Effects of alpha-fluoromethylhistidine (FMH), an irreversible inhibitor of histidine decarboxylase, on development of brain histamine and catecholamine systems in the neonatal rat

The Histaminergic System in Neuropsychiatric Disorders

Histamine does not only modulate the immune response and inflammation, but also acts as a neurotransmitter in the mammalian brain. The histaminergic system plays a significant role in the maintenance of wakefulness, appetite regulation, cognition and arousal, which are severely affected in neuropsychiatric disorders. In this review, we first briefly describe the distribution of histaminergic neurons, histamine receptors and their intracellular pathways. Next, we comprehensively summarize recent experimental and clinical findings on the precise role of histaminergic system in neuropsychiatric disorders, including cell-type role and its circuit bases in narcolepsy, schizophrenia, Alzheimer's disease, Tourette's syndrome and Parkinson's disease. Finally, we provide some perspectives on future research to illustrate the curative role of the histaminergic system in neuropsychiatric disorders.

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

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

Distribution of histamine-, 5-hydroxytryptamine-, and tyrosine hydroxylase-immunoreactive neurons and nerve fibers in developing rat brain

Although the general patterns of the developing histaminergic system in the rat brain are known, no comparative studies between the development of the brain histaminergic system and the development of other neuroactive substances have yet been published. Interestingly, separate immunohistochemical studies on the development of the 5-HT system and on the catecholaminergic system in the rat imply common features in the different aminergic systems. Therefore, the spatial distribution of histamine-immunoreactive (HA-ir) neurons and nerve fibers was compared to the distribution of 5-hydroxytryptamine (5-HT)-, and tyrosine hydroxylase-immunoreactive (TH-ir) ones in the developing rat brain between embryonic days 12 (E12) and 20 (E20) by using a double-immunostaining method. The high-pressure liquid chromatography (HPLC) fluorometric method was used for determination of histamine concentration in different brain regions during the same period of development and synthetic oligonucleotide probes complementary to the rat histidine decarboxylase (HDC) to determine the origin of HA in the brain during the development with in situ hybridization. The immunohistochemical results revealed co-localization of HA and 5-HT within a subgroup of cells in the developing raphe nuclei between E14 and E18. From E18 onwards HA immunoreactivity started to gradually disappear from the rhombencephalon, and was totally abolished by E20, while 5-HT-ir cells continued to establish their adult positions. No significant colocalization of HA and TH immunoreactivities was detected. The biochemical results were in agreement with the immunohistochemical ones and confirmed that histamine detected in the early developing brain is authentic. A positive in situ hybridization signal for HDC was detected in a small area in the ventrolateral pons in the same areas as HA- and HDC-ir cell bodies at E16, suggesting that at least some HA may be synthesized locally. These results confirm that HA is one of the first neurotransmitters to appear in the developing brain. In addition, the transient co-localization of HA and 5-HT immunoreactivities and the transient HDC expression at E16 within the developing pontine raphe nuclei may imply an interesting and a more general role for HA in modification of brain development.

The disposition of a histidine decarboxylase inhibitor (S)-alpha-fluoromethylhistidine in rats

An amino acid analyser method using ninhydrin was developed for (S)-alpha-fluoromethylhistidine (FMH) with a minimum quantitation limit of 0.2 microgram mL-1. The assay was used to study the kinetics of FMH in rat. After bolus intravenous administration of FMH hydrochloride hemihydrate (50 mg kg-1), plasma concentration decreased biexponentially with half-lives of 4.4 and 32.7 min. The distribution volumes of the central and peripheral compartments were 127.4 and 166.3 mL kg-1, respectively. The tissue concentration of FMH was highest in the kidney and also decreased biphasically. The FMH concentrations in other tissues were lower, but their tissue/plasma ratios of FMH increased continuously after FMH injection, indicating that FMH partitioned into these tissues and was lost from them very slowly.

