Inhibitory effects of histamine on the release of serotonin and noradrenaline from rat brain slices

Histamine H3 Receptor Isoforms: Insights from Alternative Splicing to Functional Complexity

Alternative splicing significantly enhances the diversity of the G protein-coupled receptor (GPCR) family, including the histamine H3 receptor (H3R). This post-transcriptional modification generates multiple H3R isoforms with potentially distinct pharmacological and physiological profiles. H3R is primarily involved in the presynaptic inhibition of neurotransmitter release in the central nervous system. Despite the approval of pitolisant for narcolepsy (Wakix®) and daytime sleepiness in adults with obstructive sleep apnea (Ozawade®) and ongoing clinical trials for other H3R antagonists/inverse agonists, the functional significance of the numerous H3R isoforms remains largely enigmatic. Recent publicly available RNA sequencing data have confirmed the expression of multiple H3R isoforms in the brain, with some isoforms exhibiting unique tissue-specific distribution patterns hinting at isoform-specific functions and interactions within neural circuits. In this review, we discuss the complexity of H3R isoforms with a focus on their potential roles in central nervous system (CNS) function. Comparative analysis across species highlights evolutionary conservation and divergence in H3R splicing, suggesting species-specific regulatory mechanisms. Understanding the functionality of H3R isoforms is crucial for the development of targeted therapeutics. This knowledge will inform the design of more precise pharmacological interventions, potentially enhancing therapeutic efficacy and reducing adverse effects in the treatment of neurological and psychiatric disorders.

Histaminergic regulation of food intake

Histamine is a biogenic amine that acts as a neuromodulator within the brain. In the hypothalamus, histaminergic signaling contributes to the regulation of numerous physiological and homeostatic processes, including the regulation of energy balance. Histaminergic neurons project extensively throughout the hypothalamus and two histamine receptors (H1R, H3R) are strongly expressed in key hypothalamic nuclei known to regulate energy homeostasis, including the paraventricular (PVH), ventromedial (VMH), dorsomedial (DMH), and arcuate (ARC) nuclei. The activation of different histamine receptors is associated with differential effects on neuronal activity, mediated by their different G protein-coupling. Consequently, activation of H1R has opposing effects on food intake to that of H3R: H1R activation suppresses food intake, while H3R activation mediates an orexigenic response. The central histaminergic system has been implicated in atypical antipsychotic-induced weight gain and has been proposed as a potential therapeutic target for the treatment of obesity. It has also been demonstrated to interact with other major regulators of energy homeostasis, including the central melanocortin system and the adipose-derived hormone leptin. However, the exact mechanisms by which the histaminergic system contributes to the modification of these satiety signals remain underexplored. The present review focuses on recent advances in our understanding of the central histaminergic system's role in regulating feeding and highlights unanswered questions remaining in our knowledge of the functionality of this system.

The Histamine H3 Receptor: Structure, Pharmacology, and Function

Among the four G protein-coupled receptors (H1-H4) identified as mediators of the biologic effects of histamine, the H3 receptor (H3R) is distinguished for its almost exclusive expression in the nervous system and the large variety of isoforms generated by alternative splicing of the corresponding mRNA. Additionally, it exhibits dual functionality as autoreceptor and heteroreceptor, and this enables H3Rs to modulate the histaminergic and other neurotransmitter systems. The cloning of the H3R cDNA in 1999 by Lovenberg et al. allowed for detailed studies of its molecular aspects. In this work, we review the characteristics of the H3R, namely, its structure, constitutive activity, isoforms, signal transduction pathways, regional differences in expression and localization, selective agonists, antagonists and inverse agonists, dimerization with other neurotransmitter receptors, and the main presynaptic and postsynaptic effects resulting from its activation. The H3R has attracted interest as a potential drug target for the treatment of several important neurologic and psychiatric disorders, such as Alzheimer and Parkinson diseases, Gilles de la Tourette syndrome, and addiction.

