A detailed mapping of histamine H1-receptors in guinea-pig central nervous system established by autoradiography with [125I]iodobolpyramine

Histamine in the neocortex: Towards integrating multiscale effectors

Histamine is a modulatory neurotransmitter, which has received relatively less attention in the central nervous system than other neurotransmitters. The functional role of histamine in the neocortex, the brain region that controls higher-order cognitive functions such as attention, learning and memory, remains largely unknown. This article focuses on the emerging roles and mechanisms of histamine release in the neocortex. We describe gaps in current knowledge and propose the application of interdisciplinary tools to dissect the detailed multiscale functional logic of histaminergic action in the neocortex ranging from sub-cellular, cellular, dendritic and synaptic levels to microcircuits and mesoscale effects.

Histaminergic System and Vestibular Function in Normal and Pathological Conditions

Most neurotransmitter systems are represented in the central and peripheral vestibular system and are thereby involved both in normal vestibular signal processing and the pathophysiology of vestibular disorders. However, there is a special relationship between the vestibular system and the histaminergic system. The purpose of this review is to document how the histaminergic system interferes with normal and pathological vestibular function. In particular, we will discuss neurobiological mechanisms such as neuroinflammation that involve histamine to modulate and allow restoration of balance function in the situation of a vestibular insult. These adaptive mechanisms represent targets of histaminergic pharmacological compounds capable of restoring vestibular function in pathological situations. The clinical use of drugs targeting the histaminergic system in various vestibular disorders is critically discussed.

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.

Dopamine and Dopamine-Related Ligands Can Bind Not Only to Dopamine Receptors

The dopaminergic system is one of the most important neurotransmitter systems in the central nervous system (CNS). It acts mainly by activation of the D₁-like receptor family at the target cell. Additionally, fine-tuning of the signal is achieved via pre-synaptic modulation by the D₂-like receptor family. Some dopamine drugs (both agonists and antagonists) bind in addition to DRs also to α₂-ARs and 5-HT receptors. Unfortunately, these compounds are often considered subtype(s) specific. Thus, it is important to consider the presence of these receptor subtypes in specific CNS areas as the function virtually elicited by one receptor type could be an effect of other—or the co-effect of multiple receptors. However, there are enough molecules with adequate specificity. In this review, we want to give an overview of the most common off-targets for established dopamine receptor ligands. To give an overall picture, we included a discussion on subtype selectivity. Molecules used as antipsychotic drugs are reviewed too. Therefore, we will summarize reported affinities and give an outline of molecules sufficiently specific for one or more subtypes (i.e., for subfamily), the presence of DR, α₂-ARs, and 5-HT receptors in CNS areas, which could help avoid ambiguous results.

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.

The Combination of Betahistine and Oxybutynin Increases Respiratory Control Sensitivity (Loop Gain) in People with Obstructive Sleep Apnea: A Randomized, Placebo-Controlled Trial

RATIONALE

There are widespread histaminergic projections throughout the brain, including hypoglossal nuclei, that modulate pharyngeal muscle tone and respiratory control. Hence, histaminergic stimulation pharmacologically may increase pharyngeal muscle tone and stabilize respiratory control (loop gain) to reduce obstructive sleep apnea (OSA) severity. Antimuscarinics also increase REM pharyngeal muscle tone in rats. Thus, a combination of histaminergic and anti-muscarinic drugs may be a novel target for OSA pharmacotherapy. However, this has not been investigated. Accordingly, we aimed to test the effects of betahistine (Beta), an H3-autoreceptor antagonist which thereby increases histamine levels, in combination with the antimuscarinic oxybutynin (Oxy), on OSA severity, OSA endotypes, polysomnography parameters and next-day sleepiness and alertness.

METHODS

Thirteen adults with OSA received either Beta-Oxy (96-5mg) or placebo according to a randomized, crossover, double-blind design, prior to polysomnography. Participants completed the Karolinska Sleep Scale and Leeds Sleep Evaluation Questionnaire and a driving simulation task to quantify next-day sleepiness and alertness. OSA endotypes were estimated through validated algorithms using polysomnography.

RESULTS

Compared to placebo, Beta-Oxy increased respiratory control sensitivity (loop gain) (0.52[0.24] vs 0.60[0.34], median [IQR], P = 0.021) without systematically changing OSA severity (34.4±17.2 vs 40.3±27.3 events/h, mean±SD, P = 0.124), sleep efficiency, arousal index or markers of hypoxemia. Beta-Oxy was well tolerated and did not worsen next-day sleepiness/alertness.

CONCLUSION

Rather than stabilize breathing during sleep, Beta-Oxy increases loop gain, which is likely to be deleterious for most people with OSA. However, in certain conditions characterized by blunted respiratory control (eg, obesity hypoventilation syndrome), interventions to increase loop gain may be beneficial.

The effect of amisulpride, olanzapine, quetiapine, and aripiprazole single administration on c-Fos expression in vasopressinergic and oxytocinergic neurons of the rat hypothalamic paraventricular nucleus

Antipsychotics, including amisulpride (AMI), quetiapine (QUE), aripiprazole (ARI), and olanzapine (OLA), are used to treat mental illnesses associated with psychotic symptoms. The effect of these drugs on c-Fos expression in vasopressinergic (AVP) and oxytocinergic (OXY) neurons was studied in the hypothalamic paraventricular nucleus (PVN) of rats. The presence of c-Fos in AVP and OXY perikarya was investigated in seven PVN cells segregations: the anterior (Ant), dorsal cup (Dc), wing-shaped (Wi), periventricular zone (Pe), circle-shaped core (Co) and shell of core (Sh), and the posterior (pPVN) after an acute treatment with AMI-20 mg/kg, QUE-15 mg/kg, ARI-10 mg/kg, and OLA-5 mg/kg/bw in rats. Ninety min after treatments, the animals were sacrificed by transcardial perfusion with fixative and the PVN area sliced into 35 μm thick coronal sections for immunohistochemistry. The c-Fos was processed by avidin-biotin-peroxidase complex intensified with nickel-enhanced 3,3'-diaminobenzidine tetrahydrochloride. Visualization of AVP- and OXY-synthesizing neurons was achieved by a fluorescent marker Alexa Flour 568. The c-Fos-AVP and c-Fos-OXY colocalizations were evaluated from c-Fos stained sections merged with AVP or OXY ones. AMI, QUE, ARI, and OLA, single administration distinctly increased the c-Fos expression in each of the PVN cells segregations. QUE induced the highest magnitude of activation of AVP and OXY neurons, while OLA and AMI had only moderate effects. Incontestable variabilities detected in c-Fos expression in PVN AVP and OXY neurons extend the knowledge of selected antipsychotics extra-striatal actions and may also be helpful in a presumption of their possible functional impact.

