Is the histaminergic neuron system a regulatory center for whole-brain activity?

Top-down modulation of the retinal code via histaminergic neurons of the hypothalamus

The mammalian retina is considered an autonomous circuit, yet work dating back to Ramon y Cajal indicates that it receives inputs from the brain. How such inputs affect retinal processing has remained unknown. We confirmed brain-to-retina projections of histaminergic neurons from the mouse hypothalamus. Histamine application ex vivo altered the activity of various retinal ganglion cells (RGCs), including direction-selective RGCs that gained responses to high motion velocities. These results were reproduced in vivo with optic tract recordings where histaminergic retinopetal axons were activated chemogenetically. Such changes could improve vision of fast-moving objects (e.g., while running), which fits with the known increased activity of histaminergic neurons during arousal. An antihistamine drug reduced optomotor responses to high-speed moving stimuli in freely moving mice. In humans, the same antihistamine nonuniformly modulated visual sensitivity across the visual field, indicating an evolutionary conserved function of the histaminergic system. Our findings expose a previously unappreciated role for brain-to-retina projections in modulating retinal function.

Histaminergic regulation of food intake

Histamine is a biogenic amine that acts as a neuromodulator within the brain. In the hypothalamus, histaminergic signaling contributes to the regulation of numerous physiological and homeostatic processes, including the regulation of energy balance. Histaminergic neurons project extensively throughout the hypothalamus and two histamine receptors (H1R, H3R) are strongly expressed in key hypothalamic nuclei known to regulate energy homeostasis, including the paraventricular (PVH), ventromedial (VMH), dorsomedial (DMH), and arcuate (ARC) nuclei. The activation of different histamine receptors is associated with differential effects on neuronal activity, mediated by their different G protein-coupling. Consequently, activation of H1R has opposing effects on food intake to that of H3R: H1R activation suppresses food intake, while H3R activation mediates an orexigenic response. The central histaminergic system has been implicated in atypical antipsychotic-induced weight gain and has been proposed as a potential therapeutic target for the treatment of obesity. It has also been demonstrated to interact with other major regulators of energy homeostasis, including the central melanocortin system and the adipose-derived hormone leptin. However, the exact mechanisms by which the histaminergic system contributes to the modification of these satiety signals remain underexplored. The present review focuses on recent advances in our understanding of the central histaminergic system's role in regulating feeding and highlights unanswered questions remaining in our knowledge of the functionality of this system.

Genetically encoded sensors for measuring histamine release both in vitro and in vivo

Histamine (HA) is a key biogenic monoamine involved in a wide range of physiological and pathological processes in both the central and peripheral nervous systems. Because the ability to directly measure extracellular HA in real time will provide important insights into the functional role of HA in complex circuits under a variety of conditions, we developed a series of genetically encoded G-protein-coupled receptor-activation-based (GRAB) HA (GRAB_HA) sensors with good photostability, sub-second kinetics, nanomolar affinity, and high specificity. Using these GRAB_HA sensors, we measured electrical-stimulation-evoked HA release in acute brain slices with high spatiotemporal resolution. Moreover, we recorded HA release in the preoptic area of the hypothalamus and prefrontal cortex during the sleep-wake cycle in freely moving mice, finding distinct patterns of HA dynamics between these specific brain regions. Thus, GRAB_HA sensors are robust tools for measuring extracellular HA transmission in both physiological and pathological processes.

Nanodelivery of Histamine H3/H4 Receptor Modulators BF-2649 and Clobenpropit with Antibodies to Amyloid Beta Peptide in Combination with Alpha Synuclein Reduces Brain Pathology in Parkinson's Disease

Parkinson's disease (PD) in military personnel engaged in combat operations is likely to develop in their later lives. In order to enhance the quality of lives of PD patients, exploration of novel therapy based on new research strategies is highly warranted. The hallmarks of PD include increased alpha synuclein (ASNC) and phosphorylated tau (p-tau) in the cerebrospinal fluid (CSF) leading to brain pathology. In addition, there are evidences showing increased histaminergic nerve fibers in substantia niagra pars compacta (SNpc), striatum (STr), and caudate putamen (CP) associated with upregulation of histamine H3 receptors and downregulation of H4 receptors in human brain. Previous studies from our group showed that modulation of potent histaminergic H3 receptor inverse agonist BF-2549 or clobenpropit (CLBPT) partial histamine H4 agonist with H3 receptor antagonist induces neuroprotection in PD brain pathology. Recent studies show that PD also enhances amyloid beta peptide (AβP) depositions in brain. Keeping these views in consideration in this review, nanowired delivery of monoclonal antibodies to AβP together with ASNC and H3/H4 modulator drugs on PD brain pathology is discussed based on our own observations. Our investigation shows that TiO₂ nanowired BF-2649 (1 mg/kg, i.p.) or CLBPT (1 mg/kg, i.p.) once daily for 1 week together with nanowired delivery of monoclonal antibodies (mAb) to AβP and ASNC induced superior neuroprotection in PD-induced brain pathology. These observations are the first to show the modulation of histaminergic receptors together with antibodies to AβP and ASNC induces superior neuroprotection in PD. These observations open new avenues for the development of novel drug therapies for clinical strategies in PD.

The Role of the Central Histaminergic System in Behavioral State Control

Histamine is a small monoamine signaling molecule that plays a role in many peripheral and central physiological processes, including the regulation of wakefulness. The tuberomammillary nucleus is the sole neuronal source of histamine in the brain, and histamine neurons are thought to promote wakefulness and vigilance maintenance - under certain environmental and/or behavioral contexts - through their diffuse innervation of the cortex and other wake-promoting brain circuits. Histamine neurons also contain a number of other putative neurotransmitters, although the functional role of these co-transmitters remains incompletely understood. Within the brain histamine operates through three receptor subtypes that are located on pre- and post-synaptic membranes. Some histamine receptors exhibit constitutive activity, and hence exist in an activated state even in the absence of histamine. Newer medications used to reduce sleepiness in narcolepsy patients in fact enhance histamine signaling by blunting the constitutive activity of these histamine receptors. In this chapter, we provide an overview of the central histamine system with an emphasis on its role in behavioral state regulation and how drugs targeting histamine receptors are used clinically to treat a wide range of sleep-wake disorders.

Histamine Neuroimaging in Stress-Related Disorders

Although histamine plays a major role in animal models of stress-related disorders, human neuroimaging data are sparse. Histamine H1 receptors in the human brain were first imaged by Professor Kazuhiko Yanai in 1992 by using ¹¹C-doxepin, a potent ligand of H1 receptors, and positron emission tomography (PET). Subsequent work revealed that H1 receptors are reduced in the prefrontal and anterior cingulate cortices in patients with major depressive disorders. A sex difference in H1 receptor binding in the brain has also been found, with women exhibiting more abundant H1 receptor binding than men. Moreover, female patients with anorexia nervosa show higher H1 receptor binding in the amygdala and lentiform nucleus. These studies also found an inverse correlation of depression scores with H1 receptor binding. Histamine is considered to play a major role in the pathophysiology of irritable bowel syndrome (IBS), a representative disorder of brain-gut interactions. Along these lines, hypnotic suggestion dramatically changes the waveforms of viscerosensory cerebral evoked potentials in response to electrical rectal stimulation and these changes are modified by the administration of H1 antagonist. The direction of the H1 antagonist-induced changes in the viscerosensory cerebral evoked potentials differs between IBS patients and healthy controls. Thus, histamine likely plays an important role in stress-related disorders. Further histamine brain imaging studies of humans are warranted.

