D1 dopamine receptors can interact with both stimulatory and inhibitory guanine nucleotide binding proteins

Age-Dependent Modulation of Layer V Pyramidal Neuron Excitability in the Mouse Primary Motor Cortex by D1 Receptor Agonists and Antagonists

The primary motor cortex (M1) receives dopaminergic (DAergic) projections from the midbrain which play a key role in modulating motor and cognitive processes, such as motor skill learning. However, little is known at the level of individual neurons about how dopamine (DA) and its receptors modulate the intrinsic properties of the different neuronal subpopulations in M1 and if this modulation depends on age. Using immunohistochemistry, we first mapped the cells expressing the DA D1 receptor across the different layers in M1, and quantified the number of pyramidal neurons (PNs) expressing the D1 receptor in the different layers, in young and adult mice. This work reveals that the spatial distribution and the molecular profile of D1 receptor-expressing neurons (D1+) across M1 layers do not change with age. Then, combining whole-cell patch-clamp recordings and pharmacology, we explored ex vivo in young and adult mice the impact of activation or blockade of D1 receptors on D1+ PN intrinsic properties. While the bath application of the D1 receptor agonist induced an increase in the excitability of layer V PNs both in young and adult, we identified a distinct modulation of intrinsic electrical properties of layer V D1+ PNs by D1 receptor antagonist depending on the age of the animal.

Mini G protein probes for active G protein-coupled receptors (GPCRs) in live cells

G protein–coupled receptors (GPCRs) are key signaling proteins that regulate nearly every aspect of cell function. Studies of GPCRs have benefited greatly from the development of molecular tools to monitor receptor activation and downstream signaling. Here, we show that mini G proteins are robust probes that can be used in a variety of assay formats to report GPCR activity in living cells. Mini G (mG) proteins are engineered GTPase domains of Gα subunits that were developed for structural studies of active-state GPCRs. Confocal imaging revealed that mG proteins fused to fluorescent proteins were located diffusely in the cytoplasm and translocated to sites of receptor activation at the cell surface and at intracellular organelles. Bioluminescence resonance energy transfer (BRET) assays with mG proteins fused to either a fluorescent protein or luciferase reported agonist, superagonist, and inverse agonist activities. Variants of mG proteins (mGs, mGsi, mGsq, and mG12) corresponding to the four families of Gα subunits displayed appropriate coupling to their cognate GPCRs, allowing quantitative profiling of subtype-specific coupling to individual receptors. BRET between luciferase–mG fusion proteins and fluorescent markers indicated the presence of active GPCRs at the plasma membrane, Golgi apparatus, and endosomes. Complementation assays with fragments of NanoLuc luciferase fused to GPCRs and mG proteins reported constitutive receptor activity and agonist-induced activation with up to 20-fold increases in luminescence. We conclude that mG proteins are versatile tools for studying GPCR activation and coupling specificity in cells and should be useful for discovering and characterizing G protein subtype–biased ligands.

G protein-coupled oestrogen receptor 1 (GPER1)/GPR30: a new player in cardiovascular and metabolic oestrogenic signalling

Oestrogens are important sex hormones central to health and disease in both genders that have protective effects on the cardiovascular and metabolic systems. These hormones act in complex ways via both genomic and non-genomic mechanisms. The genomic mechanisms are relatively well characterized, whereas the non-genomic ones are only beginning to be explored. Two oestrogen receptors (ER), ERα and ERβ, have been described that act as nuclear transcription factors but can also associate with the plasma membrane and influence cytosolic signalling. ERα has been shown to mediate both anti-atherogenic effects and pro-survival effects in pancreatic β-cells. In recent years, a third membrane-bound ER has emerged, G protein-coupled receptor 30 or G protein-coupled oestrogen receptor 1 (GPER1), which mediates oestrogenic responses in cardiovascular and metabolic regulation. Both GPER1 knock-out models and pharmacological agents are now available to study GPER1 function. These tools have revealed that GPER1 activation may have several beneficial effects in the cardiovascular system including vasorelaxation, inhibition of smooth muscle cell proliferation, and protection of the myocardium against ischaemia/reperfusion injury, and in the metabolic system including stimulation of insulin release and protection against pancreatic β-cell apoptosis. Thus, GPER1 is emerging as a candidate therapeutic target in both cardiovascular and metabolic disease.

