Central histaminergic stimulation of pituitary--adrenocortical response in the rat

Intracerebroventricular injection of histamine induces the hypothalamic-pituitary-gonadal axis activation in male rats

Brain histamine holds a key position in the regulation of behavioral states, biological rhythms, body weight, energy metabolism, thermoregulation, fluid balance, stress and reproduction in female animals. However, it is not clear whether central histamine exerts any effect on hypothalamic-pituitary-testicular in male rats and if so, the involvement of type of central histamine receptors. The current study was designed to determine the effect of centrally administrated histamine on plasma gonadotropin hormone-releasing hormone (GnRH), luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone level, and sperm parameters, and to show the mediation of the central histaminergic H1, H2 and H3/H4 receptors on histamine-evoked hormonal and sperm parameters' effects. Studies were performed in male Sprague-Dawley rats. A total of 50 or 100 nmol doses of histamine were injected intracerebroventricularly (icv). 100 nmol dose of histamine significantly caused increases in plasma GnRH, LH, FSH and testosterone levels of animals, but not 50 nmol dose of histamine. Moreover, central pretreatment with chlorpheniramine, histaminergic H1 receptor antagonist (100 nmol), ranitidine and histaminergic H2 receptor antagonist (100 nmol) completely prevented histamine evoked increase in plasma GnRH, LH, FSH and testosterone levels, while thioperamide, histaminergic H3/H4 receptor antagonist (100 nmol) pretreatment failed to reverse sex hormones responses to histamine. Both central histamine treatment alone and central histamine treatment after central histaminergic receptors antagonists' pretreatments did not alter any sperm parameters in rats. In conclusion, our findings show that centrally administered histamine increases plasma GnRH, LH, FSH and testosterone levels of conscious male rats without change any sperm parameters. Moreover, according to our findings, central histaminergic H1, and H2 receptors mediate these histamine-induced effects.

Altered methylprednisolone pharmacodynamics in healthy subjects with histamine N-methyltransferase C314T genetic polymorphism

This study investigated the potential differences in methylprednisolone pharmacodynamics between healthy subjects with different histamine N-methyltransferase (HNMT) C314T genotypes. Six individuals with C/C genotype and 4 with C/T genotype were administered a single intravenous dose of methylprednisolone 0.6 mg/kg ideal body weight in a randomized 2-period manner. Methylprednisolone plasma concentrations were fitted with a 1-compartment model. Cortisol and whole blood histamine suppression were assessed by indirect response models, with circadian baseline cortisol analyzed by Fourier analysis. The area between the baseline and effect curve and the area under the effect versus time curve suppression ratio were used to characterize plasma histamine suppression. Methylprednisolone pharmacokinetics and plasma and whole blood histamine suppression were similar between the 2 genotype groups. Median nadir of cortisol and the 50% inhibitory concentration for cortisol were significantly higher in subjects with C/T genotype than those with C/C genotype (P=.031 and .033, respectively, Wilcoxon rank sum test). Subjects who are heterozygous for the T314 variant allele thus appeared less sensitive to the suppressive effects of methylprednisolone on cortisol secretion.

Impaired drinking response in histamine H3 receptor knockout mice following dehydration or angiotensin-II challenge

Histamine H3 receptors (H3Rs) are presynaptic receptors that negatively regulate the release of histamine. The present study examined the physiological role of H3Rs in drinking behavior. In water-replete rats, intracerebroventricular (i.c.v.) administration of R-alpha-methylhistamine (RalphaMeHA), an H3R agonist, elicited drinking behavior. In contrast, i.c.v. administration of thioperamide, an H3R inverse agonist, significantly attenuated the drinking behavior elicited by either overnight dehydration or i.c.v. administration of angiotensin-II (AT-II). Inhibition of histamine release with alpha-fluoromethylhistidine, an inhibitor of histidine decarboxylase, did not elicit drinking behavior. Moreover, the inhibitory effects of thioperamide on drinking behavior in water-depleted rats were not mimicked by i.c.v. administration of histamine. These results suggest that the predominant effects of H3Rs on drinking behavior are not mediated by the modulation of histamine release. In H3R-deficient (H3RKO) mice, drinking behavior induced by overnight dehydration or i.c.v. administration of AT-II was significantly impaired compared to wild type mice. Collectively, these observations suggest that brain H3Rs play a pivotal role in drinking behavior in response to dehydration and AT-II, and these effects may be largely independent of the modulation of histaminergic tone.

