Histidine decarboxylase and histamine in discrete nuclei of rat hypothalamus and the evidence for mast-cells in the median eminence

Sodium Homeostasis, a Balance Necessary for Life

Body sodium (Na) levels must be maintained within a narrow range for the correct functioning of the organism (Na homeostasis). Na disorders include not only elevated levels of this solute (hypernatremia), as in diabetes insipidus, but also reduced levels (hyponatremia), as in cerebral salt wasting syndrome. The balance in body Na levels therefore requires a delicate equilibrium to be maintained between the ingestion and excretion of Na. Salt (NaCl) intake is processed by receptors in the tongue and digestive system, which transmit the information to the nucleus of the solitary tract via a neural pathway (chorda tympani/vagus nerves) and to circumventricular organs, including the subfornical organ and area postrema, via a humoral pathway (blood/cerebrospinal fluid). Circuits are formed that stimulate or inhibit homeostatic Na intake involving participation of the parabrachial nucleus, pre-locus coeruleus, medial tuberomammillary nuclei, median eminence, paraventricular and supraoptic nuclei, and other structures with reward properties such as the bed nucleus of the stria terminalis, central amygdala, and ventral tegmental area. Finally, the kidney uses neural signals (e.g., renal sympathetic nerves) and vascular (e.g., renal perfusion pressure) and humoral (e.g., renin-angiotensin-aldosterone system, cardiac natriuretic peptides, antidiuretic hormone, and oxytocin) factors to promote Na excretion or retention and thereby maintain extracellular fluid volume. All these intake and excretion processes are modulated by chemical messengers, many of which (e.g., aldosterone, angiotensin II, and oxytocin) have effects that are coordinated at peripheral and central level to ensure Na homeostasis.

Immunoregulatory effect of mast cells influenced by microbes in neurodegenerative diseases

When related to central nervous system (CNS) health and disease, brain mast cells (MCs) can be a source of either beneficial or deleterious signals acting on neural cells. We review the current state of knowledge about molecular interactions between MCs and glia in neurodegenerative diseases such as Multiple Sclerosis, Alzheimer's disease, Amyotrophic Lateral Sclerosis, Parkinson's disease, Epilepsy. We also discuss the influence on MC actions evoked by the host microbiota, which has a profound effect on the host immune system, inducing important consequences in neurodegenerative disorders. Gut dysbiosis, reduced intestinal motility and increased intestinal permeability, that allow bacterial products to circulate and pass through the blood-brain barrier, are associated with neurodegenerative disease. There are differences between the microbiota of neurologic patients and healthy controls. Distinguishing between cause and effect is a challenging task, and the molecular mechanisms whereby remote gut microbiota can alter the brain have not been fully elucidated. Nevertheless, modulation of the microbiota and MC activation have been shown to promote neuroprotection. We review this new information contributing to a greater understanding of MC-microbiota-neural cells interactions modulating the brain, behavior and neurodegenerative processes.

Mast cells and inflammation

Mast cells are well known for their role in allergic and anaphylactic reactions, as well as their involvement in acquired and innate immunity. Increasing evidence now implicates mast cells in inflammatory diseases where they are activated by non-allergic triggers, such as neuropeptides and cytokines, often exerting synergistic effects as in the case of IL-33 and neurotensin. Mast cells can also release pro-inflammatory mediators selectively without degranulation. In particular, IL-1 induces selective release of IL-6, while corticotropin-releasing hormone secreted under stress induces the release of vascular endothelial growth factor. Many inflammatory diseases involve mast cells in cross-talk with T cells, such as atopic dermatitis, psoriasis and multiple sclerosis, which all worsen by stress. How mast cell differential responses are regulated is still unresolved. Preliminary evidence suggests that mitochondrial function and dynamics control mast cell degranulation, but not selective release. Recent findings also indicate that mast cells have immunomodulatory properties. Understanding selective release of mediators could explain how mast cells participate in numerous diverse biologic processes, and how they exert both immunostimulatory and immunosuppressive actions. Unraveling selective mast cell secretion could also help develop unique mast cell inhibitors with novel therapeutic applications. This article is part of a Special Issue entitled: Mast cells in inflammation.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Stimulation of brain mast cells by compound 48/80, a histamine liberator, evokes renin and vasopressin release in dogs

Because degranulation of brain mast cells activates adrenocortical secretion (41, 42), we examined whether activation of such cells increases renin and vasopressin (antidiuretic hormone: ADH) secretion. For this, we administered compound 48/80 (C48/80), which liberates histamine from mast cells, to pentobarbital-anesthetized dogs. An infusion of 37.5 microg/kg C48/80 into the cerebral third ventricle evoked increases in plasma renin activity (PRA), and in plasma epinephrine (Epi) and ADH concentrations. Ketotifen (mast cell-stabilizing drug; given orally for 1 wk before the experiment) significantly reduced the C48/80-induced increases in PRA, Epi, and ADH. Resection of the bilateral splanchnic nerves (SPX) below the diaphragm completely prevented the C48/80-induced increases in PRA and Epi, but potentiated the C48/80-induced increase in ADH and elevated the plasma Epi level before and after C48/80 challenge. No significant changes in mean arterial blood pressure, heart rate, concentrations of plasma electrolytes (Na+, K+, and Cl-), or plasma osmolality were observed after C48/80 challenge in dogs with or without SPX. Pyrilamine maleate (H1 histaminergic-receptor antagonist) significantly reduced the C48/80-induced increase in PRA when given intracerebroventricularly, but not when given intravenously. In contrast, metiamide (H2 histaminergic-receptor antagonist) given intracerebroventricularly significantly potentiated the C48/80-induced PRA increase. A small dose of histamine (5 microg/kg) administered intracerebroventricularly increased PRA twofold and ADH fourfold (vs. their basal level). These results suggest that in dogs, endogenous histamine liberated from brain mast cells may increase renin and Epi secretion (via the sympathetic outflow) and ADH secretion (via the central nervous system).