Activation of the dopamine 1 and dopamine 5 receptors increase skeletal muscle mass and force production under non-atrophying and atrophying conditions

BACKGROUND

Control of skeletal muscle mass and force production is a complex physiological process involving numerous regulatory systems. Agents that increase skeletal muscle cAMP levels have been shown to modulate skeletal muscle mass and force production. The dopamine 1 receptor and its closely related homolog, the dopamine 5 receptor, are G-protein coupled receptors that are expressed in skeletal muscle and increase cAMP levels when activated. Thus we hypothesize that activation of the dopamine 1 and/or 5 receptor will increase skeletal muscle cAMP levels thereby modulating skeletal muscle mass and force production.

METHODS

We treated isolated mouse tibialis anterior (TA) and medial gastrocnemius (MG) muscles in tissue bath with the selective dopamine 1 receptor and dopamine 5 receptor agonist SKF 81297 to determine if activation of skeletal muscle dopamine 1 and dopamine 5 receptors will increase cAMP. We dosed wild-type mice, dopamine 1 receptor knockout mice and dopamine 5 receptor knockout mice undergoing casting-induced disuse atrophy with SKF 81297 to determine if activation of the dopamine 1 and dopamine 5 receptors results in hypertrophy of non-atrophying skeletal muscle and preservation of atrophying skeletal muscle mass and force production.

RESULTS

In tissue bath, isolated mouse TA and MG muscles responded to SKF 81297 treatment with increased cAMP levels. Treating wild-type mice with SKF 81297 reduced casting-induced TA and MG muscle mass loss in addition to increasing the mass of non-atrophying TA and MG muscles. In dopamine 1 receptor knockout mice, extensor digitorum longus (EDL) and soleus muscle mass and force was not preserved during casting with SKF 81297 treatment, in contrast to significant preservation of casted wild-type mouse EDL and soleus mass and EDL force with SKF 81297 treatment. Dosing dopamine 5 receptor knockout mice with SKF 81297 did not significantly preserve EDL and soleus muscle mass and force although wild-type mouse EDL mass and force was significantly preserved SKF 81297 treatment.

CONCLUSIONS

These data demonstrate for the first time that treatment with a dopamine 1/5 receptor agonist results in (1) significant preservation of EDL, TA, MG and soleus muscle mass and EDL muscle force production during periods of atrophy and (2) hypertrophy of TA and MG muscle. These effects appear to be mainly mediated by both the dopamine 1 and dopamine 5 receptors.

The calcium-sensing receptor couples to Galpha(s) and regulates PTHrP and ACTH secretion in pituitary cells

The calcium-sensing receptor (CaR or CASR as listed in the MGI Database) is a G protein-coupled receptor that binds and signals in response to extracellular calcium and other polycations. It is highly expressed on parathyroid and kidney cells, where it participates in the regulation of systemic calcium homeostasis. It is also expressed on many other cell types and is involved in a wide array of biological functions such as cell growth and differentiation, ion transport, and hormone secretion. It has been described to couple to several different G proteins including Galpha(i/0), Galpha(q/11), and Galpha(12/13). Recently, it has also been shown to stimulate cAMP production by coupling to Galpha(s) in immortalized or malignant breast cells. The CaR is expressed on cells in the anterior pituitary and had previously been described to stimulate cAMP production in these cells. In this report, we examined signaling from the CaR in murine pituitary corticotroph-derived, AtT-20 cells. We found that CaR activation led to the stimulation of cAMP production, and PTH-related protein (PTHrP or PTHLH as listed in the MGI Database) and ACTH secretion from these cells. Furthermore, manipulation of cAMP levels was able to modulate PTHrP and ACTH secretion independent of changes in extracellular calcium. Finally, we demonstrated that the CaR couples to Galpha(s) in AtT-20 cells. Therefore, in pituitary corticotroph-like cells, as in breast cancer cells, the CaR utilizes Galpha(s) and activates cAMP production to stimulate hormone secretion.

