Abnormal histidine metabolism promotes macrophage lipid accumulation under Ox-LDL condition.

Distinct macrophage populations exert highly heterogeneity and perform various functions, among which, a crucial function of lipid metabolism is highlighted. However, the role of histidine metabolism disorder in macrophage lipid metabolism remains elusive. Addressed this question, we sorted and cultured the bone marrow-derived macrophages (BMDMs) of histidine decarboxylase (Hdc) knockout (Hdc-/-) mice with an in vitro oxidized low-density lipoprotein (ox-LDL) model, and detected the intracellular lipids by Oil Red O staining as well as lipid probe staining. Astemizole, a canonical and long-acting histamine H1 receptor (H1R) antagonist, was applied to elucidate the impact of antagonizing the H1R-dependent signaling pathway on macrophage lipid metabolism. Subsequently, the differential expressed genes were screened and analyzed in the bone marrow-derived CD11b+ immature myeloid cells of Hdc-/- and Hdc+/+ mice with a high fat diet by the microarray study. The expression levels of cholesterol metabolism-related genes were examined by qRT-PCR to explore underlying mechanisms. Lastly, we used a high-sensitivity histidine probe to detect the intracellular histidine in the BMDMs after oxidative stress. The results revealed that histidine metabolism disorder and histamine deficiency aggravated lipid accumulation in the ox-LDL-treated BMDMs. The expression level of H1R gene in the BMDMs was down-regulated after ox-LDL stimulation. The disruption of the H1R-dependent signaling pathway by astemizole further exacerbated ox-LDL-induced lipid deposition in the BMDMs partly by up-regulating scavenger receptor class A (SR-A) for lipid intake, down-regulating neutral cholesteryl ester hydrolase (nCEH) for cholesterol esterification and down-regulating ATP-binding cassette transporters A1 (ABCA1) and ABCG1 for reverse cholesterol transport. The intracellular histidine increased under ox-LDL condition, which was further increased by Hdc knockout. Collectively, these results partially reveal the relationship between histidine metabolism and lipid metabolism in the BMDMs and offer a novel strategy for lipid metabolism disorder-associated diseases.

Tibialis anterior electromyographic bursts during sleep in histamine-deficient mice.

Antihistamine medications have been suggested to elicit clinical features of restless legs syndrome. The available data are limited, particularly concerning periodic leg movements during sleep, which are common in restless legs syndrome and involve bursts of tibialis anterior electromyogram. Here, we tested whether the occurrence of tibialis anterior electromyogram bursts during non-rapid eye movement sleep is altered in histidine decarboxylase knockout mice with congenital histamine deficiency compared with that in wild-type control mice. We implanted six histidine decarboxylase knockout and nine wild-type mice to record neck muscle electromyogram, bilateral tibialis anterior electromyogram, and electroencephalogram during the rest (light) period. The histidine decarboxylase knockout and wild-type mice did not differ significantly in terms of sleep architecture. In both histidine decarboxylase knockout and wild-type mice, the distribution of intervals between tibialis anterior electromyogram bursts had a single peak for intervals < 10 s. The total occurrence rate of tibialis anterior electromyogram bursts during non-rapid eye movement sleep and the occurrence rate of the tibialis anterior electromyogram bursts separated by intervals < 10 s were significantly lower in histidine decarboxylase knockout than in wild-type mice. These data do not support the hypothesis that preventing brain histamine signalling may promote restless legs syndrome. Rather, the data suggest that limb movements during sleep, including those separated by short intervals, are a manifestation of subcortical arousal requiring the integrity of brain histamine signalling.

Histamine deficiency facilitates coronary microthrombosis after myocardial infarction by increasing neutrophil-platelet interactions.