Involvement of the tuberomammillary nucleus of the hypothalamus in the modulation of nociception and joint edema in a model of monoarthritis

AIMS

Investigate the involvement of the histaminergic projections from tuberomammillary nucleus (TMN) to the spinal cord in the modulation of nociception and peripheral edema in a model of monoarthritis.

MAIN METHODS

Subacute monoarthritis was induced by an intraarticular injection of carrageenan followed by LPS 72 h later. Disability and joint edema were assessed at the 3rd hour after LPS and at every hour up to 6 h.

KEY FINDINGS

Intrathecal administration of histamine potentiated joint incapacitation and edema, while the H1R antagonist cetirizine decreased both. The H3R agonist immepip decreased both incapacitation and edema, while the H3R antagonist thioperamide had the opposite effect. The microinjection of glutamate into the ventral TMN (vTMN) caused an increase of incapacitation and articular edema, whereas the blockade of this nucleus by cobalt chloride inhibited both parameters. Intrathecal administration of cetirizine prevented the increase of incapacitation and joint edema caused by glutamate microinjection into the vTMN. Similarly, an intrathecal injection of the NKCC1 cotransporter inhibitor bumetanide prevented the effects of glutamate microinjection into the vTMN, whereas coadministration of histamine with bumetanide only inhibited the potentiation of joint edema. A microinjection of orexin B into the vTMN potentiated incapacitation and joint edema, while coadministration of the OX1/2 receptor antagonist almorexant with orexin B did not.

SIGNIFICANCE

These data support the notion that TMN participates in the modulation of a peripheral inflammatory process by means of histaminergic projections to the spinal cord, and the hypothalamus may trigger TMN activation by means of glutamate and orexin.

Histamine N-methyltransferase regulates aggression and the sleep-wake cycle

Histamine is a neurotransmitter that regulates diverse physiological functions including the sleep-wake cycle. Recent studies have reported that histaminergic dysfunction in the brain is associated with neuropsychiatric disorders. Histamine N-methyltransferase (HNMT) is an enzyme expressed in the central nervous system that specifically metabolises histamine; yet, the exact physiological roles of HNMT are unknown. Accordingly, we phenotyped Hnmt knockout mice (KO) to determine the relevance of HNMT to various brain functions. First, we showed that HNMT deficiency enhanced brain histamine concentrations, confirming a role for HNMT in histamine inactivation. Next, we performed comprehensive behavioural testing and determined that KO mice exhibited high aggressive behaviours in the resident-intruder and aggressive biting behaviour tests. High aggression in KO mice was suppressed by treatment with zolantidine, a histamine H2 receptor (H2R) antagonist, indicating that abnormal H2R activation promoted aggression in KO mice. A sleep analysis revealed that KO mice exhibited prolonged bouts of awakening during the light (inactive) period and compensatory sleep during the dark (active) period. Abnormal sleep behaviour was suppressed by treatment with pyrilamine, a H1R antagonist, prior to light period, suggesting that excessive H1R activation led to the dysregulation of sleep-wake cycles in KO mice. These observations inform the physiological roles of HNMT.

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.

The effect of histamine on changes in mental energy and fatigue after a single bout of exercise

The purpose of this research was to determine if histamine, acting on brain H1 receptors, influences changes in feelings of energy and fatigue or cognitive test performance after acute exercise. Women (n=20) with low vigor and high fatigue were administered the H1 antagonist drug doxepin hydrocholoride (6 mg) in tomato juice and tomato juice alone (placebo) in a randomized, double-blinded, cross-over experiment before performing 30 min of light intensity cycling exercise and completing energy, fatigue, sleepiness, and motivation scales, and cognitive tasks. After exercise, mental fatigue increased for the doxepin condition (p=0.014) but not placebo (p=0.700), while mental energy decreased for both PLA and DOX (p<0.001) and cognitive task performance was unaffected. It is inferred that histamine binding to H1 receptors in the brain has a role in exercise-induced reductions in mental fatigue, but not increases in energy.

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.

Involvement of histaminergic inputs in the jaw-closing reflex arc

Histamine receptors are densely expressed in the mesencephalic trigeminal nucleus (MesV) and trigeminal motor nucleus. However, little is known about the functional roles of neuronal histamine in controlling oral-motor activity. Thus, using the whole-cell recording technique in brainstem slice preparations from Wistar rats aged between postnatal days 7 and 13, we investigated the effects of histamine on the MesV neurons innervating the masseter muscle spindles and masseter motoneurons (MMNs) that form a reflex arc for the jaw-closing reflex. Bath application of histamine (100 μM) induced membrane depolarization in both MesV neurons and MMNs in the presence of tetrodotoxin, whereas histamine decreased and increased the input resistance in MesV neurons and MMNs, respectively. The effects of histamine on MesV neurons and MMNs were mimicked by an H1 receptor agonist, 2-pyridylethylamine (100 μM). The effects of an H2 receptor agonist, dimaprit (100 μM), on MesV neurons were inconsistent, whereas MMNs were depolarized without changes in the input resistance. An H3 receptor agonist, immethridine (100 μM), also depolarized both MesV neurons and MMNs without changing the input resistance. Histamine reduced the peak amplitude of postsynaptic currents (PSCs) in MMNs evoked by stimulation of the trigeminal motor nerve (5N), which was mimicked by 2-pyridylethylamine but not by dimaprit or immethridine. Moreover, 2-pyridylethylamine increased the failure rate of PSCs evoked by minimal stimulation and the paired-pulse ratio. These results suggest that histaminergic inputs to MesV neurons through H1 receptors are involved in the suppression of the jaw-closing reflex although histamine depolarizes MesV neurons and/or MMNs.

