BRET Analysis of GPCR Dimers in Neurons and Non-Neuronal Cells: Evidence for Inactive, Agonist, and Constitutive Conformations.

G-protein-coupled receptors (GPCRs) are dimeric proteins, but the functional consequences of the process are still debated. Active GPCR conformations are promoted either by agonists or constitutive activity. Inverse agonists decrease constitutive activity by promoting inactive conformations. The histamine H3 receptor (H3R) is the target of choice for the study of GPCRs because it displays high constitutive activity. Here, we study the dimerization of recombinant and brain H3R and explore the effects of H3R ligands of different intrinsic efficacy on dimerization. Co-immunoprecipitations and Western blots showed that H3R dimers co-exist with monomers in transfected HEK 293 cells and in rodent brains. Bioluminescence energy transfer (BRET) analysis confirmed the existence of spontaneous H3R dimers, not only in living HEK 293 cells but also in transfected cortical neurons. In both cells, agonists and constitutive activity of the H3R decreased BRET signals, whereas inverse agonists and GTPγS, which promote inactive conformations, increased BRET signals. These findings show the existence of spontaneous H3R dimers not only in heterologous systems but also in native tissues, which are able to adopt a number of allosteric conformations, from more inactive to more active states.

Ciproxifan, a histamine H3-receptor antagonist / inverse agonist, modulates methamphetamine-induced sensitization in mice.

The role of histamine neurons in schizophrenia and psychostimulant abuse remains unclear. Behavioural sensitization to psychostimulants is a cardinal feature of these disorders. Here, we have explored the ability of imetit and ciproxifan (CPX), a reference H3-receptor agonist and inverse agonist, respectively, to modulate locomotor sensitization induced in mice by methamphetamine (MET). Mice received saline, CPX (3 mg/kg) or imetit (3 mg/kg) 2 h before MET (2 mg/kg), once daily for 12 days, and were killed after a 2-day wash out. Imetit had no effect, but CPX induced a decrease of MET-induced locomotor activity, which became significant at Day 5, and even more at Day 10. Quantitative polymerase chain reaction was used in the sensitized mice to quantify brain-derived neurotrophic factor (BDNF) and N-methyl-D-aspartate (NMDA)-receptor subunit 1 (NR1) mRNAs, two factors that are altered in both schizophrenia and drug abuse. Imetit and CPX used alone had no effect on any marker. Sensitization by MET decreased BDNF mRNAs by 40% in the hippocampus. This decrease was reversed by CPX. Sensitization by MET also induced strong decreases of NR1 mRNAs in the cerebral cortex, hippocampus and striatum, but not hypothalamus. These decreases were also reversed by CPX. The strong modulator effect of CPX in mice sensitized to MET may result from its modulator effect on NR1 mRNAs in the cerebral cortex and striatum. The reversal by CPX of BDNF and NR1 mRNAs in the hippocampus of sensitized animals further strengthens the interest of H3-receptor inverse agonists for the long-term treatment of cognitive deficits of patients with schizophrenia.

[The histaminergic system: a target for innovative treatments of cognitive deficits]. / Le système histaminergique : une cible pour de nouveaux traitements des déficits cognitifs.

The central effects of histamine are mediated by H(1), H(2) and H(3) receptors. The H(3) receptor inhibits histamine release in brain. Therefore, H(3) receptor inverse agonists, by suppressing this brake, enhance histamine neuron activity. The histaminergic system plays a major role in cognition and H(3) receptor inverse agonists are expected to be a potential therapeutics for cognitive deficits of Alzheimer's disease (AD). They are eagerly awaited inasmuch as other treatments of the disease, such as tacrine or memantine, also enhance, through different mechanisms, histaminergic neurotransmission. An important loss of histaminergic neurons has been observed in AD. In contrast, levels of the histamine metabolite in the CSF of AD patients show that their global activity is decreased by only 25%. This indicates that activating histamine neurons in AD can be envisaged.

Molecular dynamics of the histamine H3 membrane receptor reveals different mechanisms of GPCR signal transduction.

