Dual localization of histamine in an ascending neuronal pathway and in non-neuronal cells evidenced by lesions in the lateral hypothalamic area

Investigation on beneficial role of l-carnosine in neuroprotective mechanism of ischemic postconditioning in mice: possible role of histidine histamine pathway

OBJECTIVE

The present study was undertaken to investigate the possible role of histidine-histamine pathway in the neuroprotective effects produced by L-carnosine hand in hand with ischemic postconditioning in the animal model of cerebral ischemia.

METHODS

Cerebral ischemia was induced in swiss albino mice by performing BCCAO surgery. Morris water-maze test was utilized to assess the learning ability and memory of the animals. The whole brain acetylcholinesterase (AChE) activity, TBARS, GSH levels and MPO activity were evaluated as the biochemical parameters. For histopathological evaluation of the cerebral infarct size, TTC staining was employed.

RESULTS

Administration of L-carnosine (500   mg/kg, i.p.) successfully attenuated the manifestations of cerebral ischemia. Higher levels of AChE, TBARS, and MPO were observed in BCCAO treated animals, which were successfully attenuated by treatment with L-carnosine and ischemic postconditioning. Whereas administration of L-carnosine and ischemic postconditioning significantly increased the level of GSH in BCCAO treated animals. Moreover, treatment with ranitidine, an H₂ blocker (30 NMol, i.c.v) antagonized the neuroprotective actions of L-carnosine evidenced by decrease in MWM performance, increase in the level of AChE and oxidative stress, while decrease in GSH level in brain. The cerebral infarct size was found to be more in BCCAO inflicted animals, which was improved by the administration of L-carnosine, while the cerebral infarct size worsened by treatment with ranitidine (3   nmol, i.c.v.).

CONCLUSION

It is concluded that L-carnosine exerts neuroprotective effect via involvement of histidine-histamine pathway since the beneficial effects of L-carnosine were abolished by the H₂-blocker.

Non-imidazole-based histamine H3 receptor antagonists with anticonvulsant activity in different seizure models in male adult rats

A series of twelve novel non-imidazole-based ligands (3–14) was developed and evaluated for its in vitro binding properties at the human histamine H3 receptor (hH3R). The novel ligands were investigated for their in vivo protective effects in different seizure models in male adult rats. Among the H3R ligands (3–14) tested, ligand 14 showed significant and dose-dependent reduction in the duration of tonic hind limb extension in maximal electroshock (MES)-induced seizure model subsequent to acute systemic administration (5, 10, and 20 mg/kg, intraperitoneally), whereas ligands 4, 6, and 7 without appreciable protection in MES model were most promising in pentylenetetrazole (PTZ) model. Moreover, the protective effect observed for ligand 14 in MES model was lower than that observed for the reference drug phenytoin and was entirely abrogated when rats were co-administered with the brain-penetrant H1R antagonist pyrilamine (PYR) but not the brain-penetrant H2R antagonist zolantidine (ZOL), demonstrating that histaminergic neurotransmission by activation of postsynaptically located H1Rs seems to be involved in the protective action. On the contrary, PYR and ZOL failed to abrogate the full protection provided by 4 in PTZ model and the moderate protective effect by 14 in strychnine (STR) model. Moreover, the experimental and in silico estimation of properties such as metabolism was performed for five selected test compounds. Also, lipophilicity using planar reversed-phase thin-layer chromatography method was included for better understanding of the molecular properties of the tested compounds. Additionally, the absorption, distribution, metabolism, and elimination and toxicity parameters were evaluated for the most promising compounds 2, 4, 6, 7, and 14 utilizing in vitro methods. These interesting results highlight the potential of H3R ligands as new antiepileptic drugs or as adjuvants to available epilepsy medications.

Histamine H3 receptor as a potential target for cognitive symptoms in neuropsychiatric diseases

The potential contributions of the brain histaminergic system in neurodegenerative diseases, and the possiblity of histamine-targeting treatments is attracting considerable interests. The histamine H3 receptor (H3R) is expressed mainly in the central nervous system, and is, consequently, an attractive pharmacological target. Although recently described clinical trials have been disappointing in attention deficit hyperactivity disorder (ADHD) and schizophrenia (SCH), numerous H3R antagonists, including pitolisant, demonstrate potential in the treatment of narcolepsy, excessive daytime sleepiness associated with cognitive impairment, epilepsy, and Alzheimer's disease (AD). This review focuses on the recent preclinical as well as clinical results that support the relevance of H3R antagonists for the treatment of cognitive symptoms in neuropsychiatric diseases, namely AD, epilepsy and SCH. The review summarizes the role of histaminergic neurotransmission with focus on these brain disorders, as well as the effects of numerous H3R antagonists on animal models and humans.

