Selective inhibitors of biosynthesis of aminergic neurotransmitters

Characterization of a high-affinity membrane-associated ornithine decarboxylase from the free-living nematode Caenorhabditis elegans

Ornithine decarboxylase has been identified and characterized in the free-living nematode Caenorhabditis elegans. Unlike previously described ornithine decarboxylases, the enzyme activity is membrane-associated and remains in the membrane fraction after treatment with high salt, detergents or phosphatidylinositol-specific phospholipase C. Ornithine has an apparent Km value of 2.7 microM for ornithine decarboxylase. The enzyme is competitively inhibited by arginine and lysine with Ki values of 4.0 and 24.4 microM respectively. None of the other naturally occurring amino acids inhibited more than 10% of the enzyme activity at concentrations up to 1 mM. Agmatine, putrescine, spermidine and spermine inhibit ornithine decarboxylase in a non-competitive manner with Ki values of 10, 53.5, 59 and 855 microM respectively. A similar ornithine decarboxylase activity was also identified in membrane preparations from the parasitic nematode Haemonchus contortus.

Pharmacology of alpha-fluoromethylhistidine, a specific inhibitor of histidine decarboxylase

alpha-Fluoromethyl-[S]-histidine (FMH) is a specific and potent inhibitor of histidine decarboxylase, which forms histamine from histidine. It acts selectively and irreversibly by formation of a covalent linkage, possibly with the serine residue in the active site of the enzyme. A single administration of FMH decreases the histamine content only of non-mast cells in the brain and stomach of rodents, but repeated administration gradually decreases the histamine content of mast cells in all tissues. Thus, FMH can be used to deplete histamine in pharmacological studies. As no marked side-effects have been observed during administration of FMH, it may be useful in pathological conditions, such as some allergic diseases, peptic ulcers and mastocytosis, in which excess production of histamine is involved.

Histidine decarboxylase in human basophilic leukemia (KU-812-F) cells. Characterization and induction by phorbol myristate acetate

The human leukemic cell line KU-812-F is known to differentiate into mature basophil-like cells under serum-free culture conditions. In the present study, the activity of histidine decarboxylase (HDC), a histamine-forming enzyme, in KU-812-F cells was found to be high, ranging from 10 to 57 pmol/min/mg protein. The great variation in HDC activity appeared to be due to different percentages and degrees of maturity of basophil-like cells during differentiation of this cell line. The enzyme was inhibited by alpha-fluoromethylhistidine but not by carbidopa, was unable to form dopamine from L-3,4-dihydroxyphenylalanine, and had a Km value for histidine of 0.27 mM, indicating that it was HDC and not aromatic amino acid decarboxylase. The HDC activity increased 1.8-fold when the cells were stimulated by phorbol myristate acetate, which is known to activate protein kinase C, and this increase was blocked by staurosporine, a potent inhibitor of protein kinase C.

Autoradiography of 3H-alpha-fluoromethyl histidine in mice: correlation with the kidney histidine decarboxylase activity

Tritium-alpha-fluoromethyl histidine (3H-alpha-FMH), designed as a kcat-inhibitor of mammalian histidine decarboxylase (EC 4.1.1.22), was administered intravenously in male and pregnant female mice of the NMRI strain and the distribution of tritium in the body recorded by whole-body and microautoradiography. The results showed penetration of radioactivity into most tissues within 5 min. after the injection. After 4 hrs the highest levels of radioactivity were present in the intestinal content and in the kidneys. In the pregnant animal there was also a high labelling of the foetal tissues. When whole-body sections were washed in TCA prior to the autoradiographic exposure to retain only protein-bound radioactivity, a distinct labelling pattern was seen in the kidneys of the pregnant female mice but not in those of the male mice. Microautoradiography of the kidneys showed that the cells involved were located within the proximal convoluted tubuli. In several mouse strains, including the NMRI, the activity of kidney histidine decarboxylase is low in the males but high in females during a transient period of pregnancy. Incorporation of tritium into kidney protein after treatment with 3H-alpha-FMH, was correlated to a loss in histidine decarboxylase activity. The isotopic labelling was confined mainly to a component which cofractionated with histidine decarboxylase in polyacrylamide gel electrophoresis (PAGE) under non-denaturing conditions. Our data indicate that the cells described above represent the location of kidney histidine decarboxylase.