Histamine H3 receptor activation inhibits dopamine synthesis but not release or uptake in rat nucleus accumbens

We studied the effect of activating histamine H3 receptors (H3Rs) on rat nucleus accumbens (rNAcc) dopaminergic transmission by analyzing [(3)H]-dopamine uptake by synaptosomes, and dopamine synthesis and depolarization-evoked [(3)H]-dopamine release in slices. The uptake of [(3)H]-dopamine by rNAcc synaptosomes was not affected by the H3R agonist RAMH (10(-10)-10(-6) M). In rNAcc slices perfusion with RAMH (1 μM) had no significant effect on [(3)H]-dopamine release evoked by depolarization with 30 mM K(+) (91.4 ± 4.5% of controls). The blockade of dopamine D2 autoreceptors with sulpiride (1 μM) enhanced K(+)-evoked [(3)H]-dopamine release (168.8 ± 15.5% of controls), but under this condition RAMH (1 μM) also failed to affect [(3)H]-dopamine release. Dopamine synthesis was evaluated in rNAcc slices incubated with the l-dihydroxyphenylalanine (DOPA) decarboxylase inhibitor NSD-1015 (1 mM). Forskolin-induced DOPA accumulation (220.1 ± 10.4% of controls) was significantly reduced by RAMH (41.1 ± 6.5% and 43.5 ± 9.1% inhibition at 100 nM and 1 μM, respectively), and this effect was prevented by the H3R antagonist ciproxifan (10 μM). DOPA accumulation induced by preventing cAMP degradation with IBMX (iso-butyl-methylxantine, 1 mM) or by activating receptors for the vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide (PACAP) with PACAP-27 (1 μM) was reduced (IBMX) or prevented (PACAP-27) by RAMH (100 nM). In contrast, DOPA accumulation induced by 8-Bromo-cAMP (1 mM) was not affected by RAMH (100 nM). These results indicate that in rNAcc H3Rs do not modulate dopamine uptake or release, but regulate dopamine synthesis by inhibiting cAMP formation and thus PKA activation. This article is part of the Special Issue entitled 'Histamine Receptors'.

Presynaptic receptors for dopamine, histamine, and serotonin

Presynaptic receptors for dopamine, histamine and serotonin that are located on dopaminergic, histaminergic and sertonergic axon terminals, respectively, function as autoreceptors. Presynaptic receptors also occur as heteroreceptors on other axon terminals. Auto- and heteroreceptors mainly affect Ca(2+) -dependent exocytosis from the receptor-bearing nerve ending. Some additionally subserve other presynaptic functions.Presynaptic dopamine, histamine and serotonin receptors are involved in various (patho)physiological conditions. Examples are the following:Dopamine autoreceptors play a role in Parkinson's disease, schizophrenia and drug addiction. Dopamine heteroreceptors affecting the release of acetylcholine and of amino acid neurotransmitters in the basal ganglia are also relevant for Parkinson's disease. Peripheral dopamine heteroreceptors on postganglionic sympathetic terminals influence heart rate and vascular resistance through modulation of noradrenaline release. Blockade of histamine autoreceptors increases histamine synthesis and release and may support higher CNS functions such as arousal, cognition and learning. Peripheral histamine heteroreceptors on C fiber and on postganglionic sympathetic fiber terminals diminish neuropeptide and noradrenaline release, respectively. Both inhibititory effects are beneficial in myocardial ischemia. The inhibition of neuropeptide release also explains the antimigraine effects of some agonists of presynaptic histamine receptors. Upregulation of presynaptic serotonin autoreceptors is probably involved in the pathogenesis of major depression. Correspondingly, antidepressant treatments can be linked with a reduced density of 5-HT autoreceptors. 5-HT Heteroreceptor activation diminishes acetylcholine and GABA release and may therefore increase anxiety. In the periphery, presynaptic 5-HT heteroreceptor agonists shorten migraine attacks by inhibition of the release of neuropeptides from trigeminal afferents, apart from their constrictive action on meningeal vessels.

