Histamine excites GABAergic cells in the rat substantia nigra and ventral tegmental area in vitro

Sexual dimorphism in histamine regulation of striatal dopamine

Background

Many neuropsychiatric disorders show sex differences in prevalence and presentation. For example, Tourette's Syndrome (TS) is diagnosed 3-5 times more often in males. Dopamine modulation of the basal ganglia is implicated in numerous neuropsychiatric conditions, including TS. Motivated by an unexpected genetic finding in a family with TS, we previously characterized the modulation of striatal dopamine by histamine.

Methods

We used microdialysis to analyze striatal dopamine response to the targeted infusion of histamine and histamine agonists. siRNA knockdown of histamine receptors was used to identify the cellular mediators of observed effects.

Results

Intracerebroventricular histamine reduced striatal dopamine in male mice, replicating previous work. Unexpectedly, histamine increased striatal dopamine in females. Targeted infusion of selected agonists revealed that the effect in males depends on H2R receptors in the substantia nigra pars compacta (SNc). Knockdown of H2R in SNc GABAergic neurons abrogated the effect, identifying these cells as a key locus of histamine's regulation of dopamine in males. In females, in contrast, H2R had no role; instead, H3R agonists in the striatum increased striatal dopamine. Strikingly, the effect of histamine on dopamine in females was modulated by the estrous cycle, appearing in estrus/proestrus but not in metestrus/diestrus.

Conclusions

These findings confirm the regulation of striatal dopamine by histamine but identify marked sexual dimorphism in and estrous modulation of this effect. These findings may shed light on the mechanistic underpinnings of other sex differences in the striatal circuitry, perhaps including the marked sex differences seen in TS and related neuropsychiatric conditions.

Translational Approaches to Influence Sleep and Arousal

Sleep disorders are widespread in society and are prevalent in military personnel and in Veterans. Disturbances of sleep and arousal mechanisms are common in neuropsychiatric disorders such as schizophrenia, post-traumatic stress disorder, anxiety and affective disorders, traumatic brain injury, dementia, and substance use disorders. Sleep disturbances exacerbate suicidal ideation, a major concern for Veterans and in the general population. These disturbances impair quality of life, affect interpersonal relationships, reduce work productivity, exacerbate clinical features of other disorders, and impair recovery. Thus, approaches to improve sleep and modulate arousal are needed. Basic science research on the brain circuitry controlling sleep and arousal led to the recent approval of new drugs targeting the orexin/hypocretin and histamine systems, complementing existing drugs which affect GABA_A receptors and monoaminergic systems. Non-invasive brain stimulation techniques to modulate sleep and arousal are safe and show potential but require further development to be widely applicable. Invasive viral vector and deep brain stimulation approaches are also in their infancy but may be used to modulate sleep and arousal in severe neurological and psychiatric conditions. Behavioral, pharmacological, non-invasive brain stimulation and cell-specific invasive approaches covered here suggest the potential to selectively influence arousal, sleep initiation, sleep maintenance or sleep-stage specific phenomena such as sleep spindles or slow wave activity. These manipulations can positively impact the treatment of a wide range of neurological and psychiatric disorders by promoting the restorative effects of sleep on memory consolidation, clearance of toxic metabolites, metabolism, and immune function and by decreasing hyperarousal.

Central histaminergic signalling, neural excitability and epilepsy

Epilepsy is a common neurological disorder characterized by repeated and spontaneous epileptic seizures and is not well controlled by current medication. Traditional theory suggests that epilepsy results from an imbalance of excitatory glutamate neurons and inhibitory GABAergic neurons. However, new evidence from clinical and preclinical research suggests that histamine in the CNS plays an important role in the modulation of neural excitability and in the pathogenesis of epilepsy. Many histamine receptor ligands have achieved curative effects in animal epilepsy models, among which the histamine H₃ receptor antagonist pitolisant has shown anti-epileptic effects in clinical trials. Recent studies, therefore, have focused on the potential action of histamine receptors to control and treat epilepsy. In this review, we summarize the findings from animal and clinical epilepsy research on the role of brain histamine and its receptors. We also identify current gaps in the research and suggest where further studies are most needed.

Sensitization to amphetamine psychostimulant effect: A key role for ventral tegmental area neurotensin type 2 receptors and MAP kinase pathway

Neurotensin is an endogenous neuropeptide that acts as a potent modulator of ventral tegmental area (VTA) neurotransmission. The present study was aimed at determining VTA cell population and neurotensin receptor subtype responsible for the initiation of amphetamine-induced psychomotor activity and extracellular signal-regulated kinases (ERK1/2) sensitization. During an induction phase, rats were injected intra-VTA on two occasions, every second day, with [D-Tyr¹¹ ]-neurotensin (D-Tyr-NT), SR142948 (a mix Ntsr1/Ntsr2 receptor subtype antagonist), SR48692 (a Ntsr1 antagonist), D-Tyr-NT + SR142498, D-Tyr-NT + SR48692, or the vehicle. Effects of intra-VTA drugs were evaluated at locomotor activity and ERK1/2 phosphorylation. Five days after the last VTA microinjection, the effect of a systemic injection of amphetamine was tested (sensitization test). Results show that D-Tyr-NT stimulated locomotor activity during the induction phase, an effect that was blocked by SR142948, but not SR48692. Amphetamine also induced significantly higher ambulatory activity in rats preinjected with D-Tyr-NT than in rats preinjected with the vehicle. This sensitization effect was again attenuated by SR142948, but not SR48692, hence suggesting that this effect is mediated by Ntsr2 receptors. To confirm this, we tested a highly selective Ntsr2 peptide-peptoid hybrid ligand, NT150. At the concentration tested, NT150 stimulated locomotor activity and lead to sensitized locomotor activity and a selective neurochemical (pERK1/2) response in tyrosine hydroxylase-positive neurons of the VTA. Both effects were prevented by SR142948. Taken together, these results show that neurotensin, acting on Ntsr2 receptor subtypes, stimulates locomotor activity and initiates neural changes (ERK1/2 phosphorylation) that lead to amphetamine-induced sensitization.

Compound 21, a two-edged sword with both DREADD-selective and off-target outcomes in rats

Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) represent a technical revolution in integrative neuroscience. However, the first used ligands exhibited dose-dependent selectivity for their molecular target, leading to potential unspecific effects. Compound 21 (C21) was recently proposed as an alternative, but in vivo characterization of its properties is not sufficient yet. Here, we evaluated its potency to selectively modulate the activity of nigral dopaminergic (DA) neurons through the canonical DREADD receptor hM4Di using TH-Cre rats. In males, 1 mg.kg-1 of C21 strongly increased nigral neurons activity in control animals, indicative of a significant off-target effect. Reducing the dose to 0.5 mg.kg-1 circumvented this unspecific effect, while activated the inhibitory DREADDs and selectively reduced nigral neurons firing. In females, 0.5 mg.kg-1 of C21 induced a transient and residual off-target effect that may mitigated the inhibitory DREADDs-mediated effect. This study raises up the necessity to test selectivity and efficacy of chosen ligands for each new experimental condition.