In this work, we studied the mechanisms of classical activation and inactivation of signal transduction by the histamine H3 receptor, a 7-helix transmembrane bundle G-Protein Coupled Receptor through long-time-scale atomistic molecular dynamics simulations of the receptor embedded in a hydrated double layer of dipalmitoyl phosphatidyl choline, a zwitterionic polysaturated ordered lipid. Three systems were prepared: the apo receptor, representing the constitutively active receptor; and two holo-receptors-the receptor coupled to the antagonist/inverse agonist ciproxifan, representing the inactive state of the receptor, and the receptor coupled to the endogenous agonist histamine and representing the active state of the receptor. An extensive analysis of the simulation showed that the three states of H3R present significant structural and dynamical differences as well as a complex behavior given that the measured properties interact in multiple and interdependent ways. In addition, the simulations described an unexpected escape of histamine from the orthosteric binding site, in agreement with the experimental modest affinities and rapid off-rates of agonists.

Constitutive activity of the histamine H3 receptor.

Constitutive activity has been mainly recorded for numerous overexpressed and/or mutated receptors. The histamine H(3) receptor (H(3)R) is a target of choice to study the physiological relevance of this process. In rodent brain, postsynaptic H(3)Rs show high constitutive activity, and presynaptic H(3) autoreceptors that show constitutive activity have a predominant role in inhibiting the activity of histamine neurons. H(3)R inverse agonists abrogate this constitutive brake and enhance histamine release in vivo. Some of these inverse agonists have entered clinical trials for the treatment of cognitive and food intake disorders. Studies performed in vitro and in vivo with proxyfan show that this H(3)R ligand is a 'protean agonist' - that is, a ligand with a spectrum of activity ranging from full agonism to full inverse agonism depending on the level of H(3)R constitutive activity. Consistent with its physiological and therapeutic relevance, the constitutive activity of H(3)R thus has a major function in the brain and regulates the activity of H(3)R-targeted drugs.

Histamine H3 and dopamine D2 receptor-mediated [35S]GTPgamma[S] binding in rat striatum: evidence for additive effects but lack of interactions.

The interactions in the rat striatum between H(3) receptors (H(3)Rs) and D(2) receptors (D(2)Rs) were investigated with the [(35)S]GTPgamma[S] binding assay. The H(3)R agonist (R)alpha-methylhistamine increased [(35)S]GTPgamma[S] binding to striatal membranes with an EC(50)=14+/-5 nM and a maximal effect of +19+/-1%. This effect was inhibited by the H(3)R antagonist ciproxifan with a K(i)=1.0+/-0.3 nM. The D(2)R agonist quinpirole increased [(35)S]GTPgamma[S] binding to the same membranes with an EC(50)=1.5+/-0.5 microM and a maximal effect of +28+/-2%. Its effect was blocked by haloperidol with a K(i)=0.3+/-0.1 nM. The maximal effects of the H(3)R and D(2)R agonists were additive (+46+/-3%). However, D(2)R ligands did not modify the effects of H(3)R ligands and vice versa. Ciproxifan behaved as an H(3)R inverse agonist and decreased [(35)S]GTPgamma[S] binding. Haloperidol had no effect and did not change the inverse agonist effect of ciproxifan. Administrations for 10 days of ciproxifan (1.5mg/kg/day) or haloperidol (0.5mg/kg/day) did not change the effects of quinpirole and (R)alpha-methylhistamine, respectively. These data suggest that striatal H(3)Rs and D(2)Rs do not interact through their coupling to G-proteins. However, a hyperactivity of histaminergic and dopaminergic neurons being observed in schizophrenia, the additive activations of H(3)Rs and D(2)Rs suggest that they cooperate to generate some schizophrenic symptoms. Such a postsynaptic mechanism may underlie the antipsychotic-like effects of H(3)R inverse agonists and supports their therapeutic interest, alone or as adjunctive treatment with neuroleptics.

Brain histamine and schizophrenia: potential therapeutic applications of H3-receptor inverse agonists studied with BF2.649.

