Effects of histamine H3-receptor ligands on brain monoamine oxidase in various mammalian species

Evaluation of monoamine oxidase A and B type enzyme occupancy using non-radiolabelled tracers in rat brain

Monoamine oxidase (MAO) enzymes, type A and B metabolise the amine neurotransmitters of the body. Selective inhibition of either enzyme is an approach for treating neurodegenerative and stress-induced disorders, and inhibition of an enzyme is proportional to the binding of the MAO inhibitor. Conventionally, the binding of test compounds to enzymes is assessed by radiolabelled ligands in ex vivo and in vivo occupancy assays. Regulatory restrictions and turnaround time are the limitations of the methods that use radiolabelled ligands. But the use of non-radiolabelled tracers and sensitive mass spectrometry (LC-MS/MS) based assays accelerated the determination of target occupancy in pre-clinical species. A report on use of non-radiolabelled ligand in in vivo MAO occupancy assay is not available. The objectives of the present study were to optimise non-radiolabelled harmine and deprenyl as selective tracers in MAO-A and MAO-B occupancy assays and evaluate MAO occupancy of test compounds in rat brain. Tracer optimisation resulted in a detectable, stable, and low ratio (<3.0) of tracer concentrations between any two brain tissues. In occupancy assay, tracer was intravenously administered (10 μg/kg, harmine or 60 μg/kg, L-deprenyl) after the treatment with test compound (clorgyline or tranylcypromine or pargyline or phenelzine or thioperamide). Specific brain tissues were isolated at a defined interval and tracer concentrations were quantified using LC-MS/MS method. Pre-treatment with MAO inhibitors resulted in a decrease (maximum, 80-85%) in harmine or an increase (maximum, 85-300%) in L-deprenyl concentrations. But we considered the change in tracer concentration, relative to the vehicle and positive control groups to calculate MAO occupancy. The observed selectivity and ratio of occupancies (ED₅₀) of test compound towards MAO-A and MAO-B are comparable with the results from in vitro radiolabelled ligand-based inhibition assay. The results demonstrated the application of these non-radiolabelled tracers as suitable pre-clinical tools to determine MAO occupancy.

The neglected role of histamine in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by beta-amyloid plaques accumulation and cognitive impairment. Both environmental factors and heritable predisposition have a role in AD. Histamine is a biogenic monoamine that plays a role in several physiological functions, including induction of inflammatory reactions, wound healing, and regeneration. The Histamine mediates its functions via its 4 G-protein-coupled Histamine H1 receptor (H1R) to histamine H1 receptor (H4R). The histaminergic system has a role in the treatment of brain disorders by the development of histamine receptor agonists, antagonists. The H1R and H4R are responsible for allergic inflammation. But recent studies show that histamine antagonists against H3R and regulation of H2R can be more efficient in AD therapy. In this review, we focus on the role of histamine and its receptors in the treatment of AD, and we hope that histamine could be an effective therapeutic factor in the treatment of AD.

Metabolism of histamine in tissues of primary ductal breast cancer

Histamine performs an important role in the pathologic and physiologic aspects of the breast gland. Among monoamines, histamine demonstrates the greatest proliferative activity in breast cancer. The aim of the study was to evaluate histamine concentration in plasma and tissues of breast cancer dependent on the activity of histamine metabolism enzymes in neoplasmatic tissues of the breast gland. Ninety-five women aged 38 to 70 years were divided into 2 groups. The control group (group I) consisted of 30 healthy women. Group II consisted of 65 women with primary ductal breast cancer. The concentration of histamine in plasma was assessed by immunoenzymatic method. The concentration of histamine in cancerous tissues of the breast and the metabolism of histamine enzymes, specially histidine decarboxylase, decarboxylase of aromatic L-amino acids, N-histamine methyltransferase, monoamine oxydase B, and diamine oxydase, were determined using isotope technique. In the course of 24 hours, excretion of N-methylimidazoleacetic acid was evaluated by the methods of chromatography. The statistical analysis was made based on Statistica Pl Ed (StatSoft, Cracow, Poland, 1998). A significant increase in the concentration of histamine in plasma (P < .01) and tissues of ductal breast cancers (P < .001), and in the activity of histidine decarboxylase (P < .01), aromatic L-amino acids (P < .05), and histamine methyltransferase (P < .05) was found. Activity of monoamine oxidase B (P < .01) and diamine oxidase (P < 0.01) and excretion of N-methylimidazoleacetic acid were significantly decreased compared with the control group (P < 0.001). The conclusions are as follows: (1) Concentration of histamine in the plasma of women is dependent on the concentration of histamine in the tissues of ductal breast cancers. (2) The significant increase of histamine in cancerous tissues of ductal breast cancer could suggest the participation of this monoamine in the development of breast cancer. (3) The increase of histamine concentrations in ductal breast cancer tissues can be connected with the disturbances of the balance between synthesis and enzymatic inactivation of this monoamine. (4) The concentration of histamine in the plasma of women with ductal breast cancers is dependent on the number of involved lymph nodes and the grade of histologic malignancy.

Postischemic regulation of central histamine receptors

This study characterizes changes occurring in the central histaminergic system associated with ischemia-reperfusion pathology in the rat. Specifically, after a postocclusion time period of 48 h, we have analyzed histamine H(1) receptor mRNA expression, histamine H(2) receptor protein amount and binding densities, and histamine H(3) receptor mRNA expression and binding densities in brain regions that have been suggested to be selectively vulnerable to transient global ischemia, i.e. hippocampus, thalamus, caudate-putamen, and cerebral cortex. We found an increase in H(1) receptor mRNA expression in the caudate-putamen: given that ischemia reduces glucose uptake and H(1) receptor activation has been shown to decrease this effect, an increase of expression levels may result in mitigating tissue damage due to energy failure observed in ischemia. A decrease in H(2) receptor binding densities in the caudate-putamen was also observed; the ischemia-induced decrease in H(2) receptor protein was also detectable by Western blot analysis. This phenomenon may underlie the previously reported ischemia induced striatal dopamine release. H(3) receptor mRNA expression was increased in the caudate putamen of the postischemic brain but was decreased in the globus pallidus and the thalamus; in association with this, H(3) receptor binding densities were increased in the cortex, caudate-putamen, globus pallidus, and hippocampus. The upregulation of H(3) receptor ligand binding may be involved in the previously reported continuous neuronal histamine release. Our data suggest that central histamine receptor expression and ligand binding are altered in brain ischemia in distinct areas, and may participate in neuroprotection and/or ischemia-associated neuronal damage.