Integration of the Cortical Haemodynamic Response Measured by Functional Near-Infrared Spectroscopy and Amino Acid Analysis to Aid in the Diagnosis of Major Depressive Disorder

Background: Major depressive disorder (MDD) is a debilitating condition with a high disease burden and medical comorbidities. There are currently few to no validated biomarkers to guide the diagnosis and treatment of MDD. In the present study, we evaluated the differences between MDD patients and healthy controls (HCs) in terms of cortical haemodynamic responses during a verbal fluency test (VFT) using functional near-infrared spectroscopy (fNIRS) and serum amino acid profiles, and ascertained if these parameters were correlated with clinical characteristics. Methods: Twenty-five (25) patients with MDD and 25 age-, gender-, and ethnicity-matched HCs were recruited for the study. Real-time monitoring of the haemodynamic response during completion of a VFT was quantified using a 52-channel NIRS system. Serum samples were analysed and quantified by liquid chromatography-mass spectrometry for amino acid profiling. Receiver-operating characteristic (ROC) curves were used to classify potential candidate biomarkers. Results: The MDD patients had lower prefrontal and temporal activation during completion of the VFT than HCs. The MDD patients had lower mean concentrations of oxy-Hb in the left orbitofrontal cortex (OFC), and lower serum histidine levels. When the oxy-haemoglobin response was combined with the histidine concentration, the sensitivity and specificity of results improved significantly from 66.7% to 73.3% and from 65.0% to 90.0% respectively, as compared to results based only on the NIRS response. Conclusions: These findings demonstrate the use of combination biomarkers to aid in the diagnosis of MDD. This technique could be a useful approach to detect MDD with greater precision, but additional studies are required to validate the methodology.

Thioperamide attenuates neuroinflammation and cognitive impairments in Alzheimer's disease via inhibiting gliosis

Alzheimer's disease (AD) is an age-related neurodegenerative disease, which characterized by deposition of amyloid-β (Aβ) plaques, neurofibrillary tangles, neuronal loss, and accompanied by neuroinflammation. Neuroinflammatory processes are well acknowledged to contribute to the progression of AD pathology. Histamine H3 receptor (H3R) is a presynaptic autoreceptor regulating histamine release via negative feedback way. Recently, studies show that H3R are highly expressed not only in neurons but also in microglia and astrocytes. H3R antagonist has been reported to have anti-inflammatory efficacy. However, whether inhibition of H3R is responsible for the anti-neuroinflammation in glial cells and neuroprotection on APPswe, PSEN1dE9 (APP/PS1 Tg) mice remain unclear. In this study, we found that inhibition of H3R by thioperamide reduced the gliosis and induced a phenotypical switch from A1 to A2 in astrocytes, and ultimately attenuated neuroinflammation in APP/PS1 Tg mice. Additionally, thioperamide rescued the decrease of cyclic AMP response element-binding protein (CREB) phosphorylation and suppressed the phosphorylated P65 nuclear factor kappa B (p-P65 NF-κB) in APP/PS1 Tg mice. H89, an inhibitor of CREB signaling, abolished these effects of thioperamide to suppress gliosis and proinflammatory cytokine release. Lastly, thioperamide alleviated the deposition of amyloid-β (Aβ) and cognitive dysfunction in APP/PS1 mice, which were both reversed by administration of H89. Taken together, these results suggested the H3R antagonist thioperamide improved cognitive impairment in APP/PS1 Tg mice via modulation of the CREB-mediated gliosis and inflammation inhibiting, which contributed to Aβ clearance. This study uncovered a novel mechanism involving inflammatory regulating behind the therapeutic effect of thioperamide in AD.

Different Peas in the Same Pod: The Histaminergic Neuronal Heterogeneity

The histaminergic neuronal system is recently receiving increasing attention, as much has been learned over the past 25 years about histamine role as a neurotransmitter. Indeed, this amine is crucial in maintaining arousal and provides important contributions to regulate circadian rhythms, energy, endocrine homeostasis, motor behavior, and cognition. The extent to which these distinct physiological functions are operated by independent histamine neuronal subpopulation is unclear. In the rat brain histamine neuronal cell bodies are grouped within the tuberomamillary nucleus of the posterior hypothalamus in five clusters, E1-E5, each sending overlapping axons throughout the entire central nervous system with no strict topographical pattern. These features lead to the concept that histamine regulation of a wide range of functions in the central nervous system is achieved by the histaminergic neuronal system as a whole. However, increasing experimental evidence suggesting that the histaminergic system is organized into distinct pathways modulated by selective mechanisms challenges this view. In this review, we summarized experimental evidence supporting the heterogeneity of histamine neurons, and their organization in functionally distinct circuits impinging on separate brain regions and displaying selective control mechanisms. This implies independent functions of subsets of histaminergic neurons according to their respective origin and terminal projections with relevant consequences for the development of specific compounds that affect only subsets of histamine neurons, thus increasing the target specificity.

Histamine: A Key Neuromodulator of Memory Consolidation and Retrieval

In pharmacological studies conducted on animals over the last four decades, histamine was determined to be a strong modulator of learning and memory. Activation of histamine signaling enhances memory consolidation and retrieval. Even long after learning and forgetting, it can still restore the retrieval of forgotten memories. These findings based on animal studies led to human clinical trials with histamine H₃ receptor antagonists/inverse agonists, which revealed their positive effects on learning and memory. Therefore, histamine signaling is a promising therapeutic target for improving cognitive impairments in patients with various neuropsychiatric disorders, including Alzheimer's disease. While the memory-modulatory effects of histamine receptor agonists and antagonists have been confirmed by several research groups, the underlying mechanisms remain to be elucidated. This review summarizes how the activation and inhibition of histamine signaling influence memory processes, introduces the cellular and circuit mechanisms, and discusses the relationship between the human histaminergic system and learning and memory.

