Release of histamine in rat hypothalamus and corpus striatum in vivo

A mathematical model for histamine synthesis, release, and control in varicosities

Background

Histamine (HA), a small molecule that is synthesized from the amino acid histidine, plays an important role in the immune system where it is associated with allergies, inflammation, and T-cell regulation. In the brain, histamine is stored in mast cells and other non-neuronal cells and also acts as a neurotransmitter. The histamine neuron cell bodies are in the tuberomammillary (TM) nucleus of the hypothalamus and these neurons send projections throughout the central nervous system (CNS), in particular to the cerebral cortex, amygdala, basal ganglia, hippocampus, thalamus, retina, and spinal cord. HA neurons make few synapses, but release HA from the cell bodies and from varicosities when the neurons fire. Thus the HA neural system seems to modulate and control the HA concentration in projection regions. It is known that high HA levels in the extracellular space inhibit serotonin release, so HA may play a role in the etiology of depression.

Results

We compare model predictions to classical physiological experiments on HA half-life, the concentration of brain HA after histidine loading, and brain HA after histidine is dramatically increased or decreased in the diet. The model predictions are also consistent with in vivo experiments in which extracellular HA is measured, using Fast Scan Cyclic Voltammetry, in the premammillary nucleus (PM) after a 2 s antidromic stimulation of the TM, both without and in the presence of the H₃ autoreceptor antagonist thioperamide. We show that the model predicts well the temporal behavior of HA in the extracellular space over 30 s in both experiments.

Conclusions

Our ability to measure in vivo histamine dynamics in the extracellular space after stimulation presents a real opportunity to understand brain function and control. The observed extracellular dynamics depends on synthesis, storage, neuronal firing, release, reuptake, glial cells, and control by autoreceptors, as well as the behavioral state of the animal (for example, depression) or the presence of neuroinflammation. In this complicated situation, the mathematical model will be useful for interpreting data and conducting in silico experiments to understand causal mechanisms. And, better understanding can suggest new therapeutic drug targets.

Histamine and the striatum

The neuromodulator histamine is released throughout the brain during periods of wakefulness. Combined with an abundant expression of histamine receptors, this suggests potential widespread histaminergic control of neural circuit activity. However, the effect of histamine on many of these circuits is unknown. In this review we will discuss recent evidence for histaminergic modulation of the basal ganglia circuitry, and specifically its main input nucleus; the striatum. Furthermore, we will discuss recent findings of histaminergic dysfunction in several basal ganglia disorders, including in Parkinson's disease and most prominently, in Tourette's syndrome, which has led to a resurgence of interest in this neuromodulator. Combined, these recent observations not only suggest a central role for histamine in modulating basal ganglia activity and behaviour, but also as a possible target in treating basal ganglia disorders. This article is part of the Special Issue entitled 'Histamine Receptors'.

A voltammetric and mathematical analysis of histaminergic modulation of serotonin in the mouse hypothalamus

Histamine and serotonin are neuromodulators which facilitate numerous, diverse neurological functions. Being co-localized in many brain regions, these two neurotransmitters are thought to modulate one another's chemistry and are often implicated in the etiology of disease. Thus, it is desirable to interpret the in vivo chemistry underlying neurotransmission of these two molecules to better define their roles in health and disease. In this work, we describe a voltammetric approach to monitoring serotonin and histamine simultaneously in real time. Via electrical stimulation of the axonal bundles in the medial forebrain bundle, histamine release was evoked in the mouse premammillary nucleus. We found that histamine release was accompanied by a rapid, potent inhibition of serotonin in a concentration-dependent manner. We developed mathematical models to capture the experimental time courses of histamine and serotonin, which necessitated incorporation of an inhibitory receptor on serotonin neurons. We employed pharmacological experiments to verify that this serotonin inhibition was mediated by H3 receptors. Our novel approach provides fundamental mechanistic insights that can be used to examine the full extent of interconnectivity between histamine and serotonin in the brain. Histamine and serotonin are co-implicated in many of the brain's functions. In this paper, we develop a novel voltammetric method for simultaneous real-time monitoring of histamine and serotonin in the mouse premammillary nucleus. Electrical stimulation of the medial forebrain bundle evokes histamine and inhibits serotonin release. We show voltammetrically, mathematically, and pharmacologically that this serotonin inhibition is H3 receptor mediated.

