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Arumuham A, Nour MM, Veronese M, Onwordi EC, Rabiner EA, Howes OD. The histamine system and cognitive function: An in vivo H3 receptor PET imaging study in healthy volunteers and patients with schizophrenia. J Psychopharmacol 2023; 37:1011-1022. [PMID: 37329185 PMCID: PMC10612380 DOI: 10.1177/02698811231177287] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 06/18/2023]
Abstract
BACKGROUND The histamine-3 receptor (H3R) is an auto- and heteroreceptor that inhibits the release of histamine and other neurotransmitters. Post-mortem evidence has found altered H3R expression in patients with psychotic disorders, which may underlie cognitive impairment associated with schizophrenia (CIAS). AIMS We used positron emission tomography (PET) imaging to compare brain uptake of an H3R selective tracer between patients with schizophrenia and matched controls (healthy individuals). Regions of interest included the dorsolateral prefrontal cortex (DLPFC) and striatum. We explored correlations between tracer uptake and symptoms, including cognitive domains. METHODS A total of 12 patients and 12 matched controls were recruited to the study and were assessed with psychiatric and cognitive rating scales. They received a PET scan using the H3R-specific radioligand [11C]MK-8278 to determine H3R availability. RESULTS There was no statistically significant difference in tracer uptake between patients and controls in the DLPFC (t19 = 0.79, p = 0.44) or striatum (t21 = 1.18, p = 0.25). An exploratory analysis found evidence for lower volume of distribution in the left cuneus (pFWE-corrected = 0.01). DLPFC tracer uptake was strongly correlated with cognition in controls (trail making test (TMT) A: r = 0.77, p = 0.006; TMT B: rho = 0.74, p = 0.01), but not in patients (TMT A: r = -0.18, p = 0.62; TMT B: rho = -0.06, p = 0.81). CONCLUSIONS These findings indicate H3R in the DLPFC might play a role in executive function and this is disrupted in schizophrenia in the absence of major alterations in H3R availability as assessed using a selective radiotracer for H3R. This provides further evidence for the role of H3R in CIAS.
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Affiliation(s)
- Atheeshaan Arumuham
- Department of Psychosis Studies, Institute of Psychiatry, Psychology & Neuroscience, Kings College London, London, UK
- Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London, UK
- Psychiatric Imaging Group, Medical Research Council, London Institute of Medical Sciences, Hammersmith Hospital, London, UK
| | - Matthew M Nour
- Department of Psychosis Studies, Institute of Psychiatry, Psychology & Neuroscience, Kings College London, London, UK
- Department of Psychiatry, University of Oxford, Oxford, UK
- Max Planck University College London Centre for Computational Psychiatry and Ageing Research, London, UK
| | - Mattia Veronese
- Department of Information Engineering, University of Padua, Padua, Italy
- Department of Neuroimaging, Institute of Psychiatry, Psychology and Neuroscience, King’s College London, London, UK
| | - Ellis Chika Onwordi
- Department of Psychosis Studies, Institute of Psychiatry, Psychology & Neuroscience, Kings College London, London, UK
- Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London, UK
- Psychiatric Imaging Group, Medical Research Council, London Institute of Medical Sciences, Hammersmith Hospital, London, UK
- Centre for Psychiatry and Mental Health, Wolfson Institute of Population Health, Queen Mary University of London, London, UK
| | - Eugenii A Rabiner
- Department of Neuroimaging, Institute of Psychiatry, Psychology and Neuroscience, King’s College London, London, UK
- Invicro, London, UK
| | - Oliver D Howes
- Department of Psychosis Studies, Institute of Psychiatry, Psychology & Neuroscience, Kings College London, London, UK
- Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London, UK
- Psychiatric Imaging Group, Medical Research Council, London Institute of Medical Sciences, Hammersmith Hospital, London, UK
- H Lundbeck A/s, St Albans, UK
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Xu J, Pittenger C. The histamine H3 receptor modulates dopamine D2 receptor-dependent signaling pathways and mouse behaviors. J Biol Chem 2023; 299:104583. [PMID: 36871761 PMCID: PMC10139999 DOI: 10.1016/j.jbc.2023.104583] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2022] [Revised: 02/20/2023] [Accepted: 02/21/2023] [Indexed: 03/06/2023] Open
