Mechanisms of multifunctional signalling by G protein-linked receptors

Persistent activation of histamine H1 receptors in the hippocampal CA1 region enhances NMDA receptor-mediated synaptic excitation and long-term potentiation in astrocyte- and D-serine-dependent manner

Behavioral studies using pharmacological tools have implicated histamine H₁ receptors in cognitive function via their interactions with N-methyl-D-aspartate receptors (NMDARs) in the hippocampus. However, little is known about the neurophysiological mechanism that underlies the interaction between H₁ receptors and NMDARs. To explore how H₁ receptor activation affects hippocampal excitatory neurotransmission and synaptic plasticity, this study aimed to examine the effect of H₁ receptor ligands on both NMDAR-mediated synaptic currents and long-term potentiation (LTP) at synapses between Schaffer collaterals and CA1 pyramidal neurons using acute mouse hippocampal slices. We found that the H₁ receptor antagonist/inverse agonists, pyrilamine (0.1 μM) and cetirizine (10 μM), decreased the NMDAR-mediated component of stimulation-induced excitatory postsynaptic currents (EPSCs) recorded from CA1 pyramidal neurons without affecting the AMPA receptor-mediated component of EPSCs and its paired pulse ratio. Pretreatment of slices with either the glial metabolism inhibitor, fluoroacetate (5 mM), or D-serine (100 μM) diminished the pyrilamine- or cetirizine-induced attenuation of the NMDAR-mediated EPSCs. Furthermore, the LTP of field excitatory postsynaptic potentials induced following high frequency stimulation of Schaffer collaterals was attenuated with application of pyrilamine or cetirizine. Pretreatment with D-serine again attenuated the pyrilamine-induced suppression of LTP. Our data suggest that H₁ receptors in the CA1 can undergo persistent activation induced by their constitutive receptor activity and/or tonic release of endogenous histamine, resulting in facilitation of the NMDAR activity in a manner dependent of astrocytes and the release of D-serine. This led to the enhancement of NMDA-component EPSC and LTP at the Schaffer collateral-CA1 pyramidal neuron synapses.

Support for 5-HT2C receptor functional selectivity in vivo utilizing structurally diverse, selective 5-HT2C receptor ligands and the 2,5-dimethoxy-4-iodoamphetamine elicited head-twitch response model

There are seemingly conflicting data in the literature regarding the role of serotonin (5-HT) 5-HT2C receptors in the mouse head-twitch response (HTR) elicited by the hallucinogenic 5-HT2A/2B/2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI). Namely, both 5-HT2C receptor agonists and antagonists, regarding 5-HT2C receptor-mediated Gq-phospholipase C (PLC) signaling, reportedly attenuate the HTR response. The present experiments tested the hypothesis that both classes of 5-HT2C receptor compounds could attenuate the DOI-elicited-HTR in a single strain of mice, C57Bl/6J. The expected results were considered in accordance with ligand functional selectivity. Commercially-available 5-HT2C agonists (CP 809101, Ro 60-0175, WAY 161503, mCPP, and 1-methylpsilocin), novel 4-phenyl-2-N,N-dimethyl-aminotetralin (PAT)-type 5-HT2C agonists (with 5-HT2A/2B antagonist activity), and antagonists selective for 5-HT2A (M100907), 5-HT2C (SB-242084), and 5-HT2B/2C (SB-206553) receptors attenuated the DOI-elicited-HTR. In contrast, there were differential effects on locomotion across classes of compounds. The 5-HT2C agonists and M100907 decreased locomotion, SB-242084 increased locomotion, SB-206553 resulted in dose-dependent biphasic effects on locomotion, and the PATs did not alter locomotion. In vitro molecular pharmacology studies showed that 5-HT2C agonists potent for attenuating the DOI-elicited-HTR also reduced the efficacy of DOI to activate mouse 5-HT2C receptor-mediated PLC signaling in HEK cells. Although there were differences in affinities of a few compounds at mouse compared to human 5-HT2A or 5-HT2C receptors, all compounds tested retained their selectivity for either receptor, regardless of receptor species. Results indicate that 5-HT2C receptor agonists and antagonists attenuate the DOI-elicited-HTR in C57Bl/6J mice, and suggest that structurally diverse 5-HT2C ligands result in different 5-HT2C receptor signaling outcomes compared to DOI.

Developmental profile and mechanisms of GABA-induced calcium signaling in hippocampal astrocytes

GABA (gamma-aminobutyric acid) is a transmitter with dual action. Whereas it excites neurons during the first week of postnatal development, it represents the major inhibitory transmitter in the mature brain. GABA also activates astrocytes by binding to ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. This results in glial calcium transients which can induce the release of gliotransmitters, rendering GABA an important mediator of neuron-glia interaction. Using whole-cell patch-clamp and ratiometric calcium imaging in hippocampal slices from rats at postnatal days 3-34, we have analyzed the developmental profile as well as the cellular mechanisms of calcium signals induced by GABA(A) and GABA(B) receptor activation in astrocytes. We found that GABA-evoked glial calcium transients are mediated by both GABA(A) and GABA(B) receptors. Throughout development, GABA(A)-receptor activation resulted in immediate calcium transients in the vast majority of astrocytes, most likely by influx of calcium through voltage-gated calcium channels. GABA(B) receptor activation, in contrast, resulted in delayed calcium transients, which were blocked following depletion of intracellular calcium stores and during persistent activation of heterotrimeric G-proteins. GABA(B) receptor-mediated calcium signals exhibited a clear developmental profile with less than 10% of astrocytes responding at P3 or P32-34, and about 60% of cells between P11 and P15. Our data thus indicate that GABA(B) receptor-mediated calcium transients are due to calcium release from intracellular stores following G-protein activation. Moreover, GABA(B) receptor-mediated calcium signaling in astrocytes preferentially occurs at a period during postnatal development when hippocampal networks are established.

Function and regulation of somatostatin receptor subtypes

The five known somatostatin receptors serve unique biological roles by virtue of their tissue-specific expression and particular biochemical properties. However, the function of any individual receptor in its normal physiological milieu is not understood. Studies to address this problem have been difficult because tissues and cell lines often express multiple somatostatin receptors and, in the absence of receptor-selective somatostatin analogues, the actions of individual receptors cannot be identified. Moreover, the biological and biochemical actions of somatostatin receptors depend on their cellular environment, so that the behaviour of a receptor expressed in heterologous cells does not necessarily mimic that of endogenous receptors. We have developed two approaches to examine somatostatin receptors which circumvent these problems. Using a biotinylated somatostatin analogue for affinity purification, we isolated somatostatin receptors together with associated G proteins. Subsequent analysis of the purified complex with G protein-specific antibodies showed that the somatostatin receptors in AR42J cells preferentially couple with two pertussis toxin-sensitive G proteins: Gi alpha 1 and Gi alpha 3. To examine individual receptor types, we developed receptor-specific antibodies and used them to show that both sstr1 and sstr2 proteins were present in the GH4C1 pituitary cell line whereas AR42J cells contained sstr2 but not sstr1. Immunoprecipitation of receptor-G protein complexes with GH4C1 cells showed that sstr1 and sstr2 are both coupled to pertussis toxin-sensitive G proteins, in contrast to the results observed when these receptors are overexpressed in some non-endocrine cells. We also showed that the somatostatin receptors in GH4C1 cells are subject to both homologous and heterologous hormonal regulation. The mechanisms involved in the regulation of different receptor types are now being characterized using the receptor-specific antibodies to isolate the individual receptor proteins. Elucidating signal transduction by endogenous somatostatin receptors as well as their hormonal regulation will be critical for understanding the functions of these receptors in the different physiological targets of somatostatin.

