Inhibition of noradrenaline release in the rat brain cortex via presynaptic H3 receptors

Muscarinic, adenosine A(2) and histamine H(3) receptor modulation of haloperidol-induced c-fos expression in the striatum and nucleus accumbens

It is generally believed that haloperidol exerts its motor side effects and therapeutic effects mainly by antagonizing dopamine D(2) receptors in the striatum and the nucleus accumbens, respectively. Several neurotransmitters/modulators, including glutamate, acetylcholine, adenosine and histamine, affect dopaminergic activity in these centers. We have recently shown that N-methyl-D-aspartate receptor-mediated modulation of haloperidol-induced c-fos expression differs in functionally specific regions of the striatum and the nucleus accumbens. In the present study, the entire striatum and the nucleus accumbens were comprehensively examined for the pattern of modulation of haloperidol-induced c-fos expression by adenosine A(2), histamine H(3) and muscarinic receptor antagonists. Blockade of muscarinic and H(3) receptors resulted in a profound suppression of haloperidol-induced c-fos expression in the dorsolateral part of the striatum. In addition, the H(3) receptor antagonist suppressed the effects of haloperidol in the ventrolateral aspect of the striatum and the rostral parts of the medial striatum. Muscarinic receptor antagonists suppressed haloperidol-induced c-fos expression throughout the shell and in the mid-level of the core of the nucleus accumbens while A(2) and H(3) receptor antagonists did not.We found that the muscarinic and H(3) receptor antagonists suppress the induction of c-fos by haloperidol in the dorsolateral aspect of the striatum, an area implicated in the development of extrapyramidal motor symptoms following chronic haloperidol treatment. By contrast, haloperidol-induced c-fos expression in the nucleus accumbens, an area implicated in the therapeutic effects of haloperidol, was suppressed by the muscarinic receptor antagonist, but not by the H(3) receptor antagonist. Therefore we conclude that H(3) receptor modulation may provide a useful therapeutic target in future efforts to minimize neuroleptic-induced motor side effects.

Molecular cloning and pharmacology of functionally distinct isoforms of the human histamine H(3) receptor

The pharmacology of histamine H(3) receptors suggests the presence of distinct receptor isoforms or subtypes. We herein describe multiple, functionally distinct, alternatively spliced isoforms of the human H(3) receptor. Combinatorial splicing at three different sites creates at least six distinct receptor isoforms, of which isoforms 1, 2, and 4, encode functional proteins. Detailed pharmacology on isoforms 1 (unspliced receptor), and 2 (which has an 80 amino acid deletion within the third intracellular loop of the protein) revealed that both isoforms displayed robust responses to a series of known H(3) agonists, while all agonists tested displayed increased potency at isoform 2 relative to isoform 1. Histamine, N(alpha)-methylhistamine, and R(-) and S(+)-alpha-methylhistamine are 16-23-fold more potent, while immepip and imetit are three to fivefold more potent. Antagonist experiments revealed a rank order of potency at both isoforms of clobenpropit>iodophenpropit>thioperamide, and these drugs are fivefold less potent at isoform 2 than isoform 1. To further explore the pharmacology of H(3) receptor function, we screened 150 clinically relevant neuropsychiatric drugs for H(3) receptor activity, and identified a small number of antipsychotics that possess significant antagonist activity.

Improvement of spatial memory by (R)-alpha-methylhistamine, a histamine H(3)-receptor agonist, on the Morris water-maze in rat

This work aims to clarify the role of histamine in learning and memory. This is done by studying the effect of administration of the histamine precursor, L-histidine (His), and the agonist of the H(3) receptor (R)-alpha-methylhistamine (RAMH), on acquisition and retention of spatial reference memory in rats. Treatment with RAMH (10 mg/kg i.p.) facilitates recovery of spatial memory. In contrast, administration of His (500 mg/kg i.p.) does not affect the performance of this task. Moreover, pharmacological modulation of the cerebral histaminergic system does not impair the animal's behavioral flexibility, i.e. their ability to adapt to a new learning task in the same stimular context. Improved retention of spatial memory after a reduction in cerebral histamine confirms the modulating role of this neurotransmitter in memory processes.

Comparative anatomy of the histaminergic and other aminergic systems in zebrafish (Danio rerio)

The histaminergic system and its relationships to the other aminergic transmitter systems in the brain of the zebrafish were studied by using confocal microscopy and immunohistochemistry on brain whole-mounts and sections. All monoaminergic systems displayed extensive, widespread fiber systems that innervated all major brain areas, often in a complementary manner. The ventrocaudal hypothalamus contained all monoamine neurons except noradrenaline cells. Histamine (HA), tyrosine hydroxylase (TH), and serotonin (5-HT) -containing neurons were all found around the posterior recess (PR) of the caudal hypothalamus. TH- and 5-HT-containing neurons were found in the periventricular cell layer of PR, whereas the HA-containing neurons were in the surrounding cell layer as a distinct boundary. Histaminergic neurons, which send widespread ascending and descending fibers, were all confined to the ventrocaudal hypothalamus. Histaminergic neurons were medium in size (approximately 12 microm) with varicose ascending and descending ipsilateral and contralateral fiber projections. Histamine was stored in vesicles in two types of neurons and fibers. A close relationship between HA fibers and serotonergic raphe neurons and noradrenergic locus coeruleus neurons was evident. Putative synaptic contacts were occasionally detected between HA and TH or 5-HT neurons. These results indicate that reciprocal contacts between monoaminergic systems are abundant and complex. The results also provide evidence of homologies to mammalian systems and allow identification of several previously uncharacterized systems in zebrafish mutants.

Histamine H1 receptor-mediated inhibition of potassium-evoked release of 5-hydroxytryptamine from mouse forebrains

The release of endogenous serotonin and dopamine from slices of mouse forebrains induced by high extracellular K(+) was examined in histamine H1 receptor knockout mice. The release of 5-hydroxytryptamine (5-HT) evoked by 30 mM K(+) significantly decreased in the presence of 10-50 microM histamine in wild-type mice, but was not inhibited in the mutant mice. Histamine H1 receptor-mediated inhibition of serotonin release in wild-type mice was also observed in the presence of thioperamide, an H3 antagonist. From these data, we postulate that endogenous histamine indirectly inhibits the release of 5-HT through H1 receptors in addition to H3 receptors. The treatment of 2 microM tetrodotoxin could partly abolish the effects of histamine on K(+)-evoked 5-HT release. Bicuculline, a GABA(A) antagonist, could reverse the histamine-induced inhibition of 5-HT release in wild-type mice, suggesting that H1 receptors facilitate the release of GABA, which in turn inhibits 5-HT release through GABA(A) receptors. The difference in the effects of d- and l-chlorpheniramine on K(+)-evoked 5-HT release in wild-type mice further supports the evidence of the function of H1 receptor modulating 5-HT release.

Dopamine D2 receptors mediate amylin's acute satiety effect

The anorectic effect of the pancreatic peptide amylin has been established in numerous studies. Here, we investigated the influence of a pretreatment with dopamine (DA) D1- and D2-receptor antagonists on the anorectic effect of intraperitoneally injected amylin in rats fed a medium-fat (18% fat) diet. In 24-h food-deprived rats, pretreatment with the DA D2-receptor antagonist raclopride [100 μg/kg (0.2 μmol/kg) ip] significantly attenuated amylin's (5 μg/kg ip) anorectic effect, whereas raclopride alone had no effect on food intake [i.e., food intakes 1 h after injection were ( n = 12): NaCl/NaCl 7.3 ± 0.5 g; NaCl/amylin 3.9 ± 0.6; raclopride/NaCl 7.7 ± 0.7; raclopride/amylin 5.6 ± 0.7]. Pretreatment with another DA D2 receptor antagonist, sulpiride [50 mg/kg (154 μmol/kg) ip], similarly reduced amylin's satiety effect, whereas pretreatment with the DA D1-receptor antagonist SCH-23390 [10 μg/kg (0.03 μmol/kg) ip] did not influence amylin's effect. SCH-23390, however, completely blocked the anorexia induced by d-amphetamine (0.3 mg/kg ip). These results suggest that, under the present feeding conditions, the dopaminergic system mediates part of amylin's inhibitory effect on feeding in rats when administered intraperitoneally. This seems to involve DA D2 receptors but not D1 receptors.

