Histamine-sensitive adenylate cyclase in hypothalamus of rat brain: H1 and H2 receptors

Sex differences in mouse behavior following pyrilamine treatment: role of histamine 1 receptors in arousal

Arousal, the activation of brain and behavior, is a fundamental component of behavior. While sex differences in behavior are pervasive, it is unknown whether they could be due to an underlying dimorphism in arousal mechanisms. Because histamine (HA) acting through histamine 1 (H1) receptors is one essential component of arousal neural circuitry, the aim of the current experiment was to measure sex differences in behavioral arousal following treatment with the H1 receptor antagonist, pyrilamine (PYRL). Castrated male and ovariectomized female Swiss-Webster mice were treated subcutaneously with either 15 or 35 mg/kg of PYRL. The effect of drug treatment was determined in an array of behaviors: sensory responsiveness, running wheel activity, and fearfulness. Surprisingly, the lower dose of PYRL increased some aspects of arousal, sensory responsiveness, and anxiety-like behavior, while the higher dose of PYRL resulted in decreases in arousal across tests, indicating that antagonism of histamine receptors does not have a linear relationship with arousal. Females were more sensitive to the arousal-reducing effects of PYRL than males in sensory and running wheel tasks but not in tests of emotion. In conclusion, antagonism of H1 receptors can alter arousal in a sex-dependent manner, independent of circulating gonadal steroids, in mice.

Exploratory behaviour after intra-accumbens histamine and/or histamine antagonists injection in the rat

The possible role of histamine locally applied into the nucleus accumbens on exploratory behaviours measured in 'conflictive' and 'non-conflictive' environments was studied in adult male rats. It was assumed that in conflictive environments the brain mechanisms involved in processing incentive environmental clues (novelty) were interacting with mechanisms involved in the processing of fearful or 'anxiogenic' environmental clues. As a model of conflictive environment, the elevated asymmetric-plus maze (APM) was used. As a model of a non-conflictive environment, a modified holebroad enriched with an object (OVM) was used. The exploration score in any of the arms of the APM was considered an approximate index of exploratory motivation. The permanency score (non-exploratory behaviours) was considered an inverse approximate index of emotionality. Other variables such as the frequency of entries into any arm, the latency time and central activity were also measured. In the OVM, the general motor activity and head-dipping, vertical rearing and focalized exploration were measured. Results show that histamine in the APM had a dual effect. On the one hand, an increase of exploration was observed in those arms considered more 'anxiogenic'. On the other hand, a decrease in exploration occurred in one of the arms considered less 'anxiogenic'. No changes of permanency was observed in the 'anxiogenic' arms, and a decrease of permanency took place in the arms considered less 'anxiogenic'. In the OVM, histamine did not change the overall motor activity, but head-dipping was inhibited by the imidazolamine treatment. Histamine effects on exploration parameters were counteracted by pre-treatment with H1- and H2-histamine antagonists. Nevertheless, some behaviours were not blocked by the histamine receptor antagonists. The present results give support to the role of the nucleus accumbens in the exploratory motivation mechanisms and suggest that histamine might be an endogenous regulator.

Sexual dimorphism in [3H]histamine binding sites of rat cerebral cortex

[3H]Histamine binding sites, identified on rat brain homogenate membranes, displayed sexual dimorphism with higher density and lower affinity in preparations from adult female compared to the males. The ontogenetic development of [3H]histamine binding sites was studied in male and female brain cortex neural membranes. Histamine binding sites were detectable in newborn-10 day old rats. Density increased with age, reaching adult levels at 37-45 days. No significant differences between sexes were observed in the binding constants until 37 days after birth, when the [3H]histamine binding characteristics began to differentiate according to sex. Sexual dimorphism in brain histamine binding site development appeared to depend on ovarian steroids. Binding patterns in immature female rats which were ovariectomized at 21 days and killed at 37+ were similar to those found in males. On the other hand, when oestradiol replacement was administered for 10 days to ovariectomized rats a total recovery of [3H]histamine binding pattern was obtained, which was comparable to that observed in intact female rats of the same age.

