Selective inhibitors of biosynthesis of aminergic neurotransmitters

Effects of GABA-mimetic drugs on turnover of histamine in the mouse brain

The effect of GABA-mimetics on histaminergic activity in the mouse brain was investigated. Systemic administration of muscimol (2 and 5 mg/kg), THIP (5-15 mg/kg) and aminooxyacetic acid (AOAA) (25 mg/kg) but not baclofen (2.5-15 mg/kg) inhibited the pargyline (65 mg/kg)-induced accumulation of tele-methylhistamine (t-MH). There was no regional difference in the inhibitory effect of muscimol on the t-MH accumulation by pargyline treatment. Muscimol and AOAA also inhibited decrease in the histamine (HA) level induced by alpha-fluoromethylhistidine (50 mg/kg), a specific inhibitor of histidine decarboxylase. These results suggest that these GABA-mimetics reduce the HA turnover in the mouse brain.

Histaminergic mechanism of motion sickness. Neurochemical and neuropharmacological studies in rats

Rats were rotated about two parallel axes to produce motion sickness, and the histamine levels in their hypothalamus and pons-medulla oblongata were measured. Rotation for 60 min induced kaolin intake, which is a behavioral index of motion sickness in rats, and increased the histamine levels in the hypothalamus and pons-medulla oblongata, the highest levels occurring after 15 min of rotation. In bilaterally labyrinthectomized rats, no rise in histamine level was observed. alpha-Fluoromethylhistidine (alpha-FMH), which depletes the neural component of histamine in the brain, suppressed kaolin intake induced by rotation. These findings demonstrated that the histaminergic neuron system contributes to development of motion sickness, and suggested that alpha-FMH may be effective as an anti-motion sickness drug.

Drug-induced changes in histamine and tele-methylhistamine levels in mouse peripheral tissues

To clarify the histamine (HA) dynamics in peripheral tissues, effects of drugs on the tissue HA and tele-methylhistamine (t-MH) levels were studied in mice. alpha-Fluoromethylhistidine (50 mg/kg, i.p.) significantly decreased the HA level in the stomach, but not in the liver, heart, ileum, submandibular gland and skin of mice. This compound had no significant effect on the t-MH level in any tissue examined. In non-fasted and 24-hr fasted animals, the t-MH level in the liver, heart and ileum was significantly increased by treatment with aminoguanidine (10 mg/kg, i.p.) plus pargyline (65 mg/kg, i.p.). However, in mice fasted for 48 hr, this treatment was ineffective in increasing the t-MH level in the heart and ileum, suggesting that the t-MH level in some peripheral tissues is under the influence of the food intake. Even if HA is synthetized and then metabolized in the peripheral tissues, the size of the HA pool with a rapid turnover in each tissue except for the gastric tissue seems to be very small.

Comparison of effects of phencyclidine and methamphetamine on body temperature in mice: a possible role for histamine neurons in thermoregulation

We compared the effects of phencyclidine (PCP) and methamphetamine on body temperature and brain histamine turnover in mice. Methamphetamine at 5 and 10 mg/kg produced dose-related increases in rectal temperature, whereas PCP given in the same doses had no significant effect. In mice pretreated with alpha-fluoromethylhistidine, an inhibitor of histidine decarboxylase, PCP produced a marked hyperthermia. PCP markedly accelerated brain histamine turnover, as measured by the accumulation of tele-methylhistamine, a predominant metabolite of brain histamine, following administration of pargyline. Methamphetamine had no significant effect on the histamine dynamics. These results suggest involvement of brain histaminergic neurons in the action of PCP but not methamphetamine, and the presence of a histaminergic thermoregulatory mechanism.

Brain histamine turnover enhanced by footshock

When footshock was given to mice at 15-s intervals for 30-120 min, there was a significant increase in the brain level of tele-methyl-histamine (t-MH), a predominant metabolite of brain histamine (HA). This footshock-induced elevation of the t-MH level also occurred in mice pretreated with pargyline but not in mice pretreated with metoprine. The footshock facilitated the HA depletion induced by a-fluoromethylhistidine. These results suggest that footshock increases the brain HA turnover.

Restraint stress stimulation of prolactin and ACTH secretion: role of brain histamine

The possible role of brain histamine in the release of prolactin, ACTH and corticosterone following acute restraint, was pharmacologically evaluated in adult male rats. Fifteen min of restraint caused marked increases in the plasma levels of these hormones. alpha-Fluoromethyl histidine (FH), a histidine decarboxylase inhibitor which depleted hypothalamic histamine, inhibited the enhancement of plasma prolactin levels. In contrast, plasma ACTH levels were not modified. FH treatment decreased plasma corticosterone concentrations in animals submitted to stress or in rest; this suggests a direct action of FH on the adrenal. Intraventricular (IVT) injection of ranitidine (H2 antagonist) blunted the prolactin response to restraint stress whereas its systemic administration had no effect. On the contrary, pyrilamine (H1 antagonist) given systemically decreased slightly, but significantly, the prolactin rise but when injected IVT it was ineffective. Pyrilamine was also unable to affect the ranitidine action. ACTH and corticosterone levels in plasma of restrained rats were not modified by the histamine antagonists. It is concluded that histamine is involved, mainly through central H2 receptors, in the enhancement of plasma prolactin levels produced by an acute stress. The failure of both antihistaminic compounds and a histamine depletor to alter the ACTH stimulation suggest that histamine has no participation in the hypophysio-corticoadrenal response to acute restraint.