Arousal State-Dependent Alterations in VTA-GABAergic Neuronal Activity

Decades of research have implicated the ventral tegmental area (VTA) in motivation, learning and reward processing. We and others recently demonstrated that it also serves as an important node in sleep/wake regulation. Specifically, VTA-dopaminergic neuron activation is sufficient to drive wakefulness and necessary for the maintenance of wakefulness. However, the role of VTA-GABAergic neurons in arousal regulation is not fully understood. It is still unclear whether VTA-GABAergic neurons predictably alter their activity across arousal states, what is the nature of interactions between VTA-GABAergic activity and cortical oscillations, and how activity in VTA-GABAergic neurons relates to VTA-dopaminergic neurons in the context of sleep/wake regulation. To address these, we simultaneously recorded population activity from VTA subpopulations and electroencephalography/electromyography (EEG/EMG) signals during spontaneous sleep/wake states and in the presence of salient stimuli in freely-behaving mice. We found that VTA-GABAergic neurons exhibit robust arousal-state-dependent alterations in population activity, with high activity and transients during wakefulness and REM sleep. During wakefulness, population activity of VTA-GABAergic neurons, but not VTA-dopaminergic neurons, was positively correlated with EEG γ power and negatively correlated with θ power. During NREM sleep, population activity in both VTA-GABAergic and VTA-dopaminergic neurons negatively correlated with δ, θ, and σ power bands. Salient stimuli, with both positive and negative valence, activated VTA-GABAergic neurons. Together, our data indicate that VTA-GABAergic neurons, like their dopaminergic counterparts, drastically alter their activity across sleep-wake states. Changes in their activity predicts cortical oscillatory patterns reflected in the EEG, which are distinct from EEG spectra associated with dopaminergic neural activity.

Complex patterns of dopamine-related gene expression in the ventral tegmental area of male zebra finches relate to dyadic interactions with long-term female partners

Dopaminergic projections from the ventral tegmental area (VTA) to multiple efferent targets are implicated in pair bonding, yet the role of the VTA in the maintenance of long-term pair bonds is not well characterized. Complex interactions between numerous neuromodulators modify activity in the VTA, suggesting that individual differences in patterns of gene expression in this region may explain individual differences in long-term social interactions in bonded pairs. To test this hypothesis we used RNA-seq to measure expression of over 8000 annotated genes in male zebra finches in established male-female pairs. Weighted gene co-expression network analysis identified a gene module that contained numerous dopamine-related genes with TH found to be the most connected gene of the module. Genes in this module related to male agonistic behaviors as well as bonding-related behaviors produced by female partners. Unsupervised learning approaches identified two groups of males that differed with respect to expression of numerous genes. Enrichment analyses showed that many dopamine-related genes and modulators differed between these groups, including dopamine receptors, synthetic and degradative enzymes, the avian dopamine transporter and several GABA- and glutamate-related genes. Many of the bonding-related behaviors closely associated with VTA gene expression in the two male groups were produced by the male's partner, rather than the male himself. Collectively, results highlight numerous candidate genes in the VTA that can be explored in future studies and raise the possibility that the molecular/genetic organization of the VTA may be strongly shaped by a social partner and/or the strength of the pair bond.

Ionic Mechanisms Underlying the Excitatory Effect of Orexin on Rat Subthalamic Nucleus Neurons

Central orexinergic system deficiency results in cataplexy, a motor deficit characterized with a sudden loss of muscle tone, highlighting a direct modulatory role of orexin in motor control. However, the neural mechanisms underlying the regulation of orexin on motor function are still largely unknown. The subthalamic nucleus (STN), the only excitatory structure of the basal ganglia, holds a key position in the basal ganglia circuitry and motor control. Previous study has revealed a wide distribution of orexinergic fibers as well as orexin receptors in the basal ganglia including the STN. Therefore, in the present study, by using whole-cell patch clamp recording and immunostaining techniques, the direct effect of orexin on the STN neurons in brain slices, especially the underlying receptor and ionic mechanisms, were investigated. Our results show that orexin-A elicits an excitatory effect on STN neurons in rats. Tetrodotoxin (TTX) does not block the orexin-induced excitation on STN neurons, suggesting a direct postsynaptic action of the neuropeptide. The orexin-A-induced inward current on STN neurons is mediated by the activation of both OX1 and OX2 receptors. Immunofluorescence result shows that OX1 and OX2 receptors are co-expressed and co-localized in STN neurons. Furthermore, Na⁺-Ca²⁺ exchangers (NCXs) and inward rectifier K⁺ channels co-mediate the excitatory effect of orexin-A on STN neurons. These results demonstrate a dual receptor in conjunction with the downstream ionic mechanisms underlying the excitatory action of orexin on STN neurons, suggesting a potential modulation of the central orexinergic system on basal ganglia circuitry as well as its related motor control and motor diseases.

Brain histamine modulates recognition memory: possible implications in major cognitive disorders

Several behavioural tests have been developed to study and measure emotionally charged or emotionally neutral memories and how these may be affected by pharmacological, dietary or environmental manipulations. In this review, we describe the experimental paradigms used in preclinical studies to unravel the brain circuits involved in the recognition and memorization of environmentally salient stimuli devoid of strong emotional value. In particular, we focus on the modulatory role of the brain histaminergic system in the elaboration of recognition memory that is based on the judgement of the prior occurrence of an event, and it is believed to be a critical component of human declarative memory. The review also addresses questions that may help improve the treatment of impaired declarative memory described in several affective and neuropsychiatric disorders such as ADHD, Alzheimer's disease and major neurocognitive disorder. 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.

Histamine Excites Striatal Dopamine D1 and D2 Receptor-Expressing Neurons via Postsynaptic H1 and H2 Receptors

The central histaminergic nervous system, originating from the tuberomammillary nucleus (TMN) of the hypothalamus, widely innervates almost the whole brain, including the basal ganglia. Intriguingly, the histaminergic system is altered in parkinsonian patients. Yet, little is known about the effect and mechanisms of histamine on different types of neurons in the basal ganglia circuitry. Here, by using anterograde tracing, immunostaining, patch clamp recording, and single-cell qPCR techniques, we investigate the histaminergic afferents in the striatum, the major input structure of the basal ganglia, as well as the effect of histamine on the striatal GABAergic medium spiny projection neurons (MSNs). We report a direct histaminergic projection from the hypothalamic TMN to the striatum in rats. Furthermore, histamine exerts a strong postsynaptic excitatory effect on both dopamine D1 and D2 receptor-expressing MSNs. The concentration-response curves and the EC₅₀ values for histamine on these two types of MSNs are similar. In addition, dopamine D1 and D2 receptor-expressing MSNs co-express histamine H1 and H2 receptor mRNAs. Both histamine H1 and H2 receptors are co-localized on dopamine D1 and D2 receptor-expressing MSNs and co-mediate the histamine-induced excitation on the two types of neurons. These results suggest that the histaminergic afferent inputs in the striatum may modulate both dopamine D1 and D2 receptor-expressing MSNs by activation of postsynaptic histamine H1 and H2 receptors and thus serve as an important extrastriatal modulator for biasing the direct and indirect pathways to actively regulate functions of the basal ganglia and participate in the pathogenesis and pathophysiology of basal ganglia diseases.