Comparative pharmacology of two D1-like dopamine receptors cloned from the silkworm Bombyx mori

Dopamine (DA) is a physiologically important biogenic amine in insect peripheral and nervous tissues.We recently cloned two DA receptors (BmDopR1 and BmDopR2) from the silkworm Bombyx mori and identified them as D1-like receptors, which activate adenylate cyclase to increase intracellular cAMP levels. In this study, these two receptors were stably expressed in HEK-293 cells, and the dose-responsiveness to DA and their pharmacological properties were examined using cAMP assays. BmDopR1 showed a dose-dependent increase in cAMP levels at DA concentrations up to 10(-7) M with EC(50) of 3.30 nM, while BmDopR2 required 10(-6) M DA for activation. In BmDopR1-expressing cells, DA at 10(-6)-10(-4) M induced 30-50% lower cAMP production than 10(-7) MDA. BmDopR2-expressing cells showed a standard sigmoidal dose-response, with maximum cAMP levels attained with 10(-5)-10(-4) M DA and EC(50) of 1.30 microM. Both receptors had similar agonist profiles, and the typical vertebrate D1-like receptor agonist SKF-38393 was ineffective. Experiments with antagonists revealed that BmDopR1 exhibits D1-like features. However, the pharmacology of BmDopR2 was distinct from D1-like receptors; the typical vertebrate D1-like receptor antagonist SCH-23390 was less potent than the nonselective antagonist flupenthixol and the D2-like receptor antagonist chlorpromazine. The rank order of activities of several antagonists for BmDopR1 and BmDopR2 was more similar to that of Drosophila melanogaster DA receptors than Apis mellifera DA receptors. These data suggest that DA receptors could be potential targets for specific insecticides or insectistatics.

Switching of G-protein usage by the calcium-sensing receptor reverses its effect on parathyroid hormone-related protein secretion in normal versus malignant breast cells

The calcium-sensing receptor (CaR) is a G-protein-coupled receptor that signals in response to extracellular calcium and regulates parathyroid hormone secretion. The CaR is also expressed on normal mammary epithelial cells (MMECs), where it has been shown to inhibit secretion of parathyroid hormone-related protein (PTHrP) and participate in the regulation of calcium and bone metabolism during lactation. In contrast to normal breast cells, the CaR has been reported to stimulate PTHrP production by breast cancer cells. In this study, we confirmed that the CaR inhibits PTHrP production by MMECs but stimulates PTHrP production by Comma-D cells (immortalized murine mammary cells) and MCF-7 human breast cancer cells. We found that changes in intracellular cAMP, but not phospholipase C or MAPK signaling, correlated with the opposing effects of the CaR on PTHrP production. Pharmacologic stimulation of cAMP accumulation increased PTHrP production by normal and transformed breast cells. Inhibition of protein kinase A activity mimicked the effects of CaR activation on inhibiting PTHrP secretion by MMECs and blocked the effects of the CaR on stimulating PTHrP production in Comma-D and MCF-7 cells. We found that the CaR coupled to Galphai in MMECs but coupled to Galphas in Comma-D and MCF-7 cells. Thus, the opposing effects of the CaR on PTHrP production are because of alternate G-protein coupling of the receptor in normal versus transformed breast cells. Because PTHrP contributes to hypercalcemia and bone metastases, switching of G-protein usage by the CaR may contribute to the pathogenesis of breast cancer.