Pharmacology of antidepressants

Presently in the United States, 21 compounds have been approved by the Food and Drug Administration as antidepressants. Two additional drugs marketed outside the United States as antidepressants have been approved for obsessive-compulsive disorder. Nearly one half of all these compounds became available within the past 12 years, whereas the first antidepressant was available more than 40 years ago. After the clinical aspects of depression are introduced in this article, the pharmacology of the newer generation drugs is reviewed in relationship to the older compounds. The information in this review will help clinicians treat acute depression with pharmacological agents.

Histamine and selective H3-receptor ligands: a possible role in the mechanism and management of epilepsy

The interaction of selective histamine H3-receptor agonist R(alpha)-methyl-histamine (RAMH) and antagonist thioperamide (THP) with some antiepileptic drugs [AED; phenytoin (PHT), carbamazepine (CBZ), sodium valproate (SVP), and gabapentin (GBP)] was studied on seizures induced by maximal electroshock (MES) and pentylenetetrazole (PTZ) in mice. It was found that subeffective dose of THP in combination with the subeffective doses of PHT and GBP provided protection against MES and/or PTZ-induced seizures. Further, RAMH reversed the protection afforded by either PHT or GBP on MES and/or PTZ seizures. In another set of experiments, the histamine content was measured in the whole brain and in different brain regions including cerebral cortex, hypothalamus, brain stem and cerebellum following convulsant (MES and PTZ) and AED treatment. It was seen that while MES exhibited a tendency to enhance brain histamine levels, PTZ showed the opposite effect. AEDs either increased (PHT and GBP) or decreased (SVP) brain histamine content in different regions to varying degrees. The results indicate a role for histamine in seizures and in the action of AEDs and suggest that selective H3-receptor antagonists may prove to be of value as adjuncts to conventional AEDs.

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.

New agents for the medical treatment of interstitial cystitis

Interstitial cystitis (IC) is a painful, sterile, disorder of the urinary bladder characterised by urgency, frequency, nocturia and pain. IC occurs primarily in women but also in men with recent findings indicating that chronic, abacterial prostatitis may be a variant of this condition. The prevalence of IC has ranged from about 8 - 60 cases/100,000 female patients depending on the population evaluated. About 10% of patients have severe symptoms that are associated with Hunner's ulcers on bladder biopsy; the rest could be grouped in those with or without bladder inflammation. Symptoms of IC are exacerbated by stress, certain foods and ovulatory hormones. Many patients also experience allergies, irritable bowel syndrome (IBS) and migraines. There have been various reports indicating dysfunction of the bladder glycosaminoglycan (GAG) protective layer and many publications showing a high number of activated bladder mast cells. Increasing evidence suggests that neurogenic inflammation and/or neuropathic pain is a major component of IC pathophysiology. Approved treatments so far include intravesical administration of dimethylsulphoxide (DMSO) or oral pentosanpolysulphate (PPS). New treatments focus on the combined use of drugs that modulate bladder sensory nerve stimulation (neurolytic agents), inhibit neurogenic activation of mast cells, or provide urothelial cytoprotection, together with new drugs with anti-inflammatory activity.

Histamine H3 receptor-mediated suppression of inhibitory synaptic transmission in the submucous plexus of guinea-pig small intestine

Conventional intracellular microelectrodes and marker injection techniques were used to study the actions of histamine on inhibitory synaptic transmission in the submucous plexus of guinea-pig small intestine. Bath application of histamine (1-300 microM) reversibly suppressed both noradrenergic and non-adrenergic slow inhibitory postsynaptic potentials in a concentration-dependent manner. These effects of histamine were mimicked by the selective histamine H(3) receptor agonist R(-)-alpha-methylhistamine but not the selective histamine H(1) receptor agonist, 6-[2-(4-imidazolyl)ethylamino]-N-(4-trifluoromethylphenyl) heptanecarboxamide (HTMT dimaleate), or the selective histamine H(2) receptor agonist, dimaprit. The histamine H(3) receptor antagonist, thioperamide, blocked the effects of histamine. Histamine H(1) and H(2) receptor antagonists did not change the action of histamine. Hyperpolarizing responses to focal application of norepinephrine or somatostatin by pressure ejection from micropipettes were unaffected by histamine and R(-)-alpha-methylhistamine. The results suggest that histamine acts at presynaptic histamine H(3) receptors on the terminals of sympathetic postganglionic fibers and intrinsic somatostatinergic nerves in the small intestine to suppress the release of the inhibitory neurotransmitters, norepinephrine and somatostatin.

Central histaminergic system and cognition

The neurotransmitter histamine is contained within neurons clustered in the tuberomammillary nuclei of the hypothalamus. These cells give rise to widespread projections extending through the basal forebrain to the cerebral cortex, as well as to the thalamus and pontomesencephalic tegmentum. These morphological features suggest that the histaminergic system acts as a regulatory center for whole-brain activity. Indeed, this amine is involved in the regulation of numerous physiological functions and behaviors, including learning and memory, as indicated by extensive research reviewed in this paper. Histamine effects on cognition might be explained by the modulation of the cholinergic system. However, interactions of histamine with any transmitter system, and/or a putative intrinsic procognitive role cannot be excluded. Furthermore, although experimental evidence indicates that attention-deficit hyperactivity disorder symptoms arise from impaired dopaminergic and noradrenergic transmission, recent research suggests that histamine is also involved. The possible relevance of histamine in disorders such as age-related memory deficits, Alzheimer's disease and attention-deficit hyperactivity disorder is worth of consideration, and awaits validation with clinical trials that will prove the beneficial effects of histaminergic drugs in the treatment of these diseases.

Changes in histamine H3 receptor responsiveness in mouse brain

Changes in various histamine (HA) H3 receptor-mediated responses and H3 receptor binding in brain were investigated in mice receiving single or repeated administration of ciproxifan, a potent brain-penetrating and selective H3 receptor antagonist. Blockade of the H3 autoreceptor was nearly as effective in enhancing levels of tele-methylhistamine (t-MeHA), a major HA metabolite, in brain areas when ciproxifan was administered once either at 7 a.m. or 8 p.m., in spite of the large differences of basal levels at these two phases of the circadian cycle. Blockade after a single ciproxifan administration was, however, followed by a transient decrease in striatal t-MeHA levels, possibly reflecting rapid development of autoreceptor hypersensitivity. Following a 5-day administration of ciproxifan and a 2-day drug-free period, basal t-MeHA levels were significantly decreased (approximately -20%) in three brain areas, and the ED50 values of the drug to enhance t-MeHA levels were increased by 5-15 times without significant change in maximal response, indicating that H3 autoreceptor hypersensitivity had developed. However, in synaptosomes from the cerebral cortex of these animals, the H3 receptor-mediated inhibition of K+-induced [3H]HA release was not significantly modified. Subchronic administration of ciproxifan for 10 days also resulted in an increased binding of [125I]iodoproxyfan to the H3 receptor of striatal and hypothalamic membranes by 40-54%. Hypersensitivity at H3 somatodendritic autoreceptors and at heteroreceptors attributable to an increased number of HA binding sites could account for the various changes observed in this study.