Histamine facilitates GABAergic transmission in the rat entorhinal cortex: Roles of H1 and H2 receptors, Na+ -permeable cation channels, and inward rectifier K+ channels

In the brain, histamine (HA) serves as a neuromodulator and a neurotransmitter released from the tuberomammillary nucleus (TMN). HA is involved in wakefulness, thermoregulation, energy homeostasis, nociception, and learning and memory. The medial entorhinal cortex (MEC) receives inputs from the TMN and expresses HA receptors (H1 , H2 , and H3 ). We investigated the effects of HA on GABAergic transmission in the MEC and found that HA significantly increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) with an EC50 of 1.3 µM, but failed to significantly alter sIPSC amplitude. HA-induced increases in sIPSC frequency were sensitive to tetrodotoxin (TTX), required extracellular Ca2+ , and persisted when GDP-β-S, a G-protein inactivator, was applied postsynaptically via the recording pipettes, indicating that HA increased GABA release by facilitating the excitability of GABAergic interneurons in the MEC. Recordings from local MEC interneurons revealed that HA significantly increased their excitability as determined by membrane depolarization, generation of an inward current at -65 mV, and augmentation of action potential firing frequency. Both H1 and H2 receptors were involved in HA-induced increases in sIPSCs and interneuron excitability. Immunohistochemical staining showed that both H1 and H2 receptors are expressed on GABAergic interneurons in the MEC. HA-induced depolarization of interneurons involved a mixed ionic mechanism including activation of a Na+ -permeable cation channel and inhibition of a cesium-sensitive inward rectifier K+ channel, although HA also inhibited the delayed rectifier K+ channels. Our results may provide a cellular mechanism, at least partially, to explain the roles of HA in the brain. © 2017 Wiley Periodicals, Inc.

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

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

Identification of histaminergic neurons through histamine 3 receptor-mediated autoinhibition

Using a reporter mouse model with expression of the tomato fluorescent protein under the dopamine transporter promoter (Tmt-DAT) we discovered a new group of neurons in the histaminergic tuberomamillary nucleus (TMN), which, in contrast to tuberoinfundibular dopaminergic neurons of the dorsomedial arcuate nucleus, do not express tyrosine hydroxylase but can synthesize and store dopamine. Tmt-DAT neurons located within TMN share electrophysiological properties with histaminergic neurons: spontaneous firing at a membrane potential around -50 mV and presence of hyperpolarization-activated cyclic nucleotide-gated ion channels. Histamine (30 μM) depolarizes and excites Tmt-DAT neurons through H1R activation but inhibits histaminergic neurons through H3R activation thus allowing a pharmacological identification of the different neurons. Single-cell RT-PCR revealed that all histaminergic neurons expressing histidine decarboxylase (HDC) also express H3R. This includes neurons retrogradely traced from the striatum whose inhibition by a selective H3R agonist was indistinguishable from the whole population. Prolonged depolarization reduces the autoinhibition. The potency of histamine at H3R depends on membrane potential and on extracellular and intracellular calcium. Autoinhibition can be impaired by preincubation with capsaicin, a ligand of the calcium-permeable TRPV1 channel or by blockade of Ca(2+)-ATPase with thapsigargin. The pharmacology of autoinhibition is revisited and physiological conditions for its functionality are determined. Usage of reporter mouse models for the safe identification of aminergic neurons under pathophysiological conditions is recommended. This article is part of the Special Issue entitled 'Histamine Receptors'.

Turning a Negative into a Positive: Ascending GABAergic Control of Cortical Activation and Arousal

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain. Recent technological advances have illuminated the role of GABAergic neurons in control of cortical arousal and sleep. Sleep-promoting GABAergic neurons in the preoptic hypothalamus are well-known. Less well-appreciated are GABAergic projection neurons in the brainstem, midbrain, hypothalamus, and basal forebrain, which paradoxically promote arousal and fast electroencephalographic (EEG) rhythms. Thus, GABA is not purely a sleep-promoting neurotransmitter. GABAergic projection neurons in the brainstem nucleus incertus and ventral tegmental nucleus of Gudden promote theta (4-8 Hz) rhythms. Ventral tegmental area GABAergic neurons, neighboring midbrain dopamine neurons, project to the frontal cortex and nucleus accumbens. They discharge faster during cortical arousal and regulate reward. Thalamic reticular nucleus GABAergic neurons initiate sleep spindles in non-REM sleep. In addition, however, during wakefulness, they tonically regulate the activity of thalamocortical neurons. Other GABAergic inputs to the thalamus arising in the globus pallidus pars interna, substantia nigra pars reticulata, zona incerta, and basal forebrain regulate motor activity, arousal, attention, and sensory transmission. Several subpopulations of cortically projecting GABAergic neurons in the basal forebrain project to the thalamus and neocortex and preferentially promote cortical gamma-band (30-80 Hz) activity and wakefulness. Unlike sleep-active GABAergic neurons, these ascending GABAergic neurons are fast-firing neurons which disinhibit and synchronize the activity of their forebrain targets, promoting the fast EEG rhythms typical of conscious states. They are prominent targets of GABAergic hypnotic agents. Understanding the properties of ascending GABAergic neurons may lead to novel treatments for diseases involving disorders of cortical activation and wakefulness.

Selective Modulation of Histaminergic Inputs on Projection Neurons of Cerebellum Rapidly Promotes Motor Coordination via HCN Channels

Insights into function of central histaminergic system, a general modulator originating from the hypothalamus for whole brain activity, in motor control are critical for understanding the mechanism underlying somatic-nonsomatic integration. Here, we show a novel selective role of histamine in the cerebellar nuclei, the final integrative center and output of the cerebellum. Histamine depolarizes projection neurons but not interneurons in the cerebellar nuclei via the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels coupled to histamine H2 receptors, which are exclusively expressed on glutamatergic and glycinergic projection neurons. Furthermore, blockage of HCN channels to block endogenous histaminergic afferent inputs in the cerebellar nuclei significantly attenuates motor balance and coordination. Therefore, through directly and quickly modulation on projection neurons but not interneurons in the cerebellar nuclei, central histaminergic system may act as a critical biasing force to not only promptly regulate ongoing movement but also realize a rapid integration of somatic and nonsomatic response.

Pharmacology of Ultrasonic Vocalizations in adult Rats: Significance, Call Classification and Neural Substrate

Pharmacological studies of emotional arousal and initiation of emotional states in rats measured by their ultrasonic vocalizations are reviewed. It is postulated that emission of vocalizations is an inseparable feature of emotional states and it evolved from mother-infant interaction. Positive emotional states are associated with emission of 50 kHz vocalizations that could be induced by rewarding situations and dopaminergic activation of the nucleus accumbens and are mediated by D1, D2, and partially D3 dopamine receptors. Three biologically significant subtypes of 50 kHz vocalizations have been identified, all expressing positive emotional states: (1) flat calls without frequency modulation that serve as contact calls during social interactions; (2) frequencymodulated calls without trills that signal rewarding and significantly motivated situation; and (3) frequency-modulated calls with trills or trills themselves that are emitted in highly emotional situations associated with intensive affective state. Negative emotional states are associated with emission of 22 kHz vocalizations that could be induced by aversive situations, muscarinic cholinergic activation of limbic areas of medial diencephalon and forebrain, and are mediated by M2 muscarinic receptors. Two biologically significant subtypes of 22 kHz vocalizations have been identified, both expressing negative emotional sates: (1) long calls that serve as alarm calls and signal external danger; and (2) short calls that express a state of discomfort without external danger. The positive and negative states with emission of vocalizations are initiated by two ascending reticular activating subsystems: the mesolimbic dopaminergic subsystem as a specific positive arousal system, and the mesolimbic cholinergic subsystem as a specific negative arousal system.

