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Role of histamine H3 receptors in the regulation of gastric functions

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The role of central and peripheral histamine H3 receptors in the regulation of gastric acid secretion and gastric mucosal integrity is reviewed. The activation of H3 receptors by peripheral administration of the selective agonist (R) alpha-methylhistamine reduced acid secretion in cats, dogs, rats and rabbits, while increasing it in mice. The antisecretory effects were observed against indirect stimuli that act on vagal pathways or on enterochromaffin-like (ECL) cells, such as 2-deoxy-D-glucose, food or pentagastrin, but not against histamine or dimaprit. Inhibitory effects on acid production were observed in rats after central administration of histamine or of H3 receptor agonists. In the conscious rat intragastric administration of (R) alpha-methylhistamine caused gastroprotective effects against the damage induced by absolute ethanol, HCl, aspirin and stress. The mechanism involved seems to be related to the increased mucus production, via nitric oxide-independent mechanisms. Gastroprotective effects against ethanol were also observed after central administration of histamine or its metabolite Nalpha -methylhistamine, suggesting that brain H3 receptors participate in the histamine-mediated effects on gastric functions.
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Histamine H3 receptors modulate reactive hyperemia in rat gut

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Reactive hyperemia (RH) is an abrupt blood flow increase following release from mechanical occlusion of an artery, with restoration of intra-arterial pressure. The mechanism of this postocclusion increase in blood flow in the gut is multifactorial. Relaxation of intestinal resistance vessels, observed during RH, may involve myogenic, metabolic, hormonal and neurogenic factors. Evidence exists that histamine is an important endogenous mediator of various functions of the gut, including blood flow. The vascular effects of histamine in the intestinal circulation are due its agonistic action on histamine H1, H2 and H3 receptors. In the present study the hypothesis was tested that peripheral histamine H3 receptors are involved in the mediation of RH in the intestinal circulation. In anesthetized rats, anterior mesenteric artery blood flow (MBF) was determined with ultrasonic Doppler flowmeter, and arterial pressure (AP) was determined with a transducer. The increase in the volume of blood accumulating during RH (RH-volume), the peak increase of arterial blood flow (RH-peak response) and the duration of the hyperemia (RH-duration) were used to quantify RH after occluding the anterior mesenteric artery for 30, 60 and 120 s. Hyperemia parameters were determined before and after administration of the selective histamine H3 receptor antagonist clobenpropit. Pretreatment with clobenpropit was without any effect on control MBF and AP but significantly reduced most of RH responses. These findings support the hypothesis that histamine H3 receptors do not play any role in the control of intestinal vasculature at basal conditions but these receptors participate in the intestinal hyperemic reaction in response to complete temporal intestinal ischemia.
Animal models based on N-methyl-d-aspartate (NMDA) receptor blockade have been widely reported. Ketamine and MK- 801, the two noncompetitive antagonists of NMDA receptors, produce behaviors related to schizophrenia and exacerbated symptoms in patients with schizophrenia. The study presented here investigated the effect of subchronic dosing (once-daily, 7 day) of histamine H3 receptor (H3R) antagonists, ciproxifan (CPX) (3 mg/kg, i.p.) and clobenpropit (CBP) (15 mg/kg, i.p) including clozapine (CLZ) (3.0 mg/kg, i.p.) and chlorpromazine (CPZ) (3.0 mg/kg, i.p.), the atypical and typical antipsychotic, respectively, on MK-801(0.2 mg/kg, i.p.)-induced locomotor activity, and dopamine and histamine levels in rats. Atypical and typical antipsychotic was used to serve as clinically relevant reference agents to compare the effects of the H3R antagonists. MK-801 significantly increased horizontal activity which was reduced with CPX and CBP. The attenuation of MK-801-induced locomotor hyperactivity produced CPX and CBP were comparable to CLZ and CPZ. Dopamine and histamine levels were measured in striatum and hypothalamus, respectively, of rat brain. The MK-801 induced increase of the striatal dopamine level was reduced in rats pretreated with CPX and CBP including CLZ. CPZ also significantly lowered striatal dopamine levels, though the decrease was less robust compared to CLZ, CPX and CBP. MK-801 increased histamine content although to a lesser degree. Subchronic treatment with CPX and CBP exhibited further increased histamine levels in the hypothalamus compared to MK-801 treatment alone. Histamine H3 receptor agonist, R-α methylhistamine (10 mg/kg, i.p.) counteracted the effect of CPX and CBP. The findings of the present study support our previous work showing positive effects of CPX and CBP on MK-801-induced schizophrenia like behaviors in rodents. However, clinical studies have reported no antipsychotic effects with histamine H3 receptor antagonists.
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