Blockade of histamine-stimulated alterations in cerebrovascular permeability by the H2-receptor antagonist cimetidine

Histamine has been shown previously to cause dose-dependent systemic hypotension and concurrent alterations in the permeability of the blood-brain barrier of rats. The purpose of the present study was to determine whether histamine-induced changes in cerebrovascular permeability were mediated by the histamine H2-receptor. Wistar-Kyoto (control) and spontaneously hypertensive rats were pretreated with the histamine H2-receptor antagonist cimetidine (10 mg/kg), followed by saline or histamine (1.25, 2.5 or 5.0 micrograms/kg). Premedication with cimetidine did not block histamine-induced systemic hypotension. The permeability of the blood-brain barrier was measured with 131I-labelled serum albumin (RISA) or with 99mTc-sodium pertechnetate (TcO4-). In both control and spontaneously hypertensive animals, cimetidine prevented histamine-induced changes in the permeability of the blood-brain barrier to either tracer. These findings suggest that the H2-receptor is the prime mediator of histamine-stimulated alterations in cerebrovascular permeability.

Histamine increases phospholipid methylation and H2-receptor-adenylate cyclase coupling in rat brain

Histamine stimulated the enzymatic synthesis of phosphatidylcholine from phosphatidylethanolamine in crude synaptic membranes of rat brain containing the methyl donor S-adenosyl-L-methionine (SAM). In the presence of, but not in the absence of SAM, histamine increased cyclic AMP accumulation at the concentrations that stimulate phospholipid methylation. S-Adenosyl-L-homocysteine, an inhibitor of phospholipid methyltransferases, inhibited histamine-stimulated phospholipid methylation and histamine-induced cyclic AMP accumulation in the presence of SAM in a concentration-dependent manner. Histamine-induced [3H]methyl incorporation into phospholipids exhibited a marked regional heterogeneity in rat brain in the order of cortex greater than medulla oblongata greater than hippocampus greater than striatum greater than midbrain greater than hypothalamus. The regional distribution of histamine-induced cyclic AMP accumulation exactly paralleled histamine-stimulated [3H]methyl incorporation in rat brain. Histamine-induced cyclic AMP accumulation was inhibited by the addition of cimetidine or famotidine, but not by mepyramine or diphenhydramine. The accumulation of cyclic AMP in the presence of SAM was observed by the addition of impromidine or dimaprit, but not by 2-pyridylethylamine. These results indicate that phospholipid methylation is induced by histamine and may participate in H2-receptor-mediated stimulation of adenylate cyclase in rat brain.

Histamine acting on H2 receptors stimulates phospholipid methylation in synaptic membranes of rat brain

Histamine stimulated [3H]methyl group incorporation into phospholipids in crude synaptic membranes of rat whole brain (without cerebellum) in modified Krebs-Ringer solution containing the methyl donor S-adenosyl-[methyl-3H]methionine. The transient increase of [3H]methyl incorporation into lipids peaked within 45 s after addition of histamine (5 or 10 microM) and decreased the basal level in 60 s. Histamine-stimulated [3H]methyl incorporation was increased linearly in a protein concentration-dependent manner. The stimulation was temperature and histamine concentration dependent. TLC analysis of a chloroform/methanol extract indicated that radioactive phospholipids (phosphatidylcholine, phosphatidyl-N,N-dimethylethanolamine, and phosphatidyl-N-monomethylethanolamine) accounted for 60-65% of the total radioactivity recovered. The synaptosomal fraction had the highest specific activity of all the subfractions of crude synaptic membranes (P2). Histamine-induced [3H]methyl incorporation was inhibited by addition of cimetidine (0.01-10 microM) or famotidine (0.01-1.0 microM) in a concentration-dependent manner but not by mepyramine (0.1-10 microM) or diphenhydramine (0.1-10 microM). The stimulation of [3H]methyl incorporation was also observed by addition of impromidine (0.01-10 microM) or dimaprit (1.0 microM-1.0 mM) in a concentration-dependent manner but not by 2-pyridylethylamine (1.0 microM-1.0 mM). These results indicate that phospholipid methylation is induced by histamine acting on H2 receptors in rat brain synaptosomes.

