High constitutive activity of native H3 receptors regulates histamine neurons in brain

Protean agonism at histamine H3 receptors in vitro and in vivo

G protein-coupled receptors (GPCRs) are allosteric proteins that adopt inactive (R) and active (R*) conformations in equilibrium. R* is promoted by agonists or occurs spontaneously, leading to constitutive activity of the receptor. Conversely, inverse agonists promote R and decrease constitutive activity. The existence of another pharmacological entity, referred to as "protean" agonists (after Proteus, the Greek god who could change shape), was assumed on theoretical grounds. It was predicted from the existence of constitutive activity that a same ligand of this class could act either as an agonist or an inverse agonist at the same GPCR. Here, we show that proxyfan, a high-affinity histamine H3-receptor ligand, acts as a protean agonist at recombinant H3 receptors expressed in the same Chinese hamster ovary cells. In support of the physiological relevance of the process, we show that proxyfan also behaves as a protean agonist at native H3 receptors known to display constitutive activity. On neurochemical and behavioral responses in rodents and cats, proxyfan displays a spectrum of activity ranging from full agonism to full inverse agonism. Thus, protean agonism demonstrates the existence of ligand-directed active states LR* different from, and competing with, constitutively active states R* of GPCRs, and defines a pharmacological entity with important therapeutic implications.

A new G protein-coupled receptor from a primitive metazoan shows homology with vertebrate aminergic receptors and displays constitutive activity in mammalian cells

Biogenic amine receptors mediate wide-ranging hormonal and modulatory functions in vertebrates, but are largely unknown in primitive invertebrates. In a representative of the most basal multicellular animals possessing a nervous system, the cnidarian Renilla koellikeri, aminergic-like receptors were previously characterized pharmacologically and found to engender control of the animal's bioluminescent and peristaltic reactions. Using degenerate oligonucleotides in a RT-PCR strategy, we obtained a full-length cDNA encoding a polypeptide with typical G protein-coupled receptor (GPCR) characteristics and which displayed a significant degree of sequence similarity (up to 45%) to biogenic amine receptors, particularly dopamine and adrenergic receptors. The new receptor, named Ren1, did not resemble any one specific type of amine GPCR and thus could not be identified on the basis of sequence. Ren1 was expressed transiently and stably in cultured mammalian cells, as demonstrated by immunocytochemistry and western blotting. Functional analysis of transfected HEK293, LTK- and COS-7 cells, based on both cAMP and Ca2+ signalling assays, revealed that Ren1 was not activated by any of the known biogenic amines tested and several related metabolites. The results indicated, however, that cells stably expressing Ren1 contained, on average, an 11-fold higher level of cAMP than the controls, in the absence of agonist stimulation. The high basal cAMP levels were shown to be specific for Ren1 and to vary proportionally with the level of Ren1 expressed in the transfected cells. Taken together, the data suggested that Ren1 was expressed as a constitutively active receptor. Its identification provides a basis for examination of the early evolutionary emergence of GPCRs and their functional properties.

The role of HB-donor groups in the heterocyclic polar fragment of H3-antagonists

It has been recently reported that compounds composed of an imidazole connected through an alkyl spacer to a 2-aminobenzimidazole showed high affinity towards the H(3)-receptor. The guanidine fragment of the 2-aminobenzimidazole is probably involved in hydrogen bond interactions at the binding site, and is referred to as the 'polar fragment'. In the present work, starting from 2-aminobenzimidazole derivatives with a di-methylene spacer 1 (pK(i)=7.25) or a tri-methylene one 2 (pK(i)=8.90), we investigated the importance of the hydrogen bond (HB) donor groups at the polar fragment in the interaction with the H(3)-receptor. The replacement of 2-aminobenzimidazoles with different moieties [2-aminobenzothiazole, 3, 4; 2-thiobenzimidazole, 5, 6; 2-thiobenzothiazole, 7, 8; 2-thio-4-phenyl- or 2-thio-5-phenyl-N-methylimidazoles, 9-12] highlighted the effect of the polar group basicity on the optimal length of the alkyl chain: longer spacers were preferred with polar groups of moderate basicity whereas, in the presence of neutral polar groups, the best affinity values were obtained with di-methylene chains. Moreover, N-methylation at the 2-aminobenzimidazole moiety 13-16 revealed different behaviour for compounds having different spacer lengths. In fact, methylation of the exocyclic NH group maintained high affinity for the tri-methylene 2-aminobenzimidazole derivative, while a drop in affinity was observed for the annular N-methylation. An opposite trend characterised di-methylene derivatives. These observed SAR suggest that, within this class of compounds, the number of HB-donor groups can be lowered while maintaining high receptor affinity. Since the presence of HB-donor groups strongly affects brain access, this observation could be useful to design and prepare new H(3)-antagonists.

The beta2-adrenergic receptor specifically sequesters Gs but signals through both Gs and Gi/o in rat sympathetic neurons

Beta(2)-adrenergic receptors (beta(2)-AR) and CB1 cannabinoid receptors share the property of being constitutively active. The CB1 cannabinoid receptor can also sequester G(i/o) proteins; however, it is not known whether the beta(2)-AR can also sequester G proteins. Beta(2)-ARs were heterologously expressed in rat superior cervical ganglion neurons by microinjection of cDNA and studied using the patch-clamp technique. The beta-AR agonist isoproterenol increased the Ca(2+) current 25.9+/-1.6% in neurons microinjected with 100 ng/microl beta(2)-AR cDNA but was without effect on control neurons. Pretreatment with cholera toxin (CTX) abolished the effect of isoproterenol, indicating coupling via G(s) proteins. In neurons microinjected with 200 ng/microl beta(2)-AR cDNA, isoproterenol had the opposite effect of inhibiting the Ca(2+) current 36.5+/-2.0%. Inhibition of the Ca(2+) current was sensitive to pertussis toxin, indicating beta(2)-AR coupling to G(i/o) proteins. Pretreatment with CTX resulted in a greater 54+/-3.8% inhibition of the Ca(2+) current, indicating that G(s) coupling masks the full effect of G(i/o) coupling. Expression of beta(2)-ARs abolished signaling by G(s)-coupled receptors for vasoactive intestinal polypeptide (VIP). VIP inhibited the Ca(2+) current 49.5+/-0.5% in control neurons but had no effect in neurons expressing beta(2)-ARs. In contrast, expression of beta(2)-ARs had no effect on signaling by the G(i/o)-coupled alpha(2)-adrenergic receptor. This study demonstrates that the beta(2)-AR couples to both G(s) and G(i/o) proteins but specifically sequesters G(s) proteins, preventing their interaction with another G(s)-coupled receptor. beta(2)-adrenergic receptors thus have the potential to prevent other G(s)-coupled receptors from transducing their biological signals.

Therapeutic implications of constitutive activity of receptors: the example of the histamine H3 receptor

Some G-protein-coupled receptors display constitutive activity, that is spontaneous activity in the absence of agonist: a proportion of the receptor population adopts a conformation that can bind and activate G proteins. Whereas this was mainly shown to occur with recombinant or pathologically mutated receptors, the physiological relevance of the process has remained debated. We have adressed this question in the case of the histamine H3 receptor, a presynaptic inhibitory receptor regulating histamine release in brain. Having identified a neutral antagonist and inverse agonists with variable intrinsic activity, we show that the native H3 receptor in brain displays high constitutive activity in vitro and, in vivo, controls the release of endogenous histamine. This implies that inverse agonists with high intrinsic activity should be preferred for therapeutic application as "cognitive enhancers" in several psychiatric disorders.