Histamine in dorsal and ventral hippocampus. II. Effects of H1 and H2 histamine antagonists on exploratory behavior in male rats

The effects on Hole-Board behavior of histamine (HA) microinjected into different parts of the hippocampus and the effects of pyrilamine (PYR, an H1-histamine antagonist), ranitidine (RAN, an H2-histamine antagonist) or alpha-fluoromethyl-histidine (alpha-FMH, an irreversible inhibitor of the HA synthetizing enzyme) injected into the hippocampus on behavior were studied. Forty five nMol of HA were injected stereotaxically into the dorsal or ventral hippocampus. Five min later, Hole-board behavior was measured. It was observed that HA inhibited locomotion and rearing only in the rats injected into the ventral hippocampus. In other experiments, animals were microinjected into the ventral hippocampus with 135 nMol of PYR or RAN in 1 microliter of saline solution. Ten min later, they were microinjected with 45 nMol of HA. Hole-board exploratory activity was measured 5 min thereafter. Results showed that both PYR and RAN were effective in counteracting the inhibitory effect of HA on locomotor activity, but only RAN was able to block the inhibitory action of HA on rearing behavior. Head-dipping frequency was not affected by these treatments. In rats microinjected with 20 nMol of alpha-FMH, increased scores of locomotion were observed but the other behaviors (head-dipping frequency, grooming and rearing) were not affected. The present results support the hypothesis that HA in hippocampus may be exerting a regulatory role on behavior by interaction with H1 and H2 receptors.

Changes in histamine metabolism in the mouse hypothalamus induced by acute administration of ethanol

The effect of acute ethanol administration on histamine (HA) dynamics was examined in the mouse hypothalamus. The steady-state level of HA did not change after intraperitoneal administration of ethanol (0.5-5 g/kg), whereas the level of tele-methylhistamine (t-MH), a predominant metabolite of brain HA, increased when 3 and 5 g/kg of ethanol was given. Pargyline hydrochloride (80 mg/kg, i.p.) increased the level of t-MH by 72.2% 90 min after the treatment. Ethanol at any dose given did not significantly affect the t-MH level in the pargyline-pretreated mice. Decrease in the t-MH level induced by metoprine (10 mg/kg, i.p.), an inhibitor of HA-N-methyltransferase, was suppressed by ethanol (5 g/kg), thereby suggesting inhibition of the elimination of brain t-MH. Ethanol (5 g/kg) significantly delayed the depletion of HA induced by (S)-alpha-fluoromethylhistidine (50 mg/kg, i.v.), a specific inhibitor of histidine decarboxylase. Therefore, a large dose of ethanol apparently decreases HA turnover in the mouse hypothalamus.

Inhibition of monoamine oxidase selectively in brain monoamine nerves using the bioprecursor (E)-beta-fluoromethylene-m-tyrosine (MDL 72394), a substrate for aromatic L-amino acid decarboxylase

(E)-beta-Fluoromethylene-m-tyrosine (FMMT) is a dual-enzyme-activated inhibitor of monoamine oxidase (MAO). The compound is not an inhibitor per se but is decarboxylated by aromatic L-amino acid decarboxylase (AADC) to yield a potent enzyme-activated irreversible inhibitor of MAO, (E)-beta-fluoromethylene-m-tyramine, which shows some selectivity for inhibition of MAO type A. Decarboxylation of FMMT was demonstrated in vitro using hog kidney AADC and in vivo in rats by the ability of alpha-monofluoromethyldopa (MFMD), a potent inhibitor of AADC, to prevent MAO inhibition produced by FMMT. In isolated synaptosomes, FMMT was decarboxylated by AADC, and, furthermore, the compound was actively transported into these isolated nerve endings. An active transport into the CNS has also been demonstrated in vivo by performing competition experiments with leucine. To demonstrate that FMMT is preferentially decarboxylated within monoamine nerves of the CNS, the nigrostriatal 3,4-dihydroxyphenylethylamine (dopamine) pathway of rats was unilaterally lesioned with 6-hydroxydopamine or infused with MFMD. Under these conditions, MAO inhibition produced by orally administered FMMT in the striatum ipsilateral to the lesion or infusion was markedly attenuated. Combination of FMMT with an inhibitor of extracerebral AADC, such as carbidopa, protected peripheral organs against the MAO inhibitory effects and concomitantly enhanced MAO inhibition in the CNS. Such combinations had a greatly reduced propensity to augment the cardiovascular effects of intraduodenally administered tyramine, when compared with FMMT given alone or with clorgyline, a selective inhibitor of MAO type A. The results obtained with FMMT suggest the possibility of achieving selective inhibition of MAO within monoamine nerves of the CNS and, further, suggest that combination of FMMT with an inhibitor of extracerebral AADC will reduce the propensity of this inhibitor to produce adverse interactions with tyramine.

