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

Modulation of serotonin transporter expression by escitalopram under inflammation

Selective serotonin reuptake inhibitors (SSRIs) are widely used for depression based on the monoamine deficiency hypothesis. However, the clinical use of these agents is controversial, in part because of their variable clinical efficacy and in part because of their delayed onset of action. Because of the complexities involved in replicating human disease and clinical dosing in animal models, the scientific community has not reached a consensus on the reasons for these phenomena. In this work, we create a theoretical hippocampal model incorporating escitalopram's pharmacokinetics, pharmacodynamics (competitive and non-competitive inhibition, and serotonin transporter (SERT) internalization), inflammation, and receptor dynamics. With this model, we simulate chronic oral escitalopram in mice showing that days to weeks are needed for serotonin levels to reach steady-state. We show escitalopram's chemical efficacy is diminished under inflammation. Our model thus offers mechanisms for how chronic escitalopram affects brain serotonin, emphasizing the importance of optimized dose and time for future antidepressant discoveries.

Enantioselective Pharmacokinetics of α-Fluoromethylhistidine in Rats and Its Comparison with Histidine

The enantiomer-specific pharmacokinetics of histidine and its analogue, α-fluoromethylhistidine (FMH), were investigated in rats. After bolus intravenous administration of each enantiomer of histidine or FMH at a dose of 40·3 mg kg−1 as free base equivalents, the plasma concentrations of l-histidine, d-histidine, (S)-FMH and (R)-FMH decreased biexponentially with half-lives of 39·2, 20·8, 32·8 and 25·0 min, respectively, in the elimination phase. Although the concentration of l-histidine in the plasma was lower than that of d-histidine, there was no large difference in plasma concentration-time curves of the enantiomers of FMH. The apparent total clearance of l-histidine from rat plasma was about 4 times that of d-histidine or the enantiomers of FMH. l-Histidine was quickly transferred to the peripheral tissues where the concentrations also decrease biphasically. l-Histidine penetrated more rapidly into the brain than either its d-enantiomer or a compound closely related in structure such as FMH. However, the disappearance of l-histidine from the various brain regions was very rapid. In contrast, brain/plasma ratios of d-histidine and (S)-FMH increased continuously after injection of these compounds, indicating that d-histidine or (S)-FMH partitioned into the brain and was very slowly removed from the brain; (R)-FMH was not distributed to the brain. These results suggested stereoselectivity in disposition of histidine and FMH enantiomers in rats.

Modulation of the histaminergic system and behaviour by alpha-fluoromethylhistidine in zebrafish

The functional role of histamine (HA) in zebrafish brains was studied. Zebrafish did not display a clear circadian variation in brain HA levels. Loading of zebrafish with l-histidine increased HA concentration in the brain. A single injection of the histidine decarboxylase (HDC) inhibitor, alpha-fluoromethylhistidine (alpha-FMH), gave rise to a rapid reduction in zebrafish brain HA. Low HDC activity in the brain after injections verified the effect of alpha-FMH. A reduction in the number of histaminergic fibres but not neurones and an increased expression of HDC mRNA was evident after alpha-FMH. Automated behavioural analysis after alpha-FMH injection showed no change in swimming activity, but abnormalities were detected in exploratory behaviour examined in a circular tank. No significant behavioural changes were detected after histidine loading. The time spent for performance in the T-maze was significantly increased in the first trial 4 days after alpha-FMH injections, suggesting that lack of HA may impair long-term memory. The rostrodorsal telencephalon, considered to correspond to the mammalian amygdala and hippocampus in zebrafish, is densely innervated by histaminergic fibres. These results suggest that low HA decreases anxiety and/or affects learning and memory in zebrafish, possibly through mechanisms that involve the dorsal forebrain.

Effects of intracerebroventricular injection of alpha-fluoromethylhistidine on radial maze performance in rats

The effects of alpha-fluoromethylhistidine (alpha-FMH) on spatial cognition were investigated using the eight-arm radial maze paradigm in rats. Intracerebroventricular (ICV) injection of alpha-FMH resulted in spatial memory deficits characterized by an increase in the number of total errors (TE) and a decrease in the number of initial correct responses (ICR). There was a strong correlation between increases in the number of TE and decreases in histamine contents of the cortex and hippocampus regions of the brain, which are known to participate in learning and memory. On the other hand, both histamine (50-100 ng, ICV) and thioperamide (10 microg, ICV) significantly ameliorated the memory deficit induced by alpha-FMH. However, metoprine showed no significant effect on the alpha-FMH-induced memory deficit. Pyrilamine and R-(alpha)-methylhistamine enhanced the memory deficit induced by alpha-FMH, at doses that had no appreciable effect when administered alone. In contrast, no significant influence on alpha-FMH-induced memory deficit was observed with zolantidine.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Effect of cholecystokinin-B/gastrin receptor blockade on gastric acid secretion in conscious rats