Histamine and the regulation of body weight

Energy intake and expenditure is regulated by a complex interplay between peripheral and central factors. An exhaustive list of peptides and neurotransmitters taking part in this complex regulation of body weight exists. Among these is histamine, which acts as a central neurotransmitter. In the present article we review current evidence pointing at an important role of histamine in the regulation of appetite and metabolism. Studies using both knockout mouse models as well as pharmacological studies have revealed that histamine acts as an anorexigenic agent via stimulation of histamine H(1) receptors. One effect of histamine in the regulation of appetite is to act as a mediator of the inhibitory effect of leptin on appetite. It seems that histamine may attenuate and delay the development of leptin resistance in high-fat-diet-induced obesity. Furthermore, histamine may also act to accelerate lipolysis. Based on the current evidence of the involvement of histamine in the regulation of body weight, the histaminergic system is an obvious target for the development of pharmacological agents to control obesity. At present, H(3) receptor antagonists that stimulate the histaminergic system may be the most promising histaminergic drugs for antiobesity therapy.

Cerebral ischemia and brain histamine

Cerebral ischemia induces excess release of glutamate and an increase in the intracellular Ca(2+) concentration in neurons, which provokes enzymatic process leading to irreversible neuronal injury. Histamine plays a role as a neurotransmitter in the mammalian brain, and histamine release from nerve endings is enhanced in ischemia by facilitation of histaminergic activity. Dissimilar to ischemia-induced release of glutamate, histamine release is gradual and long lasting. The enhancement may contribute to neuroprotection against ischemic damage, because suppression of histaminergic activity aggravates the histologic outcome caused by ischemia. Preischemic administration of histamine (i.c.v.) suppresses ischemic release of glutamate and ameliorates neuronal damage, whereas blockade of central histamine H(2) receptors aggravates ischemic injury. These suggest that histamine provides beneficial effects against ischemic damage through histamine H(2) receptors, when administered before induction of ischemia. Postischemic loading with histidine, a precursor of histamine, alleviates both brain infarction and delayed neuronal death. Since the alleviation is abolished by blockade of central histamine H(2) receptors, facilitation of central histamine H(2) action caused by histidine may prevent reperfusion injury after ischemic events. Because the ischemia-induced increase in the glutamate level rapidly resumes after reperfusion of cerebral blood flow, beneficial effects caused by postischemic loading with histidine may be due to other mechanisms besides suppression of excitatory neurotransmitter release. Anti-inflammatory action by histamine H(2) receptor stimulation is a likely mechanism responsible for the improvement.

Selective histamine H3 receptor antagonists for treatment of cognitive deficiencies and other disorders of the central nervous system

Evidence exists to implicate the monoamine histamine in the control of arousal and cognitive functions. Antagonists of H(3) receptors are postsynaptic and presynaptic modulators of neural transmission in a variety of neuronal circuits relevant to cognition. Accumulating neuroanatomical, neurochemical, pharmacological, and behavioral data support the idea that H(3) receptor antagonists may function to improve cognitive performances in disease states (e.g., Alzheimer's disease and mild cognitive impairment states). Thus, H(3) receptor antagonists have been shown to increase performance in attention and memory tests in nonhuman experiments and prevent the degradation in performances produced by scopolamine, MK-801, or age. In contrast, agonists of the H(3) receptor generally produce cognitive impairing effects in animal models. The role of H(3) receptors in these behavioral effects is substantiated by data indicating a central origin for their effects, the selectivity of some of the H(3) receptor antagonists studied, and the pharmacological modification of effects of H(3) receptor antagonists by selective H(3) receptor agonists. Data and issues that challenge the potential role for H(3) receptor antagonists in cognitive processes are also critically reviewed. H(3) receptor antagonists may also have therapeutic value in the management of obesity, pain, sleep disorders, schizophrenia, and attention deficit hyperactivity disorder.