Activation of CREB-mediated autophagy by thioperamide ameliorates β-amyloid pathology and cognition in Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disease, and the imbalance between production and clearance of β-amyloid (Aβ) is involved in its pathogenesis. Autophagy is an intracellular degradation pathway whereby leads to removal of aggregated proteins, up-regulation of which may be a plausible therapeutic strategy for the treatment of AD. Histamine H3 receptor (H3R) is a presynaptic autoreceptor regulating histamine release via negative feedback way. Our previous study showed that thioperamide, as an antagonist of H3R, enhances autophagy and protects against ischemic injury. However, the effect of thioperamide on autophagic function and Aβ pathology in AD remains unknown. In this study, we found that thioperamide promoted cognitive function, ameliorated neuronal loss, and Aβ pathology in APP/PS1 transgenic (Tg) mice. Interestingly, thioperamide up-regulated autophagic level and lysosomal function both in APP/PS1 Tg mice and in primary neurons under Aβ-induced injury. The neuroprotection by thioperamide against AD was reversed by 3-MA, inhibitor of autophagy, and siRNA of Atg7, key autophagic-related gene. Furthermore, inhibition of activity of CREB, H3R downstream signaling, by H89 reversed the effect of thioperamide on promoted cell viability, activated autophagic flux, and increased autophagic-lysosomal proteins expression, including Atg7, TFEB, and LAMP1, suggesting a CREB-dependent autophagic activation by thioperamide in AD. Taken together, these results suggested that H3R antagonist thioperamide improved cognitive impairment in APP/PS1 Tg mice via modulation of the CREB-mediated autophagy and lysosomal pathway, which contributed to Aβ clearance. This study uncovered a novel mechanism involving autophagic regulating behind the therapeutic effect of thioperamide in AD.

In vivo evaluation of effects of histamine H3 receptor antagonists on methamphetamine-induced hyperlocomotion in mice

A single administration with METH (3 mg/kg) induced a hyperlocomotion in male ICR mice. Pretreatment of mice with pitolisant, a histamine H₃ receptor antagonist (5 and 10 mg/kg), for 30 min showed a significant reduction of the hyperlocomotion induced by METH, as compared with vehicle (saline)-pretreated subjects. Pretreatment of mice with the histamine H₃ receptor antagonists JNJ-10181457 (5 and 10 mg/kg) or conessine (20 mg/kg), also showed similar inhibitory effects on METH-induced hyperlocomotion, similar to pitolisant. No significant change in locomotion was observed in mice pretreated with pitolisant, JNJ-10181457, or conessine alone. The pitolisant (10 mg/kg) action on METH-induced hyperlocomotion was completely abolished by the histamine H₁ receptor antagonist pyrilamine (10 mg/kg), but not by the peripherally acting histamine H₁ receptor antagonist fexofenadine (20 mg/kg), the brain-penetrating histamine H₂ receptor antagonist zolantidine (10 mg/kg), or the brain-penetrating histamine H₄ receptor antagonist JNJ-7777120 (40 mg/kg). Pretreatment with a histamine H₃ receptor agonist immepip (10 mg/kg) augmented METH--induced behavior, including hyperlocomotion and stereotyped biting, and combined pretreatment with pitolisant (10 mg/kg) significantly attenuated stereotyped biting. These observations suggest that pretreatment with histamine H₃ receptor antagonists attenuate METH-induced hyperlocomotion via releasing histamine after blocking H₃ receptors, which then bind to the post-synaptic histamine receptor H₁ (but not H₂ or H₄). It is likely that activation of brain histamine systems may be a good strategy for the development of agents, which treat METH abuse and dependence.

Neuronal histamine and the memory of emotionally salient events

In this review, we describe the experimental paradigms used in preclinical studies to unravel the histaminergic brain circuits that modulate the formation and retrieval of memories associated with aversive events. Emotionally arousing events, especially bad ones, are remembered more accurately, clearly and for longer periods of time than neutral ones. Maladaptive elaborations of these memories may eventually constitute the basis of psychiatric disorders such as generalized anxiety, obsessive-compulsive disorders and post-traumatic stress disorder. A better understanding of the role of the histaminergic system in learning and memory has not only a theoretical significance but also a translational value. Ligands of histamine receptors are among the most used drugs worldwide; hence, understanding the impact of these compounds on learning and memory may help improve their pharmacological profile and unravel unexplored therapeutic applications. 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.

NMR assignment of the in vivo daphnia magna metabolome

Daphnia (freshwater fleas) are among the most widely used organisms in regulatory aquatic toxicology/ecology, while their recent listing as an NIH model organism is stimulating research for understanding human diseases and processes.

Selective monoaminergic and histaminergic circuit dysregulation following long-term HIV-1 protein exposure

Between 30 and 60% of HIV-seropositive individuals develop symptoms of clinical depression and/or apathy. Dopamine and serotonin are associated with motivational alterations; however, histamine is less well studied. In the present study, we used fast-scan cyclic voltammetry in HIV-1 transgenic (Tg) rats to simultaneously analyze the kinetics of nucleus accumbens dopamine (DA), prefrontal cortical serotonin (5-HT), and hypothalamic histamine (HA). For voltammetry, subjects were 15 HIV-1 Tg (7 male, 8 female) and 20 F344/N (11 male, 9 female) adult rats. Both serotonergic and dopaminergic release and reuptake kinetics were decreased in HIV-1 Tg animals relative to controls. In contrast, rates of histamine release and reuptake increased in HIV-1 Tg rats. Additionally, we used immunohistochemical (IHC) methods to identify histaminergic neurons in the tuberomammillary nucleus (TMN) of the hypothalamus. For IHC, subjects were 9 HIV-1 Tg (5 male, 4 female) and 9 F344/N (5 male, 4 female) adult rats. Although the total number of TMN histaminergic cells did not differ between HIV-1 Tg rats and F344/N controls, a significant sex effect was found, with females having an increased number of histaminergic neurons, relative to males. Collectively, these findings illustrate neurochemical alterations that potentially underlie or exacerbate the pathogenesis of clinical depression and/or apathy in HIV-1.

Histamine induces intracellular Ca2+ oscillations and nitric oxide release in endothelial cells from brain microvascular circulation

The neuromodulator histamine is able to vasorelax in human cerebral, meningeal and temporal arteries via endothelial histamine 1 receptors (H₁ Rs) which result in the downstream production of nitric oxide (NO), the most powerful vasodilator transmitter in the brain. Although endothelial Ca ²⁺ signals drive histamine-induced NO release throughout the peripheral circulation, the mechanism by which histamine evokes NO production in human cerebrovascular endothelial cells is still unknown. Herein, we exploited the human cerebral microvascular endothelial cell line, hCMEC/D3, to assess the role of intracellular Ca ²⁺ signaling in histamine-induced NO release. To achieve this goal, hCMEC/D3 cells were loaded with the Ca ²⁺ - and NO-sensitive dyes, Fura-2/AM and DAF-FM/AM, respectively. Histamine elicited repetitive oscillations in intracellular Ca ²⁺ concentration in hCMEC/D3 cells throughout a concentration range spanning from 1 pM up to 300 μM. The oscillatory Ca ²⁺ response was suppressed by the inhibition of H ₁ Rs with pyrilamine, whereas H ₁ R was abundantly expressed at the protein level. We further found that histamine-induced intracellular Ca ²⁺ oscillations were initiated by endogenous Ca ²⁺ mobilization through inositol-1,4,5-trisphosphate- and nicotinic acid dinucleotide phosphate-sensitive channels and maintained over time by store-operated Ca ²⁺ entry. In addition, histamine evoked robust NO release that was prevented by interfering with the accompanying intracellular Ca ²⁺ oscillations, thereby confirming that the endothelial NO synthase is recruited by Ca ²⁺ spikes also in hCMEC/D3 cells. These data provide the first evidence that histamine evokes NO production from human cerebrovascular endothelial cells through intracellular Ca ²⁺ oscillations, thereby shedding novel light on the mechanisms by which this neuromodulator controls cerebral blood flow.