A direct, sensitive microassay for mammalian histidine decarboxylase

A microassay procedure for mammalian histidine decarboxylase based on the conversion of L-[3H]histidine to [3H]histamine, which were separated by an alkaline butanol extraction followed by thin-layer chromatography, is described. This assay is direct and simple to perform, in addition to being very sensitive and reproducible. It is useful for tissues containing high levels of endogenous histamine, because only newly formed radiolabeled histamine is measured. This report includes information on histidine decarboxylase activity at various pH levels, in different buffers, and in the presence of selected histamine active drugs. In addition, it describes histidine decarboxylase activity in several fetal rat tissues.

Effects of alpha-fluoromethylhistidine on increase in histidine decarboxylase activity of maternal mouse kidney observed during late pregnancy and evidence for its non-mast cell origin by using estrogen and W/WV mice

The increase of histidine decarboxylase (HDC) activity during late pregnancy in the whole bodies of fetal mice and the kidneys of their mothers were almost completely inhibited by i.p. administration of 25 mg/kg of alpha-fluoromethylhistidine (alpha-FMH), a suicide inhibitor of HDC, starting on day 13 of pregnancy. The increase of HDC in fetal mice was previously shown to be in mast cells [T. Watanabe et al., Proc. Natl. Acad. Sci. U.S.A. 78, 4209-4212 (1981)]. The increase of HDC in maternal kidneys was examined by using estrogen and W/WV mice, which were devoid of mast cells and infertile. Treatment of castrated mice with 17-beta-estradiol increased the HDC activity of the kidney, and this increase was antagonized by concomitant treatment with clomiphene, an antiestrogen, confirming that the increase is mediated through an estrogen receptor. HDC activity in the kidney of W/WV mice was also increased by estradiol treatment, indicating that HDC activity was associated with non-mast cells.

Effects of various compounds on histidine decarboxylase activity: active site mapping

The effect of about one hundred compounds on the activity of histidine decarboxylase partially purified from whole bodies of fetal rats was determined. Most of them at their 10 mM concentration had little effect on the enzyme activity; but 12 compounds inhibited the enzyme to a greater extent than 30%. Among these, except for alpha-methylhistidine that has been known to be a strong and specific inhibitor, DOPA, homocysteine, cysteine, methionine and urocanic acid were the best inhibitors; beta-phenyllactic acid, phenylpyruvic acid and carnosine were less strong inhibitors; valine, oxaloacetic acid and N tau-methylimidazole acetic acid were weak inhibitors. Histamine had no inhibitory action. Thus, the substrate binding site of histidine decarboxylase is very rigid and specific for L-histidine.

In vivo and in vitro inhibition of human histidine decarboxylase by (S)-alpha-fluoromethylhistidine

Histidine decarboxylase (HDC) activity in Ficoll-Hypaque purified human peripheral blood leukocytes (PBL) was determined by measuring the formation of [3H]histamine from L-[3H]histidine. HDC activity was inhibited in vitro to more than 90% by (S)-alpha-fluoromethylhistidine (alpha-FMH) at concentrations of 10(-5) M and above. Both polymorphonuclear and mononuclear cells possessed HDC activity, but on a per cell basis the former had several-fold higher enzyme activity than the latter. In safety and tolerability studies, alpha-FMH was administered orally to healthy human subjects twice daily for 7 days at doses of 2.5, 10, 50 and 100 mg per person. A dose-dependent inhibition of HDC activity was observed in PBL that were isolated both at 12 hr after administration of the first dose of alpha-FMH and after treatment for 1 week. At the 50 and 100 mg doses of alpha-FMH, there was complete inhibition of HDC activity and partial inhibition at the 10 mg dose. Twenty-four hours after the last dose, HDC activity had recovered to 64-100%, 44-46%, and 30-52% of control values in subjects that received 10, 50 and 100 mg alpha-FMH respectively.

Decrease in histamine turnover in the brain of spontaneously hypertensive rats

Histamine (HA) turnover rate in the brain of spontaneously hypertensive rats (SHR) was determined by the accumulation of telemethylhistamine after pargyline treatment. The values in these SHR were lower than in the Wistar Kyoto rats, particularly in the hypothalamus and brainstem. However, chronic treatment with L-histidine had no effect on the development of hypertension in the SHR. The functional significance of the decreased HA turnover in SHR is discussed in relation to the pathogenesis of hypertension.

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).