Dipsogenic potentiation by sodium chloride but not by sucrose or polyethylene glycol in tuberomammillary-mediated polydipsia

The aim of this study was to examine the dipsogenic mechanisms involved in the recently discovered tuberomammillary (TM)-mediated polydipsia. Rats with bilateral electrolytic lesions of each TM subnucleus underwent several dipsogenic treatments, both osmotic and volemic. Animals with ventral (E2) or medial TM lesions (E3 or E4) showed a potentiated hyperdipsic response to hypertonic sodium chloride administration but not to sucrose or polyethylene glycol treatments. The increase in response to sodium chloride was significantly greater in groups E3/E4 and E2 than in the non-lesioned group and in animals with polydipsia induced by lesion of the median eminence. As previously reported, hyperphagia was induced by lesion to ventral TM nuclei (E1 or E2), confirming a possible role for the TM complex in food intake. However, lesions in medial nuclei (E3 or E4) did not produce this increase in food intake. These results are interpreted in relation to the hypothalamic systems involved in food and water intake.

Human umbilical cord blood-derived mast cells: a unique model for the study of neuro-immuno-endocrine interactions

Findings obtained using animal models have often failed to reflect the processes involved in human disease. Moreover, human cultured cells do not necessarily function as their actual tissue counterparts. Therefore, there is great demand for sources of human progenitor cells that may be directed to acquire specific tissue characteristics and be available in sufficient quantities to carry out functional and pharmacological studies. Acase in point is the mast cell, well known for its involvement in allergic reactions, but also implicated in inflammatory diseases. Mast cells can be activated by allergens, anaphylatoxins, immunoglobulin-free light chains, superantigens, neuropeptides, and cytokines, leading to selective release of mediators. These could be involved in many inflammatory diseases, such as asthma and atopic dermatitis, which worsen by stress, through activation by local release of corticotropin-releasing hormone or related peptides. Umbilical cord blood and cord matrix-derived mast cell progenitors can be separated magnetically and grown in the presence of stem cell factor, interleukin-6, interleukin-4, and other cytokines to yield distinct mast cell populations. The recent use of live cell array, with its ability to study such interactions rapidly at the single-cell level, provides unique new opportunities for fast output screening of mast cell triggers and inhibitors.

The critical role of mast cells in allergy and inflammation

Mast cells are well known for their involvement in allergic and anaphylactic reactions, but recent findings implicate them in a variety of inflammatory diseases affecting different organs, including the heart, joints, lungs, and skin. In these cases, mast cells appear to be activated by triggers other than aggregation of their IgE receptors (FcepsilonRI), such as anaphylatoxins, immunoglobulin-free light chains, superantigens, neuropeptides, and cytokines leading to selective release of mediators without degranulation. These findings could explain inflammatory diseases, such as asthma, atopic dermatitis, coronary inflammation, and inflammatory arthritis, all of which worsen by stress. It is proposed that the pathogenesis of these diseases involve mast cell activation by local release of corticotropin-releasing hormone (CRH) or related peptides. Combination of CRH receptor antagonists and mast cell inhibitors may present novel therapeutic interventions.

The role of mast cells in migraine pathophysiology

Mast cells are critical players in allergic reactions, but they have also been shown to be important in immunity and recently also in inflammatory diseases, especially asthma. Migraines are episodic, typically unilateral, throbbing headaches that occur more frequently in patients with allergy and asthma implying involvement of meningeal and/or brain mast cells. These mast cells are located perivascularly, in close association with neurons especially in the dura, where they can be activated following trigeminal nerve, as well as cervical or sphenopalatine ganglion stimulation. Neuropeptides such as calcitonin gene-related peptide (CGRP), hemokinin A, neurotensin (NT), pituitary adenylate cyclase activating peptide (PACAP), and substance P (SP) activate mast cells leading to secretion of vasoactive, pro-inflammatory, and neurosensitizing mediators, thereby contributing to migraine pathogenesis. Brain mast cells can also secrete pro-inflammatory and vasodilatory molecules such as interleukin-6 (IL-6) and vascular endothelial growth factor (VEGF), selectively in response to corticotropin-releasing hormone (CRH), a mediator of stress which is known to precipitate or exacerbate migraines. A better understanding of brain mast cell activation in migraines would be useful and could lead to several points of prophylactic intervention.