The metabolism of histamine in rat hypothalamus and cortex after reserpine treatment

The effect of reserpine on histamine (HA) and tele-methylhistamine (N(τ)-MHA) in hypothalamus and cortex of rats was analyzed and compared to catecholamines. IP injection of reserpine (5 mg/kg) confirmed the effectiveness of reserpine treatment on noradrenaline and dopamine levels. Our in-vitro experiment with synaptosomal/crude mitochondrial fraction from hypothalamus and cortex confirmed that while mono amine oxidase (MAO) is an efficient metabolic enzyme for catecholamines, HA is not significantly affected by its enzymatic action. HMT activity after reserpine, pargyline and L-histidine treatment showed no differences compared to the control values. However HDC was significantly increased in both hypothalamus and cortex. In this study, Ws/Ws rats with deficiency of mast cells were used to clarify aspects of HA metabolism in HAergic neurons by eliminating the contribution of mast cells. The irreversible MAO-B inhibitor Pargyline (65 mg/kg) failed to accumulate N(τ)-MHA in the hypothalamus. However, when animals treated with reserpine and pargyline/reserpine were compared, the last group showed higher N(τ)-MHA values (p < 0.01). Moreover, the precursor of HA, L-histidine (1 g/kg), produced an increase of HA in the hypothalamus to 166% and the cortex to 348%. In conclusion, our results suggest that the effect of reserpine on the HA pools in the brain might be different. The neuronal HA pools are more resistant to reserpine as compared to those of catecholamine. Moreover, the HAergic pool appears to be more resistant to depletion than mast cells' pool, and thus HDC/HMT activity and its localization may play a key role in the understanding of HA metabolism in brain after reserpine treatment.

Role of histamine and its receptors in cerebral ischemia

Histamine is recognized as a neurotransmitter or neuromodulator in the brain, and it plays a major role in the pathogenic progression after cerebral ischemia. Extracellular histamine increases gradually after ischemia, and this may come from histaminergic neurons or mast cells. Histamine alleviates neuronal damage and infarct volume, and it promotes recovery of neurological function after ischemia; the H1, H2, and H3 receptors are all involved. Further studies suggest that histamine alleviates excitotoxicity, suppresses the release of glutamate and dopamine, and inhibits inflammation and glial scar formation. Histamine may also affect cerebral blood flow by targeting to vascular smooth muscle cells, and promote neurogenesis. Moreover, endogenous histamine is an essential mediator in the cerebral ischemic tolerance. Due to its multiple actions, affecting neurons, glia, vascular cells, and inflammatory cells, histamine is likely to be an important target in cerebral ischemia. But due to its low penetration of the blood-brain barrier and its wide actions in the periphery, histamine-related agents, like H3 antagonists and carnosine, show potential for cerebral ischemia therapy. However, important questions about the molecular aspects and pathophysiology of histamine and related agents in cerebral ischemia remain to be answered to form a solid scientific basis for therapeutic application.

In vivo electrochemical evidence for simultaneous 5-HT and histamine release in the rat substantia nigra pars reticulata following medial forebrain bundle stimulation

Exploring the mechanisms of serotonin [5-hydroxytryptamine (5-HT)] in the brain requires an in vivo method that combines fast temporal resolution with chemical selectivity. Fast-scan cyclic voltammetry is a technique with sufficient temporal and chemical resolution for probing dynamic 5-HT neurotransmission events; however, traditionally it has not been possible to probe in vivo 5-HT mechanisms. Recently, we optimized fast-scan cyclic voltammetry for measuring 5-HT release and uptake in vivo in the substantia nigra pars reticulata (SNR) with electrical stimulation of the dorsal raphe nucleus (DRN) in the rat brain. Here, we address technical challenges associated with rat DRN surgery by electrically stimulating 5-HT projections in the medial forebrain bundle (MFB), a more accessible anatomical location. MFB stimulation elicits 5-HT in the SNR; furthermore, we find simultaneous release of an additional species. We use electrochemical and pharmacological methods and describe physiological, anatomical and independent chemical analyses to identify this species as histamine. We also show pharmacologically that increasing the lifetime of extracellular histamine significantly decreases 5-HT release, most likely because of increased activation of histamine H-3 receptors that inhibit 5-HT release. Despite this, under physiological conditions, we find by kinetic comparisons of DRN and MFB stimulations that the simultaneous release of histamine does not interfere with the quantitative 5-HT concentration profile. We therefore present a novel and robust electrical stimulation of the MFB that is technically less challenging than DRN stimulation to study 5-HT and histamine release in the SNR.

Histamine and histamine receptors in pathogenesis and treatment of multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease associated with chronic inflammatory demyelination of the central nervous system (CNS). Due to disease complexity and heterogeneity, its pathogenesis remains unknown and despite extensive studies, specific effective treatments have not yet been developed. The factors behind the initiation of the inflammatory reactions in CNS have not been identified until now. MS is considered as a complex disease depending on genetic as well as environmental factors. Experimental autoimmune encephalomyelitis (EAE) is the preferential experimental rodent model for MS. Histamine [2-(4-imidazole) ethylamine] is a ubiquitous inflammatory mediator of diverse physiological processes including neurotransmission, secretion of pituitary hormones, and regulation of the gastrointestinal and circulatory systems which can modulate immune responses. Histamine functions are mediated through four G-protein coupled receptors that are named H1-H4 receptor. Histamine is implicated as an important factor in pathophysiology of MS and EAE. It has been shown that histamine can change the permeability of blood brain barrier, which leads to elevation of infiltrated cells in CNS and neuroinflammation. In contrast, there are evidence that show the protective role of histamine in MS and its animal model, EAE. In this review, we try to clarify the role of histamine in pathogenesis of MS, as well as we evaluate the efficacy of histamine receptors agonists and antagonists in treatment of this disease.