In vivo histamine voltammetry in the mouse premammillary nucleus

Histamine plays a major role in the mediation of allergic reactions such as peripheral inflammation. This classical monoamine is also a neurotransmitter involved in the central nervous system but its role in this context is poorly understood. Studying histamine neurotransmission is important due to its implications in many neurological disorders. The sensitivity, selectivity and high temporal resolution of fast scan cyclic voltammetry (FSCV) offer many advantages for studying electroactive neurotransmitters. Histamine has previously been studied with FSCV; however, the lack of a robust Faradaic electrochemical signal makes it difficult to selectively identify histamine in complex media, as found in vivo. In this work, we optimize an electrochemical waveform that provides a stimulation-locked and unique electrochemical signal towards histamine. We describe in vitro waveform optimization and a novel in vivo physiological model for stimulating histamine release in the mouse premammillary nucleus via stimulation of the medial forebrain bundle. We demonstrate that a robust signal can be used to effectively identify histamine and characterize its in vivo kinetics.

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.

Mast cell-derived mediators protect against oxygen-glucose deprivation-induced injury in PC12 cells and neurons

Recent reports and our previous study suggest that mast cells play a crucial role in the pathological processes that follow cerebral ischemia. In this study, the effect of mast cells on neuron injury after cerebral ischemia was determined by adding in vitro ischemia-induced supernatant from mast cells to neurons and PC12 cells under the same conditions (oxygen-glucose deprivation, OGD). The degree of cell injury was evaluated by the 3-[4,5-dimethylthiazol-2-yl]-2,5-dipheny-ltetrazolium bromide (MTT) assay. Mast cell-derived supernatant protected against OGD-induced injury of PC12 cells and neurons, and this protection was reversed by a histamine H1 antagonist and by anti-histamine serum, but not by an H2 antagonist. However, histamine and nerve growth factor (NGF) added separately or together did not have protective effects against OGD-induced injury. These results indicate that mast cell-derived protection during in vitro ischemia is histamine-dependent, and involves cooperation with other mediators, but not NGF.

Effect of cerebral ischemia on brain mast cells in rats

The purpose of this study was to investigate the effect of transient cerebral ischemia on brain mast cells in rats. The mast cells decreased significantly at 1 h, 2 h, 4 h and 7 days after ischemia. At 1 day following ischemia, the increase of the number of mast cells in the middle aspect of the thalamus (bregma -2.80 to -3.16 mm) was twice as that of other regions in the thalamus. In addition, histamine contents increased significantly in the thalamus and striatum after ischemia. These results indicate that brain mast cells participate in the pathological process after ischemia.

Experimental anxiety induced by histaminergics in mast cell-deficient and congenitally normal mice

To clarify the effect of mast cell-derived histamine release in the brain on anxiety, histaminergics-induced anxiety-like behaviors were examined by a light/dark test in mast cell-deficient (W/Wv) and congenitally normal (+/+) mice. In +/+ mice, when cimetidine (an H2 receptor antagonist) was coadministered with thioperamide (a neuronal histamine releaser acting via inhibition of H3 autoreceptors) or Compound 48/80 (C48/80, a selective histamine releaser from mast cells), the time spent in the light zone and the number of crossings between light and dark zones in a light/dark test decreased significantly, suggesting induction of anxiety. In W/Wv mice, however, experimental anxiety was induced by coadministration of thioperamide-cimetidine, but not C48/80-cimetidine. These results suggest that both nonneuronal mast cell-derived histamine and neuronal histamine play an important role in inducing experimental anxiety.

Evidence for activation of histamine H3 autoreceptors during handling stress in the prefrontal cortex of the rat

On-line microdialysis of histamine in 10-min samples of the prefrontal cortex of the conscious rat is described. The HPLC-fluorescent assay for histamine in dialysates has been significantly simplified by using only one postcolumn reagent line instead of the three reagent lines described in earlier methods. The method is selective, sensitive (detection limit: 2-3 fmol on column), and linear over a large concentration range. Basal values of histamine decreased to about 50% of basal levels during infusion of tetrodotoxin (5 x 10(-6) M). Handling rats for 15 min increased histamine in dialysates to about 300% of basal levels. When tetrodotoxin (10(-6) M) was applied during handling the increase in histamine release was strongly (about 80%) suppressed. The handling-induced increase in histamine was used as a paradigm to investigate the functional activity of histamine H3 autoreceptors during mild stress or arousal. An H3 receptor specific agonist (alpha-methylhistamine; 10(-5) M) and antagonist (thioperamide; 10(-5) M) were infused into the frontal cortex via the microdialysis probe. The effect of handling on histamine release was potentiated during infusion of thioperamide and fully suppressed during infusion of alpha-methylhistamine. These results clearly illustrate the efficacy of the H3 autoreceptor in modulating stimulated histamine release during natural stimulatory conditions.

