Paul L. Chazot

The histamine H4 receptor is functionally expressed on neurons in the mammalian CNS

Background and purpose:  The histamine H4 receptor is the most recently identified of the G protein‐coupled histamine receptor family and binds several neuroactive drugs, including amitriptyline and clozapine. So far, H4 receptors have been found only on haematopoietic cells, highlighting its importance in inflammatory conditions. Here we investigated the possibility that H4 receptors may be expressed in both the human and mouse CNS.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein–coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Molecular and biochemical pharmacology of the histamine H4 receptor

The elucidation of the human genome has had a major impact on histamine receptor research. The identification of the human H4 receptor by several groups has been instrumental for a new appreciation of the role of histamine in the modulation of immune function. In this review, we summarize the historical developments and the molecular and biochemical pharmacology of the H4 receptor.

Human Inflammatory Dendritic Epidermal Cells Express a Functional Histamine H4 Receptor

Expression of histamine H(4) receptor (H(4)R) on leukocytes suggests an immunomodulatory role of this receptor. Here we investigated the expression and function of H(4)R on human inflammatory dendritic epidermal cells (IDECs). H(4)R is expressed by IDEC of the skin. On monocyte-derived IDECs (Mo-IDECs), H(4)R is also expressed and upregulated by IFN-gamma. Functionally, histamine and H(4)R agonists clobenpropit and 4-methylhistamine downregulated the production of the Th2-linked chemokine CCL2 and the Th1 cytokine IL-12 on Mo-IDEC, whereas agonists for the other histamine receptors did not. An H(4)R-selective antagonist (JNJ7777120) blocked the effect of H(4)R agonists. Downregulation of CCL2 also led to a decreased migration of monocytes. Thus, IDEC express a functionally active H(4)R, which upon stimulation leads to downregulation of CCL2 and IL-12. This might have implications for the treatment of atopic dermatitis, since H(4)R agonists may have beneficial effects in downregulating inflammation.

Histamine H4 receptors modulate dendritic cell migration through skin – immunomodulatory role of histamine

Background:  Dendritic cells (DC) are the major antigen‐presenting cells and play a key role in adaptive immunity as they are able to activate naive T cells. It was recently described, that the histamine H4 receptor (H4R) is present on human monocyte‐derived DC and that chemotaxis and T‐helper (Th)1–Th2 polarization is mediated by this receptor. However, the distribution of histamine receptors on murine DC has not been studied yet.

Identification, molecular cloning and characterization of a novel GABA-A receptor associated protein, GRIF-1

A novel 913-amino acid protein, γ-aminobutyric acid type A (GABAA) receptor interacting factor-1 (GRIF-1), has been cloned and identified as a GABAAreceptor-associated protein by virtue of its specific interaction with the GABAA receptor β2 subunit intracellular loop in a yeast two-hybrid assay. GRIF-1 has no homology with proteins of known function, but it is the rat orthologue of the human ALS2CR3/KIAA0549 gene. GRIF-1 is expressed as two alternative splice forms, GRIF-1a and a C-terminally truncated form, GRIF-1b. GRIF-1 mRNA has a wide distribution with a major transcript size of 6.2 kb. GRIF-1a protein is only expressed in excitable tissues, i.e. brain, heart, and skeletal muscle major immunoreactive bands ofM r ∼ 115 and 106 kDa and, in muscle and heart only, an additional 88-kDa species. When expressed in human embryonic kidney 293 cells, GRIF-1a yielded three immunoreactive bands withM r ∼ 115, 106, and 98 kDa. Co-expression of GRIF-1a and α1β2γ2 GABAA receptors in mammalian cells revealed some co-localization in the cell cytoplasm. Anti-FLAG-agarose specifically precipitated GRIF-1FLAG and GABAAreceptor β2 subunits from human embryonic kidney 293 cells co-transfected with GRIF-1aFLAG and β2 subunit clones. Further, immobilized GRIF-1-(8–633) specifically precipitated in vitro GABAA receptor α1 and β2 subunit immunoreactivities from detergent extracts of adult rat brain. The respective GABAA receptor β2 subunit/GRIF-1 binding domains were mapped using the yeast two-hybrid reporter gene assays. A possible role for GRIF-1 as a GABAA receptor β2 subunit trafficking factor is proposed.

Immunohistochemical localization of histamine H3 receptors in rodent skin, dorsal root ganglia, superior cervical ganglia, and spinal cord: Potential antinociceptive targets

Abstract Activation of histamine H3 receptors (H3Rs) reduces inflammation and nociception, but the existence of H3Rs on peripheral innervation has never been demonstrated. Here we use antibodies to locate H3Rs in whisker pads, hairy and glabrous hind paw skin, dorsal root ganglia (DRGs), and spinal cords of rats, wild type mice, and H3R knockout (H3KO) mice. Although H3Rs have been hypothesized to be on C and sympathetic fibers, H3R‐like immunoreactivity (H3R‐LI) was only detected on presumptive periarterial Aδ fibers and on Aβ fibers that terminated in Meissner’s corpuscles and as lanceolate endings around hair follicles. The H3R‐positive periarterial fibers were thin‐caliber and coexpressed immunoreactivity for calcitonin gene‐related peptide (CGRP), substance P, acid sensing ion channel 3, and 200 kDa neurofilament protein (NF). H3R‐LI was also detected on epidermal keratinocytes and Merkel cells, but not on Merkel endings, C fibers, any other Aδ fibers, or sympathetic fibers. In DRGs, H3R‐LI was preponderantly on medium to large neurons coexpressing NF‐LI and mostly CGRP‐LI. In dorsal horn, CGRP‐positive fibers with and without H3R‐LI ramified extensively in lamina II; many of the former formed a plexus in lamina V. Low levels of H3R‐LI were also present on Aβ fibers penetrating superficial and into deeper laminae. The distribution of H3R‐LI was similar in rats and wild type mice, but was eliminated or strongly reduced in Aδ fibers and Aβ fibers, respectively, in H3KO mice. Taken with recently published behavioral results, the present findings suggest that periarterial, peptidergic, H3R‐containing Aδ fibers may be sources of high threshold mechanical nociception.