New role for histamine in interleukin-3-induced proliferation of hematopoietic stem cells

This study investigated the effect of histamine generated by murine bone marrow cells in response to IL-3 on one particular biological activity of this growth factor, i.e., triggering of cells forming colonies in spleen (CFU-S) into S phase. Evidence is provided that i) IL-3-induced day-8 CFU-S cell cycling, evaluated by hydroxy-urea suicide, is completely abrogated when the binding of histamine to its H2 receptors is blocked by the specific antagonist oxmetidine, whereas cetirizine, a H1 receptor antagonist, is ineffective; and ii) the entry of day-8 CFU-S into S phase in response to IL-3 is likewise abolished when the histamine synthesis promoted by the growth factor is prevented by alpha-fluoromethylhistidine, a specific inhibitor of the histamine-forming enzyme, histidine decarboxylase. Similar results are obtained with both drugs, when a progenitor-enriched bone marrow population is used instead of total cells. Furthermore, i.v. injection of recombinant (r)IL-3 results within 2 hr in a substantial increase in bone marrow cell histamine synthesis together with triggering of day-8 CFU-S into cycle, the latter being completely abolished by a simultaneous injection of the H2 histamine receptor antagonist oxmetidine. Thus, our findings support the notion that both in vitro and in vivo the proliferation of early CFU-S in response to IL-3 is modulated by histamine via its H2 receptors. This conclusion is also consistent with the observation that dimaprit, a specific agonist of these receptors not only enhances the sensitivity of day-8 CFU-S to HU after a 2 hr incubation with bone marrow cells but also increases, to the same extent as IL-3, the number of colonies formed in irradiated spleens after a 5 hr pretreatment.

Hypothalamic histamine modulates adaptive behavior of rats at high environmental temperature

Histamine content in the rat hypothalamus was lower at 4 degrees C and higher at 31 degrees C compared to that at 21 degrees C. Pretreatment with alpha-fluoromethylhistidine, a 'suicide' inhibitor of histamine decarboxylase, attenuated both the increased level of hypothalamic histamine and rat adaptive behavior at 31 degrees C. Increase of histamine content in the hypothalamus appears to be an important factor contributing to rat adaptive behavior to high environmental temperature.

Evidence for feeding elicited through antihistaminergic effects of tricyclic antidepressants in the rat hypothalamus

We studied central mechanisms of antidepressants that affect feeding behavior in rats. The tricyclic compounds amitriptyline, doxepin and imipramine significantly induced feeding after their infusion into the third cerebral ventricle in the light phase, but the tricyclic, desipramine, and the dicyclic zimelidine, did not. Drinking was not affected by any compound tested. The relative order of potency in eliciting feeding was: amitriptyline and doxepin greater than imipramine greater than desipramine and zimelidine. To clarify the involvement of neuronal histamine in antidepressant-induced feeding, alpha-fluoromethylhistidine (FMH), a "suicide" inhibitor of histidine decarboxylase, was intraperitoneally administered before infusion of amitriptyline. FMH attenuated the amitriptyline's effect. Bilateral microinfusion of amitriptyline into the ventromedial hypothalamus or the paraventricular nucleus verfied that these are loci for the modulation of feeding by amitriptyline. In the lateral hypothalamus, amitriptyline was less effective. These findings indicate that tricyclic antidepressants directly facilitate feeding, which is, at least in part, mediated by histamine in the hypothalamus.

Biochemical and physiological evidence that histamine is the transmitter of barnacle photoreceptors

We tested the hypothesis that histamine is the transmitter released by barnacle photoreceptors. Median and lateral ocelli were incubated with 3H-histidine and found to synthesize 3H-histamine, identified by high-voltage electrophoresis. Synthesis could be blocked by the histidine decarboxylase inhibitor (S)-alpha-fluoromethylhistidine. Histamine was applied to I-cells either by superfusion or by pressure ejection from a pipette (100 microM or 1 mM histamine) positioned close to the I-cell's soma. When bath-applied at concentrations ranging from 5-100 microM, histamine hyperpolarized the I-cell in a dose-dependent fashion and increased its conductance. At 100 microM, histamine abolished the I-cell's response to light. The response to a pulse of pressure-applied histamine was a hyperpolarization whose amplitude was graded with dose (determined by the duration of the pulse). This response persisted in concentrations of Co2+ and Cd2+ that blocked synaptic transmission from the photoreceptors. Cimetidine, an antagonist of mammalian H2 receptors, markedly decreased the cell's responses both to HA and to light at 100 microM and blocked both responses at 1 mM. Pyrilamine and triprolidine, H1 antagonists, had a complex effect on the I-cell's responses to histamine and to light. Neither H1 nor H2 antagonists, nor histamine itself, affected the voltage or light responses recorded in the presynaptic terminal region, or any phase of calcium-dependent action potentials induced in the terminal in the presence of tetraethylammonium ion. Thus, biochemical, immunocytochemical, and physiological evidence suggests that HA is the transmitter from these photoreceptors to the I-cells. Although gamma-aminobutyric acid (GABA) is also present in the photoreceptors, it did not affect the I-cell's responses to light or to histamine when bath-applied at 100 microM. Thus, GABA does not appear to modulate transmission from the photoreceptor to the I-cell.