Dopamine D1 receptor coupling to Gs/olf and Gq in rat striatum and cortex: a scintillation proximity assay (SPA)/antibody-capture characterization of benzazepine agonists

Cloned, human dopamine D(1) receptors recruit multiple effectors but the G-protein subtype(s) activated by cerebral populations remain poorly defined, a question addressed using a rapid immunocapture technique. In rat striatum, dopamine (DA) and four selective, benzazepine agonists at D(1) receptors concentration-dependently enhanced [(35)S]GTPgammaS binding to Galphas/olf. For all drugs, Galphaq was also recruited with similar potencies and efficacies. Comparable observations were made in the cortex wherein profiles of Galphas/olf vs Galphaq activation were also highly correlated. In contrast to Galphas/olf and Galphaq, Galphao and Galphai were activated neither in the striatum nor in the cortex, except for SKF82958. As compared to DA, both SKF81297 and SKF82958 were full agonists at Gs/olf and Gq in cortex and striatum, whereas SKF38393 behaved as a partial agonist. Likewise, the "atypical" agonist, SKF83959 only partially activated Galphaq and also Gs/olf in these two regions. In both striatum and cortex, the selective D(1) receptor antagonist, SCH23390, abolished the recruitment of Galphaq and Galphas by DA, and the action of DA was partially attenuated by SKF83959. These findings demonstrate that, in native CNS tissue, DA and other D(1) receptor agonists activate Galphas and Galphaq with similar potencies and efficacies, suggesting their recruitment via pharmacologically-indistinguishable populations of D(1) receptors, and show that SPA technology is well-adapted to study the coupling of native DA receptors.

Ghrelin Amplifies Dopamine Signaling by Cross Talk Involving Formation of Growth Hormone Secretagogue Receptor/Dopamine Receptor Subtype 1 Heterodimers

Our objective is to determine the neuromodulatory role of ghrelin in the brain. To identify neurons that express the ghrelin receptor [GH secretagogue receptor (GHS-R)], we generated GHS-R-IRES-tauGFP mice by gene targeting. Neurons expressing the GHS-R exhibit green fluorescence and are clearly evident in the hypothalamus, hippocampus, cortex, and midbrain. Using immunohistochemistry in combination with green fluorescent protein fluorescence, we identified neurons that coexpress the dopamine receptor subtype 1 (D1R) and GHS-R. The potential physiological relevance of coexpression of these two receptors and the direct effect of ghrelin on dopamine signaling was investigated in vitro. Activation of GHS-R by ghrelin amplifies dopamine/D1R-induced cAMP accumulation. Intriguingly, amplification involves a switch in G protein coupling of the GHS-R from Gα11/q to Gαi/o by a mechanism consistent with agonist-dependent formation of GHS-R/D1R heterodimers. Most importantly, these results indicate that ghrelin has the potential to amplify dopamine signaling selectively in neurons that coexpress D1R and GHS-R.

Regulation of receptor-coupling to (multiple) G proteins. A challenge for basic research and drug discovery

G protein-coupled receptors induce intracellular signals via interaction of with cytosolic/peripheral membrane proteins, mainly G proteins. There has been much debate about the mode of interaction between the receptors, G proteins and effectors, their mobility and the ways of determining the specificity of interaction. Additional complexity has been added to system upon the discovery of i) coupling of single receptors to several G proteins and ii) active direction of this by different ligands (stimulus trafficking). These data suggest that the most primary unit in the signal transduction is the receptor complexed with a specific G protein, making the investigation of the mechanism of receptor-G protein selection and interaction even more important. In this review, I will summarize the general knowledge of receptor interaction with G proteins and effectors and the ways of investigating this.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

Thiomersal enhances the binding of histamine to the H1 receptor, but not histamine-stimulated inositol phosphate formation

Thiomersal (thimerosal) was a weak inhibitor of the binding of [(3)H]mepyramine to histamine H(1) receptors in guinea-pig cerebellar membranes (11 +/- 1% inhibition at 10 microM, 32 +/- 3% inhibition at 300 microM). However, in the concentration range 3-30 microM, thiomersal enhanced the binding of histamine to the H(1) receptor, as reflected by the displacement of curves of histamine inhibition of [(3)H]mepyramine binding to lower concentrations, without any change in the Hill coefficient. The ratio of the IC50 values (the concentration giving 50% inhibition) in the absence and presence of thiomersal increased from 1.8 with 3 microM to 3.6 with 30 microM thiomersal. There was no consistent effect of thiomersal at concentrations of 30 microM and below on curves of mepyramine inhibition of [(3)H]mepyramine binding. In the presence of 10 microM thiomersal histamine-induced accumulation of inositol phosphates in U373 MG astrocytoma cells was partially inhibited (37 +/- 8% inhibition of the maximum response), without any significant change in the EC50 (the concentration giving the half maximal response) for histamine. Thus although histamine binding was potentiated by thiomersal, there was no potentiation of an H(1) receptor-mediated functional response.

