Distinct roles for Galphai2, Galphai3, and Gbeta gamma in modulation offorskolin- or Gs-mediated cAMP accumulation and calcium mobilization by dopamine D2S receptors

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.

Growth hormone-induced diacylglycerol and ceramide formation via Galpha i3 and Gbeta gamma in GH4 pituitary cells. Potentiation by dopamine-D2 receptor activation

Growth hormone (GH) secretion is regulated by indirect negative feedback mechanisms. To address whether GH has direct actions on pituitary cells, lipid signaling in GH(4)ZR(7) somatomammotroph cells was examined. GH (EC(50) = 5 nm) stimulated diacylglycerol (DAG) and ceramide formation in parallel by over 10-fold within 15 min and persisting for >3 h. GH-induced DAG/ceramide formation was blocked by pertussis toxin (PTX) implicating G(i)/G(o) proteins and was potentiated 1.5-fold by activation of G(i)/G(o)-coupled dopamine-D2S receptors, which had no effect alone. Following PTX pretreatment, only PTX-resistant Galpha(i)3, not Galpha(o) or Galpha(i)2, rescued GH-induced DAG/ceramide signaling. GH-induced DAG/ceramide formation was also blocked in cells expressing Gbetagamma blocker GRK-ct. In GH(4)ZR(7) cells, GH induced phosphorylation of JAK2 and STAT5, which was blocked by PTX and mimicked by ceramide analogue C2-ceramide or sphingomyelinase treatment to increase endogenous ceramide. We conclude that in GH(4) pituitary cells, GH induces formation of DAG/ceramide via a novel Galpha(i)3/Gbetagamma-dependent pathway. This novel pathway suggests a mechanism for autocrine feedback regulation by GH of pituitary function.

Feeding induced by food deprivation is differentially reduced by G-protein alpha-subunit antisense probes in rats

Antisense oligodeoxynucleotide (AS ODN) probes directed against the alpha-subunit of different G-proteins have been used to differentiate feeding responses in rats elicited by different opioid agonists, including morphine, beta-endorphin and dynorphin. Furthermore, antisense probes directed against G(o)alpha, but not G(s)alpha, G(q)alpha or G(i)alpha, significantly reduced nocturnal feeding in rats. The present study examined whether food intake and weight changes elicited by 24 h of food deprivation were significantly altered by ventricular administration of antisense probes directed against either G(i)alpha(1), G(i)alpha(2), G(i)alpha(3), G(s)alpha, G(o)alpha, G(q)alpha or G(x/z)alpha as well as a control nonsense probe in rats. Deprivation-induced weight loss was significantly enhanced by antisense probes directed against G(s)alpha and G(x/z)alpha, whereas weight recovery 24 h following reintroduction of food was significantly reduced by antisense probes directed against G(i)alpha(2), G(q)alpha and G(o)alpha. Selective antisense probe effects were noted for deprivation-induced intake with G(s)alpha and G(q)alpha probes exerting the greatest reductions, G(x/z)alpha, G(i)alpha(2), and G(i)alpha(3) probes exerting lesser effects, and G(i)alpha(1) and G(o)alpha probes failing to affect deprivation-induced intake. Importantly, the nonsense control probe failed to alter deprivation-induced intake or weight. The reductions in deprivation-induced intake by AS ODN probes directed against G(s)alpha or G(q)alpha were not accompanied by any evidence of a conditioned taste aversion. These data indicate important distinctions between G-protein mediation of different effector signaling pathways mediating feeding responses elicited under natural (e.g. nocturnal feeding) and regulatory challenge (e.g. food deprivation) conditions.

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.