Non-imidazole histamine H3 ligands. Part I. Synthesis of 2-(1-piperazinyl)- and 2-(hexahydro-1H-1,4-diazepin-1-yl)benzothiazole derivatives as H3-antagonists with H1 blocking activities

New 2-(1-Piperazinyl)- and 2-(hexahydro-1H-1,4-diazepin-1-yl)benzothiazoles were prepared and tested as H1- and H3-receptor antagonists. A number of compounds showed weak H1-antagonistic activity, with pA2 values ranging from 5.5 to 6.1. The simple alkyl substituted, 2-[1-(4-methyl and 4-ethyl)piperazinyl] analogues show increasing, moderate H3-antagonistic activity (pA2 = 6.0, and pA2 = 7.0). The compounds with 4-phenylalkyl substitution, for both the piperazinyl and the hexahydro-1H-1,4-diazepin-1-yl homologues series, regardless of the different physicochemical properties of the para substituents at the phenyl ring, showed weak H3-antagonistic activity with pA2 values ranging from 4.4 to 5.6.

Evaluation of [18F]VUF 5000 as a potential PET ligand for brain imaging of the histamine H3 receptor

[18F]VUF 5000 was evaluated as a potential PET ligand for the histamine H3 receptor. In the rat a high uptake of [18F]VUF 5000 was observed in liver, lung and kidney and a low uptake in the brain. In order to explain these findings we determined the LogD(oct,7.2) of [18F]VUF 5000, studied the biodistribution in the presence of carrier VUF 5000, modified [18F]VUF 5000 chemically and studied the binding of [18F]VUF 5000 to human serum albumin. From the results of these experiments it was concluded that [18F]VUF 5000 is not suitable as a PET ligand for brain imaging of the histamine H3 receptor, since [18F]VUF 5000 hardly penetrates into the brain.

Radiosynthesis and biodistribution of 123I-labeled antagonists of the histamine H3 receptor as potential SPECT ligands

We have synthesized three 123I-labeled histamine H3 receptor ligands, i.e., [123I]GR 190028, [123I]FUB 271, and [123I]iodoproxyfan, in moderate to good radiochemical yields via a Cu+-assisted I-for-123I exchange method. Biodistribution in the rat of these compounds revealed high hepatic and pulmonary uptake. Brain uptake was moderate, but for [123I]iodoproxyfan, brain uptake was high enough for a pilot single photon emission computed tomography (SPECT) study in the rabbit. However, for this compound, the cerebral uptake could not be blocked by a pretreatment with [R]-alpha-methylhistamine, a selective, high-affinity histamine H3 receptor agonist, both in the SPECT study in the rabbit and in the biodistribution study in the rat. Apparently, [123I]iodoproxyfan is binding to a non-H3 receptor binding site. None of the three investigated compounds is suitable for use as a SPECT ligand for the H3 receptor in the brain.

Major changes in the brain histamine system of the ground squirrel Citellus lateralis during hibernation

Hibernation in mammals such as the rodent hibernator Citellus lateralis is a physiological state in which CNS activity is endogenously maintained at a very low, but functionally responsive, level. The neurotransmitter histamine is involved in the regulation of diurnal rhythms and body temperature in nonhibernators and, therefore, could likely play an important role in maintaining the hibernating state. In this study, we show that histamine neuronal systems undergo major changes during hibernation that are consistent with such a role. Immunohistochemical mapping of histaminergic fibers in the brains of hibernating and nonhibernating golden-mantled ground squirrels (C. lateralis) showed a clear increase in fiber density during the hibernating state. The tissue levels of histamine and its first metabolite tele-methylhistamine were also elevated throughout the brain of hibernating animals, suggesting an increase in histamine turnover during hibernation, which occurs without an increase in histidine decarboxylase mRNA expression. This hibernation-related apparent augmentation of histaminergic neurotransmission was particularly evident in the hypothalamus and hippocampus, areas of importance to the control of the hibernating state, in which tele-methylhistamine levels were increased more than threefold. These changes in the histamine neuronal system differ from those reported for the metabolic pattern in other monoaminergic systems during hibernation, which generally indicate a decrease in turnover. Our results suggest that the influence of histamine neuronal systems may be important in controlling CNS activity during hibernation.

In vivo modulation of rat hypothalamic histamine release by the histamine H3 receptor ligands, immepip and clobenpropit. Effects of intrahypothalamic and peripheral application

We investigated the effect of the new potent and selective histamine H3 receptor agonist, immepip, and the histamine H3 receptor antagonist, clobenpropit, on in vivo neuronal histamine release from the anterior hypothalamic area of urethane-anesthetized rats, using microdialysis. Intrahypothalamic perfusion with immepip at concentrations of 1 and 10 nM reduced histamine release to 75% and 35% of its basal level, respectively. Peripheral injection of immepip (5 mg/kg) caused a sustained decrease in histamine release of 50%. Clobenpropit potently increased histamine release after intrahypothalamic perfusion. The maximal increase in histamine release was 2-fold, observed at a concentration of 10 nM clobenpropit. Peripheral injection of clobenpropit (5-15 mg/kg) increased histamine release to about 150% of the basal value. A more marked increase in histamine release was found after injection of the histamine H3 receptor antagonist, thioperamide (5 mg/kg). In conclusion, intrahypothalamic perfusion of the histamine H3 receptor agonist, immepip and the histamine H3 receptor antagonist, clobenpropit, potently and oppositely modulated in vivo histamine release from the anterior hypothalamic area. The decreased histamine release after peripheral injection of immepip indicates that this novel agonist readily crosses the blood-brain barrier, making it a potential candidate for in vivo histamine H3 receptor studies. The differential increase in histamine release after peripheral injection of clobenpropit and thioperamide is discussed.

Histamine modulates high-voltage-activated calcium channels in neurons dissociated from the rat tuberomammillary nucleus

The effects of histamine on high-voltage-activated Ca2+ channels in the histaminergic neurons acutely dissociated from the rat tuberomammillary nucleus were investigated in the nystatin-perforated patch recording mode under voltage-clamp conditions. Histamine suppressed the high-voltage-activated Ca2+ channel currents in neurons which were positive for histidine decarboxylase with immunocytochemistry. The half-maximum inhibitory concentration and maximum inhibition were 2.6 x 10(-7) M and 16.6+/-1.90%, respectively. An H3 receptor agonist, R(-)-alpha-methylhistamine, mimicked the response to histamine, and thioperamide, an H3 receptor antagonist, inhibited the response to histamine. On the other hand, neither 2-methylhistamine, an H1 receptor agonist, nor dimaprit, an H2 receptor agonist, had a significant effect on the Ca2+ channel currents. Pretreatment with pertussis toxin blocked the inhibitory effect of histamine on Ca2+ channels, suggesting the involvement of Gi/Go proteins in the action of histamine. Omega-conotoxin-GVIA, omega-agatoxin-IVA, nicardipine, and omega-conotoxin-MVIIC blocked the high-voltage-activated Ca2+ channel currents by 15.6, 4.3, 27.1, and 31.2% of the total current, respectively, suggesting the existence of N-, P-, L-, and Q-type Ca2+ channels. A current that was insensitive to these blockers was also found. This residual current, "R-type", was completely suppressed by the addition of 200 microM Cd2+. Histamine significantly inhibited both the N- and P-type current components among these five types of Ca2+ channel currents. We concluded that histamine suppresses the N- and P-type Ca2+ channels in histaminergic neurons through an H3 receptor which is linked to a pertussis toxin-sensitive G-protein.