Neuropeptide FF receptors couple to a cholera toxin-sensitive G-protein in rat dorsal raphe neurones

In rat dorsal raphe neurones, nociceptin (300 nM) reduced the peak [Ca(2+)](i) transient, triggered by depolarization, by 36.7+/-1.8% (n=46). This effect of nociceptin decreased to 16.7+/-2.9% (n=18) after pre-treatment of the neurones with pertussis toxin (5 microg/ml, 2-6 h) but was unchanged (37.4+/-2.1%, n=44) after pre-incubation with cholera toxin (5 microg/ml, 2-6 h). This suggests that, in dorsal raphe neurones, the ORL1 receptor couples to inhibitory (G(i/o)) G-proteins. The neuropeptide FF analogue, [D-Tyr1, (N-Me)Phe(3)]neuropeptide FF (10, 100, 1000 nM), acted as an anti-opioid and reduced the effect of nociceptin (300 nM, 30 s) by 62.0+/-3.3% (n=28). Following pre-incubation with cholera toxin (5 microg/ml, 2-6 h) [D-Tyr1, (N-Me)Phe3] neuropeptide FF was unable, at the three concentrations tested, to block nociceptin activity. We conclude that, in rat dorsal raphe neurones, neuropeptide FF receptors couple to stimulatory G-proteins (Gs).

Coupling of dopamine receptor subtypes to multiple and diverse G proteins

The family of five dopamine receptors subtypes activate cellular effector systems through G proteins. Historically, dopamine receptors were thought to only stimulate or inhibit adenylyl cyclase, by coupling to either G(s)alpha or G(i)alpha, respectively. Recent studies in transfected cells, reviewed here, have shown that multiple and highly diverse signaling pathways are activated by specific dopamine receptor subtypes. This multiplicity of signaling responses occurs through selective coupling to distinct G proteins and each of the receptors can interact with more than one G protein. Although some of the multiple coupling of dopamine receptors to different G proteins occurs from within the same family of G proteins, these receptors can also couple to G proteins belonging to different families. Such multiple interactions between receptors and G proteins elicits functionally distinct physiological effects which acts to enhance and subsequently suppress the original receptor response, and to activate apparently distinct signaling pathways. In the brain, where coexpression of functionally distinct receptors in heterogeneous cells further adds to the complexity of dopamine signaling, minor alterations in receptor/G protein coupling states during either development or in adults, may underlie the imbalanced signaling seen in dopaminergic-linked diseases such as schizophrenia, Parkinson's disease and attention deficit hyperactivity disorder.

Pharmacological and molecular evidence for dopamine D(1) receptor expression by striatal astrocytes in culture

The neurotransmitter dopamine (DA) at a 10 microM concentration elicited a stimulation of intracellular cyclic AMP (cAMP) accumulation in cultured astrocytes derived from embryonic rat striatum. This accumulation was partially blocked by the beta-adrenergic receptors antagonist propranolol, mimicked by the D(1) agonist SKF 38393 and by the mixed D(1)/D(2) agonist apomorphine. A regional heterogeneity in the magnitude of dopamine-induced cAMP accumulation was observed in cultured astrocytes obtained from different brain areas. The maximum effect was observed in striatal astrocytes, a lower effect in cortical astrocytes, and no increase was detected in cerebellar astrocytes. Reverse transcription-polymerase chain reaction (RT-PCR) coupled to Southern blot hybridization demonstrated that striatal astrocytes express only D(1) receptor mRNA and Western blot analysis confirmed the expression of the D(1) receptor protein in striatal astrocytes. In contrast to what found in neurons, the D(1)-dependent cAMP formation in striatal astrocytes is partially reduced by pertussis toxin (PTX) treatment. The stimulation of D(1) receptors or the activation of adenylyl cyclase by forskolin led to an increase of cytosolic and nuclear protein kinase A (PKA) catalytic activity. The presence of dopamine D(1) receptors in cultured striatal astrocytes suggests a role of dopamine in the regulation of cellular processes in striatal astrocytes.