Neutrophil-platelet interactions are responsible for thrombosis as well as inflammatory responses following acute myocardial infarction (AMI). While histamine has been shown to play a crucial role in many physiological and pathological processes, its effects on neutrophil-platelet interactions in thromboinflammatory complications of AMI remain elusive. In this study, we show a previously unknown mechanism by which neutrophil-derived histamine protects the infarcted heart from excessive neutrophil-platelet interactions and redundant arterial thrombosis. Using histamine-deficient (histidine decarboxylase knockout, HDC-/- ) and wild-type murine AMI models, we demonstrate that histamine deficiency increases the number of microthrombosis after AMI, in accordance with depressed cardiac function. Histamine-producing myeloid cells, mainly Ly6G+ neutrophils, directly participate in arteriole thrombosis. Histamine deficiency elevates platelet activation and aggregation by enhancing Akt phosphorylation and leads to dysfunctional characteristics in neutrophils which was confirmed by high levels of reactive oxygen species production and CD11b expression. Furthermore, HDC-/- platelets were shown to elicit neutrophil extracellular nucleosomes release, provoke neutrophil-platelet interactions and promote HDC-expressing neutrophils recruitment in arteriole thrombosis in vivo. In conclusion, we provide evidence that histamine deficiency promotes coronary microthrombosis and deteriorates cardiac function post-AMI, which is associated with the enhanced platelets/neutrophils function and neutrophil-platelet interactions.

Histamine deficiency delays ischaemic skeletal muscle regeneration via inducing aberrant inflammatory responses and repressing myoblast proliferation.

Histidine decarboxylase (HDC) catalyses the formation of histamine from L-histidine. Histamine is a biogenic amine involved in many physiological and pathological processes, but its role in the regeneration of skeletal muscles has not been thoroughly clarified. Here, using a murine model of hindlimb ischaemia, we show that histamine deficiency in Hdc knockout (Hdc-/- ) mice significantly reduces blood perfusion and impairs muscle regeneration. Using Hdc-EGFP transgenic mice, we demonstrate that HDC is expressed predominately in CD11b+ Gr-1+ myeloid cells but not in skeletal muscles and endothelial cells. Large amounts of HDC-expressing CD11b+ myeloid cells are rapidly recruited to injured and inflamed muscles. Hdc-/- enhances inflammatory responses and inhibits macrophage differentiation. Mechanically, we demonstrate that histamine deficiency decreases IGF-1 (insulin-like growth factor 1) levels and diminishes myoblast proliferation via H3R/PI3K/AKT-dependent signalling. These results indicate a novel role for HDC-expressing CD11b+ myeloid cells and histamine in myoblast proliferation and skeletal muscle regeneration.

Histamine regulation of microglia: Gene-environment interaction in the regulation of central nervous system inflammation.

Microglia mediate neuroinflammation and regulate brain development and homeostasis. Microglial abnormalities are implicated in a range of neuropsychiatric pathology, including Tourette syndrome (TS) and autism. Histamine (HA) is both a neurotransmitter and an immune modulator. HA deficiency has been implicated as a rare cause of TS and may contribute to other neuropsychiatric conditions. In vitro studies suggest that HA can regulate microglia, but this has never been explored in vivo. We used immunohistochemistry to examine the effects of HA deficiency in histidine decarboxylase (Hdc) knockout mice and of HA receptor stimulation in wild-type animals. We find HA to regulate microglia in vivo, via the H4 receptor. Chronic HA deficiency in Hdc knockout mice reduces ramifications of microglia in the striatum and (at trend level) in the hypothalamus, but not elsewhere in the brain. Depletion of histaminergic neurons in the hypothalamus has a similar effect. Microglia expressing IGF-1 are particularly reduced, However, the microglial response to challenge with lipopolysacchariade (LPS) is potentiated in Hdc knockout mice. Genetic abnormalities in histaminergic signaling may produce a vulnerability to inflammatory challenge, setting the state for pathogenically dysregulated neuroimmune responses.

High-amplitude theta wave bursts characterizing narcoleptic mice and patients are also produced by histamine deficiency in mice.

Histamine and orexins are wake promoters released by hypothalamic neurons. The activity of histamine neurons is increased by orexin neurons. Recently, it has been shown that orexin deficiency entails high-amplitude theta wave bursts during rapid eye movement sleep and cataplexy in narcoleptic mice. The primary aim of this study was to assess whether histamine system is involved in high-amplitude theta wave burst generation during rapid eye movement sleep. The secondary aim was to assess the effects of combined histamine and orexin deficiency on high-amplitude theta wave bursts during rapid eye movement sleep in mice. Twelve histidine-decarboxylase knockout mice with congenital histamine deficiency, seven double mutant mice with combined deficiency of orexin neurons and histamine, and 11 wild-type control mice were studied with electrodes for sleep recordings and a telemetric blood pressure transducer. High-amplitude theta wave bursts during rapid eye movement sleep were detected in each of the histidine-decarboxylase knockout and double mutant mice, whereas only one burst was found in a wild-type control mouse. High-amplitude theta wave bursts occurred significantly more often and were significantly longer in double mutant than in histidine-decarboxylase knockout mice. In conclusion, it was demonstrated that, similarly to orexin, the chronic impairment of histamine entailed high-amplitude theta wave bursts during rapid eye movement sleep. The current data also suggested a synergistic role of orexin and histamine signalling on high-amplitude theta wave bursts during rapid eye movement sleep in mice.