Activation of spinal histamine H3 receptors inhibits mechanical nociception

Previous studies have suggested a possible pain-modulatory role for histamine H(3) receptors, but the localization of these receptors and nature of this modulation is not clear. In order to explore the role of spinal histamine H(3) receptors in the inhibition of nociception, the effects of systemically (subcutaneous, s.c.) and intrathecally (i.t.) administered histamine H(3) receptor agonists were studied in rats and mice. Immepip (5 mg/kg, s.c.) produced robust antinociception in rats on a mechanical (tail pinch) test but did not alter nociceptive responses on a thermal (tail flick) test. In contrast, this treatment in mice (immepip, 5 and 30 mg/kg, s.c.) did not change either mechanically or thermally evoked nociceptive responses. When administered directly into the spinal subarachnoid space, immepip (15-50 microg, i.t.) and R-alpha-methylhistamine (50 microg, i.t.) had no effect in rats on the tail flick and hot plate tests, but produced a dose- and time-dependent inhibition (90-100%) of nociceptive responses on the tail pinch test. This attenuation was blocked by administration of thioperamide (10 mg/kg, s.c.), a histamine H(3) receptor antagonist. Intrathecally administered thioperamide also reversed antinociceptive responses induced by systemically administered immepip, which demonstrates a spinal site of action for the histamine H(3) receptor agonist. In addition, intrathecally administered immepip (25 microg) produced maximal antinociception on the tail pinch test in wild type, but not in histamine H(3) receptor knockout (H(3)KO) mice. These findings demonstrate an antinociceptive role for spinal histamine H(3) receptors. Further studies are needed to confirm the existence of modality-specific (i.e. mechanical vs. thermal) inhibition of nociception by these receptors, and to assess the efficacy of spinally delivered histamine H(3) receptor agonists for the treatment for pain.

Inverse agonism gains weight

Inverse agonism is emerging as a new endogenous principle for receptor regulation. Agouti-related protein (AgRP), following its release in the brain, stimulates food intake. AgRP binds to brain melanocortin receptors, which are involved in the regulation of body weight. In addition to antagonizing the effects of the melanocortin receptor agonist alpha-melanocyte-stimulating hormone (alpha-MSH), AgRP suppresses the constitutive activity of melanocortin MC(3) and MC(4) receptors, which characterizes AgRP as an inverse agonist rather than a neutral antagonist. The balance between the activity of AgRP-containing neurons and alpha-MSH-containing neurons determines the extent of activation of melanocortin receptors in neurons onto which they project. The identification of AgRP as an endogenous inverse agonist provides physiological relevance to inverse agonism in the control of body weight.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Two novel and selective nonimidazole histamine H3 receptor antagonists A-304121 and A-317920: I. In vitro pharmacological effects

Histamine H3 receptor (H3R) antagonists enhance neurotransmitter release and are being developed for the treatment of a variety of neurological and cognitive disorders. Many potent histamine H3R antagonists contain an imidazole moiety that limits receptor selectivity and the tolerability of this class of compounds. Here we present the in vitro pharmacological data for two novel piperazine amide ligands, A-304121 [4-(3-((2R)-2-aminopropanoyl-1-piperazinyl)propoxy)phenyl)cyclopropylmethanone] and A-317920 [N-((1R)-2-(4-(3-(4-(cyclopropylcarbonyl)phenoxy)propyl)-1-piperazinyl)-1-methyl-2-oxo-ethyl-)-2-furamide], and compare them with the imidazole H3R antagonists ciproxifan, clobenpropit, and thioperamide. Both A-304121 and A-317920 bind potently to recombinant full-length rat H3R(pKi values = 8.6 and 9.2, respectively) but have lower potencies for binding the full-length human H3R (pKi values = 6.1 and 7.0, respectively). A-304121 and A-317920 are potent antagonists at rat H3R in reversing R-alpha-methylhistamine [(R)-alpha-MeHA] inhibition of forskolin-stimulated cAMP formation (pKb values = 8.0 and 9.1) but weak antagonists at human H3Rs in cyclase (pKb values = 6.0 and 6.3) and calcium mobilization (pKb values = 6.0 and 7.3) assays in cells co-expressing Galphaqi5-protein. Both compounds potently antagonize native H3Rs by blocking histamine inhibition of potassium-evoked [3H]histamine release from rat brain cortical synaptosomes (pKb values = 8.6 and 9.3) and (R)-alpha-MeHA reversal of electric field-stimulated guinea pig ileum contractions (pA2 values = 7.1 and 8.3). A-304121 and A-317920 are also more efficacious inverse agonists in reversing basal guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTP gamma S) binding at the human H3R (pEC50 values = 5.7 and 7.0) than are the imidazole antagonists. These novel and selective piperazine amides represent useful leads for the development of H3R antagonist therapeutic agents.

Analysis of two D1-like dopamine receptors from the honey bee Apis mellifera reveals agonist-independent activity

Dopamine is found in many invertebrate organisms, including insects, however, the mechanisms through which this amine operates remain unclear. We have expressed two dopamine receptors cloned from honey bee (AmDOP1 and AmDOP2) in insect cells (Spodoptera frugiperda), and compared their pharmacology directly using production of cAMP as a functional assay. In each assay, AmDOP1 receptors required lower concentrations of dopamine and 6,7-ADTN for maximal activation than AmDOP2 receptors. Conversely, butaclamol and cis(Z)-flupentixol were more potent at blocking the cAMP response mediated through AmDOP2 than AmDOP1 receptors. Expression of AmDOP1, but not AmDOP2, receptors significantly increased levels of cAMP even in the absence of ligand. This constitutive activity was blocked by cis(Z)-flupentixol. This work provides the first evidence of a constitutively activated dopamine receptor in invertebrates and suggests that although AmDOP1 and AmDOP2 share much less homology than their vertebrate counterparts, they display a number of functional parallels with the mammalian D1-like dopamine receptors.

Novel nonimidazole histamine H3 receptor antagonists: 1-(4-(phenoxymethyl)benzyl)piperidines and related compounds

In an extension of very recently published studies on successful imidazole replacements in some series of histamine H(3) receptor antagonists, we report on a new class of lipophilic nonimidazole antagonist having an aliphatic tertiary amino moiety connected to a benzyl template substituted in the 4-position by a phenoxymethyl group. The structural modifications were performed with the intention to avoid possible negative side effects reported for other series of antagonists. The novel compounds combine different characteristics of recently developed histamine H(3) receptor antagonists. The compounds were screened for their affinity in a binding assay for the human histamine H(3) receptor stably expressed in CHO-K1 cells and tested for their in vivo potency in the central nervous system of mice after oral administration. Different substitution patterns on the phenoxy group were used to optimize in vitro and/or in vivo potency leading to some compounds with low nanomolar affinity and high oral in vivo potency. Modifications of the basic piperidino moiety were performed by ring expansion, contraction, and opening. Selected compounds exhibited selectivity in functional assays on isolated organs of guinea-pig for H(3) vs H(1) and H(2) receptors. Unexpectedly, some of the novel antagonists also showed a slight preference for the human histamine H(3) receptor compared to their affinities for the guinea-pig H(3) receptor.

Creation, expression, and characterization of a constitutively active mutant of the human serotonin 5-HT6 receptor

The serotonin 5-HT(6) receptor, a G-protein-coupled receptor, displays high affinity for antipsychotic, antidepressant, and psychotropic drugs. We created a constitutively active form of the human 5-HT(6) receptor in order to probe the molecular domains of receptor activation and to determine if inverse agonist activities of antipsychotic drugs contribute to their clinical profile. Previous studies from our laboratory support a critical role for the c-terminal region of the third intracellular loop (il3) in the activation of G(q)-coupled serotonin receptors. In the present study, PCR-based mutagenesis was used to mutate serine 267 (S6.34) in the c-terminal region of il3 to lysine (S267K). The native and S267K 5-HT(6) receptors were expressed in COS-7 cells to study the functional effects of the mutation. The S267K receptor shows 10-fold higher affinity for serotonin than the native receptor and demonstrates agonist-independent activity. Clozapine decreased the basal activity of the S267K receptor to vector control levels. Therefore, we can conclude that the S267K mutation renders the 5-HT(6) receptor constitutively active and that clozapine is an inverse agonist at the mutant 5-HT(6) receptor. These results indicate that the c-terminal region of il3 of the G(s)-coupled 5-HT(6) receptor is a key domain for G-protein coupling, similar to the G(q)-coupled 5-HT receptors. The inverse agonist action of clozapine indicates that drugs displaying competitive antagonist activity at native 5-HT(6) receptors may display inverse agonist activity at the constitutively activated form of the receptor.