The physiology of brain histamine

Histamine-releasing neurons are located exclusively in the TM of the hypothalamus, from where they project to practically all brain regions, with ventral areas (hypothalamus, basal forebrain, amygdala) receiving a particularly strong innervation. The intrinsic electrophysiological properties of TM neurons (slow spontaneous firing, broad action potentials, deep after hyperpolarisations, etc.) are extremely similar to other aminergic neurons. Their firing rate varies across the sleep-wake cycle, being highest during waking and lowest during rapid-eye movement sleep. In contrast to other aminergic neurons somatodendritic autoreceptors (H3) do not activate an inwardly rectifying potassium channel but instead control firing by inhibiting voltage-dependent calcium channels. Histamine release is enhanced under extreme conditions such as dehydration or hypoglycemia or by a variety of stressors. Histamine activates four types of receptors. H1 receptors are mainly postsynaptically located and are coupled positively to phospholipase C. High densities are found especially in the hypothalamus and other limbic regions. Activation of these receptors causes large depolarisations via blockade of a leak potassium conductance, activation of a non-specific cation channel or activation of a sodium-calcium exchanger. H2 receptors are also mainly postsynaptically located and are coupled positively to adenylyl cyclase. High densities are found in hippocampus, amygdala and basal ganglia. Activation of these receptors also leads to mainly excitatory effects through blockade of calcium-dependent potassium channels and modulation of the hyperpolarisation-activated cation channel. H3 receptors are exclusively presynaptically located and are negatively coupled to adenylyl cyclase. High densities are found in the basal ganglia. These receptors mediated presynaptic inhibition of histamine release and the release of other neurotransmitters, most likely via inhibition of presynaptic calcium channels. Finally, histamine modulates the glutamate NMDA receptor via an action at the polyamine binding site. The central histamine system is involved in many central nervous system functions: arousal; anxiety; activation of the sympathetic nervous system; the stress-related release of hormones from the pituitary and of central aminergic neurotransmitters; antinociception; water retention and suppression of eating. A role for the neuronal histamine system as a danger response system is proposed.

Dietary induced anorexia: a review of involvement of the histaminergic system

This review examines possible relationships between anorexia, dietary intake and central nervous system histaminergic activity. The hypothesis being reviewed is that one component of normal or pathophysiological neuroregulation of food intake involves histaminergic activity in the central nervous system, as influenced by concentrations and bioperiodicities of histamine and/or histamine receptors. Changes in concentrations of receptors are gender specific. Low protein quality or quantity diets elevate both central histamine and histamine receptors (H1) in rats while significantly decreasing their food intake. When injected with histaminergic antagonists, rats fed low protein diets increase food intake and have improved efficiency of weight gain. This review supports a dual hypotheses: central histaminergic activity is involved in the regulation of food intake, but food intake patterns (including dietary composition or energy content) can modify central histaminergic activity. This review also suggests that modified histamine and/or H1 receptor concentrations are potential mechanisms for elevated central histaminergic activity in food intake-related pathophysiological states. Thus, dietary interventions (clinically- or self-imposed) which modify food intake or diet composition have the potential of affecting the histaminergic system. Also, drugs with antihistaminergic properties have the potential of affecting food intake/weight gain patterns by interfering with normal neurochemical signals.

Dimaprit--induced neurotoxicity

The neurotoxicity of the histamine H2 agonist dimaprit was characterized. Dimaprit (100 micrograms administered into the lateral cerebral ventricle) induced a large area of brain necrosis 1-3 days later which was uniformly lethal. Lower doses caused dose - related effects on survival, gross brain pathology and body weight. Experiments with other H2 agonists and H2 antagonists, together with studies by others demonstrating a similar toxicity of the congener homodimaprit suggest that the neurotoxicity of dimaprit is independent of brain H2 receptors. Although dimaprit is a useful tool for the characterization of H2 receptor responses, the present results show that this agent must be used with caution, if at all, in classifying brain H2-receptor mediated events.