Brain histamine response to stress in 12 month old rats

The regional brain histamine regulation in response to stress was investigated in 12 month old Sprague-Dawley male rats. Air blast exposure (15 min) induced significant (26.5%) elevation in hypothalamic HA level; midbrain and cortical HA concentrations were not affected. Histamine methyltransferase activity was not altered by stress in any of the brain regions investigated. Plasma corticosterone levels of stressed rats were significantly elevated (6.5 fold). Hence, the response of hypothalamic HA to stress is still evident in 12 month old rats.

Brain histamine regulation following chronic diazepam treatment and stress

Chronic diazepam treatment (5 mg/kg intragastrically, twice daily for 14 days) did not influence either hypothalamic, midbrain or cortical histamine (HA) levels or histidine decarboxylase (HD) activity in male Sprague-Dawley (200-220 g) rats. However, a small but significant decrease in hypothalamic HA concentration and significantly increased HD activity was seen following diazepam withdrawal. Air blast stress induced a significant elevation in hypothalamic HA levels and HD activity in vehicle-treated controls, diazepam-treated and diazepam-withdrawn rats, but the change in HD activity was significantly greater in the last group. The latter group also displayed the greatest elevation in plasma corticosterone levels in response to stress. Hence, diazepam withdrawal in rats results in some changes in the basal hypothalamic HA regulation and may influence the hypothalamic HA and corticosterone response to stress.

Pharmacological modification of blood-brain barrier permeability following a cold lesion

The effect of desipramine, imidazole, thioridazine and trifluoperazine on blood-brain barrier (BBB) permeability after a 24 hour cold lesion was studied in rats. Changes in BBB permeability were determined using a quantitative horseradish peroxidase (HRP) assay. The four drugs tested did not alter the quantity of HRP in the cortex of control animals, or in the contralateral cortex of test animals. However, imidazole, desipramine and trifluoperazine significantly reduced the HRP extravasation in and around the cold lesion. Several mechanisms for this effect are suggested; one possible mechanism common to all these drugs is the reduction of increased vesicular transport in cortical vessels adjacent to the cold lesions.

Regulatory influence of histamine receptor activation and inhibition on the synthesis and level of hypothalamic histamine

The blockade of histamine receptors by repeated i.p. administration of 10-20 mg/kg of chloropyramine and tripelennamine, the potent H1-receptor antagonists, or by the i.c.v. administration of 2 mg/kg of metiamide and cimetidine, the highly selective H2-receptor antagonists, led to significant enhancement in the hypothalamic HD2(L-histidine carboxylase; EC 4.1.1.22.) activity and the histamine content; whereas the activation of the histamine H1-receptor by 4 mg/kg i.e.v. doses of 2-pyridylethylamine, the specific histamine H1 agonist, resulted in significant diminution in both the synthesis and level of this amine. These compounds either do not influence the hypothalamic HD in vitro or cause opposite effects in relatively high concentrations. After repeated administration of either agonists or antagonists, no significant alteration have been observed in the hypothalamic HNMT (histamine-N-methyl-transferase; EC 2.1.1.8.) activity. There were, however, two exceptions: 2 mg/kg i.c.v. doses of 2-methylhistamine and 4-methylhistamine produced remarkable inhibitions in the hypothalamic HNMT activity. These effects do not seem to correspond to the agonistic character of the compounds, but mask the indirect actions and create difficulties in the discovery of regulatory events. The regulatory influence which suppresses or stimulates the basal activity of HD under the activation or the inhibition of the functional state of histamine receptor, is assumed to be mediated through the cyclic AMP system.

Regulation of histidine decarboxylase activity in rat hypothalamus in vitro by ATP and cyclic AMP: enzyme inactivation under phosphorylating conditions

In vitro, hypothalamic HD1 from rat, could strikingly be inhibited by ATP and cyclic AMP. The enzyme inhibition is partially dependent upon Mg2+ and the circumstances favourable for a cAMP-dependent phosphorylation. An almost complete inhibition could be achieved by incubating the homogenate of the hypothalamus under phosphorylating conditions (ATP, cAMP, Mg2+ and IBMX) in the presence of a cAMP-dependent protein kinase (obtained from bovine thymus). Cyclic nucleotides and ATP alone elicit only moderate inhibitions on the hypothalamic HD activity. Neither ATP, nor cAMP, added alone or in combinations, alter the total brain or the hypothalamic HNMT from guinea-pigs or rats in concentrations up to 10(-3) M. Results suggest that hypothalamic HD is regulated through a cAMP-dependent process, probable a direct phosphorylation, via a cAMP-dependent protein kinase.

