The role of histamine in the brain

Control of circulation by cerebral catecholaminergic and histaminergic mechanisms

Catecholamines are formed, released and metabolized in the cerebral regions known to regulate the cardiovascular system. The release of catecholamines in these brain areas is associated with changes in cardiovascular function. Injections of exogenous catecholamines into certain brain regions induce cardiovascular changes and mimic the effects of the release of endogenous catecholamines in the same regions. In addition, drugs with selective effects on cerebral catecholamine functions also induce cardiovascular changes. Changes in cerebral catecholamines have also been found in arterial hypertension. Taken together, these observations provide strong evidence that cerebral catecholaminergic mechanisms have a crucial role in both the normal and pathological control of circulation. Cerebral catecholaminergic mechanisms also mediate, at least in part, the therapeutic effects of clonidine, alpha-methyldopa and some other antihypertensive drugs. Recent findings suggest that histaminergic mechanisms in the brain may also be important in the control of circulation and in the mediation of drug effects. Since the cerebral mono-aminergic systems are necessary for the physiological control of circulation, it is likely that the cerebral catecholaminergic and possibly histaminergic pathways are also involved in the mediation of the effects of psychological stress factors on the cardiovascular system.

Leakiness of rat brain microvessels to fluorescent probes following craniotomy

The effects of craniotomy and/or histamine treatment upon brain microvascular permeability was studied in Wistar rats. Extravasation of circulating Na-fluorescein (MW 376) and of FITC-albumin (MW 69,000) was observed through a cranial window using intravital fluorescence microscopy. Simple exposure of the pial microvessels induced formation of discrete spots of fluorescent material around venules, but not around arterioles or capillaries. The average number of leaky spots to Na-fluorescein and to FITC-albumin was 4.3 and 1.8 per 10 mm2, respectively, 35 min after exposure. Pretreatment of the rats with either indomethacin (a cyclo-oxygenase inhibitor) or promethazine (a histamine H1-receptor blocker) did not reduce the number of leaky sites, whereas pretreatment with a combination of the two drugs had a significant protective effect. Administration of histamine (10(-4) M) to the exposed brain surface for 5 min increased the number of leaky sites to Na-fluorescein and FITC-albumin 3.2 and 3.6 times, respectively. It is concluded that exposure of the brain surface induces release of histamine and cyclo-oxygenase metabolites, and that these inflammatory mediators elicit formation of leaky sites in brain venules.

Modulation of the spontaneous histamine release by adrenergic and cholinergic drugs

Experiments have been reported on the possible modulation of the spontaneous histamine release by adrenergic and cholinergic drugs. Adrenergic drugs increase the spontaneous histamine release in vivo, and in neoplastic mast cells, in vitro. The mechanism of histamine release appears to be dependent upon the activation of alpha-adrenoceptors. Cholinergic drugs activate the release of histamine in many secretory processes in vivo; in vitro, acetylcholine is one of the most powerful histamine releasers in isolated purified rat mast cells. The release of histamine evoked by acetylcholine in rat mast cells is a calcium-requiring, temperature-dependent exocytosis. The physiological relationship of the sympathetic, parasympathetic and histamine-containing cells are discussed.

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.

Antibodies to histamine. Specificity studies and radioimmunological assay

Antibodies against conjugated histamine were raised in rabbits. This amine was coupled to different protein carriers by a bifunctional agent, hexamethylene diisocyanate. The specificity of the antibodies was determined with radioimmunological tests in equilibrium dialysis using an iodinated ligand: 125I-labelled histamine-hexamethylene diisocyanate-glycyl-tyrosine. The latter mimicked the antigenic determinant present in immunogens. Competition experiments were established between the radiolabelled ligand and conjugated histamine, conjugated analogs or unconjugated histamine. Cross-reactivity ratios and affinity constants were calculated from displacement curves, thereby allowing the antibody site to be characterized. The antibodies were found to be highly specific and were used for the assay of histamine in biological samples. For this, polystyrene beads coated with purified antiserum were used to establish a simple and reproducible test.

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.

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.

Neurologic sequelae following the imported fire ant sting

Four cases are described in which neurologic disturbances are the presenting symptoms after the imported fire ant (IFA) sting. The neurologic sequelae include focal motor and grand mal seizures and a mononeuropathy. The possible pathogenesis of these disorders is discussed.

Response of local blood flow in the caudate nucleus of the cat to intraventricular administration of histamine

The effect of intraventricular histamine on blood flow in the caudate nucleus of the cat was studied by means of the hydrogen clearance technique. Bilateral ventriculo-cisternal perfusion was installed. After a control period during which both lateral ventricles were perfused with mock CSF with the same composition, the drug under study was added to one side (experimental side) while the other side was perfused further with the control mock SCF (control side). At each point in time, blood flow at the experimental side was compared to that at the control side. Histamine (10(-3) M) caused a severe vasodilatation and this effect was completely antagonised by the H2-receptor blocker cimetidine (10(-2) M). Cimetidine had no vasoactive effects of itself in the concentration used. The H2-receptor agonist Dimaprit (10(-3) M) had a vasodilator effect although less important than histamine. Indirect evidence was gained that H1-receptors are not active in the vascular bed under study.