The role of HB-donor groups in the heterocyclic polar fragment of H3-antagonists

It has been recently reported that compounds composed of an imidazole connected through an alkyl spacer to a 2-aminobenzimidazole showed high affinity towards the H(3)-receptor. The guanidine fragment of the 2-aminobenzimidazole is probably involved in hydrogen bond interactions at the binding site, and is referred to as the 'polar fragment'. In the present work, starting from 2-aminobenzimidazole derivatives with a di-methylene spacer 1 (pK(i)=7.25) or a tri-methylene one 2 (pK(i)=8.90), we investigated the importance of the hydrogen bond (HB) donor groups at the polar fragment in the interaction with the H(3)-receptor. The replacement of 2-aminobenzimidazoles with different moieties [2-aminobenzothiazole, 3, 4; 2-thiobenzimidazole, 5, 6; 2-thiobenzothiazole, 7, 8; 2-thio-4-phenyl- or 2-thio-5-phenyl-N-methylimidazoles, 9-12] highlighted the effect of the polar group basicity on the optimal length of the alkyl chain: longer spacers were preferred with polar groups of moderate basicity whereas, in the presence of neutral polar groups, the best affinity values were obtained with di-methylene chains. Moreover, N-methylation at the 2-aminobenzimidazole moiety 13-16 revealed different behaviour for compounds having different spacer lengths. In fact, methylation of the exocyclic NH group maintained high affinity for the tri-methylene 2-aminobenzimidazole derivative, while a drop in affinity was observed for the annular N-methylation. An opposite trend characterised di-methylene derivatives. These observed SAR suggest that, within this class of compounds, the number of HB-donor groups can be lowered while maintaining high receptor affinity. Since the presence of HB-donor groups strongly affects brain access, this observation could be useful to design and prepare new H(3)-antagonists.

Blockade of central histaminergic H2 receptors facilitates catecholaminergic metabolism and aggravates ischemic brain damage in the rat telencephalon

Blockade of central H(2) receptors aggravates ischemic neuronal damage. Since changes in the activity of the monoaminergic system are contributing factors in the development of ischemic neuronal damage, the authors evaluated the effects of ranitidine on the monoaminergic system and ischemic neuronal damage in the middle cerebral artery (MCA) occlusion model of rats. Wistar rats pretreated with saline or ranitidine (3 and 30 nmol, i.c.v.) were subjected to reversible occlusion of MCA for 2 h. The total infarct volume was determined 24 h after reperfusion. The relationship between dopaminergic activity and the histologic outcome was estimated by lesioning the substantia nigra 2 days before MCA occlusion. In a second experiment, the animals were subjected to 15 min of MCA occlusion, and the effects of ranitidine on the histologic outcome was evaluated 7 days after ischemia. In a third experiment, the tissue concentrations of monoamines and their metabolites were determined in the cerebral cortex and striatum 2 h after reperfusion following MCA occlusion for 2 h. The turnover of norepinephrine and dopamine was compared between animals treated with saline and those treated with ranitidine by estimating the alpha-methyl-p-tyrosine-induced depletion of norepinephrine and dopamine, respectively. The turnover of 5-hydroxytryptamine was evaluated by the probenecid-induced accumulation of 5-hydroxyindoleacetic acid. Treatments with ranitidine markedly increased the infarct volume 24 h after reperfusion. Ranitidine also aggravated delayed neuronal death 7 days after ischemia. The aggravation was abolished by the lesion of the substantia nigra before MCA occlusion. The MCA occlusion increased the turnover of cortical norepinephrine and striatal dopamine. The turnover was further facilitated by ranitidine. Although ranitidine suppressed the 5-hydroxytryptamine turnover in the cerebral cortex, the extent of this effect was similar in both the ischemic and non-ischemic sides. These results suggest that facilitation of the catecholaminergic systems is involved in the aggravation of ischemic neuronal damage by H(2) blockade.

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.

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.

A qualitative model for the histamine H3 receptor explaining agonistic and antagonistic activity simultaneously

A pharmacophore model for histamine H3 ligands is derived that reveals the putative interaction of both H3 agonists and antagonists with an aspartate residue of the receptor. This interaction is determined by applying the density functional theory implemented in a program package adapted for parallel computers. The model reveals a molecular determinant explaining efficacy as the conformation of the aspartic acid residue differs according to whether it is binding to agonists or antagonists. The differences in structure-activity relationships (SAR) observed for the lipophilic tails of different classes of H3 antagonists are now explained, since the model reveals two distinct lipophilic pockets available for antagonist binding.

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.