Degranulation of mast cells located in median eminence in response to compound 48/80 evokes adrenocortical secretion via histamine and CRF in dogs

The effect of intracerebroventricular infusion of compound 48/80 (C48/80), a mast cell secretagogue, on adrenal cortisol secretion was investigated in dogs under pentobarbital sodium anesthesia. A marked increase in adrenal cortisol secretion was elicited by C48/80 along with a concomitant increase in the plasma levels of cortisol and immunoreactive ACTH, but neither arterial blood pressure and heart rate nor the plasma histamine level altered significantly. Pretreatment with either anti-CRF antiserum or pyrilamine maleate (H(1) histamine-receptor antagonist) significantly attenuated the C48/80-evoked increase in cortisol secretion, but pretreatment with metiamide (H(2)-receptor antagonist) significantly potentiated it. Significant attenuation of the C48/80-evoked increase in cortisol also occurred in dogs given ketotifen, a mast cell stabilizing drug, before pharmacologic challenge. In the pars tuberalis and median eminence (ME), mast cells were highly concentrated in close association with the primary plexus of the hypophysial portal system. Degranulated mast cells were extensively found in the ME of C48/80-treated animals. These results suggest that mast cells located in these regions liberated histamine within the brain as a result of degranulation induced by C48/80 and that this led to activation of the hypothalamic-pituitary-adrenocortical axis.

Critical role of mast cells in inflammatory diseases and the effect of acute stress

Mast cells are not only necessary for allergic reactions, but recent findings indicate that they are also involved in a variety of neuroinflammatory diseases, especially those worsened by stress. In these cases, mast cells appear to be activated through their Fc receptors by immunoglobulins other than IgE, as well as by anaphylatoxins, neuropeptides and cytokines to secrete mediators selectively without overt degranulation. These facts can help us better understand a variety of sterile inflammatory conditions, such as multiple sclerosis (MS), migraines, inflammatory arthritis, atopic dermatitis, coronary inflammation, interstitial cystitis and irritable bowel syndrome, in which mast cells are activated without allergic degranulation.

Brain mast cells act as an immune gate to the hypothalamic-pituitary-adrenal axis in dogs

Mast cells perform a significant role in the host defense against parasitic and some bacterial infections. Here we show that in the dog, degranulation of brain mast cells evokes hypothalamic-pituitary-adrenal responses via histamine release. A large number of mast cells were found in a circumscribed ventral region of the hypothalamus, including the pars tuberalis and median eminence. When these intracranial mast cells were passively sensitized with immunoglobulin E via either the intracerebroventricular or intravenous route, there was a marked increase in the adrenal cortisol secretion elicited by a subsequent antigenic challenge (whether this was delivered via the central or peripheral route). Comp.48/80, a mast cell secretagogue, also increased cortisol secretion when administered intracerebroventricularly. Pretreatment (intracerebroventricularly) with anti-corticotropin--releasing factor antibodies or a histamine H(1) blocker, but not an H(2) blocker, attenuated the evoked increases in cortisol. These data show that in the dog, degranulation of brain mast cells evokes hypothalamic-pituitary-adrenal responses via centrally released histamine and corticotrophin-releasing factor. On the basis of these data, we suggest that intracranial mast cells may act as an allergen sensor, and that the activated adrenocortical response may represent a life-saving host defense reaction to a type I allergy.

Acute stress increases permeability of the blood-brain-barrier through activation of brain mast cells

Disruption of the blood-brain-barrier (BBB) is important in the pathophysiology of various inflammatory conditions of the central nervous system (CNS), such as multiple sclerosis (MS), in which breakdown of the BBB precedes any clinical or pathological findings. There is some evidence that relapsing-remitting MS attacks may be correlated with certain types of acute stressful episodes. Stress typically activates the hypothalamic-pituitary-adrenal (HPA) axis through the release of corticotropin releasing hormone (CRH), leading to production of glucocorticoids that down regulate immune responses. However, acute stress also has pro-inflammatory effects that appear to be mediated through activation of mast cells. Here we show that acute stress by immobilization increased permeability of rat BBB to intravenous 99Technetium gluceptate (99Tc). This effect was statistically significant in the diencephalon and the cerebellum, while it was absent in the cerebral cortex where there are not mast cells. Immobilization stress also induced activation of mast cells in diencephalon, the site where most mast cells are found in the rat brain. Both BBB permeability and mast cell activation were inhibited by the 'mast cell stabilizer' disodium cromoglycate (cromolyn). These results expand the pathophysiology of mast cells and implicate them in CNS disorders, that may possibly be induced or exacerbated by stress.