Abstract
The histamine H3 receptor (H3R) is highly enriched in the spiny projection neurons (SPNs) of the striatum, in both the D1 receptor (D1R)-expressing and D2 receptor (D2R)-expressing populations. A crossantagonistic interaction between H3R and D1R has been demonstrated in mice, both at the behavioral level and at the biochemical level. Although interactive behavioral effects have been described upon coactivation of H3R and D2R, the molecular mechanisms underlying this interaction are poorly understood. Here, we show that activation of H3R with the selective agonist R-(-)-α-methylhistamine dihydrobromide mitigates D2R agonist-induced locomotor activity and stereotypic behavior. Using biochemical approaches and the proximity ligation assay, we demonstrated the existence of an H3R-D2R complex in the mouse striatum. In addition, we examined consequences of simultaneous H3R-D2R agonism on the phosphorylation levels of several signaling molecules using immunohistochemistry. H3R agonist treatment modulated Akt (serine/threonine PKB)-glycogen synthase kinase 3 beta signaling in response to D2R activation via a β-arrestin 2-dependent mechanism in D2R-SPNs but not in D1R-SPNs. Phosphorylation of mitogen- and stress-activated protein kinase 1 and rpS6 (ribosomal protein S6) was largely unchanged under these conditions. As Akt-glycogen synthase kinase 3 beta signaling has been implicated in several neuropsychiatric disorders, this work may help clarify the role of H3R in modulating D2R function, leading to a better understanding of pathophysiology involving the interaction between histamine and dopamine systems.
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Affiliation(s)
- Jian Xu
- Department of Psychiatry, Yale University. ,
| | - Christopher Pittenger
- Department of Psychiatry, Yale University; Department of Psychology, Yale University; Department of Child Study Center, Yale University; Department of Interdepartmental Neuroscience Program, Yale University; Department of Wu-Tsai Institute, Yale University; Department of Center for Brain and Mind Health, Yale University.
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Xi L, Ji X, Ji W, Yang Y, Zhang Y, Long H. Jing-an oral liquid alleviates Tourette syndrome via the NMDAR/MAPK/CREB pathway in vivo and in vitro. PHARMACEUTICAL BIOLOGY 2022; 60:1790-1800. [PMID: 36102587 PMCID: PMC9487928 DOI: 10.1080/13880209.2022.2116056] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/08/2022] [Revised: 07/01/2022] [Accepted: 08/13/2022] [Indexed: 06/15/2023]
Abstract
CONTEXT Jing-an oral liquid (JA) is a Chinese herbal formula used in the treatment of Tourette syndrome (TS); however, its mechanism is unclear. OBJECTIVE To investigate the effects of JA on amino acid neurotransmitters and microglia activation in vivo and in vitro. MATERIALS AND METHODS Sixty male Sprague-Dawley rats were divided into a control group and 5 TS groups. TS was induced in rats with intraperitoneal injection of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (1 mg/kg) and in BV2 cells with lipopolysaccharide. Control and model rats were administered saline, whereas treatment groups were administered JA (5.18, 10.36, or 20.72 g/kg) or tiapride (a benzamide, 23.5 mg/kg) by gavage once daily for 21 days. Stereotypic behaviour was tested. The levels of N-methyl-d-aspartate receptor (NMDAR)/mitogen-activated protein kinase/cAMP response element-binding protein (CREB)-related proteins in striatum and BV2 cells were measured via western blots. CD11b and IBa1 levels were also measured. Ultra-high-performance liquid-chromatography was used to determine γ-aminobutyric acid (GABA), glutamic acid (Glu), and aspartic acid (ASP) levels. RESULTS JA markedly alleviated the stereotype behaviour (25.92 ± 0.35 to 13.78 ± 0.47) in rats. It also increased NMDAR1 (0.48 ± 0.09 to 0.67 ± 0.08; 0.54 ± 0.07 to 1.19 ± 0.18) expression and down-regulated the expression of p-ERK, p-JNK, p-P38, and p-CREB in BV2 cells and rat striatum. Additionally, Glu, ASP, GABA, CD11b, and IBa1 levels were significantly decreased by JA. DISCUSSION AND CONCLUSIONS JA suppressed microglia activation and regulated the levels of amino acid neurotransmitters, indicating that it could be a promising therapeutic agent for TS.