Activation of nuclear Ca(2+)/calmodulin-dependent protein kinase II and brain-derived neurotrophic factor gene expression by stimulation of dopamine D2 receptor in transfected NG108-15 cells

We identified the isoforms of Ca(2+) /calmodulin-dependent protein kinase II (CaM kinase II) subunits in rat striatum. All four subunits of CaM kinase II alpha, beta, gamma and delta were detected including the isoforms of alphaB, gammaA, gammaA', gammaA.B, delta3 and delta7 with nuclear localization signal. We established NG108-15 cells with the stably expressed dopamine D2L receptor (D2LR, long form), which is an alternative splicing variant. The cells were termed NGD2L. Immunostaining demonstrated that D2LR was localized in plasma membranes. Calcium imaging with fluo-3 AM revealed that quinpirole, a D2R agonist, increased the intracellular Ca(2+), which was blocked by treatment with sulpiride and pertussis toxin in NGD2L cells, but not in mock cells. Furthermore, stimulation of D2LR with quinpirole in NGD2L cells activated the nuclear isoform of CaM kinase II. Stimulation of D2LR increased the expression of exon III- and IV-BDNF mRNA. Overexpression of CaM kinase II delta3 increased exon IV- but not exon III-BDNF mRNA. These results suggest that D2R is involved in the activation of the nuclear isoform of CaM kinase II and thereby in stimulation of gene expression through Ca(2+) signaling.

G protein specificity: traffic direction required

This review focuses on the coupling specificity of the Galpha and Gbetagamma subunits of pertussis toxin (PTX)-sensitive G(i/o) proteins that mediate diverse signaling pathways, including regulation of ion channels and other effectors. Several lines of evidence indicate that specific combinations of G protein alpha, beta and gamma subunits are required for different receptors or receptor-effector networks, and that a higher degree of specificity for Galpha and Gbetagamma is observed in intact systems than reported in vitro. The structural determinants of receptor-G protein specificity remain incompletely understood, and involve receptor-G protein interaction domains, and perhaps other scaffolding processes. By identifying G protein specificity for individual receptor signaling pathways, ligands targeted to disrupt individual pathways of a given receptor could be developed.

Opposing changes of trimeric G protein levels during ontogenetic development of rat brain

Developmental changes in the distribution of guanine nucleotide-binding regulatory proteins (G proteins) were investigated in the rat brain during postnatal development. Using a standard or high-resolution urea-SDS-PAGE and specific polyclonal antipeptide antibodies oriented against G(i)alpha1/G(i)alpha2, G(i)alpha3, G(s)alpha, G(o)alpha1/G(o)alpha2, G(q)alpha/G(11)alpha and Gbeta subunit, all these proteins were determined by quantitative immunoblotting in homogenates prepared from cortex, thalamus, hippocampus and pituitary of 1-, 7-, 12-, 18-, 25- and 90-day-old animals. The levels of the majority of G protein alpha subunits, namely G(i)alpha1, G(i)alpha2, G(i)alpha3, G(o)alpha1, G(o)alpha2, G(q)alpha, G(11)alpha and Gbeta, were high already at birth. Whereas the short variant of G(s)alpha, G(s)alphaS, rose sharply in all tested brain regions between postnatal day (PD) 1 and 90, the long variant of G(s)alpha, G(s)alphaL, was unchanged in cortex and thalamus and slightly increased in hippocampus. An increase was observed also in expression of G(i)alpha1/G(i)alpha2 and G(o)alpha1 protein, while G(o)alpha2 remained constant. Minority protein G(o)alpha* dramatically increased in cortex and thalamus, was unchanged in hippocampus and not detectable in pituitary. By contrast, the highest levels of G(i)alpha3 and G(q)alpha/G(11)alpha were detected as early as at PD 1. During the next 90 days, the immunological signal of G(i)alpha3 almost disappeared and G(q)alpha/G(11)alpha continuously declined to the levels corresponding to 50% of the levels determined at birth. Expression of Gbeta subunit was basically unchanged during postnatal development. Our present analysis indicates that G(s)alpha, G(i)alpha/G(o)alpha and G(q)alpha/G(11)alpha proteins are differently expressed in the course of brain development. Differential expression of the individual alpha subunits of trimeric G proteins during postnatal development suggests their different roles in maturation of the brain tissue.