Gastrin, histamine and acetylcholine are physiological stimuli of gastric acid secretion. The cholecystokinin-B/gastrin receptor antagonists YM022 and RP73870 were used to study the effect of gastrin receptor blockade on acid secretion. Gastrin, histamine, insulin or bethanechol were administered to conscious gastric fistula rats with or without the concomitant intravenous infusion of YM022 or RP73870. Other rats were subjected to pylorus ligation. YM022 and RP73870 inhibited the gastrin-induced acid secretion in a dose- and time-dependent manner; maximal inhibition was observed at a dose of 0.3 mumol.kg-1.hr-1 for both YM022 and RP73870, the ID50 values being 0.02 mumol.kg-1.hr-1 and 0.05 mumol.kg-1.hr-1 for YM022 and RP7870, respectively. At a dose of 0.3 mumol.kg-1.hr-1 YM022 and RP73870 failed to inhibit basal and histamine-, bethanechol-, and insulin-evoked secretion. They also failed to affect the secretion evoked by infusion of a cocktail of maximally effective doses of gastrin-17, histamine and bethanechol. YM022 and RP73870, finally, were without effect on the acid response to pylorus ligation. We suggest that endogenous gastrin in the conscious rat does not contribute to the basal acid secretion and does not participate in the acid response to histamine or to vagus stimulation.

Gastric acid secretion after depletion of enterochromaffin-like cell histamine. A study with alpha-fluoromethylhistidine in rats

BACKGROUND

Histamine is thought to play a central role in the regulation of gastric acid secretion. In the rat oxyntic mucosa most of the histamine is synthesized and stored in enterochromaffin-like (ECL) cells, and the rest resides in mast cells. The present study examines the role of ECL-cell histamine in the control of acid secretion in the intact, conscious rat.

METHODS

Rats were treated with alpha-fluoromethylhistidine (alpha-FMH) to inhibit histamine synthesis. alpha-FMH was given by continuous subcutaneous infusion (3 mg/kg/h) for up to 9 days. An additional oral dose of alpha-FMH (50 mg/kg) was given 2 h before each acid secretion test. Acid secretion was studied in pylorus-ligated rats and in chronic gastric fistula rats stimulated with histamine, gastrin-17, or insulin after 2-6 days of alpha-FMH infusion.

RESULTS

Treatment with alpha-FMH lowered oxyntic mucosal histamine synthesis by 80%. From previous observations this is thought to reflect depletion of histamine from the ECL cells. The remaining 20% resides in mucosal and submucosal mast cells, which seem to be resistant to alpha-FMH. Basal acid secretion was inhibited by more than 60% after alpha-FMH treatment and by more than 80% by ranitidine. Histamine-stimulated secretion was unaffected by alpha-FMH and abolished by the histamine H2-receptor antagonist ranitidine. The acid response to gastrin-17 was almost abolished in histamine-depleted rats and abolished by ranitidine. Vagally induced acid secretion (provoked by the injection of insulin or by pylorus ligation) was unaffected by alpha-FMH treatment but abolished by ranitidine and by the muscarinic M1-receptor antagonist pirenzepine.

CONCLUSION

The results suggest that gastrin stimulates acid secretion by releasing histamine from ECL cells. Vagally induced acid secretion is also dependent on a histaminergic pathway but not on ECL-cell histamine.