Bioanalysis and disposition of alpha-fluoromethylhistidine, a new histidine decarboxylase inhibitor

A sensitive, selective, and rapid high-performance liquid chromatographic procedure was developed for the determination of alpha-fluoromethylhistidine (alpha-FMH) in human biological samples. The plasma assay required isolation of the drug using a weak cation-exchange resin prior to HPLC analysis with UV detection. The urine assay employed postcolumn derivatization with o-phthalaldehyde (without a thiol) and fluorescence detection. The extent of metabolism of alpha-FMH in humans was studied in four healthy volunteers using tritium-labeled material. No significant differences in the plasma and urine concentrations of radioactivity and unchanged drug were detected. In addition, the radiochromatograms of selected urine samples revealed a single peak with a retention time corresponding to the unchanged drug. The evidence presented suggests negligible biotransformation of alpha-FMH in humans.

Properties of N-acetylhistamine deacetylase in mammalian brain

Properties of N-acetylhistamine deacetylase in rat brain were studied, utilizing a sensitive coupled radioenzymatic assay. The Km for N-acetylhistamine for this deacetylase was 660 microM and its Vmax was 330 pmol/h/mg protein. N-Acetylhistamine deacetylase activity increased 80% in the presence of 1 mM Mn2+. The Km of Mn2+ was 40 microM. The enzyme is primarily a soluble enzyme with a relatively uniform regional distribution, unlike the distribution for histamine and histidine decarboxylase. Neonatal activity of this enzyme in rat brain is higher than in adult brain. alpha-Fluoromethylhistidine does not affect the activity of N-acetylhistamine, indicating that deacetylation probably does not play a regulatory role in the synthesis of brain histamine.

Catalytic irreversible inhibition of Trypanosoma brucei brucei ornithine decarboxylase by substrate and product analogs and their effects on murine trypanosomiasis

Ornithine decarboxylase from Trypanosoma brucei brucei was inhibited by several substrate (ornithine) and product (putrescine) analogs both in vitro and in vivo. Since alpha-difluoromethylornithine is effective for the treatment of experimental and clinical African trypanosomiasis, it was possible that the more potent ornithine and putrescine analogs might be more active in treating the disease. However, only alpha-monofluoromethyldehydroornithine methyl ester was more potent than alpha-difluromethylornithine against mouse trypanosomiasis and warrants further study in model infections.

Effect on mast cell histamine of inhibiting histamine formation in vivo with alpha-fluoromethylhistidine

An irreversible inhibitor of histidine decarboxylase, alpha-fluoromethylhistidine (FMH), was used to inhibit histamine formation by mast cells in vivo. Even at doses of FMH sufficient to reduce histamine formation more than 95%, the ability of mast cells to synthesize histamine recovered rapidly. It was possible, however, to sustain levels of histamine-forming activity below 10% of normal with continuous administration of FMH from subcutaneously implanted osmotic pumps. Administration of FMH under these conditions did not deplete significantly mast cell histamine but did prevent the increase in total mast cell histamine that occurs over 14 days and also prevented the reconstitution of mast cell histamine stores after depletion by treatment with polymyxin B.

Treatment of two mastocytosis patients with a histidine decarboxylase inhibitor

Alpha-fluoromethylhistidine, alpha-FMH (L-641.575, Merck, Sharp and Dohme), a specific irreversible inhibitor of the mammalian histidine decarboxylase, was investigated for its effect on possible histamine symptoms in mastocytosis. Two patients were treated for 28 and 22 weeks respectively. The first patient had systemic mastocytosis and a severe malabsorption causing weight loss, excessive fecal fat losses and electrolyte disturbances, the main symptom being frequent diarrhoea. The second patient had mainly skin manifestations diagnosed as urticaria pigmentosa and the main symptom was pruritus. There were no side effects of the drug. The first patient produced formed stools after one week of treatment, concomitant with a decrease in plasma histamine and in urinary excretion of the main histamine metabolite tele-methylimidazoleacetic acid (MeImAA). In the second patient definite benefit of the treatment was observed after two months. However, changes in histamine parameters occurred earlier. Plasma histamine and urinary MeImAA were reduced after one week and in two or three weeks reached a steady-state level of about 25% of pretreatment values. The results indicate an effective inhibition of histamine synthesis in both patients but only the diarrhoea seemed to be causally related to the change in histamine metabolism.

Effects of alpha-fluoromethylhistidine on sleep-waking parameters in rats

Effects of alpha-fluoromethylhistidine (FMH), an irreversible inhibitor of histidine decarboxylase, on the sleep-waking parameters were studied in rats for 24 hours. Intraperitoneal administration of FMH (100 mg/kg) at 11:30 hr resulted in a longer sleep latency compared with the control values. Hour-to-hour analyses revealed that wakefulness (W) time decreased (from 20:00 to 07:00 hr) and slow wave sleep (SS) time increased (from 19:00 to 06:00 hr) in the night. Paradoxical sleep (PS) time did not parallel the SS changes; it was increased significantly from 07:00 to 11:00 hr in the next morning. The influence of FMH seemed to be divided into direct, immediate action (increase of W) and late, prolonged action (decrease of W), and the results obtained support the histamine arousal hypothesis.