Effects of selective histamine H3-receptor ligands on prolactin and growth hormone secretion in the rat

The effects of intracarotid (i.a.) administration of the histamine (HA) H3-receptor agonist (R)-alpha-methyl-histamine (alpha MeHA) and of the H3-antagonist thioperamide, (THIO) on basal or morphine (M)-induced prolactin (PRL) and growth hormone (GH) secretion were studied in male rats. M was administered 3 h after the H3-drugs. Neither THIO (2.5 mg/kg) nor alpha MeHA (10 mg/kg) changed basal PRL levels and only THIO enhanced the PRL-releasing effect of M (6 mg/kg). Basal GH secretion was not modified by THIO. alpha MeHA slightly increased GH secretion. THIO significantly decreased M-stimulated GH secretion (1 mg/kg, i.a.) and alpha MeHA slightly increased it. These results, in agreement with previous evidence obtained after central HA administration, indicated that endogenous brain HA facilitates PRL and inhibits GH secretion.

The intracerebroventricularly administered mast cells degranulator compound 48/80 increases the pituitary-adrenocortical activity in rats

The effect of brain mast cells degranulation by compound 48/80 on the pituitary-adrenocortical activity, measured indirectly through corticosterone secretion, and the involvement of a histaminergic mechanism in that stimulation was investigated in conscious rats. All the drugs were given intracerebroventricularly (icv), histamine antagonists 15 min prior to compound 48/80. Compound 48/80 induced a significant dose- and time-related increase in the serum corticosterone levels. That increase, measured 1 h after administration of compound 48/80, was moderately diminished by icv pretreatment of rats with mepyramine and cimetidine, histamine H1- and H2-receptor antagonists. Three hours after administration of compound 48/80 mast cells of the thalamus and the hypothalamus were completely degranulated. At the same time the thalamus and the whole brain histamine levels were substantially higher than in the saline-treated control rats. The above results suggest that histamine liberated from the brain mast cells and central histamine receptors play a moderate role in increasing the pituitary-adrenocortical activity by compound 48/80.

The influence of alpha-fluoromethylhistidine and histamine receptor antagonists on the beta-endorphin-induced corticosterone response

Involvement of a central histaminergic mechanism in the stimulating effect of beta-endorphin (beta-End) on the pituitary-adrenocortical activity, measured indirectly through corticosterone secretion, was investigated in conscious rats. The rise in serum corticosterone levels, induced by beta-End injected intraventricularly (icv) was considerably impaired by pretreatment with naltrexone, an opioid receptor antagonist. The stimulating effect of beta-End was almost totally suppressed by a prior icv administration of mepyramine, a histamine H1-receptor antagonist, and also considerably reduced by pretreatment with cimetidine, an H2-receptor antagonist. The strongest suppression, by 83%; of the beta-End-induced corticosterone response was evoked by a prior administration of alpha-fluoromethylhistidine, an inhibitor of neuronal histamine synthesis in the brain. These results indicate that both the brain neuronal histamine and central histamine H1- and H2-receptors are considerably involved in the beta-endorphin-induced stimulation of the pituitary-adrenocortical activity.

The significance of brain histamine and its receptors in opioid stimulation of the pituitary-adrenocortical activity

The involvement of brain histamine, contained in mast cells and neurons, and central histamine receptors in stimulation of the pituitary-adrenal axis by opioids was investigated indirectly through corticosterone secretion in conscious rats. The opioid agonists leu-enkephalinamide (DADL), morphine and b-endorphin (b-End) the s- m- and putative e-opioid receptor agonists, given intracerebroventricularly (ici) induced a significant, dose-dependent increase in serum corticosterone levels. The corticosterone response to b-End and morphine was significantly impaired by pretreatment with mepyramine and cimetidine, whereas the response to DADL was insensitive to histamine antagonists. Twenty four hours after administration of compound 48/80 the corticosterone responses to the opioids were greatly diminished. Pretreatment with a-fluoromethylhistidine (a-FMH), a histamine synthesis inhibitor, almost abolished the corticosterone response to DADL, b-End and morphine. These results indicate that central neuronal histamine and histamine receptors are significantly involved in the opioid-induced stimulation of the pituitary-adrenal axis in rats.