Influence of the hippocampus on amino acid utilizing and cholinergic neurons within the nucleus accumbens is promoted by histamine via H₁ receptors

BACKGROUND AND PURPOSE

The influence of the neurotransmitter histamine on spontaneous and stimulation-evoked release of glutamate, aspartate, GABA and ACh in the nucleus accumbens (NAc) was investigated in vivo.

EXPERIMENTAL APPROACH

Using the push-pull superfusion technique, histaminergic compounds were applied to the NAc and neurotransmitter release was assessed. In some experiments, the fornix/fimbria of the hippocampus was electrically stimulated by a microelectrode and evoked potentials were monitored in the NAc.

KEY RESULTS

Superfusion of the NAc with the H1 receptor antagonist triprolidine (50 μM) decreased spontaneous outflow of glutamate, aspartate and ACh, while release of GABA remained unaffected. Superfusion with histamine elevated release of ACh, without influencing that of the amino acids. Electrical stimulation of the fornix/fimbria enhanced the output of amino acids and ACh within the NAc. The evoked outflow of glutamate and ACh was diminished on superfusion with triprolidine, while release of aspartate and GABA was not affected. Superfusion of the NAc with histamine intensified the stimulation-evoked release of glutamate and Ach. Histamine also elevated the stimulation-induced release of aspartate, without influencing that of GABA. Presuperfusion with triprolidine abolished the reinforced effect of histamine on stimulation-evoked transmitter release within the NAc.

CONCLUSION AND IMPLICATIONS

Neuronal histamine activates H1 receptors and increases spontaneous release of glutamate, aspartate and ACh within the NAc. Stimulation of the hippocampal fornix/fimbria tract also enhances release of glutamate and ACh within the NAc and this effect is intensified by H1 receptor stimulation within the NAc. The latter effects, which are mediated by hippocampal afferences, might play an important role in mnemonic performance and in emotional processes such as anxiety and stress disorders.

Postsynaptic mechanisms underlying the excitatory action of histamine on medial vestibular nucleus neurons in rats

BACKGROUND AND PURPOSE

Anti-histaminergic drugs have been widely used in the clinical treatment of vestibular disorders and most studies concentrate on their presynaptic actions. The present study investigated the postsynaptic effect of histamine on medial vestibular nucleus (MVN) neurons and the underlying mechanisms.

EXPERIMENTAL APPROACH

Histamine-induced postsynaptic actions on MVN neurons and the corresponding receptor and ionic mechanisms were detected by whole-cell patch-clamp recordings on rat brain slices. The distribution of postsynaptic histamine H₁, H₂ and H₄ receptors was mapped by double and single immunostaining. Furthermore, the expression of mRNAs for H₁, H₂ and H₄ receptors and for subtypes of Na⁺ -Ca²⁺ exchangers (NCXs) and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels was assessed by quantitative real-time RT-PCR.

KEY RESULTS

A marked postsynaptic excitatory effect, co-mediated by histamine H₁ and H₂ receptors, was involved in the histamine-induced depolarization of MVN neurons. Postsynaptic H₁ and H₂ rather than H₄ receptors were co-localized in the same MVN neurons. NCXs contributed to the inward current mediated by H₁ receptors, whereas HCN channels were responsible for excitation induced by activation of H₂ receptors. Moreover, NCX1 and NCX3 rather than NCX2, and HCN1 rather than HCN2-4 mRNAs, were abundantly expressed in MVN.

CONCLUSION AND IMPLICATIONS

NCXs coupled to H₁ receptors and HCN channels linked to H₂ receptors co-mediate the strong postsynaptic excitatory action of histamine on MVN neurons. These results highlight an active role of postsynaptic mechanisms in the modulation by central histaminergic systems of vestibular functions and suggest potential targets for clinical treatment of vestibular disorders.

Plasticity of the histamine H3 receptors after acute vestibular lesion in the adult cat

After unilateral vestibular neurectomy (UVN) many molecular and neurochemical mechanisms underlie the neurophysiological reorganizations occurring in the vestibular nuclei (VN) complex, as well as the behavioral recovery process. As a key regulator, the histaminergic system appears to be a likely candidate because drugs interfering with histamine (HA) neurotransmission facilitate behavioral recovery after vestibular lesion. This study aimed at analyzing the post-lesion changes of the histaminergic system by quantifying binding to histamine H₃ receptors (H₃R; mediating namely histamine autoinhibition) using a histamine H₃ receptor agonist ([³H]N-α-methylhistamine). Experiments were done in brain sections of control cats (N = 6) and cats submitted to UVN and killed 1 (N = 6) or 3 (N = 6) weeks after the lesion. UVN induced a bilateral decrease in binding density of the agonist [³H]N-α-methylhistamine to H₃R in the tuberomammillary nuclei (TMN) at 1 week post-lesion, with a predominant down-regulation in the ipsilateral TMN. The bilateral decrease remained at the 3 weeks survival time and became symmetric. Concerning brainstem structures, binding density in the VN, the prepositus hypoglossi, the subdivisions of the inferior olive decreased unilaterally on the ipsilateral side at 1 week and bilaterally 3 weeks after UVN. Similar changes were observed in the subdivisions of the solitary nucleus only 1 week after the lesion. These findings indicate vestibular lesion induces plasticity of the histamine H₃R, which could contribute to vestibular function recovery.

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.

