Jai K. Khandelwal

Histamine turnover in regions of rat brain

Rapid and complete inhibition of monoamine oxidase by pargyline produced linear increases in the content of the histamine metabolite, tele-methylhistamine (t-MH), in 9 regions of rat brain 2 and 4 h after drug administration. The treatment had little or no effect on the histamine content of these regions. As histamine methylation is the major metabolic pathway of histamine in brain, the rate of increase in brain t-MH content after complete inhibition of its metabolism provides an estimate of histamine turnover. Histamine turnover rates varied over 46-fold among regions, from cerebellum (0.029 nmol/g . h) to hypothalamus (1.33 nmol/g . h), similar to those reported for norepinephrine and serotonin. Turnover rates were highly correlated with control t-MH levels (r = 0.97) and control histamine levels (r = 0.84). Rate constants were highest in the caudate nucleus and frontal cortex, equivalent to a half-life of about 11 min in these regions. While hypothalamic histamine had the highest turnover rate, the rate constant for histamine in this region was among the lowest in brain, perhaps consistent with the presence of histaminergic cell bodies. Histamine turnover rates may be indicative of the activity of histamine-synthesizing neurons, and their determination will facilitate understanding of histamine in brain.

Aspects of histamine metabolism

ConclusionsProgress in learning the role of histamine in physiology and pathology has been impeded by difficulties in accurately measuring histamine and by the deficiencies of methods to measure its metabolites. The availability of specific, sensitive and rapid methods to measure histamine has helped in understanding the role of histamine in disease. Measuring histamine alone may provide an incomplete indication of the role of histamine in disease or in any other process. For histamine is metabolized by multiple pathways, and the kinetics of these enzymatic activities (as well as the rate of synthesis of histamine) determine the steady-state levels of histamine in tissue and in body fluids. Measurements of both histamine and its metabolites would contribute, and may be essential, to the understanding of the role of histamine in disease, just as measurements of the metabolites of other biogenic amines have been critical to understanding of their roles in diseases. Yet another reason that compels measurements of metabolites is evidence that some of the metabolites of histamine are pharmacologically, perhaps physiologically, active.

An improved GCMS method to measure tele-methylhistamine

The presence of tele-methylhistamine (t-MH), the metabolite of histamine in rat brain, and its relationship to putative histaminergic transmission have been the subjects of recent work. We modified the GCMS method of Hough et al. (1979) to enhance both sensitivity and reproducibility. The substitution of KOH for NaOH to extract t-MH considerably improved the recovery. Evaporation of the extract in 0.2 N HCl, as used in the earlier method, reduced the formation of heptafluorobutyryl derivative; substitution with 0.01 N HCl more than doubled the recovery of the derivative. The derivatization procedure itself was changed, the new method exhibiting significantly improved reproducibility. Standard curve of t-MH obtained at different times after derivatization were indistinguishable. The modified method is capable of measuring less than 1 pmole of t-MH. The t-MH content found in nine rat brain regions agree with previously reported values.

Inhibition of brain histamine metabolism by metoprine

To study the extent to which histamine methylation accounts for the biosynthesis of histamine metabolites in brain, the effects of the histamine methyltransferase (HMT) inhibitor metoprine were determined on the whole brain levels of tele-methylhistamine (t-MH), its oxidative metabolite tele-methylimidazoleacetic acid (t-MIAA), and brain HMT activity in albino rats. Metoprine (5-30 mg/kg) reduced brain t-MH levels by about 75% and caused a dose-dependent reduction (70-90%) in HMT activity 4 hr after administration. Furthermore, the levels of t-MH remaining in each brain after metoprine treatment were significantly positively correlated with the remaining HMT activity of that brain after all doses of drug. Although brain t-MIAA levels were reduced by only 30% 4 hr after metoprine administration, the levels were reduced by about 75% 12 hr after the drug, similar to the reduction in t-MH levels. These findings support previous suggestions that t-MH and t-MIAA in brain arise from brain histamine metabolism, and that brain t-MH synthesis is equivalent to histamine methylation.

Effects of pargyline on tele-methylhistamine and histamine in rat brain

Rapid and complete inhibition of brain MAO produced linear increases in brain t-MH levels from 30 min to 4 hr after drug treatment at a rate of 0.26 nmole/g X hr, resulting in a 3-fold increase which persisted for at least 12 hr. HA levels were slightly elevated 1 and 2 hr after drug administration but quickly returned to control levels, suggestive of sensitive regulatory mechanisms in brain. Although the slight change in HA levels precludes steady-state assumptions, the rate of increase in brain t-MH levels after MAO inhibition provides a novel estimate of the half-life of endogenous brain HA (50 min). Despite the transient effect of pargyline on brain HA content, the effect of pargyline on brain t-MH levels suggests that MAO inhibitors may produce long-term alterations in brain histaminergic dynamics.

Measurement of tele-methylhistamine and histamine in human cerebrospinal fluid, urine, and plasma

A gas chromatographic-mass spectrometric method described by us to measure tele-methylhistamine (t-MH) in brain was used to measure t-MH in human cerebrospinal fluid (CSF), urine and plasma. The presence of t-MH in these body fluids was rigorously established. No pros-methylhistamine could be detected, and it was used as internal standard to quantify t-MH in the fluids. The mean levels of t-MH were: urine, 943 pmol/mg creatinine; plasma, 12.3 pmol/ml; and CSF, 2.2 pmol/ml. Parallel measurements of histamine by a radioenzymatic method showed, respectively, 182 pmol/mg creatinine; 19.5 pmol/ml; and 388 pmol/ml. The levels of HA in CSF, much higher than those of its metabolite, t-MH, are high enough to stimulate HA receptors in the central nervous system.