Localization of putative transmitters in the hippocampal formation: with a note on the connections to septum and hypothalamus

Biochemical assays on microdissected samples, denervation studies, subcellular fractionation, and light and electron microscopic autoradiography of high affinity uptake have been performed to study the cellular localization of transmitter candidates in the rat hippocampal formation. High affinity uptake of glutamate and aspartate is localized in the terminals of several excitatory systems, such as the entorhino-dentate fibres (perforant path), mossy fibres (from granular cells) and pyramidal cell axons. Thus, in stratum radiatum and oriens of CA1, 85% of glutamate and asparate uptake and 40% of glutamate and aspartate content are lost after lesions of ipsilateral plus commissural fibres from CA3/CA4. Hippocampal efferents also take up aspartate and glutamate, since these activities are heavily reduced in the lateral septum and mamillary bodies after transection of fimbria and the dorsal fornix. The synthesis (by glutamic acid decarboxylase), content and high affinity uptake of gamma-aminobutyrate (GABA) are not reduced after lesions of these or other projection fibre systems. A localization in intrinsic neurons is confirmed by a selective loss of glutamic acid decarboxylase after local injections of kainic acid. Peak concentrations of the enzyme occur near the pyramidal and granular cell bodies, corresponding to the site of the inhibitory basket cell terminals, and in the outer parts of the molecular layers. Some 85% of glutamic acid decarboxylase is situated in 'nerve ending particles'. Acetylcholine synthesis (by choline acetyltransferase) disappears after lesions of septo-hippocampal fibres. Since 80% of the hippocampal choline acetyltransferase is in 'nerve ending particles', the characteristic topographical distribution of this enzyme should reflect the distribution of cholinergic septo-hippocampal afferents. Serotonin, noradrenaline, dopamine and histamine are located/synthesized in afferent fibre systems. Some monoamine-containing afferents to the hippocampal formation pass via the septal area, others via the amygdala. The hippocampal formation also contains nerve elements reacting with antibodies against neuroactive peptides, such as enkephalin, substance P, somatostatin and gastrin/cholecystokinin.

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

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

Reduction in brain infarction by augmentation of central histaminergic activity in rats

Inflammation is a factor in the aggravation of reperfusion injury after cerebral ischemia. Since histamine H(2) receptor stimulation suppresses inflammatory reactions, effects of the central histaminergic activation on brain infarction were examined in rats. Focal cerebral ischemia for 2 h was provoked by transient occlusion of the right middle cerebral artery, and the infarct size was determined by 2,3,5-triphenyltetrazolium chloride stain after 24 h. Effects of postischemic administration of thioperamide, an H(3) antagonist, and metoprine, an inhibitor of histamine-N-methyltransferase, were evaluated in rats treated with l-histidine, a precursor of histamine. Furthermore, effects of these agents on changes in the striatal histamine level were examined by a microdialysis procedure. Focal ischemia provoked marked damage in rats treated with l-histidine (1000 mg/kg) alone. Administration of l-histidine (1000 mg/kg) with either thioperamide (5 mg/kg) or metoprine (10 mg/kg) alleviated brain infarction. The size of brain infarction was 27% and 10% of that in animals treated solely with l-histidine, respectively. The combination treatment with thioperamide and metoprine decreased the size of brain infarction in rats given l-histidine (500 mg/kg), although protective effects were not clear without l-histidine. A marked increase in the histamine concentration was observed in the histidine plus metoprine group, the value being 363% of that in the saline-injected group after 2-3 h. The histamine concentrations in the histidine group and histidine plus thioperamide group were 188% and 248%, respectively. These findings indicate that facilitation of central histaminergic activity reduced the brain infarction.

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.

Prevention of brain infarction by postischemic administration of histidine in rats

Focal cerebral ischemia for 2 h by occlusion of the right middle cerebral artery provoked severe brain infarction in the rat brain after 24 h. Intraperitoneal administration of histidine, a precursor of histamine, immediately and 6 h after reperfusion, alleviated brain infarction. The infarct size in the histidine (200 mg/kg, 500 mg/kg, and 1000 mg/kg, each time) groups was 71%, 39%, and 7% of that in the control group, respectively. Although intracerebroventricular administration of mepyramine (3 nmol), an H1 antagonist, did not affect the morphologic outcome in histidine-treated rats, ranitidine (30 nmol), an H2 antagonist, completely abolished the alleviation caused by histidine. These findings indicate that postischemic administration of histidine prevents development of brain infarction by stimulating central histamine H2 receptors.

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

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

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.

H2 histamine receptor-phosphorylation of Kv3.2 modulates interneuron fast spiking

Histamine-containing neurons of the tuberomammilary nucleus project to the hippocampal formation to innervate H1 and H2 receptors on both principal and inhibitory interneurons. Here we show that H2 receptor activation negatively modulates outward currents through Kv3.2-containing potassium channels by a mechanism involving PKA phosphorylation in inhibitory interneurons. PKA phosphorylation of Kv3.2 lowered the maximum firing frequency of inhibitory neurons, which in turn negatively modulated high-frequency population oscillations recorded in principal cell layers. All these effects were absent in a Kv3.2 knockout mouse. These data reveal a novel pathway for histamine-dependent regulation of high-frequency oscillations within the hippocampal formation.

