mediates the decrease of depolarization induced [3H]acetylcholine release from rat striatum to a large extent through stimulation of K+ efflux (opening of K+ channels) in a cyclic adenosine-3',5'-monophosphate independent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

D-2 dopamine autoreceptor selective drugs: do they really exist?

The catecholamine dopamine plays an important role as a neurotransmitter or neurohormone in the brain and pituitary gland. Dopamine exerts its effects through activation of two types of receptors called D-1 and D-2. These receptors are distinguished by their different pharmacological characteristics and signal transduction mechanism(s). Release of dopamine inhibits the activity of dopaminergic neurons through activation of so-called dopamine autoreceptors which are of the D-2 type. In general, these receptors occur both in the soma-dendritic region of the dopaminergic neuron, where they are involved in the inhibition of the firing rate and on the dopaminergic terminals where they mediate the inhibition of dopamine synthesis and release. D-2 receptors occur also on the target cells of dopaminergic neurons both in the brain (postsynaptic D-2 receptors) and pituitary gland. On the basis of data gathered from in vivo (behavioral- as well as electrophysiological) studies it has been concluded that D-2 agonists are much more potent at dopamine autoreceptors as compared to postsynaptic D-2 receptors, indicating the possibility of a pharmacological distinction between these differentially located D-2 receptors. This concept led to the introduction of a whole group of drugs allegedly displaying a selective agonist profile at the dopamine autoreceptor. In contrast, biochemical (in vitro) studies with brain tissue as well as the pituitary gland, did not reveal any significant difference between the pharmacological profiles of autoreceptors and postsynaptic D-2 receptors. In the present minireview a balanced discussion is presented of these in vivo and in vitro findings and it is concluded that both autoreceptors as well as postsynaptic D-2 receptors are similar if not identical entities.

The role of DA1- and DA2-receptors in the control of blood pressure

1. Endogenous dopamine, acting through specific receptors on blood vessels, renal tubules and adrenal cortical cells, may play a role in the development and maintenance of the hypertensive state. 2. In hypertensive patients, activation of the DA1-receptor with intravenous fenoldopam produces a rapid and sustained reduction in blood pressure, in contrast to the tachyphylaxis seen in some rat models. 3. Activation of the DA2-receptor also represents a viable antihypertensive approach, based on the chronic efficacy of hydergine. However, none of the recently developed selective DA2-receptor agonists has been shown to reduce blood pressure in man. 4. Since animal experiments suggest qualitatively different antihypertensive profiles for quinpirole and SK&F 101468, both presumed to be selective agonists at the DA2-receptor, it may be possible to design new DA2-receptor agonists more effective as antihypertensive drugs. 5. Dopamine receptors at different sites, even if of the same subtype, can be differentially regulated; this may be a consequence of activation of multiple second messenger systems by receptor occupation.

Presence of D2-dopamine receptors in human term placenta

We studied the binding of [3H]-spiperone on human term placental membranes. This binding reached plateau level after 30 min incubation at 37 degrees C and was reversed (t1/2 approximately 5 min) by addition of an excess of unlabeled spiperone. Scatchard analysis of saturation experiments with increasing doses of [3H]-spiperone (0-25 nM) showed one class of high affinity binding sites with a dissociation constant (Kd) of 14 +/- 2 nM and a maximal binding capacity (Bmax) of 222 +/- 9 fmoles/mg protein. The affinity of 5 competitors was determined in competitive binding assays. The D2-dopamine antagonists were the most potent inhibitors: Ki for spiperone and haloperidol were 8 +/- 2 and 56 +/- 22 nM respectively. Dopamine inhibited [3H]-spiperone binding with a Ki of 570 +/- 50 microM whereas Schering 23390 (D1 antagonist) and propranolol (beta-adrenergic antagonist) were without effect. The binding was also inhibited by 100 microM GTP gamma S (38 +/- 8% inhibition), indicating that the dopamine receptor is coupled with a GTP binding protein. These results demonstrate for the first time the presence of D2-dopamine receptors in human placenta.