Spontaneous receptor-independent heterotrimeric G-protein signalling in an RGS mutant

Tripartite G-protein-coupled receptors (GPCRs) represent one of the largest groups of signal transducers, transmitting signals from hormones, neuropeptides, odorants, food and light. Ligand-bound receptors catalyse GDP/GTP exchange on the G-protein alpha-subunit (Galpha), leading to alpha-GTP separation from the betagamma subunits and pathway activation. Activating mutations in the receptors or G proteins underlie many human diseases, including some cancers, dwarfism and premature puberty. Regulators of G-protein signalling (RGS proteins) are known to modulate the level and duration of ligand-induced signalling by accelerating the intrinsic GTPase activity of the Galpha subunit, and thus reformation of the inactive GDP-bound Galpha. Here we find that even in the absence of receptor, mutation of the RGS family member Sst2 (refs 6-9) permits spontaneous activation of the G-protein-coupled mating pathway in Saccharomyces cerevisiae at levels normally seen only in the presence of ligand. Our work demonstrates the occurrence of spontaneous tripartite G-protein signalling in vivo and identifies a requirement for RGS proteins in preventing such receptor-independent activation.

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.

BML-190 and AM251 act as inverse agonists at the human cannabinoid CB2 receptor: signalling via cAMP and inositol phosphates

The aminoalkylindole BML-190 and diarylpyrazole AM251 ligands have previously been shown to bind to cannabinoid CB(2) and CB(1) receptors, respectively. In HEK-293 cells stably expressing the human CB(2) receptor, BML-190 and AM251 potentiated the forskolin-stimulated accumulation of cAMP. Moreover, the CB(2) receptor can interact productively with 16z44, a promiscuous G alpha(16/z) chimera. BML-190 and AM251 reduce the basal levels of inositol phosphate production in cells expressing the CB(2) receptor and 16z44. These results demonstrate that BML-190 and AM251 act as inverse agonists at the human CB(2) receptor acting via G alpha(i/o) and G alpha(q) family-coupled pathways.

Norepinephrine release from the ischemic heart is greatly enhanced in mice lacking histamine H3 receptors

We previously reported that histamine H(3) receptors (H(3)Rs) are present in cardiac sympathetic nerve endings (cSNE) of animals and humans, where they attenuate norepinephrine (NE) release in normal and hyperadrenergic states, such as myocardial ischemia. The recent creation of a transgenic line of mice lacking H(3)R provided us with the opportunity to assess the relevance of H(3)R in the ischemic heart. We isolated SNE from hearts of wild-type (H(3)R(+/+)) and knockout (H(3)R(-/-)) mice and found that basal NE release from H(3)R(-/-) cSNE was approximately 60% greater than that from H(3)R(+/+) cSNE. NE exocytosis evoked by K(+)-induced depolarization of cSNE from H(3)R(+/+) mice was attenuated by activation of either H(3)R or adenosine A(1) receptors (A(1)R). In contrast, NE release from cSNE of H(3)R(-/-) was unaffected by H(3)R agonists, but it was still attenuated by A(1)R activation. When isolated mouse hearts were subjected to ischemia for 20 min, NE overflow into the coronaries was 2-fold greater in the H(3)R(-/-) hearts than in those from H(3)R(+/+) mice. Furthermore, whereas stimulation of H(3)R or A(1)R reduced ischemic NE overflow from H(3)R(+/+) hearts by 50%, only A(1)R, but not H(3)R activation, reduced NE release in H(3)R(-/-). Our data demonstrate that NE release from cSNE can be modulated by various heteroinhibitory receptors (e.g., H(3)R and A(1)R) and that H(3)Rs are particularly important in modulating NE release in myocardial ischemia. Inasmuch as excessive NE release is clinically recognized as a major cause of arrhythmic cardiac dysfunction, our findings reveal a significant cardioprotective role of H(3)R on cSNE.

Molecular Analysis of the Structure and Function of the Angiotensin II Type 1 Receptor

The renin-angiotensin system hormone angiotensin II (Ang II) plays a central role in the pathophysiology of hypertension, cardiac hypertrophy, congestive heart failure, and coronary heart disease. Two distinct subtypes of Ang II receptor, type 1 (AT1) and type 2 (AT2), have been identified, and both have been shown to belong to the G-protein-coupled receptor superfamily (GPCRs). The recent Human Genome Project has revealed more than 1,000 transmembrane (TM) receptors that belong to this superfamily, and it has been estimated that 50% of all clinically used medicines modulate GPCRs activity. Recently, there have been many new insights regarding Ang II receptors and other GPCRs, such as on homo- and hetero-oligomerization, constitutive activation, movement of TM helices, internalization, desensitization and phosphorylation, trafficking, nuclear localization, intracellular protein-induced receptor activation, and receptor-associated proteins. Although AT1 receptor antagonists which prevent Ang II-induced signaling are already clinically available, we here summarize new findings regarding their structure and function, and the possibility of new therapeutic strategies for targeting Ang II receptors through molecular biological techniques.

Ciproxifan, a histamine H3-receptor antagonist/inverse agonist, modulates the effects of methamphetamine on neuropeptide mRNA expression in rat striatum

We have explored the effect of histamine H3-receptor ligands on the regulation of neuropeptide mRNA expression in the striatum by using in situ hybridization performed with proenkephalin, prodynorphin, substance P and proneurotensin riboprobes. Acute administration of ciproxifan, an H3-receptor antagonist/inverse agonist, or (R)-alpha-methylhistamine, an H3-receptor agonist, did not modify the striatal expression of the neuropeptides by itself. However, ciproxifan strongly and differentially modulated the effect of a single administration of 3 mg/kg methamphetamine on neuropeptide mRNA expression. This modulation was suppressed by the administration of (R)-alpha-methylhistamine and occurred in both the caudate-putamen and nucleus accumbens. Ciproxifan strongly potentiated the decrease of proenkephalin mRNA expression induced by methamphetamine. In contrast, it suppressed the increase in prodynorphin and substance P mRNA expression induced by methamphetamine. Methamphetamine alone or with ciproxifan did not modify proneurotensin mRNA expression. These neurochemical findings indicate that ciproxifan differentially regulates the effect of methamphetamine on the neuropeptides contained in striatonigral and striatopallidal neurons. They suggest that endogenous histamine and dopamine cooperate to modulate the activity of striatal projection neurons and strengthen the interest of H3-receptors as new targets for the treatment of psychotic disorders and drug abuse.