Histamine infusion induces a selective dopaminergic neuronal death along with an inflammatory reaction in rat substantia nigra

We have evaluated the effects of a direct infusion of histamine, as mediator of inflammatory response, in substantia nigra, striatum, medial septum, and medial lemniscus. Injection of 100 and 250 nmol of histamine in substantia nigra produced a selective damage in dopaminergic neurons evidenced by the loss of tyrosine hydroxylase mRNA-expressing cells, tyrosine hydroxylase-immunolabeled-positive cell bodies, and dopamine and 3,4-dihydroxyphenylacetic acid levels. In parallel we found an acute inflammatory response manifested by a loss of glial fibrillary acidic protein-immunolabeled astrocytes and, at precisely the same area, an activation of microglia. In the striatum, only high doses (500 nmol) produced an evident terminal degeneration. The selective neurotoxicity of histamine for dopaminergic cells was demonstrated by the unaltered transcription of glutamic acid decarboxylase mRNA in substantia nigra. Moreover, intraseptal injection of 100 nmol of histamine failed to alter the pattern of choline acetyltransferase mRNA-expressing cells, and intraparenchymal injection of histamine in medial lemniscus failed to alter the pattern of serotonin-immunolabeled cells. We conclude that the substantia nigra is highly sensitive to histamine-derived neurotoxicity, where inflammatory processes mediated by histamine could be important in the pathological changes that lead to dopaminergic neuronal damage after histamine infusion.

Cross-generalization between a cocaine cue and two antihistamines

Rats were trained to discriminate between 10 mg/kg cocaine and saline injections under a fixed ratio 10 schedule of food-motivated lever press responding. Once stimulus control was achieved, reinforced test sessions were conducted to assess the degree of generalization of a wide range of cocaine doses and the cross-generalization between the cocaine training stimulus and two over-the-counter antihistaminic drugs, diphenhydramine and doxylamine, when administered with saline or in drug combinations. Cocaine produced a dose-dependent generalization to the 10 mg/kg training stimulus. Cocaine also produced mild rate-increasing effects at low test doses and response rate suppression at higher doses. Both diphenhydramine and doxylamine produced a partial generalization to the 10 mg/kg cocaine training stimulus. Drug mixtures produced complete cross-generalization with the training cue.

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

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

Histamine does not play an essential role in electrocortical activation during waking behavior

Intraperitoneal injection of alpha-fluoromethylhistidine (alpha-FMH; 200 mg/kg), a specific inhibitor of histidine decarboxylase produced a severe depletion of neocortical and hippocampal histamine 3 h later as determined by a radioenzymatic assay. This treatment had no obvious effect on either low voltage fast activity (LVFA) in the neocortex or on rhythmical slow activity (RSA) in the hippocampus during an 8 h recording period during the rats' light cycle. Scopolamine-sensitive LVFA, scopolamine-resistant LVFA and scopolamine-resistant hippocampal RSA all appeared unaffected. This suggests that any contribution histamine makes to electrocortical activation is probably indirect, acting via other transmitter systems.

Histaminergic system in the cat hypothalamus with reference to type B monoamine oxidase

It is known that histamine (HA) and type B monoamine oxidase (MAO-B), an enzyme involved in its metabolism, are present in the posterior hypothalamus, but the sites where MAO-B intervenes in HA metabolism remain uncertain. The present study examined and compared the detailed distribution and morphology of neurons immunoreactive to HA (HA-ir) and MAO-B (MAO-B-ir) in the cat hypothalamus. HA-ir neurons were localized almost exclusively in the posterior hypothalamus with the largest group in the tuberomammillary nucleus and adjacent areas. MAO-B-ir staining was detected in the vast majority of HA-ir neurons, suggesting that the degradation of tele-methylhistamine (t-MHA), the direct metabolite of HA, may occur within these cells. Nevertheless, a few HA-ir cells showed no detectable or very weak MAO-B-ir labeling; a small group of neurons containing MAO-B alone was detected in the area dorsolateral to the caudal part of the arcuate nucleus. Numerous HA-ir axons and terminal-like structures were distributed unevenly in virtually all hypothalamic regions. One of their principal trajectories ascended through the ventrolateral part of the hypothalamus and rostrally formed an axon column, which ascended into the preoptic area and contributed fibers to the diagonal band of Broca and bed nucleus of the stria terminalis. Other HA-ir axons passed laterally, dorsal to the zona incerta or ventrally through a narrow zone dorsal to the optic tract. Numerous long HA-ir axons coursed dorsomedially from the ventrolateral posterior hypothalamus to the dorsal hypothalamic area. Many are oriented vertically to the thalamus in the midline. MAO-B-ir axons and fibers were detectable throughout the hypothalamus and overlapped the areas distributing HA-ir fibers. They were, however, weaker in staining intensity and apparently fewer than the HA-ir fibers. MAO-B-ir glial cells were numerous in all hypothalamic structures rich in HA-ir fibers. These results suggest that the metabolism of t-MHA may also occur within HA terminals and glial cells.