Role of cannabinoid CB1 receptors and Gi/o protein activation in the modulation of synaptosomal Na+,K+-ATPase activity by WIN55,212-2 and delta(9)-THC

In the present study, we evaluated the effects of the synthetic cannabinoid receptor agonist (R)-(+)-[2,3-Dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN55,212-2) and the active component of Cannabis delta-9-tetrahydrocannabinol (triangle up(9)-THC) on Na(+),K(+)-ATPase activity in synaptosomal mice brain preparation. Additionally, the potential exogenous cannabinoids and endogenous opioid peptides interaction as well as the role of G(i/o) proteins in mediating Na(+),K(+)-ATPase activation were also explored. The ouabain-sensitive Na(+),K(+)-ATPase activity was measured in whole-brain pure intact synaptosomes (obtained by Percoll gradient method) of female CF-1 mice and was calculated as the difference between the total and the ouabain (1 mM)-insensitive Na(+),K(+)-ATPase activities. Incubation in vitro of the synaptosomes with WIN55,212-2 (0.1 pM-10 microM) or triangle up(9)-THC (0.1 pM-0.1 microM), in a concentration-dependent manner, stimulated ouabain-sensitive Na(+),K(+)-ATPase activity. WIN55,212-2 was less potent but more efficacious than triangle up(9)-THC. N-(Piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM-251) (10 nM), a CB(1) cannabinoid receptor selective antagonist, had not effect per se but antagonized the enhancement of Na(+),K(+)-ATPase activity induced by both, WIN55,212-2 and triangle up(9)-THC. AM-251 produced a significant reduction in the E(max) of cannabinoid-induced increase in Na(+),K(+)-ATPase activity, but did not significantly modify their EC(50). On the other hand, co-incubation with naloxone (1 microM), an opioid receptor antagonist, did not significantly modify the effect of WIN55,212-2 and completely failed to modify the effect of triangle up(9)-THC on synaptosomal Na(+),K(+)-ATPase. Finally, pre-incubation with 0.5 microg of pertussis toxin (G(i/o) protein blocker) completely abolished the enhancement of ouabain-sensitive Na(+),K(+)-ATPase activity induced by WIN55,212-2. A lower dose, 0.25 microg, decreased the E(max) of WIN55,212-2 by 70% but did not significantly affect its EC(50). These results suggest that WIN55212-2 and triangle up(9)-THC indirectly enhance Na(+),K(+)-ATPase activity in the brain by activating cannabinoid CB(1) receptors in a naloxone-insensitive manner. In addition, the effect of WIN55,212-2 on neuronal Na(+),K(+)-ATPase is apparently due to activation of G(i/o) proteins.

DIFFERENTIAL DEVELOPMENT OF CARBACHOL-INDUCED DESENSITIZATION IN RECEPTOR-MEDIATED Ca2+INFLUX AND Ca2+RELEASE PATHWAYS IN SMOOTH MUSCLE OF GUINEA-PIG TAENIA CAECI

1. We have found that development of carbachol (CCh)-induced desensitization to receptor agonists, but not to receptor by-passed stimulation, is transiently interrupted by a Ca2+-dependent resensitization during the early stage in the smooth muscle of guinea-pig taenia caeci. To further characterize the receptor-mediated signal transduction pathways involved in this peculiar desensitization process, we examined the desensitization processes during Ca2+ influx- and Ca2+ release-mediated contractions in response to activation of muscarinic receptors or histamine H1 receptors. 2. Desensitization treatment with 10(-4) mol/L CCh for 30 min in the presence of extracellular Ca2+ resulted in desensitization to the muscarinic agonists McN-A-343 or AHR-602, which are known to induce contraction only in the presence of extracellular Ca2+ in taenia caeci. The development of desensitization to these agonists was interrupted by a transient resensitization at 1 min. In contrast, the transient resensitization phase was lost following removal of extracellular Ca2+ during the desensitization treatment with CCh; under these conditions, the desensitization developed gradually without an apparent resensitization phase. 3. Contractions to 10(-4) mol/L CCh and 10(-4) mol/L histamine in the absence of extracellular Ca2+ were gradually desensitized without a resensitization phase following the CCh desensitization treatment, irrespective of the presence or absence of extracellular Ca2+ during CCh treatment, although the onset of the desensitization was delayed under Ca2+-free conditions. 4. These results suggest that the receptor-mediated Ca2+ influx and Ca2+ release pathways are differentially desensitized to CCh and that the transient resensitization appears to regulate the desensitization process in response to Ca2+ influx-mediated contraction. Such differential processes of desensitization in receptor-mediated bifurcated signalling pathways may determine cellular responsiveness to certain types of stimuli, depending on the different Ca2+ sources required for contraction.

G-protein coupled activation of potassium channels by endogenous neuropeptides in snail neurons

Members of the mytilus inhibitory peptide (MIP) family play an important role in the modulation of many physiological processes in molluscs. The signal transduction pathways affected by the MIP effect have not, however, been elucidated. Application of guanosine 5'-[gamma-thio]triphosphate tetralithium salt (GTPgammaS), guanosine 5'-[beta-thio]diphosphate trilithium salt (GDPbetaS), the G-protein inhibitor suramin and pertussis toxin (PTX) demonstrated the involvement of the PTX-insensitive G-protein in the signal transduction pathway mediating MIP effects. Both G-protein alpha(i) and betagamma subunits were identified in D-neurons of Helix pomatia by immunoblotting. Their role in signal transduction was shown in electrophysiological experiments, which supported the notion that, in addition to the Galpha subunit, the betagamma dimer also participates in the neuropeptide-induced activation of K-channels in snail neurons. Finally, neuropeptide-activated responses were inhibited by the activation of adenylyl cyclase and by blockers of the phospholipase pathway. We suggest that bifurcation of the signal transduction takes place at the level of G-protein subunits. The alpha subunit may have a direct effect on adenylyl cyclase, while the betagamma subunit may have a direct effect on phospholipase enzymes.

Ligand-directed functional heterogeneity of histamine H1 receptors: novel dual-function ligands selectively activate and block H1-mediated phospholipase C and adenylyl cyclase signaling

The autacoid and neurotransmitter histamine activates the H(1) G protein-coupled receptor (GPCR) to stimulate predominantly phospholipase C (PLC)/inositol phosphate (IP) signaling and, to a lesser extent, adenylyl cyclase (AC)/cAMP signaling in a variety of mammalian cells and tissues, as well as H(1)-transfected clonal cell lines. This study reports that two novel H(1) receptor ligands developed in our laboratory, (-)-trans-1-phenyl-3-dimethylamino-1,2,3,4-tetrahydronaphthalene (trans-PAT) and (+/-)-cis-5-phenyl-7-dimethylamino-5,6,7,8-tetrahydro-9H-benzocycloheptane (cis-PAB), activate H(1) receptors to selectively stimulate AC/cAMP formation and PLC/IP formation, respectively, in Chinese hamster ovary cells transfected with guinea pig H(1) receptor cDNA. trans-PAT and cis-PAB also are shown to be functionally selective antagonists of H(1)-linked PLC/IP and AC/cAMP signaling, respectively. Whereas cis-PAB H(1) receptor activity is shown to be typically competitive, trans-PAT displays a complex interaction with the H(1) receptor that is not competitive regarding antagonism of saturation binding by the standard H(1) antagonist radioligand [(3)H]mepyramine or H(1)/PLC/IP functional activation by histamine. trans-PAT, however, does competitively block H(1)/PLC/IP functional activation by cis-PAB. Molecular determinants for trans-PAT versus cis-PAB differential binding to H(1) receptors, which presumably leads to differential activation of AC/cAMP versus PLC/IP signaling, likely involves stereochemical factors as well as more subtle steric influences. Results suggest the trans-PAT and cis-PAB probes will be useful to study molecular mechanisms of ligand-directed GPCR multifunctional signaling. Moreover, because most untoward cardiovascular-, respiratory-, and gastrointestinal H(1) receptor-mediated effects proceed via the PLC/IP pathway, PAT-type agonists that selectively enhance H(1)-mediated AC/cAMP signaling provide a mechanistic basis for exploiting H(1) receptor activation for drug design purposes.

Bell-shaped curves for prostaglandin-induced modulation of adenylate cyclase: two mutually opposing effects

Each of the natural prostanoid is at least one order of magnitude more potent for its specific receptor (DP, EP, FP, IP and TP) than any of the other prostanoids. However, they are able to interact also with one or more of the other classes of prostanoid receptors. The concentration-response curves for modulation of adenylate cyclase activity in rabbit mesenteric artery smooth muscle cells by different prostaglandins are not always monotonic, i.e. simple sigmoidal curves in logarithmic scale, but they are often biphasic. Prostacyclin, iloprost and prostaglandin E(1) showed a convex bell-shaped curve, i.e. adenylate cyclase activity is stimulated at lower concentrations and inhibited at higher concentrations, while the curve of prostaglandin E(2) showed a concave bell-shaped curve, i.e. adenylate cyclase is inhibited at lower concentrations and stimulated at higher concentrations. By selectively inhibiting one of the transduction mechanisms present in mesenteric smooth muscle cells, we have demonstrated that the observed responses to these prostanoids are likely due to two mutually opposing effects. Thus, the data previously published by our laboratory on a prostacyclin analog, 5(Z)-carbacyclin, might be reinterpreted more correctly in the light of this new possibility.

