Pharmacology of high-threshold calcium currents in GH4C1 pituitary cells and their regulation by activation of human D2 and D4 dopamine receptors

Dopamine receptor signaling and current and future antipsychotic drugs

All currently efficacious antipsychotic drugs have as part of their mechanism the ability to attenuate some or all of the signaling through the dopamine D(2) receptor. More recently, the dopamine D(1) receptor has been hypothesized to be a promising target for the treatment of negative and/or cognitive aspects of schizophrenia that are not improved by current antipsychotics. Although cAMP has been presumed to be the primary messenger for signaling through the dopamine receptors, the last decade has unveiled a complexity that has provided exciting avenues for the future discovery of antipsychotic drugs (APDs). We review the signaling mechanisms of currently approved APDs at dopamine D(2) receptors, and note that aripiprazole is a compound that is clearly differentiated from other approved drugs. Although aripiprazole has been postulated to cause dopamine stabilization due to its partial D(2) agonist properties, a body of literature suggests that an alternative mechanism, functional selectivity, is of primary importance. Finally, we review the signaling at dopamine D(1) receptors, and the idea that drugs that activate D(1) receptors may have use as APDs for improving negative and cognitive symptoms. We address the current state of drug discovery in the D(1) area and its relationship to novel signaling mechanisms. Our conclusion is that although the first APD targeting dopamine receptors was discovered more than a half-century ago, recent research advances offer the possibility that novel and/or improved drugs will emerge in the next decade.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Third generation antipsychotic drugs: partial agonism or receptor functional selectivity?

Functional selectivity is the term that describes drugs that cause markedly different signaling through a single receptor (e.g., full agonist at one pathway and antagonist at a second). It has been widely recognized recently that this phenomenon impacts the understanding of mechanism of action of some drugs, and has relevance to drug discovery. One of the clinical areas where this mechanism has particular importance is in the treatment of schizophrenia. Antipsychotic drugs have been grouped according to both pattern of clinical action and mechanism of action. The original antipsychotic drugs such as chlorpromazine and haloperidol have been called typical or first generation. They cause both antipsychotic actions and many side effects (extrapyramidal and endocrine) that are ascribed to their high affinity dopamine D(2) receptor antagonism. Drugs such as clozapine, olanzapine, risperidone and others were then developed that avoided the neurological side effects (atypical or second generation antipsychotics). These compounds are divided mechanistically into those that are high affinity D(2) and 5-HT(2A) antagonists, and those that also bind with modest affinity to D(2), 5-HT(2A), and many other neuroreceptors. There is one approved third generation drug, aripiprazole, whose actions have been ascribed alternately to either D(2) partial agonism or D(2) functional selectivity. Although partial agonism has been the more widely accepted mechanism, the available data are inconsistent with this mechanism. Conversely, the D(2) functional selectivity hypothesis can accommodate all current data for aripiprazole, and also impacts on discovery compounds that are not pure D(2) antagonists.

D1/D2-dopamine receptor agonist dihydrexidine stimulates inspiratory motor output and depresses medullary expiratory neurons

It is now accepted that dopamine plays an important neuromodulatory role in the central nervous control of respiration. D1, D2, and D4 subtypes of the receptor seem to be important players, but the assignment of various respiratory tasks to specific subtypes of the dopamine receptor is a work in progress. In the present investigation, dihydrexidine (DHD), a full dopamine receptor agonist with affinity for both D1- and D2-subtypes of receptor, was tested for its effects on inspiratory neurons and motor output and on membrane potential properties of medullary bulbospinal expiratory augmenting expiratory neurons in the pentobarbital anesthetized adult cat. The effects of DHD were compared with those of the highly selective D1-dopamine receptor (D1R) agonists SKF-38393 and 6-chloro-APB. DHD increased the intensity and duration of inspiratory motor output. Phrenic nerve discharge intensity was increased and prolonged, contributing to elevated inspiratory effort and duration when spontaneous breathing was monitored with tracheal pressure measurements. Intracellular recording from rostral medullary inspiratory neurons revealed that DHD, like SKF-38393, increases and prolongs inspiratory phase membrane depolarization, resulting in a longer and more intense discharge of action potentials. Remarkably, DHD had opposite effects on Aug-E neurons. Membrane potential was hyperpolarized, and action potential discharges were suppressed or abolished. In association with reduction of discharge intensity, action potential half width was reduced and after-hyperpolarization increased. The stimulatory action of DHD on inspiratory motor output is attributed to D1R effects, while the depression of Aug-E neurons seems to be linked to D2R actions on the postsynaptic membrane.