Synthesis and biological assays of new H3-antagonists with imidazole and imidazoline polar groups

New histamine H3-receptor antagonists were synthesised and tested on rat brain membranes and on electrically stimulated guinea-pig ileum. The new compounds have a central polar group represented by a 2-alkylimidazole or a 2-thioimidazoline nucleus. The effect of the polar group basicity on the optimal length of the alkyl chain, connecting this group to a 4(5)-imidazolyl ring, was investigated. The best affinity values, obtained by displacement of [3H]-RAMHA from rat brain, were obtained for the 2-alkylimidazole derivatives (2a-f) with tetramethylene chain (pKi 8.03-8.97), having an intermediate basicity between that of the previously reported 2-thioimidazoles (1a-i) and that of 2-alkylthioimidazolines (3a-h). In contrast, a general lowering of affinity (pKi 5.90-7.63) was observed for compounds of the last series (3a-h), with a complex dependence on the terminal lipophilic group and chain length.

Targeting disruption of histamine H1 receptors in mice: behavioral and neurochemical characterization

With gene targeting, one can practically knock out a gene in vivo and create a mutant organism that completely lacks the gene product. The mutant mice lacking histamine H1 receptors was generated by the method of gene targeting. In brains of homozygous mutant mice, no specific binding of [3H]pyrilamine was seen. The mutant mice showed impaired locomotor activity and exploratory behavior in an open field and activity wheel. Behaviors of the mutant mice were examined with several other tasks such as passive avoidance test, resident-intruder aggression test and formalin test to clarify the role for the H1 receptors in behaviors. Behavioral changes observed in the mutant mice are almost compatible with those obtained by the classical pharmacological tools. In correlation to the behavioral changes in the mutant mice, 5-hydroxytryptamine release was significantly increased in the brains of mutant mice.

Therapeutic potential of histamine H3 receptor agonists and antagonists

The histamine H3 receptor was discovered 15 years ago, and many potent and selective H3 receptor agonists and antagonists have since been developed. Currently, much attention is being focused on the therapeutic potential of H3 receptor ligands. In this review, Rob Leurs, Patrizio Blandina, Clark Tedford and Henk Timmerman describe the available H3 receptor agonists and antagonists and their effects in a variety of pharmacological models in vitro and in vivo. The possible therapeutic applications of the various compounds are discussed.

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.

Histamine modulation of glutamate release from hippocampal synaptosomes

Histamine is widely believed to act as a neuromodulator in the central nervous system. Histaminergic fibers arriving at the hippocampus could be involved in the modulation of glutamatergic neurotransmission. Therefore, we have investigated the effect of histamine on [3H]glutamate release from hippocampal synaptosomes by using a superfusion system. Calcium-dependent [3H]glutamate release was stimulated by KCI or 4-aminopyridine. When submaximal concentrations of the depolarizing agents were used (15 mM KCI or 50 microM 4-aminopyridine), histamine, acting via histamine H1 and H2 receptors, produced a concentration-dependent increase in the evoked release of glutamate. Maximal effect was obtained with 500 microM histamine. Histamine (up to 1 mM) did not modify basal release. These data suggest that histaminergic afferents may modulate activity-dependent glutamatergic presynaptic terminals in the hippocampus.

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.

Histamine and serotonin released from the rat perfused heart by compound 48/80 or by allergen challenge influence noradrenaline or acetylcholine exocytotic release