A detailed mapping of the histamine H(3) receptor and its gene transcripts in rat brain

The detailed distribution of histamine H(3) receptor mRNAs in rat brain was analyzed by in situ hybridization using a 33P-labelled riboprobe and was combined for the first time with the detailed autoradiographic distribution of the receptor determined in the same animals with [(125)I]iodoproxyfan, a selective radioligand. The signals generated on adjacent brain sections by each probe were quantified and/or rated and were compared in order to identify neuronal populations expressing the receptor. In addition, the cellular localization of the transcripts within various brain structures was analyzed in sections dipped in a photographic emulsion. In the cerebral cortex, the strong mRNA expression in intermediate and deep layers indicates the presence of H(3) receptors on several types of neurons. The binding is dense except in layer V, suggesting that H(3) receptors are located on granule cells and apical dendrites of pyramidal cells. In addition to their localization on monoaminergic afferents, the dense binding in layer IV and strong mRNA expression in thalamic nuclei suggest the presence of heteroreceptors on thalamocortical projections. In the hippocampus, the strong mRNA expression but low binding in pyramidal layers of the CA1 and ventral CA3 fields suggest that H(3) receptors are abundant on efferent projections of pyramidal cells. In the dentate gyrus, some binding sites in the molecular layer may correspond to H(3) receptors synthesized in granule cells and coexpressed with H(1) and H(2) receptors in their dendrites. In the basal ganglia, H(3) receptors are highly expressed in the striatal complex and olfactory tubercles but not in islands of Calleja. Some of the striatal binding sites may correspond to presynaptic receptors present on afferents. The mRNAs in cortical layer V may encode for heteroreceptors on corticostriatal neurons. The presence of mRNAs in the substantia nigra pars compacta suggests that H(3) receptors are located upon nigrostriatal afferents. However, the absence of any signal in the ventral tegmental area indicates that some but not all dopaminergic neurons express H(3) receptors. In addition, the homogeneous mRNA expression within the caudate putamen and nucleus accumbens suggests that many striatal H(3) receptors are present on medium-sized, spiny projection neurons of both the direct and indirect movement pathways. In agreement, a dense binding, but low mRNA expression, is observed in external and internal pallidum and in substantia nigra pars reticulata. In the amygdala, the dense binding and mRNA expression indicate the presence of receptors on both afferents and projections. In the thalamus, the binding in some association nuclei may correspond to receptors present on neurons emanating from the deep cortical layers that strongly express the mRNAs, as well as receptors on the visual systems. However, the low binding and high mRNA expression in most nuclei indicate that many receptors are present upon thalamic projections. In the hypothalamus, the mRNA expression parallels the density of binding sites and is the highest in the tuberomammillary nucleus. Further investigation is needed to know if the dense binding and mRNA expression observed in other nuclei such as the paraventricular, ventromedial and medial tuberal nuclei correspond to pre- and/or postsynaptic receptors. mRNAs are also observed in several areas projecting to the tuberomammillary nucleus, such as the ventrolateral preoptic nucleus. In the lower brainstem, the high mRNA expression and very low binding in the locus coeruleus and raphe nuclei indicate that presynaptic rather than somatodendritic receptors regulate noradrenaline and serotonin release, respectively. A similar pattern in vestibular nuclei suggests that receptors located on projections account for the anti-vertigo properties of H(3) receptor antagonists. In the cerebellum, binding is hardly detectable but a strong mRNA expression is found in most, if not all, Purkinje cells as well as in several central cerebellar nuclei, suggesting the presence of H(3) receptors on efferent projections. The present study reports the first detailed quantification and/or rating of H(3) receptor mRNAs in the brain. The comparison, performed in the same animals, with the distribution of the H(3) receptor protein provides evidence for the presence of H(3) receptors on many neuronal perikarya, dendrites and projections. Although some localizations, mainly as auto- or heteroreceptors, are consistent with previous functional studies, the physiological role, if any, of most of these presynaptic or postsynaptic receptors remains to be established.

Characteristics of recombinantly expressed rat and human histamine H3 receptors

Human and rat histamine H(3) receptors were recombinantly expressed and characterized using receptor binding and a functional cAMP assay. Seven of nine agonists had similar affinities and potencies at the rat and human histamine H(3) receptor. S-alpha-methylhistamine had a significantly higher affinity and potency at the human than rat receptor, and for 4-[(1R*,2R*)-2-(5,5-dimethyl-1-hexynyl)cyclopropyl]-1H-imidazole (Perceptin) the preference was the reverse. Only two of six antagonists had similar affinities and potencies at the human and the rat histamine H(3) receptor. Ciproxifan, thioperamide and (1R*,2R*)-trans-2-imidazol-4 ylcyclopropyl) (cyclohexylmethoxy) carboxamide (GT2394) had significantly higher affinities and potencies at the rat than at the human histamine H(3) receptor, while for N-(4-chlorobenzyl)-N-(7-pyrrolodin-1-ylheptyl)guanidine (JB98064) the preference was the reverse. All antagonists also showed potent inverse agonism properties. Iodoproxyfan, Perceptin, proxyfan and GR175737, compounds previously described as histamine H(3) receptor antagonists, acted as full or partial agonists at both the rat and the human histamine H(3) receptor.

Ciproxifan, a histamine H3-receptor antagonist/inverse agonist, potentiates neurochemical and behavioral effects of haloperidol in the rat

By using double in situ hybridization performed with proenkephalin and H3-receptor riboprobes on the same sections from rat brain, we show that histamine H3 receptors are expressed within striatopallidal neurons of the indirect movement pathway. The majority ( approximately 70%) of striatal enkephalin neurons express H3-receptor mRNAs. This important degree of coexpression of proenkephalin and H3-receptor mRNAs prompted us to explore the effect of H3-receptor ligands on the regulation of enkephalin mRNA expression in the striatum. Acute administration of ciproxifan, a H3-receptor antagonist/inverse agonist, did not modify the expression of the neuropeptide by itself but strongly increased the upregulation of its expression induced by haloperidol. This potentiation (1) was suppressed by the administration of (R)-alpha-methylhistamine, a H3-receptor agonist, (2) occurred both in the caudate-putamen and nucleus accumbens, and (3) was also observed with a similar pattern on c-fos and neurotensin mRNA expression. Similarly, whereas it was devoid of any motor effect when used alone, ciproxifan strongly potentiated haloperidol-induced locomotor hypoactivity and catalepsy, two behaviors in which striatal neurons are involved. The strong H3-receptor mRNA expression in enkephalin neurons suggests that the synergistic neurochemical and motor effects of ciproxifan and haloperidol result from direct H3/D2-receptor interactions, leading to an enhanced activation of striatopallidal neurons of the indirect movement pathway. The potentiation of the effects of haloperidol by ciproxifan strengthens the potential interest of H3-receptor antagonists/inverse agonists to improve the symptomatic treatment of schizophrenia.

Influence of bulky substituents on histamine h(3) receptor agonist/antagonist properties

Novel derivatives of 3-(1H-imidazol-4-yl)propanol were designed on the basis of lead compounds belonging to the carbamate or ether series possessing (partial) agonist properties on screening assays of the histamine H(3) receptor. One pair of enantiomers in the series of alpha-methyl-branched chiral carbamates was stereoselectively prepared in high optical yields. Enantiomeric purity was checked by Mosher amide derivatives of precursors and capillary electrophoresis of the final compounds with trimethyl-beta-cyclodextrin as chiral selector, and was determined to be >/=95%. The novel compounds were investigated in various histamine H(3) receptor assays in vitro and in vivo. Some compounds displayed partial agonist activity on synaptosomes of rat brain cortex, whereas others exhibited antagonist properties only. Selected compounds were investigated in [(125)I]iodoproxyfan binding studies on the human histamine H(3) receptor and showed high affinity in the nanomolar concentration range. Under in vivo conditions after oral administration to mice, some of the compounds exhibited partial or full agonist activity in the brain at low dosages. The (S)-enantiomer of one pair of chiral carbamates (9) proved to be the eutomer; thus, the (S)-enantiomer was selected for further pharmacological studies. In a peripheral in vivo test model in rats, measuring the level of inhibition of capsaicin-induced plasma extravasation, (S)-9 again proved its high oral agonist potency with full intrinsic activity (ED(50) values of 0.07-0.1 mg/kg depending on tissue).