Histamine and the hypothalamus

The chemical tools that could be used to examine the function of histamine in the brain are considered together with the evidence linking histamine specifically with the hypothalamus. The distribution of histamine and the enzymes responsible for its synthesis and metabolism is consistent with there being both mast cells and histaminergic nerve terminals within the hypothalamus. Iontophoresis, mepyramine binding and histamine-stimulated adenylate cyclase studies suggest that both histamine H1- and H2- receptors are present in the hypothalamus. In addition, intracerebroventricularly injected histamine receptor agonists and antagonists affect many functions associated with the hypothalamus such as cardiovascular control, food intake, body temperature control, and pituitary hormones whose release is mediated via the hypothalamus, such as corticotropin, growth hormone, thyroid stimulating hormone, prolactin, gonadotropins and vasopressin. However, only in the case of thyroliberin release, prolactin release, body fluid control and blood pressure control is there evidence yet that such effects are mediated via histamine receptors actually in the hypothalamus. The effects of enzyme inhibitors suggest endogenous histamine may be involved in the physiological control of thyroid stimulating hormone, growth hormone and blood pressure, and the effects of receptor antagonists support a role for endogenous histamine in prolactin control. Otherwise, there is little evidence for a physiological role for endogenous, as against exogenous, histamine whether it be from histaminergic terminals or mast cells. In addition, few studies have tried to distinguish possible effects on presynaptic receptors, postsynaptic receptors, hypothalamic blood vessels or the hypophyseal portal blood vessels. It is concluded that although there is good evidence now linking histamine and the hypothalamus more specific studies are required, for instance using microinjection or in vitro techniques and the more specific chemical tools now available, to enable a clearer understanding of the physiological role of histamine in the hypothalamus.

Brain histamine-plasma corticosterone interactions

Significant elevation in plasma corticosterone of rats achieved by intraperitoneal (i.p.) administration of corticosterone (2.4 mg/kg) was associated with a rapid (2.5 min) and significant increase in hypothalamic histamine (HA) levels which persisted for 60 min. Midbrain and cortical HA concentrations were not affected. Significant and prolonged elevation of hypothalamic, midbrain and cortical HA levels was achieved by L-histidine administration (500 mg/kg i.p.). The most significant increase was noted in the hypothalamus and persisted for 10 hours. The elevated brain HA levels were associated with significant increase in plasma corticosterone levels which lasted for 120 mins. Present data supports the involvement of central HA in endocrine function.

Effect of diphenhydramine on stress-induced changes in brain histidine decarboxylase activity, histamine and plasma corticosterone levels

Exposure of rats to platform stress induced a significant elevation in hypothalamic histamine levels. Air blast-stress resulted in a significant increase in hypothalamic histamine concentration and in histidine decarboxylase activity. No significant changes were noted either in the enzyme activity or in histamine levels in the midbrain or cortex of stressed rats. In the nonstressed rats, diphenhydramine (7.5 mg/kg intragastrically), a H1-receptor antagonist, did not influence histidine decarboxylase activity or histamine concentration in any of the three brain regions investigated. However, diphenhydramine pretreatment prevented the increase in histidine decarboxylase activity induced by air blasts. In untreated rats, plasma corticosterone levels were significantly elevated following either platform stress (4.5-fold) or air blasts (7.8-fold). A significant increase was also noted in saline and diphenhydramine-treated animals following these stressors, however, the increase in saline or diphenhydramine treated rats following air blasts was significantly less than that seen in untreated stressed controls.