Morphological and functional demonstration of rat dura mater mast cell-neuron interactions in vitro and in vivo

Mast cells derive from a distinct bone marrow precursor and mature in tissues under the influence of stem cell factor, nerve growth factor (NGF) and certain interleukins. Intracranial mast cells first appear in the meninges and are located perivascularly close to neurons. They can be activated by antidromic stimulation of the trigeminal nerve, as well as by acute immobilization stress. Substance P (SP) and corticotropin-releasing hormone (CRH) are particularly potent in stimulating mast cell release of vasoactive, inflammatory and nociceptive molecules. These findings have suggested that mast cells may be involved in neuroinflammatory conditions, such as migraines. In this study, dura mast cells were shown to have characteristics of connective tissue mast cells (CTMC) as they contained histamine, heparin and rat mast cell protease I (RMCP-I). Mast cells were localized close to SP-positive neurons immunocytochemically and mast cell-neuron contacts were also documented using scanning electron microscopy. Dura stimulated by SP and carbachol in situ released histamine. Preincubation of dura with estradiol slightly augmented histamine release by SP, an effect possibly mediated through estrogen receptors identified on dura mast cells. Acute stress by immobilization led to dura mast cell degranulation which was prevented by pretreatment with a neutralizing antibody to CRH or a CRH receptor antagonist. The present results further clarify the biology of intracranial mast cells and support their involvement in the pathophysiology of migraines which are precipitated or worsened by stress.

Definitive characterization of rat hypothalamic mast cells

Mast cells have previously been identified in mammalian brain by histochemistry and histamine fluorescence, particularly in the rat thalamus and hypothalamus. However, the nature of brain mast cells has continued to be questioned, especially because the electron microscopic appearance often shows secretory granule morphology distinct from that of typical connective tissue mast cells. Here we report that mast cells in the rat hypothalamus, identified based on metachromatic staining with Toluidine Blue, fluoresced after staining with berberine sulfate, indicating the presence of heparin. These cells were also positive immunohistochemically for histamine, as well as for rat mast cell protease I, an enzyme characteristically present in rat connective tissue mast cells. In addition, these same cells showed a very strong signal with in situ hybridization for immunoglobulin E binding protein messenger RNA. However, use of antibodies directed towards immunoglobulin E or its binding protein did not label any cells, which may mean either the binding protein is below the level of detection of the techniques used or that it is not expressed except in pathological conditions when the blood-brain barrier becomes permeable. At the ultrastructural level, perivascular mast cells contained numerous, intact, electron-dense granules which were labeled by gold-labeled anti-rat mast cell protease I. These results clearly demonstrate the presence of perivascular mast cells in the rat hypothalamus, where they may participate in homeostatic processes.

Neuronal histamine in the hypothalamus suppresses food intake in rats

Using probes to manipulate hypothalamic neuronal histamine, we report here that changes in neuronal histamine modulate physiological feeding behavior in rats. Infusion of alpha-fluoromethylhistidine (FMH), a "suicide" inhibitor of histidine decarboxylase (HDC), into the third cerebroventricle induced feeding in the early light phase when the histamine synthesis was most accelerated. FMH at an optimum 2.24 mumol dose elicited feeding in 100% of rats. Treatment of FMH specifically and selectively decreased concentration of histamine without affecting concentrations of catecholamines in the hypothalamus. Immediately before the dark phase, when the histamine synthesis was normally lower, FMH infusion did not affect feeding-related parameters such as meal size, meal duration or latency to eat. Conversely, thioperamide, which facilitates both synthesis and release of neuronal histamine by blocking presynaptic autoinhibitory H3 receptors, significantly decreased food intake after infusion of a 100-nmol dose into the third cerebroventricle. The effect of thioperamide was abolished with i.p. injection of 26 mumol/kg chlorpheniramine, an H1antagonist. FMH at 224 nmol was microinfused bilaterally into the feeding-related nuclei in the hypothalamus. The ventromedial nucleus (VMH) and the paraventricular nucleus (PVN), but not the lateral hypothalamus, the dorsomedial hypothalamus or the preoptic anterior hypothalamus were identified as the active sites for the modulation. Neuronal histamine may convey suppressive signals of food intake through H1 receptors in the VMH and the PVN with diurnal fluctuation.

Effect of a single dose of TCDD on the level of histamine in discrete nuclei in rat brain

Histaminergic neurones may be involved in the regulation of feed intake. Since TCDD causes anorexia in rats, histamine concentrations were measured in several hypothalamic nuclei involved in feeding regulation. Histamine concentrations were not changed in medial or lateral accumbens, suprachiasmatic, paraventricular, arcuate, ventromedial, dorsomedial or perifornical nuclei, or in lateral hypothalamic area, cortex, or pineal gland. There was, however, an increase in histamine concentration in median eminence 25 h after the administration of TCDD.

Intraocular hypothalamic transplants containing histaminergic neurons: innervation of host iris and hippocampal cografts