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Affiliation(s)
- Leying Xi
- Department of Pediatrics, Nanjing Hospital of Chinese Medicine Affiliated to Nanjing University of Chinese Medicine, Nanjing, China
- Department of Pediatric, Nanjing University of Chinese Medicine, Nanjing, China
| | - Xixi Ji
- Department of Pediatrics, Nanjing Hospital of Chinese Medicine Affiliated to Nanjing University of Chinese Medicine, Nanjing, China
- Department of Pediatric, Nanjing University of Chinese Medicine, Nanjing, China
| | - Wenxiu Ji
- Department of Pediatrics, Nanjing Hospital of Chinese Medicine Affiliated to Nanjing University of Chinese Medicine, Nanjing, China
- Department of Pediatric, Nanjing University of Chinese Medicine, Nanjing, China
| | - Yue’e Yang
- Department of Pediatrics, Nanjing Hospital of Chinese Medicine Affiliated to Nanjing University of Chinese Medicine, Nanjing, China
| | - Yajie Zhang
- Central Laboratory, Nanjing Hospital of Chinese Medicine Affiliated to Nanjing University of Chinese Medicine, Nanjing, China
- Clinical Biobank of Nanjing Hospital of Chinese Medicine, Nanjing Hospital of Chinese Medicine, Nanjing University of Chinese Medicine, Nanjing, China
| | - Hongyan Long
- Department of Pediatrics, Nanjing Hospital of Chinese Medicine Affiliated to Nanjing University of Chinese Medicine, Nanjing, China
- Department of Pediatric, Nanjing University of Chinese Medicine, Nanjing, China
- Central Laboratory, Nanjing Hospital of Chinese Medicine Affiliated to Nanjing University of Chinese Medicine, Nanjing, China
- Clinical Biobank of Nanjing Hospital of Chinese Medicine, Nanjing Hospital of Chinese Medicine, Nanjing University of Chinese Medicine, Nanjing, China
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Xu L, Zhang C, Zhong M, Che F, Guan C, Zheng X, Liu S. Role of histidine decarboxylase gene in the pathogenesis of Tourette syndrome. Brain Behav 2022; 12:e2511. [PMID: 35114079 PMCID: PMC8933785 DOI: 10.1002/brb3.2511] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/01/2021] [Revised: 11/18/2021] [Accepted: 01/06/2022] [Indexed: 12/15/2022] Open
Abstract
Tourette syndrome (TS) is caused by complex genetic and environmental factors and is characterized by tics. Histidine decarboxylase (HDC) mutation is a rare genetic cause with high penetrance in patients with TS. HDC-knockout (KO) mice have similar behavioral and neurochemical abnormalities as patients with TS. Therefore, HDC-KO mice are considered a valuable TS pathophysiological model as it reveals the underlying pathological mechanisms that cannot be obtained from patients with TS, thus advancing the development of treatment strategies for TS and other tic disorders. This review summarizes some of the recent research hotspots and progress in HDC-KO mice, aiming to deepen our understanding of brain mechanisms relevant to TS. Furthermore, we encapsulate the possible brain nerve cell changes in HDC-KO mice and their potential roles in TS to provide multiple directions for the future research on tics.