Histamine deficiency decreases atherosclerosis and inflammatory response in apolipoprotein E knockout mice independently of serum cholesterol level.

OBJECTIVE: Histamine and histamine receptors are found in atherosclerotic lesions, and their signaling and subsequent proatherogenic or proinflammatory gene expression are involved in atherogenesis. In the present study, we generated apolipoprotein E (apoE) and histamine synthesizing histidine decarboxylase double knockout (DKO) mice on a C57BL/6J (wild-type mice) background to clarify the roles of histamine in atherosclerosis. METHODS AND RESULTS: Wild-type, apoE knockout (KO), and DKO mice were fed a high-cholesterol diet to analyze hyperlipidemia-induced atherosclerosis. Compared with wild-type mice, apoE-KO mice showed increased expression of histamine and its receptors, corresponding to increased atherosclerotic lesion areas and expression of inflammatory regulators, such as nuclear factor-κB, scavenger receptors, inflammatory cytokines, and matrix metalloproteinases. Histamine deficiency after deletion of histidine decarboxylase reduced atherosclerotic areas and expression of a range of the inflammation regulatory genes, but serum cholesterol levels of DKO mice were higher than those of apoE-KO mice. CONCLUSIONS: These results indicate that histamine is involved in the development of atherosclerosis in apoE-KO mice by regulating gene expression of inflammatory modulators, an action that appears to be independent of serum cholesterol levels. In addition to acute inflammatory response, histamine participates in chronic inflammation, such as hyperlipidemia-induced atherosclerosis, and might be a novel therapeutic target for the treatment of atherosclerosis.

Histamine Deficiency Promotes Myofibroblasts Transformation from HDC-Expressing CD11b+ Myeloid Cells in Injured Hearts Post Myocardial Infarction.

Myocardial infarction (MI) is a significant contributor to the development of heart failure. Histidine decarboxylase (HDC), the unique enzyme that converts L-histidine to histamine, is highly expressed in CD11b+ immature myeloid cells. However, the relationship between HDC-expressing macrophages and cardiac myofibroblasts remains to be explained. Here, we demonstrate that the GFP (green fluorescent protein)-labeled HDC+CD11b+ myeloid precursors and their descendants could differentiate into fibroblast-like cells in myocardial interstitium. Furthermore, we prove that CD11b+Ly6C+ monocytes/macrophages, but not CD11b+Ly6G+ granulocytes, are identified as the main cellular source for bone marrow-derived myofibroblast transformation, which could be regulated via histamine H1 and H2 receptor-dependent signaling pathways. Using HDC knockout mice, we find that histamine deficiency promotes myofibroblast transformation from Ly6C+ macrophages and cardiac fibrosis partly through upregulating the expression of Krüppel-like factor 5 (KLF5). Taken together, our data uncover a central role of HDC in regulating bone marrow-derived macrophage-to-myofibroblast transformation but also identify a histamine receptor (HR)-KLF5 related signaling pathway that mediates myocardial fibrosis post-MI. CD11b+Ly6C+ monocytes/macrophages are the main cellular source for bone marrow-derived myofibroblast transformation. Histamine inhibits myofibroblasts transformation via H1R and H2R-dependent signaling pathways, and ameliorates cardiac fibrosis partly through upregulating KLF5 expression.

Cutting edge: histamine receptor H4 activation positively regulates in vivo IL-4 and IFN-gamma production by invariant NKT cells.