Inhibition of histamine synthesis in vitro and in vivo by S-alpha-fluoromethylhistidine

(S)-alpha-Fluoromethylhistidine (alpha-FMH) is a Kcat or "suicide-substrate" inhibitor of partially purified mammalian histidine decarboxylase; i.e. the agent is converted enzymatically to a more active form which effects a time-dependent, irreversible inhibition. Incubation of a alpha-FMH[4-3H] with enzyme and pyridoxal phosphate resulted in an apparently irreversible labeling of protein, with no demonstratable formation of free-amine product, suggesting a very low to non-existent turnover ratio. alpha-FMH was accumulated in isolated mastocytoma cells and effected a time-dependent inhibition of the conversion histidine[3H]----histamine[3H], the latter product having a markedly different distribution between cells and medium than the pre-existing histamine pool. Inhibition of whole-body histidine decarboxylase activity, as specifically measured by alpha-methylhistidine-14COOH----14CO2, was also time dependent. Concomitant reduction in histamine levels was seen only in the rapidly turning-over pools of stomach and brain. However, over the course of 13 weeks of chronic treatment, depletion of the relatively inert mast-cell histamine pool(s) was seen as well.

Histidine decarboxylase inhibition: a novel approach towards the development of an effective and safe gastric anti-ulcer drug

The involvement of histamine in mediating gastric function under normal and pathological conditions has been largely established. The relationship between gastric acid production and peptic ulcer diathesis is also well known. Recently, endogenous histamine formation and its release from mast cells has been implicated in the pathogenesis of human and experimental gastric ulcers produced by restraint and pyloric ligation. It has also been implicated in the gastric mucosal damage produced by drugs like aspirin, phenylbutazone and reserpine. These observations suggest that histidine decarboxylase inhibitors may be useful in the prevention of such lesions. Our studies on the evaluation of some histidine decarboxylase inhibitors show that these compounds have a promising potential for developing an effective and safe anti-ulcer drug. This mini-review incorporates the results of our studies which have been adequately supported by other studies as well.

Histamine turnover in the brain of different mammalian species: implications for neuronal histamine half-life

The turnover of neuronal histamine (HA) in nine brain regions and the spinal cord of the guinea pig and the mouse was estimated and the values obtained were compared with data previously obtained in rats. The size of the neuronal HA pool was determined from the decrease in HA content, as induced by (S)-alpha-fluoro-methylhistidine (alpha-FMH), a suicide inhibitor of histidine decarboxylase. The ratios of neuronal HA to the total differed with the brain region. Pargyline hydrochloride increased the tele-methylhistamine (t-MH) levels linearly up to 2 h after administration in both the guinea pig and the mouse whole brain. Regional differences in the turnover rate of neuronal HA, calculated from the pargyline-induced accumulation of t-MH, as well as in the size of the neuronal HA pool, were more marked in the mouse than in the guinea pig brain. The hypothalamus showed the highest rate in both species. There was a good correlation between the steady-state t-MH levels and the turnover rate in different brain regions. Neither the elevation of the t-MH levels by pargyline nor the reduction of HA by alpha-FMH was observed in the spinal cord, thereby suggesting that the HA present in this region is of mast cell origin. The half-life of neuronal HA in different brain regions was in the range of 13-38 min for the mouse and 24-37 min for the guinea pig, except for HA from the guinea pig hypothalamus, which had an extraordinarily long value of 87 min. These results suggest that there are species differences in the function of the brain histaminergic system.

Neuronal histamine inhibits methamphetamine-induced locomotor hyperactivity in mice

Whether central histaminergic (HAergic) neurons mediate the regulation of methamphetamine (MAMP)-induced hyperactivity was clarified. L-histidine (HIS; 500 and 1000 mg/kg i.p.) reduced the locomotor hyperactivity induced by MAMP (1 mg/kg i.p.) in mice, and the effect was significant only at 1000 mg/kg. HIS significantly elevated brain histamine (HA) levels, in both doses, whereas telemethylhistamine (t-MH) levels were elevated only at 1000 mg/kg. Pretreatment with alpha-fluoromethylhistidine, a histidine decarboxylase inhibitor, suppressed both behavioral and biochemical effects of HIS. Metoprine, a HA-N-methyltransferase inhibitor, increased brain HA levels, decreased t-MH levels and suppressed the MAMP-induced locomotor hyperactivity. It is concluded that central HAergic systems may play an inhibitory role on the MAMP-induced locomotor hyperactivity.

Enhancement of histamine synthesis in mouse skin by epinephrine

Intravenous administration of 10-40 micrograms/Kg epinephrine to mice leads to a transient 50% increase in skin histamine, followed by an increase in blood histamine. Delayed inhibition by alpha-fluorometyl histidine (alpha FMH), suggests that these changes follow stimulation of pre-formed tissue histidine decarboxylase.