Dopamine modulates the function of group II and group III metabotropic glutamate receptors in the substantia nigra pars reticulata

Recent findings have shown that dendritically released dopamine (DA) plays an important modulatory role in the substantia nigra pars reticulata (SNr). It is therefore possible that the loss of DA observed in Parkinson's disease (PD) could hold important consequences for nigral function. Previously, we have shown that activation of presynaptically localized group II metabotropic glutamate receptors (mGluRs) inhibits excitatory transmission at the subthalamic nucleus (STN)-SNr synapse and that activation of presynaptically localized group III mGluRs decreases excitatory and inhibitory transmission in the SNr. To test the hypothesis that nigral DA can modulate mGluR function in the SNr, we performed whole-cell patch-clamp recordings from gamma-aminobutyric acidergic SNr neurons in slices obtained from rats that were acutely reserpinized. In slices obtained from reserpinized animals, the effect of group II mGluR activation by the selective agonist (+)-2-aminobicyclo[3.1.0]-hexane-2,6-dicarboxylate monohydrate (LY354740) (100 nM), but not group III mGluR activation [L-(+)-2-amino-4-phosphonobutyric acid, L-AP4, 500 microM], at STN-SNr synapses is significantly decreased. This effect could be mimicked in control slices by prior bath application of haloperidol (20 microM) and R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH23390) (20 microM) but not sulpiride (50 microM). Furthermore, application of dopamine (100 microM) and (+/-)-6-chloro-7,8-dyhydroxy-3allyl-1-phenyl-2,3,4,5-tetra-hydro-1H-benzazepine (SKF82958) (1 microM) but not quinpirole (10 microM) could rescue the group II mGluR effect in reserpinized slices. The effect of group III mGluR activation (L-AP4, 100 microM) on inhibitory synaptic transmission was also significantly reduced in slices from reserpine-treated animals. This effect was mimicked by haloperidol (20 microM), SCH23390 (20 microM), and sulpiride (50 microM) in control slices. Thus, in a Parkinsonian state, the loss of nigral DA may add to the overall pathophysiological changes in basal ganglia output.

Role of G-protein availability in differential signaling by alpha 2-adrenoceptors

The impact of G-protein expression on the coupling specificity of the human alpha(2B)-adrenergic receptor (alpha(2B)-AR) was studied in Sf9 cells. The alpha(2B)-AR was shown to activate both coexpressed G(s)- and G(i)-proteins in a [(35)S]GTPgammaS binding assay. Noradrenaline and the synthetic agonist UK14,304 were equally potent and efficacious in stimulating G(i) activation. At the effector level (adenylyl cyclase), both ligands stimulated cAMP production. In the presence of forskolin, the effects of the agonists were more complex. Noradrenaline stimulated cAMP production, while UK14,304 showed a biphasic concentration-response curve with inhibition of stimulated cAMP production at low agonist concentrations and further stimulation at high agonist concentrations. G(s) coexpression caused a monophasic stimulatory response with both ligands. Coexpression with G(i) resulted in a biphasic concentration-response curve for noradrenaline and a monophasic inhibition with UK14,304. Experiments with a panel of agonists demonstrated that the more efficacious an agonist is in stimulating cAMP production, the weaker is its ability to couple to inhibition of cAMP accumulation via exogenous G(i). To be able to explain the mechanistic consequences of dual G-protein coupling described above, we developed a mathematical model based on the hypothesis that an agonist induces different conformations of the receptor having different affinity for different G-proteins. The model reproduced the profiles seen in the concentration-response curves with G(s) and G(i) coexpression. The model predicts that the affinity of the receptor conformation for G-proteins as well as the availability of G-proteins will determine the ultimate response of the receptor.

GTP-binding protein Gq mediates muscarinic-receptor-induced inhibition of the inwardly rectifying potassium channel IRK1 (Kir 2.1)

The inwardly rectifying potassium channel IRK1, a member of the Kir 2.0 family, is inhibited by m1 muscarinic receptor stimulation. In this study the mechanism of action underlying the muscarinic response was investigated by identification of the subtype of heterotrimeric G-protein involved in transduction of the signal. tsA201 cells were simultaneously transfected with cDNAs encoding IRK1, m1 and the Galpha subunit of either G(q), G(12) or G(13). The whole-cell patch-clamp technique was used to study the effects of G-protein transfection. Antibodies generated against the C-terminal regions of Galpha(q/11) and Galpha(12) were used to confirm G-protein expression by Western blot. When challenged with carbachol, IRK1 currents recorded from cells co-transfected with Galpha(q) were potently inhibited compared with controls. Conversely, co-transfection with Galpha(12) or Galpha(13) subunits had no effect on muscarinic-receptor-induced inhibition of IRK1. Concentration response curves revealed that carbachol was 16 times more potent at inhibiting IRK1 currents in cells co-transfected with Galpha(q) as compared with Galpha(12) co-transfected cells. Immunoblotting illustrated low levels of endogenous Galpha(q/11) and Galpha(12) in untransfected tsA cells. Transfection with Galpha(q) or Galpha(12) cDNAs greatly increased the levels of G-protein expression in both cell populations. G-protein expression did not interfere with m1 muscarinic receptor expression levels. These findings suggest that the m1 muscarinic-receptor-induced inhibition of IRK1 is mediated by the heterotrimeric G-protein, Galpha(q), in tsA cells.

Receptors coupling to G proteins: is there a signal behind the sequence?

Upon the binding of their ligands, G protein-coupled receptors couple to the heterotrimeric G proteins to transduce a signal. One receptor family may couple to a single G protein subtype and another family to several ones. Is there a signal in the receptor sequence that can give an indication of the G protein subtype selectivity? We used a sequence analysis method on biogenic amine and adenosine receptors and concluded that a weak signal can be detected in receptor families where specialization for coupling to a given G protein occurred during a recent divergent evolutionary process. Proteins 2000;41:448-459.

Use of a dual firefly and Renilla luciferase reporter gene assay to simultaneously determine drug selectivity at human corticotrophin releasing hormone 1 and 2 receptors

The aim of this study was to investigate and validate the use of a dual glow-signal luciferase reporter gene assay to simultaneously evaluate drug activity at two different seven-transmembrane receptor subtypes. Stable cell lines (CHO) transfected with either human corticotrophin releasing hormone 1 (hCRH1) receptors and a firefly luciferase reporter gene or hCRH2 and a Renilla luciferase reporter gene were created to provide different luciferase readouts for CRH1 and CRH2 receptors, respectively. Cells were combined for stimulation and measurement of luciferase luminescence in a 96-well plate format assay. The nonselective CRH agonists rat/human CRH and sauvagine caused concentration-dependent increases in luminescence via activation of CRH1 (firefly luciferase; pEC50 = 8.40 +/- 0.06 and 8.39 +/- 0.08, respectively, n = 8) and CRH2 (Renilla luciferase; pEC50 = 8.89 +/- 0.14 and 8.92 +/- 0.13, respectively, n = 8) receptors. The nonselective CRH antagonist astressin blocked these agonist-induced increases in luciferase at both CRH1 and CRH2 receptors. The selective CRH1 antagonist CP154,526 blocked r/hCRH- and sauvagine-induced increases in luciferase at CRH1 receptors only. These data report the expected pharmacology for CRH1 and CRH2 receptors. This assay enabled two receptor subtypes to be studied simultaneously in the same 96-well plate and generated robust data with low variability. It has the potential advantage of significant time and cost savings, with application to both basic research and compound screening.

Vascular β-adrenoceptor function in hypertension and in ageing

Functional β-adrenoceptors (β-AR) have been identified and characterized in blood vessels under in vivo conditions as well as in vascular smooth muscle cells (SMC) grown in culture. Agonist occupancy of β-AR activates adenylyl cyclase (AC) via the stimulatory guanine nucleotide-binding protein (Gs) and leads to elevations in intracellular adenosine 3',5'-cyclic monophosphate levels (cAMP). Increased cAMP activates the cAMP-dependent protein kinase (PKA), with subsequent phosphorylation of various target proteins. This β-AR pathway interacts with several other intracellular signalling pathways via cross-talk, so that activation by β-AR agonists may also modulate other second messengers and protein kinases. SMC β-AR play an important role in SMC function. In intact blood vessels they mediate SMC relaxation by various intracellular mechanisms, ultimately causing a decrease in intracellular Ca2+ levels. In cultured SMC, activation of the β-AR pathway results in inhibition of cellular proliferation, the development of SMC polyploidy, and SMC apoptosis. Blood vessels from hypertensive animals are characterized by an increase in SMC cell mass, a greater incidence of SMC polyploidy in the aorta, and an impairment in the β-agonist-mediated SMC relaxation. Some of these changes may result from an attenuation of β-AR function due to agonist-induced receptor desensitization caused by the uncoupling of receptors from the Gs-AC system. The phosphorylated β-AR may in turn trigger new signals and activate different intracellular pathways. However, the details of these mechanisms are still unresolved. Since functional β-AR play such a prominent and multi-faceted role in SMC function, it is important to understand how these diverse physiological effects are mediated by this receptor system, and how they contribute to the development of hypertension. With ageing, a decrease in β-AR-Gs-AC coupling is observed, and this is implicated in the reduced responsiveness of SMC. The similarities in SMC β-AR functional changes in hypertension and in ageing suggest that the underlying mechanisms are also analogous.Key words: smooth muscle, β-adrenoceptors, cyclic AMP, protein kinase A, cell proliferation, polyploidy, relaxation, apoptosis, hypertension, ageing.