Pharmacological characterisation of the histamine H3 receptor in the rat hippocampus

The purpose of this report was to pharmacologically characterise the histamine H3 in the rat hippocampus using radioligand binding studies with the H3 receptor antagonist [125I]iodophenpropit and the H3 receptor mediated inhibition of [3H]noradrenaline release. A dissociation constant of 0.33 nM and a maximal number of binding sites of 125 fmol/mg protein were found for [125I]iodophenpropit. Competition studies showed stereoselectivity for the (R) and (S) enantiomers of alpha-methylhistamine and 10 microM of GTPgammaS shifted the curve of (R)-alpha-methylhistamine rightwards. Up to 1 microM, (R)-alpha-methylhistamine displaced only 30% whereas the tested H3-antagonists displaced 50-60% of the total [125I]iodophenpropit bound. This indicates the presence of an additional non-H3 receptor binding site(s) for [125I]iodophenpropit in the rat hippocampus. This secondary site shows low affinity for H3 agonists, but high affinity for the tested H3 antagonists. Electrically evoked [3H]acetylcholine release was shown in slices of rat hippocampus. No H3 receptor modulation of [3H]acetylcholine release from hippocampal slices was detectable. However, H3 receptor activation inhibited 42% of the electrically-evoked [3H]noradrenaline release in rat hippocampal slices. The inhibition of [3H]noradrenaline release was effectively antagonized by the H3 antagonists thioperamide and burimamide. We describe the pharmacological identification of the histamine H3 receptor in the rat hippocampus and its similarities and differences from the cortical H3 receptor. These studies enable us to investigate changes in density and functionality of the hippocampal H3 receptor under (patho)physiological conditions.

Histamine H3 receptor desensitization in the guinea-pig ileum

Histamine H3 receptor ligands are usually tested in guinea-pig intestine preparations. A possible desensitization of agonist-induced twitch inhibition was studied in longitudinal muscle-myenteric plexus from ileal segments. A cumulative concentration-response curve for R-alpha-methylhistamine was made; when a second curve was made 30 min afterwards, a marked decrease of pD2 and a more modest decrease of Emax were observed without changes in tissue sensitivity to electrical stimulation or morphine inhibition. At 120 min, pD2 and Emax were not different from those for the first curve. Receptor desensitization seems homologous and reversible and could interfere with repetitive testing of histamine H3 receptor ligands.

Histamine H3 receptor activation selectively inhibits dopamine D1 receptor-dependent [3H]GABA release from depolarization-stimulated slices of rat substantia nigra pars reticulata

The release of [3H]GABA from slices of rat substantia nigra pars reticulata induced by increasing extracellular K+ from 6 to 15 mM in the presence of 10 microM sulpiride was inhibited by 73 +/- 3% by 1 microM SCH 23390, consistent with a large component of release dependent upon D1 receptor activation. The histamine H3 receptor-selective agonist immepip (1 microM) and the non-selective agonist histamine (100 microM) inhibited [3H]GABA release by 78 +/- 2 and 80 +/- 2%, respectively. The inhibition by both agonists was reversed by the H3 receptor antagonist thioperamide (1 microM). However, in the presence of 1 microM SCH 23390 depolarization-induced release of [3H]GABA was not significantly decreased by 1 microM immepip. In rats depleted of dopamine by pretreatment with reserpine, immepip no longer inhibited control release of [3H]GABA, but in the presence of 1 microM SKF 38393, which produced a 7 +/- 1-fold stimulation of release, immepip reduced the release to a level not statistically different from that in the presence of immepip alone. Immepip (1 microM) also inhibited the depolarization-induced release of [3H]dopamine from substantia nigra pars reticulata slices, by 38 +/- 3%. The evidence is consistent with the proposition that activation of histamine H3 receptors leads to the selective inhibition of the component of depolarization-induced [3H]GABA release in substantia nigra pars reticulata slices which is dependent upon D1 receptor activation. This appears to be largely an action at the terminals of the striatonigral GABA projection neurons, which may be enhanced by a partial inhibition of dendritic [3H]dopamine release.

Pharmaco-ontogeny of bombesin's suppression of food intake and its attenuation by histamine H3 receptor agonists

Bombesin (BN) administration has been shown to suppress food intake across diverse species. Preliminary results have shown that BN elicits a satiety-like state from postnatal day (PD) 1, through unknown mechanism(s). We have recently shown that in adult rats, alpha-methyl histamine (alpha-MH), a selective H3 receptor agonist that inhibits the release and synthesis of histamine, blocks the feeding suppressant effects of BN. The objective of this study was to determine if such a mechanism was operation at birth or whether it developed over time. Thus effects of histamine H3 receptor agonists as well as BN-histamine interactions in the regulation of food intake were assessed during early development. On PD 1, 5, 10 and 15, groups of food deprived Sprague-Dawley rat pups (n = 8-12) were injected with BN alone (0 (saline), 0.006, 0.06 or 0.6 mg/kg, s.c.), H3 receptor agonists alone (alpha-MH or Imetit (3 or 5 mg/kg s.c.)) or the combination of BN and H3 receptor agonists, and their ingestive behavior was monitored. Results confirmed that pups were sensitive to feeding suppressant effects of BN starting from PD 1. Imetit or alpha-MH either failed to affect food intake or at certain time points enhanced food intake. Pretreatment with the H3 receptor agonists significantly attenuated the feeding suppressant effects of BN, suggesting that early in ontogeny, BN may suppress food ingestion possibly by facilitating histamine release at some relevant site(s).

Mutual interactions of the presynaptic histamine H3 and prostaglandin EP3 receptors on the noradrenergic terminals in the mouse brain

We studied whether interactions between the presynaptic histamine H3 and prostaglandin EP3 receptors on the noradrenergic neurons of the mouse brain cortex occur. Cerebral cortex slices from the mouse (and, in few experiments, from the rat) were preincubated with [3H]noradrenaline and then superfused with a physiological salt solution. Tritium overflow was evoked electrically, either at 0.3 or 3 Hz (2 min) (standard stimulation protocol) or at 100 Hz (eight pulses) (stimulation protocol under which almost no activation of the presynaptic alpha2-adrenoceptors by endogenous noradrenaline occurs). In another set of experiments, Ca2+ ions were introduced into Ca2+-free K+-rich medium containing tetrodotoxin to evoke tritium overflow. The electrically-evoked tritium overflow (0.3 Hz) was inhibited by histamine or the H3 receptor agonist imetit, acting via H3 receptors. and by prostaglandin E2 or the EP3 receptor agonist sulprostone, acting via EP3 receptors. When histamine or imetit was given first (at concentrations causing the maximum effect at H3 receptors), the effect of prostaglandin E2 on the evoked tritium overflow was attenuated by 5-10%. When prostaglandin E2 or sulprostone was given first (at concentrations causing the maximum effect at EP3 receptors), the effect of histamine or imetit on the evoked overflow was attenuated by almost 50%. The previous administration of prostaglandin E2 also blunted the effect of histamine on the evoked tritium overflow evoked at 3 Hz; the degree of attenuation was identical when the current strength was 25 mA or was increased to 100 or 200 mA in order to partially compensate for the inhibitory effect of prostaglandin E2 on the evoked overflow. In addition, prostaglandin E2 attenuated the effect of histamine when tritium overflow was evoked (i) by 100 Hz, eight pulses or (ii) by Ca2+ ions or (iii) when rat (instead of mouse) brain cortex slices were used. An interaction of prostaglandin E2 or sulprostone with the H3 receptor recognition site could be excluded since both prostanoids did not affect the specific binding of the H3 agonist radioligand [3H]N(alpha)-methylhistamine to rat brain cortex membranes. In conclusion, mutual interactions occur between the presynaptic H3 and EP3 receptors involved in the inhibition of noradrenaline release in the mouse brain cortex. Pre-activation of the H3 receptor slightly attenuates the EP3 receptor-mediated effect whereas pre-activation of the EP3 receptor more markedly attenuates the H3 receptor-mediated effect. The interactions may occur between the receptors themselves or at a step behind the receptors (e.g., at the level of G proteins). The physiological significance of these interactions may be to limit the total extent of inhibition of noradrenaline release in a scenario under which both receptors are activated simultaneously.