Antagonistic action of naloxone on central histamine receptors-stimulated corticosterone secretion in rats under stress

In rats under a mild stress of restraint the interaction between central opioid receptors and histaminergic stimulation of the pituitary-adrenocortical activity was investigated indirectly through corticosterone secretion. In order to avoid a possible direct action on adrenal glands, all the tested drugs were administered intracerebroventricularly (icv). Naloxone an opioid antagonist and cimetidine, a H2- and mepyramine a H1-receptor antagonists were given 15 min before histamine and histamine agonists. One hour after histaminergic drug injection the rats were restrained for 10 min and decapitated. Histamine, 2-pyridylethylamine (PEA), a histamine H1-receptor agonist, and 4-methylhistamine (MeHA) and dimaprit, H2-receptor agonists, significantly intensified the stress-induced increase in serum corticosterone levels. Naloxone, given alone icv or ip, did not substantially alter the stress-induced corticosterone response. Like mepyramine naloxone abolished the corticosterone response to PEA in stressed rats. Naloxone also decreased significantly, though not totally, the corticosterone response to MeHA, dimaprit and histamine, its efficiency being similar to that of cimetidine, a H2-receptor antagonist. These results suggest that in stressed rats central opioid receptors are considerably involved in the histamine H1-receptor - and, to a lesser degree, in the H2-receptor stimulation of the hypothalamo-pituitary-adrenocortical axis.

Naloxone antagonizes a central histaminergic stimulation of corticosterone secretion in rats

The interaction of central opioid receptors with histaminergic stimulation of the hypothalamo-pituitary-adrenocortical axis, evaluated indirectly through corticosterone secretion, was investigated in conscious unstressed rats. To avoid any possible direct action on the adrenal cortex, all drugs were given intracerebroventricularly (i.c.v.). Histamine, 2-pyridylethylamine (PEA), a histamine H1-receptor agonist, and 4-methyl-histamine (MeHA) and dimaprit, the H2-receptor agonists, considerably increased the serum corticosterone levels 1 h after administration. Naloxone, an opioid receptor antagonist, almost abolished the corticosterone response to PEA and considerably reduced the responses to MeHA, dimaprit and histamine. The maximum inhibitory effects of naloxone on corticosterone responses induced by histamine and histamine agonists were comparable with those of the H1- and H2-receptor antagonists, mepyramine and cimetidine. These results strongly suggest that a major part of the histaminergic stimulation of the hypothalamo-pituitary-adrenal axis is mediated by central opioid receptors.

Involvement of central histamine receptors in corticosterone secretion induced by intraventricular administration of morphine

The effect of intracerebroventricular (i.c.v.) administration of morphine on corticosterone secretion was studied in conscious, unstressed rats. A dose-dependent increase in serum corticosterone levels was observed 1 h after morphine injection. The corticosterone response to morphine was antagonized in a dose-dependent manner, and at larger dose almost abolished, by i.c.v. pretreatment of rats with naloxone, an opioid receptor antagonist. Intraventricular pretreatment of rats with mepyramine and cimetidine, the histamine H1- and H2-receptor antagonists, significantly diminished the corticosterone response to morphine. These results suggest that central opioid receptors are involved in the stimulating effect of morphine on the hypothalamo-pituitary-adrenocortical axis. Central histamine H1- and H2-receptors seem to be substantially involved in the stimulatory effect of morphine on corticosterone secretion in conscious, unstressed rats.

Evidence of correlations between mast-cell histamine and beta-endorphin (ir) from NIL-pituitary in the homeostasis

This study was designed to characterize physiological events related to a single or repeated experimental anaphylactoid reactions (by Compound 48/80) in non-stressed or cold-restrained rats. Acute treatment with Compound 48/80 (1 mg/kg, i.p.) increases Beta-endorphin(ir) content in the neurointermediate lobe (NIL) of rat pituitary. Moreover, repeated treatment with Compound 48/80 showed tolerance effects. These animals, exposed to stressful conditions, exhibited a fully evident paw oedema following carrageenin oedema-test, whereas saline-pretreated rats, under the same experimental conditions, showed reduced local inflammation. Since Compound 48/80 produces characteristic, systemic, anaphylactoid reaction in the rat, with a very high degree of selectivity in its action, our results suggest that mast-cell histamine and Beta-endorphin from NIL pituitary are involved in the body's reactivity to stressful stimuli.