The probable role of histamine in the rostral hypothalamus on the prolactin and luteinizing hormone release induced by estrogen in conscious spayed rats

The participation of histamine (HA) sensitive sites in the preoptic-anterior hypothalamic area (POA-AHA) on prolactin (PRL) and luteinizing hormone (LH) surge induced by estrogen was studied in ovariectomized rats. Different groups of animals were subjected to the following experimental schedule: On day "0" rats were stereotaxically implanted into the POA-AHA with microinjection cannulae. On day "1", rats were injected s.c. with estrogen. On days "2" and "3", animals were microinjected into the POA-AHA with different drugs, according to the type of experiment, and at day "4", through a silastic cannula implanted previously in the jugular vein, blood samples were taken each hour between 15:00-21:00 h. In the plasma, PRL and LH concentrations were measured by RIA. Four experiments were performed. In Experiment 1, animals at 12:00 h were injected into the POA-AHA with pyrilamine maleate (an H 1-histamine antagonist), metiamide (an H2-histamine antagonist) or saline as control. In Experiment 2, rats at 12:00 h were injected into the POA-AHA with alpha-fluormethyl-histidine (an inhibitor of histamine synthesis) or the combined administration of pyrilamine and metiamide. In Experiment 3, rats previously microinjected with the histamine synthesis inhibitor were microinjected with 4-methyl-histamine (an H 2-histamine agonists) or 2-pyridilethyl-amine (an H 1-histamine agonist) and in Experiment 4, rats were microinjected at 09:00 h with metiamide, pyrilamine, fluor-methyl-histidine or saline as control. Results showed that in animals treated with pyrilamine or metiamide at noon the prolactin surge induced by estrogen was affected (inhibited by metiamide and shortened by pyrilamine, Experiment 1) and LH surge slightly affected. Rats that received FMH or the combined administration of the histamine antagonists the prolactin and LH surge were abolished (Experiment 2). Only the treatment of the H 2-histamine agonist was able to reproduce the prolactin increase in rats treated with FMH. Nor the H 1 or H 2-histamine agonists were effective in reproducing the LH surge in these animals (Experiment 3). Animals that received saline at 09:00 h into the POA-AHA, the prolactin and LH surges were abolished. Results confirm that histamine in the POA-AHA is important for the expression of prolactin and LH surge induced by estrogen and suggest that H 1- and H 2-histamine receptors are involved in the complex timing mechanisms of the rostral hypothalamus that control both hormone release in rats.

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.

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.

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.

Inhibitors of histidine decarboxylase decrease basal gastric acid secretion in the rat

We examined the ability of two specific inhibitors of histidine decarboxylase, (s)-alpha-fluoromethylhistidine (FMHd) and (s)-alpha-fluoromethylhistamine (FMHm), to inhibit basal gastric acid secretion. The two highest doses of FMHd administered, 50 and 100 mg/kg, decreased basal gastric acid secretion and total secretion volume but did not affect intraluminal pH. FMHm decreased gastric acid secretion, raised intraluminal pH, and to a lesser degree decreased total secretion volume. Neither compound changed the severity of gastric ulcers produced by cold restraint stress.

Histamine sensitive sites in hippocampus: their probable role on prolactin release in male rats

The effect of histamine (HA), 3-methyl-histamine (3MHA), HA antagonists or drugs interfering with HA synthesis, microinjected into the hippocampus (HPC), on prolactin (PRL) secretion were studied in rats. Three experiments were performed. In Experiment 1, increasing doses of HA or 3-MHA (9-90 nmol) were microinjected stereotaxically into the ventral HPC of adult male rats. In Experiment 2, 135 nmol of pyrilamine (PYR, an H1-HA-antagonist) or ranitidine (RAN, an H2-HA-antagonist) were administered locally into the ventral HPC. Fifteen min later, the rats were microinjected again with 45 nmol of HA. In Experiment 3, rats were microinjected with different doses of HA-antagonists or with 20 nmol of alpha-fluormethyl-histidine (FMH, an inhibitor of the enzyme of HA synthesis) and later subjected to an immobilization stress of 15 min duration. In all cases, the PRL plasma concentrations were measured in blood samples taken at different time intervals (0-120 min) after the last brain injection. Results showed that HA applied locally in ventral HPC induced an increase in PRL levels which was statistically significant from saline-injected rats between 5-30 min after the HPC stimulation. On the contrary, local applications of 3-MHA did not change significantly the PRL blood levels (Experiment 1). Only PYR did block partially the PRL response due to HA in basal conditions. RAN in these later conditions had no effect (Experiment 2). When animals were subjected to stress neither PYR nor RAN, alone or in combination, locally applied were able to block the PRL increase due to stress. Only FMH blunted significantly the hormone response.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

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.

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.