Dopamine inhibits basal prolactin release in pituitary lactotrophs through pertussis toxin-sensitive and -insensitive signaling pathways

Dopamine D2 receptors signal through the pertussis toxin (PTX)-sensitive G(i/o) and PTX-insensitive G(z) proteins, as well as through a G protein-independent, beta-arrestin/glycogen synthase kinase-3-dependent pathway. Activation of these receptors in pituitary lactotrophs leads to inhibition of prolactin (PRL) release. It has been suggested that this inhibition occurs through the G(i/o)-alpha protein-mediated inhibition of cAMP production and/or G(i/o)-betagamma dimer-mediated activation of inward rectifier K(+) channels and inhibition of voltage-gated Ca(2+) channels. Here we show that the dopamine agonist-induced inhibition of spontaneous Ca(2+) influx and release of prestored PRL was preserved when cAMP levels were elevated by forskolin treatment. We further observed that dopamine agonists inhibited both spontaneous and depolarization-induced Ca(2+) influx in untreated but not in PTX-treated cells. This inhibition was also observed in cells with blocked inward rectifier K(+) channels, suggesting that the dopamine effect on voltage-gated Ca(2+) channel gating is sufficient to inhibit spontaneous Ca(2+) influx. However, agonist-induced inhibition of PRL release was only partially relieved in PTX-treated cells, indicating that dopamine receptors also inhibit exocytosis downstream of voltage-gated Ca(2+) influx. The PTX-insensitive step in agonist-induced inhibition of PRL release was not affected by the addition of wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and lithium, an inhibitor of glycogen synthase kinase-3, but was attenuated in the presence of phorbol 12-myristate 13-acetate, which inhibits G(z) signaling pathway in a protein kinase C-dependent manner. Thus, dopamine inhibits basal PRL release by blocking voltage-gated Ca(2+) influx through the PTX-sensitive signaling pathway and by desensitizing Ca(2+) secretion coupling through the PTX-insensitive and protein kinase C-sensitive signaling pathway.

Dopamine D2 receptor partial agonists display differential or contrasting characteristics in membrane and cell-based assays of dopamine D2 receptor signaling

Clinical evidence suggests that dopamine D(2) receptor partial agonists must have a sufficiently low intrinsic activity to be effective as antipsychotics. Here, we used dopamine D(2) receptor signaling assays to compare the in vitro functional characteristics of the antipsychotic aripiprazole with other dopamine D(2) receptor partial agonists (7-{3-[4-(2,3-dimethylphenyl)-piperazinyl]propoxy}-2(1H)-quinolinone [OPC-4392], (-)-3-(3-hydroxy-phenyl)-N-n-propylpiperidine [(-)3-PPP] and (+)terguride) and dopamine D(2) receptor antagonists. Aripiprazole and OPC-4392 were inactive in a guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding assay using Chinese Hamster Ovary (CHO) cell membranes expressing cloned human dopamine D(2Long) (hD(2L)) receptors, whereas (-)3-PPP and (+)terguride displayed low intrinsic activity. Aripiprazole also had no effect on [(35)S]GTPgammaS binding to CHO-hD(2L) cells, while OPC-4392, (-)3-PPP and (+)terguride were partial agonists. In contrast, aripiprazole, OPC-4392, (-)3-PPP, and (+)terguride were inactive in a [(35)S]GTPgammaS binding assay using rat striatal membranes. However, at a more downstream level of CHO-hD(2L) cell signalling, these drugs all behaved as dopamine hD(2L) receptor partial agonists, with aripiprazole displaying an intrinsic activity 2 to 3-fold lower (inhibition of forskolin-induced adenosine 3',5'-cyclic monophosphate accumulation) and almost half as high (enhancement of adenosine triphosphate-stimulated [(3)H]arachidonic acid release) as OPC-4392, (-)3-PPP and (+)terguride. Dopamine activity was blocked in each case by (-)raclopride, which was inactive on its own in every assay, as were the antipsychotics haloperidol, olanzapine, ziprasidone and clozapine. Together, these data, whilst preclinical in nature, are consistent with clinical evidence suggesting the favorable antipsychotic profile of aripiprazole, compared with the other clinically ineffective partial agonists, is dependent on its low intrinsic activity at dopamine D(2) receptors. This study also highlights the limitations of using [(35)S]GTPgammaS binding assays to identify dopamine D(2) receptor partial agonists.

Signaling Mechanisms of the D3Dopamine Receptor

A substantial body of evidence shows the capacity of the dopamine D3 receptor to couple functionally to G proteins when expressed in an appropriate milieu in heterologous expression systems. In these systems, activation of D3 receptors inhibits adenylate cyclase, modulates ion flow through potassium and calcium channels, and activates kinases, most notably mitogen-activated protein kinase. Coupling to Gi/Go is implicated in many of these effects, but other G proteins may contribute. Studies with chimeric receptors implicate the third intracellular loop in the mediation of agonist-induced signal transduction. Finally, D3-preferring drugs modulate expression of c-fos in neuronal cultures and brain. Signaling mechanisms of the D3 receptor in brain, however, remain to be definitively determined.