Neurochemical mechanisms of motion sickness

Three kinds of neurotransmitters (histamine, acetylcholine, and catecholamine) are thought to be important in the neural processes of motion sickness because antihistaminics, scopolamine, and amphetamine are effective in preventing motion sickness. In this study, we examined the neurochemical and neuropharmacologic features of motion sickness in rats. Based on our results, we propose the following hypotheses for the neurochemical mechanisms of motion sickness: (1) the histaminergic neuron system is involved in the signs and symptoms of motion sickness, including vomiting; (2) the acetylcholinergic neuron system is involved in the processes of habituation to motion sickness, including neural store mechanisms; and (3) the catecholaminergic neuron system in the brain stem is not related to the development of motion sickness.

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.

Hypothalamic sites of neuronal histamine action on food intake by rats

To identify sites of histaminergic modulation of food intake, histamine H1-receptor antagonist was microinfused into the rat hypothalamus, the ventromedial hypothalamus (VMH), the lateral hypothalamus (LHA), the paraventricular nucleus (PVN), the dorsomedial hypothalamus (DMH), or the preoptic anterior hypothalamus (POAH), during the early light period. Feeding, but not drinking, was elicited in 100% of the rats (P less than 0.01) that were bilaterally microinfused with 26 nmol chlorpheniramine into the VMH. Unilateral infusion into the VMH did not affect food intake at doses of 26 or 52 nmol. Feeding was also induced by bilateral microinfusion into the PVN, but only the 52 nmol dose was effective. Bilateral infusions into the LHA, the DMH or the POAH did not affect ingestive behavior. Feeding induced by an H1-antagonist was completely abolished in all 7 rats tested when endogenous neuronal histamine was decreased by pretreatment with alpha-fluoromethylhistidine (100 mg/kg). The findings suggest that H1-receptors in the VMH and the PVN, but not in the LHA, the DMH or the POAH, may be involved in histaminergic suppression of food intake.

Effects of the histamine H3-agonist (R)-alpha-methylhistamine and the antagonist thioperamide on histamine metabolism in the mouse and rat brain

To study the feedback control by histamine (HA) H3-receptors on the synthesis and release of HA at nerve endings in the brain, the effects of a potent and selective H3-agonist, (R)-alpha-methylhistamine, and an H3-antagonist, thioperamide, on the pargyline-induced accumulation of tele-methylhistamine (t-MH) in the brain of mice and rats were examined in vivo. (R)-alpha-Methylhistamine dihydrochloride (6.3 mg free base/kg, i.p.) and thioperamide (2 mg/kg, i.p.), respectively, significantly decreased and increased the steady-state t-MH level in the mouse brain, whereas these compounds produced no significant changes in the HA level. When administered to mice immediately after pargyline (65 mg/kg, i.p.), (R)-alpha-methylhistamine (3.2 mg/kg, i.p.) inhibited the pargyline-induced increase in the t-MH level almost completely during the first 2 h after treatment. Thioperamide (2 mg/kg, i.p.) enhanced the pargyline-induced t-MH accumulation by approximately 70% 1 and 2 h after treatment. Lower doses of (R)-alpha-methylhistamine (1.3 mg/kg) and thioperamide (1 mg/kg) induced significant changes in the pargyline-induced t-MH accumulation in the mouse brain. In the rat, (R)-alpha-methylhistamine (3.2 mg/kg, i.p.) and thioperamide (2 mg/kg, i.p.) also affected the pargyline-induced t-MH accumulation in eight brain regions and the effects were especially marked in the cerebral cortex and amygdala. These results indicate that these compounds have potent effects on HA turnover in vivo in the brain.

alpha-Fluoromethylhistidine decreases the histamine content of the rat right atrium under the influence of sympathetic activity

alpha-Fluoromethylhistidine (alpha-FMH; 65 mg/kg, i.p.), a specific inhibitor of histidine decarboxylase, significantly decreased the histamine content of the rat right atrium in a time-dependent manner; the maximal decrease of 22.2% was observed 4 h after injection. However, alpha-FMH had no significant effect on the histamine content of the left atrium or the ventricles. The alpha-FMH-induced decrease in the right atrial histamine content was not observed in rats pretreated with 6-hydroxydopamine (25 mg/kg, i.p.). Two i.p. injections of 10 and 5 mg/kg of propranolol and the cardioselective beta 1-adrenoceptor antagonist metoprolol almost completely inhibited the alpha-FMH-induced histamine decrease. On the other hand, phentolamine (10 mg/kg, i.p.) had no influence on the histamine-decreasing action of alpha-FMH. These results suggest that in the rat right atrium there is a histamine pool where a rapid turnover of histamine is maintained by normal sympathetic activity.