Brain Histamine Modulates the Antidepressant-Like Effect of the 3-Iodothyroacetic Acid (TA1)

3-iodothyroacetic acid (TA1), an end metabolite of thyroid hormone, has been shown to produce behavioral effects in mice that are dependent on brain histamine. We now aim to verify whether pharmacologically administered TA1 has brain bioavailability and is able to induce histamine-dependent antidepressant-like behaviors. TA1 brain, liver and plasma levels were measured by LC/MS-MS in male CD1 mice, sacrificed 15 min after receiving a high TA1 dose (330 μgkg^-1). The hypothalamic mTOR/AKT/GSK-β cascade activation was evaluated in mice treated with 0.4, 1.32, 4 μgkg^-1 TA1 by Western-blot. Mast cells were visualized by immuno-histochemistry in brain slices obtained from mice treated with 4 μgkg^-1 TA1. Histamine release triggered by TA1 (20-1000 nM) was also evaluated in mouse peritoneal mast cells. After receiving TA1 (1.32, 4 or 11 μgkg^-1; i.p.) CD1 male mice were subjected to the forced swim (FST) and the tail suspension tests (TST). Spontaneous locomotor and exploratory activities, motor incoordination, and anxiolytic or anxiogenic effects, were evaluated. Parallel behavioral tests were also carried out in mice that, prior to receiving TA1, were pre-treated with pyrilamine (10 mgkg^-1; PYR) or zolantidine (5 mgkg^-1; ZOL), histamine type 1 and type 2 receptor antagonists, respectively, or with p-chloro-phenylalanine (100 mgkg^-1; PCPA), an inhibitor of serotonin synthesis. TA1 given i.p. to mice rapidly distributes in the brain, activates the hypothalamic mTOR/AKT and GSK-3β cascade and triggers mast cells degranulation. Furthermore, TA1 induces antidepressant effects and stimulates locomotion with a mechanism that appears to depend on the histaminergic system. TA1 antidepressant effect depends on brain histamine, thus highlighting a relationship between the immune system, brain inflammation and the thyroid.

Activation of orexinergic and histaminergic pathway involved in therapeutic effect of histamine H4 receptor antagonist against cisplatin-induced anorexia in mice

We previously reported that hypothalamic tumor necrosis factor-alpha (TNF-α) mRNA expression via histamine H₄ receptors contributes to the development of cisplatin-induced anorexia; however, its precise mechanisms remain unclear. It has been reported that chemotherapeutic agents induce the suppression of orexin neuron activity, and the administration of orexin inhibits chemotherapeutic agent-induced gastric discomfort. Other studies demonstrated that the central administration of TNF-α impairs the orexinergic system, and that orexin excites the histaminergic system. We investigated the involvement of orexinergic and histaminergic systems in the therapeutic effect of an H₄ receptor antagonist against cisplatin-induced anorexia. Cisplatin decreased the expression of prepro-orexin mRNA, which encodes precursors of orexin, in the hypothalamus of mice. The period of expression decreased in parallel with the onset of anorexia, and treatment with an H₄ receptor antagonist (JNJ7777120, 10 mg/kg) inhibited the decrease in expression. The effect of the H₄ receptor antagonist on cisplatin-induced anorexia in mice was antagonized by an orexin OX₂ receptor antagonist (JNJ10397049, 5 mg/kg) rather than an orexin OX₁ receptor antagonist (SB408124, 30 mg/kg). Although an OX₂ receptor agonist (YNT-185, 20 mg/kg) or a histamine H₃ receptor inverse agonist (ciproxifan, 1 mg/kg) inhibited the cisplatin-induced anorexia, the inhibitory effect of the OX₂ receptor agonist was antagonized by an H₃ receptor silent antagonist (VUF5681, 5 mg/kg). The combination of JNJ7777120 (10 mg/kg) and ciproxifan (0.5 mg/kg) completely resolved the cisplatin-induced anorexia. These results suggest that activation of the orexinergic and histaminergic pathway is involved in the therapeutic effect of an H₄ receptor antagonist against cisplatin-induced anorexia.

The histamine H3R antagonist DL77 attenuates autistic behaviors in a prenatal valproic acid-induced mouse model of autism

Autistic spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairment in social communication and restricted/repetitive behavior patterns or interests. Antagonists targeting histamine H3 receptor (H3R) are considered potential therapeutic agents for the therapeutic management of different brain disorders, e.g., cognitive impairments. Therefore, the effects of subchronic treatment with the potent and selective H3R antagonist DL77 (5, 10, or 15 mg/kg, i.p.) on sociability, social novelty, anxiety, and aggressive/repetitive behavior in male Tuck-Ordinary (TO) mice with ASD-like behaviors induced by prenatal exposure to valproic acid (VPA, 500 mg/kg, i.p.) were evaluated using the three-chamber test (TCT), marble burying test (MBT), nestlet shredding test (NST), and elevated plus maze (EPM) test. The results showed that VPA-exposed mice exhibited significantly lower sociability and social novelty preference compared to VPA-exposed mice that were pretreated with DL77 (10 or 15 mg/kg, i.p.). VPA-exposed mice presented a significantly higher percentage of buried marbles in MBT and shredded nestlet significantly more in NST compared to the control groups. However, VPA-exposed animals pretreated with DL77 (10 or 15 mg/kg, i.p.) buried a reduced percentage of marbles in MBT and presented a significantly lower percentage of shredding behavior in NST. On the other hand, pretreatment with DL77 (5, 10, or 15 mg/kg, i.p.) failed to restore the disturbed anxiety levels and hyperactivity observed in VPA-exposed animals in EPM, whereas the reference drug donepezil (DOZ, 1 mg/kg, i.p.) significantly palliated the anxiety and reduced the hyperactivity measures of VPA-exposed mice. Furthermore, pretreatment with DL77 (10 or 15 mg/kg, i.p.) modulated oxidative stress status by increasing GSH and decreasing MDA, and it attenuated the proinflammatory cytokines IL-1β, IL-6 and TNF-α exacerbated by lipopolysaccharide (LPS) challenge, in VPA-exposed mouse brain tissue. Taken together, these results provide evidence that modulation of brain histaminergic neurotransmission, such as by subchronic administration of the H3R antagonist DL77, may serve as an effective pharmacological therapeutic target to rescue ASD-like behaviors in VPA-exposed animals, although further investigations are necessary to corroborate and expand these initial data.