Histamine H(3) receptor-mediated inhibition of depolarization-induced, dopamine D(1) receptor-dependent release of [(3)H]-gamma-aminobutryic acid from rat striatal slices

1. A study was made of the regulation of [(3)H]-gamma-aminobutyric acid ([(3)H]-GABA) release from slices of rat striatum by endogenous dopamine and exogenous histamine and a histamine H(3)-agonist. Depolarization-induced release of [(3)H]-GABA was Ca(2+)-dependent and was increased in the presence of the dopamine D(2) receptor family antagonist, sulpiride (10 microM). The sulpiride-potentiated release of [(3)H]-GABA was strongly inhibited by the dopamine D(1) receptor family antagonist, SCH 23390 (1 microM). Neither antagonist altered basal release. 2. The 15 mM K(+)-induced release of [(3)H]-GABA in the presence of sulpiride was inhibited by 100 microM histamine (mean inhibition 78+/-3%) and by the histamine H(3) receptor-selective agonist, immepip, 1 microM (mean inhibition 81+/-5%). The IC(50) values for histamine and immepip were 1.3+/-0.2 microM and 16+/-2 nM, respectively. The inhibitory effects of histamine and immepip were reversed by the H(3) receptor antagonist, thioperamide, 1 microM. 3. The inhibition of 15 mM K(+)-induced [(3)H]-GABA release by immepip was reversed by the H(3) receptor antagonist, clobenpropit, K(d) 0.11+/-0.04 nM. Clobenpropit alone had no effect on basal or stimulated release of [(3)H]-GABA. 4. Elevated K(+) caused little release of [(3)H]-GABA from striatal slices from reserpinized rats, unless the D(1) partial agonist, R(+)-SKF 38393, 1 microM, was also present. The stimulated release in the presence of SKF 38393 was reduced by 1 microM immepip to the level obtained in the absence of SKF 38393. 5. These observations demonstrate that histamine H(3) receptor activation strongly inhibits the dopamine D(1) receptor-dependent release of [(3)H]-GABA from rat striatum; primarily through an interaction at the terminals of GABA neurones.

The physiology of brain histamine

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

Anxiolytic-like effect of saiboku-to, an oriental herbal medicine, on histaminergics-induced anxiety in mice

Effect of saiboku-to, an oriental herbal medicine, on anxiety in mice was investigated using a light/dark test. Anxiogenic- and anxiolytic-like effects were evaluated on the basis of shortened and prolonged time spent in the light zone of the test. Subacute administration (once a day for 7 days) of saiboku-to (0.5-2.0 g/kg, p.o.) induced anxiolytic-like effect. To assess the effect of saiboku-to on brain histaminergic system in a state of anxiety, Compound 48/80 (1.0 microg/2 microl, i.c.v.), a non-neuronal mast cell histamine releaser, or thioperamide (10.0 mg/kg, i.p.), a neuronal histamine releaser possessing the inhibitory effect of histamine H(3) autoreceptors, induced decrease in the time spent in the light zone by co-injection with cimetidine (10.0 microg/2 microl, i.c.v.), a H(2) inhibitor, suggesting anxiety-like effect. These histaminergics-induced experimental anxieties were inhibited by pre-treatment with subacute administration of saiboku-to, as well as single treatment with diazepam. The results suggest that saiboku-to exhibits anxiolytic-like effect closely related to histaminergic system in the brain.

Histamine depolarizes cholinergic interneurones in the rat striatum via a H(1)-receptor mediated action

1. Whole-cell patch clamp recordings were made from rat striatal cholinergic interneurones in slices of brain tissue in vitro. Bath application of histamine (EC(50) 6.3 microM) was found to rapidly and reversibly depolarize these neurones through the induction of an inward current at -60 mV. 2. The effects of histamine were mimicked by the H(1) receptor agonist 2-thiazolylethylamine (50 microM) and selectively inhibited by pre-incubation with the H(1) receptor antagonist triprolidine (1 microM). 3. Ion substitution experiments under voltage clamp conditions revealed that the histamine activated current was comprised of two components. One component was sensitive to the concentration of extracellular Na(+), whilst the other component was inhibited by intracellular Cs(+) or extracellular Ba(2+). 4. In situ hybridization experiments revealed that the majority of cholinergic interneurones in the rat striatum express the histamine H(1) receptor but few neurones express H(2) receptors. These findings were confirmed using single cell RT - PCR. 5. It is concluded that histamine depolarizes cholinergic interneurones in the rat striatum via a H(1)-receptor mediated mechanism.