Identification of histamine H3 receptors in the tail artery from normotensive and spontaneously hypertensive rats

We examined the possible existence of prejunctional histamine H3 receptors on sympathetic nerve fibers innervating rat tail artery. The stimulation-evoked tritium outflow from isolated vessels preincubated with [3H]-noradrenaline and perfused/superfused in the presence of the alpha2-adrenoceptor antagonist rauwolscine, 3 microM, was inhibited by histamine 10 microM (by 8%) and the H3 agonists R-(-)-alpha-methylhistamine, 10 microM (by 18%), and imetit, 0.1-10 microM (by < or =20%). The inhibitory effect of imetit, which did not occur in the absence of rauwolscine, was counteracted by thioperamide, 1 microM. In the presence of rauwolscine, 3 microM, the inhibitory effect of imetit also occurred when the current strength or the Ca2+ concentration in the medium was reduced to compensate for the increase in tritium overflow elicited by rauwolscine, indicating that the inhibitory action of imetit is not associated with the increase in noradrenaline release produced by rauwolscine. In spontaneously hypertensive rats (SHRs), imetit also inhibited the overflow of tritium. This inhibitory effect was comparable to that observed in Wistar-Kyoto (WKY) rats and indicates that the sympathetic nerves of the rat tail artery in SHRs, like those in normotensive rats, are endowed with prejunctional histamine H3 receptors.

Effects of clobenpropit (VUF-9153), a histamine H3-receptor antagonist, on learning and memory, and on cholinergic and monoaminergic systems in mice

The effects of clobenpropit (VUF-9153), a potent histamine H3-receptor antagonist, on a scopolamine-induced learning deficit in the step-through passive avoidance test was studied in mice. Clobenpropit (10 and 20 mg/kg) alone showed a tendency to ameliorate the scopolamine-induced learning deficit, and clobenpropit (10 mg/kg) in combination with zolantidine (20 mg/kg), a histamine H2-receptor antagonist, ameliorated the scopolamine-induced effect. This ameliorating effect was antagonized by (R)-alpha-methylhistamine (20 mg/kg), a histamine H3-receptor agonist and pyrilamine (20 mg/kg), a histamine H1-receptor antagonist, suggesting that clobenpropit in combination with zolantidine showed the ameliorating effect via histamine H3 receptors and/or histamine H1 receptors. We also studied the effects of clobenpropit on cholinergic and monoaminergic systems. Clobenpropit did not show any significant effect on these neuronal systems except the activation of noradrenergic system. The present results suggest that the effect of clobenpropit might be partially involved with the activation of noradrenergic system, and the histaminergic system may play certain important roles in learning and memory.

Effects of imidazolines on noradrenaline release in brain: an investigation into their relationship to imidazoline, alpha 2 and H3 receptors

The present study was carried out to clarify whether the imidazolines clonidine, moxonidine and cirazoline as well as the guanidine aganodine inhibit noradrenaline release in the rat and rabbit brain via imidazoline receptors, alpha 2-adrenoceptors and/or histamine H3 receptors. Slices or synaptosomes from the rat or the rabbit brain were incubated with 3H-noradrenaline and exposed to phenoxybenzamine, which irreversibly blocks presynaptic alpha 2-adrenoceptors and, at considerably lower potency, imidazoline receptors. Tritium overflow in the superfused preparations was evoked electrically (3 Hz; slices) or by K+ 15 mmol/l (synaptosomes). Noradrenaline and rauwolscine, which possess low affinity, if any, for imidazoline receptors, were used as reference drugs. The evoked overflow in rat brain cortex slices and synaptosomes and in rat medulla oblongata slices, not exposed to phenoxybenzamine, was inhibited by clonidine, moxonidine and noradrenaline. Phenoxybenzamine markedly attenuated the effect of each drug to about the same extent. In rabbit brain cortex slices, not exposed to phenoxybenzamine, the evoked overflow was inhibited by clonidine, moxonidine, aganodine and noradrenaline, facilitated by BDF 6143 (4-chloro-2-(2-imidazoline-2-yl-amino)-isoindoline), idazoxan and rauwolscine and not affected by cirazoline. In slices exposed to phenoxybenzamine, the inhibitory effects of the imidazolines, of aganodine and of noradrenaline were again attenuated by about the same high degree, the facilitatory effects of BDF 6143, idazoxan and rauwolscine were abolished and cirazoline produced a slight inhibition of the evoked overflow. The latter effect was not affected by high concentrations of rauwolscine and idazoxan (at which these drugs act antagonistic at imidazoline receptors in other models). The specific binding of 3H-N alpha-methylhistamine to H3 receptors in rat brain cortex membranes was displaced only by high concentrations of moxonidine (pKi = 6.16) and at even lower affinity by aganodine, BDF 6143, cirazoline, clonidine and idazoxan (pKi < 5). Histamine, which was used as a reference drug, proved to be very potent (pKi = 8.20). In conclusion, imidazolines affect noradrenaline release in the rat and rabbit brain cortex and medulla oblongata via alpha 2-adrenoceptors but not via imidazoline receptors resembling the presynaptic imidazoline receptors previously identified in peripheral tissues of the rabbit. In addition, the involvement of I1- or I2-imidazoline binding sites or of H3 receptors is very improbable in view of the low affinity of aganodine, moxonidine and/or clonidine for these recognition sites and/or incompatibility of the rank order of their affinities with the potencies of the drugs in inhibiting noradrenaline release.

Brain penetration of the histamine H3 receptor antagonists thioperamide and clobenpropit in rat and mouse, determined with ex vivo [125I]iodophenpropit binding

We investigated the brain penetration of the histamine H3 receptor antagonists thioperamide and clobenpropit using ex vivo [125I]iodophenpropit binding. Homogenates of the rat cortex, striatum and mouse whole brain were prepared 1 h after subcutaneous injection of the H3 antagonists and incubated with [125I]iodophenpropit, a radiolabeled H3 receptor antagonist, to determine the H3 receptor occupancy. Specific [125I]iodophenpropit binding to the rat cortex and striatum was inhibited by thioperamide with IC30 values of 1.0 and 1.5 mg/kg, respectively. Clobenpropit also inhibited [125I]iodophenpropit binding, but was less potent (IC30: 18 and 19 mg/kg in the rat cortex and striatum, respectively) than thioperamide. Similar results were obtained in experiments with mouse whole brain (3.5 and 13 mg/kg for thioperamide and clobenpropit), indicating that there is no important species differences in the brain penetration of these drugs between rats and mice. These findings suggest that after peripheral injection both in rat and mouse thioperamide penetrates the blood-brain barrier more efficiently compared to clobenpropit.

Inhibition of cortical acetylcholine release and cognitive performance by histamine H3 receptor activation in rats

1. The effects of histamine and agents at histamine receptors on spontaneous and 100 mM K(+)-evoked release of acetylcholine, measured by microdialysis from the cortex of freely moving, rats, and on cognitive tests are described. 2. Local administration of histamine (0.1-100 microM) failed to affect spontaneous but inhibited 100 mM K(+)-stimulated release of acetylcholine up to about 50%. The H3 receptor agonists (R)-alpha-methylhistamine (RAMH) (0.1-10 microM), imetit (0.01-10 microM) and immepip (0.01-10 microM) mimicked the effect of histamine. 3. Neither 2-thiazolylethylamine (TEA), an agonist showing some selectivity for H1 receptors, nor the H2 receptor agonist, dimaprit, modified 100 mM K(+)-evoked release of acetylcholine. 4. The inhibitory effect of 100 microM histamine was completely prevented by the highly selective histamine H3 receptor antagonist, clobenpropit but was resistant to antagonism by triprolidine and cimetidine, antagonists at histamine H1 and H2 but not H3 receptors. 5. The H3 receptor-induced inhibition of K(+)-evoked release of acetylcholine was fully sensitive to tetrodotoxin (TTX). 6. The effects of intraperitoneal (i.p.) injection of imetit (5 mg kg-1) and RAMH (5 mg kg-1) were tested on acetylcholine release and short term memory paradigms. Both drugs reduced 100 mM K(+)-evoked release of cortical acetylcholine, and impaired object recognition and a passive avoidance response. 7. These observations provide the first evidence of a regulatory role of histamine H3 receptors on cortical acetylcholine release in vivo. Moreover, they suggest a role for histamine in learning and memory and may have implications for the treatment of degenerative disorders associated with impaired cholinergic function.