BF2.649, a high affinity and selective non-imidazole histamine H(3)-receptor antagonist/inverse agonist, was found to easily enter the brain after oral administration to mice: it displayed a ratio of brain/plasma levels of about 25 when considering either C(max) or AUC values. At low oral doses (2.5-20mg/kg), it elicited in mice a dose-dependent wakening effect accompanied with a shift towards high frequency waves of the EEG, a sign of cortical activation. DOPAC/dopamine ratios were enhanced in the prefrontal cortex but not in the striatum, indicating a selective activation of a sub-population of dopaminergic neurons. BF2.649 showed significant inhibitory activity in several mouse models of schizophrenia. It reduced locomotor hyperactivity elicited by methamphetamine or dizolcipine without significantly affecting spontaneous locomotor activity when administered alone. It also abolished the apomorphine-induced deficit in prepulse inhibition. These observations suggest that H(3)-receptor inverse agonists/antagonists deserve attention as a novel class of antipsychotic drugs endowed with pro-cognitive properties.

Mechanistic characterization of S 38093, a novel inverse agonist at histamine H3 receptors.

Histaminergic H3 inverse agonists, by stimulating central histamine release, represent attractive drug candidates to treat cognitive disorders. The present studies aimed to describe the mechanistic profile of S 38093 a novel H3 receptors inverse agonist. S 38093 displays a moderate affinity for rat, mouse and human H3 receptors (Ki=8.8, 1.44 and 1.2µM, respectively) with no affinity for other histaminergic receptors. In cellular models, the compound was able to antagonize mice H3 receptors (KB=0.65µM) and to suppress cAMP decrease induced by an H3 agonist via human H3 receptors (KB=0.11µM). The antagonism properties of the compound were confirmed by electrophysiological studies on rat hippocampal slices (from 0.1µM). In cells expressing a high H3 density, S 38093 behaved as a moderate inverse agonist at rat and human H3 receptors (EC50=9 and 1.7µM, respectively). S 38093 was rapidly absorbed in mouse and rat (Tmax=0.25-0.5h), slowly in monkey (2h), with a bioavailability ranging from 20% to 60% and t1/2 ranging from 1.5 to 7.4h. The compound was widely distributed with a moderate volume of distribution and low protein binding. The brain distribution of S 38093 was rapid and high. In mice, S 38093 significantly increased ex vivo N-tele-Methylhistamine cerebral levels from 3mg/kg p.o. and antagonized R-α-Methylhistamine-induced dipsogenia from 10mg/kg i.p. Taken together, these data suggest that S 38093, a novel H3 inverse agonist, is a good candidate for further in vivo evaluations, in particular in animal models of cognition.

Compared pharmacology of human histamine H3 and H4 receptors: structure-activity relationships of histamine derivatives.

Various histamine derivatives were investigated at the human H3 receptor (H3R) and H4 receptor (H4R) stably expressed in human embryonic kidney (HEK)-293 cells using [125I]iodoproxyfan and [3H]histamine binding, respectively. In Tris buffer, [3H]histamine binding to membranes of HEK(hH4R) cells was monophasic (K(D) of 3.8+/-0.8 nM). In phosphate buffer, the Hill coefficient was decreased (n(H) = 0.5+/-0.1) and a large fraction of the binding was converted into a low-affinity component (K(D) = 67+/-27 nM). The inhibition of [3H]histamine binding by two agonists, a protean agonist and five antagonists/inverse agonists confirms that the potency of many H3R ligands is retained or only slightly reduced at the H4R. Histamine derivatives substituted with methyl groups in alpha, beta or N(alpha) position of the side chain retained a nanomolar potency at the H3R, but their affinity was dramatically decreased at the H4R. With relative potencies to histamine of 282 and 0.13% at the H3R and H4R, respectively, (+/-)-alpha,beta-dimethylhistamine is a potent and selective H3R agonist. Chiral alpha-branched analogues exhibited a marked stereoselectivity at the H3R and H4R, the enantiomers with a configuration equivalent to L-histidine being preferred at both receptors. The methylsubstitution of the imidazole ring was also studied. The relative potency to histamine of 4-methylhistamine (4-MeHA) at the H4R (67%) was similar to that reported at H2 receptors but, owing to its high affinity at the H4R (Ki = 7.0+/-1.2 nM) and very low potency at H1- and H3-receptors, it can be considered as a potent and selective H4R agonist. On inhibition of forskolin-induced cAMP formation, all the compounds tested, including 4-MeHA, behaved as full agonists at both receptors. However, the maximal inhibition achieved at the H4R (approximately -30%) was much lower than at the H3R (approximately -80%). Thioperamide behaved as an inverse agonist at both receptors and increased cAMP formation with the same maximal effect (approximately +25%). In conclusion, although the pharmacological profiles of the human H3R and H4R overlap, the structure-activity relationships of histamine derivatives at both receptors strongly differ and lead to the identification of selective compounds.

