Histamine H1 receptors are activated by endogenous histamine, which is released by neurons that have their cell bodies in the tuberomammillary nucleus of the hypothalamus. The histaminergic neurons of the tuberomammillary nucleus become active during the 'wake' cycle, firing at approximately 2 Hz; during slow wave sleep, this firing rate drops to approximately 0.5 Hz. Finally, during REM sleep, histaminergic neurons stop firing altogether. It has been reported that histaminergic neurons have the most wake-selective firing pattern of all known neuronal types.
The tuberomammillary nucleus is a histaminergic nucleus that strongly regulates the sleep-wake cycle. H1-antihistamines that cross the blood-brain barrier inhibit H1 receptor activity on neurons that project from the tuberomammillary nucleus. This action is responsible for the drowsiness effect associated with these drugs.
^ abCanonica GW, Blaiss M (Feb 2011). "Antihistaminic, anti-inflammatory, and antiallergic properties of the nonsedating second-generation antihistamine desloratadine: a review of the evidence". The World Allergy Organization Journal. 4 (2): 47–53. doi:10.1097/WOX.0b013e3182093e19. PMC3500039. PMID23268457. The H1-receptor is a transmembrane protein belonging to the G-protein coupled receptor family. Signal transduction from the extracellular to the intracellular environment occurs as the GCPR becomes activated after binding of a specific ligand or agonist. A subunit of the G-protein subsequently dissociates and affects intracellular messaging including downstream signaling accomplished through various intermediaries such as cyclic AMP, cyclic GMP, calcium, and nuclear factor kappa B (NF-κB), a ubiquitous transcription factor thought to play an important role in immune-cell chemotaxis, proinflammatory cytokine production, expression of cell adhesion molecules, and other allergic and inflammatory conditions.1,8,12,30–32 ... For example, the H1-receptor promotes NF-κB in both a constitutive and agonist-dependent manner and all clinically available H1-antihistamines inhibit constitutive H1-receptor-mediated NF-κB production ...
Importantly, because antihistamines can theoretically behave as inverse agonists or neutral antagonists, they are more properly described as H1-antihistamines rather than H1-receptor antagonists.15
^Passani MB, Lin JS, Hancock A, Crochet S, Blandina P (Dec 2004). "The histamine H3 receptor as a novel therapeutic target for cognitive and sleep disorders". Trends in Pharmacological Sciences. 25 (12): 618–25. doi:10.1016/j.tips.2004.10.003. PMID15530639.
^Malenka RC, Nestler EJ, Hyman SE (2009). "Chapter 6: Widely Projecting Systems: Monoamines, Acetylcholine, and Orexin". In Sydor A, Brown RY. Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (2nd ed.). New York: McGraw-Hill Medical. pp. 175–176. ISBN9780071481274. Within the brain, histamine is synthesized exclusively by neurons with their cell bodies in the tuberomammillary nucleus (TMN) that lies within the posterior hypothalamus. There are approximately 64000 histaminergic neurons per side in humans. These cells project throughout the brain and spinal cord. Areas that receive especially dense projections include the cerebral cortex, hippocampus, neostriatum, nucleus accumbens, amygdala, and hypothalamus. ... While the best characterized function of the histamine system in the brain is regulation of sleep and arousal, histamine is also involved in learning and memory ... It also appears that histamine is involved in the regulation of feeding and energy balance.
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