Dopamine Receptor Signaling

The D1-like (D1, D5) and D2-like (D2, D3, D4) classes of dopamine receptors each has shared signaling properties that contribute to the definition of the receptor class, although some differences among subtypes within a class have been identified. D1-like receptor signaling is mediated chiefly by the heterotrimeric G proteins Galphas and Galphaolf, which cause sequential activation of adenylate cyclase, cylic AMP-dependent protein kinase, and the protein phosphatase-1 inhibitor DARPP-32. The increased phosphorylation that results from the combined effects of activating cyclic AMP-dependent protein kinase and inhibiting protein phosphatase 1 regulates the activity of many receptors, enzymes, ion channels, and transcription factors. D1 or a novel D1-like receptor also signals via phospholipase C-dependent and cyclic AMP-independent mobilization of intracellular calcium. D2-like receptor signaling is mediated by the heterotrimeric G proteins Galphai and Galphao. These pertussis toxin-sensitive G proteins regulate some effectors, such as adenylate cyclase, via their Galpha subunits, but regulate many more effectors such as ion channels, phospholipases, protein kinases, and receptor tyrosine kinases as a result of the receptor-induced liberation of Gbetagamma subunits. In addition to interactions between dopamine receptors and G proteins, other protein:protein interactions such as receptor oligomerization or receptor interactions with scaffolding and signal-switching proteins are critical for regulation of dopamine receptor signaling.

Dopamine receptors for every species: gene duplications and functional diversification in Craniates

The neuromodulatory effects of dopamine on the central nervous system of craniates are mediated by two classes of G protein-coupled receptors (D1 and D2), each comprising several subtypes. A systematic isolation and characterization of the D1 and D2-like receptors was carried out in most of the Craniate groups. It revealed that two events of gene duplications took place during vertebrate evolution, before or simultaneously to the emergence of Gnathostomes. It led to the conservation of two-to-four paralogous receptors (subtypes), depending on the species. Additional duplication of dopamine receptor gene occurred independently in the teleost fish lineage. Duplicated genes were maintained in most of the vertebrate groups, certainly by the acquisition of a few functional characters, specific of each subtypes, as well as by discrete changes in their expression territories in the brain. The evolutionary scenario elaborated from these data suggests that receptor gene duplications were the necessary conditions for the expansion of vertebrate forebrain to occur, allowing dopamine systems to exert their fundamental role as modulator of the adaptive capabilities acquired by vertebrate species.

Functional coupling of the human dopamine D2 receptor with G alpha i1, G alpha i2, G alpha i3 and G alpha o G proteins: evidence for agonist regulation of G protein selectivity

(1) The human dopamine D(2long) (D(2L)) receptor was expressed with four different G proteins in Sf9 cells using the baculovirus expression system. When co-expressed with G(i)/G(o) G proteins (G(i1)alpha, G(i2)alpha, G(i3)alpha, or G(o)alpha, plus Gbeta(1) and Ggamma(2)), the receptor displayed a high-affinity binding site for the agonists (dopamine and NPA), which was sensitive to GTP (100 micro M), demonstrating interaction between the receptor and the different G proteins. (2) The receptor to G protein ratio (R : G ratio) was evaluated using [(3)H]-spiperone saturation binding (R) and [(35)S]-GTPgammaS saturation binding (G). R : G ratios of 1 : 12, 1 : 3, 1 : 14 and 1 : 5 were found for G(i1), G(i2), G(i3), and G(o) preparations, respectively. However, when R : G ratios of 1 : 2 and 1 : 12 were compared for G(i2) and G(o), no difference was found for the stimulation of [(35)S]-GTPgammaS binding. (3) Several agonists were tested for their ability to stimulate [(35)S]-GTPgammaS binding to membranes co-expressing the receptor and various G proteins. All the compounds tested showed agonist activity in preparations expressing G(i3) and G(o). However, for G(i2) and G(i1) preparations, compounds such as S-(-)-3-PPP and p-tyramine were unable to stimulate [(35)S]-GTPgammaS binding. (4) Most of the compounds showed higher relative efficacies (compared to dopamine) and higher potencies in the preparation expressing G(o). Comparison of the effects of different agonists in the different preparations showed that each agonist differentially activates the four G proteins. (5) We conclude that the degree of selectivity of G protein activation by the D(2L) receptor can depend on the conformation of the receptor stabilised by an agonist.