Dual-transmitter systems regulating arousal, attention, learning and memory

An array of neuromodulators, including monoamines and neuropeptides, regulate most behavioural and physiological traits. In the past decade, dramatic progress has been made in mapping neuromodulatory circuits, in analysing circuit dynamics, and interrogating circuit function using pharmacogenetic, optogenetic and imaging methods This review will focus on several distinct neural networks (acetylcholine/GABA/glutamate; histamine/GABA; orexin/glutamate; and relaxin-3/GABA) that originate from neural hubs that regulate wakefulness and related attentional and cognitive processes, and highlight approaches that have identified dual transmitter roles in these behavioural functions. Modulation of these different neural networks might be effective treatments of diseases related to arousal/sleep dysfunction and of cognitive dysfunction in psychiatric and neurodegenerative disorders.

Sleep and Sedative States Induced by Targeting the Histamine and Noradrenergic Systems

Sedatives target just a handful of receptors and ion channels. But we have no satisfying explanation for how activating these receptors produces sedation. In particular, do sedatives act at restricted brain locations and circuitries or more widely? Two prominent sedative drugs in clinical use are zolpidem, a GABA_A receptor positive allosteric modulator, and dexmedetomidine (DEX), a selective α2 adrenergic receptor agonist. By targeting hypothalamic neuromodulatory systems both drugs induce a sleep-like state, but in different ways: zolpidem primarily reduces the latency to NREM sleep, and is a controlled substance taken by many people to help them sleep; DEX produces prominent slow wave activity in the electroencephalogram (EEG) resembling stage 2 NREM sleep, but with complications of hypothermia and lowered blood pressure—it is used for long term sedation in hospital intensive care units—under DEX-induced sedation patients are arousable and responsive, and this drug reduces the risk of delirium. DEX, and another α2 adrenergic agonist xylazine, are also widely used in veterinary clinics to sedate animals. Here we review how these two different classes of sedatives, zolpidem and dexmedetomideine, can selectively interact with some nodal points of the circuitry that promote wakefulness allowing the transition to NREM sleep. Zolpidem enhances GABAergic transmission onto histamine neurons in the hypothalamic tuberomammillary nucleus (TMN) to hasten the transition to NREM sleep, and DEX interacts with neurons in the preoptic hypothalamic area that induce sleep and body cooling. This knowledge may aid the design of more precise acting sedatives, and at the same time, reveal more about the natural sleep-wake circuitry.

Bottom-Up versus Top-Down Induction of Sleep by Zolpidem Acting on Histaminergic and Neocortex Neurons

Zolpidem, a GABA_A receptor-positive modulator, is the gold-standard drug for treating insomnia. Zolpidem prolongs IPSCs to decrease sleep latency and increase sleep time, effects that depend on α2 and/or α3 subunit-containing receptors. Compared with natural NREM sleep, zolpidem also decreases the EEG power, an effect that depends on α1 subunit-containing receptors, and which may make zolpidem-induced sleep less optimal. In this paper, we investigate whether zolpidem needs to potentiate only particular GABAergic pathways to induce sleep without reducing EEG power. Mice with a knock-in F77I mutation in the GABA_A receptor γ2 subunit gene are zolpidem-insensitive. Using these mice, GABA_A receptors in the frontal motor neocortex and hypothalamic (tuberomammillary nucleus) histaminergic-neurons of γ2I77 mice were made selectively sensitive to zolpidem by genetically swapping the γ2I77 subunits with γ2F77 subunits. When histamine neurons were made selectively zolpidem-sensitive, systemic administration of zolpidem shortened sleep latency and increased sleep time. But in contrast to the effect of zolpidem on wild-type mice, the power in the EEG spectra of NREM sleep was not decreased, suggesting that these EEG power-reducing effects of zolpidem do not depend on reduced histamine release. Selective potentiation of GABA_A receptors in the frontal cortex by systemic zolpidem administration also reduced sleep latency, but less so than for histamine neurons. These results could help with the design of new sedatives that induce a more natural sleep.

SIGNIFICANCE STATEMENT

Many people who find it hard to get to sleep take sedatives. Zolpidem (Ambien) is the most widely prescribed "sleeping pill." It makes the inhibitory neurotransmitter GABA work better at its receptors throughout the brain. The sleep induced by zolpidem does not resemble natural sleep because it produces a lower power in the brain waves that occur while we are sleeping. We show using mouse genetics that zolpidem only needs to work on specific parts and cell types of the brain, including histamine neurons in the hypothalamus, to induce sleep but without reducing the power of the sleep. This knowledge could help in the design of sleeping pills that induce a more natural sleep.

Threat effects on human oculo-motor function

Neuro-anatomical evidence supports the potential for threat-related factors, such as fear, anxiety and vigilance, to influence brainstem motor nuclei controlling eye movements, as well as the vestibular nuclei. However, little is known about how threat influences human ocular responses, such as eye saccades (ES), smooth pursuit eye tracking (SP), and optokinetic nystagmus (OKN), and whether these responses can be facilitated above normal baseline levels with a natural source of threat. This study was designed to examine the effects of height-induced postural threat on the gain of ES, SP and OKN responses in humans. Twenty participants stood at two different surface heights while performing ES (ranging from 8° to 45° from center), SP (15, 20, 30°/s) and OKN (15, 30, 60°/s) responses in the horizontal plane. Height did not significantly increase the slope of the relationship between ES peak velocity and initial amplitude, or the gain of ES amplitude. In contrast height significantly increased SP and OKN gain. Significant correlations were found between changes in physiological arousal and OKN gain. Observations of changes with height in OKN and SP support neuro-anatomical evidence of threat-related mechanisms influencing both oculo-motor nuclei and vestibular reflex pathways. Although further study is warranted, the findings suggest that potential influences of fear, anxiety and arousal/alertness should be accounted for, or controlled, during clinical vestibular and oculo-motor testing.

Locus Ceruleus Norepinephrine Release: A Central Regulator of CNS Spatio-Temporal Activation?