Aggravation of ischemic neuronal damage in the rat hippocampus by impairment of histaminergic neurotransmission

Delayed damage to hippocampal CA1 pyramidal cells was observed in rats subjected to cerebral ischemia caused by 10 min of 4-vessel occlusion. Animals pretreated with alpha-fluoromethylhistidine, a suicide inhibitor of histidine decarboxylase, showed significantly more necrotic cells than did control animals. Mepyramine (H1-antagonist) and (R) alpha-methylhistamine (H3-agonist), but not zolantidine (H2-antagonist), significantly aggravated the delayed neuronal death. These results suggest that histaminergic neurons have a protective role, probably via H1-receptors, in the development of delayed neuronal death caused by cerebral ischemia.

Effects of season and estradiol on concentrations of histamine within discrete brain regions of ovariectomized ewes

Histamine has been implicated as a neuromodulator of secretion of gonadotropins in several species. Concentrations of histamine were analyzed within discrete brain regions and endocrine tissues to help determine whether this amine has the potential to exert a similar function in ewes expected to have dramatically different serum concentrations of LH. Following collection of blood samples at 12-min intervals for 4-hr, ovariectomized (OVX) and OVX-estradiol treated (OVX-E) ewes were slaughtered during the breeding and anestrous seasons (five animals/group). Concentrations of LH were depressed by treatment with estradiol (E; P < .01), but to a greater extent (P < .05) during the anestrous season compared to the breeding season. Concentrations of histamine in tissues (ng/mg) differed (P < .01) between the breeding and anestrous seasons, in the medial thalamus (39.2 +/- 14.1 vs 109.9 +/- 13.0), posterior pituitary gland (247.6 +/- 50.7 vs 23.0 +/- 9.1) and midbrain tegmentum (10.4 +/- 5.6 vs 50.7 +/- 3.9). Estradiol containing implants decreased (P < .05) concentrations of histamine in the midbrain tegmentum (20.3 +/- 7.1 vs 37.7 +/- 7.8) and posterior pituitary gland (87.3 +/- 24.0 vs 258.2 +2- 67.5) compared to non-estradiol treated controls. Histamine concentrations in the pineal and anterior pituitary glands and brain regions; stalk-median eminence, medial basal hypothalamus, preoptic area, cerebellum, parietal neocortex, were not (P > .05) affected solely by either season or E. An interaction between effects of season and estradiol on concentrations of histamine occurred (P < .05) in the posterior pituitary gland and the preoptic area.(ABSTRACT TRUNCATED AT 250 WORDS)

A detailed autoradiographic mapping of histamine H3 receptors in rat brain areas

[3H](R)alpha-methylhistamine, a selective histamine H3-receptor ligand, was used to perform binding studies with membranes and generate light microscopic autoradiograms in sections of the rat brain. High densities of H3 receptors were found in membranes from the anterior part of the cerebral cortex, the accumbens nucleus, the striatum, the olfactory tubercles and the substantia nigra. Autoradiography of sagittal and frontal sections evidenced specific labelling in a number of gray matter areas over a very low background, as determined using thioperamide, a selective H3-receptor antagonist, as competing drug. Labelled areas were identified by comparison with adjacent Nissl-stained sections and their labelling was rated visually. H3 receptors are heterogeneously distributed among areas known to receive histaminergic projections. In the cerebral cortex, H3 receptors are present in all areas and layers, with a rostrocaudal gradient and a higher density in deep layers (laminae IV-VI). In the hippocampal formation, H3 receptors are the most abundant in the dentate gyrus and the subiculum. In the amygdaloid complex, the highest densities are found in the central, lateral and basolateral groups of nuclei. In the basal forebrain, the accumbens nucleus, the striatum, the olfactory tubercles and the globus pallidus are highly labelled. In the thalamus in which histaminergic fibres are scarce, H3 receptors are present in a rather high density, particularly in the midline, median and intralaminar groups of nuclei. In the hypothalamus, where the densest network of histaminergic fibres is found, H3 receptors occur in moderate density, being slightly more abundant in the anterior and medial part. They are also present at the level of the tuberomammillary nuclei where they may reside on histaminergic perikarya. In mesencephalon and lower brainstem, H3 receptors are abundant in the reticular part of the substantia nigra and central gray. They are present in low density in areas of noradrenergic and serotoninergic perikarya and in the spinal cord, where a faint specific labelling is detected in the gray matter, particularly in the external layers of the dorsal horn. In the cerebellum and pituitary gland, H3 receptors are scarce. Kainic acid infusions into the striatum were followed by marked local decreases in H3 receptors evidenced in both membrane binding and autoradiographic studies. Unilateral interruption of the ascending histaminergic pathways via electrocoagulation of the lateral hypothalamic area was followed by ipsilateral increase in striatal [3H](R)alpha-methylhistamine binding, a process consistent with denervation up regulation of postsynaptic H3 receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

A highly sensitive assay for histamine using ion-pair HPLC coupled with postcolumn fluorescent derivatization: its application to biological specimens