Dopamine-D2S receptor inhibition of calcium influx, adenylyl cyclase, and mitogen-activated protein kinase in pituitary cells: distinct Galpha and Gbetagamma requirements

The G protein specificity of multiple signaling pathways of the dopamine-D2S (short form) receptor was investigated in GH4ZR7 lactotroph cells. Activation of the dopamine-D2S receptor inhibited forskolin-induced cAMP production, reduced BayK8644- activated calcium influx, and blocked TRH-mediated p42/p44 MAPK phosphorylation. These actions were blocked by pretreatment with pertussis toxin (PTX), indicating mediation by G(i/o) proteins. D2S stimulation also decreased TRH-induced MAPK/ERK kinase phosphorylation. TRH induced c-Raf but not B-Raf activation, and the D2S receptor inhibited both TRH-induced c-Raf and basal B-Raf kinase activity. After PTX treatment, D2S receptor signaling was rescued in cells stably transfected with individual PTX-insensitive Galpha mutants. Inhibition of adenylyl cyclase was partly rescued by Galpha(i)2 or Galpha(i)3, but Galpha(o) alone completely reconstituted D2S-mediated inhibition of BayK8644-induced L-type calcium channel activation. Galpha(o) and Galpha(i)3 were the main components involved in D2S-mediated p42/44 MAPK inhibition. In cells transfected with the carboxyl-terminal domain of G protein receptor kinase to inhibit Gbetagamma signaling, only D2S-mediated inhibition of calcium influx was blocked, but not inhibition of adenylyl cyclase or MAPK. These results indicate that the dopamine-D2S receptor couples to distinct G(i/o) proteins, depending on the pathway addressed, and suggest a novel Galpha(i)3/Galpha(o)-dependent inhibition of MAPK mediated by c-Raf and B-Raf-dependent inhibition of MAPK/ERK kinase.

G protein preferences for dopamine D2 inhibition of prolactin secretion and DNA synthesis in GH4 pituitary cells

Dopamine is the primary inhibitory regulator of lactotroph proliferation and prolactin (PRL) secretion in vivo, acting via dopamine D2 receptors (short D2S and long D2L forms). In GH4C1 pituitary cells transfected with D2S or D2L receptor cDNA, dopamine inhibits PRL secretion and DNA synthesis. These actions were blocked by pertussis toxin, implicating G(i)/G(o) proteins. To address roles of specific G(i)/G(o)4 proteins in these actions a series of GH4C1 cell lines specifically depleted of individual Galpha subunits was examined. D2S-mediated inhibition of BayK8644-stimulated PRL secretion was primarily dependent on G(o) over G(i), as observed for BayK8644-induced calcium influx. By contrast, inhibitory coupling of the D2S receptor to TRH-induced PRL secretion was partially impaired by depletion of any single G protein, but especially G(i)3. Inhibitory coupling of D2L receptors to PRL secretion required G(o), but not G(i)2, muscarinic receptor coupling was resistant to depletion of any G(i)/G(o) protein, whereas the 5-HT1A and somatostatin receptors required G(i)2 or G(i)3 for coupling. The various receptors also demonstrated distinct G protein requirements for inhibition of DNA synthesis: depletion of any G(i)/G(o) subunit completely uncoupled the D2S receptor, the D2L receptor was uncoupled by depletion of G(i)2, and muscarinic and somatostatin receptors were resistant to depletion of G(i)2 only. These results demonstrate distinct receptor-G protein preferences for inhibition of TRH-induced PRL secretion and DNA synthesis.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

Amphetamine blocks long-term synaptic depression in the ventral tegmental area

The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse are thought to usurp the normal functioning of this pathway. A growing body of evidence suggests that glutamatergic synapses on dopamine neurons in the ventral tegmental area (VTA) are modified during exposure to addictive drugs, producing sensitization, a progressive augmentation in the rewarding properties of psychostimulant drugs with repeated exposure. We have tested the hypothesis that psychostimulant exposure interferes with the synaptic plasticity of glutamatergic inputs to the VTA. We find that excitatory synapses onto VTA dopamine neurons exhibit long-term depression (LTD) in response to low-frequency stimulation and modest depolarization. LTD in the VTA is NMDA receptor-independent but is dependent on intracellular Ca(2+) and can be induced by driving Ca(2+) into the dopamine neuron. Brief exposure to amphetamine entirely blocks LTD at glutamatergic synapses in the VTA, by releasing endogenous dopamine that acts at D2 dopamine receptors. The block of LTD is selective, because amphetamine has no effect on hippocampal LTD. The LTD we have discovered in the VTA is likely to be an important component of excitatory control of the reward pathway; amphetamine will inhibit LTD, removing this normal brake on the glutamatergic drive to dopamine neurons. This effect of amphetamine represents an important mechanism by which normal function of the brain reward system may be impaired during substance abuse.