Functional coupling of G proteins to endothelin receptors is ligand and receptor subtype specific

1. The aims of the present study were (a) to determine the identity of the G proteins with which the endothelin receptor interacts and whether this interaction is subtype specific and (b) to determine whether agonist exposure can result in specific coupling between the endothelin receptor and G proteins. 2. Coupling between endothelin A (ET(A)) or endothelin B (ET(B)) receptors and G proteins was assessed in two fibroblast cell lines, each expressing one receptor subtype. Four ligands, ET-1, ET-3, SRTXb, and SRTXc, were used for receptor stimulation. The G protein alpha-subunit coupled to the receptor was identified by immunoprecipitation with an antibody against the endothelin receptor and immunoblotting with specific antibodies against different G protein alpha-subunits. 3. Unstimulated ET(A) and ET(B) receptors (ET(A)R and ET(B)R, respectively) were barely coupled to Go(alpha). The unstimulated ET(A)R coimmunoprecipitated with Gi3alpha, whereas the unstimulated ETBR was much less strongly coupled to Gi3alpha. The coupling of ETBR to Gi1Gi2 alpha-subunits was much stronger than the coupling of ET(A)R to these alpha-subunits. Stimulation with the different ET agonists also resulted in differential coupling of G proteins to the receptor subtypes. All four ligands caused a strong increase in coupling of the ET(B)R to Gi3alpha, whereas coupling of the ET(A)R to this subunit was not affected by ET-1 and was even decreased by SRTXc. On the other hand, all four ligands caused a much greater increase in the coupling of ET(A)R to G(q)alpha/G11alpha than in the coupling of ET(B)R to these alpha-subunits. Ligand-induced coupling between the receptors and the Gi1 and Gi2 alpha-subunits is similar for the two receptor subtypes. The same was true for ligand-induced coupling of the receptors to Go(alpha), except that ET-3 increased the coupling of this alpha-subunit to ET(B)R and decreased the coupling to ET(A)R. Taken together, the results of this study show that coupling between ET receptors and G proteins is ligand and receptor subtype specific. 4. It remains to be established whether this diversity of receptor-G protein coupling is of relevance for the various endothelin signaling pathways and/or pathological states.

Inverse agonism at heptahelical receptors: concept, experimental approach and therapeutic potential

Inverse agonists (negative antagonists) are ligands that stabilize the inactive conformation (R) of receptors according to the two-state receptor model. The active conformation (R*) of heptahelical receptors, i.e. G protein-coupled receptors, has high affinity for G proteins. According to ternary complex models of receptor activation, the R*G complex is in equilibrium with R + G, with spontaneous activity in the absence of agonist. Inverse agonists, having a higher affinity for R, shift R*G towards R + G, decreasing the spontaneous activity of receptors. Agonists have the opposite effect, with a higher affinity for R*. Neutral antagonists have the same affinity for R and R* and compete for both agonists and inverse agonists. Inverse agonists have been recently proposed for a variety of heptahelical receptors. Methods to detect inverse agonists among antagonists are based on the determination of ligand affinity at R and R* with binding experiments, and on the modulation of G protein activity (GTP binding and hydrolysis) or of effector activity. Receptor inverse agonists, but also G protein antagonists and GTPase inhibitors, decrease spontaneous G protein activity corresponding to R*G. Receptor agonists, G protein agonists and GTPase inhibitors increase effector basal activity, but receptor inverse agonists decrease it. The therapeutic potential of inverse agonists is proposed in human diseases ascribed to constitutively active mutant receptors and may be extended to diseases related to wild-type receptor over-expression leading to the increase of R*. Some of the therapeutic effects of presently used receptor antagonists may be related to their inverse agonist properties. Inverse agonists lead to receptor upregulation, offering new approaches to tolerance and dependence to drugs.

Pituitary adenylate cyclase-activating polypeptides directly stimulate sympathetic neuron neuropeptide Y release through PAC(1) receptor isoform activation of specific intracellular signaling pathways

Pituitary adenylate cyclase-activating polypeptides (PACAP) have potent regulatory and neurotrophic activities on superior cervical ganglion (SCG) sympathetic neurons with pharmacological profiles consistent for the PACAP-selective PAC(1) receptor. Multiple PAC(1) receptor isoforms are suggested to determine differential peptide potency and receptor coupling to multiple intracellular signaling pathways. The current studies examined rat SCG PAC(1) receptor splice variant expression and coupling to intracellular signaling pathways mediating PACAP-stimulated peptide release. PAC(1) receptor mRNA was localized in over 90% of SCG neurons, which correlated with the cells expressing receptor protein. The neurons expressed the PAC(1)(short)HOP1 receptor but not VIP/PACAP-nonselective VPAC(1) receptors; low VPAC(2) receptor mRNA levels were restricted to ganglionic nonneuronal cells. PACAP27 and PACAP38 potently and efficaciously stimulated both cAMP and inositol phosphate production; inhibition of phospholipase C augmented PACAP-stimulated cAMP production, but inhibition of adenylyl cyclase did not alter stimulated inositol phosphate production. Phospholipase C inhibition blunted neuron peptide release, suggesting that the phosphatidylinositol pathway was a prominent component of the secretory response. These studies demonstrate preferential sympathetic neuron expression of PACAP-selective receptor variants contributing to regulation of autonomic function.

Involvement of cGMP-dependent protein kinase in adrenergic potentiation of transmitter release from the calyx-type presynaptic terminal

I have previously reported that norepinephrine (NE) induces a sustained potentiation of transmitter release in the chick ciliary ganglion through a mechanism pharmacologically distinct from any known adrenergic receptors. Here I report that the adrenergic potentiation of transmitter release was enhanced by a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) and by zaprinast, an inhibitor of cGMP-selective phosphodiesterase. Exogenous application of the membrane-permeable cGMP, 8-bromo-cGMP (8Br-cGMP), potentiated the quantal transmitter release, and after potentiation, the addition of NE was no longer effective. On the other hand, 8Br-cAMP neither potentiated the transmitter release nor occluded the NE-induced potentiation. The NE-induced potentiation was blocked by neither nitric oxide (NO) synthase inhibitor nor NO scavenger. The quantal transmitter release was not potentiated by NO donors, e.g., sodium nitroprusside. The NE-induced potentiation and its enhancement by IBMX was antagonized by two inhibitors of protein kinase G (PKG), Rp isomer of 8-(4-chlorophenylthio) guanosine-3', 5'-cyclic monophosphorothioate and KT5823. As with NE-induced potentiation, the effects of 8Br-cGMP on both the resting intraterminal [Ca2+] ([Ca2+]i) and the action potential-dependent increment of [Ca2+]i (DeltaCa) in the presynaptic terminal were negligible. The reduction of the paired pulse ratio of EPSC is consistent with the notion that the NE- and cGMP-dependent potentiation of transmitter release was attributable mainly to an increase of the exocytotic fusion probability. These results indicate that NE binds to a novel adrenergic receptor that activates guanylyl cyclase and that accumulation of cGMP activates PKG, which may phosphorylate a target protein involved in the exocytosis of synaptic vesicles.