Alpha-fluoromethylhistidine in the treatment of idiopathic cold urticaria

Alpha-fluoromethylhistidine (alpha-FMH), a new irreversible inhibitor of mammalian histidine decarboxylase, was tested in the treatment of idiopathic cold urticaria in 11 patients. In the initial trial with 50 mg b.i.d., a significant decrease (about 30%) in the total blood histamine level was found after 3 weeks of treatment but clinically there was no improvement in the symptoms of ten cold urticaria patients nor in the responses to the ice-cube test. In the second trial with three patients suffering from severe idiopathic cold urticaria, a higher dose of up to 500 mg b.i.d. of alpha-FMH for 3 weeks resulted in a marked decrease in the total blood histamine level as well as in an apparent inhibition of histamine synthesis in the skin previously exposed several times to cold water. The symptoms of cold urticaria and the responses in the ice-cube tests also decreased simultaneously. No clinical side effects nor changes in laboratory analysis were seen during the treatment with alpha-FMH. These results suggest that alpha-FMH may be useful in the treatment of severe cold urticaria especially in combination with histamine exhaustion of mast cells using cold water.

Histamine is an intracellular messenger mediating platelet aggregation

Inhibition of human platelet aggregation by N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine-HCl (DPPE), a novel antagonist of histamine binding, suggested that histamine might serve a critical role in cell function. Phorbol-12-myristate-13-acetate (PMA) or collagen was found to increase platelet histamine content in parallel with promotion of aggregation. Inhibitors of histidine decarboxylase (HDC) suppressed both aggregation and the elevation of histamine content, whereas DPPE inhibited aggregation only. In saponin-permeabilized platelets, added histamine reversed the inhibition by DPPE or HDC inhibitors on aggregation induced by PMA or collagen. The results indicate a role for histamine as an intracellular messenger, which in platelets promotes aggregation.

A critical role of the posterior hypothalamus in the mechanisms of wakefulness determined by microinjection of muscimol in freely moving cats

In order to determine critical sites within the hypothalamus responsible for the induction and maintenance of wakefulness (W), we performed microinjections of muscimol, a potent gamma-aminobutyric acid (GABA) agonist, in various lateral hypothalamic regions of freely moving cats. We found that bilateral injections of a small amount of muscimol (0.1-1.0 micrograms/0.5 microliters) in the preoptic and anterior hypothalamus and rostral mesencephalic tegmentum resulted in increased vigilance and insomnia. In contrast, microinjections of muscimol in the middle and anterior parts of the posterior hypothalamus induced long-lasting behavioral and electroencephalographic signs of sleep with short latency. The hypersomnia was characterized by a significant increase in both light and deep slow wave sleep (SWS), and a nearly complete suppression of paradoxical sleep (PS). Animals with muscimol microinjections in the ventrolateral part of the posterior hypothalamus, however, exhibited increased SWS followed by a significant increase in PS. When injected into the posterior hypothalamus of insomniac cats pretreated with p-chlorophenylalanine (PCPA), muscimol induced not only SWS but also PS with short latency. The present data thus support the hypotheses that the posterior hypothalamus plays a critical role in the mechanisms of W and that sleep might result from functional blockade of the hypothalamic waking center.

pros-methylimidazoleacetic acid in rat brain: its regional distribution and relationship to metabolic pathways of histamine

pros-Methylimidazoleacetic acid (p-MIAA; 1-methylimidazole-5-acetic acid), an isomer of the histamine metabolite, tele-methylimidazoleacetic acid (t-MIAA), is present in brain and CSF. Its relationship to histamine synthesis and catabolism was assessed in brains of rats. p-MIAA distribution in brain regions was heterogeneous although the concentrations in regions with the highest (hypothalamus) and the lowest (medulla-pons) levels differed less than four-fold. There was no significant correlation between the regional distributions of p-MIAA with those of histamine or its metabolites. pros-Methylhistidine (1 g/kg, i.p.) produced a 20-fold increase in mean levels of p-MIAA and up to a 50-fold increase in levels of pros-methylhistamine (p-MH), a putative intermediate; levels of histamine and its metabolites were unaltered. L-Histidine (1 g/kg, i.p.) or alpha-fluoromethylhistidine (100 mg/kg, i.p.), the irreversible inhibitor of histamine synthesis, did not alter the levels of p-MIAA in brain. Like the levels of t-MIAA, the levels of p-MIAA were unaltered after probenecid administration. Contrary to its effects in lowering t-MIAA levels, pargyline (75 mg/kg, i.p.) produced a slight rise in levels of p-MIAA in all regions. These findings suggest that, in brain, the metabolic pathways of histamine are independent of pathways that generate p-MIAA. Further, since brain is capable of p-MH formation, its use as an internal standard in analytical methods merits caution.