Norepinephrine (NE) is synthesized in the Locus Coeruleus (LC) of the brainstem, from where it is released by axonal varicosities throughout the brain via volume transmission. A wealth of data from clinics and from animal models indicates that this catecholamine coordinates the activity of the central nervous system (CNS) and of the whole organism by modulating cell function in a vast number of brain areas in a coordinated manner. The ubiquity of NE receptors, the daunting number of cerebral areas regulated by the catecholamine, as well as the variety of cellular effects and of their timescales have contributed so far to defeat the attempts to integrate central adrenergic function into a unitary and coherent framework. Since three main families of NE receptors are represented-in order of decreasing affinity for the catecholamine-by: α2 adrenoceptors (α2Rs, high affinity), α1 adrenoceptors (α1Rs, intermediate affinity), and β adrenoceptors (βRs, low affinity), on a pharmacological basis, and on the ground of recent studies on cellular and systemic central noradrenergic effects, we propose that an increase in LC tonic activity promotes the emergence of four global states covering the whole spectrum of brain activation: (1) sleep: virtual absence of NE, (2) quiet wake: activation of α2Rs, (3) active wake/physiological stress: activation of α2- and α1-Rs, (4) distress: activation of α2-, α1-, and β-Rs. We postulate that excess intensity and/or duration of states (3) and (4) may lead to maladaptive plasticity, causing-in turn-a variety of neuropsychiatric illnesses including depression, schizophrenic psychoses, anxiety disorders, and attention deficit. The interplay between tonic and phasic LC activity identified in the LC in relationship with behavioral response is of critical importance in defining the short- and long-term biological mechanisms associated with the basic states postulated for the CNS. While the model has the potential to explain a large number of experimental and clinical findings, a major challenge will be to adapt this hypothesis to integrate the role of other neurotransmitters released during stress in a centralized fashion, like serotonin, acetylcholine, and histamine, as well as those released in a non-centralized fashion, like purines and cytokines.

Brain Histamine N-Methyltransferase As a Possible Target of Treatment for Methamphetamine Overdose

Stereotypical behaviors induced by methamphetamine (METH) overdose are one of the overt symptoms of METH abuse, which can be easily assessed in animal models. Currently, there is no successful treatment for METH overdose. There is increasing evidence that elevated levels of brain histamine can attenuate METH-induced behavioral abnormalities, which might therefore constitute a novel therapeutic treatment for METH abuse and METH overdose. In mammals, histamine N-methyltransferase (HMT) is the sole enzyme responsible for degrading histamine in the brain. Metoprine, one of the most potent HMT inhibitors, can cross the blood-brain barrier and increase brain histamine levels by inhibiting HMT. Consequently, this compound can be a candidate for a prototype of drugs for the treatment of METH overdose.

Wakefulness Is Governed by GABA and Histamine Cotransmission

Histaminergic neurons in the tuberomammilary nucleus (TMN) of the hypothalamus form a widely projecting, wake-active network that sustains arousal. Yet most histaminergic neurons contain GABA. Selective siRNA knockdown of the vesicular GABA transporter (vgat, SLC32A1) in histaminergic neurons produced hyperactive mice with an exceptional amount of sustained wakefulness. Ablation of the vgat gene throughout the TMN further sharpened this phenotype. Optogenetic stimulation in the caudate-putamen and neocortex of “histaminergic” axonal projections from the TMN evoked tonic (extrasynaptic) GABA_A receptor Cl⁻ currents onto medium spiny neurons and pyramidal neurons. These currents were abolished following vgat gene removal from the TMN area. Thus wake-active histaminergic neurons generate a paracrine GABAergic signal that serves to provide a brake on overactivation from histamine, but could also increase the precision of neocortical processing. The long range of histamine-GABA axonal projections suggests that extrasynaptic inhibition will be coordinated over large neocortical and striatal areas.

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Histaminergic axons corelease GABA into the neocortex and striatum

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The released GABA produces slow tonic inhibition

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Reducing vgat expression in histaminergic neurons increases wakefulness

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Histamine-GABA axons will coordinate tonic inhibition over large cortical areas

The metabolism of histamine in rat hypothalamus and cortex after reserpine treatment

The effect of reserpine on histamine (HA) and tele-methylhistamine (N(τ)-MHA) in hypothalamus and cortex of rats was analyzed and compared to catecholamines. IP injection of reserpine (5 mg/kg) confirmed the effectiveness of reserpine treatment on noradrenaline and dopamine levels. Our in-vitro experiment with synaptosomal/crude mitochondrial fraction from hypothalamus and cortex confirmed that while mono amine oxidase (MAO) is an efficient metabolic enzyme for catecholamines, HA is not significantly affected by its enzymatic action. HMT activity after reserpine, pargyline and L-histidine treatment showed no differences compared to the control values. However HDC was significantly increased in both hypothalamus and cortex. In this study, Ws/Ws rats with deficiency of mast cells were used to clarify aspects of HA metabolism in HAergic neurons by eliminating the contribution of mast cells. The irreversible MAO-B inhibitor Pargyline (65 mg/kg) failed to accumulate N(τ)-MHA in the hypothalamus. However, when animals treated with reserpine and pargyline/reserpine were compared, the last group showed higher N(τ)-MHA values (p < 0.01). Moreover, the precursor of HA, L-histidine (1 g/kg), produced an increase of HA in the hypothalamus to 166% and the cortex to 348%. In conclusion, our results suggest that the effect of reserpine on the HA pools in the brain might be different. The neuronal HA pools are more resistant to reserpine as compared to those of catecholamine. Moreover, the HAergic pool appears to be more resistant to depletion than mast cells' pool, and thus HDC/HMT activity and its localization may play a key role in the understanding of HA metabolism in brain after reserpine treatment.

Expression pattern of histaminergic neurons in the human fetal hypothalamus at second and third trimester

Background

The hypothalamic nuclei constitute that part of the corticodiencephalic mechanism that activates, controls and integrates the peripheral, autonomic mechanisms, endocrine activity and many somatic functions. Their full integration into behavior requires the neocortex, in particular the limbic system.

Purpose

An antiserum against histamine was used to reveal the location and time of appearance of the amine in developing fetal hypothalamus. Although the expression pattern of histamine is well studied in mammals, less is known about it in humans. The neurotransmitter histamine plays a crucial role in co-ordinating mutiple inputs from various brain centres.

Methods

In the present investigations the hypothalamus was studied in the human fetus ranging from 19 weeks of gestation (GW) to term by using immunocytochemistry. Distinct neurons that stained with Pischinger’s methylene blue were obsereved from 19 GW to term. Sections adjacent to those that contained neurons were stained for histamine labeling. Immunoreactive neurons in the hypothalamus at 19 GW of gestation show relatively meager population.

Results

Histamine immunoreactive (His-ir) neurons of the hypothalamus were divisible in lateral and ventrolateral subgroups at 19 GW to 24 GW At 32 GW along with the neuronal cell bodies some beaded fibers were visible. There was progressive increase in the histamine expressing neurons as the fetus grows. In term fetal specimens, two to three small groups of his-ir neurons gradually merged in a single large ventrolateral group.

Conclusion

The findings of the present study provide for a better understanding of the chemoneuroarchitecture of histamine containing neurons in hypothalamus during second and third trimester of human fetal development.

Histamine impairs midbrain dopaminergic development in vivo by activating histamine type 1 receptors

Background

Histamine (HA) regulates the sleep-wake cycle, synaptic plasticity and memory in adult mammals. Dopaminergic specification in the embryonic ventral midbrain (VM) coincides with increased HA brain levels. To study the effect of HA receptor stimulation on dopamine neuron generation, we administered HA to dopamine progenitors, both in vitro and in vivo.