Induction of prolonged, continuous slow-wave sleep by blocking cerebral H₁ histamine receptors in rats

BACKGROUND AND PURPOSE

Classic H(1) histamine receptor (H(1)R) antagonists are non-selective for H(1)R and known to produce drowsiness. Modern antihistamines are more selective for H(1)R, and are 'non-drowsy' presumably due to reduced permeability through the blood-brain barrier. To characterize both histaminergic sleep regulation and the central actions of antihistamines, in the present study we analysed the effect of classic and modern antihistamines on rats' sleep using continuous i.c.v. infusions.

EXPERIMENTAL APPROACH

Effects of classic (d-chlorpheniramine; d-CPA) and second-generation (cetirizine) antihistamines on sleep were compared after i.p. injections or continuous i.c.v. infusions into rats. Fluorescent cetirizine/DBD-pz was synthesized to trace the approximate distribution of cerebral cetirizine. Furthermore, the effects of H(1) R antagonists on cultured preoptic neurons were examined using calcium imaging.

KEY RESULTS

d-CPA 4 mg·kg(-1) i.p. increased non-rapid eye movement (REM) sleep whereas 10-40 mg·kg(-1) d-CPA decreased non-REM sleep at dark onset time. Nocturnal i.c.v. infusions of d-CPA (10 µmol·100 µL(-1)·10 h(-1)) increased drowsiness but not non-REM sleep, whereas the same i.c.v. infusions of cetirizine significantly increased non-REM sleep, abolished REM sleep, and decreased wakefulness for more than 10 h. The medial preoptic area contained the greatest fluorescent labelling after i.c.v. cetirizine/DBD-pz infusions. Histamine-induced Ca(2+) increases in medial preoptic neurons were blocked by d-CPA or cetirizine, whereas d-CPA, but not cetirizine, increased Ca(2+) irrespective of antihistaminergic activity at ≥ 100 µM.

CONCLUSION AND IMPLICATIONS

The excitatory action of d-CPA may explain the seemingly inconsistent actions of d-CPA on sleep. Cerebral H(1)R inhibition by cetirizine induces synchronization of cerebral activity and prolonged, continuous slow-wave sleep.

Pharmaceutical countermeasures have opposite effects on the utricles and semicircular canals in man

INTRODUCTION

Sensory conflicts in the vestibular system lead to motion sickness of which space motion sickness (SMS) is a special case. SMS affects up to 70% of the astronauts during the first 3 days in space. The search for effective countermeasures has led to several nonpharmacological and pharmacological approaches. The current study focuses on the effects of lorazepam (1 mg), meclizine (25 mg), promethazine (25 mg), and scopolamine (0.4 mg) on the vestibular system, with special focus on the canal and otolith functions separately.

METHODS

The study had a placebo-controlled, single blind, repeated measures design. Sixteen healthy volunteers were subjected to a total of 7 test sessions, the first and last being without intake of medication. Semicircular canal function was evaluated by means of electronystagmography and otolith function with unilateral centrifugation. The horizontal semicircular canal function was characterized by the vestibulo-ocular reflex (VOR) gain measured during earth vertical axis rotation as well as the total caloric response. The function of the utricles was represented by the utricular sensitivity, reflecting the ocular counter roll relative to the virtual induced head tilt.

RESULTS

Promethazine significantly decreased the semicircular canal and utricular parameters. Both scopolamine and lorazepam caused only a decrease in the utricular sensitivity, whereas meclizine only decreased the semicircular canal-induced VOR gain.

DISCUSSION

The results show that the drugs affected different areas of the vestibular system and that the effects can thus be attributed to the specific pharmacological properties of each drug. Meclizine, as an antihistaminergic and weak anticholinergic drug, only affected the VOR gain, suggesting a central action on the medial vestibular nucleus. The same site of action is suggested for the anticholinergic scopolamine since acetylcholine receptors are present and utricular fibers terminate here. The global vestibular suppression caused by promethazine is probably a consequence of its anticholinergic, antihistaminergic, and antidopaminergic properties. Based on the fact that lorazepam increased the affinity of gamma-aminobutyric acid (GABA) for the GABA(A)-receptor and its effects on the utriculi, the site of action seems to be the lateral vestibular nucleus.

CONCLUSION

Meclizine, scopolamine, and lorazepam selectively suppress specific parts of the vestibular system. Selective suppression of different parts of the vestibular system may be more beneficial for alleviating (space) motion sickness than general suppressive agents. Additionally, this knowledge may help the clinician in his therapeutic management of patients with either semicircular canal or otolith dysfunction.

Atypical antipsychotic-induced weight gain: insights into mechanisms of action

Prescriptions for second-generation antipsychotics (SGAs) have surpassed those for first-generation agents in the treatment of schizophrenia and bipolar disorder. While SGAs have the benefit of a much reduced risk of causing movement disorders, they have been associated with weight gain and metabolic effects. These adverse reactions are not uncommon, and threaten to have a significant impact on the patient's health over the long-term treatment that the patient requires. Currently, the aetiology of these effects is not known. This article reviews the data exploring the weight gain phenomenon. The literature was reviewed from searches of PubMed and the references of major articles in the field. The SGAs present a heterogeneous risk for weight gain. In addition, different individuals receiving the same drug can exhibit substantially different weight changes. This pattern suggests that a group of factors are associated with the weight gain phenomenon rather than a single mechanism. Coupled with the genetic profile that the patient brings to the treatment, the risk for SGA-induced weight gain will be different for different drugs and different individuals. Targets for exploration of the weight gain phenomenon include receptor interactions involving serotonin, histamine, dopamine, adrenergic, cannabinoid and muscarinic receptors. The association of SGA-induced weight gain and the role of orexigenic and anorexigenic peptides are reviewed. Also, a brief discussion of genetic factors associated with SGA-induced weight gain is presented, including that of the serotonin 5-HT(2C) receptor gene (HTR2C) and the cannabinoid 1 receptor gene (CNR1). The most promising data associated with SGA-induced weight gain include investigations of the histamine H(1), 5-HT(2A), 5-HT(2C), muscarinic M(3) and adrenergic receptors. In addition, work in the genetic area promises to result in a better understanding of the variation in risk associated with different individuals.