Influence of histamine depletion on learning and memory recollection in rats

To clarify the role of endogenous histamine in learning and memory, the effect of alpha-fluoromethylhistidine on active avoidance response in rats was studied. alpha-Fluoromethylhistidine (20-100 mg/kg or 10-50 micrograms) significantly (P < 0.05 or P < 0.01) prolonged the response latency in active avoidance response when administered by either intraperitoneal or intracerebroventricular injection. These effects were dose-related and long lasting. A prolongation of the response latency induced by an intraperitoneal injection of alpha-fluoromethylhistidine (100 mg/kg) was antagonized by intracerebroventricular injection of histamine (10 and 20 ng) in a dose-dependent manner. In addition, the acquisition of this response was retarded by a consecutive intracerebroventricular injection of alpha-fluoromethylhistidine (50 micrograms), whereas histamine (100 ng) facilitated the response acquisition when administered by the same route. Both intraperitoneal (100 mg/kg) and intracerebroventricular injection of alpha-fluoromethylhistidine (50 micrograms) significantly (P < 0.05 or P < 0.01) decreased the brain histamine content, especially in the hippocampus and hypothalamus. When alpha-fluoromethylhistidine (50 micrograms) was injected intracerebroventricularly, there is a high correlation between a prolongation of the response latency and a decrease in histamine content of these brain areas. Based on these findings, it was concluded that an intimate relation may exist between a prolongation of response latency in the active avoidance response and a decrease in the brain histamine content; endogenous histamine may play an important role in learning and memory recollection in rats.

Enterochromaffin-like cells in the rat stomach: effect of alpha-fluoromethylhistidine-evoked histamine depletion. A chemical, histochemical and electron-microscopic study

In the rat, gastric histamine is stored predominantly in the enterochromaffin-like (ECL) cells, which are located basally in the oxyntic mucosa. The functional significance of histamine in the ECL cells is a matter of speculation. In this study the effect of depletion of histamine on the properties and ultrastructure of the ECL cells was examined. Histamine synthesis was inhibited with alpha-fluoromethylhistidine (3 mg.kg-1.h-1) given via osmotic minipumps over a period of 24 h. The treatment reduced the histidine decarboxylase activity (approximately 20% remaining) and histamine concentration (less than 20% remaining) in the oxyntic mucosa, as well as the intensity of histamine- and chromogranin A-immunostaining in the ECL cells, compared to control rats. The cytoplasmic (secretory) granules/vesicles were greatly reduced in number and size following alpha-fluoromethylhistidine administration. The histamine immunostaining of the mast cells, which occurs at the mucosal surface and in the submucosa, appeared unaffected. We conclude that ECL cell histamine accounts for at least 80% of the total oxyntic mucosal histamine in the rat and that it represents a more mobile pool than mast cell histamine. The reduction in the number and size of the ECL cell granules/vesicles following histamine depletion is in accord with the idea that they represent the storage site for histamine.

Hyperplasia of histamine-depleted enterochromaffinlike cells in rat stomach using omeprazole and alpha-fluoromethylhistidine

In the rat, gastric histamine is stored mainly in the enterochromaffinlike cells. Gastrin releases histamine from these cells, and long-term hypergastrinemia results in hyperplasia. The effect of sustained hypergastrinemia on histamine-depleted enterochromaffinlike cells was studied by measuring histidine decarboxylase activity and histamine concentrations and by using quantitative histology. Hypergastrinemia maintained for 6 weeks was induced by inhibition of gastric acid secretion with omeprazole (400 mumol.kg-1.day-1) given orally, and histamine synthesis was inhibited for the same length of time with alpha-fluoromethylhistidine (3 mg.kg-1.h-1) given via osmotic minipumps. In rats given omeprazole alone, the effects of the resulting hypergastrinemia on the enterochromaffinlike cells was reflected in increased histidine decarboxylase activity, increased histamine concentration, and increased number of enterochromaffinlike cells. The general trophic effects on the stomach were seen as increased stomach and oxyntic mucosal weight and increased mucosal thickness. Treatment with alpha-fluoromethylhistidine plus omeprazole markedly reduced the histidine decarboxylase activity and histamine concentration, but the weight of the stomach and oxyntic mucosa, the enterochromaffinlike cell density, and intensity of histidine decarboxylase immunostaining were increased to at least the same extent as after omeprazole alone. These observations indicate that enterochromaffinlike cell histamine is not important for a full expression of gastrin-evoked trophic effects in the stomach.

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.

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.

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.

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.

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.

Comparative effects of polymyxin B and compound 48/80 on histamine metabolism in rat muscle and gastric tissue

Polymyxin B, administered in vivo, increased histidine decarboxylase (HDC) activity and histamine (HM) concentrations in muscle tissue homogenates and supernatants. When administered in vitro it increased HDC activity and HM concentrations in both muscle and gastric tissue. The stimulatory effect on muscle was similar to that obtained with compound 48/80, but 48/80, unlike polymyxin B, did not affect gastric tissue. In vitro additions of alpha-fluoromethylhistidine inhibited both in vivo and in vitro stimulatory effects of polymyxin B. The results of these studies show that the action of compound 48/80 and of polymyxin B are similar, and that both affect HM synthesis in a manner that requires further elucidation.