The contribution of mast cells to the histamine content of the central nervous system: a regional analysis

W/WV mice are severely deficient in mast cells. The absence of mast cells in skin and salivary glands was found to be paralleled by a drastic decrease of the histamine levels in these tissues when compared to non-anemic +/+ control mice. Brains of W/WV mice are also devoid of mast cells. A comparison of the histamine concentrations in several brain regions of W/WV mice and controls revealed a moderate decrease in cerebral cortex, thalamus, hypothalamus and midbrain but no change in pons, medulla and cerebellum. These findings provide strong evidence that mast cells contribute to the histamine content in forebrain regions but not in hindbrain regions. It is speculated that there may exist histaminergic neurons intrinsic to the medulla and pons.

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-induced rise in core temperature of chloral-anaesthetized rats: mediation by H2-receptors located in the preopticus area of hypothalamus

Intracerebroventricular (i.c.v.) administration of histamine in chloral anaesthetized rats exposed to an ambient temperature of 22 C elicited a rise in their colonic temperature associated with a shivering. This effect was shared by the H2 receptor agonists dimaprit and impromidine. Impromidine is, in this respect, a partial agonist with an ED50 much lower than histamine. The histamine-induced rise in core temperature was antagonized by cimetidine administered either centrally (in doses of 25-40 micrograms, i.c.v.) or peripherally (large doses greater than or equal to 50 mg/kg i.p.) This constitutes an indication for the crossing of the blood-brain barrier by cimetidine. The H2 histamine receptors involved in this effect seem to be located mainly in the preopticus medialis nucleus (p.o.m.n.) of the hypothalamus since bilateral microinjections of histamine (5 ng) into this nucleus induced the effect, whereas cimetidine injected into the p.o.m.n., antagonised the relative hyperthermia elicited by an intracerebroventricular administration of histamine.

The role of central histamine H1- and H2-receptors in hypothermia induced by histamine in the rat

Histamine administered intraventricularly or into the anterior hypothalamic preoptic region induced dose-dependent hypothermia in rats with chronic i.c.v. cannula. This hypothermia was almost totally abolished by both the histamine H1- and H2-receptor antagonists, mepyramine or chloropyramine and metiamide or cimetidine, respectively, give i.c.v. prior to histamine. In behavioural thermoregulation studies histamine considerably diminished the mean duration of dwelling of the rat under the heat lamp. This effect was abolished by histamine H1- but not by H2-receptor antagonists. It is concluded that histamine induces hypothermia by lowering the set point of the hypothalamic thermostat by means of H1-receptors. Histamine H2-receptor blockers antagonized the increase in tail skin temperature after histamine administration, suggesting that h2-receptors are involved in a heat loss mechanism.

Phylogeny of histamine in vertebrate brain

Measurement of histamine and its metabolizing enzymes in a variety of chordate species indicated that histamine and histamine methyltransferase were present in brain of all vertebrate species with a recognizable brain structure. Diamine oxidase was absent in mammalian brain but was present in brain of lower vertebrates. Histamine levels were especially high in amphibia and fish brains, in which the phylogenetically newer parts of the brain were less well-developed. In the spiny dogfish (as in mammals), brain histamine levels were highest in the midbrain regions. In contrast to brain, histamine levels were low in muscle, skin and intestine of all aquatic species.

Effect of diazepam on stress induced changes in brain histamine

Acute treatment with diazepam (2.5, 5 and 10 mg/kg) did not affect the basal histamine concentration in the hypothalamus, midbrain or in the cortex of the rat. The increase in the hypothalamic histamine level caused by 15 min of "platform stress" was significantly attenuated by diazepam (5 or 10 mg/kg) pretreatment, but the elevation induced by 15 min of air blasts remained unchanged. Diazepam significantly reduced the rise in plasma corticosterone concentration in response to air blast stress but did not affect the increase caused by "platform stress." Thus, in addition to the already known effects of diazepam on stress induced changes in other central neurotransmitters or neuromodulators, diazepam may also affect the hypothalamic histamine elevation induced by certain type of stress.

Electrically-evoked release of [3H]-histamine from the guinea-pig hypothalamus

1 [3H]-histamine was taken up by slices of guinea-pig hypothalamus against a concentration gradient. 2 Electrical field stimulation of the superfused slices resulted in an increased efflux of radioactivity, the major part of which was shown to be associated with histamine by paper chromatography. 3 The evoked release of histamine was dependent on calcium ions in the superfusate and was increased by 56% when the frequency of stimulation was doubled from 5 to 10 Hz.

Brain histamine: plasma corticosterone spontaneous locomotor activity and temperature

Hypothalamic histamine exhibited circadian fluctuations in male Sprague-Dawley rats; low values were found during the dark period when spontaneous locomotor activity (S.L.A.) and temperature were elevated. A relatively high hypothalamic histamine level was observed during the early period of the light cycle and was associated with decreased S.L.A. and temperature. Histamine concentration was high when corticosterone levels were low at the end of the dark cycle and during the morning hours (4 a.m.-1 p.m.); but histamine levels were relatively constant while corticosterone concentration dropped during afternoon and early night hours (4 p.m.-10 p.m.). Furthermore, the lowest hypothalamic histamine level (at 1 a.m.) was associated with the average plasma corticosterone value, thus no consistent relationship between histamine and corticosterone levels could be observed. Circadian fluctuations in brain histamine may support its role in brain function.

