![Figure][1] Among the four G protein–coupled receptors (H1–H4) identified as mediators of the biologic effects of histamine, the H3 receptor (H3R) is distinguished for its almost exclusive expression in the nervous system and the large variety of isoforms generated by alternative splicing of the corresponding mRNA. Additionally, it exhibits dual functionality as autoreceptor and heteroreceptor, and this enables H3Rs to modulate the histaminergic and other neurotransmitter systems. The cloning of the H3R cDNA in 1999 by Lovenberg et al. allowed for detailed studies of its molecular aspects. In this work, we review the characteristics of the H3R, namely, its structure, constitutive activity, isoforms, signal transduction pathways, regional differences in expression and localization, selective agonists, antagonists and inverse agonists, dimerization with other neurotransmitter receptors, and the main presynaptic and postsynaptic effects resulting from its activation. The H3R has attracted interest as a potential drug target for the treatment of several important neurologic and psychiatric disorders, such as Alzheimer and Parkinson diseases, Gilles de la Tourette syndrome, and addiction. [1]: pending:yes

Particulate matter (PM) adverse effects on health include lung and heart damage. The renin-angiotensin-aldosterone (RAAS) and kallikrein-kinin (KKS) endocrine systems are involved in the pathophysiology of cardiovascular diseases and have been found to impact lung diseases. The aim of the present study was to evaluate whether PM exposure regulates elements of RAAS and KKS. Sprague–Dawley rats were acutely (3 days) and subchronically (8 weeks) exposed to coarse (CP), fine (FP) or ultrafine (UFP) particulates using a particulate concentrator, and a control group exposed to filtered air (FA). We evaluated the mRNA of the RAAS components At1, At2r and Ace, and of the KKS components B1r, B2r and Klk-1 by RT-PCR in the lungs and heart. The ACE and AT1R protein were evaluated by Western blot, as were HO-1 and γGCSc as indicators of the antioxidant response and IL-6 levels as an inflammation marker. We performed a binding assay to determinate AT1R density in the lung, also the subcellular AT1R distribution in the lungs was evaluated. Finally, we performed a histological analysis of intramyocardial coronary arteries and the expression of markers of heart gene reprogramming (Acta1 and Col3a1). The PM fractions induced the expression of RAAS and KKS elements in the lungs and heart in a time-dependent manner. CP exposure induced Ace mRNA expression and regulated its protein in the lungs. Acute and subchronic exposure to FP and UFP induced the expression of At1r in the lungs and heart. All PM fractions increased the AT1R protein in a size-dependent manner in the lungs and heart after subchronic exposure. The AT1R lung protein showed a time-dependent change in subcellular distribution. In addition, the presence of AT1R in the heart was accompanied by a decrease in HO-1, which was concomitant with the induction of Acta1 and Col3a1 and the increment of IL-6. Moreover, exposure to all PM fractions increased coronary artery wall thickness. We demonstrate that exposure to PM induces the expression of RAAS and KKS elements, including AT1R, which was the main target in the lungs and the heart.

Background and Purpose: An alanine to valine exchange at amino acid position 280 (A280V) in the third intracellular loop of the human histamine H₃ receptor was first identified in a patient suffering from Shy–Drager syndrome and later reported as a risk factor for migraine. Here, we have compared the pharmacological and signalling properties of wild‐type (hH₃R_WT) and A280V mutant (hH₃R_A280V) receptors stably expressed in CHO‐K1 cells.Experimental Approach: The hH₃R_A280V cDNA was created by overlapping extension PCR amplification. Receptor expression and affinity were assessed by radioligand (N‐α‐[methyl‐³H]‐histamine) binding to cell membranes, and receptor function by the inhibition of forskolin‐induced cAMP accumulation and stimulation of ERK1/2 phosphorylation in intact cells, as well as stimulation of [³⁵S]‐GTPγS binding to cell membranes.Key Results: Both receptors were expressed at similar levels with no significant differences in their affinities for H₃ receptor ligands. Upon activation the hH₃R_WT was significantly more efficacious to inhibit forskolin‐induced cAMP accumulation and to stimulate [³⁵S]‐GTPγS binding, with no difference in pEC₅₀ estimates. The hH₃R_WT was also more efficacious to stimulate ERK1/2 phosphorylation, but this difference was not significant. The inverse agonist ciproxifan was more efficacious at hH₃R_WT to reduce [³⁵S]‐GTPγS binding but, for both receptors, failed to enhance forskolin‐induced cAMP accumulation.Conclusions and Implications: The A280V mutation reduces the signalling efficacy of the human H₃ receptor. This effect may be relevant to the pathophysiology of disorders associated with the mutation.Linked Articles: This article is part of a themed issue on Histamine Pharmacology Update. To view the other articles in this issue visit http://dx.doi.org/ 10.1111/bph.2013.170.issue‐1