Effects of drugs acting as histamine releasers or histamine receptor blockers on an experimental anxiety model in mice

Experimental anxiety in mice was evaluated using a light/dark test at 60 min after injection of various histaminergics. Thioperamide, a histamine H(3) receptor inhibitor (5-20 mg/kg, intraperitoneal [IP]), Compound 48/80, a mast cell degranulator (0.1-10 microg/2 microl, intracerebroventricularly [ICV]), mepyramine, a histamine H(1) receptor antagonist (0.1-10 microg/2 microl, ICV) or cimetidine, a histamine H(2) receptor antagonist (0.1-10 microg/2 microl, ICV) alone did not affect the locomotive activity, the time spent in the light zone, and number of shuttle crossings in the light/dark test. However, the time spent in the light zone and the number of shuttle crossings significantly decreased only when cimetidine (0.1-10 microg/2 microl, ICV) was co-treated with either thioperamide (10 mg/10 ml/kg, IP) or Compound 48/80 (1.0 microg/2 microl, ICV). The decrease in these behavioral parameters suggests induced experimental anxiety in mice. The experimental anxiety was antagonized by mepyramine (10 microg/2 microl, ICV). These results suggest that not only neuronal histamine release induced by thioperamide but also non-neuronal (mast cells) histamine release induced by Compound 48/80 play an important role in inducing experimental anxiety via post-synaptic H(1) and H(2) receptors. In addition, it is likely that the anxiety may be mediated by the stimulation of H(1) receptors, while H(2) receptors may inhibit the anxiety produced by the activation of H(1) receptors.

Brain-derived mast cells could mediate histamine-induced inhibition of food intake in neonatal chicks

In the present study, the effect of intracerebroventricular (i.c.v.) administration of histamine on food intake of neonatal chicks was examined over 2 h. Histamine (100, 200 or 400 nmol, respectively) was injected in the lateral ventricle of 2-day-old chicks, and cumulative food intakes were measured. i.c.v. injection of histamine significantly inhibited food intake in a dose-dependent manner. In addition, compound 48/80, which causes degranulation of mast cells and release of histamine, or thioperamide, which is an antagonist of the histamine H3 autoreceptor and increases histamine release from histaminergic nerve terminals, was injected i.c.v. to clarify whether mast cell- or neuron-derived histamine in the central nervous system of chicks is essential to the feeding inhibition. Central administration of compound 48/80 inhibited food intake with a dose-dependent manner, but thioperamide had no effect on feeding. An inhibitor of mast cell degranulation, sodium cromoglycate, somewhat attenuated food intake inhibited by compound 48/80. These results suggest that brain-derived mast cells could be a major source of histamine in the inhibition of food intake of neonatal chicks.

Autoregulation of histamine release in medulla oblongata via H3-receptors in rabbits

The release of histamine (HA) from the rostral ventrolateral medulla (RVL), the raphe nuclei (nR), and the solitary nucleus (nTS) was investigated in anesthetized rabbits using microdialysis and high-performance liquid chromatography. HA release upon electrical stimulation of the posterior hypothalamus (PH), where histaminergic cell bodies are located, was increased to 168% of the baseline level in the RVL (n = 6), 139% of the baseline level in the nR (n = 5), and 166% of the baseline level in the nTS (n = 4). Upon perfusion of thioperamide, an H3-receptor antagonist, via a microdialysis probe, HA release from the RVL, nR and nTS increased. The increase in HA release from the RVL, nR and nTS following thioperamide perfusion was suppressed by co-perfusion of thioperamide and an H3-receptor agonist, imetit. We found that HA is released from the RVL, nR and nTS, that the HA release from all three areas is increased upon stimulation of the PH, and that the HA release is locally influenced in opposite directions by thioperamide and imetit. These results suggest that HA release in the medulla oblongata is controlled by the PH and that H3-receptors participate in the autoregulation of HA release by providing negative feedback locally. Autoregulation of HA release via H3-receptors may be important for maintaining tonic output to the sympathetic nervous system.