Possible inhibitory role of histamine H2 receptors in the control of basal TSH secretion in male rats: studies with dimaprit, a selective H2 receptor agonist

Dimaprit,a selective H2 receptor agonist, induced a dose-related decrease of serum TSH basal levels in male rats. This effect was blocked in animals pretreated with cimetidine, a blocker of H2 receptors. Furthermore, dimaprit, added to the anterior pituitary in vitro, failed to induce any change of TSH release. These results are consistent with the hypothesis of an inhibitory role played by H2 receptors located on the median eminence in the control of TSH secretion in male rats.

Hyperlipemic responses induced by centrally administered histamine H1- and H2-receptor agonists in rats

The H1-receptor agonist 2-pyridylethylamine (PEA), and the H2-receptor agonists dimaprit and impromidine administered intracerebroventricularly (i.c.v.) to conscious rats considerably increased the concentration of free fatty acids (FFA) in the blood serum. The hyperlipemic effect of PEA was abolished by mepyramine, a H1-receptor antagonist. Responses to dimaprit and impromidine were not antagonized by cimetidine. These results suggest that central H1-receptor stimulation elicits the hyperlipemic responses in rats. The role of central H2-receptors in this reaction is not clear.

Adrenalectomy modifies the effect of intracerebral histamine on the cold-stimulated TSH secretion in male rats

Cold-stimulated TSH secretion remained normal after adrenalectomy in conscious male Sprague-Dawley rats, but the inhibitory effect of a small dose of histamine (1.0 micrograms/rat into the 3rd ventricle, i.c.v.) on the TSH secretion was abolished. Adrenaline (0.01-1.0 mg/kg s.c.) inhibited dose-dependently the cold-stimulated TSH secretion. However, although adrenalectomy causes a prominent decrease in releasable adrenaline, a larger dose of histamine (2.5 micrograms/rat i.c.v.) decreased the TSH secretion. The effect of histamine was not modified after pretreatment with either corticosterone or dexamethasone, irrespective of whether intact or adrenalectomized rats were studied. Corticosterone decreased and dexamethasone increased the cold-stimulated TSH secretion when given intraperitoneally. Chlorisondamine (10 mg/kg i.p.), a peripheral ganglionic blocking drug, suppressed the TSH cold-response in intact rats. Histamine (1.0 microgram/rat i.c.v.) had no additional inhibitory effect after chlorisondamine. The results suggest that the effect of intracerebral histamine on cold-stimulated TSH secretion is caused neither by stimulation of the hypothalamus-pituitary-adrenocortical axis nor by increased adrenomedullary catecholamine release. Further, the effect of intracerebral histamine is obviously not due to enhanced neurosympathetic activity. The effect of histamine is modified by adrenalectomy, but the adrenal glands are not essential for it.

The interaction of ethanol with central histaminergic stimulation of the pituitary-adrenocortical and hyperlipemic activity

Ethanol introduced intragastrically (i.g.) in rats increased the pituitary-adrenocortical activity, measured indirectly through corticosterone concentration in blood serum. Since this increase reached only about 40% of the maximum hormone levels observed in that species after another stimuli, ethanol may be considered as a relatively weak stimulus. Ethanol induced also a significant decrease in serum free fatty acid (FFA) levels which was blocked totally by a prior intracerebroventricular (i.c.v.) administration of either H1- or H2-histamine receptor antagonists, mepyramine or metiamide and cimetidine. The ethanol-induced increase in serum corticosterone was insensitive to a central histamine H1- and H2-receptors blockade. Ethanol abolished the rise in serum FFA levels induced by an i.c.v. administration of histamine, pyridylethylamine (PEA)-a H1-receptor agonist, and dimaprit--a H2-receptor agonist. The histamine- and histamine-agonists induced increases of serum corticosterone were generally slightly intensified by a prior i.g. administration of ethanol.