Histamine: correlative studies in nucleus accumbens

The role of histamine as a neurotransmitter has been the subject of considerable controversy. Recent evidence suggests it to be involved in such complex activities as arousal and affect. The purpose of the present study is to examine the possible source, function, and pharmacology of histamine in the nucleus accumbens, an area of the brain also implicated in complex activities such as affect. The anatomical studies suggest that the most probable source of the histamine in nucleus accumbens is the complex region lateral to the mammillary nuclei. These areas are the intercalated nucleus and the tuberomammillary nucleus (nuclei gemini hypothalami). To a lesser degree, the supramammillary complex may also contribute histamine-containing axons to the accumbens area. Adenylate cyclase in the rabbit nucleus accumbens displayed activation in response to histamine agonists (histamine, 2-Me-histamine, and 4-Me-histamine). The action of the H1 antagonist promethazine was greater than the H2 antagonist metiamide in reducing enzyme activation by histamine and 2-Me-histamine. In contrast, metiamide was more potent than promethazine toward antagonism of the action of 4-Me-histamine. However, no additive effects were noted when agonists were added in combination. Based upon these data, it is suggested that activation of adenylate cyclase in the rabbit nucleus accumbens is mediated in part by mixed H1 and H2 receptors or cellular disruption reflects the loss of receptor specificity. Physiological studies demonstrated that the H2 agonist 4-Me-histamine had an inhibitory effect on the activity of neurons driven by stimulation of the fimbria. The magnitude of the effect was frequency dependent. The H1 agonist 2-Me-histamine had no significant effect. Iontophoretic application of 4-Me-histamine had minimal effect upon low frequency volleys (0.5 Hz) but had a pronounced effect upon higher frequency volleys (6.0 Hz). These effects were antagonized by metiamide. Iontophoretic application of metiamide alone produced an effect only upon the P component of the field response, which is also bicuculline sensitive. Bicuculline coadministration was also effective in antagonizing the 4-Me-histamine effect. The physiological data suggest that histamine works through H2 receptors in nucleus accumbens, perhaps by potentiating the effects of gamma-aminobutyric acid (GABA). Thus, histamine in nucleus accumbens appears to function as a modulatory substance whose effect is dependent upon the activity of other transmitter and afferent systems.

Histamine-activated adenylate cyclase in brain homogenates of several species

Histamine has been shown to activate cyclic AMP synthesis in brain slices and homogenates of certain species, although less is known about species differences in brain homogenates. Dutch Belted and New Zealand White rabbit brain homogenates contained a histamine-responsive adenylate cyclase similar to that of the guinea pig. In contrast, adenylate cyclase of gerbil and hamster brain exhibited little or no stimulation by histamine. Male rat hypothalamic homogenates contained adenylate cyclase, but also exhibited minimal stimulation by histamine, in disagreement with some recent reports. Detailed studies of the conditions of assay failed to resolve this discrepancy.

Additional data on the function of hypothalamic histamine

In a preliminary study, the stimulatory effect of histamine on an adenylate cyclase system in a solubilized cell-free preparation of the rat hypothalamus was established. The effect was dose dependent, and the histamine concentration required for half-maximal activation (Ka) was determined at 0.1 muM. At a 10-fold higher concentration, both chloropyramine, the classical histamine H1 antagonist, and metiamide, the selective H2-receptor blocker, partially blocked this action. Experiments carried out in hypothalamic slices showed a stimulatory effect of both the H1-agonist, 2-(2-pyridyl)-ethylamine, and the H2-antagonist, dimaprit, on adenylate cyclase in the range of histamine action. These effects could be reversed completely by the H1-antagonist, mepyramine, and the H2-receptor blocker cimetidine. In an additional study, histamine, histamine agonists and antagonists were tested on the spontaneous and the potassium-activated outflow of 3H-noradrenaline from rat hypothalamic slices. Histamine did not modify this outflow significantly, whereas the H1-agonist 2-(2-pyridyl)-ethylamine, produced a marked, dose-related increase in both the spontaneous and the potassium-stimulated release of noradrenaline. The H2-receptor blocker, cimetidine, also exerted a moderate but statistically significant stimulatory effect in this system. In combination studies, the noradrenaline-releasing action of these compounds could not he reversed by the selectively acting histaminic or antihistaminic agents, showing that this effect does not relate to the histaminic or antihistaminic property of the compound. It is becoming clear that histamine exerts a direct stimulatory effect on hypothalamic adenylate cyclase. The noradrenaline-releasing potency of some histaminic and antihistaminic agents showed that these compounds might modify the clear histamine effects through the release of other transmitter amines.