Histamine is required for H₃ receptor-mediated alcohol reward inhibition, but not for alcohol consumption or stimulation

BACKGROUND AND PURPOSE

Conflicting data have been published on whether histamine is inhibitory to the rewarding effects of abused drugs. The purpose of this study was to clarify the role of neuronal histamine and, in particular, H₃ receptors in alcohol dependence-related behaviours, which represent the addictive effects of alcohol.

EXPERIMENTAL APPROACH

Alcohol-induced conditioned place preference (alcohol-CPP) was used to measure alcohol reward. Alcohol-induced locomotor stimulation, alcohol consumption and kinetics were also assessed. mRNA levels were quantified using radioactive in situ hybridization.

KEY RESULTS

Low doses of H₃ receptor antagonists, JNJ-10181457 and JNJ-39220675, inhibited alcohol reward in wild-type (WT) mice. However, these H₃ receptor antagonists did not inhibit alcohol reward in histidine decarboxylase knock-out (HDC KO) mice and a lack of histamine did not alter alcohol consumption. Thus H₃ receptor antagonists inhibited alcohol reward in a histamine-dependent manner. Furthermore, WT and HDC KO mice were similarly stimulated by alcohol. The expression levels of dopamine D₁ and D₂ receptors, STEP61 and DARPP-32 mRNA in striatal subregions were unaltered in HDC KO mice. No differences were seen in alcohol kinetics in HDC KO compared to WT control animals. In addition, JNJ-39220675 had no effect on alcohol kinetics in WT mice.

CONCLUSIONS AND IMPLICATIONS

These data suggest that histamine is required for the H₃ receptor-mediated inhibition of alcohol-CPP and support the hypothesis that the brain histaminergic system has an inhibitory role in alcohol reward. Increasing neuronal histamine release via H₃ receptor blockade could therefore be a novel way of treating alcohol dependence.

Procognitive properties of drugs with single and multitargeting H3 receptor antagonist activities

The histamine H3 receptor (H3 R) is an important modulator of numerous central control mechanisms. Novel lead optimizations for H3 R antagonists/inverse agonists involved studies of structure-activity relationships, cross-affinities, and pharmacokinetic properties of promising ligands. Blockade of inhibitory histamine H3 autoreceptors reinforces histaminergic transmission, while antagonism of H3 heteroreceptors accelerates the corticolimbic liberation of acetylcholine, norepinephrine, glutamate, dopamine, serotonin and gamma-aminobutyric acid (GABA). The H3 R positioned at numerous neurotransmission crossroads indicates therapeutic applications of small-molecule H3 R modulators in a number of psychiatric and neurodegenerative diseases with various clinical candidates available. Dual target drugs displaying H3 R antagonism/inverse agonism with inhibition of acetylcholine esterase (AChE), histamine N-methyltransferase (HMT), or serotonin transporter (SERT) are novel class of procognitive agents. Main chemical diversities, pharmacophores, and pharmacological profiles of procognitive agents acting as H3 R antagonists/inverse agonists and dual H3 R antagonists/inverse agonists with inhibiting activity on AChE, HMT, or SERT are highlighted here.

Identification and characterization of ZEL-H16 as a novel agonist of the histamine H3 receptor

The histamine H3 receptor (H3R) has been recognized as a promising target for the treatment of various central and peripheral nervous system diseases. In this study, a non-imidazole compound, ZEL-H16, was identified as a novel histamine H3 receptor agonist. ZEL-H16 was found to bind to human H3R with a Ki value of approximately 2.07 nM and 4.36 nM to rat H3R. Further characterization indicated that ZEL-H16 behaved as a partial agonist on the inhibition of forskolin-stimulated cAMP accumulation (the efficacy was 60% of that of histamine) and activation of ERK1/2 signaling (the efficacy was 50% of that of histamine) at H3 receptors, but acted as a full agonist just like histamin in the guinea-pig ileum contraction assay. These effects were blocked by pertussis toxin and H3 receptor specific antagonist thioperamide. ZEL-H16 showed no agonist or antagonist activities at the cloned human histamine H1, H2, and H4 receptors and other biogenic amine GPCRs in the CRE-driven reporter assay. Furthermore, our present data demonstrated that treatment of ZEL-H16 resulted in intensive H3 receptor internalization and delayed recycling to the cell surface as compared to that of control with treatment of histamine. Thus, ZEL-H16 is a novel and potent nonimidazole agonist of H3R, which might serve as a pharmacological tool for future investigations or as possible therapeutic agent of H3R.

Histamine and motivation

Brain histamine may affect a variety of different behavioral and physiological functions; however, its role in promoting wakefulness has overshadowed its other important functions. Here, we review evidence indicating that brain histamine plays a central role in motivation and emphasize its differential involvement in the appetitive and consummatory phases of motivated behaviors. We discuss the inputs that control histaminergic neurons of the tuberomamillary nucleus (TMN) of the hypothalamus, which determine the distinct role of these neurons in appetitive behavior, sleep/wake cycles, and food anticipatory responses. Moreover, we review evidence supporting the dysfunction of histaminergic neurons and the cortical input of histamine in regulating specific forms of decreased motivation (apathy). In addition, we discuss the relationship between the histamine system and drug addiction in the context of motivation.

Rescue of the genetically engineered Cul4b mutant mouse as a potential model for human X-linked mental retardation

Mutation in CUL4B, which encodes a scaffold protein of the E3 ubiquitin ligase complex, has been found in patients with X-linked mental retardation (XLMR). However, early deletion of Cul4b in mice causes prenatal lethality, which has frustrated attempts to characterize the phenotypes in vivo. In this report, we successfully rescued Cul4b mutant mice by crossing female mice in which exons 4-5 of Cul4b were flanked by loxP sequences with Sox2-Cre male mice. In Cul4b-deficient (Cul4b(Δ)/Y) mice, no CUL4B protein was detected in any of the major organs, including the brain. In the hippocampus, the levels of CUL4A, CUL4B substrates (TOP1, β-catenin, cyclin E and WDR5) and neuronal markers (MAP2, tau-1, GAP-43, PSD95 and syn-1) were not sensitive to Cul4b deletion, whereas the number of parvalbumin (PV)-positive GABAergic interneurons was decreased in Cul4b(Δ)/Y mice, especially in the dentate gyrus (DG). Some dendritic features, including the complexity, diameter and spine density in the CA1 and DG hippocampal neurons, were also affected by Cul4b deletion. Together, the decrease in the number of PV-positive neurons and altered dendritic properties in Cul4b(Δ)/Y mice imply a reduction in inhibitory regulation and dendritic integration in the hippocampal neural circuit, which lead to increased epileptic susceptibility and spatial learning deficits. Our results identify Cul4b(Δ)/Y mice as a potential model for the non-syndromic model of XLMR that replicates the CUL4B-associated MR and is valuable for the development of a therapeutic strategy for treating MR.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Effects of the histamine H₁ receptor antagonist and benztropine analog diphenylpyraline on dopamine uptake, locomotion and reward

