The neuronal histamine H(1) and pro-opiomelanocortin-melanocortin 4 receptors: independent regulation of food intake and energy expenditure

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

Are Polyunsaturated Fatty Acids Implicated in Histaminergic Dysregulation in Bipolar Disorder?: AN HYPOTHESIS

Bipolar disorder (BD) is an extremely disabling psychiatric disease, characterized by alternate states of mania (or hypomania) and depression with euthymic states in between. Currently, patients receive pharmacological treatment with mood stabilizers, antipsychotics, and antidepressants. Unfortunately, not all patients respond well to this type of treatment. Bipolar patients are also more prone to heart and metabolic diseases as well as a higher risk of suicide compared to the healthy population. For a correct brain function is indispensable a right protein and lipids (e.g., fatty acids) balance. In particular, the amount of fatty acids in the brain corresponds to a 50–70% of the dry weight. It has been reported that in specific brain regions of BD patients there is a reduction in the content of unsaturated n-3 fatty acids. Accordingly, a diet rich in n-3 fatty acids has beneficial effects in BD patients, while their absence or high levels of saturated fatty acids in the diet are correlated to the risk of developing the disease. On the other hand, the histamine system is likely to be involved in the pathophysiology of several psychiatric diseases such as BD. Histamine is a neuromodulator involved in arousal, motivation, and energy balance; drugs acting on the histamine receptor H3 have shown potential as antidepressants and antipsychotics. The histaminergic system as other neurotransmission systems can be altered by fatty acid membrane composition. The purpose of this review is to explore how polyunsaturated fatty acids content alterations are related to the histaminergic system modulation and their impact in BD pathophysiology.

Betahistine ameliorates olanzapine-induced weight gain through modulation of histaminergic, NPY and AMPK pathways

Olanzapine is widely used to treat schizophrenia and other disorders, but causes adverse obesity and other metabolic side-effects. Both animal and clinical studies have shown that co-treatment with betahistine (a histaminergic H1 receptor agonist and H3 receptor antagonist) is effective for ameliorating olanzapine-induced weight gain/obesity. To reveal the mechanisms underlying these effects, this study investigated the effects of co-treatment of olanzapine and betahistine (O+B) on expressions of histaminergic H1 receptor (H1R), AMP-activated protein kinase (AMPK), neuropeptide Y (NPY), and proopiomelanocortin (POMC) in the hypothalamus associated with reducing olanzapine-induced weight gain. Olanzapine significantly upregulated the mRNA and protein expressions of H1R, while O+B co-treatment significantly downregulated the H1R levels, compared to the olanzapine-only treatment group. The NPY mRNA expression was significantly enhanced by olanzapine, but it was significantly reversed by O+B co-treatment. The hypothalamic H1R expression was positively correlated with total food intake, and NPY expression. Olanzapine also increased AMPKα activation measured by the AMPKα phosphorylation (pAMPKα)/AMPKα ratio compared with controls, whereas O+B co-treatment decreased the pAMPKα/AMPKα ratio, compared with olanzapine only treatment. The pAMPKα/AMPKα ratio was positively correlated with total food intake and H1R expression. Although olanzapine administration decreased the POMC mRNA level, this level was not affected by O+B co-treatment. Therefore, these results suggested that co-treatment with betahistine may reverse olanzapine-induced body weight gain via the H1R-NPY and H1R-pAMPKα pathways.

Alterations to melanocortinergic, GABAergic and cannabinoid neurotransmission associated with olanzapine-induced weight gain

Background/Aim

Second generation antipsychotics (SGAs) are used to treat schizophrenia but can cause serious metabolic side-effects, such as obesity and diabetes. This study examined the effects of low to high doses of olanzapine on appetite/metabolic regulatory signals in the hypothalamus and brainstem to elucidate the mechanisms underlying olanzapine-induced obesity.

Methodology/Results

Levels of pro-opiomelanocortin (POMC), neuropeptide Y (NPY) and glutamic acid decarboxylase (GAD₆₅, enzyme for GABA synthesis) mRNA expression, and cannabinoid CB1 receptor (CB1R) binding density (using [³H]SR-141716A) were examined in the arcuate nucleus (Arc) and dorsal vagal complex (DVC) of female Sprague Dawley rats following 0.25, 0.5, 1.0 or 2.0 mg/kg olanzapine or vehicle (3×/day, 14-days). Consistent with its weight gain liability, olanzapine significantly decreased anorexigenic POMC and increased orexigenic NPY mRNA expression in a dose-sensitive manner in the Arc. GAD₆₅ mRNA expression increased and CB1R binding density decreased in the Arc and DVC. Alterations to neurotransmission signals in the brain significantly correlated with body weight and adiposity. The minimum dosage threshold required to induce weight gain in the rat was 0.5 mg/kg olanzapine.