Piperidino-hydrocarbon compounds as novel non-imidazole histamine H(3)-receptor antagonists

In search for novel non-imidazole histamine H(3)-receptor antagonists, piperidino-hydrocarbon compounds were synthesized using the known non-imidazole histamine H(3)-receptor antagonist FUB 637 (3-phenylpropyl 3-piperidinopropyl ether) as lead structure. Piperidino-alkyl derivatives containing highly flexible side chains (2, 4-7) were prepared via N-alkylation. Compounds containing unsaturated alkyl groups were synthesized in order to investigate the impact of rigidifying the side chain (8-16). Terminal alkynes were prepared by alkylation of lithium acetylide-ethylenediamine complex, disubstituted alkynes were synthesized by alkylation of the appropriate acetylene in the presence of n-butyllithium-N,N,N',N'-tetramethylene-ethylene-diamine complex. The novel compounds were investigated in an in vitro functional assay on the guinea-pig ileum, in which N-(7-phenylhept-3-ynyl)piperidine (14) proved to be of good potency in this class (pA(2)=7.21). In an in vivo assay the compounds were additionally screened for their abilities to influence central H(3)-histaminergic neuron activity in mice with regard to their oral availabilities and distribution properties. In this screening, N-pent-4-ynylpiperidine (9) and N-hex-5-ynylpiperidine (10) proved to be highly potent and orally available histamine H(3)-receptor antagonists. The ED(50) values for 9 and 10 were 1.3 and 1.4mg/kg po, respectively, which is in the potency range of the reference antagonist thioperamide.

The use of constitutively active GPCRs in drug discovery and functional genomics

The complete sequencing of the human genome has afforded researchers the opportunity to identify novel G-protein-coupled receptors (GPCRs) that are expressed in human tissues. The successful identification of hundreds of GPCRs represents the single greatest opportunity for novel drug development today. However, the lack of identified ligands for these GPCRs has limited their utility for traditional drug discovery approaches that focus on ligand-based assay methods to discover and pharmacologically characterize drug candidates. Here, we review the use of constitutively activated GPCRs in the discovery pathway, both as a means to overcome the limitations of traditional drug discovery at novel GPCRs and as a tool to investigate the functionality of these receptors.

Betahistine dihydrochloride interaction with the histaminergic system in the cat: neurochemical and molecular mechanisms

Drugs interfering with the histaminergic system facilitate behavioral recovery after vestibular lesion, likely by increasing histamine turnover and release. The effects of betahistine (structural analogue of histamine) on the histaminergic system were tested by quantifying messenger RNA for histidine decarboxylase (enzyme synthesizing histamine) by in situ hybridization and binding to histamine H(3) receptors (mediating, namely, histamine autoinhibition) using a histamine H(3) receptor agonist ([(3)H]N-alpha-methylhistamine) and radioautography methods. Experiments were done in brain sections of control cats (N=6) and cats treated with betahistine for 1 (N=6) or 3 (N=6) weeks. Betahistine treatment induced symmetrical changes with up-regulation of histidine decarboxylase mRNA in the tuberomammillary nucleus and reduction of [(3)H]N-alpha-methylhistamine labeling in both the tuberomammillary nucleus, the vestibular nuclei complex and nuclei of the inferior olive. These findings suggest that betahistine upregulates histamine turnover and release, very likely by blocking presynaptic histamine H(3) receptors, and induces histamine H(3) receptor downregulation. This action on the histaminergic system could explain the effectiveness of betahistine in the treatment of vertigo and vestibular disease.

Homer as both a scaffold and transduction molecule

Increasing evidence shows that scaffold proteins not only control membrane assembly of receptors and channels, but also modulate intracellular signaling by assembled receptors. The Homer family of proteins act as scaffolds to bind clusters of proteins and glutamate receptors at postsynaptic sites. We review results of cloning and gene expression of this protein family, and summarize roles in glutamate receptor function and intracellular signaling in neurons. Homer proteins trigger the localization of metabotropic glutamate receptor subtype 5 (mGlu5 receptor) to the postsynaptic plasma membrane. They can also alter the kinetics and peak amplitude of the intracellular Ca2+ responses of mGlu1 and mGlu5 receptors. Homer proteins can either prevent or promote spontaneous activation of these receptors, depending on the type of Homer protein isoform expressed.

Differential in vivo effects of H3 receptor ligands in a new mouse dipsogenia model

The selective H(3) receptor agonist (R)-alpha-methylhistamine [(R)-alpha-MeHA] stimulates drinking in the adult rat. In the present study, we investigated the role of the H(3) receptor in mediating this behavior in a new dipsogenia model using the CD-1 mouse. In addition, the putative inverse agonists ciproxifan, thioperamide and clobenpropit; the reported antagonist (1R,2R)-4-[2-(5,5-dimethylhex-1-ynyl)cyclopropyl]imidazole (GT-2331); and the putative neutral antagonist/weak partial agonist proxyfan were evaluated for possible differences in pharmacological activity in this new model. Water intake increased over baseline in a dose-related manner following intraperitoneal administration of 80, 160 or 240 micromol/kg (R)-alpha-MeHA, but this effect was dependent on age (P30<P60<P80=P120). [3H]-N-alpha-methylhistamine binding studies showed no change in H(3) receptor density for the whole mouse brain at these ages. All subsequent studies employed P80 mice dosed with 240 micromol/kg (R)-alpha-MeHA. Ciproxifan (0.001-30 micromol/kg), thioperamide (0.01-10 micromol/kg), clobenpropit (0.1-30 micromol/kg) and GT-2331 (0.03-10 micromol/kg) attenuated drinking dose-dependently, blocking the response completely at the highest doses in each case. In contrast, proxyfan (0.001-10 micromol/kg) only partially attenuated drinking elicited by (R)-alpha-MeHA: coadministration of proxyfan and ciproxifan resulted in an attenuation of ciproxifan's effects. This new dipsogenia model provides the first in vivo behavioral evidence for possible pharmacological differences between three putative H(3) receptor inverse agonists, GT-2331 and proxyfan.

Polycystin-1 activation of c-Jun N-terminal kinase and AP-1 is mediated by heterotrimeric G proteins

Functional analysis of polycystin-1, the product of the gene most frequently mutated in autosomal dominant polycystic kidney disease, has revealed that this protein is involved in the regulation of diverse signaling pathways such as the activation of the transcription factor AP-1 and modulation of Wnt signaling. However, the initial steps involved in the activation of such cascades have remained unclear. We demonstrated previously that the C-terminal cytosolic tail of polycystin-1 binds and activates heterotrimeric G proteins in vitro. To test if polycystin-1 can activate cellular signaling cascades via heterotrimeric G protein subunits, polycystin-1 C-terminal tail-mediated c-Jun N-terminal kinase (JNK) and AP-1 activities were assayed in transiently transfected 293T cells in the presence of dominant-negative, G protein inhibiting constructs, and in the presence of cotransfected Galpha subunits. The results showed that polycystin-1-mediated JNK/AP-1 activation is mediated by Galpha and Gbetagamma subunits. Polycystin-1-mediated AP-1 activity could be significantly augmented by cotransfected Galpha(i), Galpha(q), and Galpha(12/13) subunits, suggesting that polycystin-1 can couple with and activate several heterotrimeric G protein families.

Progress in the proxifan class: heterocyclic congeners as novel potent and selective histamine H(3)-receptor antagonists

Histamine H(3) receptors are critically involved in the pathophysiology of several disorders of the central nervous system (CNS). Among other families of H(3)-receptor ligands, the proxifan class has recently been described to contain numerous potent histamine H(3)-receptor antagonists, e.g. ciproxifan or imoproxifan. In the present study, we report on the design of novel heterocyclic proxifan analogues and their antagonist potencies at histamine H(3) receptors. The new compounds were tested for in vitro and in vivo H(3)-receptor antagonist potencies in different species as well as for H(3)-receptor selectivity vs. H(1) and H(2) receptors. In vitro, all compounds investigated proved to be potent H(3)-receptor antagonists in the rat as well as in the guinea-pig. In addition, they showed good to high oral CNS potency in vivo in mice. Especially, oxadiazole derivatives 24-26 displayed nanomolar antagonist activity in vitro and high potency in vivo (ED(50)=0.47-0.57 mg/kg). The results show that the additional heteroaromatic moieties might act as bioisosteres of the ketone or oxime moieties of ciproxifan or imoproxifan, respectively, and might cause divergent pharmacokinetic properties. Thus, these novel H(3)-receptor antagonists are interesting leads for further development.