Involvement of central histaminergic and cholinergic systems in the morphine-induced increase in blood-brain barrier permeability to sodium fluorescein in mice

Morphine (5 mg/kg, s.c.) caused a submaximal increase in the brain level of sodium fluorescein administered i.v. Histamine H1-antagonists, diphenhydramine and mepyramine, given either i.p. or i.c.v., had no significant influence on the effect of morphine. H2-Antagonists, cimetidine and ranitidine, administered i.c.v., but not i.p., significantly inhibited the morphine effect. alpha-Fluoromethylhistidine, a specific histidine decarboxylase inhibitor (given i.p. and i.c.v.) and antimuscarinic drugs, atropine and biperiden, but not methylatropine (given i.p.) also significantly reduced the morphine effect. Physostigimine (i.p.) significantly enhanced the effects of 0.5 and 1 mg/kg of morphine. Similar effects of histaminergic and cholinergic drugs were also observed on the buprenorphine- and DAGO-induced increase in blood-brain barrier (BBB) permeability to sodium fluorescein. None of the treatments with 6-hydroxydopamine, alpha-methyltyrosine, 5,7-dihydroxytryptamine or p-chlorophenylamine had any significant effect on the morphine-induced increase in BBB permeability. These findings suggest that the activation of brain H2-receptors by neuronal histamine and muscarinic receptors by acetylcholine is involved in the increase in BBB permeability to sodium fluorescein caused by mu opioid receptor agonists.

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.

Long-term depletion of histamine in guinea-pigs by administration of alpha-fluoromethylhistidine, a specific inhibitor of histidine decarboxylase; effect on the sensitivity of histamine receptors

The effect of intraperitoneal administration of alpha-fluoromethylhistidine (alpha-FMH), a specific inhibitor of histidine decarboxylase, at a dose of 100 mg/kg twice a day for 6 weeks on the sensitivity of histamine receptors in the guinea-pigs was examined. The histamine contents in the ileum, heart and brain after the treatment decreased to about 54%, 83% and 9-62% (depending on the region), respectively, of those of controls. However, there were no significant difference in the histamine-induced contraction of the ileum and atrium, the maximal binding (Bmax) and the dissociation constant (Kd) of [3H]-mepyramine binding to membrane fractions of the ileum and brain (except the cortex) and the increase of cyclic AMP formation induced by histamine in membranes of the heart and brain (except the cortex) between alpha-FMH-treated- and untreated animals. These results suggest that long-term histamine depletion dose not significantly affect the sensitivity of histamine receptors of guinea-pig except in the brain cortex.

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.

Feeding induced by blockade of histamine H1-receptor in rat brain

Histamine antagonists were infused into the third ventricle of the cerebrum in rats. All the H1-, but none of the H2-antagonists tested, induced initial feeding during the early portion of the light phase when histamine level was highest. No periprandial drinking was observed. Ambulation increased during feeding. The effect on feeding was attenuated when brain histamine was normally low during the early portion of the dark phase, or was decreased by alpha-fluoromethylhistidine. Hypothalamic neuronal histamine may suppress food intake through H1-receptors, and diurnal fluctuations of food intake may mirror neuronal histamine levels.

Blockade of the histamine H1-receptor in the rat ventromedial hypothalamus and feeding elicitation

All H1-, but no H2-antagonists infused into the rat third cerebroventricle, induced feeding during the early light, but not during the early dark, reflecting a concentration of hypothalamic histamine. Bilateral microinfusion identified the ventromedial hypothalamus (VMH), but not the lateral hypothalamus or the paraventricular nucleus, as a main locus for the induction of feeding by an H1-antagonist. The effect was completely abolished when brain histamine was decreased by pretreatment with alpha-fluoromethylhistidine. Hypothalamic neuronal histamine suppresses food intake, at least in part, through H1-receptors in the VMH.

Evidence for histaminergic arousal mechanisms in the hypothalamus of cat

Polygraphic 23-hr recordings were carried out in 25 adult cats in order to examine the effects of both systemic and local injections of various histaminergic and antihistaminergic drugs on sleep-waking cycles. alpha-Fluoromethylhistidine (alpha-FMH), a specific inhibitor of histidine decarboxylase, when injected intraperitoneally at a dose of 20 mg/kg, induced a significant increase in deep slow wave sleep (S2) and a decrease in wakefulness (W), without modifying light slow wave sleep (S1) and paradoxical sleep (PS). Intraperitoneal injections of mepyramine (1 mg and 5 mg/kg), a well-known histamine H1-receptor antagonist, increased deep slow wave sleep and decreased wakefulness, as well as paradoxical sleep. Bilateral injections of alpha-FMH (50 micrograms/1 microliter) into the ventrolateral posterior hypothalamus, where histamine immunoreactive neurones have been recently identified, resulted in a significant decrease in wakefulness and increase in deep slow wave sleep. Similarly, injections of mepyramine (120 micrograms/1 microliter) in the same structures caused a significant decrease in wakefulness and an increase in deep slow wave and paradoxical sleep as well. In contrast, local injections of SKF-91488 (50 micrograms/1 microliter), a specific inhibitor of histamine-N-methyltransferase, led to a significant increase in wakefulness and decrease in both slow wave sleep (SWS) and paradoxical sleep. Injections of histamine, at doses of 5, 30 and 60 micrograms/1 microliter, also increased wakefulness and decreased slow wave sleep dose dependently, while these effects were completely blocked by pretreatment with mepyramine. The results suggest that histaminergic systems in the hypothalamus play an important role in arousal mechanisms and their actions are mediated through H1-receptors.