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Affiliation(s)
- Lulu Xu
- Department of Geriatric Medicine, The Affiliated Hospital of Qingdao University, Qingdao, Shandong, China
| | - Cheng Zhang
- Department of Neurology, The Eleventh Clinical Medical College of Qingdao University, Linyi People's Hospital, Linyi, Shandong, China
| | - Meixiang Zhong
- Department of Geriatric Medicine, The Affiliated Hospital of Qingdao University, Qingdao, Shandong, China
| | - Fengyuan Che
- Department of Neurology, The Eleventh Clinical Medical College of Qingdao University, Linyi People's Hospital, Linyi, Shandong, China
| | - Chengcheng Guan
- Department of Medical Cenetics, The Affiliated Hospital of Qingdao University, Qingdao, Shandong, China
| | - Xueping Zheng
- Department of Geriatric Medicine, The Affiliated Hospital of Qingdao University, Qingdao, Shandong, China
| | - Shiguo Liu
- Department of Medical Cenetics, The Affiliated Hospital of Qingdao University, Qingdao, Shandong, China
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Protective effect of Asarum sieboldii essential oil on ovalbumin induced allergic rhinitis in rat. Biosci Rep 2021; 40:224117. [PMID: 32395767 PMCID: PMC7268255 DOI: 10.1042/bsr20191370] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2019] [Revised: 04/24/2020] [Accepted: 05/07/2020] [Indexed: 02/05/2023] Open
Abstract
BACKGROUND The study was aimed to investigate the protective effect of Asarum sieboldii Miq. essential oil (AEO) on ovalbumin (OVA)-induced allergic rhinitis (AR) in rats. METHODS AND RESULTS Sixty Sprague-Dawley male rats were randomly divided into six groups (n=10): control, model, cetirizine (Cet, 4.65 g/kg), and AEO (0.5, 1.5, 3 g/kg) groups. All animals except the control group received repeated intranasal instillation with 20 μl of 20% OVA in Al(OH)3 saline solvent for 15 days. The control group was intranasally instilled with 5 mg/ml of Al(OH)3 instead of the same procedure. In the 15 days, Cet and AEO were orally administrated for 28 days. At the end of the drug administration, 20 μl of 5% OVA was given to animals to stimulate allergic reaction, then the rat behavioral detection, assessment of the patho-morphological changes in nasal mucosa, and the serum biomarkers were determined. The result showed that AEO could significantly reduce the amount of nasal secretions, sneezing, and the degree of nasal scratching in AR rats with EC50 = 1.5 and 2.8 g/kg, respectively. The degree of nasal mucosal inflammation in AEO group improved, the levels of immunoglobulin E (IgE), histamine, IL-4, IL-5, IL-17 were decreased, and the level of IFN-γ was increased obviously with EC50 = 2 g/kg. CONCLUSION The study suggested that the possible mechanism might be related with the inhibition of histamine release and regulation of the cytokine levels, which plays an important role in the treatment of AR.
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Manz KM, Becker JC, Grueter CA, Grueter BA. Histamine H 3 Receptor Function Biases Excitatory Gain in the Nucleus Accumbens. Biol Psychiatry 2021; 89:588-599. [PMID: 33012522 PMCID: PMC7865000 DOI: 10.1016/j.biopsych.2020.07.023] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/09/2020] [Revised: 07/15/2020] [Accepted: 07/16/2020] [Indexed: 01/04/2023]
Abstract
BACKGROUND Histamine (HA), a wake-promoting monoamine implicated in stress-related arousal states, is synthesized in histidine decarboxylase-expressing hypothalamic neurons of the tuberomammillary nucleus. Histidine decarboxylase-containing varicosities diffusely innervate striatal and mesolimbic networks, including the nucleus accumbens (NAc). The NAc integrates diverse monoaminergic inputs to coordinate motivated behavior. While the NAc expresses various HA receptor subtypes, mechanisms by which HA modulates NAc circuit dynamics are undefined. METHODS Using male D1tdTomato transgenic reporter mice, whole-cell patch-clamp electrophysiology, and input-specific optogenetics, we employed a targeted pharmacological approach to interrogate synaptic mechanisms recruited by HA signaling at glutamatergic synapses in the NAc. We incorporated an immobilization stress protocol to assess whether acute stress engages these mechanisms at glutamatergic synapses onto D1 receptor-expressing [D1(+)] medium spiny neurons (MSNs) in the NAc core. RESULTS HA negatively regulates excitatory gain onto D1(+)-MSNs via presynaptic H3 receptor-dependent long-term depression that requires Gβγ-directed Akt-GSK3β signaling. Furthermore, HA asymmetrically regulates glutamatergic transmission from the prefrontal cortex and mediodorsal thalamus, with inputs from the prefrontal cortex undergoing robust HA-induced long-term depression. Finally, we report that acute immobilization stress attenuates this long-term depression by recruiting endogenous H3 receptor signaling in the NAc at glutamatergic synapses onto D1(+)-MSNs. CONCLUSIONS Stress-evoked HA signaling in the NAc recruits H3 heteroreceptor signaling to shift thalamocortical input onto D1(+)-MSNs in the NAc. Our findings provide novel insight into an understudied neuromodulatory system within the NAc and implicate HA in stress-associated physiological states.