Diphenylpyraline hydrochloride (DPP) is an internationally available antihistamine that produces therapeutic antiallergic effects by binding to histamine H₁ receptors. The complete neuropharmacological and behavioral profile of DPP, however, remains uncharacterized. Here we describe studies that suggest DPP may fit the profile of a potential agonist replacement medication for cocaine addiction. Aside from producing the desired histamine reducing effects, many antihistamines can also elicit psychomotor activation and reward, both of which are associated with increased dopamine concentrations in the nucleus accumbens (NAc). The primary aim of this study was to investigate the potential ability of DPP to inhibit the dopamine transporter, thereby leading to elevated dopamine concentrations in the NAc in a manner similar to cocaine and other psychostimulants. The psychomotor activating and rewarding effects of DPP were also investigated. For comparative purposes cocaine, a known dopamine transporter inhibitor, psychostimulant and drug of abuse, was used as a positive control. As predicted, both cocaine (15 mg/kg) and an equimolar dose of DPP (14 mg/kg) significantly inhibited dopamine uptake in the NAc in vivo and produced locomotor activation, although the time-course of pharmacological effects of the two drugs was different. In comparison to cocaine, DPP showed a prolonged effect on dopamine uptake and locomotion. Furthermore, cocaine, but not DPP, produced significant conditioned place preference, a measure of drug reward. The finding that DPP functions as a potent dopamine uptake inhibitor without producing significant rewarding effects suggests that DPP merits further study as a potential candidate as an agonist pharmacotherapy for cocaine addiction.

In vivo electrochemical evidence for simultaneous 5-HT and histamine release in the rat substantia nigra pars reticulata following medial forebrain bundle stimulation

Exploring the mechanisms of serotonin [5-hydroxytryptamine (5-HT)] in the brain requires an in vivo method that combines fast temporal resolution with chemical selectivity. Fast-scan cyclic voltammetry is a technique with sufficient temporal and chemical resolution for probing dynamic 5-HT neurotransmission events; however, traditionally it has not been possible to probe in vivo 5-HT mechanisms. Recently, we optimized fast-scan cyclic voltammetry for measuring 5-HT release and uptake in vivo in the substantia nigra pars reticulata (SNR) with electrical stimulation of the dorsal raphe nucleus (DRN) in the rat brain. Here, we address technical challenges associated with rat DRN surgery by electrically stimulating 5-HT projections in the medial forebrain bundle (MFB), a more accessible anatomical location. MFB stimulation elicits 5-HT in the SNR; furthermore, we find simultaneous release of an additional species. We use electrochemical and pharmacological methods and describe physiological, anatomical and independent chemical analyses to identify this species as histamine. We also show pharmacologically that increasing the lifetime of extracellular histamine significantly decreases 5-HT release, most likely because of increased activation of histamine H-3 receptors that inhibit 5-HT release. Despite this, under physiological conditions, we find by kinetic comparisons of DRN and MFB stimulations that the simultaneous release of histamine does not interfere with the quantitative 5-HT concentration profile. We therefore present a novel and robust electrical stimulation of the MFB that is technically less challenging than DRN stimulation to study 5-HT and histamine release in the SNR.

L-Dopa activates histaminergic neurons

L-Dopa is the most effective treatment of early and advanced stages of Parkinson's disease (PD), but its chronic use leads to loss of efficiency and dyskinesia. This is delayed by lower dosage at early stages, made possible by additional treatment with histamine antagonists. We present here evidence that histaminergic tuberomamillary nucleus (TMN) neurons, involved in the control of wakefulness, are excited under L-Dopa (EC50 15 μM), express Dopa decarboxylase and show dopamine immunoreactivity. Dopaergic excitation was investigated with patch-clamp recordings from brain slices combined with single-cell RT-PCR analysis of dopamine receptor expression. In addition to the excitatory dopamine 1 (D1)-like receptors, TMN neurons express D2-like receptors, which are coupled through phospholipase C (PLC) to transient receptor potential canonical (TRPC) channels and the Na+/Ca2+ exchanger. D2 receptor activation enhances firing frequency, histamine release in freely moving rats (microdialysis) and wakefulness (EEG recordings). In histamine deficient mice the wake-promoting action of the D2 receptor agonist quinpirole (1 mg kg⁻¹, I.P.) is missing. Thus the histamine neurons can, subsequent to L-Dopa uptake, co-release dopamine and histamine from their widely projecting axons. Taking into consideration the high density of histaminergic fibres and the histamine H3 receptor heteromerization either with D1 or with D2 receptors in the striatum, this study predicts new avenues for PD therapy.

Involvement of the brain histaminergic system in addiction and addiction-related behaviors: a comprehensive review with emphasis on the potential therapeutic use of histaminergic compounds in drug dependence

Neurons that produce histamine are exclusively located in the tuberomamillary nucleus of the posterior hypothalamus and send widespread projections to almost all brain areas. Neuronal histamine is involved in many physiological and behavioral functions such as arousal, feeding behavior and learning. Although conflicting data have been published, several studies have also demonstrated a role of histamine in the psychomotor and rewarding effects of addictive drugs. Pharmacological and brain lesion experiments initially led to the proposition that the histaminergic system exerts an inhibitory influence on drug reward processes, opposed to that of the dopaminergic system. The purpose of this review is to summarize the relevant literature on this topic and to discuss whether the inhibitory function of histamine on drug reward is supported by current evidence from published results. Research conducted during the past decade demonstrated that the ability of many antihistaminic drugs to potentiate addiction-related behaviors essentially results from non-specific effects and does not constitute a valid argument in support of an inhibitory function of histamine on reward processes. The reviewed findings also indicate that histamine can either stimulate or inhibit the dopamine mesolimbic system through distinct neuronal mechanisms involving different histamine receptors. Finally, the hypothesis that the histaminergic system plays an inhibitory role on drug reward appears to be essentially supported by place conditioning studies that focused on morphine reward. The present review suggests that the development of drugs capable of activating the histaminergic system may offer promising therapeutic tools for the treatment of opioid dependence.