Reserpine- and tetrabenazine-sensitive transport of (3)H-histamine by the neuronal isoform of the vesicular monoamine transporter

The transport of (3)H-histamine by the endocrine-specific (VMAT1) and neuronal (VMAT2) isoforms of the vesicular monoamine transporter has been evaluated in digitonin-permeabilized fibroblasts transfected with either VMAT1 or VMAT2. Transport of (3)H-histamine by both VMAT1 and VMAT2 was reserpine-sensitive but only transport by VMAT2 was inhibited by tetrabenazine. Maximal equilibrated levels of (3)H-histamine accumulation by VMAT2 (K(m) 300 mu M) were approximately three times greater than that mediated by VMAT1 when using a subsaturating concentration of exogenous (3)H-histamine (50 mu M). The expression of VMAT2 in histaminergic neurons in the rat brain was examined with polyclonal antipeptide antibodies specific for VMAT1 or VMAT2. VMAT2-positive and tyrosine hydroxylase-negative immunoreactive cell bodies were localized to the ventral part of the posterior hypothalamus in the region of the mamillary nuclei. The transport properties of VMAT2 and the distribution of VMAT2 in cell bodies in the tuberomammillary nucleus of the posterior hypothalamus reported here and the apparent absence of VMAT1 and VMAT2 in tissue mast cells support previous findings of reserpine-sensitive and reserpine-resistant pools of histamine in brain and peripheral tissues.

Cholinergic nucleus basalis neurons are excited by histamine in vitro

Considerable evidence has shown that both cholinergic and histaminergic neurons in the brain may act to facilitate processes of cortical activation that occur during wakefulness. In the present study, the potential influence of histaminergic neurons upon cholinergic neurons of the basal forebrain was investigated in guinea-pig basal forebrain slices. We found that electrophysiologically identified and immunohistochemically verified cholinergic neurons of the nucleus basalis were depolarized and excited by histamine, as manifested by an increase in tonic firing. The depolarization was associated with an increase in membrane input resistance. The effect of histamine persisted in the presence of either tetrodotoxin or a high-magnesium/low-calcium solution, indicating that it is postsynaptic. By a process of elimination, the participation in this response of the three described histamine receptors was examined. Involvement of H3 receptors was excluded on the basis that the H3 agonist (R)-alpha-methyl-histamine had no direct effect, and the H3 antagonist, thioperamide, did not block the effect of histamine. In contrast, the presence of a small response to impromidine, a selective agonist of H2 receptors, and the partial block of the response to histamine by the H2 receptor antagonist, cimetidine, indicated the participation of H2 receptors. Finally, the complete elimination of histamine's effect occurred when low doses of the H1 antagonist, mepyramine, were added to the H2 antagonist, cimetidine, indicating the involvement and predominance of H1 receptors in the response. Our data thus suggest that histamine excites nucleus basalis cholinergic neurons by a concomitant activation of H1 and H2 receptors. Histaminergic tuberomammillary neurons may accordingly facilitate tonic firing of cholinergic neurons during wakefulness. Cholinergic basalis neurons could thus act in tandem with histaminergic neurons during periods of arousal to collectively promote widespread cortical activation.

Neurochemical organization of circadian rhythm in the suprachiasmatic nucleus

The circadian rhythm in mammals is under control of the pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus. This tiny nucleus contains a number of neurochemicals, including peptides, amines and amino acids. Heterogeneous distribution of these neurochemicals defines the substructures of the SCN. In the present review, functional significance of such neurochemical heterogeneity in the SCN is discussed in the light of circadian patterns of the concentrations of these neurochemicals in the SCN and their effects on SCN neurons in in vitro slice preparation. In particular, the hypothesis that the dorsomedial SCN is involved in maintaining the circadian rhythm, while the ventrolateral SCN is involved in adjusting the phase of the rhythm, is critically discussed. These considerations suggest that distinct sub-components of the SCN as marked by neurochemicals, interact with each other and this organizational architecture could be the basis of the proper operation of the circadian time keeping system in this nucleus.

Effects of histamine on 5-hydroxytryptaminergic neuronal activity in the rat hypothalamus

Effects of pharmacological manipulations which mimic or enhance histaminergic neuronal transmission were determined on the activity of 5-hydroxytryptaminergic neurons projecting to the hypothalamus of male rats. Intracerebroventricular administration of histamine decreased 5-hydroxytryptamine (5-HT) and increased 5-hydroxyindoleacetic acid (5-HIAA) concentrations in several hypothalamic nuclei; these effects were blocked by the histamine H1 receptor antagonist mepyramine but not the histamine H2 receptor antagonist zolantidine. Blockade of the 5-HT reuptake system by fluoxetine did not prevent histamine-induced decreases in 5-HT concentrations suggesting that histamine is not transported into nerve terminals via the 5-HT reuptake system to subsequently displace 5-HT stores. These data suggest that exogenous histamine increases 5-hydroxytryptaminergic neuronal activity through an action at histamine H1 receptors. In contrast, neither the histamine H3 receptor antagonist thioperamide, the histamine-N-methyltransferase inhibitor metoprine, nor combined thioperamide-metoprine treatment affected concentrations of 5-HT or 5-HIAA suggesting these agents, which purportedly enhance endogenous histaminergic transmission, do not affect 5-hydroxytryptaminergic neuronal activity. These results reveal that procedures commonly employed to study central actions of histamine differentially affect 5-hydroxytryptaminergic neuronal activity in the rat hypothalamus.