Terminal nerve fibres of the autonomic nervous system closely approach mast cells in peripheral organs, and mutual influences between release of neurotransmitters or mast cell mediators may cause neuro-immunological interactions. We have studied the influence of mast cell degranulation on the release of endogenous noradrenaline and newly incorporated acetylcholine (such as 14C-choline/acetylcholine overflow) evoked by stimulation of extrinsic postganglionic sympathetic or preganglionic vagal nerves in the rat Langendorff heart perfused with Tyrode solution. Compound 48/80 perfused in normal hearts, or ovalbumin infused into hearts from rats sensitized to ovalbumin, enhanced the overflow of endogenous histamine and serotonin. Both stimuli increased the release of mediators to a similar extent and with fast kinetics. Maximum average concentrations in the perfusate of histamine were about 800 nmol/l, and of serotonin 40 nmol/l, in a sample collected within 4 min after mast cell degranulation. Stimulation of autonomic nerves did not affect basal histamine or serotonin overflow. Whereas basal overflows were unaffected, the stimulation-evoked releases of both noradrenaline and acetylcholine, were facilitated when compound 48/80 was perfused before and during nerve stimulation. The facilitation of noradrenaline overflow was more pronounced (by 60%) when compound 48/80-induced mediator overflow started 4 min before nerve stimulation as compared to 30 s (15%), and was reduced by cocaine (by 50%), and, in the presence of cocaine, abolished by cimetidine (but was unaffected by mepyramine and thioperamide) and NG-nitro-(L)-(-)-arginine. In the presence of cimetidine and cocaine, when the facilitatory components were abolished, the evoked noradrenaline overflow observed 30 s after the start of infusion of compound 48/80 was inhibited, and the inhibition was partly reduced by methiotepin and ketanserin. Ovalbumin infusion in hearts from sensitized animals caused an inhibition of evoked noradrenaline overflow sensitive to methiotepin and also partly to ketanserin, and no facilitation was observed. The facilitation (> 100%) of evoked overflow of acetylcholine observed at 4 min after the start of perfusion with compound 48/80 was partly reduced by thioperamide (but not mepyramine or cimetidine) and to a comparable extent either by tropisetron (3 mumol/l) alone or by tropisetron plus methiotepin. In conclusion, degranulation of immunological cells is followed by histamine and serotonin release in the rat heart and may affect the release of autonomic neurotransmitters in rather unusual ways, by i) an uptake1-dependent and ii) an H2-mediated facilitation which probably involves nitric oxide as a permissive mediator, and iii) a serotonergic inhibition, of noradrenaline release, and iv) an H3- and serotonergic facilitation of acetylcholine release.

Pituitary-adrenocortical responsiveness to histaminergic stimulation during social stress of crowding in rats

Social stress of crowding for 3, 7, 14 and 21 days drastically reduces the serum corticosterone response to intracerebroventricular administration of dimaprit, a histamine H2-receptor agonist, moderately diminishes the response to pyridylethylamine, an H1-receptor agonist, and does not change significantly the corticosterone response to histamine. These results suggest that social stress of crowding considerably desensitizes central histamine H2-receptors involved in stimulation of the hypothalamic-pituitary-adrenal axis.

Modulation of neurotransmitter release via histamine H3 heteroreceptors

Presynaptic H3 receptors occur on histaminergic neurones of the CNS (autoreceptors) and on non-histaminergic neurones of the central and autonomic nervous system (heteroreceptors). H3 heteroreceptors, most probably located on the postganglionic sympathetic nerve fibres innervating the resistance vessels and the heart, have been identified in the model of the pithed rat. Furthermore, we could show in superfusion experiments that H3 heteroreceptors also occur on the sympathetic neurones supplying the human saphenous vein and the vasculature of the pig retina and on the serotoninergic, dopaminergic and noradrenergic neurones in the brain of various mammalian species, including man. The effects of three recently described H3 receptor ligands were studied in superfused mouse brain cortex slices. The potency of the novel H3 receptor agonist imetit exceeded that of R-(-)-alpha-methylhistamine (the reference H3 receptor agonist) by one log unit and that of histamine by almost two log units. Clobenpropit was shown to be a competitive H3 receptor antagonist, exhibiting a pA2 as high as 9.6 (exceeding the pA2 of the reference H3 receptor antagonist thioperamide by one log unit). The irreversible antagonism of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was also studied. Interactions of the H3 heteroreceptor with the dopamine autoreceptor in mouse striatal slices and the alpha 2-autoreceptor in mouse brain cortex slices could be demonstrated. Activation of alpha 2-autoreceptors decreases the H3 receptor-mediated effect. Blockade of alpha 2-autoreceptors increases the H3 receptor-mediated effect only if the alpha 2-autoreceptors are simultaneously activated by endogenous noradrenaline. The H3 receptor-mediated inhibition of noradrenaline release in mouse brain cortex slices was attenuated by the K+ channel blocker tetraethylammonium but this attenuation was abolished by reduction of the Ca2+ concentration in the medium (to compensate for the facilitatory effect of tetraethylammonium on noradrenaline release). Accordingly, we assume that the H3 receptors are not coupled to voltage-sensitive K+ channels. Pertussis toxin and N-ethylmaleimide attenuated the H3 receptor-mediated effect in the mouse brain cortex, suggesting that the H3 receptors are coupled to a G protein (eg Gi or Go). However, negative coupling to an adenylate cyclase does not appear to exist since an H3 receptor-mediated inhibition of cAMP accumulation was not obtained in mouse brain cortex membranes. H3 receptor ligands are currently undergoing clinical testing and might become new remedies for the treatment of disease of the gastrointestinal and bronchial system and the CNS.