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.

The potential use of mechanism-based enzyme inactivators in medicine

Mechanism-based enzyme inactivator, alanine racemase, S-adenosylhomocysteine hydrolase, D-amino acid aminotransferase, gamma-aminobutyric acid aminotransferase, arginine decarboxylase, aromatase, L-aromatic amino acid decarboxylase, dihydrofolate reductase, dihydroorotate dehydrogenase DNA polymerase I, dopamine beta-hydroxylase, histidine decarboxylase, beta-lactamase, monoamine oxidase, ornithine decarboxylase, serine proteases, testosterone 5 alpha-reductase, thymidylate synthetase, xanthine oxidase.

Modulation of neuronal histamine in control of food intake

Neuronal histamine affects physiological functions of the hypothalamus. To investigate involvement of histamine receptors in feeding, histamine antagonists were infused into the rat third cerebroventricle. All H1- but no H2-antagonists tested, induced transient feeding during the early light when concentration of hypothalamic histamine was highest. No periprandial drinking was observed. Ambulation concomitantly increased during feeding. The effect on feeding was attenuated when brain histamine was normally low during the early dark or was decreased by alpha-fluoromethylhistidine (alpha-FMH). Bilateral microinjection indicated that the ventromedial hypothalamus, but not the lateral hypothalamus or the paraventricular nucleus, was a main locus for the induction of feeding by an H1-antagonist. The effect was completely abolished when brain histamine was decreased by pretreatment with alpha-FMH. Hypothalamic neuronal histamine suppresses food intake, at least in part, through H1-receptors in the VMH, and diurnal fluctuations of food intake may mirror neuronal histamine level.

Histidine decarboxylase from rat mast cells. Enhanced recovery in cell-free extracts and isotopic labelling

A procedure for obtaining rat mast-cell histidine decarboxylase in greater than 50% yield in cell-free extracts was developed. The enzyme was found in the supernatant fractions from a 3,500 g and a 105,000 g centrifugation step and was demonstrated to be sensitive to inhibition by alpha-fluoromethylhistidine but not by phenylalanine. Although the enzyme shows a half-life of only 3 h in cell-free extract, the initial high recovery of activity allowed for active-site labelling of the enzyme by [3H]histidine and NaBH4. Labelled protein migrated on non-denaturing polyacrylamide-gradient-gel electrophoresis as a 55,000 Da species.

Transition-state analogues as inhibitors of L-dopa decarboxylase

1. A series of compounds has been prepared which are analogues of the transition state of the reaction catalysed by L-dopa decarboxylase (EC 4.1.1.28). 2. These compounds are reduced adducts of the substrate (L-dopa) and coenzyme (pyridoxal phosphate), as well as analogues of these substances (D-dopa, pyridoxal and salicaldehyde). 3. Compounds were also prepared with an oxazine link between the 3'-oxygen and the nitrogen attached to the 4'-carbon of the aldehyde moiety. 4. None of the D-dopa adducts produced any significant inhibition, but the L-dopa adducts were all active at millimolar levels, with the oxazine derivatives being more active than their parent compounds. 5. Inhibition was competitive with respect to L-dopa, but was neither competitive nor non-competitive with respect to pyridoxal phosphate. 6. The most active compound tested was the oxazine derivative of the L-dopa/salicaldehyde adduct, with an estimated Ki of 58.0 microM. 7. Increased inhibitory activity was observed when enzyme depleted of pyridoxal phosphate was used.

The effect of a histamine synthesis inhibitor on the immediate nasal allergic reaction

We studied the effect of alpha-fluoromethyl histidine, an irreversible histamine synthesis inhibitor, on the immediate nasal reaction to antigen challenge in a double-blind, placebo controlled, randomized, parallel study using 13 subjects. The patients received either active drug 100 mg twice daily or placebo, for 3 weeks. A nasal allergen challenge was performed before and after at weekly intervals. Symptoms at challenge were assessed and the levels of histamine, TAME-esterase activity and kinins were measured in nasal lavages before and after antigen challenge. Skin tests were also performed at weekly intervals. In addition, the urinary excretion of the main histamine metabolite, telemethylimidazole acetic acid, was measured before and after 3 weeks of treatment. The active treatment induced 60% reduction in histamine levels in the lavage fluids before and after antigen challenge, as well as a reduction in the histamine levels in the lavage fluids before and after antigen challenge, as well as a reduction in the main urinary histamine metabolite. However, no reduction was found in nasal symptoms obtained after antigen challenge. The levels of kinins and TAME-esterase activity were not significantly reduced.

