Lindsay B. Hough

Mast cells in rat thalamus: Nuclear localization, sex difference and left-right asymmetry

Mast cells were positively identified in rat brain by a combination of staining and histochemical procedures. These cells stained positively with toluidine blue and Astrablau at low pH, indicating the presence of a proteoglycan similar to that found in peripheral mast cells. Brain mast cells also fluoresced after o-phthalaldehyde exposure, indicating that they contain histamine. Mast cells varied greatly in number among brains, but their distribution was almost exclusively thalamic; within thalamus, the ventral complex, medial dorsal, lateral, and paraventricular nuclei contained the most mast cells. Mast cell numbers were greater in brains of females than of males, and greater in left than in right hemispheres. These findings suggest that mast cells have a specialized function in thalamus and/or that the vascular environment of the thalamus is particularly conducive to mast cell accumulation.

Histamine and Its Receptors in the Nervous System

Before much was known about histamine in the brain-even before its con centration in brain was accurately measured-histamine by chance sparked the development of modern psychopharmacology and therefore modern biological psychiatry. Phenothiazines were antihistamines (i.e., H1 antagonists) that were observed to produce a “euphoric quietude”1,2; and from the phenothiazines developed the antidepressant drugs, which had also been designed as H1 antagonists.3 Histamine was dismissed as having a role in either the pharmacological or therapeutic effects of either neuroleptic or antidepressant drugs. As summarized below (Sections 12.2 and 12.3), these drugs do in fact interact with histamine receptors at drug concentrations that are found in plasma of patients and in brain of experimental animals. Other psychotropic drugs affect brain histamine (Section 12). Neither the pharmacological nor the therapeutic consequences of these effects are clear because the functions of histamine in brain are not certain.

Rat Brain Mast Cells: Contribution to Brain Histamine Levels

Recent studies have shown that mast cells (MCs) are present in rat brain, that they have a predominantly thalamic localization, and that they contain histamine (HA). However, the degree to which these cells contribute to brain HA levels has remained unclear. Our recent studies of the precise distribution of rat brain MCs permitted us to develop a method to determine both the MC numbers and HA content from the same brain. Thalamic MC numbers were highly correlated with both the amount (ng) and the concentration (ng/g) of thalamic HA in both sexes (p < 0.005). Slopes of these regression lines, suggestive of the HA content of thalamic MCs, were 2.5 and 1.3 pg/cell in males and females, respectively, substantially less than the HA levels in peritoneal MCs. Thalamic MC numbers were not correlated with HA (ng) outside of thalamus, but were significantly (p < 0.005) correlated with whole brain HA amounts (ng) and levels (ng/g). These results are direct biochemical evidence for a contribution by MCs to brain HA levels, and indicate that thalamic MCs contribute up to 90% of the HA in thalamus, and up to 50% of whole brain HA levels.

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.

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.

A role for histamine and histamine H2-receptors in non-opiate footshock-induced analgesia.

Scrambled DC current applied to the hind paws of rats caused an analgesic response that was inhibited by the histamine H2-receptor antagonists cimetidine, ranitidine and oxmetidine, but not by high doses of naloxone (the opiate antagonist), or other transmitter receptor antagonists. In contrast, AC current applied to all paws produced analgesia that was blocked by naloxone, but not cimetidine, showing the independence of these systems. These findings indicate a specific role for histamine and H2-receptors as mediators of endogenous non-opiate analgesia. In addition, a combination of cimetidine and naloxone did not abolish either form of footshock analgesia, implying the existence of a non-opiate, non-H2, endogenous pain-relieving system. These results also suggest that drugs capable of penetrating the brain and stimulating H2-receptors might have analgesic properties.