Amphetamine blocks long-term synaptic depression in the ventral tegmental area

The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse are thought to usurp the normal functioning of this pathway. A growing body of evidence suggests that glutamatergic synapses on dopamine neurons in the ventral tegmental area (VTA) are modified during exposure to addictive drugs, producing sensitization, a progressive augmentation in the rewarding properties of psychostimulant drugs with repeated exposure. We have tested the hypothesis that psychostimulant exposure interferes with the synaptic plasticity of glutamatergic inputs to the VTA. We find that excitatory synapses onto VTA dopamine neurons exhibit long-term depression (LTD) in response to low-frequency stimulation and modest depolarization. LTD in the VTA is NMDA receptor-independent but is dependent on intracellular Ca(2+) and can be induced by driving Ca(2+) into the dopamine neuron. Brief exposure to amphetamine entirely blocks LTD at glutamatergic synapses in the VTA, by releasing endogenous dopamine that acts at D2 dopamine receptors. The block of LTD is selective, because amphetamine has no effect on hippocampal LTD. The LTD we have discovered in the VTA is likely to be an important component of excitatory control of the reward pathway; amphetamine will inhibit LTD, removing this normal brake on the glutamatergic drive to dopamine neurons. This effect of amphetamine represents an important mechanism by which normal function of the brain reward system may be impaired during substance abuse.

Class A calcium channel variants in pancreatic islets and their role in insulin secretion

The initiation of insulin release from rat islet beta cells relies, in large part, on calcium influx through dihydropyridine-sensitive (alpha1D) voltage-gated calcium channels. Components of calcium-dependent insulin secretion and whole cell calcium current, however, are resistant to L-type channel blockade, as well as to omega-conotoxin GVIA, a potent inhibitor of alpha1B channels, suggesting the expression of additional exocytotic calcium channels in the islet. We used a reverse transcription-polymerase chain reaction-based strategy to ascertain at the molecular level whether the alpha1A calcium channel isoform was also present. Results revealed two new variants of the rat brain alpha1A channel in the islet with divergence in a putative extracellular domain and in the carboxyl terminus. Using antibodies and cRNA probes specific for alpha1A channels, we found that the majority of cells in rat pancreatic islets were labeled, indicating expression of the alpha1A channels in beta cells, the predominant islet cell type. Electrophysiologic recording from isolated islet cells demonstrated that the dihydropyridine-resistant current was sensitive to the alpha1A channel blocker, omega-agatoxin IVA. This toxin also inhibited the dihydropyridine-resistant component of glucose-stimulated insulin secretion, suggesting functional overlap among calcium channel classes. These findings confirm the presence of multiple high voltage-activated calcium channels in the rat islet and implicate a physiologic role for alpha1A channels in excitation-secretion coupling in beta cells.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Effects of dopamine on L-type Ca2+ current in single atrial and ventricular myocytes of the rat

1. The effects of dopamine on the L-type Ca2+ current (ICa,L) of both atrial and ventricular single myocytes and on the force of contraction of atrial trabeculae in rat heart were investigated. 2. Dopamine increased atrial ICa,L at concentrations higher than 1 microM, but had little or no effect on ICa,L at lower concentrations. The increase in ICa,L at high concentrations was reversed by propranolol and acetylcholine, but not by phentolamine. Activation and inactivation kinetics of ICa,L were not altered by dopamine. 3. In rat ventricular myocytes in which the D4 receptor mRNA does not express, dopamine (20-100 microM) also increased the ICa,L amplitude and propranolol reversed this effect. 4. Clozapine, a potent D4 receptor antagonist, blocked the augmenting effect of dopamine on ICa,L. However, this effect could be explained by beta-antagonism, since clozapine also inhibited the isoprenaline effect. 5. In the atrial trabeculae, the increase in contraction by dopamine (1 to 30 microM) was reversed by 1 microM propranolol, but not by 2 microM phentolamine. Low doses of dopamine (0.01 to 0.3 microM) did not affect the contraction in the controls or during a modest stimulation of the beta-adrenoceptor with 0.01 microM isoprenaline. 6. These results indicate that the positive inotropic action of dopamine is mediated through direct stimulation of the beta-adrenoceptor in both atrial and ventricular myocytes. Involvement of D4 receptor appears unlikely in the regulation of the atrial contraction.

Schizophrenia and L-745,870, a novel dopamine D4 receptor antagonist

The discovery of a novel high-affinity and selective dopamine D4 receptor antagonist, L-745,870, and the results of clinical trials with this compound are reviewed. Despite several lines of evidence which suggest that a selective D4 receptor antagonist may be an effective antipsychotic agent with a lower propensity to induce extrapyramidal side-effects, L-745,870 was ineffective as an antipsychotic in humans.