Enhanced expression of melanocortin-1 receptor (MC1-R) in normal human keratinocytes during differentiation: evidence for increased expression of POMC peptides near suprabasal layer of epidermis

Immunohistochemical staining of human skin specimen showed the stronger localization of proopiomelanocortin peptides near the suprabasal layer of the epidermis, where keratinocytes are mostly differentiated. To test the possibilities of whether the production of proopiomelanocortin peptides or their receptor-binding activity or both is increased during differentiation of keratinocytes, we treated the cells in culture with Ca2+ to induce their differentiation. The production of proopiomelanocortin peptides and its gene expression were not induced significantly, but the binding ability of melanocortin receptor, as well as its gene expression were stimulated by Ca2+. Ultraviolet B irradiation, an inducer of differentiation, stimulated both proopiomelanocortin production and melanocortin receptor expression. These data show that normal human keratinocytes express melanocortin receptor similar to melanocytes, and that it is induced during differentiation.

The cloned guinea pig neuropeptide Y receptor Y1 conforms to other mammalian Y1 receptors

We have cloned the guinea pig neuropeptide Y (NPY) Y1 receptor and found it to be 92-93% identical to other cloned mammalian Y1 receptors. Porcine NPY and peptide YY (PYY) displayed affinities of 43 pM and 48 pM, respectively. NPY2-36 and NPY3-36 had 6- and 46-fold lower affinity, respectively, than intact NPY. Functional coupling was measured by using a microphysiometer. Human NPY and PYY were equipotent in causing extracellular acidification with EC50 values of 0.59 nM and 0.69 nM, respectively, whereas NPY2-36 and NPY3-36 were about 15-fold and 500-fold less potent, respectively, than NPY. The present study shows that the cloned guinea pig Y1 receptor is very similar to its orthologues in other mammals, both with respect to sequence and pharmacology. Thus, results from previous studies on guinea pig NPY receptors might imply the existence of an additional Y1-like receptor sensitive to B1BP3226.

Norepinephrine-stimulated MAP kinase activity enhances cytokine-induced NO production by rat cardiac myocytes

The effect of norepinephrine (NE) on cytokine-stimulated nitric oxide (NO) production by cardiac myocytes has not been previously reported. NE alone caused no significant increase in NO-2 levels over vehicle. Addition of NE to interleukin-1beta (IL-1beta) significantly increased inducible NO synthase (iNOS) mRNA expression, iNOS protein, and NO-2 production vs. IL-1beta alone. Addition of the alpha-adrenergic blocker prazosin or the beta-adrenergic blocker propranolol partially reduced the NE-mediated increase in iNOS mRNA expression and NO-2 production. Addition of prazosin and propranolol together completely abolished the NE-induced increase in iNOS mRNA expression and NO-2 production. NE significantly enhanced mitogen-activated protein (MAP) kinase activity that was reduced by prazosin, propranolol, and PD-98059, a selective MAP kinase kinase inhibitor. Addition of PD-98059 reduced the NE-mediated increase in iNOS mRNA expression and NO-2 production. We report for the first time that NE enhances IL-1beta-stimulated NO production by activation of alpha- and beta-adrenergic receptors through a novel MAP kinase mechanism.

Functional cross-talk between endothelial muscarinic and alpha2-adrenergic receptors in rabbit cerebral arteries

Interactions between two classes of receptors have been observed in several cell lines and preparations. The aim of this work was to assess the impact of simultaneous stimulation of endothelial muscarinic and alpha2-adrenergic receptors (alpha2-AR) on vascular reactivity. Rabbit middle cerebral arteries were isolated and changes in isometric tension were recorded in the presence of indomethacin. Inhibition of nitric oxide (NO) synthase with Nomega-nitro-L-arginine (L-NOARG, 100 micromol l(-1)) revealed alpha-AR-dependent contractions. Pre-addition of acetylcholine (ACH, 1 micromol l(-1)) augmented oxymetazoline (OXY, 10 micromol l(-1), alpha2-AR agonist)-, but decreased phenylephrine (PE, 10 micromol(-1), alpha1-AR agonist)-induced contraction (P<0.05). The effects of ACH were endothelium-dependent. Vessels were precontracted with 40 mmol l(-1) KCl-physiological salt solution (PSS) in the absence of L-NOARG, or PE or OXY in the presence of L-NOARG. In the presence of high external K+ or PE, ACH induced a potent relaxation (P<0.05). In the presence of OXY, however, ACH mediated contraction (P<0.05). After pertussis toxin (PTX, inactivator of Galpha(i/o) proteins) pre-treatment, alpha2-AR-dependent contractions were abolished. Forty mmol l(-1) KCl-PSS induced contraction was not altered by PTX whereas ACH-induced relaxation was augmented (P<0.05). To investigate if endothelin-1 (ET-1) intervened in the endothelium-dependent contractile response to ACH in the presence of OXY-dependent tone, vessels were incubated in the presence of BQ123 (1 micromol l(-1)), an ETA receptor antagonist. OXY-mediated tone was not affected by BQ123; however, ACH-induced contraction was reversed to a relaxation (P<0.05). These data indicate that activation of endothelial alpha2-AR triggers an endothelium-dependent, ET-1 mediated, contraction to ACH. This suggests that activation of alpha2-AR affects muscarinic receptor/G protein coupling leading to an opposite biological effect.

P2Y and P2U receptors differentially release intracellular Ca2+ via the phospholipase c/inositol 1,4,5-triphosphate pathway in astrocytes from the dorsal spinal cord

In astrocytes, raising intracellular Ca2+ concentration is a principal mechanism for transducing extracellular signals following activation of cell-surface receptors. Receptors that may be activated by purine nucleotides, P2 receptors, are known to be expressed by astrocytes from dorsal spinal cord; these astrocytes express two distinct subtypes of P2 receptor, P2Y and P2U. A main goal of the present study was to determine the intracellular signalling pathways mediating the Ca2+ responses produced by stimulating these receptors. Experiments were done using cultured astrocytes from rat dorsal spinal cord. Ca2+ responses were evoked by 2-methylthio-ATP or UTP, nucleotides previously shown to selectively activate P2Y and P2U receptors, respectively, in these cells. P2Y- and P2U-evoked Ca2+ responses were found not to depend upon extracellular Ca2+ and were blocked by thapsigargin, a Ca2+-ATPase inhibitor known to deplete inositol 1,4,5-triphosphate-sensitive Ca2+ stores. Intracellular application of the inositol 1,4,5-triphosphate-sensitive receptor antagonist, heparin, or of the G-protein inhibitor guanosine 5'-O-(2-thiodiphosphate), blocked the P2Y- and P2U-evoked Ca2+ responses. Moreover, the responses were prevented by the phospholipase C inhibitor, U-73122, but were unaffected by the inactive analogue, U-73343. These results indicate that P2Y and P2U receptors on dorsal spinal astrocytes are linked via G-protein coupling to release of intracellular Ca2+ via the phospholipase C/inositol 1,4,5-triphosphate pathway. When we assessed the releasable pools of intracellular Ca2+, by repeated agonist applications in zero extracellular Ca2+, we found that the pool accessed by activating P2U receptors was only a subpool of that accessed by activating P2Y receptors. This implies that there are separable inositol 1,4,5-triphosphate-releasable pools of Ca2+ in dorsal spinal astrocytes and that these may be differentially released by activating distinct metabotropic P2 receptors. This differential release of Ca2+ may be important for physiological as well as pathophysiological events occurring within the spinal cord.

P2Y receptor subtypes differentially couple to inwardly-rectifying potassium channels

Subtypes of P2Y receptors are well characterized with respect to their agonist profile but little is known about differences in their intracellular signalling properties. When expressed in Xenopus oocytes, both P2Y2 and P2Y6 receptors effectively couple to endogenous Ca2+-dependent Cl--channels. However, only P2Y2 receptors increased currents mediated by inward-rectifier K+ channels of the Kir3.0 subfamily. This increase in Kir-current was sensitive to pertussis toxin, while activation of Ca2+-dependent Cl--channels was not. In contrast, suramin, a P2 receptor antagonist, inhibited activation of both channels. These observations suggest that, in contrast to P2Y6, P2Y2 receptors couple to two different classes of G proteins.