Hypothalamic tissue containing the tuberomammillary nucleus was dissected from fetuses of Embryonic Day 17 and inserted into the anterior chamber of the eye of young adult recipient rats. The growth of hypothalamic grafts was monitored through the translucent cornea and transplants were found to double in size over the 8 first weeks in oculo. After 4 weeks fetal hippocampal formation (Embryonic Day 18) was inserted into the eye chamber in half of the previously grafted animals and placed in contact with the first grafts. Double grafts were allowed to mature for up to 18 weeks before sacrifice. Recipient rats were anesthetized and superfused with carbodiimide and paraformaldehyde, after which transplants were removed, frozen, sectioned on a cryostat, and incubated with histamine antibodies. Immunohistochemical evaluations revealed a large number of histamine-positive nerve cell bodies with processes innervating the entire hypothalamic graft with a dense plexus of varicose fibers. Such histamine-positive fibers were also seen to invade the surrounding host iris in some cases with thick axon bundles as well as with single fibers. When hypothalamic transplants were combined with hippocampal grafts numerous histamine-immunoreactive fibers invaded the hippocampal tissue to form a plexus of varicose terminals throughout the cografts. After 4 weeks in oculo only a sparse histamine-positive innervation of hippocampal grafts was found, while 18-week-old double grafts contained a considerably larger amount of immunoreactive neurites.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitive radioimmunoassays for histamine and tele-methylhistamine in the brain

Serum albumin conjugates of histamine or tele-methylhistamine, a major catabolite, were prepared using 1,4-benzoquinone as the coupling agent and used to raise polyclonal antibodies in rabbits. The same reagent was used to prepare the [125I]iodinated tracer and treat tissue extracts submitted to the radioimmunoassays. The IC50 values of prederivatized histamine and tele-methylhistamine in the radioimmunoassays were 0.3 nM and 0.5 nM, respectively, whereas nonderivatized histamine or tele-methylhistamine, histidine, a variety of histamine derivatives, amines, etc., had at least 1,000-fold higher IC50 values. Application of the radioimmunoassays to nonpurified extracts of rat brain allowed the quantification of the two amine immunoreactivities in samples corresponding to less than 1 mg of hypothalamus. The tissue immunoreactivity corresponded to authentic histamine or tele-methylhistamine, as shown by (a) the parallel 125I-tracer displacement curves, (b) the similar elution patterns from HPLC columns, (c) the regional levels of histamine and tele-methylhistamine in brain, similar to those obtained with other methods, and (d) the clearcut effects of treatments with inhibitors of L-histidine decarboxylase or monoamine oxidase. The two radioimmunoassays appear as simple and sensitive tools to evaluate steady-state levels and turnover rates of histamine and tele-methylhistamine.

Histamine excites arcuate neurons in vitro through H1 receptors

The electrophysiological effects of histamine on neurons of the hypothalamic arcuate nucleus of the female rat were tested with extracellular single unit recordings in an in vitro slice preparation. Histamine increased the spontaneous neuronal firing rate in 63% of the arcuate cells tested. An inhibitory response to histamine was seen in only one of the 117 neurons tested. The excitatory response to histamine showed dose dependency and was stable during synaptic blockade (by high magnesium and low calcium concentrations) and across a temperature range of 29-37 degrees C. Administration of histaminergic type 1 (pyrilamine and chlorpheniramine) and type 2 (cimetidine) receptor blockers revealed that the excitatory responses to histamine were mediated by type 1 receptors. The same neurons were also tested for responses to norepinephrine, serotonin, acetylcholine and substance P. A significant correlation was found between responses to histamine and substance P: all units excited by substance P were also excited by histamine. This subclass of histamine-responsive arcuate neurons may play a role in the regulation of the anterior pituitary, since histamine and substance P have similar effects on LH and prolactin secretion.

tele-Methylhistamine levels and histamine turnover in nuclei of the rat hypothalamus and amygdala

An HPLC method using fluorescence detection for the determination of tele-methylhistamine (t-MH) was improved to a sensitivity level which enabled the detection of 0.05 pmol of tissue t-MH. The t-MH contents and the histamine turnover rates in various nuclei of the rat hypothalamus and amygdala were subsequently measured. The histamine turnover rates were estimated from pargyline-induced t-MH accumulation. Both the t-MH levels and the histamine turnover rates were shown to be relatively high in the nuclei dorsomedialis and premammillaris ventralis of the hypothalamus, and also in the nucleus medialis of the amygdala. The steady-state t-MH levels in various nuclei of the hypothalamus and amygdala correlated well with the histamine turnover rates in these nuclei.

A detailed mapping of histamine H1-receptors in guinea-pig central nervous system established by autoradiography with [125I]iodobolpyramine

[125I]Iodobolpyramine, a potent and selective histamine H1-receptor antagonist derived from mepyramine, was used to generate light microscopic autoradiograms on sections of guinea-pig brain and spinal cord. Histamine H1-receptors were labelled with high sensitivity over a low background as determined using mianserin or other H1-receptor antagonists as competing agents. An atlas of H1-receptors was established using five sagittal sections and 39 frontal sections, the latter serially prepared at 50 micron intervals. Labelled areas were identified by comparison with corresponding, classically stained sections and their density was rated according to an arbitrary scale. Autoradiographic grains were detected in a large variety of gray matter areas whereas they were generally absent from white matter areas. In the cerebral cortex, H1-receptors are present in all areas and layers with a higher density in lamina IV. In the hippocampal formation, H1-receptors display a laminated pattern of distribution and are the most abundant in the dentate gyrus (hilus and molecular layer) and in several areas of the subiculum and commissural complex. In the amygdaloid complex, the highest densities are found in the medial group of nuclei. In the basal forebrain, the striatum is moderately labelled whereas the nucleus accumbens, islands of Calleja and most septal nuclei are highly labelled. In the thalamus, H1-receptors are present in high density, particularly in the anterior, median and lateral groups of nuclei. In the hypothalamus the labelling is highly heterogeneous with high densities in, for example, medial preoptic area, dorsomedial, ventromedial and most posterior nuclei, including the tuberomammillary complex in which histaminergic perikarya and short axons are present.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoinhibition of histamine synthesis mediated by presynaptic H3-receptors