A simple and highly sensitive method for the determination of histamine (HA) was developed using ion-pair, reversed-phase HPLC coupled with postcolumn o-phthalaldehyde derivatization fluorometry, and it was applied to the unpurified extracts of human and rat plasma, and brains of rats and mice. The HA concentrations both in the plasma and brains determined by the present method were well consistent with the values obtained by cation-exchange HPLC with postcolumn fluorescent derivatization currently in use. The present method was more advantageous than the assay using cation-exchange HPLC: (1) it was three to four times more sensitive (the detection limit was 0.5 pg of HA), and (2) it enabled the measurement of HA in samples containing (R)alpha-methylhistamine, a potent and specific H3-receptor agonist, which could not be separated from HA by cation-exchange chromatography. Using the present method coupled with intracerebral microdialysis, we found in the rat hypothalamus that (R)alpha-methylhistamine (5 mg/kg i.p.) markedly decreased the extracellular concentration of HA with a maximal effect (83% reduction) during 30-60 min after injection, suggesting that most of HA in the microdialysate fraction is neuronal in origin.

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

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

Distribution of histaminergic neurons and fibers in rat brain. Comparison with noradrenergic and serotonergic innervation of the vestibular system

Histamine-immunoreactive cell bodies are strictly confined to the ventral part of the posterior hypothalamus and the region of the mamillary nuclei. They can be divided into two main cell groups with two major ascending projections and one minor descending projection. The ascending histaminergic fiber tracts innervate almost all regions of the di- and telencephalon with high numbers in for instance the median eminence, the nucleus of the diagonal tract of Broca, the caudate-putamen complex and cortical structures. The arrangement of the histaminergic neuronal system, i.e. a compact cell group with a widespread distribution of fibers, resembles that of other monoaminergic (i.e. serotonergic and catecholaminergic) systems. The vestibular system receives a high innervation of serotonergic fibers, a moderate density of noradrenergic fibers and only scattered histaminergic fibers.

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

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

Role of histamine in traumatic brain edema. An experimental study in the rat

The possibility that histamine plays a role in the formation of traumatic brain edema was investigated in the rat. A 3 mm deep and 3 mm long stab injury was performed in the right parietal cortex under urethane anaesthesia. The brain water content and histamine levels in plasma and brain were measured at the end of 1, 2 and 5 h periods after trauma. There was a 3.46% increase in brain water content in the traumatized hemisphere from the value in the control group at 5 h. The histamine content was increased by 107% in plasma and 51% in the traumatized brain hemisphere from the control value at this time period. The increased brain water content as well as the elevated plasma and brain histamine levels were prevented by prior treatment with the histamine H2-receptor antagonist cimetidine. Mepyramine (a histamine H1-receptor antagonist) failed to reduce the increased brain water content and the histamine levels in plasma and brain remained high. The results strongly indicate that histamine has a role in the formation of early traumatic brain edema and that this reaction can be influenced by pharmacological procedures.

Alpha 2-adrenoceptor-mediated inhibition of histamine release from rat cerebral cortical slices

1. Depolarization of rat cerebral cortical slices, prelabelled with [3H]-histidine, in high potassium (40 mM KCl) medium stimulated the release of [3H]-histamine. The K+-evoked release of [3H]-histamine was attenuated by incubation in calcium-free medium and prevented by prior incubation of brain slices with the selective histidine decarboxylase inhibitor S-(alpha)-fluoromethylhistidine. 2. The K+-evoked release of [3H]-histamine was significantly (P less than 0.001) reduced following stimulation of histamine H3-receptors with R-(alpha)-methylhistamine (1 microM) and this effect was antagonized by the H3-antagonist thioperamide (1 microM). 3. Noradrenaline and the alpha 2-selective adrenoceptor agonists clonidine and UK-14,304 inhibited the K+-evoked release of [3H]-histamine in a concentration-dependent manner yielding EC50 values of 2.5, 0.8 and 1.2 microM, respectively. However, the maximum response to clonidine was only 52 +/- 8% of that obtained with noradrenaline. 4. The inhibitory effect of noradrenaline was antagonized by the non-selective alpha-antagonist phentolamine and by the selective alpha 2-antagonists yohimbine and idazoxan. However, the response to noradrenaline was not inhibited by the alpha 1-antagonist prazosin at concentrations up to 1 microM. 5. These results suggest that both histamine H3-receptors and alpha 2-adrenoceptors are present on histamine-containing nerve terminals in rat cerebral cortex and can exert an inhibitory influence on neurotransmitter release.

Histamine modulates local inhibition in the rat hippocampal slice

1. These experiments investigated the action of histamine on local inhibition in the CA1 region of the in vitro hippocampal slice preparation using a paired-pulse paradigm. 2. We observed that histamine produced a concentration-dependent and reversible attenuation of paired-pulse inhibition. This effect was reduced by the H2 receptor antagonist, cimetidine, and mimicked by the H2 receptor agonist, impromidine. 3. We also observed that histamine produced concentration-dependent effects on the amplitude of the population spike that could be correlated with alterations in the field excitatory postsynaptic potential (EPSP) amplitude and input fiber volley. High concentrations of histamine produced a reduction in the amplitude of the population spike which was always accompanied by a reduction in the EPSP and fiber volley amplitude. 4. These results suggest that histamine, through the occupancy of H2 receptors, acts to modulate the efficacy of the local synaptic circuitry which is involved in producing paired-pulse inhibition in the hippocampus.