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).

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.

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.

4-(omega-(alkyloxy)alkyl)-1H-imidazole derivatives as histamine H(3) receptor antagonists/agonists.

In an effort to develop new histamine H(3) receptor antagonists usable as pharmacological tools we present here novel unsymmetrical ether derivatives. Etherification of different omega-(1H-imidazol-4-yl)alkyl scaffolds led to compounds containing alkyl chains of increasing lengths either with or without unsaturated termini, cycloalkyl or arylalkyl moieties, or additional heteroatoms. When investigated in an in vitro assay on rat synaptosomes, the majority of compounds displayed potencies in the low nanomolar concentration range at the H(3) receptor, e.g., 4-(3-(3-cyclopentylpropyloxy)propyl)-1H-imidazole (27, K(i) = 7 nM). FUB 465, 4-(3-(ethoxy)propyl)-1H-imidazole (14), a useful tool for the characterization of constitutive activity of H(3) receptors in vivo in rodents, proved to be of high oral in vivo potency in mice (ED(50) = 0.26 mg/kg). Further, the influence of chosen compounds on specific [(35)S]GTPgammaS binding was assayed on HEK293 cell membranes expressing the human histamine H(3) receptor revealing partial agonism of the compounds in this particular model. These distinct responses are further hints for "protean agonism" in this class of compounds. Additionally, selected compounds were functionally investigated in vitro on isolated organs of the guinea-pig at H(3), H(1), and H(2) receptors.

Meta-substituted aryl(thio)ethers as potent partial agonists (or antagonists) for the histamine H3 receptor lacking a nitrogen atom in the side chain.

4-(3-Aryloxypropyl)-1H-imidazoles, which possess a meta-positioned substituent in the aryl ring, have been synthesized and tested for activity at histamine H(3) receptors. The compounds having a CN, Me, or Br substituent were found to be antagonists, whereas CF(3), Et, i-Pr, t-Bu, COCH(3), or NO(2) substituents remarkably afforded partial agonists when tested in vitro on rat cerebral cortex synaptosomes for inhibition of [(3)H]histamine release. The compounds were also active in vivo, and furthermore, the CF(3)-substituted compound trifluproxim (UCL 1470, 7) acted as a potent full agonist in vivo, having ED(50) = 0.6 +/- 0.3 mg/kg per os in mice for inhibition of brain N(tau)-methylhistamine formation. Related structures have also been investigated; homologues 4-[4-(3-(trifluoromethyl)phenoxy)butyl]-1H-imidazole and 4-[2-(3-(trifluoromethyl)phenylthio)ethyl]-1H-imidazole are shown to be partial agonists, whereas the O isostere 4-[2-(3-(trifluoromethyl)phenoxy)ethyl]-1H-imidazole is an antagonist as is the S homologue 4-[3-(3-(trifluoromethyl)phenylthio)propyl]-1H-imidazole and its CH(2) isostere 4-[4-(3-(trifluoromethyl)phenyl)butyl]-1H-imidazole.

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.

Involvement of histamine receptors in the atypical antipsychotic profile of clozapine: a reassessment in vitro and in vivo.

RATIONALE: The basis of the unique clinical profile of the antipsychotic clozapine is not yet elucidated. Brain histamine receptors may play a role in schizophrenia and its treatment, but their involvement in the profile of clozapine remained unknown. OBJECTIVES: We explored the properties of clozapine and its two metabolites, N-desmethylclozapine (NDMC) and clozapine N-oxide, at the four human histaminergic receptors. We compared their active concentrations with their blood concentrations in patients treated by clozapine. We investigated the changes in receptor densities induced in rat brain by repeated administration of a therapeutic dose of clozapine. RESULTS: Clozapine and NDMC behaved as very potent, and partial, H(1)-receptor inverse agonists, weak, and full, H(2)-receptor inverse agonists, moderate, and protean, H(3)-receptor agonists, and moderate, and partial, H(4)-receptor agonists. Taking into account their micromolar mean blood concentrations found in 75 treated patients, and assuming that they are enriched in human brain as they are in rat brain, a full occupation of H(1)-, H(3)-, and H(4)-receptors, and a partial occupation of H(2) receptors, is expected. In agreement, repeated administration of clozapine at a therapeutic dose (20 mg/kg/day for 20 days) induced an up-regulation of H(1)- and H(2)-receptors in rat brain. CONCLUSIONS: Clozapine and its active metabolite NDMC interact with the four human histamine receptors at clinically relevant concentrations. This interaction may substantiate, at least in part, the atypical antipsychotic profile of clozapine, as well as its central and peripheral side effects such as sedation and weight gain.