Coupling of D1 and D5 dopamine receptors to multiple G proteins: Implications for understanding the diversity in receptor-G protein coupling

Dopamine receptors are a subclass of the super family of G protein-coupled receptors, that transduce their effects by coupling to specific G proteins. Within the dopamine receptor family, the adenylyl cyclase stimulatory receptors include the D1 and D5 subtypes. The D1 and D5 dopamine receptors are genetically distinct, sharing >80% sequence homology within the highly conserved seven transmembrane spanning domains, but displaying only 50% overall homology at the amino acid level. When expressed in transfected GH4C1 rat pituitary cells, both D1 and D5 receptors stimulate adenylyl cyclase and have identical affinities toward dopaminergic agonists and antagonists. In order to analyze specific signaling pathways mediated by activation of either D1 or D5 receptors, we have identified the G proteins that are coupled to these receptors. Through functional analyses and competition binding studies, and from immunoprecipitation techniques, using antisera against the various alpha subunits of G proteins, we have established that both D1 and D5 receptors couple to G(s)alpha. In addition, D1 receptors are also coupled to G(o)alpha. Since G(o)alpha has been implicated in the regulation of Ca2+, K+, and Na+ channels, this finding would suggest that D1 receptors can mediate the functional activity of these ion channels. There is also evidence to indicate that D5 receptors couple to G(z)alpha, a novel G protein abundantly expressed in neurons. Thus, despite similar pharmacological properties, such differential coupling of D1 and D5 receptors to G proteins other than G(s)alpha, indicates that dopamine can transduce varied signaling responses upon the simultaneous stimulation of both these receptors.

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.

Dysfunctional D1A receptor-G-protein coupling in proximal tubules of spontaneously hypertensive rats is not due to abnormal G-proteins

BACKGROUND

Dysfunctional dopamine neurotransmission and defective D1A receptor-G protein coupling exist in renal proximal tubules (RPT) of the spontaneously hypertensive rat (SHR).

OBJECTIVE

To determine whether the G proteins in SHR are abnormal, preventing formation of agonist high affinity sites in SHR.

METHODS

We examined the expression levels of the alpha-subunits of G proteins, as well as D1A receptor receptor coupling to exogenously added normal G proteins, in RPT of SHR and the normotensive Wister-Kyoto (WKY) rat.

RESULTS

In the presence of 110 mmol/l NaCl, the D1A dopamine receptor-selective agonist SKF R-38393 binds both to high- and to low-affinity sites on solubilized and reconstituted D1A receptors extracted from renal proximal tubules of normotensive Wistar-Kyoto (WKY) rats. In the spontaneously hypertensive rat (SHR), SKF R-38393 bound to a single site on the reconstituted receptor with affinity values corresponding to the low-affinity state of the receptor. Western blot analyses indicated that the alpha-subunit of the guanine nucleotide binding protein (G-protein), Gs, was expressed at similar levels, whereas G(o)alpha was not expressed in proximal tubule membranes from WKY rats and SHR. Pretreatment of proximal tubule membranes with the alkylating agent N-ethylmaleimide in the presence of SKF R-38393 inactivated alpha-subunits of endogenous G-proteins, but not D1A receptors, resulting in loss of high-affinity binding sites in WKY rats. These N-ethylmaleimide-treated D1A receptors from WKY rats, when reconstituted with exogenous sources of G-proteins, were able to couple to these exogenous G-proteins, with complete restoration of high-affinity sites. Moreover, the affinity values and the proportion of these hybrid sites were similar to those of untreated receptors, and these affinity sites were regulated by guanine nucleotide analogs. Reconstitution of D1A receptors from SHR with the same exogenous G-proteins failed to similarly induce formation of the high-affinity binding sites in the hybrid reconstituted systems, and SKF R-38393 continued to bind in a single low-affinity state of the receptor.