No effects of anti-motion sickness drugs on vestibular evoked myogenic potentials outcome parameters

OBJECTIVE

To investigate the effects of meclizine (50 mg), baclofen (10 mg), cinnarizine (20 mg) + dimenhydrinate (40 mg), and promethazine (25 mg) + dextro-amphetamine (5 mg) on the parameters of the vestibular evoked myogenic potential (VEMP) test.

STUDY DESIGN

Double-blind placebo-controlled prospective randomized trial.

SETTING

University hospital.

SUBJECTS

Twenty-four (first block: baclofen versus placebo) and 20 healthy male subjects (second block: meclizine, cinnarizine + dimenhydrinate and promethazine + dextro-amphetamine versus placebo).

INTERVENTIONS

VEMP test.

MAIN OUTCOME MEASURES

Threshold, p13 and n23 latencies, p13-n23 latency difference, p13-n23 peak-to-peak amplitude, mean rectified voltage of the sternocleidomastoid muscle contraction and the corrected amplitude.

RESULTS

There were no clinically significant pharmacologic effects on the VEMP outcome parameters. However, there was a statistically significant left-right asymmetry after intake of the combination promethazine + d-amphetamine for the parameters p13 and latency difference.

CONCLUSION

The absence of clinically significant effects can be explained by the predominant presence of the target receptors for the applied drugs in the medial vestibular nucleus, which receives the lowest grade of saccular projections. It also can be hypothesized that the VEMP methodology and techniques in general do not allow determining pharmacologic effects in a healthy group of subjects because of a too small discriminative power. The left-right asymmetry can be explained by a depressive action of the drugs on the central compensation mechanisms. Because there were no significant differences between the VEMP parameters obtained after intake of the placebos of both blocks, we concluded that there were no training effects.

Dopamine transporter-dependent and -independent striatal binding of the benztropine analog JHW 007, a cocaine antagonist with low abuse liability

The benztropine analog N-(n-butyl)-3α-[bis(4'-fluorophenyl)methoxy]-tropane (JHW 007) displays high affinity for the dopamine transporter (DAT), but unlike typical DAT ligands, has relatively low abuse liability and blocks the effects of cocaine, including its self-administration. To determine sites responsible for the cocaine antagonist effects of JHW 007, its in vitro binding was compared with that of methyl (1R,2S,3S,5S)-3-(4-fluorophenyl)-8-methyl-8-azabicyclo[3.2.1]octane-2-carboxylate (WIN 35428) in rats, mice, and human DAT (hDAT)-transfected cells. A one-site model, with K(d) values of 4.21 (rat) and 8.99 nM (mouse) best fit the [(3)H]WIN 35428 data. [(3)H]JHW 007 binding best fit a two-site model (rat, 7.40/4400 nM; mouse, 8.18/2750 nM), although a one-site fit was observed with hDAT membranes (43.7 nM). Drugs selective for the norepinephrine and serotonin transporters had relatively low affinity in competition with [(3)H]JHW 007 binding, as did drugs selective for other sites identified previously as potential JHW 007 binding sites. The association of [(3)H]WIN 35428 best fit a one-phase model, whereas the association of [(3)H]JHW 007 best fit a two-phase model in all tissues. Because cocaine antagonist effects of JHW 007 have been observed previously soon after injection, its rapid association observed here may contribute to those effects. Multiple [(3)H]JHW 007 binding sites were obtained in tissue from mice lacking the DAT, suggesting these as yet unidentified sites as potential contributors to the cocaine antagonist effects of JHW 007. Unlike WIN 35428, the binding of JHW 007 was Na(+)-independent. This feature of JHW 007 has been linked to the conformational status of the DAT, which in turn may contribute to the antagonism of cocaine.

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.

Histamine-1 receptor is not required as a downstream effector of orexin-2 receptor in maintenance of basal sleep/wake states

AIM

The effect of orexin on wakefulness has been suggested to be largely mediated by activation of histaminergic neurones in the tuberomammillary nucleus (TMN) via orexin receptor-2 (OX(2)R). However, orexin receptors in other regions of the brain might also play important roles in maintenance of wakefulness. To dissect the role of the histaminergic system as a downstream mediator of the orexin system in the regulation of sleep/wake states without compensation by the orexin receptor-1 (OX(1)R) mediated pathways, we analysed the phenotype of Histamine-1 receptor (H(1)R) and OX(1)R double-deficient (H(1)R(-/-);OX(1)R(-/-)) mice. These mice lack OX(1)R-mediated pathways in addition to deficiency of H(1)R, which is thought to be the most important system in downstream of OX(2)R.

METHODS

We used H(1)R deficient (H(1)R(-/-)) mice, H(1)R(-/-);OX(1)R(-/-) mice, OX(1)R and OX(2)R double-deficient (OX(1)R(-/-);OX(2)R(-/-)) mice, and wild type controls. Rapid eye movement (REM) sleep, non-REM (NREM) sleep and awake states were determined by polygraphic electroencephalographic/electromyographic recording.

RESULTS

No abnormality in sleep/wake states was observed in H(1)R(-/-) mice, consistent with previous studies. H(1)R(-/-);OX(1)R(-/-) mice also showed a sleep/wake phenotype comparable to that of wild type mice, while OX(1)R(-/-); OX(2)R(-/-) mice showed severe fragmentation of sleep/wake states.

CONCLUSION

Our observations showed that regulation of the sleep/wake states is completely achieved by OX(2)R-expressing neurones without involving H(1)R-mediated pathways. The maintenance of basal physiological sleep/wake states is fully achieved without both H(1) and OX(1) receptors. Downstream pathways of OX(2)R other than the histaminergic system might play an important role in the maintenance of sleep/wake states.

Effects of L-histidine depletion and L-tyrosine/L-phenylalanine depletion on sensory and motor processes in healthy volunteers

BACKGROUND AND PURPOSE

Animal studies show that histamine plays a role in cognitive functioning and that histamine H3-receptor antagonists, which increase histaminergic function through presynaptic receptors, improve cognitive performance in models of clinical cognitive deficits. In order to test such new drugs in humans, a model for cognitive impairments induced by low histaminergic functions would be useful. Studies with histamine H1-receptor antagonists have shown limitations as a model. Here we evaluated whether depletion of L-histidine, the precursor of histamine, was effective in altering measures associated with histamine in humans and the behavioural and electrophysiological (event-related-potentials) effects.