Histamine H3 receptor-mediated depression of synaptic transmission in the dentate gyrus of the rat in vitro

1. The effects of histamine on excitatory synaptic transmission in the dentate gyrus region of rat hippocampal slices were examined using extracellular and whole-cell patch-clamp recording techniques. The GABAA receptor antagonist picrotoxin (50 microM) was present in the bath in all experiments. 2. Histamine (0.7-70 microM) reversibly depressed field excitatory postsynaptic potentials (fEPSPs) or excitatory postsynaptic currents (EPSCs) recorded intracellularly by up to 30%. The presynaptic fibre volley and EPSC reversal potential were unaffected by histamine, as were responses following pressure ejection of the glutamate receptor agonist S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (S-AMPA) into the slice. 3. Histamine (7 microM) depressed equally the AMPA and N-methyl-D-aspartate (NMDA) components of the dual-component EPSC, recorded at -40 mV. 4. In addition to depressing synaptic transmission, histamine also reduced the magnitude of paired-pulse depression (PPD; 40 ms interpulse interval) of the medial perforant path EPSC. 5. Histamine depressed medial perforant path EPSCs more strongly than lateral perforant path EPSCs. Paired-pulse facilitation (PPF; 40 ms interpulse interval) in the lateral perforant path was enhanced by histamine. 6. The effects of histamine on synaptic transmission and PPD were mimicked by the selective H3 receptor agonist R-alpha-methylhistamine (0.1-10 microM) but not by the selective H2 receptor agonist dimaprit (10 microM). Similarly, the H3 receptor antagonist thioperamide (10 microM) blocked the effect of histamine whereas the H1 antagonist mepyramine (1 microM) and the H2 receptor antagonist cimetidine (50 microM) were ineffective. 7. Histamine actions on synaptic transmission and PPD were not occluded by application of the metabotropic glutamate agonist L-2-amino-4-phosphonobutyrate (AP4). 8. The results indicate that histamine depresses synaptic transmission in the dentate gyrus by binding to histamine H3 receptors located on perforant path terminals. The mechanism by which histamine depresses transmission is independent of that used by class III metabotropic glutamate receptors.

Hemodynamic profile of activation of histamine H3 receptors by R-alpha-methylhistamine in the guinea pig

1. The effect of R-alpha-methylhistamine, a histamine H1 receptor agonist, was studied on cardiovascular hemodynamics in bilateral vagotomized, anesthetized guinea pigs. 2. R-alpha-methylhistamine (100 micrograms/kg, IV) a dose that selectively activates histamine H3 receptors, produced hypotension and bradycardia. Total peripheral resistance (TPR) and rate pressure product (RPP) were also decreased at this dose. 3. Pretreatment with the ganglionic blocker hexamethonium (20 mg/kg, IV) blocked the blood pressure (BP), heart rate (HR), TPR, and RPP effects of R-alpha-methylhistamine (100 micrograms/kg, IV). Hexamethonium did not block the hypotensive and TPR lowering actions of the muscarinic agonist methacholine (1 and 3 micrograms/kg, IV). 4. Pretreatment with the alpha 1-adrenoceptor antagonist prazosin (0.5 mg/kg IV), blocked R-alpha-methylhistamine's (100 micrograms/kg, IV) effects on BP, TPR, and RPP. Prazosin did not antagonize the bradycardia effect of R-alpha-methylhistamine. 5. Pretreatment with the beta-adrenoceptor antagonist atenolol (1 mg/kg, IV) did not alter the BP, TPR, or RPP actions of R-alpha-methylhistamine. The HR effects of R-alpha-methylhistamine were blocked by atenolol. 6. The hemodynamic effects of R-alpha-methylhistamine were compared to the hemodynamic profile of the calcium channel blocker, verapamil (0.5 mg/kg, IV). Verapamil had little effect on TRP and had a greater cardiac depressant effect as evidenced by a significant reduction in HR and cardiac output (CO). 7. In summary, these results show that activation of prejunctional H3 receptors with R-alpha-methylhistamine decreases basal, BP, HR, TPR, and RPP in anesthetized guinea pigs. The fall in BP is mediated by a decrease in TPR. Furthermore, the inhibitory effects of R-alpha-methylhistamine on sympathetic control of the vasculature appears to impart a greater physiologic effect on the H3-histamine mediated hypotension than its inhibitory effects on sympathetic agents to the heart.

[3H]-thioperamide as a radioligand for the histamine H3 receptor in rat cerebral cortex

1. The purpose of the present study was to characterize the binding of the histamine H3 receptor antagonist, [3H]-thioperamide, to rat cerebral cortical membranes. 2. The binding of [3H]-thioperamide to rat cerebral cortical membranes reached equilibrium after incubation with [3H]-thioperamide after 8-10 h at 4 degrees C. Equilibrium was maintained for up to 18 h of incubation. Addition of 1 microM (R)-alpha-methylhistamine rapidly dissociated [3H]-thioperamide from its binding sites. From these kinetic experiments a dissociation constant of 0.3 nM was obtained for [3H]-thioperamide. 3. Saturation experiments with [3H]-thioperamide using 1 microM (R)-alpha-methylhistamine to define nonspecific binding were best analysed according to a single site model. A dissociation constant (KD) of 0.80 +/- 0.06 nM (n = 3) and a maximal number of binding sites (Bmax) of 73 +/- 20 fmol mg-1 protein (n = 3) were obtained for the binding of [3H]-thioperamide to rat cerebral cortical membranes. 4. Saturation experiments with [3H]-thioperamide using 0.3 microM iodophenpropit to define nonspecific binding were best analysed according to a two site model. For the high affinity [3H]-thioperamide site a KD value of 1.1 +/- 0.3 nM (n = 3) and Bmax value of 162 +/- 108 fmol mg-1 protein (n = 3) were obtained whereas KD and Bmax values for the low affinity site were 96 +/- 19 nM and 4346 +/- 3092 fmol mg-1 protein (n = 3), respectively. 5. Using 5 nM [3H]-thioperamide, the binding was hardly displaced by H3 agonists within concentration-ranges expected to bind to the histamine H3 receptor. Under these conditions, [3H]-thioperamide binding was fully displaced by various H3-antagonists, yet most H3 antagonists showed Ki values different from those expected for the histamine H3 receptor. 6. Using 0.3 nM [3H]-thioperamide, 50-60% of the total binding was potently displaced by the H3 agonists histamine, (R)-alpha-methylhistamine, (S)-alpha-methylhistamine, imetit and immepip. Displacement of the binding of 0.3 nM [3H]-thioperamide binding exhibited clear stereoselectivity for the R and S isomers of alpha-methylhistamine. 7. Binding of 0.3 nM [3H]-thioperamide was completely displaced by several H3 antagonists (thioperamide, iodophenpropit, iodoproxyfan, and burimamide) and biphasic displacement curves were obtained; the Ki values for the high affinity site corresponded well with the expected values for the H3 receptor. Antagonists fully displaced the binding of 5 nM [3H]-thioperamide with affinities comparable to the low affinity site found with 0.3 nM [3H]-thioperamide. 8. Ondansetron and haloperidol did not displace binding of 5 nM [3H]-thioperamide at concentrations at which the former are known to bind to 5-HT3 or sigma receptors, respectively. On the other hand, nonselective cytochrome P450 inhibitors displaced the binding of 5 nM [3H]-thioperamide from both rat cerebral cortical membranes and rat liver microsomes. 9. It is concluded that the histamine H3 antagonist, [3H]-thioperamide, can be used as a radioligand to study the histamine H3 receptor in rat brain, provided that subnanomolar concentrations are used in displacement studies. Moreover, the specific binding should be defined with an H3 agonist, since most H3 antagonists share with [3H]-thioperamide a low affinity, high density, non-H3 receptor binding site(s) in rat brain. The latter is probably due to binding to cytochrome P450 isoenzymes.