Lack of a Precursor‐Product Relationship Between Histamine and Its Metabolites in Brain After Histidine Loading

Abstract: Levels of histamine and its major metabolites in brain, tele‐methylhistamine (t‐MH) and tele‐methylimidazoleacetic acid (t‐MIAA), were measured in rat brains up to 12 h after intraperitoneal administration of l‐histidine (His), the precursor of histamine. Compared with saline‐treated controls, mean levels of histamine were elevated at 1 h (+ 102%) after a 500 mg/kg dose; levels of t‐MH did not increase. Following a 1,000 mg/kg dose; levels mean histamine levels were increased for up to 7 h, peaked at 3 h, and returned to control levels within 12 h. In contrast, levels of t‐MH showed a small increase only after 3 h; levels of t‐MIAA remained unchanged after either dose. Failure of most newly formed histamine to undergo methylation, its major route of metabolism in brain, suggested that histamine was metabolized by another mechanism possibly following nonspecific decarboxylation. To test this hypothesis, other rats were injected with α‐fluoromethylhistidine (α‐FMHis; 75 mg/kg, i.p.), an irreversible inhibitor of specific histidine decarboxylase. Six hours after rats received α‐FMHis, the mean brain histamine level was reduced 30% compared with saline‐treated controls. Rats given His (1,000 mg/kg) 3 h after α‐FMHis (75 mg/kg) and examined 3 h later had a higher (+112%) mean level of histamine than rats given α‐FMHis, followed by saline. Levels of t‐MH and t‐MIAA did not increase. These results imply that high doses of His distort the simple precursor‐product relationship between histamine and its methylated metabolites in brain. The possibility that some His may undergo nonspecific decarboxylation in brain after His loading is discussed. These findings, and other actions of His independent of histamine, raise questions about the validity of using His loading as a specific probe of brain histaminergic function.

Diurnal fluctuation in levels of histamine metabolites in cerebrospinal fluid of rhesus monkey.

In samples of ventricular cerebrospinal fluid (CSF) that were collected from a conscious, restrained rhesus monkey at intervals of 30–90 min, levels of the histamine metabolites,tele-methylhistamine (t-MH) andtele-methylimidazoleacetic acid (t-MIAA), were determined by gas chromatography-mass spectrometry. Levels of t-MH and t-MIAA each showed time-related fluctuations. Peak and trough concentrations of t-MIAA, the product of t-MH, paralleled, but lagged about 2 h behind, the levels of t-MH. Within the first 3 h of illumination, metabolite levels increased more than 3-fold; they fell sharply within the first 3 h of darkness. Mean levels of t-MH and t-MIAA were significantly higher during periods of illumination than of darkness. Fluctuations in the levels ofpros-methylimidazoleacetic acid (p-MIAA), an endogenous isomer of t-MIAA that is not a histamine metabolite, were markedly different from those of t-MH or t-MIAA; p-MIAA levels peaked only at the middle of the dark period. The time-related fluctuations in levels of t-MH and t-MIAA, but not p-MIAA, are similar to the daily rhythmic changes observed in monkey CSF for the levels of other central neurotransmitters and peptide neurohormones.

Influence of age and gender on the levels of histamine metabolites and pros-methylimidazoleacetic acid in human cerebrospinal fluid

The metabolites of histamine, tele-methylhistamine (t-MH) and tele-methylimidazoleacetic acid (t-MIAA), were measured in cerebrospinal fluid (CSF) from 47 subjects with neurological disorders and healthy controls. In lumbar CSF, concentrations of these metabolites were significantly correlated. Levels of t-MH, t-MIAA and their sum (which represents virtually all histamine metabolized in brain) were significantly higher in CSF from older subjects and were positively correlated with age. Females had higher levels of histamine metabolites than males. Males had higher levels of pros-methylimidazoleacetic acid (p-MIAA), an isomer of t-MIAA that is not a metabolite of histamine. Levels of p-MIAA increased with age among men. These results are in contrast to those of age-related effects on levels of other aminergic transmitter metabolites in CSF and suggest that metabolic activity of histamine in brain may increase with age.

Ontogeny and Subcellular Distribution of Rat Brain Tele-Methylhistamine

Abstract: The whole brain content and subcellular distribution of histamine and its metabolite, tele‐methylhistamine, were studied during postnatal development of the rat. Brain methylhistamine levels were similar to or greater than histamine levels, indicating that histamine methylation is a major metabolic pathway in neonatal brain, as it is in adults. When calculated per brain, histamine, methylhistamine, and histamine methyltransferase were all maximal 10 days after birth. In neonates, brain histamine was found almost entirely in nuclear fractions, whereas methylhistamine was found almost exclusively in supernatant fractions. By day 20, however, a greater proportion of both amines was localized in subcellular fractions containing synaptosomes, a finding consistent with histamines suggested transmitter role. The ontogenic pattern of brain methylhistamine questions the mast cell origin of neonatal histamine, but may be consistent with a role for histamine in brain development.