EXPERIMENTAL APPROACH

Seventeen healthy volunteers completed a three-way, double-blind, crossover study with L-histidine depletion, L-tyrosine/L-phenylalanine depletion (active control) and placebo as treatments. Interactions with task manipulations in a choice reaction time task were studied. Task demands were increased using visual stimulus degradation and increased response complexity. In addition, subjective and objective measures of sedation and critical tracking task performance were assessed.

KEY RESULTS

Measures of sedation and critical tracking task performance were not affected by treatment. L-histidine depletion was effective and enlarged the effect of response complexity as measured with the response-locked lateralized readiness potential onset latency.

CONCLUSIONS AND IMPLICATIONS

L-histidine depletion affected response- but not stimulus-related processes, in contrast to the effects of H1-receptor antagonists which were previously found to affect primarily stimulus-related processes. L-histidine depletion is promising as a model for histamine-based cognitive impairment. However, these effects need to be confirmed by further studies.

Endocannabinoid-mediated control of synaptic transmission

The discovery of cannabinoid receptors and subsequent identification of their endogenous ligands (endocannabinoids) in early 1990s have greatly accelerated research on cannabinoid actions in the brain. Then, the discovery in 2001 that endocannabinoids mediate retrograde synaptic signaling has opened up a new era for cannabinoid research and also established a new concept how diffusible messengers modulate synaptic efficacy and neural activity. The last 7 years have witnessed remarkable advances in our understanding of the endocannabinoid system. It is now well accepted that endocannabinoids are released from postsynaptic neurons, activate presynaptic cannabinoid CB(1) receptors, and cause transient and long-lasting reduction of neurotransmitter release. In this review, we aim to integrate our current understanding of functions of the endocannabinoid system, especially focusing on the control of synaptic transmission in the brain. We summarize recent electrophysiological studies carried out on synapses of various brain regions and discuss how synaptic transmission is regulated by endocannabinoid signaling. Then we refer to recent anatomical studies on subcellular distribution of the molecules involved in endocannabinoid signaling and discuss how these signaling molecules are arranged around synapses. In addition, we make a brief overview of studies on cannabinoid receptors and their intracellular signaling, biochemical studies on endocannabinoid metabolism, and behavioral studies on the roles of the endocannabinoid system in various aspects of neural functions.

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.

Short- and long-term effects of antipsychotic drug treatment on weight gain and H1 receptor expression

The present study investigated body weight gain, food intake, open-field activity and brain histamine H1 receptor mRNA and protein expression in rats treated with three types of antipsychotics. Rats were divided into eight groups and treated with aripiprazole (2.25mg/kg/day), olanzapine (1.5mg/kg/day), haloperidol (0.3mg/kg/day) or vehicle (as control) for 1 or 12 weeks. Administration of olanzapine for 1 week led to a threefold increase in body weight gain and a 35% increase in fat deposits compared to controls (p<0.05). In the 12-week olanzapine treatment group, accumulative food intake was significantly higher in the first 7 weeks of treatment compared to controls (p<0.018), while body weight gain was significantly greater in the first 8 weeks compared to controls (p<0.045). Using in situ hybridization, we found that olanzapine treatment, but not aripiprazole or haloperidol treatment, significantly reduced H1 receptor mRNA expression in the arcuate hypothalamic nucleus (Arc: -18%, p=0.006, 1 week; -20%, p=0.008, 12 weeks) and ventromedial hypothalamic nucleus (VMH: -22%, p=0.006, 1 week; -19%, p=0.042, 12 weeks) compared to controls. The quantitative autoradiography data showed a reduction in VMH H1 receptor binding density after 1 (-12%, p=0.040) and 12 (-10%, p=0.094) weeks of olanzapine treatment. There were significant negative correlations between the levels of H1 receptor mRNA expression, and body weight gain and energy efficiency in the Arc and VMH after 1- and 12-week antipsychotic treatments in all groups. In addition, H1 receptor mRNA expression in the Arc showed a significant negative correlation with food intake and fat mass in all groups. Furthermore, there were negative correlations between H1 receptor binding density in the VMH and total fat mass and body weight gain after 1 week of antipsychotic treatment. The present study suggests that downregulated VMH and Arc H1 receptor expression may be a key factor contributing to olanzapine-induced obesity.

An inverse agonist of the histamine H(3) receptor improves wakefulness in narcolepsy: studies in orexin-/- mice and patients

Narcolepsy is characterized by excessive daytime sleepiness (EDS), cataplexy, direct onsets of rapid eye movement (REM) sleep from wakefulness (DREMs) and deficiency of orexins, neuropeptides that promote wakefulness largely via activation of histamine (HA) pathways. The hypothesis that the orexin defect can be circumvented by enhancing HA release was explored in narcoleptic mice and patients using tiprolisant, an inverse H(3)-receptor agonist. In narcoleptic orexin(-/-) mice, tiprolisant enhanced HA and noradrenaline neuronal activity, promoted wakefulness and decreased abnormal DREMs, all effects being amplified by co-administration of modafinil, a currently-prescribed wake-promoting drug. In a pilot single-blind trial on 22 patients receiving a placebo followed by tiprolisant, both for 1 week, the Epworth Sleepiness Scale (ESS) score was reduced from a baseline value of 17.6 by 1.0 with the placebo (p>0.05) and 5.9 with tiprolisant (p<0.001). Excessive daytime sleep, unaffected under placebo, was nearly suppressed on the last days of tiprolisant dosing. H(3)-receptor inverse agonists could constitute a novel effective treatment of EDS, particularly when associated with modafinil.