The effect of dopamine D1 receptor stimulation on the up-regulation of histamine H3-receptors following destruction of the ascending dopaminergic neurones

1. The binding of [3H]-(R)alpha-methylhistamine and [3H]-N alpha-methylhistamine to histamine H3-receptors, [3H]-SCH23390 to dopamine D1-receptors, and [3H]-YM09151-2 to dopamine D2-receptors was investigated by quantitative receptor autoradiography in the rat brain following 6-hydroxydopamine injection into the substantia nigra. 2. The levels of [3H]-(R)alpha-methylhistamine binding sites in the denervated striatum and substantia nigra were significantly higher than those in the contralateral side from 1 week to 12 weeks after nigral lesions. The H3-receptor binding was maximal at 3 weeks after nigral lesions and maintained until 12 weeks. 3. The increased number of histamine H3-receptors was decreased to the level of the contralateral side by chronic treatment with a selective dopamine D1 agonist, SKF38393, but not modified by a selective dopamine D2 agonist, quinpirole. 4. Dopamine D1- and D2-receptors in the striatum were similarly up-regulated after unilateral nigral lesion. On the other hand, the number of dopamine D2-receptors in the substantia nigra was markedly decreased after administration of 6-hydroxydopamine. 5. The treatment with (S)alpha-fluoromethylhistidine increased the H3-receptor binding in both the ipsilateral and contralateral sides. As a result, the magnitude of the ratio of the H3-receptor binding between ipsilateral and contralateral sides was partially attenuated by treatment with (S)-alpha-fluoromethylhistidine. 6. These results strongly suggest that the expression of histamine H3-receptors in the striatum and substantia nigra is influenced through D1-receptors by tonic nigrostriatal dopaminergic inputs.

The histaminergic, but not the serotoninergic, system mediates amylin's anorectic effect

In the present study, we investigated the influence of blockade of the serotoninergic and histaminergic neurotransmitter system on the anorectic effect of IP-injected amylin in rats. In 12- or 24-h food-deprived rats, blockade of central and peripheral serotonin (5-HT) receptors with the 5-HT1 and 5-HT2 receptor antagonist metergoline (0.5 or 0.05 mg/kg, IP, respectively) did not seem to influence the anorectic effect of IP injected amylin (1 microgram/kg). Similarly, inhibition of 5-HT synthesis and release with the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (200 micrograms/kg, IP) did not diminish amylin's (5 micrograms/kg, IP) anorectic effect in 24-h food-deprived rats whereas that of CCK (3 micrograms/kg, IP) was blocked under comparable conditions. Pretreatment of rats with the histamine H3 receptor agonists R-alpha-methylhistamine (MH: 3 mg/kg, IP) and Imerit (3 mg/kg, IP), which block transmission in the histaminergic system by inhibiting release of endogenous histamine, attenuated amylin's (1 microgram/kg) anorectic effect in 24-h food-deprived rats. These results suggest that the histaminergic system in involved in transduction of IP amylin's inhibitory effect on feeding in rats. In contrast, the serotoninergic system does not seem to be involved in mediating amylin's anorectic effect.

Possible participation of histamine H3 receptors in the modulation of noradrenaline release from rat spinal cord slices

Rat spinal cord slices prelabelled with [3H]noradrenaline were superfused with a medium containing 1 mu M desipramine plus 0.3 mu M phentolamine. Histamine (0.01-10 mu M) and the selective histamine H3 receptor agonist R-(-)-alpha-methylhistamine (0.001-10 mu M) caused a concentration-dependent decrease in the release of radioactivity evoked by electrical field stimulation (0.8 Hz, 20 mA, 2 min). The inhibitory effect of histamine was not modified by either pyrilamine (1 mu M) or ranitidine (10 mu M), but it was antagonized by burimamide (1 mu M). The inhibitory action of histamine (1 mu M) was attenuated by pertussis toxin (3 mu g/ml) and was abolished by N-ethylmaleimide (30 mu M). Neither forskolin (10 mu M) nor rolipram (100 mu M), nor the combination of both drugs, modified the inhibitory effect of histamine. Histamine (1 mu M) did not modify the overflow of tritium induced by electrical stimulation in the absence of phentolamine. The present results suggest that in the rat spinal cord the release of noradrenaline elicited by electrical stimulation is negatively modulated by histamine, probably through the activation of histamine H3 receptors. This modulatory mechanism is likely to involve the participation of regulatory Go/Gi proteins.

Effects of thioperamide, a histamine H3-receptor antagonist, on a scopolamine-induced learning deficit using an elevated plus-maze test in mice

We examined the effects of thioperamide and (R)-alpha-methylhistamine, a histamine H3-receptor antagonist and an agonist, respectively, on a scopolamine-induced learning deficit using an elevated plus-maze test in mice. Thioperamide alone slightly improved the learning deficit induced by scopolamine, and pretreatment with zolantidine, a histamine H2-receptor antagonist, significantly enhanced the effect of thioperamide in this test. (R)-alpha-Methylhistamine, pyrilamine, ketotifen, terfenadine, and zolantidine alone at the doses tested had no effect. Moreover, the improvement by thioperamide plus zolantidine was antagonized by pretreatment with histamine H1-receptor antagonists such as pyrilamine or ketotifen, but not by terfenadine. Thus, thioperamide improved the scopolamine-induced learning deficit through central histamine H1 receptors in mice. The present results supported the hypothesis that histamine may play an important role in learning and memory.

Ontogenetic development of histamine receptor subtypes in rat brain demonstrated by quantitative autoradiography

The postnatal ontogenetic development of the histamine receptor subtypes was studied in rat brain by quantitative receptor autoradiography with highly sensitive imaging plates. H1 receptor binding sites labeled with [3H]pyrilamine were detected on postnatal day 2 (P2) and increased very slowly until P9, and then rapidly reaching the adult levels in the hypothalamus, hippocampus, and amygdala by P16. The densities of H1 receptor binding sites in the cortex, striatum, thalamus, and substantia nigra were relatively low during development. H3 receptor binding sites labeled with [3H](R) alpha-methylhistamine were not detectable until P9. On P9, their density was higher in the substantia nigra than in other regions. Subsequently, H3 receptor binding increased, reaching the adult levels in the substantia nigra on P16 and in the other regions on P23. The histamine concentration was initially very high, but decreased to the adult level by P16. On the contrary, the activity of L-histidine decarboxylase of whole brain tissue was low on P5, and increased markedly from P16 to P23, to the adult level on P30. Administration of (S) alpha-fluoromethylhistidine (FMH), a specific inhibitor of L-histidine decarboxylase (HDC), significantly decreased both the HDC activity and histamine concentration during postnatal development. FMH treatment did not change H1 receptor binding in any brain region, but significantly increased H3 receptors in the substantia nigra and striatum on P23. Unilateral injection of 6-hydroxydopamine into the striatum on P2 resulted in up-regulation of H3 receptor binding sites in the dorsomedial (11%) and dorsolateral (18%) regions of the striatum and substantia nigra (31%) on P23, but no change in the H3 receptor density in the nucleus accumbens or frontal cortex on P11 and P23. These results demonstrate that the developmental patterns of H1 and H3 receptors are heterogeneous and independent of each other. There are marked mismatches of presynaptic and postsynaptic markers of the histaminergic neuron system as in other aminergic systems.