Histamine (HA) is a biogenic amine with multiple activities in the immune system. In this study we demonstrate that histamine-free histidine decarboxylase-deficient (HDC(-/-)) mice present a numerical and functional deficit in invariant NK T (iNKT) cells as evidenced by a drastic decrease of IL-4 and IFN-gamma production. This deficiency was established both by measuring cytokine levels in the serum and intracellularly among gated iNKT cells. It resulted from the lack of HA, because a single injection of this amine into HDC(-/-) mice sufficed to restore normal IL-4 and IFN-gamma production. HA-induced functional recovery was mediated mainly through the H4 histamine receptor (H4R), as assessed by its abrogation after a single injection of a selective H4R antagonist and the demonstration of a similar iNKT cell deficit in H4R(-/-) mice. Our findings identify a novel function of HA through its H4R and suggest that it might become instrumental in modulating iNKT cell functions.

Excitation of histaminergic tuberomamillary neurons by thyrotropin-releasing hormone.

The histaminergic tuberomamillary nucleus (TMN) controls arousal and attention, and the firing of TMN neurons is state-dependent, active during waking, silent during sleep. Thyrotropin-releasing hormone (TRH) promotes arousal and combats sleepiness associated with narcolepsy. Single-cell reverse-transcription-PCR demonstrated variable expression of the two known TRH receptors in the majority of TMN neurons. TRH increased the firing rate of most (ca 70%) TMN neurons. This excitation was abolished by the TRH receptor antagonist chlordiazepoxide (CDZ; 50 mum). In the presence of tetrodotoxin (TTX), TRH depolarized TMN neurons without obvious change of their input resistance. This effect reversed at the potential typical for nonselective cation channels. The potassium channel blockers barium and cesium did not influence the TRH-induced depolarization. TRH effects were antagonized by inhibitors of the Na(+)/Ca(2+) exchanger, KB-R7943 and benzamil. The frequency of GABAergic spontaneous IPSCs was either increased (TTX-insensitive) or decreased [TTX-sensitive spontaneous IPSCs (sIPSCs)] by TRH, indicating a heterogeneous modulation of GABAergic inputs by TRH. Facilitation but not depression of sIPSC frequency by TRH was missing in the presence of the kappa-opioid receptor antagonist nor-binaltorphimine. Montirelin (TRH analog, 1 mg/kg, i.p.) induced waking in wild-type mice but not in histidine decarboxylase knock-out mice lacking histamine. Inhibition of histamine synthesis by (S)-alpha-fluoromethylhistidine blocked the arousal effect of montirelin in wild-type mice. We conclude that direct receptor-mediated excitation of rodent TMN neurons by TRH demands activation of nonselective cation channels as well as electrogenic Na(+)/Ca(2+) exchange. Our findings indicate a key role of the brain histamine system in TRH-induced arousal.

Decreased CSF histamine in narcolepsy with and without low CSF hypocretin-1 in comparison to healthy controls.

STUDY OBJECTIVE: To examine whether cerebrospinal fluid (CSF) histamine contents are altered in human narcolepsy and whether these alterations are specific to hypocretin deficiency, as defined by low CSF hypocretin-1. METHODS: Patients meeting the ICSD-2 criteria for narcolepsy with and without cataplexy and who had CSF hypocretin-1 results available were selected from the Stanford Narcolepsy Database on the basis of CSF availability and adequate age and sex matching across 3 groups: narcolepsy with low CSF hypocretin-1 (n=34, 100% with cataplexy), narcolepsy without low CSF hypocretin-1 (n=24, 75% with cataplexy), and normal controls (n=23). Low CSF hypocretin-1 was defined as CSF < or =110 pg/mL (1/3 of mean control values). Six of 34 patients with low CSF hypocretin-1, six of 24 subjects with normal CSF hypocretin-1, and all controls were unmedicated at the time of CSF collection. CSF histamine was measured in all samples using a fluorometric HPLC system. RESULTS: Mean CSF histamine levels were: 133.2 +/- 20.1 pg/mL in narcoleptic subjects with low CSF hypocretin-1, 233.3 +/- 46.5 pg/mL in patients with normal CSF hypocretin-1 (204.9 +/- 89.7 pg/mL if only patients without cataplexy are included), and 300.5 +/- 49.7 pg/mL in controls, reaching statistically significant differences between the 3 groups. CONCLUSION: CSF histamine levels are reduced in human narcolepsy. The reduction of CSF histamine levels was more evident in the cases with low CSF hypocretin-1, and levels were intermediate in other narcolepsy cases. As histamine is a wake-promoting amine known to decrease during sleep, decreased histamine could either passively reflect or partially mediate daytime sleepiness in these pathologies.

