Inhibition of Astrocytic Histamine N-Methyltransferase as a Possible Target for the Treatment of Alzheimer’s Disease

Alzheimer’s disease (AD) represents the principal cause of dementia among the elderly. Great efforts have been established to understand the physiopathology of AD. Changes in neurotransmitter systems in patients with AD, including cholinergic, GABAergic, serotoninergic, noradrenergic, and histaminergic changes have been reported. Interestingly, changes in the histaminergic system have been related to cognitive impairment in AD patients. The principal pathological changes in the brains of AD patients, related to the histaminergic system, are neurofibrillary degeneration of the tuberomammillary nucleus, the main source of histamine in the brain, low histamine levels, and altered signaling of its receptors. The increase of histamine levels can be achieved by inhibiting its degrading enzyme, histamine N-methyltransferase (HNMT), a cytoplasmatic enzyme located in astrocytes. Thus, increasing histamine levels could be employed in AD patients as co-therapy due to their effects on cognitive functions, neuroplasticity, neuronal survival, neurogenesis, and the degradation of amyloid beta (Aβ) peptides. In this sense, the evaluation of the impact of HNMT inhibitors on animal models of AD would be interesting, consequently highlighting its relevance.

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.

The role of the central histaminergic system in emergence from propofol anesthesia

Background: The mechanisms of emergence from general anesthesia remain to be elucidated. Recent studies indicate that the central histaminergic system plays a critical role in maintaining wakefulness. Methods: Role of the central histaminergic system in emergence from propofol anesthesia using microinjections and single-unit recordings in rats was evaluated. Results: Intracerebroventricular (icv) microinjections of histamine decreased the emergence time in a dose-dependent manner and had an excitatory effect on the firing activity of medial prefrontal cortex (mPFC) neurons, while the decrease of emergence time was completely reversed by the pre-treatment with triprolidine (80 μg/5 μl) but not cimetidine (100 μg/5 μl). Moreover, the presumed histaminergic neurons fired in a state-dependent manner, and there was a dramatic increase in firing activity before regain of righting reflex. Furthermore, bidirectional manipulations of emergence were achieved through the microinjection of GABA (10 μg/side) and a potent H3 receptor inverse agonist ciproxian (1 μg/side) into the posterior hypothalamus, where the tuberomammillary nucleus (TMN) resides. Conclusion: Combining the behavioral and neurophysiologic evidence, the central histaminergic system promotes emergence from propofol anesthesia in rats. Our findings suggest an important role of the central histaminergic system in a broader field of state transitions, such as emergence from propofol anesthesia.

Biogenic amines in the colon

The gastrointestinal (GI) tract contains the highest concentration of biogenic amines in the human body. Neurons located in the GI tract, modulated by biogenic amines and various peptide and non-peptide transmitters, are called Enteric Nervous System (ENS). That explains why many medications used in neurology and psychiatry present side effects from the gut. Serotonin (5-hyroxytrypatamine, 5-HT), 95% of which is synthesized in the gut, is the most important amine (beside epinephrine and norepinephrine) colon functionality but another substances such as histamine, dopamine and melatonin are also potent in modulating intestine’s actions. Over 30 receptors for 5-HT were described in the human body, and 5-HT3, 5-HT4 and 5-HT7 are known to have the highest influence on motility and are a potent target for the drugs for treatment GI disorders, such as Irritable Bowel Syndrome (IBS) and Inflammatory Bowel Diseases (IBD). Histamine is a key biogenic amine for pathogenesis of allergy also in the colon. Alteration in histaminergic system is found in patients with diarrhea and allergic enteropathy. Dopamine affects functions of the large intestine but its modulating actions are more presented in the upper part of GI tract. Melatonin is best known for regulating circadian circle, but may also be a potent anti-inflammatory agent within the gut. Despite many years of research, it seems that more studies are needed to fully understand human colon neurochemistry.

Analysis of histaminergic system genes expression level in the early symptom stage of Parkinson’s disease

Целью данной работы было изучение изменений экспрессии генов гистаминергической системы в развитии патологических процессов при болезни Паркинсона (БП) на ранней симптомной стадии. На МФТП-индуцированной модели ранней симптомной стадии БП (мыши-самцы линии C57BL/6 по 10 шт. в контрольной и опытной группах) проанализирована экспрессия 9 генов, имеющих отношение к гистаминергической системе, методом обратной транскрипции и ПЦР в реальном времени (TaqMan). В результате были выявлены изменения на уровне мРНК для ряда генов в тканях мозга мышей, которые могут указывать на вовлеченность гистаминергической системы в патогенез БП на ранней стадии. The goal of this work was to study changes of histaminergic system genes expression level in the early symptom stage of PD. We were use MPTP mouse model of early symptom stage of PD in this study. Male mice C57BL/6 were used (n=10 in the control and experimental groups). For analysis we were choose nine genes, which associated with histaminergic system. Measuring of expression levels was carried out using reverse transcription reaction and real-time PCR (TaqMan). Changes at the mRNA level for a number of genes in the brain tissues of mice with MPTP-induced PD were founded. Based on these data, it can be suggested that histaminergic system is involve in early stage PD pathogenesis.