Importance of the gamma-aminobutyric acid(B) receptor C-termini for G-protein coupling

Functional gamma-aminobutyric acid(B) (GABA(B)) receptors assemble from two subunits, GABA(B(1)) and GABA(B(2).) This heteromerization, which involves a C-terminal coiled-coil interaction, ensures efficient surface trafficking and agonist-dependent G-protein activation. In the present study, we took a closer look at the implications of the intracellular C termini of GABA(B(1)) and GABA(B(2)) for G-protein coupling. We generated a series of C-terminal mutants of GABA(B(1)) and GABA(B(2)) and tested them for physical interaction, surface trafficking, coupling to adenylyl cyclase, and G-protein-gated inwardly rectifying potassium channels in human embryonic kidney (HEK) 293 cells as well as on endogenous calcium channels in sympathetic neurons of the superior cervical ganglion (SCG). We found that the C-terminal interaction contributes only partly to the heterodimeric assembly of the subunits, indicating the presence of an additional interaction site. The described endoplasmic reticulum retention signal within the C terminus of GABA(B(1)) functioned only in the context of specific amino acids, which constitute part of the GABA(B(1)) coiled-coil sequence. This finding may provide a link between the retention signal and its shielding by the coiled coil of GABA(B(2).) In HEK293 cells, we observed that the two well-known GABA(B) receptor antagonists [S-(R*,R*)]-[3-[[1-(3,4-dichlorophenyl)ethyl]amino]-2-hydroxypropyl](cyclohexylmethyl) phosphinic acid (CGP54626) and (+)-(2S)-5,5-dimethyl-2-morpholineacetic acid (SCH50911) CGP54626 and SCH50911 function as inverse agonists. The C termini of GABA(B(1)) and GABA(B(2)) strongly influenced agonist-independent G-protein coupling, although they were not necessary for agonist-dependent G-protein coupling. The C-terminal GABA(B) receptor mutants described here demonstrate that the active receptor conformation is stabilized by the coiled-coil interaction. Thus, the C-terminal conformation of the GABA(B) receptor may determine its constitutive activity, which could be a therapeutic target for inverse agonists.

Enhanced detection of receptor constitutive activity in the presence of regulators of G protein signaling: applications to the detection and analysis of inverse agonists and low-efficacy partial agonists

Fusion proteins between the human 5-hydroxytryptamine (5-HT)(1A) receptor and either wild type or certain pertussis toxin-resistant forms of G(o1)alpha and G(i1)alpha display constitutive GTPase activity that can be inhibited by the inverse agonist spiperone. Addition of recombinant regulator of G protein signaling (RGS) 1 or RGS16 to membranes expressing these fusion proteins resulted in elevation of this constitutive GTPase activity without significantly altering the binding affinity of antagonist/inverse agonist ligands. For a 5-HT(1A) receptor-(Cys(351)Ile)G(o1)alpha fusion protein the increase in basal GTPase activity was greater than 4-fold. Enzyme kinetic analysis demonstrated that the effect of RGS1 was as a GTPase-activating protein for the fusion construct. In the presence of the RGS proteins, both agonists and inverse agonists produced much more robust regulation of high-affinity GTPase activity than in their absence. This allowed detection of the partial agonist nature of WAY100635, which has been described previously as a neutral antagonist at the 5-HT(1A) receptor. Of a range of ligands studied, only haloperidol functioned as a neutral ligand in the presence of RGS1. These studies show that addition of a recombinant RGS protein provides a simple and novel means to elevate the fraction of basal membrane GTPase activity contributed by the constitutive activity of a receptor. By so doing, it also greatly enhances the ability to detect and analyze the effects of inverse agonists and to discriminate between neutral ligands and those with low levels of positive intrinsic efficacy.

Overexpression of alpha7 nicotinic acetylcholine receptor prevents G1-arrest and DNA fragmentation in PC12 cells after hypoxia

We investigated the neuroprotective function of alpha7 nicotinic acetylcholine receptor (alpha7nAChR) after transient hypoxia (12 h) and reoxygenation (0-72 h), comparing rat pheochromocytoma (PC12) cells overexpressing FLAG-tagged alpha7nAChR (alpha7pCMV cells) and control PC12 cells (non-transfected or transfected with vector only) in medium with and without nicotine. Plasma membrane degradation in the early phase after hypoxia was inhibited in PC12 cells with nicotine, and more profoundly in alpha7pCMV cells with nicotine. Inhibition of DNA fragmentation in the late phase after hypoxia was most remarkable in alpha7pCMV cells with nicotine, but, surprisingly, it was more remarkable in alpha7pCMV cells without nicotine than in PC12 cells with nicotine. G1-arrest of the cell cycle, observed in control PC12 cells at 12 h after hypoxia, preceding DNA fragmentation, was not evident in alpha7pCMV cells, with or without nicotine. Furthermore, in alpha7pCMV cells with and without nicotine, the basal expression levels of total Akt were approximately 1.5-fold higher, and the up-regulation of Akt phosphorylated at Ser473 after hypoxia was strikingly enhanced, compared with control PC12 cells. These findings suggest that alpha7nAChR functions constitutively in PC12 cells, that its overexpression raises tolerance against G1-arrest and DNA fragmentation after hypoxia, and that it can be considered a candidate target for treatment against hypoxia-induced acute membrane degradation and delayed DNA fragmentation in neurons.

Effects of histamine H(3) receptor ligands GT-2331 and ciproxifan in a repeated acquisition avoidance response in the spontaneously hypertensive rat pup

Histamine H(3) receptor antagonists have been proposed as potentially useful therapeutic agents for the treatment of several disorders including attention deficit, schizophrenia, depression, and Alzheimer's disease. We have developed a repeated acquisition version of an inhibitory avoidance task using spontaneously hypertensive rat (SHR) pups that we believe provides a reproducible measure of the cognitive and attention deficits often characteristic of these disease states, and evaluated two H(3) receptor antagonists. Male SHR, Wistar (WI) and Wistar Kyoto (WKY) rat pups (20--24 days old) were trained to avoid a mild footshock (0.1 mA, 1 s duration), delivered when the pup had transferred from a brightly lit to a darkened compartment. After the first trial, the pup was removed and returned to its home cage. One minute later, the same pup was replaced in the brightly-lit compartment and the training process repeated. A total of five trials were recorded. SHR pups performed significantly more poorly than WI or WKY pups using this training schedule, and SHR pups were used for all subsequent studies. Methylphenidate and ABT-418, both clinically active in attention deficit hyperactivity disorder (ADHD), were tested to validate the model. Methylphenidate (1 and 3 mg/kg s.c.) and ABT-418 (0.03 mg/kg s.c.) significantly improved SHR pup performance. The H(3) receptor antagonists GT-2331 (1 mg/kg s.c.) and ciproxifan (3 mg/kg s.c.), also significantly, and in a dose-related manner, enhanced performance of the SHR pups. (R)-alpha-methylhistamine (3 mg/kg s.c.) blocked the pro-cognitive effects of ciproxifan, suggesting an H(3) receptor site of action for this compound. This model is useful for evaluating the cognition/attention-enhancing potential of H(3) receptor antagonists.