Histamine pharmacology and new CNS drug targets.

During the last decade, the identification of a number of novel drug targets led to the development of promising new compounds which are currently under evaluation for their therapeutic prospective in CNS related disorders. Besides the established pleiotropic regulatory functions in the periphery, the interest in the potential homeostatic role of histamine in the brain was revived following the identification of H3 and H4 receptors some years ago. Complementing classical CNS pharmacology, the development of selective histamine receptor agonists, antagonists, and inverse agonists provides the lead for the potential exploitation of the histaminergic system in the treatment of brain pathologies. Although no CNS disease entity has been associated directly to brain histamine dysfunction until now, the H3 receptor is recognized as a drug target for neuropathic pain, sleep‐wake disorders, including narcolepsy, and cognitive impairment associated with attention deficit hyperactivity disorder, schizophrenia, Alzheimer’s, or Parkinsons disease, while the first H3 receptor ligands have already entered phase I–III clinical trials. Interestingly, the localization of the immunomodulatory H4 receptor in the nervous system exposes attractive perspectives for the therapeutic exploitation of this new drug target in neuroimmunopharmacology. This review focuses on a concise presentation of the current “translational research” approach that exploits the latest advances in histamine pharmacology for the development of beneficial drug targets for the treatment of neuronal disorders, such as neuropathic pain, cognitive, and sleep‐wake pathologies. Furthermore, the role of the brain histaminergic system(s) in neuroprotection and neuroimmunology/inflammation remains a challenging research area that is currently under consideration.

Immunohistochemical localization of N-methyl-D-aspartate receptor subunits in the adult murine hippocampal formation: evidence for a unique role of the NR2D subunit.

NMDA receptors were immunopurified from adult mouse forebrain and screened by immunoblotting. NR1 was co-associated with NR2A, NR2B and NR2D but not NR2C, nor was NR2C detected in adult mouse hippocampal membranes. The anatomical distribution of NR1, 2A, 2B and 2D was mapped in the adult murine hippocampal formation. NR1-like immunoreactivity was localised to cell bodies of pyramidal neurons, granule cells and hilar cells of the dentate gyrus. Apical dendrites of the CA subfields and hilar cells were also immunopositive. NR2A- and NR2B-like immunoreactivity essentially co-localised with that of NR1 implying co-assembly of all three subunits in this brain structure. NR2D-like immunoreactivity was distinct, being totally excluded from pyramidal, granule and hilar cell bodies. Strong, punctate staining was restricted to the oriens layer of CA1 and the stratum lucidum of CA3 consistent with labelling of presynaptic receptors. Less intense staining was also observed in the internal third of the molecular layer of the dentate gyrus.

A Double-Blind Placebo-Controlled Randomized Trial of Melissa officinalis Oil and Donepezil for the Treatment of Agitation in Alzheimer’s Disease

Background/Aims: Behavioural and psychological symptoms (BPSD) are frequent in people with Alzheimer’s disease and cause considerable stress to patients and their carers. Antipsychotics have been widely used as a first-line treatment, resulting in an estimated 1,800 excess strokes and 1,600 excess deaths in the UK alone. Safe and effective alternatives are urgently needed. Based upon preliminary evidence from clinical trials, aromatherapy with melissa oil may be such an alternative, but initial studies have been modest in size, and adequate blinding has been problematic. Our objective was to assess the efficacy of melissa aromatherapy in the treatment of agitation in people with Alzheimer’s disease in an adequately powered and robustly blinded randomized controlled trial comparing it with donepezil, an anticholinesterase drug used with some benefit to treat BPSD. Methods and Findings: The study was a double-blind parallel-group placebo-controlled randomized trial across 3 specialist old age psychiatry centres in England. Participants had probable or possible Alzheimer’s disease, were resident in a care home, had clinically significant agitation (defined as a score of 39 or above on the Cohen Mansfield Agitation Inventory) and were free of antipsychotics and/or anticholinesterase for at least 2 weeks. Participants were allocated to 1 of 3 groups: placebo medication and active aromatherapy; active medication and placebo aromatherapy or placebo of both. Main Outcome: The primary outcome measure was reduction in agitation as assessed by the Pittsburgh Agitation Scale (PAS) at 4 weeks. This is an observational scale, and raters were required to wear nose clips to ensure that full blinding was maintained. The PAS, Neuropsychiatric Inventory (NPI; another measure of BPSD) and other outcome measures were completed at baseline, 4-week and 12-week follow-ups. 114 participants were randomized, of whom 94 completed the week 4 assessment and 81 completed the week 12 assessment. Aromatherapy and donepezil were well tolerated. There were no significant differences between aromatherapy, donepezil and placebo at week 4 and week 12, but importantly there were substantial improvements in all 3 groups with an 18% improvement in the PAS and a 37% improvement in the NPI over 12 weeks. Conclusion: When assessed using a rigorous design which ensures blinding of treatment arms, there is no evidence that melissa aromatherapy is superior to placebo or donepezil, in the treatment of agitation in people with Alzheimer’s disease. However, the sizeable improvement in the placebo group emphasizes the potential non-specific benefits of touch and interaction in the treatment of agitation in people with Alzheimer’s disease.