Stimulation of histamine synthesis in rat mast cells by compound 48/80 in vitro

The histamine content of rat peritoneal fluid cells is doubled within 20 min by 0.5 microgram/ml of compound 48/80. Histamine catabolism inhibitors do not reproduce this effect; cells pre-incubated with alpha-fluoromethylhistidine are unresponsive to compound 48/80 which therefore activates pre-formed histidine decarboxylase rather than 'inducing' it. Non-mast cells showed no change after treatment with compound 48/80.

Stimulation by epinephrine of histamine synthesis by rat peritoneal fluid mast cells in vitro

In vitro exposure of mast but not of other cells of rat peritoneal fluid to epinephrine leads, within 1 min, to progressing levels of histamine in both fluid and sedimentable phases of the incubates, which present no increase in their free/total histamine ratio. Histamine increase was blocked by alpha-fluoromethyl histidine (alpha FMH), acting after a significant lag period. When compared with controls under the electron microscope, epinephrine-treated mast cells show less electron-dense, swollen intracellular granules, apparent maintenance of cell membrane continuity and an apparent decrease of peripheral finger-like projections. Histamine accumulation by epinephrine-treated mast cells may be the result of an enhanced ability of pre-formed mast cell histidine decarboxylase to attack its cell-borne substrate, consequent to an unfolding of the cell membrane during cell tumefaction evoked by epinephrine.

New centrally acting antihypertensive drugs related to methyldopa and clonidine

It has been well established that the antihypertensive drugs clonidine and methyldopa lower blood pressure by acting on postsynaptic alpha 2-adrenergic receptors within cardiovascular control centers of the brain. A number of novel agents designed as lipophilic and highly selective alpha 2-adrenergic stimulants have been synthesized and in general the pharmacological features of these agents resemble clonidine or alpha-methylnorepinephrine, the principal metabolite of methyldopa. The clonidine analogs, ICI-106,270, UK-14,304, piclonidine (LR-99,853), and the bridge analogs (ST-1913, ST-1966,ST-1967) exhibit varying activity on the central cardiovascular control centers. ICI-106,270 is of interest because relative to clonidine it appears to exert fewer CNS side effects. Azepexole (BHT-933) is also of interest because, although structurally unrelated to clonidine, it appears to interact with central alpha-adrenergic receptors in a manner similar to that of clonidine. In contrast, central administration of ST-1966, a monoatomic bridge analog of clonidine, lowers blood pressure in animals treated with an alpha 2-antagonist, which suggests other mechanisms may be involved in its action. Novel antihypertensive agents structurally similar to methyldopa have not been described, although viable pro-drugs of methyldopa such as 2-oxo-1,3-dioxol-4-yl-methyl and pivaloyloxyethyl esters have been shown to have greater oral activity than methyldopa, presumably because they are more lipophilic than the parent moiety.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative inhibition of bacterial and microsomal 3-ketodihydrosphingosine synthetases by L-cycloserine and other inhibitors

Eleven compounds were examined for their capacity to inhibit the first enzyme of the sphingolipid pathway, 3-ketodihydrosphingosine synthetase. Of these, L-cycloserine was the most potent, affecting both bacterial and brain microsomal enzymes to a significant degree at 0.04 mM. D- and L-cycloserine irreversibly inactivated the enzyme, indicating a suicide substrate mode of action. L-Cycloserine was a more potent inhibitor of the growth of Bacteroides levii than was D-cycloserine, indicating that inhibition of sphingolipid synthesis could be a factor in the growth inhibition.

A method for measuring monoamine turnover in animals using an irreversible inhibitor of aromatic L-amino acid decarboxylase, DL-alpha-mono fluoromethyldopa

DL-alpha-monofluoromethyldopa (MFMD) is a potent enzyme-activated irreversible inhibitor of aromatic L-amino acid decarboxylase which, when given to rats or mice at 100 and 250 mg/kg i.p., respectively, causes a linear accumulation of L-DOPA and 5HTP and an exponential decline of noradrenaline, dopamine, and 5HT concentrations in the brain. Rates of change of these parameters can be used to calculate the turnover of the three principle monoamine neurotransmitters. Experiments with haloperidol (1 mg/kg s.c.) and the central 5HT agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (0.25 mg/kg s.c.), have been performed to validate the use of MFMD to measure monoamine turnover. MFMD has several advantages over classical methods for the determination of comparative turnovers using enzyme inhibitors.

A coupled assay for histidine decarboxylase: in vivo turnover of this enzyme in mouse brain

A sensitive coupled assay for histidine decarboxylase has been developed. This method involved conversion of [3H]histidine into [3H]histamine by the enzyme sample, with methylation of this product in situ, catalysed by the enzyme histamine N-methyltransferase, to yield [3H]N-tele-methylhistamine. The radioactive product was separated from the substrate by (i) extraction into chloroform, (ii) ion-exchange chromatography and (iii) liquid cation-exchange extraction. The "no tissue" assay blank comprised 0.0007% of the substrate radioactivity. Sample material with a histidine decarboxylase activity of as little as 0.14 fmol/min/ml (measured at 1 microM histidine) gave double the blank value. More than 50 assays could be performed in one day. This assay was used to determine the in vivo changes in mouse brain histidine decarboxylase activity following irreversible inhibition with (+) alpha-fluoromethylhistidine (alpha-FMH). From the time course of recovery of enzyme activity the half-life of histidine decarboxylase in vivo was calculated to be 53 h.