The histaminergic system in human thalamus: correlation of innervation to receptor expression

The mRNA expression of three histamine receptors (H1, H2 and H3) and H1 and H3 receptor binding were mapped and quantified in normal human thalamus by in situ hybridization and receptor binding autoradiography, respectively. Immunohistochemistry was applied to study the distribution of histaminergic fibres and terminals in the normal human thalamus. mRNAs for all three histamine receptors were detected mainly in the dorsal thalamus, but the expression intensities were different. Briefly, H1 and H3 receptor mRNAs were relatively enriched in the anterior, medial, and part of the lateral nuclei regions; whereas the expression level was much lower in the ventral and posterior parts of the thalamus, and the reticular nucleus. H2 receptor mRNA displayed in general very low expression intensity with slightly higher expression level in the anterior and lateropolar regions. H1 receptor binding was mainly detected in the mediodorsal, ventroposterolateral nuclei, and the pulvinar. H3 receptor binding was detected mainly in the dorsal thalamus, predominantly the periventricular, mediodorsal, and posterior regions. Very high or high histaminergic fibre densities were observed in the midline nuclear region and other nuclei next to the third ventricle, ventroposterior lateral nucleus and medial geniculate nucleus. In most of the core structures of the thalamus, the fibre density was very low or absent. The results suggest that histamine in human brain regulates tactile and proprioceptory thalamocortical functions through multiple receptors. Also, other, e.g. visual areas and those not making cortical connections expressed histamine receptors and contained histaminergic nerve fibres.

H1-antihistamines: inverse agonism, anti-inflammatory actions and cardiac effects

This review addresses novel concepts of histamine H1-receptor function and attempts to relate them to the anti-inflammatory effects of H1-antihistamines. Furthermore, the molecular mechanisms underlying the cardiotoxic effects of H1-antihistamines are discussed. H1-receptors are G-protein-coupled-receptors (GPCRs), the inactive and active conformations of which coexist in equilibrium. The degree receptor activation in the absence of histamine is its 'constitutive activity'. In this two-state model, histamine acts as an agonist by combining with and stabilizing the activated conformation of the H1-receptor to shift the equilibrium towards the activated state. Drugs classified previously as antagonists act as either inverse agonists or neutral antagonists. Inverse agonists combine with and stabilize the inactive conformation of the receptor to shift the equilibrium towards the inactive state. Thus, they may down-regulate constitutive receptor activity, even in the absence of histamine. Neutral antagonists combine equally with both conformations of the receptor, do not affect basal receptor activity but do interfere with agonist binding. All H1-antihistamines examined to date are inverse agonists. As the term 'H1-receptor antagonists' is obviously erroneous, we suggest that it be replaced by 'H1-antihistamines'. The observations that H1-receptors modulate NF-kappaB activation and that there are complex interactions between GPCRs, has allowed us to postulate receptor dependent-mechanisms for some anti-inflammatory effects of H1-antihistamines, e.g. inhibition of ICAM-1 expression and the effects of bradykinin. Finally, the finding that blockade of HERG1 K+ channels is the mechanism by which some H1-antihistamines may cause cardiac arrhythmias has allowed the development of preclinical tests to predict such activity.

Development of a new class of nonimidazole histamine H(3) receptor ligands with combined inhibitory histamine N-methyltransferase activity

In search of novel ways to enhance histaminergic neurotransmission in the central nervous system, a new class of nonimidazole histamine H(3) receptor ligands were developed that simultaneously possess strong inhibitory activity on the main histamine metabolizing enzyme, histamine N-methyltransferase (HMT). The novel compounds contain an aminoquinoline moiety, which is an important structural feature for HMT inhibitory activity, connected by different spacers to a piperidino group (for H(3) receptor antagonism). Variation of the spacer structure provides two different series of compounds. One series, having only an alkylene spacer between the basic centers, led to highly potent HMT inhibitors with moderate to high affinity at human histamine H(3) receptors. The second series possesses a p-phenoxypropyl spacer, which may be extended by another alkylene chain. This latter series also showed strong inhibitory activity on HMT, and in most cases, the H(3) receptor affinity even surpassed that of the first series. One of the most potent compounds with this dual mode of action is 4-(4-(3-piperidinopropoxy)phenylamino)quinoline (34) (hH(3), K(i) = 0.09 nM; HMT, IC(50) = 51 nM). This class of compounds showed high antagonist potency and good H(3) receptor selectivity in functional assays in guinea pig on H(1), H(2), and H(3) receptors. Because of low or missing in vivo activity of two selected compounds, the proof of concept of these valuable pharmacological tools for the supposed superior overall enhancing effect on histaminergic neurotransmission failed to appear hitherto.

The rat cytomegalovirus R33-encoded G protein-coupled receptor signals in a constitutive fashion

The rat cytomegalovirus (RCMV) R33 gene is conserved among all betaherpesviruses and encodes a protein (pR33) that shows sequence similarity with chemokine-binding G protein-coupled receptors (GPCRs). Previously, the physiological significance of the R33 gene was demonstrated by the finding that an RCMV strain with R33 deleted is severely attenuated in vivo and is unable to either enter or replicate in the salivary glands of infected rats. Here, we report that RCMV pR33 is expressed as a functional GPCR that signals in an agonist-independent manner in both COS-7 and Rat2 cells. Transient expression of pR33 in COS-7 cells results in constitutive activation of phospholipase C (PLC) due to coupling to G proteins of the G(q) class. Interestingly, PLC activation is partially inhibited by cotransfection with G(alpha)-transducin subunits, which indicates the involvement of G(betagamma) as well as Galpha subunits in pR33-mediated signaling. Surprisingly, PLC activation is also partially inhibited by addition of pertussis toxin (PTX), suggesting that pR33 activates not only G(q) but also G(i/0) proteins. The constitutive activation of G(i/0) proteins by pR33 is further demonstrated by the PTX-sensitive decrease of CRE-mediated transcription and the PTX-sensitive increase of both NF-kappaB- and SRE-mediated transcription. In contrast to its homolog of human herpesvirus 6B (pU12), pR33 does not bind RANTES.

Mechanisms of inverse agonism at G-protein-coupled receptors

Many drugs with important therapeutic actions that had been assumed to be antagonists at G-protein-coupled receptors (GPCRs) have been shown to be inverse agonists. For both basic pharmacology and drug design it is important to understand the mechanisms whereby these drugs achieve their effects. It had been assumed that these drugs achieved their effects by stabilizing an inactive state of the receptor (R) at the expense of a partially activated state (R*). In this article, I consider this and other mechanisms that could explain inverse agonist actions, and conclude that more than one mechanism can apply to inverse agonism at GPCRs.

Efficacy at G-protein-coupled receptors

At present, the drug-discovery process centres on ligands that either block or produce physiological responses. However, there are therapeutic uses for ligands that do neither of these things, but which still affect receptors in other ways. This review discusses the intimate relationship between the affinity of a ligand for its receptor, and the probability that the binding of the ligand will produce some change in the receptor, resulting in efficacy. This, in turn, argues that ligands that have affinity should be tested more broadly, for a wider range of efficacies, to detect hidden therapeutic activities.

Acute and chronic effects of methamphetamine on tele-methylhistamine levels in mouse brain: selective involvement of the D(2) and not D(3) receptor

We have explored the role of endogenous dopamine in the control of histaminergic neuron activity in mouse brain regions evaluated by changes in tele-methylhistamine (t-MeHA) levels. In vitro, methamphetamine released [(3)H]noradrenaline but failed to release [(3)H]histamine from synaptosomes. In vivo, methamphetamine enhanced t-MeHA levels by about 2-fold with ED(50) values of approximately 1 mg/kg in caudate putamen, nucleus accumbens, cerebral cortex, and hypothalamus. This response selectively involved the D(2) and not the D(3) receptor as indicated by its blockade by haloperidol and by its persistence after administration of nafadotride, a D(3) receptor preferential ligand, or in (-/-) D(3) receptor-deficient mice. The t-MeHA response to methamphetamine was delayed compared with the locomotor-activating effect of this drug, suggesting that it is of compensatory nature. In agreement, ciproxifan, an inverse agonist known to enhance histamine neuron activity, decreased the hyperlocomotion induced by methamphetamine. Repeated methamphetamine administration resulted in the expected sensitization to the hyperlocomotor effect of the drug but did not modify either the ED(50) or the E(max) regarding t-MeHA levels. However, it resulted in an enhanced basal t-MeHA level (+30-40%), which was sustained for at least 11 days after withdrawal in hypothalamus, striatum, and cerebral cortex and suppressed by haloperidol. Hence, both the acute and chronic administration of methamphetamine enhance histamine neuron activity, presumably in a compensatory manner. Repeated methamphetamine administration also resulted in a modified balance in the opposite influences of dopamine and serotonin on histaminergic neurons as revealed by the enhanced response to haloperidol and abolished response to ketanserin, respectively.