Effects of iodoproxyfan, a potent and selective histamine H3 receptor antagonist, on alpha 2 and 5-HT3 receptors

We determined the affinity and/or potency of the novel H3 receptor antagonist iodoproxyfan at alpha 2 and 5-HT3 receptors. Iodoproxyfan and rauwolscine (a reference alpha 2 ligand) (i) monophasically displaced 3H-rauwolscine binding to rat brain cortex membranes (pKi 6.79 and 8.59); (ii) facilitated the electrically evoked tritium overflow from superfused mouse brain cortex slices preincubated with 3H-noradrenaline (pEC50 6.46 and 7.91) and (iii) produced rightward shifts of the concentration-response curve (CRC) of (unlabelled) noradrenaline for its inhibitory effect on the evoked overflow (pA2 6.65 and 7.88). In the guinea-pig ileum, iodoproxyfan 6.3 mumol/l failed to evoke a contraction by itself but depressed the maximum of the CRC of 5-hydroxytryptamine (pD'2 5.24). Tropisetron (a reference 5-HT3 antagonist) produced rightward shifts of the CRC of 5-hydroxytryptamine (pA2 7.84). In conclusion, the affinity/potency of iodoproxyfan at H3 receptors (range 8.3-9.7 [1]) exceeds that at alpha 2 receptors by at least 1.5 log units and that at 5-HT3 receptors by at least 3 log units.

Characterization of histamine H3 receptors regulating acetylcholine release in rat entorhinal cortex

1. The pharmacological properties and location of H3 receptors modulating acetylcholine release have been investigated in non-superfused slices and synaptosomes of rat entorhinal cortex preloaded with [3H]-choline. 2. (R)alpha-methylhistamine, an H3-receptor agonist, potently inhibited the K(+)-evoked tritium release from slices, an effect antagonized by thioperamide, an H3-receptor antagonist, with nanomolar potency. 3. The K(+)-evoked tritium release from synaptosomes remained unaltered in the presence of the potent and selective H3-receptor agonists, imetit and (R)alpha-methylhistamine, suggesting that H3 receptors modulating acetylcholine release are not presynaptically located on cholinergic nerve terminals. 4. Phenylbutanoylhistamine and phenylpropylhistamine, two H3-receptor antagonists of moderate potency, failed to antagonize the inhibitory effects of (R)alpha-methylhistamine observed in slices. Unexpectedly, both compounds when used alone, inhibited tritium release from slices and synaptosomes with micromolar potency and to the same extent (by approximately 50% when added at a final concentration of 200 microM). This inhibitory effect did not involve H1, H2 or H3 receptors and was not mediated by an unknown histamine receptor site, since histamine used at a high concentration neither reproduced nor antagonized the effect of phenylbutanoylhistamine. It remained unaltered in the presence of scopolamine and was neither mimicked nor antagonized by vasoactive intestinal peptide, previously shown to be colocalized with acetylcholine in some neurones. 5. It is concluded that acetylcholine release in rat entorhinal cortex is modulated by H3 receptors presumably not located on cholinergic axon terminals. It remains to be established whether these H3 receptors belong to a receptor subtype different from those previously described since the potency ofphenylbutanoylhistamine and phenylpropylhistamine as H3-receptor antagonists was probably greatly underestimated by additional properties of both drugs.

Regulation of substance P release mediated via prejunctional histamine H3 receptors

The involvement of the histamine H3 receptor in the regulation of substance P release in neurogenic inflammation was studied by using rat hindpaw skin. R-(-)-alpha-Methylhistamine, a specific histamine H3 receptor agonist, significantly inhibited the increased vascular permeability induced by antidromic electrical stimulation of the sciatic nerve in a dose-dependent manner at doses of 0.5-3 mg/kg (i.v.), and thioperamide (2 mg/kg i.p.), a specific histamine H3 receptor antagonist, prevented the inhibitory effect of R-(-)-alpha-methylhistamine. The antidromic stimulation also caused a significant increase in immunoreactive substance P release in the subcutaneous (s.c.) perfusate in the rat hindpaw. R-(-)-alpha-Methylhistamine (0.25-2 mg/kg) dose dependently inhibited the increase in release of immunoreactive substance P, and thioperamide (2 mg/mg i.p.) antagonized it. Perfusion of histamine (10(-3) M) elicited a significant increase of immunoreactive substance P release in the perfusate, which was reduced by R-(-)-alpha-methylhistamine and the antagonism of thioperamide was also observed. Histamine (in the presence of histamine H1 and H2 receptor antagonists) had an inhibitory effect on the electrically evoked release of immunoreactive substance P. These results strongly support the hypothesis that histamine regulates substance P release via prejunctional histamine H3 receptors that are located on peripheral endings of sensory nerves.

Differential effect of sodium ions and guanine nucleotides on the binding of thioperamide and clobenpropit to histamine H3-receptors in rat cerebral cortical membranes

1. Conflicting reports in the literature over heterogeneity (West et al., 1990) or homogeneity (Arrange et al., 1990) of histamine H3-receptor binding sites may be attributed to the use of different incubation conditions. In the present study we have investigated the extent to which the binding of H3-receptor ligands to rat cerebral cortical membranes can be modified by both sodium ions and guanine nucleotides. 2. The H3-selective antagonist, thioperamide, discriminated between two specific binding sites for [3H]-N alpha-methylhistamine (IC50 1 = 2.75 +/- 0.87 nM, IC50 2 101.6 +/- 12.0 nM, % site 1 = 24 +/- 2%) in 50 mM Tris HCl buffer, but showed homogeneity of binding in 50 mM Na/K phosphate buffer. 3. Sodium ions markedly altered the binding characteristics of thioperamide (i.e. heterogeneity was lost and IC50 value shifted towards the high affinity site). The competition curves for a second H3-antagonist, clobenpropit and the H3-agonist N alpha-methylhistamine however, were unaltered in the presence of sodium ions. 4. Guanylnucleotides displaced only 60% of specific [3H]-N alpha- methylhistamine binding and modulated thioperamide binding in the same way as sodium ions. 5. These data suggest that the H3-receptor can exist in different conformations for which thioperamide, but not N alpha-methylhistamine and clobenpropit, show differential affinity. 6. The potential nature of these sites, and the implications of this apparent receptor heterogeneity for H3-receptor antagonism by thioperamide, are discussed.

Characterization of the binding of the first selective radiolabelled histamine H3-receptor antagonist, [125I]-iodophenpropit, to rat brain

1. The binding of the first selective radiolabelled histamine H3-receptor antagonist [125I]-iodophenpropit to rat cerebral cortex membranes was characterized. 2. [125I]-iodophenpropit, radiolabelled to a high specific activity of 1900 Ci mmol-1, saturably bound to a single class of sites with a KD of 0.57 +/- 0.16 nM (n = 4) and Bmax of 268 +/- 119 fmol mg-1 protein. 3. Specific binding at a concentration below 1 nM represented 50 to 60% of total binding. 4. Binding of [125I]-iodophenpropit to rat cerebral cortex membranes was readily displaced by histamine H3-agonists and antagonists. In contrast, the inhibitory potencies of selective histamine H1- and H2-receptor ligands were very low. 5. [125I]-iodophenpropit was biphasically displaced by the histamine H3-receptor antagonists, burimamide and dimaprit, which may indicate the existence of histamine H3-receptor subtypes. Other histamine H3-receptor antagonists showed a monophasic displacement. 6. Competition binding curves of H3-agonists were biphasic and showed a rightward shift upon the addition of the nonhydrolysable GTP analogue, guanosine 5'-o-(3-thio) triphosphate (GTP gamma S; 100 microM) which implicates the interaction of histamine H3-receptors with G-proteins. The affinities of the H3-receptor antagonists iodophenpropit, thioperamide and burimamide were not altered by GTP gamma S. 7. Histamine competition binding curves were shifted to the right by different nucleotides (100 microM) with a rank order of potency GTP gamma S > Gpp(NH)p, GTP. 8 In vitro autoradiographic studies revealed a heterogeneous distribution of [125I]-iodophenpropitbinding sites in rat brain, with highest densities observed in specific cerebral cortical areas and layers,the caudate-putamen complex, the olfactory tubercles, the hippocampal formation, the amygdala complex, the hypothalamic area and the mammillary bodies.9 It is concluded that the histamine H3-receptor antagonist, [125I]-iodophenpropit, meets the criteria fo ra suitable radioligand for histamine H3-receptor binding studies in rat brain.