Multiple D2 dopamine receptors produced by alternative RNA splicing

Dopamine receptor belong to a large class of neurotransmitter and hormone receptors that are linked to their signal transduction pathways through guanine nucleotide binding regulatory proteins (G proteins). Pharmacological, biochemical and physiological criteria have been used to define two subcategories of dopamine receptors referred to as D1 and D2. D1 receptors activate adenylyl cyclase and are coupled with the Gs regulatory protein. By contrast, activation of D2 receptors results in various responses including inhibition of adenylyl cyclase, inhibition of phosphatidylinositol turnover, increase in K+ channel activity and inhibition of Ca2+ mobilization. The G protein(s) linking the D2 receptors to these responses have not been identified, although D2 receptors have been shown to both copurify and functionally reconstitute with both Gi and Go related proteins. The diversity of responses elicited by D2-receptor activation could reflect the existence of multiple D2 receptor subtypes, the identification of which is facilitated by the recent cloning of a complementary DNA encoding a rat D2 receptor. This receptor exhibits considerable amino-acid homology with other members of the G protein-coupled receptor superfamily. Here we report the identification and cloning of a cDNA encoding an RNA splice variant of the rat D2 receptor cDNA. This cDNA codes for a receptor isoform which is predominantly expressed in the brain and contains an additional 29 amino acids in the third cytoplasmic loop, a region believed to be involved in G protein coupling.

Cloning of the cDNA and gene for a human D2 dopamine receptor

A clone encoding a human D2 dopamine receptor was isolated from a pituitary cDNA library and sequenced. The deduced protein sequence is 96% identical with that of the cloned rat receptor with one major difference: the human receptor contains an additional 29 amino acids in its putative third cytoplasmic loop. Southern blotting demonstrated the presence of only one human D2 receptor gene. Two overlapping phage containing the gene were isolated and characterized. DNA sequence analysis of these clones showed that the coding sequence is interrupted by six introns and that the additional amino acids present in the human pituitary receptor are encoded by a single exon of 87 base pairs. The involvement of this sequence in alternative splicing and its biological significance are discussed.

ADP and, indirectly, ATP are potent inhibitors of cAMP production in intact isoproterenol-stimulated C6 glioma cells

When added to intact C6 glioma cells in the micromolar range of concentrations, ADP and ATP induce an inhibition of the isoproterenol-elicited cAMP responses. ATP is rapidly hydrolyzed by the ectonucleotidases present on these cells, with an apparent Km of 50 microM and a Vmax of 1.1 nmol/min/10(6) cells. cAMP responses are also inhibited by millimolar concentrations of either ATP in the presence of an ATP-regenerating system to prevent ADP accumulation or AMP-PCP. These observations show that, in C6 glioma cells, ADP is a more potent inhibitor of cAMP production than ATP, the latter acting indirectly, via its rapid hydrolysis to ADP. The additive inhibition of isoproterenol-elicited cAMP responses induced, on one hand, by the treatment of the cells with a phorbol ester and by addition of ADP to the cells, and, on the other hand, by the progressive disappearance of the effects of ADP and ATP when cells are treated with increasing concentrations of Pertussis toxin, demonstrate that ADP and ATP exert their action in C6 glioma cells via a P2 purinoceptor probably negatively coupled to adenylate cyclase and a G regulatory protein.

Receptors that inhibit phosphoinositide breakdown

Calcium-mobilizing receptors are believed to activate phospholipase C. Joel Linden and Thérèse Mary Delahunty summarize recent reports which indicate that activation of some receptors that inhibit the accumulation of Ca2+ within cells - notably receptors for adenosine, dopamine and several other neurotransmitters - can inhibit phosphoinositide metabolism. Two types of mechanism may be involved in these responses. Many instances of receptor-mediated inhibition of phosphoinositide breakdown can be detected only after a period of several minutes and may be secondary to receptor-mediated events that lower [Ca2+]i or activate certain protein kinases. In other instances the activation of receptors rapidly (within seconds) inhibits phosphoinositide breakdown, possibly via the activation of guanine nucleotide binding proteins that either directly, or by a rapid indirect action, inhibit phospholipase C. Putative mechanisms for direct and indirect regulation of phospholipase C are discussed.