Monoaminergic and histaminergic strategies and treatments in brain diseases

The monoaminergic systems are the target of several drugs for the treatment of mood, motor and cognitive disorders as well as neurological conditions. In most cases, advances have occurred through serendipity, except for Parkinson's disease where the pathophysiology led almost immediately to the introduction of dopamine restoring agents. Extensive neuropharmacological studies first showed that the primary target of antipsychotics, antidepressants, and anxiolytic drugs were specific components of the monoaminergic systems. Later, some dramatic side effects associated with older medicines were shown to disappear with new chemical compounds targeting the origin of the therapeutic benefit more specifically. The increased knowledge regarding the function and interaction of the monoaminergic systems in the brain resulting from in vivo neurochemical and neurophysiological studies indicated new monoaminergic targets that could achieve the efficacy of the older medicines with fewer side-effects. Yet, this accumulated knowledge regarding monoamines did not produce valuable strategies for diseases where no monoaminergic drug has been shown to be effective. Here, we emphasize the new therapeutic and monoaminergic-based strategies for the treatment of psychiatric diseases. We will consider three main groups of diseases, based on the evidence of monoamines involvement (schizophrenia, depression, obesity), the identification of monoamines in the diseases processes (Parkinson's disease, addiction) and the prospect of the involvement of monoaminergic mechanisms (epilepsy, Alzheimer's disease, stroke). In most cases, the clinically available monoaminergic drugs induce widespread modifications of amine tone or excitability through neurobiological networks and exemplify the overlap between therapeutic approaches to psychiatric and neurological conditions. More recent developments that have resulted in improved drug specificity and responses will be discussed in this review.peer-reviewe

Effects of Betahistine at Histamine H3 Receptors: Mixed Inverse Agonism/Agonism In Vitro and Partial Inverse Agonism In Vivo

International audienceWe previously suggested that therapeutic effects of betahistine in vestibular disorders result from its antagonist properties at histamine H3 receptors (H3Rs). However, H3Rs exhibit constitutive activity and most H3R antagonists act as inverse agonists. Here, we have first investigated the effects of betahistine at recombinant H3R isoforms. On inhibition of cAMP formation and [3H]arachidonic acid release, betahistine behaved as a nanomolar inverse agonist and a micromolar agonist. Both effects were suppressed by pertussis toxin, were found at all isoforms tested, and were not detected in mock cells, confirming interactions at H3Rs. The inverse agonist potency of betahistine and its affinity on [125I]iodoproxyfan binding were similar in rat and human. We have then investigated the effects of betahistine on histamine neuron activity, by measuring tele-methylhistamine (t-MeHA) levels in the brain of mice. Its acute intraperitoneal administration increased t-MeHA levels with an ED50 of 0.4 mg/kg, indicating inverse agonism. At higher doses, t-MeHA levels gradually returned to basal levels, a profile probably resulting from agonism. After acute oral administration, betahistine increased t-MeHA levels with an ED50 of 2 mg/kg, a rightward shift likely due to almost complete first-pass metabolism. In each case, the maximal effect of betahistine was lower than that of ciproxifan, indicating partial inverse agonism. After an oral 8-day treatment, the only effective dose of betahistine was of 30 mg/kg, indicating that a tolerance had developed. These data strongly suggest that therapeutic effects of betahistine result from an enhancement of histamine neuron activity induced by inverse agonism at H3 autoreceptors

Potential molecular interactions between the histamine H3 receptor and the dopamine D2 receptor in the rat striatum

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Histamine H 3 -receptor-mediated [ 35 S]GTPγ[S] binding: evidence for constitutive activity of the recombinant and native rat and human H 3 receptors

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Pharmacological evidence for transactivation within melatonin MT2 and serotonin 5-HT2C receptor heteromers in mouse brain

Association of G protein-coupled receptors into heterodimeric complexes has been reported for over 50 receptor pairs in vitro but functional in vivo validation remains a challenge. Our recent in vitro studies defined the functional fingerprint of heteromers composed of Gi-coupled melatonin MT2 receptors and Gq-coupled serotonin 5-HT2C receptors, in which melatonin transactivates phospholipase C (PLC) through 5-HT2C. Here, we identified this functional fingerprint in the mouse brain. Gq protein activation was probed by [35S]GTPγS incorporation followed by Gq immunoprecipitation, and PLC activation by determining the inositol phosphate levels in brain lysates of animals previously treated with melatonin. Melatonin concentration-dependently activated Gq proteins and PLC in the hypothalamus and cerebellum but not in cortex. These effects were inhibited by the 5-HT2C receptor-specific inverse agonist SB-243213, and were absent in MT2 and 5-HT2C knockout mice, fully recapitulating previous in vitro data and indicating the involvement of MT2/5-HT2C heteromers. The antidepressant agomelatine had a similar effect than melatonin when applied alone but blocked the melatonin-promoted Gq activation due to its 5-HT2C antagonistic component. Collectively, we provide strong functional evidence for the existence of MT2/5-HT2C heteromeric complexes in mouse brain. These heteromers might participate in the in vivo effects of agomelatine

Constitutive activity of the recombinant and native histamine H3 receptor

International audienceAlthough constitutive activity was shown to occur with many recombinant and/or mutated G-protein-coupled receptors, the physiological relevance of the process has remained debated. We have further explored this important issue with the histamine H3 receptor (H3R), a presynaptic receptor regulating histamine neuron activity in the brain. Constitutive activity of the recombinant receptor was studied using [3H]arachidonic acid release, [35S]GTPγ[S] binding and inhibition of cAMP accumulation. Evidence for constructive activity was obtained in these three functional assays with two isoforms of the rat H3 receptor, as well as with the human H3 receptor, expressed at physiological densities. Several standard H3-receptor antagonists, such as thioperamide and ciproxifan, were in fact acting as potent inverse agonists. Proxyfan opposed both agonists and inverse agonists and was therefore identified as a neutral antagonist. Using these drugs, we show high constitutive activity of native receptors. [35S]GTPγ[S] binding demonstrated constitutive activity of H3 receptors expressed at a normal level in mouse or rat brain. Constitutive activity of presynaptic H3 autoreceptors modulates histamine release from cortical synaptosomes in vitro and controls histamine neuron activity in vivo. This implies that inverse agonists rather than neutral antagonists may find therapeutic applications

S.16.04 Histamine H3-receptor ligands: Effects of inverse agonists and protean ligands

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

International audienceSome 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