Effects of (S)-alpha-fluoromethylhistidine and (R)-alpha-methylhistamine on locomotion of W/Wv mice

We studied the effects of inactivators of the central histaminergic neuron system, (R)-alpha-methylhistamine, a histamine H3 receptor agonist, and (S)-alpha-fluoromethylhistidine, a histamine synthesis inhibitor, on locomotor activity and brain histamine content of mast cell-deficient W/Wv mice using a recently developed high-performance liquid chromatography system coupled with a fluorometric detector. IP injection of (R)-alpha-methylhistamine (6-50 mg/kg) increased brain histamine content after 1 h but caused no significant change in locomotor activity. IP injection of (S)-alpha-fluoromethylhistidine decreased brain histamine content at doses of 6-50 mg/kg and locomotor activity at doses of 12.5-50 mg/kg. However, locomotor activity was decreased significantly (in Student's t-test) by sequential administrations of (S)-alpha-fluoromethylhistidine (6 mg/kg) and (R)-alpha-methylhistamine (12.5 or 25 mg/kg), but not by (S)-alpha-fluoromethylhistidine (6 mg/kg) and other doses of (R)-alpha-methylhistamine (6 or 50 mg/kg). These results support the hypothesis that the central histaminergic neuron system is involved in the control of spontaneous locomotion or alertness.

In vivo microdialysis measurement of histamine in rat blood effects of compound 48/80 and histamine receptor antagonists

An in vivo microdialysis method combined with a highly sensitive HPLC method which was developed for the analysis of the mediators in the CNS has been applied to assay histamine concentrations in the blood. The technique was used to study the effects of compound 48/80 and histamine receptor antagonists on histamine release in the blood of rats. The mean basal level of histamine in the blood measured by in vivo microdialysis was 177.8 +/- 11.1 pmol/mL. This level was not affected significantly by intraperitoneal (i.p.) injection of saline, and remained at the constant level for at least 8 hr after injection of saline. After i.p. injection of histamine (0.5 mg/kg), histamine was quickly detected in the blood of the jugular vein. Moreover, because the recovered histamine in the dialysate is directly proportional to the free fraction in the blood, the in vivo microdialysis method of blood is a reliable method of examining histamine release into the blood. In our experiments, the histamine level in dialysates from rat jugular vein was markedly increased by compound 48/80 (2.0 mg/kg, i.p.), demonstrating the histamine release into the blood from mast cells. However, there was no increase in histamine concentration after an i.p. injection of histamine receptor antagonists, such as pyrilamine (2.0 mg/kg), d-chlorpheniramine (2.0 mg/kg), cimetidine (10 mg/kg), or thioperamide (10 mg/kg). Thus, the present results suggested that these histamine receptor antagonists might not have an influence on histamine release into the blood.

Altered histamine H3 binding in rat forebrain after reserpine treatment

Groups of 6 rats were treated for 5 days with either reserpine hydrochloride (5 mg/kg i.p., per diem), or saline. Regional binding of the histamine H3 agonist N alpha-[3H]methyl-histamine ([3H]NAMH) was determined in forebrain sections by quantitative autoradiography and Scatchard analysis. Highest maximal binding was in nucleus accumbens (107 +/- 18 fmol/mg) and corpus striatum (58 +/- 9 fmol/mg), where the apparent affinity was close to 4 nM. Maximal binding of [3H]NAMH in the insular cortex (39 +/- 6 fmol/mg) was higher than in other cortical areas examined. Reserpine treatment produced a 50% decrease in both the Bmax and the apparent Kd in the corpus striatum and nucleus accumbens, but binding parameters in the cortex and septum were unaltered. Therefore, the response of H3 receptors in rat forebrain to reserpine treatment for 5 days was regionally heterogenous such that maximal [3H]NAMH binding was typically higher in insular cortex (36 +/- 6 fmol/mg) than in corpus striatum (24 +/- 3 fmol/mg) of reserpine-treated rats.