Autoradiographic localization of H1-histamine receptors in brain using 3H-mepyramine: preliminary studies

3H-Mepyramine (3H-MEP) binds to slide-mounted tissue sections with characteristics indicating the labelling of a H1-histamine receptor. It is saturable (Bmax 12 pg/g tissue), of a high affinity (KD 0.5 mM) and possesses a drug specificity similar to that observed for H1 receptors in a variety of systems. In agreement with other biochemical studies, the association constant and the number of binding sites appear lower in the rat than in the guinea pig. Once the optimal conditions for labelling the receptors were found in the biochemical studies, tissue sections were prepared for autoradiographic studies. The receptor density varied markedly in the different brain regions. For example, there was a high density of H1 receptors in the molecular layer of the guinea pig cerebellum. In the guinea pig hippocampal formation, the highest densities were found in the molecular layer of the dentate gyrus while lower levels were found in the dendritic fields of the pyramidal cells. Significant densities of receptors were found in other areas of the rat and guinea pig brain as well.

The dilating effect of histamine on pial arteries of cats and its mediation by H2 receptors

We studied the effect of histamine and H1 or H2 blockers on the diameter of pial arteries (39-227 micron) using microapplication into the perivascular space. Concentration-response curves for histamine showed dilations which started at 10(-7) M and were maximal at 10(-5) and 10(-4) M. The H2 blocker, cimetidine, induced no vascular reaction over the whole concentration range tested (10(-7) to 10(-3) M). The H1 blocker, mepyramine, was not vasoactive in the concentration range from 10(-7) to 5 X 10(-5) M and evoked dilations at higher concentrations. The concentration-response curve for histamine was only slightly displaced by 10(-5) M mepyramine but was significantly shifted to the right by 10(-5) M cimetidine. The dilating effect of histamine could be reduced in a stepwise manner by increasing concentrations of cimetidine. These findings are in accordance with a selective antagonism between histamine and cimetidine at the H2 receptors of smooth muscle cells of pial arteries. The insignificant role of H1 receptors in histamine-induced dilations is supported by the finding that a combination of H1 and H2 blockers resulted in the same reduction of histamine-induced dilation as did the application of the H2 blocker.

Modulation by histamine of the efflux of radiolabeled catecholamines from rat brain slices

Histamine induced a dose-dependent stimulation of 3H-catecholamine(CA) efflux (superfusion procedure) from hypothalamic, striatal, hippocampal and cortical slices. The extra-hypothalamic regions were the most sensitive to histamine. Efflux of 14C-GABA and 14C-(acetyl)choline was not affected. The effect of histamine on 3H-CA efflux developed slowly, reaching its maximum after 15-20 min. Histamine was inefffective with tissue from reserpinized animals. The major part of the radioactivity released by histamine consisted of CA metabolites. Histamine apparently does not enter catecholaminergic neurons, since tyramine and the CA had no effect on the efflux of 3H-histamine previously taken up by brain slices. After incubation of slices with 3H-CA in the presence of histamine and subsequent superfusion, tyramine or K+ -depolarization induced much less 3H-CA release than from control slices not incubated with histamine. It is suggested that histamine may act as a modulator of presynaptic catecholaminergic processes in the central nervous system by causing a depletion of the transmitter stores in the nerve terminals.

Antagonism by some antihistamines of the amino acid-evoked responses recorded from the lobster muscle fibre and the frog spinal cord

1 The effects of some antihistamines on the lobster muscle fibre and the frog spinal cord were investigated using intracellular and extracellular recordings, respectively. 2. On lobster muscle, histamine H1-blockers reversibly antagonized responses to bath-applied glutamate, aspartate and quisqualate but not responses to gamma-aminobutyric acid (GABA). Iontophoretic glutamate potentials were also reduced. Histamine (up to 1 mM) had no effect on this preparation. 3 The H1-antagonists produced a small increase in muscle membrane conductance and a slight hyperpolarization. These effects were largely unchanged in a low C1- bathing solution. Procaine (1 mM) decreased membrane conductance and did not affect responses to GABA or glutamate. 4 The H2-antagonist burimamide blocked both glutamate and GABA-evoked responses on the lobster muscle without affecting resting potential or conductance. 5 In the frog cord, bath-applied histamine produced ventral root depolarizations and dorsal root hyperpolarizations (sometimes biphasic responses). These effects were reduced by tetrodotoxin (TTX) but not by antazoline (H1-blocker) or burimamide; the latter reversibly antagonized responses to both glutamate and GABA on TTX-treated cords while antazoline was ineffective. 6 It is suggested that antihistamines can act as non-specific amino acid antagonists by interacting at the level of the receptor-coupled ionophores.