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Affiliation(s)
- Kevin M Manz
- Medical Scientist Training Program, Vanderbilt University, Nashville, Tennessee; Vanderbilt Brain Institute, Vanderbilt University, Nashville, Tennessee; Department of Anesthesiology, Vanderbilt University Medical Center, Nashville, Tennessee
| | - Jennifer C Becker
- Department of Anesthesiology, Vanderbilt University Medical Center, Nashville, Tennessee
| | - Carrie A Grueter
- Vanderbilt Brain Institute, Vanderbilt University, Nashville, Tennessee; Department of Anesthesiology, Vanderbilt University Medical Center, Nashville, Tennessee
| | - Brad A Grueter
- Vanderbilt Brain Institute, Vanderbilt University, Nashville, Tennessee; Vanderbilt Center for Addiction Research, Vanderbilt University, Nashville, Tennessee; Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee; Department of Anesthesiology, Vanderbilt University Medical Center, Nashville, Tennessee.
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Xu J, Liu RJ, Fahey S, Frick L, Leckman J, Vaccarino F, Duman RS, Williams K, Swedo S, Pittenger C. Antibodies From Children With PANDAS Bind Specifically to Striatal Cholinergic Interneurons and Alter Their Activity. Am J Psychiatry 2021; 178:48-64. [PMID: 32539528 PMCID: PMC8573771 DOI: 10.1176/appi.ajp.2020.19070698] [Citation(s) in RCA: 33] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
OBJECTIVE Pediatric obsessive-compulsive disorder (OCD) sometimes appears rapidly, even overnight, often after an infection. Pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections, or PANDAS, describes such a situation after infection with Streptococcus pyogenes. PANDAS may result from induced autoimmunity against brain antigens, although this remains unproven. Pilot work suggests that IgG antibodies from children with PANDAS bind to cholinergic interneurons (CINs) in the striatum. CIN deficiency has been independently associated with tics in humans and with repetitive behavioral pathology in mice, making it a plausible locus of pathology. The authors sought to replicate and extend earlier work and to investigate the cellular effects of PANDAS antibodies on cholinergic interneurons. METHODS Binding of IgG to specific neurons in human and mouse brain slices was evaluated ex vivo after incubation with serum from 27 children with rigorously characterized PANDAS, both at baseline and after intravenous immunoglobulin (IVIG) treatment, and 23 matched control subjects. Binding was correlated with symptom measures. Neural activity after serum incubation was assessed in mouse slices using molecular markers and electrophysiological recording. RESULTS IgG from children with PANDAS bound to CINs, but not to several other neuron types, more than IgG from control subjects, in three independent cohorts of patients. Post-IVIG serum had reduced IgG binding to CINs, and this reduction correlated with symptom improvement. Baseline PANDAS sera decreased activity of striatal CINs, but not of parvalbumin-expressing GABAergic interneurons, and altered their electrophysiological responses, in acute mouse brain slices. Post-IVIG PANDAS sera and IgG-depleted baseline sera did not alter the activity of striatal CINs. CONCLUSIONS These findings provide strong evidence for striatal CINs as a critical cellular target that may contribute to pathophysiology in children with rapid-onset OCD symptoms, and perhaps in other conditions.