Hypoxia and ischemia modifies histamine metabolism and transport in brain synaptosomes

Histamine level (HA), the activities of the HA synthetizing enzyme--histidine decarboxylase (HD) and HA metabolizing enzyme--histamine methyltransferase (HMT) and the uptake and release of histidine and histamine were analyzed in synaptosomal preparations obtained from rats with brain hypoxia and ischemia. Hypoxia produced only non-significant changes in all the parameters studied, whereas ischemia induced increase of both enzyme activities and histidine release, with simultaneous decreased of histidine uptake and HA level. The effect of ischemia appeared to be reversible; the changes retreated within 1 h of resuscitation together with the vital functions of rats.

The connections between the septum-diagonal band complex and histaminergic neurons in the posterior hypothalamus of the rat. Anterograde tracing with Phaseolus vulgaris-leucoagglutinin combined with immunocytochemistry of histidine decarboxylase

The connections between nuclei of the septum-diagonal band complex and the clusters of histaminergic neurons in the posterior hypothalamic region were studied with a dual-labeling procedure in which anterograde neuroanatomical tracing with Phaseolus vulgaris-leucoagglutinin was combined with immunohistochemistry of histidine decarboxylase. Phaseolus vulgaris-leucoagglutinin was injected in the medial and lateral septal nuclei, and in various parts of the nuclei of the diagonal band of Broca. The fibers arising from the medial and lateral septal nuclei traverse the vertical limb of the diagonal band and, in part, join the medial forebrain bundle in the preoptic area. Other fibers descend diffusely through the lateral hypothalamus to the posterior hypothalamus, or course in a bundle of fibers ensheathing the fornix. The nuclei of the diagonal band project via the medial forebrain bundle and the diffuse pathway to the posterior hypothalamic region. All the nuclei of the septum-diagonal band complex, with the exception of the medial and lateral parts of the nucleus of the horizontal limb of the diagonal band, project to clusters of histaminergic neurons. These projections exhibit the following arrangement: along the axis lateral septal nucleus-medial septal nucleus-vertical limb of the diagonal band-medial part of the horizontal limb of the diagonal band, the septohypothalamic fibers decrease in density and distribute to fewer clusters of histaminergic neurons. Varicosities on the labeled fibers are formed in close proximity to the cell bodies and dendrites of the histaminergic neurons.

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

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

Effects of a-fluoromethylhistidine on sleep and wakefulness in the rat. Short note

The histamine synthesis inhibitor a-fluoromethylhistidine (a-FMH, 50 mg/kg, i.p.) significantly reduced wakefulness (W) and light sleep and increased slow wave sleep (SWS) and REM sleep during the light period in rats housed under 12 h light/12 h dark conditions (12L/12D). When animals were housed under 16 h light/8 h dark (16L/8D) they remained awake for a longer period of time during the dark as compared to the 12L/12D lighting cycle. Under this condition a-FMH 50 mg/kg significantly decreased W and increased SWS. Our results tend to indicate that histamine intervenes in sleep-wakefulness regulation. In addition, histamine could be partly involved in the abnormally increased incidence of W observed during the dark in rats housed under 16L/8D conditions.

Comparison of the size of neuronal and non-neuronal histamine pools in the brain of different rat strains

The size of the neuronal and non-neuronal histamine pools in the brain of three different strains of rats was measured by assuming that the alpha-fluoromethylhistidine-induced maximal decrement of histamine represents the size of the neuronal pool. Although the total histamine levels in the brain showed a considerable interstrain variation, no significant interstrain difference was observed in the neuronal histamine level. These results suggest that the size of the neuronal histamine pool in the brain is relatively stable, whereas the size of the non-neuronal histamine pool is variable.

En bloc immunohistochemistry reveals extensive distribution of histidine decarboxylase-immunoreactive neurons on the ventral surface of the rat hypothalamus

En bloc immunohistochemistry was used to examine the distribution of L-histidine decarboxylase (HDC)-immunoreactive neurons on the ventral surface of the rat hypothalamus. Following standard paraformaldehyde fixation, the ventral hypothalamus containing the tuber cinereum was carefully dissected free and incubated en bloc with antisera to HDC followed by standard avidin-biotin complex immunohistochemistry. Microscopic examination of these en block preparations revealed the existence of an extensive plexus of HDC-immunoreactive neurons located directly upon the ventral hypothalamic surface. HDC-immunoreactive neurons were largely restricted to the caudal half of the hypothalamic surface, and were multipolar, with 2-5 dendrites radiating in all directions from the soma. The proximity of these neurons to the ventral surface of the brain suggests that histaminergic neurons might be capable of responding to cerebrospinal fluid borne substances. Thus, one form of input to hypothalamic histaminergic neurons may be humoral rather than synaptic.