Super-additive interaction of the reinforcing effects of cocaine and H1-antihistamines in rhesus monkeys

Histamine H1 receptor antagonists can be sedating and have behavioral effects, including reinforcing and discriminative stimulus effects in non-humans, that predict abuse liability. Previous research has suggested that antihistamines can enhance the effects of some drugs of abuse. We have reported a synergistic interaction between cocaine and diphenhydramine (DPH) in a self-administration assay with monkeys. The present study was designed to extend those findings to other combinations of cocaine and DPH, and to the mixture of cocaine and another H1-antihistamine, pyrilamine. Rhesus monkeys were prepared with chronic i.v. catheters and allowed to self-administer cocaine, DPH or pyrilamine alone or as mixtures under a progressive-ratio schedule of reinforcement. Cocaine, DPH and pyrilamine alone maintained self-administration and cocaine was the stronger reinforcer. When cocaine was combined with DPH or pyrilamine in a 1:1, 1:2 or 2:1 ratio of the ED(50)s, the combinations were super-additive as reinforcers. Reinforcing strength of the combinations was greater than that of the antihistamines alone but not greater than cocaine. The data support the prediction that the combination of cocaine and histamine H1 receptor antagonists could have enhanced potential for abuse relative to either drug alone. The interaction may involve dopamine systems in the CNS.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Histamine depolarizes neurons in the dorsal vagal complex

We sought to determine whether histamine has effects on single neurons in the dorsal vagal complex of the brainstem since previous studies have suggested a role for histamine receptors in this region. Using whole-cell patch clamp recordings from neurons within the nucleus of the tractus solitarius (NTS) and the dorsal vagal nucleus (DVN), histamine (20 microM) depolarized a small proportion of neurons in these regions accompanied by a decrease in input resistance. Although few neurons were depolarized (21% of NTS neurons and 15% of DVN neurons), those that were affected showed robust depolarizations of 13 mV. These depolarizations were antagonized by the histamine H1 receptor antagonist triprolidine (2 microM) and were subject to a level of desensitization. Neither histamine nor the H3 receptor agonist imetit caused any change in the amplitudes of excitatory or inhibitory postsynaptic potentials elicited in NTS neurons by stimulation of the solitary tract. These data indicate that histamine has a restricted but profound effect on neurons in the dorsal vagal complex.

Blockade of brain histamine metabolism alters methamphetamine-induced expression pattern of stereotypy in mice via histamine H1 receptors

The administration of methamphetamine (METH, 10 mg/kg, i.p.) to male ICR mice induced stereotyped behavior consisting of nail and/or wood chip biting (86.0%), continuous sniffing (12.0%), head bobbing (1.1%), and circling (1.0%) during the observation period of 1 h. Pretreatment of the mice with metoprine (2, 10, and 20 mg/kg, i.p.), a selective inhibitor of histamine N-methyltransferase (HMT), which metabolizes histamine in the brain, significantly increased and decreased METH-induced continuous sniffing (20.5, 51.3, and 80.3%) and nail and/or wood chip biting (77.4, 45.3, and 14.2%), respectively, in a dose-dependent manner. The hypothalamic contents of histamine and its metabolite N(tau)-methylhistamine were significantly increased and decreased by metoprine (10 mg/kg, i.p.), respectively. The metoprine action on METH-induced behavior was completely abolished by pyrilamine (10 and 20 mg/kg) and ketotifen (10 mg/kg), selective, centrally acting histamine H(1) receptor antagonists, but not by fexofenadine (20 mg/kg), zolantidine (10 mg/kg) and thioperamide (10 mg/kg), a peripherally acting histamine H(1) receptor antagonist and a selective, brain-penetrating antagonist for histamine H(2) and H(3) receptors, respectively. The metoprine action was mimicked by SKF 91488 (100 microg/animal, i.c.v.), another HMT inhibitor, and the action of SKF 91488 was also blocked by pyrilamine. The frequency of the expression of METH-induced total stereotypic patterns was unchanged after metoprine pretreatment. Mice pretreated with metoprine displayed no anxiety-like behavior in the elevated plus maze test. These results suggest that brain histamine, increased by agents such as metoprine and SKF 91488, binds to histamine H(1) receptors in the brain, resulting in the modulation of dopaminergic transmission associated with stereotyped behavioral patterns induced by METH.

Self-administration of cocaine-antihistamine combinations: super-additive reinforcing effects

Histamine H1 receptor antagonists have some behavioral effects that predict abuse liability. In the present study, diphenhydramine and cocaine each maintained i.v. self-administration under a progressive-ratio schedule in rhesus monkeys. When cocaine and DPH were combined in a 1:1 ratio of the ED50s, the combination was super-additive in all monkeys. The data predict that the combination of cocaine and histamine H1 receptor antagonists would have enhanced potential for abuse relative to either drug alone.

Opposite Functions of Histamine H1 and H2 Receptors and H3 Receptor in Substantia Nigra Pars Reticulata

The substantia nigra pars reticulata (SNr) is a key basal ganglia output nucleus. Inhibitory outputs from SNr are encoded in spike frequency and pattern of the inhibitory SNr projection neurons. SNr output intensity and pattern are often abnormal in movement disorders of basal ganglia origin. In Parkinson’s disease, histamine innervation and histamine H3 receptor expression in SNr may be increased. However, the functional consequences of these alterations are not known. In this study, whole cell patch-clamp recordings were used to elucidate the function of different histamine receptors in SNr. Histamine increased SNr inhibitory projection neuron firing frequency and thus inhibitory output. This effect was mediated by activation of histamine H1 and H2 receptors that induced inward currents and depolarization. In contrast, histamine H3 receptor activation hyperpolarized and inhibited SNr inhibitory projection neurons, thus decreasing the intensity of basal ganglia output. By the hyperpolarization, H3 receptor activation also increased the irregularity of the interspike intervals or changed the pattern of SNr inhibitory neuron firing. H3 receptor–mediated effects were normally dominated by those mediated by H1 and H2 receptors. Furthermore, endogenously released histamine provided a tonic, H1 and H2 receptor–mediated excitation that helped keep SNr inhibitory projection neurons sufficiently depolarized and spiking regularly. These results suggest that H1 and H2 receptors and H3 receptor exert opposite effects on SNr inhibitory projection neurons. Functional balance of these different histamine receptors may contribute to the proper intensity and pattern of basal ganglia output and, as a consequence, exert important effects on motor control.

Stimulating effects of the antihistamine fexofenadine: testing the dopamine transporter hypothesis

RATIONALE

First- and second-generation antihistamines are known to produce different degrees of sedation. However, a few studies have shown that the H1-antagonist fexofenadine produces mild stimulating effects. One hypothesis suggests that this is due to fexofenadine producing an increase in dopamine levels by blocking the dopamine transporter.

OBJECTIVE

In this study, it was investigated whether a high dose of fexofenadine blocks the dopamine transporter in the striatum. In addition, the effect of fexofenadine on cognitive performance and motor impulsivity was investigated.

METHODS

Sixteen healthy subjects were given either placebo or fexofenadine 360 mg. The binding potential of N-w-fluoropropyl-2beta-carbomethoxy-3beta-[4-iodophenyl] nortropane ([123I]FP-CIT) was measured using single-photon emission computed tomography (SPECT). Cognitive performance was measured in 40 subjects (20 placebo, 20 fexofenadine) using a digit symbol substitution test (DSST) and a stop signal task. In addition, subjective and physiological effects of fexofenadine were observed.

RESULTS

The SPECT data demonstrated that there was no difference in the binding potential of FP-CIT at the dopamine transporter in the striatum between the placebo- and fexofenadine-treated subjects. The behavioral results showed that fexofenadine improved performance on the DSST at T (max) of the drug. Fexofenadine did not affect motor impulsivity, subjective experience, or physiological measures.

CONCLUSION

No evidence was provided to support the hypothesis that fexofenadine stimulates performance by blocking the dopamine transporter. The behavioral data suggest that a high dose of fexofenadine can stimulate performance in cognitive tasks.