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Affiliation(s)
- Jian Xu
- Department of Psychiatry, Yale University School of
Medicine, 34 Park Street, New Haven, CT 06519
| | - Rong-Jian Liu
- Department of Psychiatry, Yale University School of
Medicine, 34 Park Street, New Haven, CT 06519
| | - Shaylyn Fahey
- Department of Psychiatry, Yale University School of
Medicine, 34 Park Street, New Haven, CT 06519
| | - Luciana Frick
- Department of Psychiatry, Yale University School of
Medicine, 34 Park Street, New Haven, CT 06519,Current address: Hunter James Kelly Research Institute,
University at Buffalo
| | - James Leckman
- Child Study Center, Yale University School of
Medicine,Department of Pediatrics, Yale University School of
Medicine
| | - Flora Vaccarino
- Child Study Center, Yale University School of
Medicine,Department of Neuroscience, Yale University School of
Medicine
| | - Ronald S. Duman
- Department of Psychiatry, Yale University School of
Medicine, 34 Park Street, New Haven, CT 06519
| | - Kyle Williams
- Department of Psychiatry, Yale University School of
Medicine, 34 Park Street, New Haven, CT 06519,Current address: Department of Psychiatry, Massachusetts
General Hospital and Harvard Medical School
| | - Susan Swedo
- Pediatrics and Developmental Neuroscience Branch, National
Institute of Mental Health,PANDAS Physicians Network
| | - Christopher Pittenger
- Department of Psychiatry, Yale University School of
Medicine, 34 Park Street, New Haven, CT 06519,Child Study Center, Yale University School of
Medicine,Interdepartmental Neuroscience Program, Yale
University,Address correspondence to: Christopher Pittenger,
Yale University School of Medicine, 34 Park Street 333b, New Haven, CT 06519.
Phone: 203-974-7675.
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Abdurakhmanova S, Semenova S, Piepponen TP, Panula P. Abnormal behavior, striatal dopamine turnover and opioid peptide gene expression in histamine‐deficient mice. GENES BRAIN AND BEHAVIOR 2019; 18:e12595. [DOI: 10.1111/gbb.12595] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/07/2019] [Revised: 06/11/2019] [Accepted: 06/11/2019] [Indexed: 12/17/2022]
Affiliation(s)
| | | | - T. Petteri Piepponen
- Division of Pharmacology and PharmacotherapyUniversity of Helsinki Helsinki Finland
| | - Pertti Panula
- Department of AnatomyUniversity of Helsinki Helsinki Finland
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Pittenger C. The histidine decarboxylase model of tic pathophysiology: a new focus on the histamine H 3 receptor. Br J Pharmacol 2019; 177:570-579. [PMID: 30714121 DOI: 10.1111/bph.14606] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2018] [Revised: 12/12/2018] [Accepted: 01/07/2019] [Indexed: 12/16/2022] Open
Abstract
Histamine dysregulation was implicated as a rare cause of Tourette syndrome and other tic disorders a decade ago by a landmark genetic study in a high density family pedigree, which implicated a hypomorphic mutation in the histidine decarboxylase (Hdc) gene as a rare but high penetrance genetic cause. Studies in Hdc knockout (KO) mice have confirmed that this mutation causes tic-relevant behavioural and neurochemical abnormalities that parallel what is seen in patients and thus validate the KO as a potentially informative model of tic pathophysiology. Recent studies have focused on the potential role of the histamine H3 receptor in this model, and by association in tic disorders and related neuropsychiatric conditions. The H3 receptor is up-regulated in the striatum in Hdc KO mice. As the H3 receptor has constitutive activity in the absence of ligand, this receptor up-regulation may have significant cellular effects despite the absence of neurotransmitter histamine in these mice. Activation in vivo of H3 receptors in wild type mice regulates signalling in striatal medium spiny neurons (MSNs) that interacts non-linearly with dopamine receptor signalling. Baseline signalling alterations in MSNs in Hdc KO mice resemble those seen after H3 receptor agonist treatment in wild type animals. H3 receptor agonist treatment in the KOs further accentuates most of these signalling abnormalities and produces behavioural stereotypy. Together, these data suggest the intriguing hypothesis that constitutive signalling by up-regulated H3 receptors explains many of the molecular and behavioural abnormalities seen in these animals. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.
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