Mechanism of inactivation of mammalian L-histidine decarboxylase by (S)-alpha-fluoromethylhistidine

The mechanism of inactivation by (S)-alpha-fluoromethylhistidine (FMH) of L-histidine decarboxylase (HDC, L-histidine carboxy-lyase, EC 4.1.1.22) purified from whole bodies of fetal rats was studied. FMH inhibited the activities of HDC purified from fetal HDC as well as HDCs from the brain and stomach of adult rats. The activity was not restored by extensive dialysis, indicating that the inhibition was irreversible. The inactivation of HDC was time and concentration dependent and followed pseudo first-order kinetics. L-Histidine, a substrate, protected HDC against inactivation, but D-histidine did not. Apo-HDC was not inactivated by FMH. On labeling of HDC with [3H]FMH, a correlation was found between the extent of incorporation of radioactivity into the enzyme and the degree of inactivation. Two moles of the inhibitor were incorporated into one mole of HDC (108,000 daltons). Experiments with [carboxyl-14C]FMH and [ring 2-14C]FMH showed that decarboxylation was necessary for the inactivation and that one molecule of FMH moiety was incorporated into an HDC monomer during every three decarboxylations of FMH.

Sensitive gas-liquid chromatographic procedure for urinary N tau-methylimidazole acetic acid, an index of histamine turnover

A simple and specific gas-liquid chromatographic procedure, compatible with both nitrogen-phosphorous and electron-capture detection, and employing conventional packed columns, has been devised for urinary 1-methylimidazole-4-acetic acid (N tau-MeImAA), an index of whole-body histamine turnover. N tau-MeImAA is isolated by ion-exchange on Dowex 1 (CH3COO-), esterified by reaction with chloroethanol-boron trichloride and, depending upon detection employed, chromatographed on base-deactivated SP-2401 or SP-2250. [3H]N tau-MeImAA serves as internal recovery standard.

The significance of mast cells as a source of histamine in the mouse brain

Knowledge of the relative contributions of mast cells and neurons to the overall pool of histamine in the brain is a prerequisite to determining the significance and role of this amine in brain function. Consequently, we analyzed the levels of brain histamine in four genotypes (+/+, W/+, Wv/+, and W/Wv) of WBB6F1 mice, whose numbers of brain-associated mast cells vary in a genotypically specific manner. Although mast cell numbers ranged from a total absence of mast cells (W/Wv) to an average of about 500 mast cells/brain (W/+), no significant differences between genotypes were found in the quantities of histamine in whole brains, brain regions, or crude subcellular fractions. Thus, in this strain of mice, mast cells are not a significant source of histamine in the brain. This suggests that most of the histamine is of neuronal origin. Since neuronal histamine levels are maintained only by continued histidine decarboxylase activity, complete inhibition of this enzyme by alpha-fluoromethylhistidine, a "suicide" inhibitor of histidine decarboxylase, would totally deplete W/Wv mice of brain histamine. This was not found to occur in the W/Wv mice, suggesting that neuronal stores of histamine can be maintained in the absence of histidine decarboxylase, or that an additional nonneuronal, non-mast cell source of histamine exists in the W/Wv mouse brain.

Regional differences in the turnover of neuronal histamine in the rat brain

The turnover rate of histamine (HA) and the half-life of neuronal HA were estimated in 9 regions of the rat brain following pargyline-induced accumulation of tele-methylhistamine (t-MH). The turnover rate was the highest in the hypothalamus (108.7 ng/g/hr). The striatum also showed a high turnover rate (80.2 ng/g/hr) despite much lower levels of HA and t-MH, as compared with the levels in the hypothalamus. The turnover rate was relatively high in the thalamus, cerebral cortex, amygdala and midbrain, but it was very low in the cerebellum. t-MH accumulation in the spinal cord was nil. The HA levels were reduced to various degrees (from nil to less than 40% of the control) by (S)-alpha-fluoromethylhistidine, depending on the regions studied. The neuronal HA content of each brain region was subsequently estimated, and the half-life of neuronal HA in each region was calculated. The half-life of neuronal HA was the shortest (7.7 min) in the striatum, while it was long (about 50 min) in the hypothalamus and thalamus. Half-life values of about 20 min were obtained in other regions. These results show the high levels of histaminergic activity in some parts of the telencephalon, thalamus and midbrain as well as the hypothalamus.