Inhibition of the T-type Ca2+ current by the dopamine D1 receptor in rat adrenal glomerulosa cells: requirement of the combined action of the G betagamma protein subunit and cyclic adenosine 3',5'-monophosphate

Modulation of ionic Ca2+ currents by dopamine (DA) could play a pivotal role in the control of steroid secretion by the rat adrenal glomerulosa cells. In the present study, we report that DA decreases the T-type Ca2+ current amplitude in these cells. The use of pharmacological agonists and antagonists reveals that this effect is mediated by activation of the D1-like receptors. Modulation by cAMP is complex inasmuch as preincubation of the cells with 8-Br-cAMP or the specific adenylyl cyclase inhibitor, 2',3'-dideoxyadenosine, have no effect per se, but prevent the DA-induced inhibition. The inhibitory effect of DA was abolished by addition of GDPbetaS to the pipette medium but not by pertussis toxin. If a cell is dialyzed with medium containing G alpha(s)-GDP, the inhibitory effect is reduced and cannot be recovered by the addition of GTPgammaS, indicating that the alpha(s) is not involved, but rather the betagamma-subunit. Indeed, DA-induced inhibition was mimicked by G betagamma in the pipette and 8-Br-cAMP in the bath. Similarly, G betagamma release from the activation of the AT1 receptor of angiotensin II did affect the current amplitude only in the presence of 8-Br-cAMP in the bath. The mitogen-activated protein kinase cascade, which can be activated by receptors coupled to Gs, was not involved as shown by the lack of activation of p42mapk by DA and the absence of effect of the mitogen-activated protein kinase inhibitor, PD 098059, on the DA-induced inhibition. Because the binding of G betagamma-subunits to various effectors involves the motif QXXER, we therefore tested the effect of the QEHA peptide on the inhibition of the T-type Ca2+ current induced by DA. The peptide, added to the medium pipette (200 microM), abolished the effect of DA. We conclude that the presence of the G betagamma and an increase in cAMP concentration are both required to inhibit the T-type Ca2+ current in rat adrenal glomerulosa cells.

Review the role of dopamine D4 receptors in schizophrenia and antipsychotic action

For the past 20 years the most enduring explanation for schizophrenia has been the dopamine hypothesis, which proposes that the dopaminergic system is overactive in this widespread disease. Classically, the D2 receptor formed the core of the dopamine hypothesis since there was considerable evidence for elevations of D2 receptor levels in the brains of schizophrenic patients, and because these receptors served as the primary target in mediating antipsychotic effects of most neuroleptics. However, the dopamine D4 receptor has recently received particular attention in this context. This is because the atypical antipsychotic, clozapine, which is effective in treating refractory schizophrenics without the side-effect profile of typical neuroleptics, displays a 10-fold higher affinity for D4 compared to D2 or D3 receptors. Furthermore, the concentration in plasma water of clinical doses ofclozapine correlates well with its in vitro binding affinity for D4, but not D2 or D3 receptors, suggesting that D4 is a potential target in mediating clozapine's antipsychotic effects. As well, marked elevations in the level of a D4-like site (not identical to the D4 receptor) has been seen in the striatum of postmortem schizophrenic brains, but not in control brains. Finally, the most interesting feature of the D4 receptor is perhaps the array of polymorphisms associated with it, creating structural diversity in this receptor that supercedes all other known catecholamine receptors. The existence of these D4 polymorphisms raises the possibility that structural variations of this receptor may be associated with an increased susceptibility to schizophrenia, or observed variations in individual response to clozapine treatment. However, several studies aimed at investigating these hypotheses could not establish a direct role of D4 in schizophrenia. Furthermore, no association was evident between the polymorphic forms of D4 and susceptibility to schizophrenia, or variable clozapine response. Nevertheless, investigations surrounding this receptor has been far from futile. The observations which support the idea that D4 might serve as a target for clozapine have significantly modified and extended our understanding of mechanisms underlying atypical antipsychotic treatment of schizophrenia, as well as the dopamine hypothesis for schizophrenia. Further characterization of this receptor may prove to be crucial in designing highly effective antipsychotic drugs with minimal contraindications.