Histamine inhibits dopamine release in the mouse striatum via presynaptic H3 receptors

In superfused mouse striatal slices preincubated with [3H]dopamine 25 nmol/l, the electrically (3 Hz) evoked tritium overflow was inhibited by histamine 10 mumol/l by 18%. The degree of inhibition was increased to 38% by haloperidol but not affected by (1) atropine, (2) reducing the stimulation frequency to 0.3 Hz or (3) increasing the concentration of [3H]dopamine (used for preincubation) to 100 nmol/l. The effect of histamine was mimicked by the H3 agonist R-(-)-alpha-methylhistamine; it was not affected by the H1 antagonist dimetindene and the H2 antagonist ranitidine but abolished by the H3 antagonist thioperamide. Tritium overflow evoked by Ca2+ ions (introduced into Ca(2+)-free, K(+)-rich medium containing tetrodotoxin) was not affected by histamine 10 mumol/l in the absence, but inhibited (by 30%) in the presence of haloperidol; the effect of histamine was abolished by thioperamide. In conclusion, the dopaminergic nerve terminals in the mouse striatum are endowed with presynaptic H3 receptors. Simultaneous blockade of dopamine autoreceptors increases the extent of the H3 receptor-mediated inhibition of dopamine release.

Mutual interaction of histamine H3-receptors and ?2-adrenoceptors on noradrenergic terminals in mouse and rat rain cortex

Brain cortex slices were preincubated with 3H-noradrenaline and superfused with physiological salt solution containing desipramine. We studied the inhibition of the electrically evoked tritium overflow caused by histamine in the presence of alpha-adrenoceptor ligands (mouse and rat brain cortex), and the inhibition caused by talipexole (the former B-HT 920) in the presence of H3-receptor ligands (mouse brain cortex). In mouse brain cortex slices, the inhibitory effect of histamine on the tritium overflow evoked by 36 pulses, 0.3 Hz was not changed by the alpha 1-adrenoceptor antagonist prazosin, but increased by the alpha 2-adrenoceptor antagonist rauwolscine. When the current strength or the duration of electrical pulses was reduced to compensate for the increase in evoked tritium overflow produced by rauwolscine, the latter still enhanced the effect of histamine. The histamine-induced inhibition of tritium overflow evoked by 360 pulses, 3 Hz was not affected by the alpha 1-adrenoceptor agonist phenylephrine but attenuated by the alpha 2-adrenoceptor agonist talipexole. Finally, the inhibition by histamine of the tritium overflow evoked by 3 pulses, 100 Hz was attenuated by talipexole but not affected by rauwolscine. Conversely, the inhibitory effect of talipexole on tritium overflow elicited by 360 pulses, 3 Hz was slightly attenuated by the H3-receptor agonist R-(-)-alpha-methylhistamine but not affected by the H3-receptor antagonist thioperamide. In rat brain cortex slices, histamine only tended to inhibit tritium overflow evoked by 360 pulses, 3 Hz, both in the absence of alpha-adrenoceptor antagonists and in the presence of prazosin. However, histamine markedly inhibited the evoked overflow in the presence of rauwolscine.(ABSTRACT TRUNCATED AT 250 WORDS)