CSF histamine contents in narcolepsy, idiopathic hypersomnia and obstructive sleep apnea syndrome.

STUDY OBJECTIVE: To (1) replicate our prior result of low cerebrospinal fluid (CSF) histamine levels in human narcolepsy in a different sample population and to (2) evaluate if histamine contents are altered in other types of hypersomnia with and without hypocretin deficiency. DESIGN: Cross sectional studies. SETTING AND PATIENTS: Sixty-seven narcolepsy subjects, 26 idiopathic hypersomnia (IHS) subjects, 16 obstructive sleep apnea syndrome (OSAS) subjects, and 73 neurological controls were included. All patients were Japanese. Diagnoses were made according to ICSD-2. RESULTS: We found significant reductions in CSF histamine levels in hypocretin deficient narcolepsy with cataplexy (mean +/- SEM; 176.0 +/- 25.8 pg/mL), hypocretin non-deficient narcolepsy with cataplexy (97.8 +/- 38.4 pg/mL), hypocretin non-deficient narcolepsy without cataplexy (113.6 +/- 16.4 pg/mL), and idiopathic hypersomnia (161.0 +/- 29.3 pg/ mL); the levels in OSAS (259.3 +/- 46.6 pg/mL) did not statistically differ from those in the controls (333.8 +/- 22.0 pg/mL). Low CSF histamine levels were mostly observed in non-medicated patients; significant reductions in histamine levels were evident in non-medicated patients with hypocretin deficient narcolepsy with cataplexy (112.1 +/- 16.3 pg/ mL) and idiopathic hypersomnia (143.3 +/- 28.8 pg/mL), while the levels in the medicated patients were in the normal range. CONCLUSION: The study confirmed reduced CSF histamine levels in hypocretin-deficient narcolepsy with cataplexy. Similar degrees of reduction were also observed in hypocretin non-deficient narcolepsy and in idiopathic hypersomnia, while those in OSAS (non central nervous system hypersomnia) were not altered. The decrease in histamine in these subjects were more specifically observed in non-medicated subjects, suggesting CSF histamine is a biomarker reflecting the degree of hypersomnia of central origin.

[Role of histamine and histamine receptors in the pathogenesis of malaria]. / Rôle de l'histamine et des récepteurs histaminiques dans la pathogenèse du paludisme.

A hallmark of the host response to Plasmodium parasite is an inflammatory reaction characterized by elevated histaminemia levels. Since histamine, which acts through four different receptors and which synthesis is under the control of the histidine decarboxylase (HDC), is endowed with pro-inflammatory and immunosuppressive activities, we hypothesized that this vaso-active amine may participe to malaria pathogenesis. Combining genetic and pharmacologic approaches by using H1R(-/-), H2R(-/-), H3R(-/-), HDC(-/-) mice and H1R, H2R-, and H3R-antagonists, respectively, we found that cerebral malaria-associated pathogenetic processes such as blood brain barrier disruption, and T lymphocyte sequestration to cerebral vascular endothelium in mice were associated with histamine production. The identification of this novel inflammatory pathway and its implication in Plasmodium infection may lead to novel strategies to manipulate the anti-Plasmodium immune response and may provide new therapeutic tools to alleviate malaria disease.

An infection of Enterobacter ludwigii affects development and causes age-dependent neurodegeneration in Drosophila melanogaster.

The effects of teeth-blackening bacteria Enterobacter ludwigii on the physiological system were investigated using the model organism Drosophila melanogaster. The bacteria were mixed with the fly food, and its effect was checked on the growth, development and behaviour of Drosophila. Microbes generate reactive oxygen species (ROS) within the haemolymph of the larvae once it enters into the body. The increased amount of ROS was evidenced by the NBT assay and using 2',7'-dichlorofluorescin diacetate dye, which indicates the mitochondrial ROS. The increased amount of ROS resulted in a number of abnormal nuclei within the gut. Besides that larvae walking became sluggish in comparison with wild type although the larvae crawling path did not change much. Flies hatched from the infectious larvae have the posterior scutellar bristle absent from the thorax and abnormal mechanosensory hairs in the eye, and they undergo time-dependent neurodegeneration as evidenced by the geotrophic and phototrophic assays. To decipher the mechanism of neurodegeneration, flies were checked for the presence of four important bioamines: tyramine, cadaverine, putrescine and histamine. Out of these four, histamine was found to be absent in infected flies. Histamine is a key molecule required for the functioning of the photoreceptor as well as mechanoreceptors. The mechanism via which mouth infectious bacteria E. ludwigii can affect the development and cause age-dependent neurodegeneration is explained in this paper.