Dorsal raphe vs. median raphe serotonergic antagonism. Anatomical, physiological, behavioral, neuroendocrinological, neuropharmacological and clinical evidences: relevance for neuropharmacological therapy

Monoaminergic neurons located in the central nervous system (CNS) are organized into complex circuits which include noradrenergic (NA), adrenergic (Ad), dopaminergic (DA), serotonergic (5-HT), histaminergic (H), GABA-ergic and glutamatergic systems. Most of these circuits are composed of more than one and often several types of the above neurons. Such physiologically flexible circuits respond appropriately to both external and internal stimuli which, if not modulated adequately, can trigger pathophysiologic responses. A great deal of research has been devoted to mapping the multiple functions of the CNS circuitry, thereby forming the basis for effective neuropharmacological therapeutic approaches. Such lineal strategies that seek to normalize complex and mixed physiological disorders, however, meet only partial therapeutic success and are often followed by undesirable side effects and/or total failure. In light of these, we have worked to develop possible models of CNS circuitry that are less affected by physiological interaction using the models to design more effective therapeutic approaches. In the present review, we cite and present evidence supporting the dorsal raphe versus median raphe serotonergic circuitry as one model of a reliable paradigm, necessary to the clear understanding and therapy of many psychiatric and even non-psychiatric disturbances.

Histaminergic neurons protect the developing hippocampus from kainic acid-induced neuronal damage in an organotypic coculture system

The central histaminergic neuron system inhibits epileptic seizures, which is suggested to occur mainly through histamine 1 (H1) and histamine 3 (H3) receptors. However, the importance of histaminergic neurons in seizure-induced cell damage is poorly known. In this study, we used an organotypic coculture system and confocal microscopy to examine whether histaminergic neurons, which were verified by immunohistochemistry, have any protective effect on kainic acid (KA)-induced neuronal damage in the developing hippocampus. Fluoro-Jade B, a specific marker for degenerating neurons, indicated that, after the 12 h KA (5 microM) treatment, neuronal damage was significantly attenuated in the hippocampus cultured together with the posterior hypothalamic slice containing histaminergic neurons [HI plus HY (POST)] when compared with the hippocampus cultured alone (HI) or with the anterior hypothalamus devoid of histaminergic neurons. Moreover, alpha-fluoromethylhistidine, an inhibitor of histamine synthesis, eliminated the neuroprotective effect in KA-treated HI plus HY (POST), and extracellularly applied histamine (1 nM to 100 microM) significantly attenuated neuronal damage only at 1 nM concentration in HI. After the 6 h KA treatment, spontaneous electrical activity registered in the CA1 subregion contained significantly less burst activity in HI plus HY (POST) than in HI. Finally, in KA-treated slices, the H3 receptor antagonist thioperamide enhanced the neuroprotective effect of histaminergic neurons, whereas the H1 receptor antagonists triprolidine and mepyramine dose-dependently decreased the neuroprotection in HI plus HY (POST). Our results suggest that histaminergic neurons protect the developing hippocampus from KA-induced neuronal damage, with regulation of neuronal survival being at least partly mediated through H1 and H3 receptors.

Excitatory effect of histamine on neuronal activity of rat globus pallidus by activation of H2 receptors in vitro

Previous studies have revealed distribution of histaminergic fibers and presence of histamine receptors in globus pallidus (GP). In this study, the brain slice preparation of adult rats was used to examine the effect of histamine on the spontaneous unitary discharge of GP neurons and the underlying receptor mechanism. Ninety-five GP neurons were extracellularly recorded from 42 slices containing the GP, of which 87 (91.6%) were excited by the stimulation of histamine. The histamine-induced excitation was concentration-dependent and persisted in low Ca2+/high Mg2+ medium (n = 9), demonstrating that the action of histamine on the GP neurons was postsynaptic. The excitatory effect of histamine on the GP neurons was not blocked by selective histamine H1 receptor antagonist triprolidine (n = 16) or chlorpheniramine (n = 6), but was effectively suppressed by ranitidine, a highly selective histamine H2 receptor antagonist (n = 21). On the other hand, highly selective histamine H2 receptor agonist dimaprit mimicked the excitatory effect of histamine on the GP neurons (n = 23), while histamine H1 receptor agonists, including 2-pyridylethylamine (n = 22), 2-thiazolyethylamine (n = 9) and betahistine (n = 9), did not cause GP neurons any response. The dimaprit-induced GP neuronal excitation was effectively antagonized by selective histamine H2 receptor antagonist ranitidine (n = 14) but not influenced by selective histamine H1 receptor antagonist triprolidine (n = 12). Moreover, adenylate cyclase (AC) activator forskolin (n = 7) was observed to evoke GP neurons an excitatory response, whereas the histamine-induced excitation was effectively reduced by H-89 (n = 9), a selective and potent inhibitor of protein kinase A (PK(A)). Finally, it was noted that neurons of both subdivisions of the GP, the internal (GPi, n = 35) and external (GPe, n = 60) segment, showed no differences in their responses to stimulations of the tested histaminergic reagents. These results demonstrated that histamine excited GP (including GPi and GPe) neurons via histamine H2 receptors and H2 receptors linked intracellular G-protein-AC-PK(A) signaling pathway, suggesting that the hypothalamic histaminergic afferent fibers innervating GP may play an important modulatory role in motor control through its excitatory effect on GP neurons.

Neuro-modulation, aminergic neuro-disinhibition and neuro-degeneration. Draft of a comprehensive theory for Alzheimer disease

A comprehensive theory for Alzheimer disease (AD) which can provide a clue to the neuronal selective vulnerability (pathoklisis) is still missing. Based upon evidence from the current literature, the present work is aimed at proposing such a theory, namely the 'aminergic disinhibition theory' of AD. It includes data-based hypotheses as to the pathoklisis, mechanisms of neuro-degeneration and dementia as well as the aetiology of the disease. Alzheimer disease is regarded as a disorder of neural input modulation caused by the degeneration of four modulatory amine transmitter (MAT) systems, namely the serotoninergic, the noradrenergic, the histaminergic, and the cholinergic systems with ascending projections. MATs modulate cognitive processing including arousal, attention, and synaptic plasticity in learning and memory, not only through direct, mostly inhibitory impact on principal neurones but also partially through interaction with local networks of GABA-ergic inter-neurones. The distribution and magnitude of the pathology in AD roughly correlate with the distribution and magnitude of MAT modulation: Regions more densely innervated by ascending MAT projections are, as a rule, more severely affected than areas receiving less MAT innervation. Because the global effect of MATs in the forebrain is inhibition, the degeneration of four MAT systems, some related peptidergic systems and a secondary alleviation of the GABA-ergic transmission means a fundamental loss of inhibitory impact in the neuronal circuitry resulting in neuronal (aminergic) disinhibition. Clearly, the basic mechanism promoting neuronal death in AD is thought to be a chronic disturbance of the inhibition-excitation balance to the advantage of excitation. Chronic over-excitation is conceived to result in Ca2+ dependent cellular excito-toxicity leading to neuro-degeneration including amyloid-beta production and NFT formation. Disinhibited neurons will degenerate while less excited (relatively over-inhibited) neurones will survive. Because the decline of aminergic transmission in AD is likely to start at the receptor level, it is hypothesized that early impairment by a molecular 'hit' to an MAT receptor (or a group of receptors) initiates a pathogenetic cascade that develops in an avalanche-like manner. Based on experimental evidence from the literature, the 'hit' might be the attachment of a targeted pathogen like a small roaming amino acid sequence to the receptor(s), e.g., the serotoninergic 5-HT2A-R. Referential sequence analysis could be a means to identify such a small pathogen hidden in a large receptor molecule.