The regulation of histamine synthesis was studied on rat brain slices or synaptosomes labeled with L-[3H]histidine. Depolarization by increased extracellular K+ concentration enhanced by about twofold the [3H]histamine formation in slices of cerebral cortex. This stimulation was also observed, although to a lesser extent, in synaptosomes from cerebral cortex and slices from the posterior hypothalamus where most histaminergic cell-bodies are located, suggesting that it may occur in nerve endings as well as in perikarya. In the presence of exogenous histamine in increasing concentrations the K+-induced stimulation was progressively reduced by up to 60-70%. The effect of exogenous histamine appears to be receptor-mediated as shown by its saturable character, high pharmacological specificity and competitive reversal by histamine antagonists. The EC50 value of histamine for synthesis reduction (0.34 +/- 0.03 microM) was similar to its EC50 value for release inhibition known to be mediated by H3-receptors. In addition, whereas mepyramine and tiotidine, two potent antagonists at H1- and H2-receptors, respectively, were poorly effective, the H3-receptor antagonists burimamide and impromidine reversed the histamine effect in an apparently competitive manner. These effects were observed in slices of cerebral cortex or posterior hypothalamus as well as in cortical synaptosomes. Furthermore, even in the absence of added histamine, H3-receptor antagonists enhanced the depolarization-induced stimulation of [3H]histamine synthesis, indicating a participation of released endogenous histamine in the synthesis control process. The potencies of H3-receptor antagonists were similar to those of these agents at presynaptic autoreceptors controlling [3H]histamine release. It is concluded that H3-receptors control not only release but also synthesis of histamine at the level of nerve endings and also, presumably, of perikarya. A relationship between the two regulatory processes, possibly via intracellular calcium, seems likely but remains to be investigated at the molecular level.

Electrophysiological properties of cortically projecting histamine neurons of the rat hypothalamus

The tuberomammillary (TM) nuclei of the hypothalamus appear to be the sole histaminergic cell group in the brain. The extracellular electrophysiological properties of cortically projecting TM neurons were studied in the urethane-anesthetized rat. TM neurons, antidromically activated from either ipsi- or contralateral cerebral cortex, displayed relatively slow conduction velocities, consistent with reports suggesting that TM neurons possess unmyelinated axons. Spontaneous activity was slow and regular, with action potentials of long duration. There was often a noticeable delay between initial segment and somatodendritic portions of spontaneous action potentials, and complete loss of the somatodendritic portion of the second antidromic action potential was commonly seen when double pulse paradigms were employed. These data demonstrate that in addition to anatomical and biochemical similarities, TM neurons share a constellation of physiological properties with other central aminergic neurons.

Biochemical and morphological correlates of transmitter type in C2, an identified histaminergic neuron in Aplysia

There is compelling evidence that histamine serves as a neurotransmitter in C2, a pair of symmetrical neurons in the cerebral ganglion of Aplysia californica. These cells had previously been shown to contain high concentrations both of histamine and of its biosynthetic enzyme, histidine decarboxylase; in addition, 3H-histamine injected intrasomatically was found to move along C2's axons by fast transport. Furthermore, several actions of C2 on identified follower cells were simulated by the application of histamine. We have now characterized this identified neuron further. C2 converts 3H-histidine to histamine: 16% of the labeled precursor was converted to histamine 1 hour after intrasomatic injection. Synthesis of 3H-histamine is specific, since no conversion occurred after injection of other identified Aplysia neurons that are known to use other neurotransmitter substances. We also examined the fine structure of C2's cell body, axons, and axon terminals within the cerebral ganglion and in the nerves that carry its three peripheral branches, identified after injection of Lucifer Yellow, 3H-histamine, or horseradish peroxidase. Characteristic dense-core vesicles are present in all regions of the neuron, and are labeled after intrasomatic injection of 3H-histamine. These 100-nm vesicles together with 60-nm electron-lucent vesicles fill the varicose extensions of C2's neurites that are widely distributed within the ganglion, but only the smaller vesicles cluster at the membrane specializations presumed to be active zones that make contact with many neurons. The widespread distribution of axon terminals and varicosities is consistent with the idea that C2 is modulatory in function; 3H-histamine is taken up selectively by the cell body and axons of C2 and of several other putative histaminergic neurons in a Na+ -dependent manner. Characterization of these biochemical and morphological features of C2 adds to the large amount of information already available to make this identified cell a standard for identifying other neurons that use histamine as a transmitter.