D2 dopamine receptors reduce N-type Ca2+ currents in rat neostriatal cholinergic interneurons through a membrane-delimited, protein-kinase-C-insensitive pathway

Dopamine has long been known to regulate the activity of striatal cholinergic interneurons and the release of acetylcholine. Yet, the cellular mechanisms by which this regulation occurs have not been elucidated. One way in which dopamine might act is by modulating voltage-dependent Ca2+ channels. To test this hypothesis, the impact of dopaminergic agonists on Ca2+ channels in neostriatal cholinergic interneurons was studied by combined whole cell voltage-clamp recording and single-cell reverse transcription-polymerase chain reactions. Cholinergic interneurons were identified by the presence of choline acetyltransferase mRNA. Nearly, all interneurons tested (90%, n = 17) coexpressed D2 (short and long isoforms) and D1b (D5) dopamine receptor mRNAs. D1a receptor mRNA was found in only a small subset (20%) of the sample and D3 and D4 receptor mRNAs were undetectable. D2 receptor agonists rapidly and reversibly reduced N-type Ca2+ currents. D1b/D1a receptor activation had little or no effect on Ca2+ currents. The D2 receptor antagonist sulpiride blocked the effect of D2 agonists. Dialysis with guanosine-5'-O-(2-thiodiphosphate) or brief exposure to the G protein (Gi/o) alkylating agent N-ethylmaleimide also blocked the D2 modulation. The reduction in N-type currents was neither accompanied by kinetic slowing nor significantly reversed by depolarizing prepulses. The D2 receptor effects were mediated by a membrane-delimited pathway, because the modulation was not seen in cell-attached patches when agonist was applied to the bath and was not disrupted by perturbations in cytosolic signaling pathways known to be linked to D2 receptors. Activation of M2 muscarinic receptors occluded the D2 modulation, suggesting a shared signaling element. However, activation of protein kinase C attenuated the M2 modulation without significantly affecting the D2 modulation. Taken together, our results suggest that activation of D2 dopamine receptors in cholinergic interneurons reduces N-type Ca2+ currents via a membrane-delimited, Gi/o class G protein pathway that is not regulated by protein kinase C. This signaling pathway may underlie the ability of D2 receptors to reduce striatal acetylcholine release.

In vitro pharmacological profile of YM-43611, a novel D2-like receptor antagonist with high affinity and selectivity for dopamine D3 and D4 receptors

1. We investigated some neurochemical properties of a novel benzamide, YM-43611, [(S)-N-(1-benzyl-3-pyrrolidinyl)-5-chloro-4-cyclopropylcarbonylamino+ ++-2- methoxybenzamide] in comparison with putative D2-like receptor antagonists using both rat and human cloned dopamine D2-like receptors in vitro. 2. Receptor binding studies revealed that YM-43611 had appropriately potent affinities for both rat and human D2-like receptors, with moderate selectivity for D3 receptors and high selectivity for D4 receptors over D2 receptors (Ki values (nM) for rat receptors: D2, 165; D3, 35.5; D4, 1.85, and for human receptors: D2, 42.9; D3, 11.2; D4, 2.10). 3. YM-43611 displayed weak or negligible affinity for other neurotransmitter receptors, namely D1, D5, alpha(1), alpha(2), beta, 5-HT1A, 5-HT2A, 5-HT3, H1, M1 and M2 receptors. 4. Dopamine stimulated low-Km GTPase activity on membranes from Chinese hamster ovary (CHO) cells expressing the human D2-like receptor subtype. This response to dopamine of low-Km GTPase activity was inhibited by use of putative D2-like receptor antagonists. YM-43611 showed a moderate selectivity for D3 receptors (Ki = 45.5 nM) and a high selectivity for D4 receptors (Ki = 3.28 nM) over D2 receptors (Ki = 70.6 nM). 5. Dopamine inhibited forskolin-stimulated adenylate cyclase in intact CHO cells expressing the human D2-like receptor subtype. YM-43611 shifted the inhibition curve of dopamine on respective D2-like receptor subtype-mediated cyclic AMP formation to the right in a parallel fashion, showing a pA2 value of 7.42 (38.1 nM) for D2 receptors, a pKB value of 8.06 (8.68 nM) for D3 receptors, and a pA2 value of 8.42 (3.77 nM) for D4 receptors. 6. YM-43611 but not the other D2-like receptor antagonists exhibited good selectivity with respect to dual antagonism for D3 and D4 receptors in both receptor binding and functional assays. 7. These results indicate that YM-43611 is a novel D2-like receptor antagonist with high potency and selectivity for both D3 and D4 receptors. YM-43611 is therefore expected to be valuable in exploration of the physiological role of D3 and D4 receptors.