Abnormal behavior, striatal dopamine turnover and opioid peptide gene expression in histamine-deficient mice.

Hypothalamic histaminergic neurons regulate a variety of homeostatic, metabolic and cognitive functions. Recent data have suggested a modulatory role of histamine and histamine receptors in shaping striatal activity and connected the histaminergic system to neuropsychiatric disorders. We characterized exploratory behavior and striatal neurotransmission in mice lacking the histamine producing enzyme histidine decarboxylase (Hdc). The mutant mice showed a distinct behavioral pattern during exploration of novel environment, specifically, increased frequency of rearing seated against the wall, jumping and head/body shakes. This behavioral phenotype was associated with decreased levels of striatal dopamine and serotonin and increased level of dopamine metabolite DOPAC. Gene expression levels of dynorphin and enkephalin, opioids released by medium spiny neurons of striatal direct and indirect pathways respectively, were lower in Hdc mutant mice than in control animals. A low dose of amphetamine led to similar behavioral and biochemical outcomes in both genotypes. Increased striatal dopamine turnover was observed in Hdc KO mice after treatment with dopamine precursor l-Dopa. Overall, our study suggests a role for striatal dopamine and opioid peptides in formation of distinct behavioral phenotype of Hdc KO mice.

Genetic lesioning of histamine neurons increases sleep-wake fragmentation and reveals their contribution to modafinil-induced wakefulness.

Acute chemogenetic inhibition of histamine (HA) neurons in adult mice induced nonrapid eye movement (NREM) sleep with an increased delta power. By contrast, selective genetic lesioning of HA neurons with caspase in adult mice exhibited a normal sleep-wake cycle overall, except at the diurnal start of the lights-off period, when they remained sleepier. The amount of time spent in NREM sleep and in the wake state in mice with lesioned HA neurons was unchanged over 24 hr, but the sleep-wake cycle was more fragmented. Both the delayed increase in wakefulness at the start of the night and the sleep-wake fragmentation are similar phenotypes to histidine decarboxylase knockout mice, which cannot synthesize HA. Chronic loss of HA neurons did not affect sleep homeostasis after sleep deprivation. However, the chronic loss of HA neurons or chemogenetic inhibition of HA neurons did notably reduce the ability of the wake-promoting compound modafinil to sustain wakefulness. Thus, part of modafinil's wake-promoting actions arise through the HA system.

Histamine deficiency aggravates cardiac injury through miR-206/216b-Atg13 axis-mediated autophagic-dependant apoptosis.

Histamine is a widely distributed biogenic amine involved in the regulation of an array of biological processes. Serum histamine level is markedly elevated in the early stages of acute myocardial infarction, whereas the role it plays remains unclear. Histidine decarboxylase (HDC) is the unique enzyme responsible for histamine production, and cardiac injury is significantly aggravated in HDC knockout mice (HDC-/-), in which histamine is deficient. We also observed that autophagy was highly activated in cardiomyocytes of HDC-/- mice post acute myocardial infarction (AMI), which was abolished by compensation of exogenous histamine. The in vivo and in vitro results showed that acting through histamine 1 receptor, histamine increased miR-206 and miR-216b, which worked in concert to target to Atg13, resulting in the reduction of autophagy activation under hypoxia and AMI condition. Further study revealed that Atg13 interacted with FADD to promote the activation of caspase-8 and cell apoptosis. Taken together, these data unveil a novel intracellular signaling pathway involved in histamine regulating myocardial autophagy and apoptosis under hypoxia and AMI condition, which might help to more comprehensively evaluate the usage of histamine receptor antagonists and to develop new therapeutic targets for myocardial infarction.

Histamine-deficient mice do not respond to the antidepressant-like effects of oleoylethanolamide.