Conclusions

Olanzapine-induced weight gain is associated with reduced appetite-inhibiting POMC and increased NPY. This study also supports a role for the CB1R and GABA in the mechanisms underlying weight gain side-effects, possibly by altering POMC transmission. Metabolic dysfunction can be modelled in the female rat using low, clinically-comparable olanzapine doses when administered in-line with the half-life of the drug.

Effects of simvastatin and 6-hydroxydopamine on histaminergic H1 receptor binding density in rat brains

Statins have been widely used for the treatment of a variety of medical conditions including psychoneurological disorders beyond their original use in lowering cholesterol. Histamine receptors play an important role in the regulation of neural activity, however, it is unknown whether statins act on histamine receptors, particularly for their neural regulatory effects. This study examined the effects of simvastatin and 6-hydroxydopamine (6-OHDA) lesions on histamine H1 receptors using [(3)H] pyrilamine binding autoradiography. Compared to the saline group, simvastatin (1 mg/kg/day) significantly decreased H1 receptor bindings in the primary motor cortex (M1), ventromedial hypothalamic nucleus (VMH), caudate putamen (CPu), accumbens core (AcbC) and prefrontal cortex (PfC) (all p<0.05); however 10 mg/kg/day simvastatin increased H1 receptor density only in the medial amygdaloid nucleus (Mep) (p<0.05), but had no significant effect in other regions examined. The 6-OHDA lesion did not alter H1 receptor binding density in most brain areas, except a trend decrease in the hippocampus (p=0.07) and a trend increase in the cingulate cortex (p=0.06). These results suggested that simvastatin has different effects on the H1 receptors in different rat brain regions depending on the doses. Therefore, simvastatin can modulate histaminergic neurotransmission in the brain, and support the role of H1 receptors in psychoneurological disorders.

The role of histaminergic H1 and H3 receptors in food intake: a mechanism for atypical antipsychotic-induced weight gain?

Atypical antipsychotics such as olanzapine and clozapine are effective at treating the multiple domains of schizophrenia, with a low risk of extra-pyramidal side-effects. However a major downfall to their use is metabolic side-effects particularly weight gain/obesity, which occurs by unknown mechanisms. The present paper explores the potential candidature of histaminergic neurotransmission in the mechanisms of atypical antipsychotic-induced weight gain, with a focus on the histaminergic H1 and H3 receptors. Olanzapine and clozapine have a high affinity for the H1 receptor, and meta-analyses show a strong correlation between risk of weight gain and H1 receptor affinity. In addition, olanzapine treatment decreases H1 receptor binding and mRNA expression in the rat hypothalamus. Furthermore, a complex role is emerging for the histamine H3 receptor in the control of hunger. The H3 receptor is a pre-synaptic autoreceptor that inhibits the synthesis and release of histamine, and a heteroreceptor that inhibits other neurotransmitters such as serotonin (5-HT), noradrenaline (NA) and acetylcholine (ACh), which are also implicated in the regulation of food intake. Thus, the H3 receptor is in a prime position to regulate food intake, both through its control of histamine and its influence on other feeding pathways. We proposed that a mechanism for atypical antipsychotic-induced weight gain may be partly through the H3 receptor, as a drug-induced decrease in H1 receptor activity may decrease histamine tone through the H3 autoreceptors, compounding the weight gain problem. In addition, atypical antipsychotics may affect food intake by influencing 5-HT, NA and ACh release via interactions with the H3 heteroreceptor.

Effects of clozapine and olanzapine on cytokine systems are closely linked to weight gain and drug-induced fever

The second generation antipsychotics clozapine and olanzapine are known to cause weight gain. However, only clozapine is associated with drug-induced fever. Cytokines have been linked to the induction of both weight gain and drug-induced fever. We investigated these potential side effects of clozapine and olanzapine and studied their differential effects on cytokine secretion. Thirty patients suffering from schizophrenia, schizophreniform disorder or schizoaffective disorder were treated with either clozapine (mean modal dose: 266.7+/-77.9mg) or olanzapine (21.2+/-2.5mg) in a randomized, double-blind, 6-week study. Body mass index (BMI), tympanic temperature, and plasma levels of leptin and cytokines (tumor necrosis factor-alpha (TNF-alpha), soluble TNF receptor 1 and 2 (sTNFR-1/2), soluble interleukin-2 receptors (sIL-2R), interleukin-6) were determined weekly. BMI, leptin and cytokines significantly increased over time, except interleukin-6 and sTNFR-1 in the olanzapine group. All cytokines numerically increased compared to baseline already during the first week of treatment in both groups. Leptin, TNF-alpha, sTNFR-1, sTNFR-2 and sIL-2R levels correlated with the BMI. Five patients who received clozapine (33%) developed drug-induced fever (>/=38 degrees C). In these, interleukin-6 peak levels were significantly (p<0.01) higher than in those patients treated with clozapine who did not develop fever. In conclusion, increase of BMI appears to be related to clozapine's and olanzapine's similar effects on cytokine systems, whilst drug-induced fever appears to be related to clozapine's differential effects on interleukin-6.