Dopamine receptors and brain function

In the central nervous system (CNS), dopamine is involved in the control of locomotion, cognition, affect and neuroendocrine secretion. These actions of dopamine are mediated by five different receptor subtypes, which are members of the large G-protein coupled receptor superfamily. The dopamine receptor subtypes are divided into two major subclasses: the D1-like and D2-like receptors, which typically couple to Gs and Gj mediated transduction systems. In the CNS, the various receptor subtypes display specific anatomical distributions, with D1-like receptors being mainly post-synaptic and D2-like receptors being both pre- and post-synaptic. D1 and D2 dopamine receptors, the most abundant subtypes in the CNS, appear to be expressed largely in distinct neurons. Substance P and dynorphin, which are expressed in D1 receptor-containing neurons, as well as pre-proenkephalin in D2 receptor-containing neurons, have been used as monitors of dopaminergic activity in the CNS. Expression of immediate early genes, in particular fos, has also been found to correlate with dopaminergic transmission. Dopamine released from the hypothalamus controls the synthesis and secretion of prolactin from the anterior pituitary via D2 dopamine receptors. As yet none of the dopamine receptor subtypes have been associated with the etiology of psychotic disorders, such as schizophrenia. However, the recent characterization of D3 and D4 receptors which are, interestingly, expressed in areas of the CNS mediating cognition and affect or showing increased affinity for certain neuroleptics, have renewed the interest and hope of finding effective neuroleptics devoid of side effects. Finally, the recent production of genetically-derived animals lacking several of these receptor genes should help elucidate which specific physiological paradigms the receptors mediate.

Activation of dopamine D4 receptor inhibits an L-type calcium current in cerebellar granule cells

The functions of the D4 receptor, a newly cloned D2-like receptor, as well as the identity of cells expressing it, are still poorly defined. Using quantitative polymerase chain reaction we detected the messenger RNA of the D4, but not other D2-like receptor, in cultured granule cells from neonatal rat cerebellum. In these neurons, dopamine reduced high-voltage-activated calcium current, with a pharmacology corresponding to that of the D4 receptor. The response declined from one to three days, when calcium currents were mostly sensitive to nifedipine, to 15 days, when nifedipine-insensitive calcium currents were also present and D4 receptor messenger RNA had declined. The dopamine response was abolished after pretreatment of the cells by pertussis toxin, was potentiated and made irreversible by infusion of guanosine 5'-O-(3-thiotriphosphate) but persisted in the presence of cyclic AMP and isobutylmethylxanthine. These results indicate the presence in the neonatal cerebellum of a functional D4 receptor inhibiting an L-type calcium current, an action involving a Gi/Go protein but independent from adenylate cyclase inhibition.

Novel dopamine receptors half a decade later

Only half a decade ago, the effects of dopamine were all attributed to activation of two receptor subtypes, the D1 and D2, with opposing effects on adenylate cyclase, and for which apparently selective ligands were available. From the end of 1988, however, the application of homology cloning techniques starting from sequences of the seven transmembrane domain catecholamine receptors, particularly that of the D2 receptor, led to the identification of 'novel', previously uncharacterized dopamine receptors. In this article, Pierre Sokoloff and Jean-Charles Schwartz discuss the functional significance of such diversity, as well as the new therapeutic perspectives it offers.

Phosphorylation- and voltage-dependent inhibition of neuronal calcium currents by activation of human D2(short) dopamine receptors

1. Activation of human D2(s) dopamine receptors with quinpirole (10 nM) inhibits omega-conotoxin GVIa-sensitive, high-threshold calcium currents when expressed in differentiated NG108-15 cells (55% inhibition at +10 mV). This inhibition was made irreversible following intracellular dialysis with the non-hydrolysable guanosine triphosphate analogue GTP-gamma-S (100 microM), and was prevented by pretreatment with pertussis toxin (1 microgram ml-1 for 24 h). 2. Stimulation of protein kinase C with the diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol (100 microM), also attenuated the inhibition of the sustained calcium current but did not affect the receptor-mediated decrease in rate of current activation. Similarly, okadaic acid (100 nM), a protein phosphatase 1/2A inhibitor, selectively occluded the inhibition of the sustained current. 3. The depression of calcium currents by quinpirole (10 nM) was enhanced following intracellular dialysis with 100 microM cyclic adenosine monophosphate (cyclic AMP, 72.8 +/- 9.8% depression), but was not mimicked by the membrane permeant cyclic GMP analogue, Sp-8-bromoguanosine-3',5':cyclic monophosphorothioate (100 microM). 4. Inhibition of calcium currents was only partly attenuated by 100 ms depolarizing prepulses to +100 mV immediately preceding the test pulse. However, following occlusion of the sustained depression with okadaic acid (100 nM) the residual kinetic slowing was reversed in a voltage-dependent manner (P < 0.05). 5. Thus pertussis toxin-sensitive G-proteins liberated upon activation of human D2(short) dopamine receptors inhibited high-threshold calcium currents in two distinct ways. The decrease in rate of calcium current activation involved a voltage-dependent pathway, whereas the sustained inhibition of calcium current involved, in part, the voltage-resistant phosphorylation by cyclic AMP-dependent protein kinases and subsequent dephosphorylation by protein phosphatases 1/2A.