Evidence for excitatory actions of histamine on supraoptic neurons in vitro: mediation by an H1-type receptor

The effects of histamine on the firing of supraoptic neurosecretory neurons in the rat were examined in vitro using acutely prepared, hypothalamo-neurohypophysial explants perifused with an artificial cerebrospinal fluid. Extracellular action potentials meeting the criteria of antidromic invasion from neurohypophysial stalk stimulation were recorded from 135 neurons in the tuberal portion of the supraoptic nucleus, which lies superficially along the tuber cinereum and consists of mostly vasopressin-containing neurons. Units could be classified as slow/silent (76.3%), phasic (21.5%) or continuous (2.2%) on the basis of their spontaneous activity. Histamine applied briefly to the perifusate excited approximately one-third of the slow silent neurons and approximately two-thirds of the phasic neurons, with a wide range (10(-3)-10(-9)) in the effective concentration across neurons. The H1-receptor agonists 2-pyridylethylamine and 2-thiazolylethylamine mimicked these excitations in 10 of 12 and 3 of 6 neurons tested, respectively. The H2-receptor agonists dimaprit (4 neurons) and impromidine (5 neurons) failed to excite any of the tested neurons previously excited by histamine. The H1-receptor antagonist promethazine antagonized histamine's excitatory effect in 8 of 9 cells, while the H2-receptor antagonist cimetidine had little effect on the 9 cells tested. Histamine also modified bursts of activity induced in some slow/silent neurons by antidromic stimulation without having an observable effect in the absence of an antidromic burst. In 10 of 18 neurons histamine produced an elongation of burst duration and a modest increase in intraburst firing rate when applied during an antidromically evoked burst. In an additional 5 of 17 neurons, which had neither previously responded to histamine nor shown an antidromically-evoked burst, the pairing of histamine application and antidromic shocks resulted in an antidromically evoked burst. The effects of histamine on evoked bursts also appeared to be mediated by an H1-receptor. Histamine's excitation of supraoptic neurons is thus dependent on the electrical activity expressed by the neuron at the time of testing. Conductances activated by depolarization of the neuron may be modified by histamine or this compound may alter the threshold for burst generation. Considered with data showing H1-receptor localization and histamine-immunoreactive fibers within the supraoptic nucleus, the present results, as well as those showing the potency of centrally applied histamine in releasing vasopressin, suggest histamine may act physiologically by altering the electrical activity of vasopressin-secreting neurons.

Morphine-induced changes in histamine dynamics in mouse brain

The effect of the acute morphine treatment on histamine (HA) pools in the brain and the spinal cord was examined in mice. Morphine (1-50 mg/kg, s.c.) administered alone caused no significant change in the steady-state levels of HA and its major metabolite, tele-methylhistamine (t-MH), in the brain. However, depending on the doses tested, morphine significantly enhanced the pargyline (65 mg/kg, i.p.)-induced accumulation of t-MH and this effect was antagonized by naloxone. A specific inhibitor of histidine decarboxylase, alpha-fluoromethylhistidine (alpha-FMH) (50 mg/kg, i.p.), decreased the brain HA level in consequence of the almost complete depletion of the HA pool with a rapid turnover. Morphine further decreased the brain HA level in alpha-FMH-pretreated mice. Morphine administered alone significantly reduced the HA level in the spinal cord, an area where the turnover of HA is very slow. These results suggest that the acute morphine treatment increases the turnover of neuronal HA via opioid receptors, and this opiate also releases HA from a slowly turning over pool(s).

Monoclonal antibody against L-histidine decarboxylase for localization of histaminergic cells

The immunochemical and immunohistochemical properties of a monoclonal antibody (Mab HI 113-12) developed in mice against a partially purified preparation of rat gastric L-histidine decarboxylase (HD) were studied. The Mab recognised the HD activity from the antigen and from crude tissue extracts with a high histamine (HA)-synthesizing capacity (stomach, hypothalamus, striatum, mastocytoma). In contrast, neither a bacterial HD nor other decarboxylases (glutamic and DOPA decarboxylases) were immunoprecipitated. In preliminary immunohistochemical studies, staining of the cytoplasm of mastocytoma as well as of hypothalamic neurons, particularly magnocellular ones in the mamillary region, were observed. The latter presumably correspond to the cells of origin of a long ascending histaminergic pathway.

Histamine turnover in the brain of different mammalian species: implications for neuronal histamine half-life

The turnover of neuronal histamine (HA) in nine brain regions and the spinal cord of the guinea pig and the mouse was estimated and the values obtained were compared with data previously obtained in rats. The size of the neuronal HA pool was determined from the decrease in HA content, as induced by (S)-alpha-fluoro-methylhistidine (alpha-FMH), a suicide inhibitor of histidine decarboxylase. The ratios of neuronal HA to the total differed with the brain region. Pargyline hydrochloride increased the tele-methylhistamine (t-MH) levels linearly up to 2 h after administration in both the guinea pig and the mouse whole brain. Regional differences in the turnover rate of neuronal HA, calculated from the pargyline-induced accumulation of t-MH, as well as in the size of the neuronal HA pool, were more marked in the mouse than in the guinea pig brain. The hypothalamus showed the highest rate in both species. There was a good correlation between the steady-state t-MH levels and the turnover rate in different brain regions. Neither the elevation of the t-MH levels by pargyline nor the reduction of HA by alpha-FMH was observed in the spinal cord, thereby suggesting that the HA present in this region is of mast cell origin. The half-life of neuronal HA in different brain regions was in the range of 13-38 min for the mouse and 24-37 min for the guinea pig, except for HA from the guinea pig hypothalamus, which had an extraordinarily long value of 87 min. These results suggest that there are species differences in the function of the brain histaminergic system.