CSF levels of the histamine metabolite tele-methylhistamine are only slightly decreased in Alzheimer's disease.

Neuropathological studies have reported a strong neurofibrillary degeneration of the tuberomamillary nucleus, the region of origin of histamine neurons, in Alzheimer's disease (AD). Histaminergic neurons enhance cognition and memory, suggesting that their degeneration may contribute to the cognitive decline of AD. Besides neurons, the brain histaminergic system comprises mast cells and microglia that can also produce histamine. The level of activity of this histaminergic system in AD remained unknown. In the present study, we have measured the levels of the main histamine metabolite in brain, tele-methylhistamine (t-MeHA), an index of histaminergic system activity, in the cerebrospinal fluid (CSF) of 97 non-AD (controls) and 91 AD patients, males or females. t-MeHA levels in CSF of controls tended to be higher, although non-significantly, in females than in males. t-MeHA levels of controls and AD significantly increased with age (1.66 ± 0.13, 2.04 ± 0.12, and 2.76 ± 0.12 pmol/ml at 40, 60 and 80 years, respectively). In spite of the strong degeneration of histamine neurons in the disease, t-MeHA levels in CSF were only slightly decreased in AD compared to controls (2.14 ± 0.10 vs 2.76 ± 0.13 pmol/ml, -22%, p < 0.01). This decrease was similar whatever the age, and was slightly higher in females than in males. The increase observed with age, and the limited magnitude of the decrease in AD even at late stages may result from the compensatory activation of spared neurons, as well as the neuroinflammation-induced activation of microglia occurring in senescence and AD.

Modulation of prepulse inhibition and stereotypies in rodents: no evidence for antipsychotic-like properties of histamine H3-receptor inverse agonists.

RATIONALE: H(3)-receptor inverse agonists raise a great interest as innovative therapeutics in several central disorders. Whereas their procognitive properties are well established, their antipsychotic-like properties are still debated. OBJECTIVES: We further explored the effect of maximal doses (3-10 mg/kg) of ciproxifan, BF2.649, and ABT-239, three selective H(3)-receptor inverse agonists, on deficits of prepulse inhibition (PPI) induced by apomorphine, MK-801, and phencyclidine (PCP). Their effect was also investigated on stereotypies induced by apomorphine and methamphetamine. RESULTS: Ciproxifan, BF2.649, and ABT-239 did not reverse the PPI impairment produced by apomorphine (0.5 mg/kg, subcutaneous) in rats. Ciproxifan and BF2.649 did not reverse the impairment induced in mice by MK-801 (0.3 mg/kg). Ciproxifan and BF2.649 also failed to reverse the disruption induced in mice by PCP (5-10 mg/kg). Low to moderate doses of haloperidol (0.1-0.4 mg/kg, intraperitoneal), alone or co-administered with BF2.649, did not reverse MK-801-induced PPI disruption. A high dose (1 mg/kg) of haloperidol partially reversed the MK-801-induced deficit and BF2.649 tended to increase this effect, although nonsignificantly. Whereas stereotypies induced in mice by apomorphine and methamphetamine were totally suppressed by haloperidol, the decrease induced by ciproxifan was partial against apomorphine and very low, if any, against methamphetamine. CONCLUSIONS: Their total absence of effect in several validated animal models of the disease does not support antipsychotic properties of H(3)-receptor inverse agonists. However, their positive effects previously reported in behavioral tasks addressing learning, attention, and memory maintain the interest of H(3)-receptor inverse agonists for the treatment of cognitive symptoms of schizophrenia as adjunctive medications.