Effects of pentagastrin and carbachol on the gastric histamine level in alpha-fluoromethylhistidine-treated mice and rats

The gastric mucosal histamine level in mice increased by about 80% and 100% after fasting for 24 and 48 h, respectively. In non-fasted mice, alpha-fluoromethylhistidine (alpha-FMH), a specific histidine decarboxylase inhibitor, significantly decreased the histamine level, the reduction amounting to 35% and 49%, 2 h and 4 h after treatment, respectively. In mice fasted for 24 h, a significant decrease of 42% was observed 4 h after treatment. However, in mice fasted for 48 h, no significant decrease was seen even 4 h after alpha-FMH treatment. Therefore, the histamine-releasing effect of re-feeding and drugs on the gastric mucosa was examined in vivo, using animals fasted for 48 h and subsequently treated with alpha-FMH. Food given simultaneously with alpha-FMH to 48-h fasted mice significantly decreased the histamine level 4 h later. Pentagastrin and carbachol administered alone (0.25-2.0 mg/kg, i.p.) had no significant effect on the histamine level. However, the combined treatment with these drugs significantly decreased the histamine level. In rats fasted for 48 h and treated with alpha-FMH, pentagastrin (0.25 and 0.5 mg/kg, i.p.) but not carbachol (0.125-0.5 mg/kg, i.p.) caused a significant decrease in the mucosal histamine level. In contrast to mice, the effect of the combined treatment with pentagastrin and carbachol was not synergistic in rats. These findings suggest that gastrin acts synergistically with acetylcholine in the histamine release from the gastric mucosa in mice, whereas such synergism may not occur in rats.

Feeding-related circadian variation in tele-methylhistamine levels of mouse and rat brains

Circadian changes in the brain histamine (HA) and tele-methylhistamine (t-MH) levels were studied in mice and rats after adaptation to an alternating 12-h light/dark cycle (lights on at 0600). Although there was no significant circadian fluctuation of the brain HA levels, the levels of t-MH, a major metabolite of brain HA, showed a marked circadian variation. In mice, the t-MH levels were about 80 ng/g from 1200 to 1800 but about two times higher values were obtained from 2400 to 0600 of the next morning. In rats, the t-MH levels ranged from 24 to 28 ng/g at 0600 and 1200, slightly increased at 1800, and reached at 2400 a peak twice as high as the levels seen during the light period. The t-MH levels again rapidly decreased during the subsequent 3 h. In mice fasted from 1200, the t-MH levels did not increase during the period of darkness. When mice were fed at 1200 after a 24-h fast, a significant increase in the t-MH levels was observed at 1800. There was no significant circadian variation of the HA and t-MH levels in the plasma of mice and rats. These results suggest that circadian variation in brain t-MH levels is related to feeding and possible subsequent changes in elimination of t-MH from the brain and/or turnover of HA in the brain. This phenomenon should be given due attention when HA dynamics in the brain are being assessed.

Effects of halothane, enflurane and pentobarbital on brain histamine dynamics in mice

The effects of halothane, enflurane, ketamine and pentobarbital on brain histamine dynamics were examined in mice. Brain histamine and tele-methylhistamine, a predominant metabolite of brain histamine, were simultaneously measured by high-performance liquid chromatography with fluorescence detection. Anaesthesia with the four agents had no effect on brain histamine content. The tele-methylhistamine content significantly increased during 1 h and 2 h anaesthesia with halothane (0.051 mmol/l or 0.76 mol/l) and 2 h anaesthesia with enflurane (0.11 mol/l or 0.16 mol/l). Enflurane and pentobarbital significantly inhibited the histamine depletion induced by alpha-fluoromethylhistidine (50 mg/kg, intraperitoneally), a specific inhibitor of histidine decarboxylase, suggesting that these agents decrease the histamine turnover. However, halothane and ketamine were ineffective in this respect. These results emphasize that various anaesthetics have different influences on brain histamine dynamics. Since there have been findings suggesting that brain histaminergic systems are involved in physiological functions such as regulation of blood pressure, body temperature and hormone secretion, changes in the brain histamine turnover should be given due attention with regard to physiological changes during anaesthesia.

Chiral separation of enantiomers of substituted alpha- and beta-alanine and gamma-aminobutyric acid analogues by gas chromatography and high-performance liquid chromatography

Gas chromatography (GC) with a chiral stationary phase, Chirasil-Val, has been used for separation of the enantiomers of several analogues of alpha- and beta-alanine as their N-trifluoroacetyl isopropyl esters. The same chiral phase GC procedure has been applied to the enantiomeric separation of various substituted gamma-aminobutyric acid analogues (GABA). Reversed-phase high-performance liquid chromatography (HPLC) with the chiral copper-L-proline complex allowed a clear resolution of all the alpha-amino acids in their underivatized forms. It yielded somewhat smaller separation coefficients for the substituted beta-alanines and no resolution for the GABA analogues. The influence of the nature of the amino acid, alpha, beta or gamma, and the effects of the different substituents on the separation coefficients obtained by GC and HPLC are discussed.