Attempts to antagonize the effect of histamine on the cold-stimulated thyrotropin secretion in male rats

Histamine given directly into the 3rd ventricle inhibits the enhancement of thyrotropin secretion induced by cold-exposure (4 degrees, 30 min.) in male rats. This effect was antagonized neither by mepyramine, a H1-receptor antagonist, nor by cimetidine, a H2-receptor antagonist. Histamine would not have an indirect mechanism of action through adrenergic alpha 1- or alpha 2-receptors in rat hypothalamus, since pretreatment with neither phenoxybenzamine nor yohimbine had any effect on histamine suppressed TSH cold-response. Further, it was also tested if histamine could decrease the TSH secretion through cholinergic-, GABA-, serotonergic- or dopaminergic receptors but no results supporting such a mechanism of action were obtained. The effect of histamine was not modified by pretreatment with naloxone or desipramine either. Imidazole acetic acid, IAA, a metabolite of histamine, had no effect on cold-induced TSH secretion. It is concluded that the effect of exogenous histamine on cold-stimulated TSH secretion is not mediated through H1- or H2-receptors. Histamine may decrease brain noradrenergic activity which is important in the generation of TSH cold-response. In addition, the effect of exogenous histamine might be due to decreased endogenous histaminergic activity in rat brain.

Studies on the site and mechanism of the inhibitory action of intracerebral histamine on the cold-stimulated thyrotropin secretion in male rats

The locus of the inhibitory action of histamine on cold-stimulated TSH secretion was studied in male rats. Various histaminergic drugs were given either into the median eminence (ME), the third ventricle, the rostral and caudal part of anterior hypothalamus (rAH and cAH) or the posterior hypothalamus (PH) of conscious rats. When infused into ME, histamine (5 and 10 micrograms/rat) decreased and mepyramine (2.5 micrograms) increased the cold-stimulated TSH secretion while a larger dose of mepyramine (10 micrograms/rat), impromidine (0.05 and 0.1 microgram/rat), 2-pyridylethylamine (2-PEA; 5 and 10 micrograms/rat) and cimetidine (2.5 and 7.5 micrograms/rat) had no effect. The inhibitory action of histamine (5 micrograms/rat) was most marked 30 min after the infusion into ME and it remained significant for at least 50 min. Neither cimetidine (50 and 100 mg/kg i.p.) nor mepyramine (10 and 20 mg/kg i.p.) antagonized the inhibitory action of histamine (5 micrograms/rat) in ME. Histamine (10 micrograms/side) inhibited the cold-stimulated TSH secretion likewise when infused bilaterally either into rAH, cAH or PH. TSH secretion induced by exogenous TRH (100 ng i.p.) was also inhibited by histamine (1 microgram/rat) given into the third ventricle. In conclusion, histamine seems to have an inhibitory action on the cold-stimulated TSH secretion. This action is apparently mediated through areas close to the third ventricle. The mechanism of this action seems to be fairly non-specific, i.e. it is mediated through neither H1- nor H2-receptors.

Central histaminergic stimulation of hyperlipemic response in rats under stress

In rats subjected to a mild stress of immobilization histamine, the H1-receptor agonist 2-pyridylethylamine (PEA), and the H2-receptor agonists 4-methyl histamine (4-MeHA) and impromidine administered intracerebroventricularly (i.c.v.) 1 h prior to stress, intensified the stress-induced increase in serum free fatty acid (FFA) levels. Impromidine was far more potent than histamine and its agonists in increasing hyperlipemia in stressed rats. The hyperlipemic response to histamine was abolished by i.c.v. pretreatment of rats with mepyramine, a H1-receptor antagonist, but was unchanged in rats pretreated with cimetidine or metiamide, H2-receptor antagonists. The increase in serum FFA levels induced in stressed rats by PEA was abolished by mepyramine but the hyperlipemic responses to 4-MeHA and impromidine were not antagonized by cimetidine. These results suggest that central H1-receptor mediate the histamine-stimulated hyperlipemic response in stressed rats.