Molecular cloning and functional characterization of a rat somatostatin sst2(b) receptor splice variant

1. The mouse somatostatin (SRIF) sst2 receptor exists in two splice variants, sst2(a) and sst2(b), which differ in their intracellular carboxy-termini only. The murine sst2(b) receptor was reported to be less prone to agonist-induced desensitization as compared with the sst2(a) receptor. To determine whether a sst2(b) splice variant with similar functional characteristics exists in the rat, we have isolated a cDNA fragment from rat gastric mucosa encoding a sst2(b) receptor and expressed the full-length protein in CHO-K1 cells for functional characterization. 2. This study provides the first evidence for the occurrence in the rat of the sst2(b) receptor, which has a 15 amino acid carboxy-terminus differing in composition to the 38 amino acid C-terminus of the rat sst2(a) receptor. 3. In CHO-K1 cells expressing rat recombinant sst2(a) or sst2(b) receptors, SRIF caused concentration-dependent increases in extracellular acidification rates (EAR) with pEC50 values of 9.0 and 9.9, respectively. Pre-treatment with pertussis toxin (Ptx) caused a rightward displacement of the SRIF concentration-effect curves with pEC50 values of 8.3 (sst2(a) and 8.4 (sst2(b)). 4. SRIF (3 pM-3 nM) also caused concentration-dependent inhibition of forskolin-stimulated cyclic AMP formation in CHO-sst2(a) cells (pIC50 10.5) and CHO-sst2(b) cells (pIC50 10.4). The degree of inhibition was less with higher concentrations of SRIF resulting in bell-shaped concentration-effect curves. Following pre-treatment with Ptx, the inhibitory effect of SRIF was abolished and SRIF caused only increases in cyclic AMP formation. 5. Both the SRIF-induced increases in EAR and inhibition of cyclic AMP formation were susceptible to agonist-induced desensitization, but this was less apparent following pre-treatment with Ptx. 6. This demonstrates that the operational characteristics of the recombinant rat sst2(a) and sst2(b) receptors are broadly similar. Both isoforms couple to Ptx-sensitive as well as -insensitive G proteins and are equally prone to agonist-induced desensitization.

Stimulation of type 1 and type 8 Ca2+/calmodulin-sensitive adenylyl cyclases by the Gs-coupled 5-hydroxytryptamine subtype 5-HT7A receptor

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) plays an important regulatory role in developing and adult nervous systems. With the exception of the 5-HT3 receptor, all of the cloned serotonin receptors belong to the G protein-coupled receptor superfamily. Subtypes 5-HT6 and 5-HT7 couple to stimulation of adenylyl cyclases through Gs and display high affinities for antipsychotic and antidepressant drugs. In the brain, mRNA for 5-HT6 is found at high levels in the hippocampus, striatum, and nucleus accumbens. 5-HT7 mRNA is most abundant in the hippocampus, neocortex, and hypothalamus. To better understand how serotonin might control cAMP levels in the brain, we coexpressed 5-HT6 or 5-HT7A receptors with specific isoforms of adenylyl cyclase in HEK 293 cells. The 5-HT6 receptor functioned as a typical Gs-coupled receptor in that it stimulated AC5, a Gs-sensitive adenylyl cyclase, but not AC1 or AC8, calmodulin (CaM)-stimulated adenylyl cyclases that are not activated by Gs-coupled receptors in vivo. Surprisingly, serotonin activation of 5-HT7A stimulated AC1 and AC8 by increasing intracellular Ca2+. 5-HT also increased intracellular Ca2+ in primary neuron cultures. These data define a novel mechanism for the regulation of intracellular cAMP by serotonin.

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.

Radioligand binding characteristics of the endothelin receptor in the rabbit iris

We previously suggested the presence of functionally atypical endothelin (ET) A receptors in the rabbit iris sphincter. Here, we further characterized the ET receptor by a radioligand-receptor binding study utilizing a membrane fraction of the rabbit iris. In addition, we functionally confirm the presence of an atypical ET(A) receptor in the iris dilator similar to that in the iris sphincter. In binding experiments, [125I]ET-1 was completely displaced by ET-3 in a biphasic fashion, but only partially by BQ-123 and ET(B) ligands. In the presence of RES-701, ET-3 and sarafotoxin (SRTX)-b completely displaced [125I]ET-1 in a monophasic fashion, but with shallow slopes. Moreover, ET-1, ET-3 and SRTX-b completely displaced [3H]BQ-123 with IC50 values of 0.8, 81 and 4.4 nM, respectively, but with slopes of ET-3 and SRTX-b being again shallow. In iris dilator muscles, ET-3 showed lower and SRTX-b showed higher contractile activities than ET-1. SRTX-c was inactive. BQ-123 more preferentially antagonized ET-3 and SRTX-b than ET-1, with the Schild plot slope of SRTX-b being shallow. Thus, functional experiments suggested the presence of atypical ET(A) receptors in the iris dilator similar to the iris sphincter. However, the binding experiments suggested the presence of rather typical ET(A)- and ET(B)-like receptors. Therefore, we apparently failed to show ET binding sites corresponding to functionally atypical ET(A) receptors.

G protein-coupled-receptor cross-talk: the fine-tuning of multiple receptor-signalling pathways

Signalling via the large family of G protein-coupled receptors (GPCRs) can lead to many cellular responses, ranging from regulation of intracellular levels of cAMP to stimulation of gene transcription. Members of this receptor family have been grouped into different categories dependent on the particular G protein subtypes that they predominantly interact with. Thus, receptors that couple to GS proteins will stimulate adenylate cyclase in many cells, while Gq/11-coupled receptors can mobilize intracellular Ca2+ via activation of phospholipase C. There is accumulating evidence, however, that activation of one particular signalling pathway by a GPCR can amplify intracellular signalling within a parallel but separate pathway. In this article Lisa Selbie and Stephen Hill review some of the evidence for these synergistic interactions and suggest that they may have an important role in finetuning signals from multiple receptor signalling pathways.

Functional characterization and mRNA expression of pituitary adenylate cyclase activating polypeptide (PACAP) type I receptors in rat peritoneal macrophages

The present work characterizes the mRNA expression of PACAP type I receptors in rat peritoneal macrophages but not in peritoneal lymphocytes by both retrotranscriptase and polymerase chain reaction (RT-PCR) and homologous Southern hybridization and the stimulation by PACAP27, PACAP38 and vasoactive intestinal peptide (VIP) of sn-1,2-diacylglycerol production in rat peritoneal macrophage membranes. The binding of [125I]PACAP27 was time and cell concentration dependent. Scatchard analysis of displacement of the bound tracer by unlabeled PACAP27 indicates the existence of two classes of binding sites. The dissociation constant (Kd) was 0.64 +/- 0.08 nM and the maximal binding capacity (Bmax) was 8.85 +/- 1.45 fmol/10(6) cells for the high affinity binding site. The low affinity binding site had a Kd of 0.10 +/- 0.06 microM with a Bmax of 300 +/- 21.9 fmol/10(6) cells. Scatchard analysis of VIP displacement data indicated the presence of two classes of binding sites with a Kd and Bmax different to those of PACAP27. These results suggest that PACAP binds to two binding sites, PACAP type I receptors and PACAP type II receptors. The PACAP27-stimulated diacylglycerol production was not affected by treatment with pertussis toxin. However, the presence of GTP partially inhibited this PACAP27 stimulation of 1,2-diacylglycerol in a dose dependent manner, although GTP alone stimulates diacylglycerol accumulation. In conclusion, for the first time we demonstrate by biochemical and molecular biology criteria the existence of PACAP type I receptors on rat peritoneal macrophages and the evidence for coupling with a pertussis toxin-insensitive G regulatory protein.

Measurement of agonist efficacy using an alpha2A-adrenoceptor-Gi1alpha fusion protein

A fusion protein was constructed between the porcine alpha2A-adrenoceptor and a pertussis toxin-insensitive (Cys351Gly) form of the alpha subunit of the G protein Gi1. Addition of agonist ligands to membranes of COS-7 cells transiently transfected to express this construct, and treated with pertussis toxin prior to cell harvest, resulted in stimulation of both high affinity GTPase activity and enhanced binding of [35S]GTPgammaS. By considering the fusion protein as an agonist-activated enzyme and measuring Vmax of GTP hydrolysis a range of agonist ligands displayed varying efficacy in their capacity to activate the receptor-associated G protein with adrenaline = noradrenaline = alpha-methylnoradrenaline > UK14304 > BHT933 > or = xylazine = clonidine. A similar rank order was observed following independent co-expression of the alpha2A-adrenoceptor and Cys351Gly-Gi1alpha. These data demonstrate the utility and applicability of using a receptor-G protein fusion protein approach, in which the stoichiometry of receptor and G protein is fixed at 1:1, to measure and further understand the nature of agonist efficacy.