Histamine excites noradrenergic neurons in locus coeruleus in rats

Histamine is implicated in the control of many brain functions, in particular the control of arousal. Histaminergic neurons send dense projections through the entire brain, including the locus coeruleus (LC)--the main noradrenergic (NAergic) nucleus. In this study, we have examined the effect of bath-applied histamine on cells in the LC by single-unit recordings in slices and the expression of histamine receptors in this area by single-cell RT-PCR. Histamine (10 microM) increased the firing of NAergic cells to 130+/-9% of control, 100 microM to 256+/-58% of control. This excitation was unaffected by blocking synaptic transmission. Histamine-mediated excitation was blocked by an H1 receptor antagonist, mepyramine, in 78% of cells and by cimetidine, an H2 receptor antagonist, in 42% of cells, but not by the H3 receptor antagonist, thioperamide. RT-PCR revealed that mRNA for the H1 receptor was expressed in 77% of isolated LC neurons, mRNA for the H2 receptor in 41% of LC neurons and H3 receptors in 29%. These findings underline the coordination between aminergic systems and suggest that the arousal induced by the histamine system could involve excitation of noradrenergic neurons in the locus coeruleus.

On the paradox of ion channel blockade and its benefits in the treatment of Alzheimer disease

The surprisingly beneficial effects in Alzheimer disease (AD) of ion channel blockers (ICB) like memantine that act on NMDA- and other aminergic transmitter receptors are yet poorly understood. NMDA receptor levels and binding were shown to be significantly decreased in AD, in which highly NMDA receptor and Ca(2+) dependent synaptic plasticity and re-modelling are severely compromised. Thus, how could one expect to improve AD by further suppressing NMDA channels with antagonists. Nevertheless, clinical trials with NMDA blockers revealed in moderate to advanced AD surprisingly positive effects. The present paper tries to provides a hypothetical explanation of that paradoxical success of ICBs. Based on evidence from current data, emphasis is put on a profound impairment in the AD brain of the inhibition-excitation balance in the neuronal circuitry to the advantage of excitation. This imbalance is conceived to result from a degeneration of four modulatory aminiergic transmitter systems (serotonin, noradrenalin, acetylcholine, histamine) and related peptidergic systems, the decline of which causes a profound loss of inhibitory impact in the forebrain neuronal circuitry leading to disinhibition of principal neurones ("aminergic disinhibition"). Subsequent Ca(2+) excito-toxicity and its sequelae are suggested to be the basic promotors of the neuro-degeneration and the related mental decline in AD. Re-adjustment of the inhibition-excitation imbalance by decreasing excitation is conceived to be the mechanism that renders ion channel blockade therapeutically successful. Putatively, attempts to increase inhibition, e.g., by application of GABA mimetics that stimulate the production GABA from preserved but "lazy" GABA neurones lacking aminergic facilitation, might be an even better way to achieve the re-balance.

Histaminergic modulation of excitatory synaptic transmission in the rat basolateral amygdala

The effects of histamine on excitatory synaptic transmission between the external capsule and basolateral amygdala (BLA) were examined using intracellular and field potential recordings in rat amygdala slices. Bath application of histamine (20 microM) suppressed intracellular excitatory postsynaptic potentials (EPSPs; 70.3+/-5.1% of control amplitude) in 43 of 64 BLA neurons, and potentiated EPSPs (341+/-81% of control amplitude) in 21 neurons, without changing resting membrane potential or input resistance. The histamine-induced suppression of EPSPs was accompanied by an increase in paired-pulse facilitation of the slopes of EPSPs, suggesting a presynaptic locus of the action. The suppressive effect could be blocked by the selective H3 antagonist thioperamide, and mimicked by the selective H3 agonist R-alpha-methylhistamine, indicating that the suppressive effect is mediated by the presynaptic H3 receptor. The potentiating effect of histamine on EPSPs was not accompanied by the change of paired-pulse facilitation and was not affected by the presence of H1, H2 or H3 receptor antagonists. In addition, the effective concentration of agonist to produce 50% of maximal response (EC50) of the potentiating action of histamine is 49 nM, much lower than the EC50 (470 nM) of the H3 receptor-mediated suppressive effect characterized here. These observations suggest a novel, high affinity and postsynaptically mediated effect of histamine. In extracellular recordings, histamine, at low concentration (200 nM), consistently potentiated field potentials. At high concentration (20 microM), histamine suppressed field potentials, but potentiated field potentials when H3 receptors were blocked. Taken together, these results revealed that histamine, via the presynaptic H3 receptor and a currently unknown mechanism, decreases or increases excitatory synaptic transmission in the BLA respectively. This specific histaminergic modulation of neuronal activity in the amygdala may play an important role in amygdala-mediated physiological and pathophysiological processes, such as fear, emotional learning and memory, temporal lobe epilepsy, and affective disorders.

The biochemical basis of the neurobehavioral abnormalities in the Lesch–Nyhan syndrome: a hypothesis

Lesch-Nyhan syndrome (LNS) is a rare X-recessive disorder that leads to virtually complete deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). Partial HPRT deficiency results in uric acid overproduction with subsequent hyperuricemia, nephrolithiasis, renal failure and gouty arthritis. In contrast, at complete HPRT deficiency, besides overproduction of uric acid neurological problems appear including spasticity, choreoathetosis, mental retardation, and compulsive self-mutilation. The cause for the uric acid overproduction has been clarified, but the connection between the enzyme deficiency and the neurological manifestations in LNS remains unclear. A hypothesis, which explains this relation, is proposed in the paper. The hypothesis has several important points most substantial of which is the accelerated biosynthesis of semiessential amino acid histidine that against the background of accelerated purine de novo biosynthesis results in 5-aminoimidazole-4-carboxamideribotide (AICAR) and histamine accumulation. The histamine and AICAR were determined to be the compounds that cause the neurobehavioral symptoms of LNS for several reasons. First, in the basal ganglia a balance between the direct (activating) and the indirect (inhibiting) pathways arising on the basis of the antagonistic and reciprocal dopamine-adenosine interactions normally exists. This balance can tonically regulate smooth voluntary movements and the activity of the thalamus, which, in turn, processes the afferent sensorimotor signals from the whole body to the all areas of the cerebral cortex and is concerned to modulate mental development and bring sensory information into awareness. Second, histamine is known to induce a selective damage in dopaminergic neurons inhibiting the direct dopaminergic pathway, which could lead to muscular rigidity, and slowness in initiating movements as well as tremor that are characteristic of Parkinsonism in LNS. Third, AICAribosid (AICAR breakdown product) is a potent adenosine A2a receptor antagonist inhibiting the indirect dopamine-adenosinergic pathway and, therefore, could be responsible for the choreoathetosis, dystonia and ballismus found in LNS. The excitatory-inhibitory disbalance in the basal ganglia could result in inadequate modification of the thalamus activity with subsequent mental retardation and symptoms that include the patients not being aware for their own bodies that could give rise to self-mutilation. Finally, a possibility for the creation of a new animal model that could exactly match the human LNS is proposed in the paper.