Receptor signaling and structure: insights from serotonin-1 receptors

Studies of the serotonin (5-HT) receptors have illustrated several important concepts in G-protein-mediated signaling. These concepts include G-protein specificity and cellular specificity of signaling; mechanisms of transactivation; receptor states and constitutive receptor activity; and the structural basis of coupling. The 5-HT1 receptors couple via specific G(i)/G(o) proteins to inhibitory pathways [inhibition of adenylyl cyclase (AC) activity and regulation of ion channels], but also to stimulate phospholipase C, ACII, and the mitogen-activated protein kinase (MAPK) growth-signaling pathway. 5-HT1 receptors initiate novel endocytotic and Ca(2+)-dependent pathways to activate MAPK acutely, but can downregulate MAPK on chronic activation. These pathways are often mediated via distinct G(i)/G(o)-protein subtypes. Desensitization by multiple protein kinases via receptor phosphorylation is pathway selective. Structural determination of 5-HT1 receptor and G-protein domains that mediate G-protein-specific coupling and desensitization could lead to the development of highly selective ligands that directly regulate receptor-G-protein coupling.

Regulation of 5-HT(1A) and 5-HT(1B) receptor systems by phospholipid signaling cascades

The 5-HT(1A) and 5-HT(1B) receptor systems play central roles in the control of serotonergic neurotransmission and feature prominently in many behaviors and physiological functions. In addition, the regulation of these receptors and their effector mechanisms has been the focus of intense interest because of their potential importance in the therapeutic actions of anxiolytic and antidepressant drugs. Here we describe the regulation of 5-HT(1A) and 5-HT(1B) receptor-mediated inhibition of adenylyl cyclase activity by receptors which activate phospholipid signaling cascades. Although it might be expected that these two highly homologous Gi-coupled receptors would be regulated similarly by activation of phospholipase C (PLC) and phospholipase A(2) (PLA(2)), we have found that the regulation differs markedly between these receptor systems. Further, our data suggest that the modulation of agonist efficacy at these receptor subtypes is dependent on the nature of receptor coupling to PLC and PLA(2) activation. Moreover, regulation at the level of the effector (e.g., adenylyl cyclase) appears to play a significant role in the regulation of both the 5-HT(1A) and 5-HT(1B) receptor systems by the PLA(2) signaling cascade. Such data illustrate multiple levels for control of biochemical signaling cascades within cells and demonstrate that although different receptors may couple to the same effector pathways, the ultimate cellular effects produced by these receptors may differ due to differential cross-talk regulation.

Agonist regulation of D(2) dopamine receptor/G protein interaction. Evidence for agonist selection of G protein subtype

The D(2) dopamine receptor has been expressed in Sf21 insect cells together with the G proteins G(o) and G(i2), using the baculovirus system. Expression levels of receptor and G protein (alpha, beta, and gamma subunits) in the two preparations were similar as shown by binding of [(3)H]spiperone and quantitative Western blot, respectively. For several agonists, binding data were fitted best by a two-binding site model in either preparation, showing interaction of expressed receptor and G protein. For some agonists, binding to the higher affinity site was of higher affinity in D(2)/G(o) than in the D(2)/G(i2) preparation. Some agonists exhibited binding data that were best fitted by a two-binding site model in D(2)/G(o) and a one-binding site model in D(2)/G(i2). Therefore, receptor/G protein interaction seemed to be stronger in the D(2)/G(o) preparation. Agonist stimulation of [(35)S]GTP gamma S (guanosine 5'-3-O-(thio)triphosphate) binding in the two preparations also gave evidence for higher affinity D(2)/G(o) interaction. In the D(2)/G(o) preparation, agonist stimulation of [(35)S]GTP gamma S binding occurred at higher potency for several agonists, and a higher stimulation (relative to dopamine) was achieved in D(2)/G(o) compared with D(2)/G(i2). Some agonists were able to stimulate [(35)S]GTP gamma S binding in the D(2)/G(o) preparation but not in D(2)/G(i2). The extent of D(2) receptor selectivity for G(o) over G(i2) is therefore dependent on the agonist used, and thus agonists may stabilize different conformations of the receptor with different abilities to couple to and activate G proteins.