Results

Cultured embryonic day 12 (E12) VM neural stem/progenitor cells expressed transcripts for HA receptors H₁R, H₂R and H₃R. These undifferentiated progenitors increased intracellular calcium upon HA addition. In HA-treated cultures, dopamine neurons significantly decreased after activation of H₁R. We performed intrauterine injections in the developing VM to investigate HA effects in vivo. HA administration to E12 rat embryos notably reduced VM Tyrosine Hydroxylase (TH) staining 2 days later, without affecting GABA neurons in the midbrain, or serotonin neurons in the mid-hindbrain boundary. qRT-PCR and Western blot analyses confirmed that several markers important for the generation and maintenance of dopaminergic lineage such as TH, Lmx1a and Lmx1b were significantly diminished. To identify the cell type susceptible to HA action, we injected embryos of different developmental stages, and found that neural progenitors (E10 and E12) were responsive, whereas differentiated dopaminergic neurons (E14 and E16) were not susceptible to HA actions. Proliferation was significantly diminished, whereas neuronal death was not increased in the VM after HA administration. We injected H₁R or H₂R antagonists to identify the receptor responsible for the detrimental effect of HA on dopaminergic lineage and found that activation of H₁R was required.

Conclusion

These results reveal a novel action of HA affecting dopaminergic lineage during VM development.

What optogenetic stimulation is telling us (and failing to tell us) about fast neurotransmitters and neuromodulators in brain circuits for wake-sleep regulation

In the last eight years optogenetic tools have been widely used to identify functional synaptic connectivity between specific neuronal populations. Most of our knowledge comes from the photo-activation of channelrhodopsin-2 (ChR2) expressing inputs that release glutamate and GABA. More recent studies have been reporting releases of acetylcholine and biogenic amines but direct evidence for photo-evoked released of neuropeptides is still limited particularly in brain slice studies. The high fidelity in the responses with photo-evoked amino-acid transmission is ideal for ChR2-assisted circuit mapping and this approach has been successfully used in different fields of neuroscience. Conversely, neuropeptides employ a slow mode of communication and might require higher frequency and prolonged stimulations to be released. These factors may have contributed to the apparent lack of success for optogenetic release of neuropeptides. In addition, once released, neuropeptides often act on multiple sites and at various distances from the site of release resulting in a greater complexity of postsynaptic responses. Here, we focus on what optogenetics is telling us-and failing to tell us-about fast neurotransmitters and neuropeptides.

Antihistamine induced blood oxygenation level dependent response changes related to visual processes during sensori-motor performance

The histaminergic involvement in selective processes underlying its role in human sensori-motor performance is largely unknown. Recently, selective effects of central H₁-inverse agonism on sensory visual processes were observed in electrophysiological--but not behavioral data; a discrepancy suggested to result from speeded response-choice related processes. This study attempts to establish the effects on visual processes and identify putative compensatory mechanisms related to increased visual and response-choice task demands by assessing H₁-inverse agonism induced changes in blood oxygenation level dependent (BOLD) response. Twelve participants received oral doses of dexchlorpheniramine 4 mg, lorazepam 1 mg, and placebo in a three-way crossover designed study. Brain activity was assessed for choice reaction time task performance in a 3 T magnetic resonance scanner 2 h after drug administration. Participants responded with their left or right hand and index or middle finger as indicated by the laterality of stimulus presentation and identity of the stimulus, respectively. Stimuli were intact or visually degraded and responses were compatible or incompatible with the laterality of stimulus presentation. Both dexchlorpheniramine and lorazepam affected the BOLD response in the occipital cortex indicating affected visual information processing. Dexchlorpheniramine decreased BOLD response in the dorsal precuneus and left precentral gyrus as part of a motor network, which however might not be interpreted as a compensatory mechanism, but may be the upstream consequence of impaired visual processing.

Effect of histamine H1 and H2 receptor antagonists, microinjected into cerebellar vermis, on emotional memory consolidation in mice

This study investigated the effects of histamine H1 or H2 receptor antagonists on emotional memory consolidation in mice submitted to the elevated plus maze (EPM). The cerebellar vermis of male mice (Swiss albino) was implanted using a cannula guide. Three days after recovery, behavioral tests were performed in the EPM on 2 consecutive days (T1 and T2). Immediately after exposure to the EPM (T1), animals received a microinjection of saline (SAL) or the H1 antagonist chlorpheniramine (CPA; 0.016, 0.052, or 0.16 nmol/0.1 µL) in Experiment 1, and SAL or the H2 antagonist ranitidine (RA; 0.57, 2.85, or 5.7 nmol/0.1 µL) in Experiment 2. Twenty-four hours later, mice were reexposed to the EPM (T2) under the same experimental conditions but they did not receive any injection. Data were analyzed using one-way ANOVA and the Duncan test. In Experiment 1, mice microinjected with SAL and with CPA entered the open arms less often (%OAE) and spent less time in the open arms (%OAT) in T2, and there was no difference among groups. The results of Experiment 2 demonstrated that the values of %OAE and %OAT in T2 were lower compared to T1 for the groups that were microinjected with SAL and 2.85 nmol/0.1 µL RA. However, when animals were microinjected with 5.7 nmol/0.1 µL RA, they did not show a reduction in %OAE and %OAT. These results demonstrate that CPA did not affect behavior at the doses used in this study, while 5.7 nmol/0.1 µL RA induced impairment of memory consolidation in the EPM.

The mediation of the central histaminergic system in the pressor effect of intracerebroventricularly injected melittin, a phospholipase A2 activator, in normotensive rats

Melittin is a polypeptide component of bee venom that leads to an increase in arachidonic acid release and subsequently in prostaglandin synthesis by activating phospholipase A(2). Recently we demonstrated that centrally or peripherally administrated melittin caused pressor effect and central thromboxane A(2) (TXA(2)) and cholinergic system mediated these effects of melittin. Also centrally injected histamine leads to pressor and bradycardic response by activating central histamine receptors in normotensive rats and central cholinergic system involved the effects of histamine. The present study demonstrates an involvement of the central histaminergic system in melittin-induced cardiovascular effect in normotensive rats. Experiments were carried out in male Sprague Dawley rats. Intracerebroventricularly (i.c.v.) injected melittin (0.5, 1 and 2 nmol) caused dose- and time-dependent increases in mean arterial pressure (MAP) and decrease in heart rate (HR) as we reported previously. Moreover, H(2) receptor antagonist ranitidine (50 nmol; i.c.v.) almost completely and H(3)/H(4) receptor antagonist thioperamide (50 nmol; i.c.v.) partly blocked melittin-evoked cardiovascular effects, whereas H(1) receptor blocker chlorpheniramine (50 nmol; i.c.v.) had no effect. Also centrally injected melittin was accompanied by 28% increase in extracellular histamine concentration in the posterior hypothalamus, as shown in microdialysis studies. In conclusion, results show that centrally administered melittin causes pressor and bradycardic response in conscious rats. Moreover, according to our findings, there is an involvement of the central histaminergic system in melittin-induced cardiovascular effects.