Developmental effects of alpha-fluoromethylhistidine, an irreversible inhibitor of histidine decarboxylase, on growth and on levels and turnover of catecholamines

To examine the potential participation of histamine in cellular development, neonatal rats were given daily 50 mg/kg doses of alpha-fluoromethylhistidine (FMH), an irreversible inhibitor of histidine decarboxylase; previous studies have shown this regimen to deplete both neurotransmitter and nonneurotransmitter pools of histamine. No inhibition of growth was observed for either body weight, brain weight, heart weight or kidney weight; indeed, kidney weights tended to become supranormal toward weaning in the FMH-treated pups. Similarly, FMH failed to affect protein synthesis, confirming the lack of systemic toxicity of this amino acid as well as indicating that maintenance of histamine levels is not required for growth to proceed. In contrast, FMH did have a deleterious effect on development of the cardiac-sympathetic axis, with deficits in norepinephrine levels appearing during the third postnatal week. The deficits were not present in other catecholaminergic systems (brain noradrenergic or dopaminergic neurons and renal sympathetic neurons). The subnormal cardiac norepinephrine levels were preceded by a sharp increase in the turnover of norepinephrine at precisely the age at which central control of sympathetic tone first appears. The developmental effects of FMH indicate that, although it is unlikely that histamine participates in a major way in general control of cellular maturation, a more selective role for histamine as a trophic agent or neurotransmitter may exist during defined periods in nervous system development.

Effect of prolonged inhibition of histidine decarboxylase on tissue histamine concentrations

In rats, chronic infusion of alpha-fluoromethyl histidine, a selective irreversible inhibitor of mammalian histidine decarboxylase, caused a marked depletion of histamine in all tissues examined. There were no gross pharmacological effects associated with this depletion.

Evidence for the presence of a histaminergic neuron system in the rat brain: an immunohistochemical analysis

Histamine-containing cells in rats were identified by indirect immunofluorescent histochemistry using an antibody raised against histidine decarboxylase (HDC), the enzyme forming histamine, which was purified from fetal rat liver. HDC-like immunoreactive (HDCI) structures could be detected in the brain as well as in peritoneal mast cells and basal-granulated cells in deep crypts of the gastric mucosa of rats. Numerous HDCI neurons were found in the posterior hypothalamic area and HDCI nerve fibers with a varicose appearance of fluorescence were widely distributed in various regions of the brain.

Histamine and the hypothalamus

The chemical tools that could be used to examine the function of histamine in the brain are considered together with the evidence linking histamine specifically with the hypothalamus. The distribution of histamine and the enzymes responsible for its synthesis and metabolism is consistent with there being both mast cells and histaminergic nerve terminals within the hypothalamus. Iontophoresis, mepyramine binding and histamine-stimulated adenylate cyclase studies suggest that both histamine H1- and H2- receptors are present in the hypothalamus. In addition, intracerebroventricularly injected histamine receptor agonists and antagonists affect many functions associated with the hypothalamus such as cardiovascular control, food intake, body temperature control, and pituitary hormones whose release is mediated via the hypothalamus, such as corticotropin, growth hormone, thyroid stimulating hormone, prolactin, gonadotropins and vasopressin. However, only in the case of thyroliberin release, prolactin release, body fluid control and blood pressure control is there evidence yet that such effects are mediated via histamine receptors actually in the hypothalamus. The effects of enzyme inhibitors suggest endogenous histamine may be involved in the physiological control of thyroid stimulating hormone, growth hormone and blood pressure, and the effects of receptor antagonists support a role for endogenous histamine in prolactin control. Otherwise, there is little evidence for a physiological role for endogenous, as against exogenous, histamine whether it be from histaminergic terminals or mast cells. In addition, few studies have tried to distinguish possible effects on presynaptic receptors, postsynaptic receptors, hypothalamic blood vessels or the hypophyseal portal blood vessels. It is concluded that although there is good evidence now linking histamine and the hypothalamus more specific studies are required, for instance using microinjection or in vitro techniques and the more specific chemical tools now available, to enable a clearer understanding of the physiological role of histamine in the hypothalamus.

Effect of alpha-fluoromethylhistidine on the histamine content of the brain of W/Wv mice devoid of mast cells: turnover of brain histamine

In the brains of W/Wv mutant mice that have no mast cells, the histidine decarboxylase (HDC) level is as high as in the brain of congenic normal mice (+/+), but the histamine content is 53% of that of +/+ mice. The effects of alpha-fluoromethylhistidine (alpha-FMH) on the HDC activity and histamine content of the brain of W/Wv and +/+ mice were examined. In both strains, 30 min after i.p. injection of alpha-FMH the HDC activity of the brain had decreased to 10% of that in untreated mice. The histamine content decreased more gradually, and after 6 h about half of the control level remained in +/+ mice, whereas histamine had disappeared almost completely in W/Wv mice. It is concluded that the portion of the histamine content that was depleted by HDC inhibitor in a short time is derived from non-mast cells, probably neural cells. The half-life of histamine in the brain of W/Wv mice was estimated from the time-dependent decrease in the histamine content of the brain after administration of alpha-FMH: 48 min in the forebrain, 103 min in the midbrain, and 66 min in the hindbrain.