Pulsatile release of histamine in the hypothalamus of conscious rats

The pattern of histamine release was investigated in the hypothalamus of the conscious, freely moving rat over 20 h. Under anaesthesia, a guide cannula was stereotaxically inserted into the posterior hypothalamus. In the conscious animal, the stylet of the guide cannula was replaced by a push-pull cannula, and the hypothalamus was superfused with artificial cerebrospinal fluid. Histamine was determined radioenzymatically in the superfusate which was continuously collected in time periods of 20 min. The release rate of histamine fluctuated according to an ultradian rhythm (frequency: 1 cycle per 83 min) and a circadian rhythm with the highest release rate of histamine between 11:00 p.m. and 1:00 a.m. The release rate of histamine during darkness was higher than that during the light period. The results demonstrate that, in the brain, neuronal histamine is released according to rhythms with various frequencies.

Direct evidence for increased continuous histamine release in the striatum of conscious freely moving rats produced by middle cerebral artery occlusion

Extracellular histamine in the stratum of conscious freely moving rats collected by intracerebral microdialysis 1 day after implantation of a U-shaped dialysis probe was measured by HPLC coupled with postcolumn o-phthalaldehyde derivatization fluorometry. The basal fractional histamine outputs were almost constant from 1 to 7 h after the start of perfusion (5.9-8.4 pg/30 min). Depolarization by perfusion with a high K+ (100 mM)-containing medium produced a significant (124%) increase and neuronal blockade by perfusion with a tetrodotoxin (1 microM)-containing medium resulted in a 68% reduction in the histamine output. The histamine output was markedly reduced by intraperitoneal injection of alpha-fluoromethylhistidine (100 mg/kg), an irreversible inhibitor of histidine decarboxylase, or (R)-alpha-methylhistamine (5 mg/kg), a potent and specific H3-receptor agonist. After middle cerebral artery (MCA) occlusion, the histamine output gradually increased, and reached four times the control value 8 h later. When rats were pretreated with metoprine (10 mg/kg), a histamine N-methyltransferase inhibitor, there was no significant difference in the histamine output between the MCA-occluded and the sham-operated groups during the first 3.5 h after the operation, but the histamine output gradually increased thereafter in the MCA-occluded group. In rats treated with alpha-fluoromethylhistidine, MCA occlusion failed to cause an increase in the histamine output. These results demonstrate that MCA occlusion induces a long-lasting increase in neuronal histamine release in the rat striatum.

Morphine-induced increases of extracellular histamine levels in the periaqueductal grey in vivo: a microdialysis study

The effect of morphine on extracellular histamine levels in two regions of the rat midbrain was studied in vivo by microdialysis. Morphine (5.6 and 12.8 mg/kg, s.c.) significantly and dose-dependently increased extracellular histamine levels in the periaqueductal grey, while no significant effect was observed in the reticular formation. In addition, no significant effect of sequential saline injections was observed on extracellular histamine levels in the periaqueductal grey. Since morphine has no effect on histamine catabolism, these results suggest that morphine increases histamine release in the rat PAG, a site where morphine and histamine are known to have analgesic action. Taken with earlier studies showing the ability of H2 antagonists to block morphine analgesia, these results support the hypothesis that histamine and H2 receptors are important in mediating morphine analgesia in the rat periaqueductal grey. The cellular origin of the extracellular histamine, and the mechanism of this morphine effect remain to be determined.

Circadian rhythm of histamine release from the hypothalamus of freely moving rats

Using an in vivo microdialysis technique coupled with HPLC-fluorometry, the release of neuronal histamine from the anterior hypothalamic area was monitored continuously in conscious, freely moving rats under a 12:12 h light:dark cycle. Spontaneous locomotor activity of the rats was measured simultaneously using a locomotor activity counter. Histamine release gradually increased in the second half of the light period (1400-2000) and the average histamine release during the dark period (2000-0800, 0.20 +/- 0.02 pmol/30 min) was significantly higher than that during the light period (0.12 +/- 0.01 pmol/30 min). This clear circadian change in the release suggests that the central histaminergic system is related to the circadian rhythm of rats.

Effects of histamine on spontaneous electrical activity of neurons in rat suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus (SCN) is thought to be a light-entrained pacemaker in mammals, inducing a wide range of endogenous circadian events. In rat brain, histaminergic (HAergic) fibres are particularly rich in the hypothalamus. This prompted an investigation of the influence of bath-applied HAergic compounds on the spontaneous electrical activity of SCN neurons, recorded extracellularly in the hypothalamic slice preparation. Cells activated by bath application of HA (n = 28) outnumbered those inhibited by HA (n = 6). 48% of cells tested (n = 28) were unresponsive to HA application. HA-induced activation of SCN neurons' discharge rate could be suppressed by the H1-antagonist mepyramine, but not by the H2-antagonist cimetidine. HAergic effects were still present when synaptic transmission was blocked, indicating a postsynaptic site of action for HA within SCN. Due to the anatomical distribution of HA-responsive SCN neurons and the independence of HA-effects with respect to circadian time, HA seems not to endow a pivotal role within generation and maintenance of circadian rhythm. Although data were obtained from SCN deprived of neural input, results show clear evidence for a modulatory impact of HA on the spontaneous electrical activity of part of SCN neurons.