Contrasting effects of carbachol, McN-A-343 and AHR-602 on Ca(2+)-mobilization and Ca(2+)-influx pathways in taenia caeci

1. We compared the binding profiles and contractile mechanisms of putative muscarinic M1 agonists McN-A-343 and AHR-602 with those of carbachol in smooth muscle of guinea-pig taenia caeci. 2. McN-A-343 and AHR-602, as well as carbachol, completely displaced the atropine-sensitive binding of [3H]-quinuclidinyl benzilate to muscarinic receptors present in the membrane preparation. The potency order for the affinity of these agents for muscarinic receptors was carbachol > McN-A-343 >> AHR-602. 3. In the presence of 2.2 mM extracellular Ca2+, McN-A-343 and AHR-602 induced contraction corresponding to 79 and 85%, respectively, of the maximal contraction to 0.1 mM carbachol. Contractions induced by these agents were mediated via activation of the muscarinic receptor subtype that had a high affinity for 4-DAMP (M3 selective) but a low affinity for pirenzepine (M1 selective) and AF-DX 116 (M2 selective). These contractions were inhibited by an L-type Ca2+ channel blocker, verapamil. 4. In Ca(2+)-free solution containing 2 mM EGTA, carbachol elicited a transient contraction whereas no contraction was observed in response to McN-A-343 and AHR-602. Application of McN-A-343 or AHR-602 inhibited the carbachol-induced contraction in Ca(2+)-free solution, and this inhibition was surmounted by a higher concentration of carbachol. 5. The EC50 value for carbachol-induced contraction in the presence of extracellular Ca2+ was approximately 175 times lower than that in the absence of Ca2+. After treatment with propylbenzilylcholine mustard, carbachol induced contraction only in the presence of extracellular Ca2+. 6. The results suggest that in the taenia caeci there is a greater receptor reserve for muscarinic M3 receptor-mediated Ca2+ influx than for M3 mediated Ca2+ release. The compounds McN-A-343 and AHR-602 are agonists of the Ca2+ influx pathway, but do not appear to stimulate the Ca2+ release pathway.

Novel insights into histamine H2 receptor biology

Histamine exerts multiple biological actions through one of three receptor subtypes (H1, H2, and H3). This review focuses on new developments regarding the structure and function of the H2 receptor. In addition to the important role this receptor plays in stimulating gastric acid secretion, recent studies have demonstrated that it is also involved in regulating gastrointestinal motility and intestinal secretion. The potential role of the H2 receptor in regulating cell growth and differentiation has also been added to the list of actions this biogenic amine may exert in both normal and transformed tissues. Molecular cloning of the gene indicates that it has the structural characteristics of a heptahelical G protein-linked receptor. Site-directed mutagenesis studies of this receptor reveal the presence of key amino acids within the third and fifth transmembrane domains that are critical for ligand recognition. Molecular approaches have also shed light on the structural components of the H2 receptor important in regulating desensitization and internalization. Although the H2 receptor was classically thought to couple to the adenylate cyclase pathway, recent work with the cloned receptor indicates that it can also activate the phosphoinositide signaling cascade through an independent G protein-dependent mechanism. The novel observation that histamine may stimulate c-fos gene expression lends further support to the possible role of this receptor in regulating cell growth and differentiation.

Localization of D1 and D2 dopamine receptors in the rat mesencephalic trigeminal nucleus by immunocytochemistry and in situ hybridization

The rat mesencephalic trigeminal nucleus (MTN) receives a dopaminergic innervation. In order to identify and localize dopaminoceptive cells within this nucleus, expression of D1 and D2 dopamine (DA) receptors was examined by immunocytochemistry with subtype-specific antibodies and in situ hybridization with digoxigenin-labeled cRNA probes. Immunocytochemical labeling was restricted to neuronal perikarya and proximal processes whereas the hybridization signal was confined to MTN cell bodies. Cells immunopositive for D1 were located throughout the entire MTN whereas cells labeled with D2 antibodies were concentrated in its caudal portion. D1 receptor message was found in relatively low levels. In contrast, high levels of mRNA for D2 were seen in MTN neurons. The distribution of DA receptor mRNA-containing cells were very similar to those of DA receptor immunoreactivity. Neurons expressing the D1 receptor gene were localized in both rostral and caudal portions, which receive inputs from masticatory muscle spindles and from spindles and periodontal ligament receptors, respectively. D2 receptors were limited to ventrocaudally located cells. These results suggest that processing of proprioceptive information in the MTN is controlled by the DAergic input through different postsynaptic mechanisms.

Both overlapping and distinct signaling pathways for somatostatin receptor subtypes SSTR1 and SSTR2 in pituitary cells

To elucidate the signaling events mediated by specific somatostatin receptor (SSTR) subtypes, we expressed SSTR1 and SSTR2 individually in rat pituitary GH12C1 and F4C1 cells, which lack endogenous somatostatin receptors. In transfected GH12C1 cells, both SSTR1 and SSTR2 coupled to inhibition of Ca2+ influx and hyperpolarization of membrane potential via a pertussis toxin (PTx)-sensitive mechanism. These effects reflected modulation of ion channel activities which are important for regulation of hormone secretion. Somatostatin analogs MK678 and CH275 acted as subtype selective agonists as expected. In transfected F4C1 cells, both SSTR1 and SSTR2 mediated somatostatin-induced inhibition of adenylyl cyclase via a PTx-sensitive pathway. In addition, activation of SSTR2 in F4C1 cells, but not SSTR1, stimulated phospholipase C (PLC) activity and an increase in [Ca2+]i due to release of Ca2+ from intracellular stores. Unlike adenylyl cyclase inhibition, the PLC-mediated response was only partially sensitive to PTx. To determine the structural determinants in SSTR2 necessary for activation of PLC, we constructed chimeric receptors in which domains of SSTR2 were introduced into SSTR1. Chimeric receptors containing only the third intracellular loop, or all three intracellular loops from SSTR2, mediated inhibition of adenylyl cyclase, but failed to stimulate PLC activity as did wild-type SSTR2. Furthermore, the C-terminal tail of SSTR2 was not required for coupling to PLC. Thus, by expressing individual somatostatin receptor subtypes in pituitary cells, we have identified both overlapping and distinct signaling pathways for SSTR1 and SSTR2, and have shown that sequences other than simply the intracellular domains are required for SSTR2 to couple to the PLC signaling pathway.

Is promiscuity of G protein interaction an issue in the classification of receptors?

It is clear that the details of binding of different classes of agonist ligands to the same receptor may be distinct. This could result in subtle differences in the profile of G protein activation by individual ligands at the same receptor due to agonist-induced selection of conformational states of the receptor which favor interaction and ternary-complex formation with different G proteins. This can result in differences in the details of receptor pharmacology when measured at the level of effector output. Such differences are also likely to be dependent upon both the levels of expression of the receptor and the relevant G proteins and the G protein and effector enzyme isoform expression profile of a particular tissue. It would be foolish, however, to use such differences, in isolation, as a means to expand the classification of G protein-coupled receptors particularly when more molecular approaches to receptor classification are readily available.

Characterization of metabotropic glutamate receptors in rat C6 glioma cells

Metabotropic glutamate receptors in rat C6 glioma cells have been characterized by pharmacological and kinetic binding experiments, using both L-[3H]glutamate and [3H(+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid ([3H](+/-)-trans-ACPD) radioligands. Saturation experiments revealed a single binding site with a Kd = 1250 +/- 101 nM and Bmax = 12.1 +/- 1.8 pmol/mg protein when the assays were performed with L-[3H]glutamate as radioligand in the presence of AMPA, kainate, NMDA and DL-threo-beta-hydroxyaspartic acid. When [3H](+/-)-trans-ACPD was used as radioligand, the kinetic parameters obtained were Kd = 2605 +/- 1042 nM and Bmax = 13.66 +/- 5.01 pmol/mg protein. Pharmacological characterization indicated that specific binding of L-[3H]glutamate was sensitive to different agonists of mGlu receptors, showing a rank order of affinity L-glutamate > L-quisqualic acid > (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) > ibotenic acid >>> (2S, 'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I). Specific binding of L-[3H]glutamate to mGlu receptors is regulated by guanine nucleotides. Guanylyl imidodiphosphate (Gpp(NH)p) causes an affinity shift on the L-glutamate dose-response curve, increasing the IC50 value. These results support the evidence that metabotropic glutamate receptors are present in rat C6 glioma cells and they are coupled to a G-protein.

5-HT receptor classification and nomenclature: towards a harmonization with the human genome

Molecular biology has dramatically advanced our knowledge and understanding of receptors for 5-hydroxytryptamine (5-HT). The existence of multiple 5-HT receptors defined using traditional pharmacological and biochemical approaches has now been amply confirmed, but gene products encoding putative "new" 5-HT receptors have also been discovered. In some cases, the absence of suitably selective agonists and antagonists has hampered determination of a physiological role for these gene products. This makes their classification as formally recognised receptors premature.