Heterodimerization of mineralocorticoid and glucocorticoid receptors at a novel negative response element of the 5-HT1A receptor gene

Negative regulation of neuronal serotonin (5-HT1A) receptor levels by glucocorticoids in vivo may contribute to depression. Both types I (mineralocorticoid) and II (glucocorticoid) receptors (MR and GR, respectively) participate in corticosteroid-induced transcriptional repression of the 5-HT1A gene; however, the precise mechanism is unclear. A direct repeat 6-base pair glucocorticoid response element (GRE) half-site 5'-TGTCCT separated by 6 nucleotides was conserved in human, mouse, and rat 5-HT1A receptor promoters. In SN-48 neuronal cells that express MR, GR, and 5-HT1A receptors, deletion or inactivation of the nGRE (negative GRE) eliminated negative regulation of the rat 5-HT1A or heterologous promoters by corticosteroids, whereas its inclusion conferred corticosteroid-induced inhibition to a heterologous promoter. Bacterially expressed recombinant MR and GR preferentially bound to the nGRE as a heterodimer, as identified in nuclear extracts of MR/GR-transfected COS-7 cells, and with higher affinity than MR or GR homodimers. In SN48 and COS-7 cells, concentration-dependent coactivation of MR and GR was required for maximal inhibitory action by corticosteroids and was abrogated in the L501P-GR mutant lacking DNA binding activity. Corticosteroid-mediated transcriptional inhibition was greater for MR/GR in combination than for MR or GR alone. These data represent the first identification of an nMRE/GRE and indicate that heterodimerization of MR and GR mediates direct corticosteroid-induced transrepression of the 5-HT1A receptor promoter.

The role of members of the pertussis toxin-sensitive family of G proteins in coupling receptors to the activation of the G protein-gated inwardly rectifying potassium channel

Inwardly rectifying potassium (K(+)) channels gated by G proteins (Kir3.x family) are widely distributed in neuronal, atrial, and endocrine tissues and play key roles in generating late inhibitory postsynaptic potentials, slowing the heart rate and modulating hormone release. They are directly activated by G(betagamma) subunits released from G protein heterotrimers of the G(i/o) family upon appropriate receptor stimulation. Here we examine the role of isoforms of pertussis toxin (PTx)-sensitive G protein alpha subunits (G(ialpha1-3) and G(oalphaA)) in mediating coupling between various receptor systems (A(1), alpha(2A), D(2S), M(4), GABA(B)1a+2, and GABA(B)1b+2) and the cloned counterpart of the neuronal channel (Kir3.1+3.2A). The expression of mutant PTx-resistant G(i/oalpha) subunits in PTx-treated HEK293 cells stably expressing Kir3.1+3.2A allows us to selectively investigate that coupling. We find that, for those receptors (A(1), alpha(2A)) known to interact with all isoforms, G(ialpha1-3) and G(oalphaA) can all support a significant degree of coupling to Kir3.1+3.2A. The M(4) receptor appears to preferentially couple to G(ialpha2) while another group of receptors (D(2S), GABA(B)1a+2, GABA(B)1b+2) activates the channel predominantly through G(betagamma) liberated from G(oA) heterotrimers. Interestingly, we have also found a distinct difference in G protein coupling between the two splice variants of GABA(B)1. Our data reveal selective pathways of receptor activation through different G(i/oalpha) isoforms for stimulation of the G protein-gated inwardly rectifying K(+) channel.