Stimulation of brain mast cells by compound 48/80, a histamine liberator, evokes renin and vasopressin release in dogs

Because degranulation of brain mast cells activates adrenocortical secretion (41, 42), we examined whether activation of such cells increases renin and vasopressin (antidiuretic hormone: ADH) secretion. For this, we administered compound 48/80 (C48/80), which liberates histamine from mast cells, to pentobarbital-anesthetized dogs. An infusion of 37.5 microg/kg C48/80 into the cerebral third ventricle evoked increases in plasma renin activity (PRA), and in plasma epinephrine (Epi) and ADH concentrations. Ketotifen (mast cell-stabilizing drug; given orally for 1 wk before the experiment) significantly reduced the C48/80-induced increases in PRA, Epi, and ADH. Resection of the bilateral splanchnic nerves (SPX) below the diaphragm completely prevented the C48/80-induced increases in PRA and Epi, but potentiated the C48/80-induced increase in ADH and elevated the plasma Epi level before and after C48/80 challenge. No significant changes in mean arterial blood pressure, heart rate, concentrations of plasma electrolytes (Na+, K+, and Cl-), or plasma osmolality were observed after C48/80 challenge in dogs with or without SPX. Pyrilamine maleate (H1 histaminergic-receptor antagonist) significantly reduced the C48/80-induced increase in PRA when given intracerebroventricularly, but not when given intravenously. In contrast, metiamide (H2 histaminergic-receptor antagonist) given intracerebroventricularly significantly potentiated the C48/80-induced PRA increase. A small dose of histamine (5 microg/kg) administered intracerebroventricularly increased PRA twofold and ADH fourfold (vs. their basal level). These results suggest that in dogs, endogenous histamine liberated from brain mast cells may increase renin and Epi secretion (via the sympathetic outflow) and ADH secretion (via the central nervous system).

Histamine-induced excitatory responses in mouse ventromedial hypothalamic neurons: ionic mechanisms and estrogenic regulation

Histamine is capable of modulating CNS arousal states by regulating neuronal excitability. In the current study, histamine action in the ventromedial hypothalamus (VMH), its related ionic mechanisms, and its possible facilitation by estrogen were investigated using whole cell patch-clamp recording in brain slices from ovariectomized female mice. Under current clamp, a bath application of histamine (20 microM) caused membrane depolarization, associated with an increased membrane resistance. In some cells, the depolarization was accompanied by action potentials. Histamine application also significantly reduced the latency of action potential evoked by current steps. Histamine-induced depolarization was not affected by either tetrodotoxin or Cd(2+). However, after blocking K(+) channels with tetraethylammonium, 4-aminopyridine, and Cs(+), depolarization was significantly decreased. Under voltage clamp, histamine-induced depolarization was associated with an inward current. The current-voltage relationship revealed that this inward current reversed near E(K). The histamine effect was mimicked by a histamine receptor 1 (H(1)) agonist, but not a histamine receptor 2 (H(2)) agonist. An H(1) antagonist, but not H(2) antagonist, abolished histamine responses. When ovariectomized mice were treated with estradiol benzoate (E2), histamine-induced depolarization was significantly enhanced with an increased percentage of cells showing action potential firing. These results suggest that histamine depolarized VMH neurons by attenuating a K(+) leakage current and this effect was mediated by H(1) receptor. E2 facilitated histamine-induced excitation of VMH neurons. This histamine effect may present a potential mechanism by which estrogens modulate the impact of generalized CNS arousal on a sexual arousal-related neuronal group.

Modification of formalin-induced nociception by different histamine receptor agonists and antagonists

The present study evaluated the effects of different histamine receptor agonists and antagonists on the nociceptive response in the mouse formalin test. Intracerebroventricular (20-40 microg/mouse i.c.v.) or subcutaneous (1-10 mg/kg s.c.) injection of HTMT (H(1) receptor agonist) elicited a dose-related hyperalgesia in the early and late phases. Conversely, intraperitoneal (20 and 30 mg/kg i.p.) injection of dexchlorpheniramine (H(1) receptor antagonist) was antinociceptive in both phases. At a dose ineffective per se, dexchlorpheniramine (10 mg/kg i.p.) antagonized the hyperalgesia induced by HTMT (40 mug/mouse i.c.v. or 10 mg/kg s.c.). Dimaprit (H(2) receptor agonist, 30 mg/kg i.p.) and ranitidine (H(2) receptor antagonist, 20 and 40 mg/kg i.p.) reduced the nociceptive responses in the early and late phases. No significant change in the antinociceptive activity was found following the combination of dimaprit (30 mg/kg i.p.) with ranitidine (10 mg/kg i.p.). The antinociceptive effect of dimaprit (30 mg/kg i.p.) was prevented by naloxone (5 mg/kg i.p.) in the early phase or by imetit (H(3) receptor agonist, 25 mg/kg i.p.) in both early and late phases. The histamine H(3) receptor agonist imetit was hyperalgesic following i.p. administration of 50 mg/kg. Imetit-induced hyperalgesia was completely prevented by treatment with a dose ineffective per se of thioperamide (H(3) receptor antagonist, 5 mg/kg i.p.). The results suggest that histamine H(1) and H(3) receptor activations increase sensitivity to nociceptive stimulus in the formalin test.

Selective cognitive dysfunction in mice lacking histamine H1 and H2 receptors

Previous pharmacological experiments provide conflicting findings that describe both facilitatory and inhibitory effects of neuronal histamine on learning and memory. Here, we examined learning and memory and synaptic plasticity in mice with a null mutation of gene coding histamine H1 or H2 receptor in order to clarify the role of these receptors in learning and memory processes. Learning and memory were evaluated by several behavioral tasks including object recognition, Barnes maze and fear conditioning. These behavioral tasks are highly dependent on the function of prefrontal cortex, hippocampus or amygdala. Object recognition and Barnes maze performance were significantly impaired in both H1 receptor gene knockout (H1KO) and H2 receptor gene knockout (H2KO) mice when compared to the respective wild-type (WT) mice. Conversely, both H1KO and H2KO mice showed better auditory and contextual freezing acquisition than their respective WT mice. Furthermore, we also examined long-term potentiation (LTP) in the CA1 area of hippocampus in H1KO and H2KO mice and their respective WT mice. LTP in the CA1 area of hippocampus was significantly reduced in both H1KO and H2KO mice when compared with their respective WT mice. In conclusion, our results demonstrate that both H1 and H2 receptors are involved in learning and memory processes for which the frontal cortex, amygdala and hippocampus interact.