Physiological role of brain endothelin in the central autonomic control: from neuron to knockout mouse

Although endothelin (ET) was discovered as a potent vascular endothelium-derived constricting peptide, its presumed physiological and pathophysiological roles are now considered much more diverse than originally though. Endothelin in the brain is thought to be deeply involved in the central autonomic control and consequent cardiorespiratory homeostasis, possibly as a neuromodulator or a hormone that functions locally in an autocrine/paracrine manner or widely through delivery by the cerebrospinal fluid (CSF). This notion is based on the following lines of evidence. (1) Mature ET, its precursors, converting enzymes, and receptors all are detected at strategic sites in the central nervous system (CNS), especially those controlling the autonomic functions. (2) The ET is present in the CSF at concentrations higher than in the plasma. (3) There is a topographical correspondence of ET and its receptors in the CNS. (4) The ET is released by primary cultures of hypothalamic neurons. (5) When ET binds to its receptors, intracellular calcium channels. (6) An intracerebroventricular or topical application of ET to CNS sites elicits a pattern of cardiorespiratory changes accompanied by responses of vasomotor and respiratory neurons. (7) Recently generated knockout mice with disrupted genes encoding ET-1 exhibited, along with malformations in a subset of the tissues of neural crest cell lineage, cardiorespiratory abnormalities including elevation of arterial pressure, sympathetic overactivity, and impairment of the respiratory reflex. Definitive evidence is expected from thorough analyses of knockout mice by applying conventional experimental methods.

Nomenclature and classification of transmitter receptors: an integrated approach

This short assay is an attempt to rationalize an integrated approach to transmitter receptor nomenclature and classification based on three criteria: structural information (gene structure/amino acid sequence), operational 1 information (pharmacological/ ligand-defined/recognitory) and transductional information (receptor-effector coupling) are proposed to be used when considering the naming of existing or newly discovered receptors. It should be recognized that none of these criteria has primacy and that in an ideal situation, as much information as feasible on these three aspects should be collected before the naming of a receptor can be agreed upon.

Bombesin and thrombin affect discrete pools of intracellular calcium through different G-proteins

In mouse NIH 3T3 cells, the mitogens bombesin and thrombin induced Ca2+ release from intracellular stores. Ca2+ release induced by bombesin was inhibited by the Ca(2+)-ATPase inhibitor thapsigargin, while Ca2+ release induced by thrombin was unaffected by this agent. The Ca(2+)-release response to bombesin was not affected by pertussis toxin, but the response to thrombin was abolished by the toxin. Stable transfectants overexpressing the G-protein subunit type alpha 9 showed an accentuated response to bombesin, indicating that the bombesin receptor was coupled to a Gq-like G-protein. Together, these results show that the two mitogenic receptors are coupled to distinct G-proteins that affect functionally different pools of Ca2+. Organization of signalling pathways in this manner may allow cells to differentially encode information from different signals.

Cholera and pertussis toxins reveal multiple regulation of cAMP levels in the rabbit carotid body

It is known that hypoxia (PO2 approximately equal to 66-18 mm Hg), acting via unknown receptors, increases carotid body cAMP levels in Ca(2+)-free solutions, indicating that low PO2 activates adenylate cyclases independently of the action of the released neurotransmitters. The aim of the present work was to investigate the involvement of G proteins in the genesis of the basal level of cAMP and on the increase in cAMP induced by low PO2. In carotid body homogenates, cholera toxin- and pertussis toxin-induced [32P]ADP-ribosylation of two protein bands of approximately equal to 42 and 45 kDa, and approximately equal to 39 and 40 kDa respectively; in both cases, prior incubation of the carotid bodies with the toxins reduced [32P]ADP-ribosylation by > 90%. In intact carotid bodies, cholera toxin treatment increased cAMP levels more in normoxic than in hypoxic organs, indicating that hypoxia releases neurotransmitters acting on receptors negatively coupled to adenylate cyclases. Cholera toxin-treated carotid bodies incubated in Ca(2+)-free solution had identical cAMP levels in normoxia and in hypoxia. In pertussis toxin-treated normoxic carotid bodies the cAMP level was close to control, but in pertussis toxin-treated hypoxic carotid bodies cAMP rose to a level similar to those seen in normoxic cholera toxin-treated organs, indicating that low PO2 releases neurotransmitters acting on receptors positively coupled to adenylate cyclases. Pertussis toxin-treated carotid bodies incubated in Ca(2+)-free solution lost their capacity to increase cAMP in response to hypoxia, indicating that a G protein sensitive to pertussis toxin is needed for this response. This implies that the carotid bodies express a pertussis toxin-sensitive G protein positively coupled to adenylate cyclases, or that a Gs protein requiring the cooperative action of Go/Gi donated beta gamma subunits mediates the increase in cAMP level produced by hypoxia.

GTP-binding protein regulates the contractile response elicited by the phorbol ester (PMA)-induced activation of protein kinase C in the isolated rat aorta

1. The aim of the present study was to investigate the involvement of GTP-binding protein in the contractile response induced by activation of protein kinase C (PKC) in isolated rat aorta. The rats were treated with islet-activating protein (IAP) for 4 days prior to the experiments. 2. In the aorta from control rats, phorbol 12-myristate 13-acetate (PMA) produced biphasic contractions; twitch contraction superimposed on the slowly developing contraction. The twitch contraction was abolished by the removal of external Ca2+ or by treatment with nicardipine. In the aorta pretreated with IAP, PMA produced only a slowly developing contraction, and no twitch contraction was induced. 3. The application of Ca2+ to aortic strips in a Ca(2+)-free solution, that had been treated with 10(-6) M PMA caused concentration-dependent contraction, and the contraction was completely inhibited by IAP. 4. Pretreatment with IAP inhibited Ca(2+)-induced contraction of the aorta in Ca(2+)-free medium in the presence of 10(-6) M clonidine, but did not affect the Ca(2+)-induced contraction in the medium treated with 10(-6) M phenylephrine and 10(-7) M nicardipine. 5. These results suggest that the activation of PKC by PMA produces biphasic contractions in the rat aorta. The twitch contraction may be induced by the activation of voltage-dependent Ca(2+)-channels and the activation may be regulated by IAP-sensitive GTP-binding protein.

Signal transduction pathways modulated by the D2 subfamily of dopamine receptors

The D2 subfamily of dopamine receptors includes D2A, D2B, D3, and D4 dopamine receptors. These receptors activate cellular effector systems, principally through pertussis toxin-sensitive G-proteins. Historically, D2-like receptors in brain tissues were recognized as the dopamine receptor subtypes that inhibit adenylyl cyclase. Recent studies, reviewed here, have shown that multiple effector systems can be activated by these receptors, and the potential involvement of these in dopaminergic neutrotransmission is discussed.

Dual signal transduction through delta opioid receptors in a transfected human T-cell line

Opiates are known to function as immunomodulators, in part by effects on T cells. However, the signal transduction pathways mediating the effects of opiates on T cells are largely undefined. To determine whether pathways that regulate free intracellular calcium ([Ca2+]i) and/or cAMP are affected by opiates acting through delta-type opioid receptors (DORs), a cDNA encoding the neuronal DOR was expressed in a stably transfected Jurkat T-cell line. The DOR agonists, deltorphin and [D-Ala2, D-Leu5]-enkephalin (DADLE), elevated [Ca2+]i, measured by flow cytofluorometry using the calcium-sensitive dye, Fluo-3. At concentrations from 10(-11)-10(-7) M, both agonists increased [Ca2+]i from 60 nM to peak concentrations of 400 nM in a dose-dependent manner within 30 sec (ED50 of approximately 5 x 10(-9) M). Naltrindole, a selective DOR antagonist, abolished the increase in [Ca2+]i, and pretreatment with pertussis toxin was also effective. To assess the role of extracellular calcium, cells were pretreated with EGTA, which reduced the initial deltorphin-induced elevation of [Ca2+]i by more than 50% and eliminated the second phase of calcium mobilization. Additionally, the effect of DADLE on forskolin-stimulated cAMP production was determined. DADLE reduced cAMP production by 70% (IC50 of approximately equal to 10(-11) M), and pertussis toxin inhibited the action of DADLE. Thus, the DOR expressed by a transfected Jurkat T-cell line is positively coupled to pathways leading to calcium mobilization and negatively coupled to adenylate cyclase. These studies identify two pertussis toxin-sensitive, G protein-mediated signaling pathways through which DOR agonists regulate the levels of intracellular messengers that modulate T-cell activation.