Cannabinoid CB1 receptor-mediated inhibition of prolactin release and signaling mechanisms in GH4C1 cells

The GH4C1 cell line was used to study the cellular mechanisms of cannabinoid-mediated inhibition of PRL release. Cannabinoid CB1 receptor activation inhibited vasoactive intestinal polypeptide- and TRH-stimulated PRL release, but not its basal secretion. The cannabinoid-mediated inhibition of TRH-stimulated PRL release was reversed by the CB1 receptor-specific antagonist, SR141,716A, and was abolished by pertussis toxin pretreatment, indicating that G alpha subunits belonging to the G(i)alpha and G(o)alpha family were involved in the signaling. Photoaffinity labeling using [alpha-32P] azidoaniline GTP showed that cannabinoid receptor stimulation in cell membranes produced activation of four G alpha subunits (G(i)alpha2, G(i)alpha3, G(o)alpha1, and G(o)alpha2), which was also reversed by SR141,716A. The CB1 receptor agonists, WIN55,212-2 and CP55,940, inhibited cAMP formation and calcium currents in GH4C1 cells. The subtypes of calcium currents inhibited by WIN55,212-2 were characterized using holding potential sensitivity and calcium channel blockers. WIN55,212-2 inhibited the omega-conotoxin GVIA (Conus geographus)- and omega-agatoxin IVA (Aigelenopsis aperta)-sensitive calcium currents, but not the nisoldipine-sensitive calcium currents, suggesting the inhibition of N- and P-type, but not L-type, calcium currents. Taken together, the present findings indicate that CB1 receptors can couple through pertussis toxin-sensitive G alpha subunits to inhibit adenylyl cyclase and calcium currents and suppress PRL release from GH4C1 cells.

Regulation of adenylyl cyclase in the central nervous system

Adenylyl cyclases (ACs) are a family of enzymes that synthesize one of the major second messengers, cAMP, upon stimulation. Since the report of the first adenylyl cyclase (AC) gene in 1989, tremendous efforts have been devoted to identifying and characterizing more AC isozymes. In the past decade, significant knowledge regarding the basic structure, tissue distribution, and regulation of AC isozymes has been accumulated. Because members of the AC superfamily are tightly controlled by various signals, one of the most important impacts of these AC isozymes is their contribution to the complexity and fine-tuning of cellular signalling, especially in the central nervous system (CNS) where multiple signals constantly occur. This review focuses on recent progress toward understanding the physiological roles of ACs in the CNS.

Distinct roles for Galpha(i)2 and Gbetagamma in signaling to DNA synthesis and Galpha(i)3 in cellular transformation by dopamine D2S receptor activation in BALB/c 3T3 cells

Control of cell proliferation depends on intracellular mediators that determine the cellular response to external cues. In neuroendocrine cells, the dopamine D2 receptor short form (D2S receptor) inhibits cell proliferation, whereas in mesenchymal cells the same receptor enhances cell proliferation. Nontransformed BALB/c 3T3 fibroblast cells were stably transfected with the D2S receptor cDNA to study the G proteins that direct D2S signaling to stimulate cell proliferation. Pertussis toxin inactivates G(i) and G(o) proteins and blocks signaling of the D2S receptor in these cells. D2S receptor signaling was reconstituted by individually transfecting pertussis toxin-resistant Galpha(i/o) subunit mutants and measuring D2-induced responses in pertussis toxin-treated cells. This approach identified Galpha(i)2 and Galpha(i)3 as mediators of the D2S receptor-mediated inhibition of forskolin-stimulated adenylyl cyclase activity; Galpha(i)2-mediated D2S-induced stimulation of p42 and p44 mitogen-activated kinase (MAPK) and DNA synthesis, whereas Galpha(i)3 was required for formation of transformed foci. Transfection of toxin-resistant Galpha(i)1 cDNA induced abnormal cell growth independent of D2S receptor activation, while Galpha(o) inhibited dopamine-induced transformation. The role of Gbetagamma subunits was assessed by ectopic expression of the carboxyl-terminal domain of G protein receptor kinase to selectively antagonize Gbetagamma activity. Mobilization of Gbetagamma subunits was required for D2S-induced calcium mobilization, MAPK activation, and DNA synthesis. These findings reveal a remarkable and distinct G protein specificity for D2S receptor-mediated signaling to initiate DNA synthesis (Galpha(i)2 and Gbetagamma) and oncogenic transformation (Galpha(i)3), and they indicate that acute activation of MAPK correlates with enhanced DNA synthesis but not with transformation.