Histamine actions on activity of cultured hypothalamic neurons: evidence for mediation by H1- and H2-histamine receptors

Tuberal hypothalamic tissue cultures were used to investigate the actions of histaminergic agents on neuronal activity using extracellular glass micropipettes. Histamine and H1- and H2-agonists were applied locally onto single active neurons by iontophoresis, while histaminergic antagonists were perfused through the bathing medium. Peri-event histogram and ratemeter analysis showed histamine to both excite and depress unit activity. Excitations were only antagonized by putative H1- and not H2-histamine antagonists, whereas inhibitions were antagonized by H2- and H2-antagonists. Dimaprit, a specific H2-agonist, elicited inhibitions of activity, while 2-(2-pyridyl)ethylamine, a putative H1-agonist, elicited both excitations and inhibitions. Two pharmacologically distinct populations of histamine receptors may exit in the hypothalamus: excitatory H2-receptors and inhibitory H2-receptors.

Regulation of histamine synthesis: altered synthesis and level of histamine in the hypothalamus of rats by repeated administration of histamine H1 and H2 receptor antagonists

Repeated administration of histamine H1 and H2 receptor antagonists resulted in significant alterations in the hypothalamic histamine level of rats and similar changes in the HD activity of the hypothalami were observed after these treatment. The changes in the level run parallel with the modifications in the HD activity. The effects of classical antihistaminergic agents were however opposite to that of the specific H2 antagonists, namely chloropyramine and tripelennamine markedly enhanced while metiamide and cimetidine diminished the hypothalamic level of histamine.

Histamine H1-receptors in the brain of the guinea-pig and the rat: differences in ligand binding properties and regional distribution

1 The equilibrium dissociation constant, K(d), for mepyramine binding to a particulate fraction from rat brain, 9.1 nM, determined from inhibition of the binding of 1 nM [(3)H]-mepyramine, was distinctly higher than that, 0.83 nM, measured on an equivalent preparation from guinea-pig brain.2 In rat brain the dissociation constant for mepyramine, determined from the binding of [(3)H]-mepyramine sensitive to inhibition by 2 x 10(-6) M promethazine, was higher than the constant obtained from the inhibition of the binding of 1 nM [(3)H]-mepyramine by non-radioactive mepyramine. This suggests that the promethazine-sensitive binding of [(3)H]-mepyramine includes a lower affinity non-receptor component, which becomes apparent at higher concentrations of [(3)H]-mepyramine.3 In the guinea-pig the dissociation constant for mepyramine determined from inhibition of [(3)H]-mepyramine binding was in good agreement with the value obtained from inhibition of the contractile response of intestinal smooth muscle to histamine. No similar comparison was possible in the rat. Rat ileum was much less sensitive to histamine and the contraction produced was not inhibited by 10(-6) M mepyramine, indicating that it is not mediated by H(1)-receptors.4 Low levels of promethazine-sensitive [(3)H]-mepyramine binding were present in membrane fractions prepared from the longitudinal muscle from rat small intestine, but the characteristics of this binding suggest that it may be largely to lower affinity, non-receptor sites.5 Promethazine was practically equipotent as an inhibitor of [(3)H]-mepyramine binding in rat and guinea-pig brain. Chlorpheniramine showed stereospecificity in the rat as in the guinea-pig, although the potency of the (+)-isomer in the rat was only a tenth of that in the guinea-pig. Histamine had nearly the same IC(50) in both species.6 The evidence suggests that the high-affinity [(3)H]-mepyramine binding sites in rat brain can be described as H(1)-receptors, but that these differ structurally from H(1)-receptors in the guinea-pig.7 The regional distribution of [(3)H]-mepyramine binding in rat brain was not the same as that in guinea-pig brain, the most notable difference being the very much lower level in rat cerebellum compared to guinea-pig cerebellum.