Low brain histamine content affects ethanol-induced motor impairment

The effect of ethanol on motor performance in humans is well established but how neural mechanisms are affected by ethanol action remains largely unknown. To investigate whether the brain histaminergic system is important in it, we used a genetic model consisting of rat lines selectively outbred for differential ethanol sensitivity. Ethanol-sensitive rats had lower levels of brain histamine and lower densities of histamine-immunoreactive fibers than ethanol-insensitive rats, although both rat lines showed no changes in histamine synthesizing neurons. Lowering the high brain histamine content of the ethanol-insensitive rats with alpha-fluoromethylhistidine before ethanol administration increased their ethanol sensitivity in a behavioral motor function test. Higher H3 receptor ligand binding and histamine-induced G-protein activation was detected in several brain regions of ethanol-naive ethanol-sensitive rats. Brain histamine levels and possibly signaling via H3 receptors may thus correlate with genetic differences in ethanol-induced motor impairment.

Presynaptic H3 autoreceptors modulate histamine synthesis through cAMP pathway

Histamine H3 receptors modulate histamine synthesis, although little is known about the transduction mechanisms involved. To investigate this issue, we have used a preparation of rat brain cortical miniprisms in which histamine synthesis can be modulated by depolarization and by H3 receptor ligands. When the miniprisms were incubated in presence of forskolin, dibutyryl-cAMP, or 3-isobutyl-1-methylxanthine (IBMX), histamine synthesis was stimulated in 34, 29, and 47%, respectively. These stimulations could be prevented by the selective cAMP protein kinase blocker Rp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPs). Preincubation with the H3 receptor agonist imetit prevented IBMX- (100% blockade) and forskolin- (70% blockade) induced stimulation of histamine synthesis. The H3 inverse agonist thioperamide enhanced histamine synthesis in the presence of 1 mM IBMX or 30 mM potassium (+47 and +45%, respectively). Similarly, the H3 antagonist clobenpropit enhanced histamine synthesis in the presence of 30 mM potassium (+ 59%). The cAMP-dependent protein kinase blockers Rp-cAMPs and PKI14-22 could impair the effects of thioperamide and clobenpropit, respectively. These results indicate that the adenylate cyclase-protein kinase A pathway is involved in the modulation of histamine synthesis by H3 autoreceptors present in histaminergic nerve terminals.

Histamine H3-receptor-mediated [35S]GTP gamma[S] binding: evidence for constitutive activity of the recombinant and native rat and human H3 receptors

Constitutive activity of the recombinant and native rat and human H(3) receptors (H(3)Rs) was studied using H(3)R-mediated [(35)S]GTPgamma[S] binding and [(3)H]-arachidonic acid release. Ciproxifan, an inverse agonist at the rat H(3)R (rH(3)R), decreased [(3)H]arachidonic acid release from CHO cells expressing moderate densities (approximately 200 - 300 fmol mg(-1) protein) of the human H(3)R (hH(3)R). This effect occurred with the same magnitude than at the rH(3)R. The expression of the hH(3)R was associated with an increase in [(35)S]GTPgamma[S] binding to membranes of CHO cells. Ciproxifan decreased [(35)S]GTPgamma[S] binding to membranes of CHO (hH(3)R) cells. Both effects were correlated to receptor density and revealed that constitutive activity of the hH(3)R, although lower than that of the rH(3)R in this assay, was again observed at physiological densities (<500 fmol mg(-1) protein). Ciproxifan was less potent at the human than the rat receptor, not only as an antagonist (K(i)=45 nM), but also as an inverse agonist (EC(50)=15 nM). Constitutive activity of the hH(3)R was also evidenced using inhibition of [(35)S]GTPgamma[S] binding by unlabelled GTPgammaS. The expression of the hH(3)R generated a high affinity binding for GTPgammaS which was increased by imetit, but partially decreased by ciproxifan, therefore acting as a partial inverse agonist. [(35)S]GTPgamma[S] binding to rat brain membranes was decreased in several regions by thioperamide, ciproxifan and FUB 465, three inverse agonists at the H(3)R, whose effects were blocked by proxyfan, a neutral antagonist. [(35)S]GTPgamma[S] binding was also decreased by an A(1)-adenosine receptor inverse agonist, but remained unchanged in the presence of inverse agonists at D(2)/D(3) dopamine, H(1) and H(2) histamine, alpha(2)-adrenergic and delta opioid receptors. In conclusion, the present study shows that the recombinant rat and human H(3) receptors expressed at physiological densities display constitutive activity and suggests that constitutive activity of native H(3)Rs is one of the highest among G-protein-coupled receptors present in rat brain.

Histamine H3 receptor activation inhibits glutamate release from rat striatal synaptosomes

The release of glutamate from striatal synaptosomes induced by depolarisation with 4-aminopyridine (4-AP) was studied by a method based on the fluorescent properties of the NAPDH formed by the metabolism of the neurotransmitter by glutamate dehydrogenase.Ca(2+)-dependent, depolarisation-induced glutamate release was inhibited in a concentration-dependent manner by the selective histamine H(3) agonist immepip. Best-fit estimates were: maximum inhibition 60+/-10% and IC(50) 68+/-10 nM. The effect of 300 nM immepip on depolarisation-evoked glutamate release was reversed by the selective H(3) antagonist thioperamide in a concentration-dependent manner (EC(50) 23 nM, K(i) 4 nM). In fura-2-loaded synaptosomes, the increase in the intracellular concentration of Ca(2+) ([Ca(2+)](i)) evoked by 4-AP-induced depolarisation (resting level 167+/-14 nM; Delta[Ca(2+)](i) 88+/-15 nM) was modestly, but significantly reduced (29+/-5% inhibition) by 300 nM immepip. The action of the H(3) agonist on depolarisation-induced changes in [Ca(2+)](i) was reversed by 100 nM thioperamide. Taken together, our results indicate that histamine modulates the release of glutamate from corticostriatal nerve terminals. Inhibition of depolarisation-induced Ca(2+) entry through voltage-dependent Ca(2+) channels appears to account for the effect of H(3) receptor activation on neurotransmitter release. Modulation of glutamatergic transmission in rat striatum may have important consequences for the function of basal ganglia and therefore for the control of motor behaviour.

Application of genomics to drug design: the example of the histamine H3 receptor

The histamine H(3) receptor was characterized in the 1980s as an autoreceptor regulating histamine release in brain. Since then, selective drugs have been designed, many of them displaying a high potency in vivo, and used in many studies to delineate the implications of cerebral histaminergic systems in physiological functions such as arousal or cognitive functions. The recent cloning of the H(3) receptor, more than 15 years later, has allowed to start molecular studies that led to important findings for optimization of drug design. In agreement some ligands display distinct affinities for the recombinant rat and human H(3) receptors, a difference that we assign to two amino acids in the third transmembrane domain. In addition, H(3) autoreceptors present in the brain display high constitutive activity including in vivo. As a consequence, inverse agonists enhance histamine neuron activity and constitute a novel potential therapeutic approach to schizophrenia and Alzheimer's disease.