Constitutive G(i2)-dependent activation of adenylyl cyclase type II by the 5-HT1A receptor. Inhibition by anxiolytic partial agonists

The 5-HT1A receptor is implicated in depression and anxiety. This receptor couples to G(i) proteins to inhibit adenylyl cyclase (AC) activity but can stimulate AC in tissues (e.g. hippocampus) that express ACII. The role of ACII in receptor-mediated stimulation of cAMP formation was examined in HEK-293 cells transfected with the 5-HT1A receptor, which mediated inhibition of basal and G(s)-induced cAMP formation in the absence of ACII. In cells cotransfected with 5-HT1A receptor and ACII plasmids, 5-HT1A agonists induced a 1. 5-fold increase in cAMP level. Cotransfection of 5-HT1A receptor, ACII, and Galpha(i2), but not Galpha(i1), Galpha(i3), or Galpha(o), resulted in an agonist-independent 6-fold increase in the basal cAMP level, suggesting that G(i2) preferentially coupled the receptor to ACII. The 5-HT1B receptor also constitutively activated ACII. Constitutive activity of the 5-HT1A receptor was blocked by pertussis toxin and the Gbetagamma antagonist, betaCT, suggesting an important role for Gbetagamma-mediated activation of ACII. The Thr-149 --> Ala mutation in the second intracellular domain of the 5-HT1A receptor disrupted Gbetagamma-selective activation of ACII. Spontaneous 5-HT1A receptor activity was partially attenuated by 5-HT1A receptor partial agonists with anxiolytic activity (e.g. buspirone and flesinoxan) but was not altered by full agonists or antagonists. Thus, anxiolytic activity may involve inhibition of spontaneous 5-HT1A receptor activity.

The receptor for the orexigenic peptide melanin-concentrating hormone is a G-protein-coupled receptor

Gene-knockout studies of melanin-concentrating hormone (MCH) and its effect on feeding and energy balance have firmly established MCH as an orexigenic (appetite-stimulating) peptide hormone. Here we identify MCH as the ligand for the orphan receptor SLC-1. The rat SLC-1 is activated by nanomolar concentrations of MCH and is coupled to the G protein G alpha i/o. The pattern of SLC-1 messenger RNA expression coincides with the distribution of MCH-containing nerve terminals and is consistent with the known central effects of MCH. Our identification of an MCH receptor could have implications for the development of new anti-obesity therapies.

Stimulation of cAMP synthesis by Gi-coupled receptors upon ablation of distinct Galphai protein expression. Gi subtype specificity of the 5-HT1A receptor

The three Galphai subunits were independently depleted from rat pituitary GH4C1 cells by stable transfection of each Galphai antisense rat cDNA construct. Depletion of any Galphai subunit eliminated receptor-induced inhibition of basal cAMP production, indicating that all Galphai subunits are required for this response. By contrast, receptor-mediated inhibition of vasoactive intestinal peptide (VIP)-stimulated cAMP production was blocked by selective depletions for responses induced by the transfected serotonin 1A (5-HT1A) (Galphai2 or Galphai3) or endogenous muscarinic-M4 (Galphai1 or Galphai2) receptors. Strikingly, receptor activation in Galphai1-depleted clones (for the 5-HT1A receptor) or Galphai3-depleted clones (for the muscarinic receptor) induced a pertussis toxin-sensitive increase in basal cAMP production, whereas the inhibitory action on VIP-stimulated cAMP synthesis remained. Finally, in Galphai2-depleted clones, activation of 5-HT1A receptors increased VIP-stimulated cAMP synthesis. Thus, 5-HT1A and muscarinic M4 receptor may couple dominantly to Galphai1 and Galphai3, respectively, to inhibit cAMP production. Upon removal of these Galphai subunits to reduce inhibitory coupling, stimulatory receptor coupling is revealed that may involve Gbetagamma-induced activation of adenylyl cyclase II, a Gi-stimulated cyclase that is predominantly expressed in GH4C1 cells. Thus Gi-coupled receptor activation involves integration of both inhibitory and stimulatory outputs that can be modulated by specific changes in alphai subunit expression level.