Histamine-containing neurons in the rat hypothalamus

A specific antiserum against histamine was produced in rabbits, and an immunohistochemical study of histamine-containing cells was carried out in rat brain. The antiserum bound histamine in a standard radioimmunoassay and stained mast cells located in various rat and guinea pig tissues. Enterochromaffin-like cells in the stomach and neurons in the posterior hypothalamic area could be detected with this antiserum. The staining was highly specific and was not abolished by preabsorption with histidine, histidine-containing peptides, serotonin, or catecholamines, whereas preabsorption with histamine completely abolished the staining. Immunoglobulins of this antiserum purified by affinity chromatography stained the same cells as did the crude antiserum, whereas the serum fraction, which was not absorbed by histamine-affinity ligand, failed to stain any neuron. Histamine-immunoreactive neuronal cell bodies were found only in the hypothalamic and premammillary areas of colchicine-treated rats. The largest group of cells was seen in the caudal magnocellular nucleus and medially on the dorsal and ventral aspects of the ventral premammillary nucleus. Immunoreactive nerve fibers, but no cell bodies, were detected in other parts of the brain. Histamine-immunoreactive mast cells were found in the median eminence and pituitary gland. The results suggest that histamine-containing neurons are located only in a small area of the posterior hypothalamus, and these cells are probably the source of ascending and descending fibers detected in other brain areas.

Distribution of the histaminergic neuron system in the central nervous system of rats; a fluorescent immunohistochemical analysis with histidine decarboxylase as a marker

The distribution of histidine decarboxylase-like immunoreactivity (HDCI) in the rat central nervous system was studied by the indirect immunofluorescence technique. HDCI cell bodies were concentrated in the posterior hypothalamic area, such as in the tuberal magnocellular nucleus, caudal magnocellular nucleus, posterior hypothalamic nucleus and lateral hypothalamus just lateral to the fasciculus mammillothalamicus at the level of the posterior hypothalamic nucleus. Extensive networks of HDCI fibers of various densities were found in many areas of the brain; they were particularly dense in the hypothalamus but were also found in the following areas: rostrally in the cerebral cortex, olfactory nuclei, medial amygdaloid nucleus, n. tractus diagonalis, and bed nucleus of the stria terminalis, and caudally in the central gray matter of the midbrain and pons, auditory system, n. vestibularis medialis, n. originis nervi facialis, n. parabrachialis, n. commissuralis, n. tractus solitarii, and n. raphe dorsalis.

Histamine turnover in regions of rat brain

Rapid and complete inhibition of monoamine oxidase by pargyline produced linear increases in the content of the histamine metabolite, tele-methylhistamine (t-MH), in 9 regions of rat brain 2 and 4 h after drug administration. The treatment had little or no effect on the histamine content of these regions. As histamine methylation is the major metabolic pathway of histamine in brain, the rate of increase in brain t-MH content after complete inhibition of its metabolism provides an estimate of histamine turnover. Histamine turnover rates varied over 46-fold among regions, from cerebellum (0.029 nmol/g . h) to hypothalamus (1.33 nmol/g . h), similar to those reported for norepinephrine and serotonin. Turnover rates were highly correlated with control t-MH levels (r = 0.97) and control histamine levels (r = 0.84). Rate constants were highest in the caudate nucleus and frontal cortex, equivalent to a half-life of about 11 min in these regions. While hypothalamic histamine had the highest turnover rate, the rate constant for histamine in this region was among the lowest in brain, perhaps consistent with the presence of histaminergic cell bodies. Histamine turnover rates may be indicative of the activity of histamine-synthesizing neurons, and their determination will facilitate understanding of histamine in brain.

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.

Release of endogenous histamine in the hypothalamus of anaesthetized cats and conscious, freely moving rabbits

The hypothalamus of anaesthetized cats and conscious, freely moving rabbits was superfused with CSF through double-walled, push-pull cannulae and the release of endogenous histamine was determined in the superfusates by a radioenzymatic assay. In the posterior hypothalamic area of the anaesthetized cat, the rate of release of endogenous histamine varied rhythmically; phases of high rate of release appeared at 60 min cycles. The release of histamine was increased by electrical stimulation of the superfused area, as well as by hypothalamic superfusion with potassium-rich CSF. In the conscious rabbit, the anterior hypothalamic area and the posterior hypothalamic nucleus were superfused simultaneously. In both regions, the resting release of histamine varied rhythmically at approximately 70 min cycles. Phases of high or low-rate of release in the anterior hypothalamic area coincided with the corresponding phases in the posterior hypothalamic nucleus. The rhythmic release of endogenous histamine in the hypothalamus, as well as the ability of depolarizing stimuli to enhance the release of the amine support the idea that histamine acts as a neurotransmitter in the central nervous system.