Effect of monofluoromethyldopa (MFMD) on trace amine levels

The concentrations of the trace amines, m-tyramine, p-tyramine, phenylethylamine and tryptamine, were measured in the striatum of the brain and in the kidney of adult rats treated with alpha-monofluoromethyldopa (MFMD), an inhibitor of aromatic amino acid decarboxylase. While MFMD decreased the levels of all four amines in the kidney, only phenylethylamine and tryptamine levels were decreased in the striatum compared to control. Striatal p-tyramine levels were not affected, while striatal m-tyramine levels were increased by MFMD. When the rats were injected with a monoamine oxidase (MAO) inhibitor before MFMD administration, similar changes in striatal and kidney trace amine levels were observed compared to MFMD alone.

Suppression and potentiation of 5-hydroxytryptophan-induced hypoglycaemia by alpha-monofluoromethyldopa: correlation with the accumulation of 5-hydroxytryptamine in the liver

Experiments were done to examine whether the accumulation of 5-hydroxytryptamine (5-HT) in the liver is responsible for the hypoglycaemia induced in mice by 5-hydroxytryptophan (5-HTP) and lipopolysaccharides (LPS). (+/-)-alpha-Monofluoromethyldopa (FMD), a potent irreversible inhibitor of aromatic amino acid decarboxylase, suppressed the 5-HTP-induced accumulation of 5-HT in the liver at a dose of 2 mg kg-1 or more, but potentiated the accumulation at lower dose of 0.4 mg kg-1. Corresponding to these effects, the hypoglycaemic response was prevented by the higher doses of FMD and potentiated by the lower dose. These contrasting effects of FMD were explicable by the amounts of 5-HTP entering the liver. In contrast, FMD did not prevent either the hypoglycaemia or the accumulation of 5-HT in the liver induced by LPS. These results further support the hypothesis that the accumulation of 5-HT in the liver is causally related to the hypoglycaemia induced by 5-HTP and indicate that the LPS-induced 5-HT accumulation in the liver is not derived from stimulation of 5-HT synthesis. It is still not clear whether the accumulation of 5-HT in the liver is involved in the hypoglycaemic response to LPS.

The role of cytoplasmic (newly synthesized) dopamine for the spontaneous and electrically evoked release of dopamine and its metabolites from the isolated neurointermediate lobe of the rat pituitary gland in vitro

Isolated rat NILs were incubated in Krebs-HEPES solution. The release of dopamine and its metabolites (DOPAC, HVA and MOPET) was determined by HPLC with electrochemical detection. The spontaneous release of the sum of metabolites was about 40 times that of dopamine. The spontaneous outflow of dopamine metabolites was unaffected after inhibition of dopamine uptake (by GBR 12921) or after pretreatment with reserpine (5 mg/kg, 12 h before the experiments), but it was reduced by 50% after preincubation with the irreversible DOPA decarboxylase inhibitor, (MFMD, 10 microM, for 10 min). The combination of pretreatment with reserpine and preincubation with MFMD resulted in an 80% inhibition of the spontaneous outflow of dopamine metabolites. Treatment with reserpine caused a 98% depletion of the dopamine tissue content, whereas 60 min after exposure to MFMD the dopamine tissue content was decreased by 40%. Electrical stimulation of the pituitary stalk (3-15 Hz, in the presence of GBR 12921) caused a frequency-dependent release of dopamine. Stimulation at 7 or 15 Hz caused also a significant release of dopamine metabolites. After pretreatment with reserpine, the release of dopamine evoked by stimulation at 15 Hz was abolished, whereas the evoked release of the metabolites was only reduced by about 55%. After MFMD, the evoked release of dopamine decreased by a percentage similar to that of dopamine tissue content, but the reduction of the evoked release of metabolites was more pronounced. In conclusion, the spontaneous release of dopamine metabolites from the dopaminergic nerve endings in the NIL largely reflects the catabolism of newly synthesized dopamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of opioid and non-opioid mechanisms in footshock-induced enhancement of brain histamine turnover in mice

Naloxone (1-10 mg/kg, i.p.) dose-dependently inhibited the footshock-induced elevation in levels of tele-methylhistamine (t-MH), a predominant metabolite of brain histamine (HA), although this compound had no effect on the HA dynamics in the non-shocked control mice. Footshock significantly enhanced the HA depletion induced by alpha-fluoromethylhistidine, a specific inhibitor of histidine decarboxylase. However, in mice treated with naloxone (5 mg/kg, i.p.) footshock did not significantly facilitate the alpha-fluoromethylhistidine-induced HA depletion. In mice which had been rendered morphine-tolerant following an s.c. implantation of a pellet containing 50 mg of morphine base 3 days before, footshock produced no significant elevation of the t-MH level. The treatment with alpha-methyl-p-tyrosine, p-chlorophenylalanine or atropine had no significant influence on the footshock-induced t-MH elevation. The t-MH elevation was the most marked in the midbrain. In the hypothalamus and pons-medulla oblongata, no significant change in the t-MH level was produced by footshock. These results suggest that footshock increases the HAergic activity in the mouse brain partly through activation of opioid-related mechanisms and that alterations in HA dynamics differ with region of the brain.