Emotional memory consolidation impairment induced by histamine is mediated by H1 but not H2 receptors

Histaminergic fibers are present in the molecular and granular layers of the cerebellum and have high density in the vermis and flocculus. Evidence indicates that the cerebellar vermis is involved in memory consolidation. Recently, we demonstrated that when histamine is microinjected into the cerebellar vermis it results in impaired emotional memory consolidation in mice that are submitted to the elevated plus maze (EPM). This study investigated whether histamine impairment was mediated by the H(1) or H(2) receptors. The cerebellar vermis of male mice (Swiss Albino) were implanted using a guide cannula. Three days after recovery, behavioral tests were performed in the EPM on two consecutive days (Trial 1 and Trial 2). Immediately after exposure to the EPM (Trial 1), animals received a microinjection of histaminergic drugs. In Experiment 1, saline (SAL) or histamine (HA, 4.07 nmol/0.1 μl) was microinjected 5 min after pretreatment with the H(1) antagonist chlorpheniramine (CPA, 0.16 nmol/0.1μl) or SAL. In Experiment 2, SAL or HA was microinjected into the mice 5 min after pretreatment with the H(2) antagonist ranitidine (RA, 2.85 nmol/0.1 μl) or SAL. Twenty-four hours later, the mice were re-exposed to the EPM (Trial 2) under the same experimental conditions but did not receive an injection. On both days, the test sessions were recorded to enable analysis of the behavioral measures. The decrease in open arm exploration (% entries and % time spent in the open arms) in Trial 2 relative to Trial 1 was used as a measure of learning and memory. The data were analyzed using the two-way analysis of variance (ANOVA) and Duncan's tests. In Experiment 1, the Duncan's test indicated that the mice entered the open arms less often (%OAE) and spent less time in the open arms (%OAT) in Trial 2 after being microinjected with SAL+SAL, SAL+CPA and CPA+HA. However, the animals that received SAL+HA did not enter the open arms less frequently or spend less time in them, which was significantly different from the CPA+HA group. The results of Experiment 2 demonstrated that the %OAE and %OAT in Trial 2 were different from Trial 1 for the groups that were microinjected with SAL+SAL and SAL+RA. The animals that were microinjected with RA+HA or with SAL+HA did not show a reduction in %OAE. These results demonstrate that the animals treated with HA did not avoid the open arms less on retesting and indicated that CPA did not alter the behavior parameters but did revert the histamine-induced impairment of memory consolidation. Furthermore, the H(2) antagonist RA, at the dose used in this study, did not affect memory consolidation and failed to revert histamine-induced impairment.

Origins of arousal: roles for medullary reticular neurons

The existence of a primitive CNS function involved in the activation of all vertebrate behaviors, generalized arousal (GA), has been proposed. Here, we provide an overview of the neuroanatomical, neurophysiological and molecular properties of reticular neurons within the nucleus gigantocellularis (NGC) of the mammalian medulla, and propose that the properties of these neurons equip them to contribute powerfully to GA. We also explore the hypothesis that these neurons may have evolved from the Mauthner cell in the medulla of teleost fish, although NGC neurons have a wider range of action far beyond the specific escape network served by Mauthner cells. Understanding the neuronal circuits that control and regulate GA is central to understanding how motivated behaviors such as hunger, thirst and sexual behaviors arise.

Histamine neurons in the tuberomamillary nucleus: a whole center or distinct subpopulations?

Histamine axons originate from a single source, the tuberomamillary nucleus (TMN) of the posterior hypothalamus, to innervate almost all central nervous system (CNS) regions. This feature, a compact cell group with widely distributed fibers, resembles that of other amine systems, such as noradrenaline or serotonin, and is consistent with a function for histamine over a host of physiological processes, including the regulation of the sleep-wake cycle, appetite, endocrine homeostasis, body temperature, pain perception, learning, memory, and emotion. An important question is whether these diverse physiological roles are served by different histamine neuronal subpopulation. While the histamine system is generally regarded as one single functional unit that provides histamine throughout the brain, evidence is beginning to accumulate in favor of heterogeneity of histamine neurons. The aim of this review is to summarize experimental evidence demonstrating that histamine neurons are heterogeneous, organized into functionally distinct circuits, impinging on different brain regions, and displaying selective control mechanisms. This could imply independent functions of subsets of histamine neurons according to their respective origin and terminal projections.

Adenosine inhibits glutamatergic input to basal forebrain cholinergic neurons

Adenosine has been proposed as an endogenous homeostatic sleep factor that accumulates during waking and inhibits wake-active neurons to promote sleep. It has been specifically hypothesized that adenosine decreases wakefulness and promotes sleep recovery by directly inhibiting wake-active neurons of the basal forebrain (BF), particularly BF cholinergic neurons. We previously showed that adenosine directly inhibits BF cholinergic neurons. Here, we investigated 1) how adenosine modulates glutamatergic input to BF cholinergic neurons and 2) how adenosine uptake and adenosine metabolism are involved in regulating extracellular levels of adenosine. Our experiments were conducted using whole cell patch-clamp recordings in mouse brain slices. We found that in BF cholinergic neurons, adenosine reduced the amplitude of AMPA-mediated evoked glutamatergic excitatory postsynaptic currents (EPSCs) and decreased the frequency of spontaneous and miniature EPSCs through presynaptic A(1) receptors. Thus we have demonstrated that in addition to directly inhibiting BF cholinergic neurons, adenosine depresses excitatory inputs to these neurons. It is therefore possible that both direct and indirect inhibition may synergistically contribute to the sleep-promoting effects of adenosine in the BF. We also found that blocking the influx of adenosine through the equilibrative nucleoside transporters or inhibiting adenosine kinase and adenosine deaminase increased endogenous adenosine inhibitory tone, suggesting a possible mechanism through which adenosine extracellular levels in the basal forebrain are regulated.

Modulation of Fgfr1a signaling in zebrafish reveals a genetic basis for the aggression-boldness syndrome

Behavioral syndromes are suites of two or more behaviors that correlate across environmental contexts. The aggression-boldness syndrome links aggression, boldness, and exploratory activity in a novel environment. Although aggression-boldness has been described in many animals, the mechanism linking its behavioral components is not known. Here we show that mutation of the gene encoding fibroblast growth factor receptor 1a (fgfr1a) simultaneously increases aggression, boldness, and exploration in adult zebrafish. We demonstrate that altered Fgf signaling also results in reduced brain histamine levels in mutants. Pharmacological increase of histamine signaling is sufficient to rescue the behavioral phenotype of fgfr1a mutants. Together, we show that a single genetic locus can underlie the aggression-boldness behavioral syndrome. We also identify one of the neurotransmitter pathways that may mediate clustering of these behaviors.