CONCLUSION

These results demonstrate that the absence of G-protein coupling in SHR is due to intrinsic defects within the receptor protein, rather than to any abnormalities of the endogenous G-proteins themselves.

Coupling of human D-1 dopamine receptors to different guanine nucleotide binding proteins. Evidence that D-1 dopamine receptors can couple to both Gs and G(o)

Coupling between D-1 dopamine receptors and G proteins in cell lines expressing human D-1 receptors and different G proteins was examined. Pertussis toxin (PTX) treatment of rat pituitary GH4C1 cells significantly reduced, but did not abolish, agonist high affinity binding sites of the D-1 dopamine receptor; in SK-N-MC neuroblastoma cells, PTX failed to have any effect on D-1 high affinity sites. Cholera toxin (CTX) treatment of GH4C1 cells reduced but did not abolish the high affinity sites of D-1 receptors, while in SK-N-MC cells, treatment with CTX abolished all the high affinity sites. Western blot analyses with specific antisera indicated that Gs alpha, Gi1 alpha, Gi3 alpha, and Gq alpha were expressed in both cell lines, while Gi2 alpha and G(o) alpha were expressed in GH4C1 but not SK-N-MC cells. Antisera NEI-805 (anti-Gs alpha) and 9072 (anti-G(o) alpha) immunoprecipitated 24 +/- 4.3 and 34.4 +/- 6.9%, respectively, of G protein-associated D-1 dopamine receptors. Antisera 3646 (anti-Gi1 alpha), 1521 (anti-Gi2 alpha), 1518 (anti-Gi3 alpha), and 0941 (anti-Gq alpha) failed to coimmunoprecipitate appreciable levels of soluble receptors. These data indicate that D-1 dopamine receptors are coupled to both Gs alpha and G(o) alpha but not to Gq alpha.

Quantitative autoradiography of 5-HT1E binding sites in rodent brains: effect of lesion of serotonergic neurones

Binding sites corresponding to 5-HT1E receptors were labelled in mouse, rat, and guinea-pig brains by using [3H]5-hydroxytryptamine ([3H]5-HT) in the presence of 5-carboxamidotryptamine (5-CT) (0.1 microM), and their distribution within the brain was studied by quantitative autoradiography. The results obtained with mouse brain show that 5-HT1E binding sites are particularly present in the cortex, caudate-putamen and claustrum, where they showed the highest density. Lower densities were measured in other regions. Saturation experiments showed that the affinity of [3H]5-HT for 5-HT1E binding sites (nanomolar range) was very similar in the different structures. The distribution of 5-HT1E binding sites was similar in rat and guinea-pig brains. In rat brain, selective lesioning of serotonergic fibres by intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT), a specific 5-HT neurotoxin, did not affect the density of 5-HT1E binding, indicating that these receptors are mainly localized on non-serotonergic neurones.

Direct coupling of opioid receptors to both stimulatory and inhibitory guanine nucleotide-binding proteins in F-11 neuroblastoma-sensory neuron hybrid cells

Evidence is presented for linkage of opioid receptors directly to the stimulatory G protein (guanine nucleotide-binding protein), Gs, in addition to the generally accepted linkage to the inhibitory and "other" G proteins, gi and Go, in F-11 (neuroblastoma-dorsal root ganglion neuron) hybrid cells. Treatment of intact F-11 cells with cholera toxin decreased specific binding of the opioid agonist [D-Ala2,D-Leu5]enkephalin to F-11 cell membranes by 35%, with the remaining binding retaining high affinity for agonist. Under these conditions cholera toxin influenced the alpha subunit of Gs (Gs alpha) but had no effect on the alpha subunit of Gi/o (Gi/o alpha), based on ADP-ribosylation studies. Pertussis toxin treatment decreased high-affinity opioid agonist binding by about 50%; remaining binding was also of high affinity, even though pertussis toxin had inactivated Gi/o alpha selectively and essentially completely. Simultaneous treatment with both toxins had an additive effect, reducing specific binding by about 80%. While opioid agonists inhibited forskolin-stimulated adenylate cyclase activity of F-11 cells as expected, opioids also stimulated basal adenylate cyclase activity, indicative of interaction with Gs as well as Gi. Cholera toxin treatment attenuated opioid-stimulation of basal adenylate cyclase, whereas pertussis toxin treatment enhanced stimulation. In contrast, inhibition by opioid of forskolin-stimulated activity was attenuated by pertussis toxin but not by cholera toxin. It is concluded that a subset of opioid receptors may be linked directly to Gs and thereby mediate stimulation of adenylate cyclase. This Gs-adenylate cyclase interaction is postulated to be responsible for the novel excitatory electrophysiologic responses to opioids found in our previous studies of sensory neurons and F-11 cells.