Impaired ventilation and metabolism response to hypoxia in histamine H1 receptor-knockout mice

The role of central histamine in the hypoxic ventilatory response was examined in conscious wild-type (WT) and histamine type1 receptor-knockout (H1RKO) mice. Hypoxic gas (7% O(2) and 3% CO(2) in N(2)) exposure initially increased and then decreased ventilation, referred to as hypoxic ventilatory decline (HVD). The initial increase in ventilation did not differ between genotypes. However, H1RKO mice showed a blunted HVD, in which mean inspiratory flow was greater than that in WT mice. O(2) consumption (V(O2)) and CO(2) excretion were reduced 10min after hypoxic gas exposure in both genotypes, but (V(O2)) was greater in H1RKO mice than in WT mice. The ratio of minute ventilation to (V(O2)) during HVD did not differ between genotypes, indicating that ventilation is adequately controlled according to metabolic demand in both mice. Peripheral chemoreceptor sensitivity did not differ between genotypes. We conclude that central histamine contributes via the H1 receptor to changes in metabolic rate during hypoxia to increase HVD in conscious mice.

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.

Altered sleep-wake characteristics and lack of arousal response to H3 receptor antagonist in histamine H1 receptor knockout mice

Histaminergic neurons play an important role in the regulation of sleep-wake behavior through histamine H(1) receptors (H(1)R). Blockade of the histamine H(3) receptor (H(3)R) is proposed to induce wakefulness by regulating the release of various wake-related transmitters, not only histamine. In the present study, we characterized sleep-wake cycles of H(1)R knockout (KO) mice and their arousal responses to an H(3)R antagonist. Under baseline conditions, H(1)R KO mice showed sleep-wake cycles essentially identical to those of WT mice but with fewer incidents of brief awakening (<16-sec epoch), prolonged durations of non-rapid eye movement (NREM) sleep episodes, a decreased number of state transitions between NREM sleep and wakefulness, and a shorter latency for initiating NREM sleep after an i.p. injection of saline. The H(1)R antagonist pyrilamine mimicked these effects in WT mice. When an H(3)R antagonist, ciproxifan, was administered i.p., wakefulness increased in WT mice in a dose-dependent manner but did not increase at all in H(1)R KO mice. In vivo microdialysis revealed that the i.p. application of ciproxifan increased histamine release from the frontal cortex in both genotypes of mice. These results indicate that H(1)R is involved in the regulation of behavioral state transitions from NREM sleep to wakefulness and that the arousal effect of the H(3)R antagonist completely depends on the activation of histaminergic systems through H(1)R.

Functional genomics of sex hormone-dependent neuroendocrine systems: specific and generalized actions in the CNS

Sex hormone effects on hypothalamic neurons have been worked out to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been determined, and several functional genomic regulations have been discovered and conceptualized. With that knowledge in hand, we approach deeper problems of explaining sexual arousal and generalized CNS arousal. After a brief summary of arousal mechanisms, we focus on three chemical systems which signal generalized arousal and impact hormone-dependent hypothalamic neurons of behavioral importance: histamine, norepinephrine and enkephalin.

The anatomy of the vestibular nuclei

The vestibular portion of the eighth cranial nerve informs the brain about the linear and angular movements of the head in space and the position of the head with respect to gravity. The termination sites of these eighth nerve afferents define the territory of the vestibular nuclei in the brainstem. (There is also a subset of afferents that project directly to the cerebellum.) This chapter reviews the anatomical organization of the vestibular nuclei, and the anatomy of the pathways from the nuclei to various target areas in the brain. The cytoarchitectonics of the vestibular brainstem are discussed, since these features have been used to distinguish the individual nuclei. The neurochemical phenotype of vestibular neurons and pathways are also summarized because the chemical anatomy of the system contributes to its signal-processing capabilities. Similarly, the morphologic features of short-axon local circuit neurons and long-axon cells with extrinsic projections are described in detail, since these structural attributes of the neurons are critical to their functional potential. Finally, the composition and hodology of the afferent and efferent pathways of the vestibular nuclei are discussed. In sum, this chapter reviews the morphology, chemoanatomy, connectivity, and synaptology of the vestibular nuclei.

Dose- and duration-dependent effects of betahistine dihydrochloride treatment on histamine turnover in the cat

Drugs interacting with the histaminergic system are currently used for vertigo treatment and it was shown in animal models that structural analogues of histamine like betahistine improved the recovery process after vestibular lesion. This study was aimed at determining the possible dose and duration effects of betahistine treatment on histamine turnover in normal adult cats, as judged by the level of messenger RNA for histidine decarboxylase (enzyme synthesizing histamine) in the tuberomammillary nuclei. Experiments were conducted on betahistine-treated cats receiving daily doses of 2, 5, 10, or 50 mg/kg during 1 week, 3 weeks, 2 months, or 3 months. The 1-week, 3-week, and 2- and 3-month treatments correspond to the acute, compensatory, and sustained compensatory stages of vestibular compensation, respectively. The lowest dose (2 mg/kg) given the longest time (3 months) was close to the dosage for vestibular defective patients. Data from the experimental groups were compared to control, untreated cats and to placebo-treated animals. The results clearly show that betahistine dihydrochloride administered orally in the normal cat interferes with histamine turnover by increasing the basal expression level of histidine decarboxylase mRNA of neurons located in the tuberomammillary nuclei of the posterior hypothalamus. The effects were both dose- and time-dependent. In conclusion, compensation of both static and dynamic deficits is subtended by long-term adaptive mechanisms that could be facilitated pharmacologically using betahistine dihydrochloride.