It has been suggested that the bioactive lipid mediator oleoylethanolamide (OEA), a potent agonist of the peroxisome proliferator-activated receptor-alpha (PPAR-α) possesses anti-depressant-like effects in several preclinical models. We recently demonstrated that several of OEA's behavioural actions require the integrity of the brain histaminergic system, and that an intact histaminergic neurotransmission is specifically required for selective serotonin re-uptake inhibitors to exert their anti-depressant-like effect. The purpose of our study was to test if OEA requires the integrity of the histaminergic neurotransmission to exert its antidepressant-like effects. Immobility time in the tail suspension test was measured to assess OEA's potential (10 mg/kg i.p.) as an antidepressant drug in histidine decarboxylase null (HDC-/-) mice and HDC+/+ littermates, as well as in PPAR-α+/+ and PPAR-α-/- mice. CREB phosphorylation was evaluated using Western blot analysis in hippocampal and cortical homogenates, as pCREB is considered partially responsible for the efficacy of antidepressants. Serotonin release from ventral hippocampi of HDC+/+ and HDC-/- mice was measured with in-vivo microdialysis, following OEA administration. OEA decreased immobility time and increased brain pCREB levels in HDC+/+ mice, whereas it was ineffective in HDC-/- mice. Comparable results were obtained in PPAR-α+/+ and PPAR-α-/- mice. Microdialysis revealed a dysregulation of serotonin release induced by OEA in HDC-/- mice. Our observations corroborate our hypothesis that brain histamine and signals transmitted by OEA interact to elaborate appropriate behaviours and may be the basis for the efficacy of OEA as an antidepressant-like compound.

Prolonged histamine deficiency in histidine decarboxylase gene knockout mice affects Leydig cell function.

The present study focuses on histaminergic regulation of Leydig cell physiology, since limited information is available so far. To evaluate the dependency of Leydig cells on histamine (HA), we performed experiments using highly purified Leydig cells in culture, isolated from wild type (WT) and histidine decarboxylase (Hdc) gene knockout (HDC KO)-so HA-deprived-mice. HDC KO Leydig cells showed lower basal and human choriogonadotropin (hCG)-induced testosterone production compared to WT Leydig cells, presumably due to altered P450scc gene (Cyp11a1) expression levels. Moreover, in HDC KO cells, hCG did not increase basal expression levels of HA H1 and H2 receptor genes, while the hormone showed a significant inducing effect in WT cells. Based on these findings, we propose that prolonged HA deficiency in HDC KO mice affects various aspects of Leydig cell physiology, most importantly the response to hCG, providing definite evidence that HA plays a role as direct modulator of Leydig cell function and steroid synthesis in the testis. Also, the results presented herein constitute the first molecular evidence for the expression of HA H1 and H2 receptor subtypes in isolated Leydig cells.

Brain histamine depletion enhances the behavioural sequences complexity of mice tested in the open-field: Partial reversal effect of the dopamine D2/D3 antagonist sulpiride.

Markers of histaminergic dysregulation were found in several neuropsychiatric disorders characterized by repetitive behaviours, thoughts and stereotypies. We analysed the effect of acute histamine depletion by means of i. c.v. injections of alpha-fluoromethylhistidine, a blocker of histidine decarboxylase, on the temporal organization of motor sequences of CD1 mice behaviour in the open-field test. An ethogram encompassing 9 behavioural components was employed. Durations and frequencies were only slightly affected by treatments. However, as revealed by multivariate t-pattern analysis, histamine depletion was associated with a striking increase in the number of behavioural patterns. We found 42 patterns of different composition occurring, on average, 520.90 ± 50.23 times per mouse in the histamine depleted (HD) group, whereas controls showed 12 different patterns occurring on average 223.30 ± 20.64 times. Exploratory and grooming behaviours clustered separately, and the increased pattern complexity involved exclusively exploratory patterns. To test the hypothesis of a histamine-dopamine interplay on behavioural pattern phenotype, non-sedative doses of the D2/D3 antagonist sulpiride (12.5-25-50 mg/kg) were additionally administered to different groups of HD mice. Sulpiride counterbalanced the enhancement of exploratory patterns of different composition, but it did not affect the mean number of patterns at none of the doses used. Our results provide new insights on the role of histamine on repetitive behavioural sequences of freely moving mice. Histamine deficiency is correlated with a general enhancement of pattern complexity. This study supports a putative involvement of histamine in the pathophysiology of tics and related disorders.