CNS signal transduction in the pathophysiology and pharmacotherapy of affective disorders and schizophrenia

Until recently, research on the neurochemical basis of affective disorders (AD) and schizophrenia (SCZ) focused on detecting postulated disturbances in presynaptic neurotransmitter release and metabolism, or postsynaptic receptor function. New insights into the molecular mechanisms involved in the propagation of neurotransmitter signals across biological membranes and in the regulation of neuronal responses have allowed the development of novel hypotheses, which may explain the altered postsynaptic neuroreceptor responsivity thought to be integral to the pathophysiology of these disorders. In this review we evaluate evidence from both basic science and clinical research implicating disturbances in postreceptor signal transduction in the pathophysiology and pharmacotherapy of AD and SCZ. Specific findings regarding potential postreceptor sites of pathophysiology are highlighted in each of these disorders, together with the growing body of data on the possible postreceptor loci of psychotropic drug action, especially lithium and antidepressants.

Effects of chronic treatment of haloperidol and clozapine on levels of G-protein subunits in rat striatum

Chronic administration of typical neuroleptic drugs, such as haloperidol, causes the supersensitivity of brain dopamine D2 receptor in striatum and limbic regions, while the atypical neuroleptic clozapine does not. In order to understand the mechanism of their action at a molecular level, studies were carried out to assess the effects of chronic treatment of these drugs on the levels of G-proteins in the rat striatum using the Western blot method. Results of the present study demonstrate that the treatment with haloperidol or clozapine, respectively, down-regulate or up-regulate the levels of G proteins. Quantitative immunoblotting, using site-directed specific antisera, demonstrated that chronic treatment with haloperidol down-regulates Gi alpha, Gs alpha, and beta subunits while chronic treatment with clozapine upregulates Gi alpha, Gs alpha, and beta subunits. Neither of these drugs has any effect on the levels of Go alpha.

Persistent defective coupling of dopamine-1 receptors to G proteins after solubilization from kidney proximal tubules of hypertensive rats

The natriuretic effect of dopamine-1 (DA-1) agonists is reduced in spontaneously hypertensive rat (SHR), partly because of defective DA-1 receptor-adenylate cyclase (AC) coupling in renal proximal convoluted tubules. To investigate this defective coupling, DA-1 dopamine receptors from renal proximal tubules were solubilized and reconstituted into phospholipid vesicles. The binding of DA-1-selective ligand [125I]SCH 23982 was specific and saturable, with no differences in receptor density or Kd between SHR and normotensive rats (Wistar-Kyoto rats; WKY). Competition experiments of the reconstituted DA-1 dopamine receptors in WKY with a DA-1-selective agonist, SKF R-38393, revealed the presence of high- (Kh = 350 +/- 209 nM) and low-affinity (Kl = 70,500 +/- 39,500 nM) binding sites. 100 microM Gpp(NH)p abolished the agonist high-affinity sites, converting them to a low-affinity state (Ki = 33,650 +/- 10,850 nM). In SHR, one affinity site was noted (Ki = 13,800 +/- 500) and was not modulated by Gpp(NH)p (Ki = 11,505 +/- 2,295). The absence of guanine nucleotide-sensitive agonist high-affinity sites may explain the defective DA-1/AC coupling mechanism in the SHR.