Projection from the prefrontal cortex to histaminergic cell groups in the posterior hypothalamic region of the rat. Anterograde tracing with Phaseolus vulgaris leucoagglutinin combined with immunocytochemistry of histidine decarboxylase

We investigated the projection from the infralimbic division of the prefrontal cortex (area 25) to histaminergic neurons in the posterior hypothalamic area. Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected in the prefrontal cortex of rats. Frozen brain sections were subjected to combined PHA-L and histidine decarboxylase (HDC)-peroxidase immunocytochemistry, using nickel-enhanced diaminobenzidine (blue reaction product) to visualize the transported PHA-L, and diaminobenzidine (brown reaction product) to visualize simultaneously the HDC-containing neurons. PHA-L-labeled fibers could be seen coursing in the capsula interna, leaving the telencephalon via the anterior thalamic radiation and the medial forebrain bundle. In the lateral and posterior hypothalamic areas, PHA-L-labeled fibers leave the medial forebrain bundle and traverse the nuclei containing HDC-immunoreactive neurons. Varicosities on the PHA-L-labeled fibers, the majority of which occur en passant, could be observed in close association with the HDC-immunoreactive neurons. The results suggest that the hypothalamic histaminergic neurons receive afferent synaptic input from neurons of the infralimbic division of the prefrontal cortex.

Histaminergic neurons in the rat brain: correlative immunocytochemistry, Golgi impregnation, and electron microscopy

Histamine-containing neurons were visualized in Vibratome--sections of rat brain with the indirect peroxidase-antiperoxidase immunocytochemical method of Sternberger (Immunocytochemistry, 2nd edition, New York: John Wiley and Sons, pp. 1-354, '79) by utilizing a primary antibody directed against L-histidine decarboxylase (HDC). Cell bodies of HDC-immunoreactive neurons are located exclusively in the posterior hypothalamus: tuberal magnocellular nucleus (TM), caudal magnocellular nucleus (CM), and post-mammillary magnocellular nucleus (PCM). With the light microscope, all the HDC-immunoreactive neurons in CM and PCM and the majority of the HDC-immunoreactive neurons in TM appear to be large neurons, with a short, thick dendrite emerging from each pole of the long axis of the oval perikaryon and one or more, thinner, nonpolar primary dendrites. In the electron microscope, it can be seen that the immunoreaction product is diffusely dispersed in the cytoplasm. The ultrastructural features of all investigated (70) HDC-immunoreactive neurons in the three nuclei, independent of their light microscopic characteristics, are remarkably similar: large, unindented, pale nucleus; a high proportion of cytoplasm to nucleus (with the exception of the medium-sized HDC-immunoreactive neurons in TM); large, perinuclear array of Golgi apparatus; numerous mitochondria; endoplasmic reticulum fragmented into numerous small cisterns; thick initial portions of the primary dendritic trunks; few axosomatic synaptic contacts. Twenty-one Golgi-Kopsch-impregnated neurons taken from CM, PCM, and TM were embedded in epoxy resin, serially sectioned, and investigated in the electron microscope. The ultrastructural characteristics typical of HDC-immunoreactive neurons were observed in all three nuclei in neurons with large cell bodies tapering into two thick, sparsely spinous primary dendrites that subsequently dichotomize into very long (up to 100 microns), nontapering, aspinous secondary dendrites. In sections taken from the posterior hypothalamic area of rats prepared in a conventional way for electron microscopy, distinct populations of large cells can be observed in TM, CM, and PCM displaying the same set of ultrastructural characteristics as the HDC-immunoreactive neurons.

Histaminergic projections from the premammillary and posterior hypothalamic region to the caudate-putamen complex in the rat

Immunofluorescence, using antibodies to histamine and to histidine decarboxylase, was combined with retrograde axonal tracing by injecting Granular Blue into the caudate-putamen complex. Evidence is presented for the existence of histaminergic as well as non-histaminergic projections from the posterior hypothalamus and the premammillary region to the caudate-putamen complex. The majority of the histaminergic neurons projecting to this brain region are localized in the nuclei caudalis magnocellularis and caudalis magnocellularis postmammillaris. Roughly 20-25% of the histaminergic neurons in these cell groups innervate the caudate-putamen complex.

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

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

Effects of H1- and H2-histamine receptor agonists and antagonists on sleep and wakefulness in the rat

The H1-receptor agonist 2-thiazolylethylamine (2-TEA) given by i.c.v. route dose-dependently increased wakefulness (W) and decreased NREM sleep (NREMS) and REM sleep (REMS) in rats prepared for chronic sleep recordings. The H1-receptor antagonists pyrilamine and diphenhydramine given by i.p. route decreased W and increased NREMS. Pyrilamine prevented the increase of W and decrease of NREMS produced by 2-TEA. However, REMS reduction was not antagonized, what tends to suggest that two different mechanisms could be involved in the 2-TEA-induced effects on NREMS and REMS. Cimetidine which blocks H2-receptors, when given by i.p. route showed no significant effects on sleep and W. Administration of the H2-receptor agonist dimaprit and the H2-receptor antagonists cimetidine, metiamide and ranitidine by i.c.v. route induced the appearance of high voltage spikes at cortical leads, thus leaving inconclusive the matter of their effects on sleep and wakefulness. Our results tend to support the proposal that the H1-receptor intervenes in sleep-wakefulness regulation. Limitations in the available H2-receptor agonists and antagonists presently preclude a more detailed analysis of the role of H2-receptors on sleep and W.

Morphine-induced changes in histamine dynamics in mouse brain

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