Characterization of extracellular histamine in the striatum and bed nucleus of the stria terminalis of the rat: an in vivo microdialysis study

The intracerebral microdialysis technique, coupled with a sensitive radioenzymatic assay, was employed to study histamine release in the striatum and in the bed nucleus of the stria terminalis (BNST) in conscious, freely moving rats. In these brain regions, extracellular histamine concentrations decreased by 20% when calcium was omitted from the perfusion solution. Extracellular histamine was insensitive to the addition of tetrodotoxin to the perfusion medium. In striatum, extracellular histamine concentrations declined in an apparent biexponential manner after the administration of alpha-fluoromethylhistidine, an inhibitor of histamine synthesis. The half-lives for the disappearance of histamine were 32 min and 7.7 h, indicating the presence of at least two histamine pools. Histidine loading resulted in a nearly twofold increase in histamine outflow in striatum. In the BNST, yohimbine increased the extracellular histamine content by 50%, suggesting that histamine release is subject to alpha 2-adrenergic regulation in vivo. The extent to which histamine detected in cerebral microdialysis samples is of neurogenic origin remains to be established.

Feeding-induced increase in the extracellular concentration of histamine in rat hypothalamus as measured by in vivo microdialysis

The extracellular concentration of histamine (HA) in the hypothalamus of conscious and freely moving rats was measured by in vivo microdialysis and the effects of fasting and feeding on the HA concentration were examined. In non-fasted rats, the basal HA concentration was almost constant from 11.00 to 17.00 h on the day following implantation of the dialysis probe, the mean value being 11.1 pg/30 min. No significant change in the HA concentration was observed in rats deprived of food for 24 h. In 24-h fasted rats, feeding for 15 min produced a transient and significant increase in the HA concentration. These results suggest that histaminergic activity in the rat hypothalamus increases during feeding.

In vivo release of neuronal histamine in the hypothalamus of rats measured by microdialysis

Using an in vivo intracerebral microdialysis method coupled with an HPLC-fluorometric method, we investigated the extracellular level of endogenous histamine in the anterior hypothalamic area of urethane-anaesthetized rats. The basal rate of release of endogenous histamine in the anterior hypothalamic area measured by this method was 0.09 +/- 0.01 pmol/20 min. When the anterior hypothalamic area was depolarized by infusion of 100 mM K+ through the dialysis membrane or electrical stimulation at 200 mu A was applied through an electrode implanted into the ipsilateral tuberomammillary nucleus, histamine release increased to 175% and 188%, respectively, of the basal level. These increases were completely suppressed by removal of extracellular Ca2+. The basal release of histamine was also suppressed after infusion of 10(-6) M tetrodotoxin or i.p. administration of 100 mg/kg of alpha-fluoromethylhistidine. On the other hand, 3-fold increase in the basal release was observed after i.p. administration of 5 mg/kg thioperamide. These results clearly indicate that both the basal and evoked release of histamine measured by our method are of neuronal origin.

Effects of thioperamide, a histamine H3 receptor antagonist, on locomotor activity and brain histamine content in mast cell-deficient W/Wv mice

The purpose of this study was to examine the effects of thioperamide, a histamine H3 antagonist, on the locomotor activity and the brain histamine content in mast-cell-deficient W/Wv mice. Thioperamide (12.5 and 25 mg/kg) showed significant increase in the locomotor activity of W/Wv mice, measured by a photo-beam system, 1 hr after the intraperitoneal injection. However, more than 75 mg/kg of thioperamide showed not only the reduction of the locomotor activity but also the inhibition of motor coordination measured by the rotarod performance. The increase in the locomotor activity by thioperamide was blocked by i. p. pretreatment with (R)-alpha-methyl-histamine, an H3 agonist, or pyrilamine, an H1 antagonist, or zolantidine, an H2 antagonist. The brain histamine content was decreased by thioperamide (12.5-75.0 mg/kg), 1 hr after administration. Thus, the blockade of histamine H3 receptor by thioperamide showed the activation of locomotor activity of mice, which may be mediated by H1 and/or H2 receptors. The present data support the hypothesis that central histaminergic neurons may be involved in the control of state of wakefulness.