Effects of a-fluoromethylhistidine on sleep and wakefulness in the rat. Short note

Samelisant (SUVN-G3031), a potent, selective and orally active histamine H3 receptor inverse agonist for the potential treatment of narcolepsy: pharmacological and neurochemical characterisation

RATIONALE

Samelisant (SUVN-G3031) is a potent and selective histamine H3 receptor (H3R) inverse agonist with good brain penetration and oral bioavailability.

OBJECTIVES

Pharmacological and neurochemical characterisation to support the utility of Samelisant (SUVN-G3031) in the treatment of sleep-related disorders like narcolepsy.

METHODS

Samelisant (SUVN-G3031) was tested in rat brain microdialysis studies for evaluation of modulation in histamine, dopamine and norepinephrine. Sleep EEG studies were carried out in orexin knockout mice to study the effects of Samelisant (SUVN-G3031) on the sleep-wake cycle and cataplexy.

RESULTS

Samelisant (SUVN-G3031) has a similar binding affinity towards human (hH3R; K_i = 8.7 nM) and rat (rH3R; K_i = 9.8 nM) H3R indicating no inter-species differences. Samelisant (SUVN-G3031) displays inverse agonist activity and it exhibits very high selectivity towards H3R. Samelisant (SUVN-G3031) treatment in mice produced a dose-dependent increase in tele-methylhistamine levels indicating the activation of histaminergic neurotransmission. Apart from increasing the levels of histamine, Samelisant (SUVN-G3031) also modulates dopamine and norepinephrine levels in the cerebral cortex while it has no effects on dopamine levels in the striatum or nucleus accumbens. Treatment with Samelisant (SUVN-G3031; 10 and 30 mg/kg, p.o.) produced a significant increase in wakefulness with a concomitant decrease in NREM sleep in orexin knockout mice subjected to sleep EEG. Samelisant (SUVN-G3031) also produced a significant decrease in Direct REM sleep onset (DREM) episodes, demonstrating its anticataplectic effects in an animal model relevant to narcolepsy. Modulation in cortical levels of histamine, norepinephrine and dopamine provides the neurochemical basis for wake-promoting and anticataplectic effects observed in orexin knockout mice.

CONCLUSIONS

Pre-clinical studies of Samelisant (SUVN-G3031) provide a strong support for utility in the treatment of sleep-related disorders related to EDS and is currently being evaluated in a phase 2 proof of concept study in the USA for the treatment of narcolepsy with and without cataplexy.

The role of co-neurotransmitters in sleep and wake regulation

Sleep and wakefulness control in the mammalian brain requires the coordination of various discrete interconnected neurons. According to the most conventional sleep model, wake-promoting neurons (WPNs) and sleep-promoting neurons (SPNs) compete for network dominance, creating a systematic "switch" that results in either the sleep or awake state. WPNs and SPNs are ubiquitous in the brainstem and diencephalon, areas that together contain <1% of the neurons in the human brain. Interestingly, many of these WPNs and SPNs co-express and co-release various types of the neurotransmitters that often have opposing modulatory effects on the network. Co-transmission is often beneficial to structures with limited numbers of neurons because it provides increasing computational capability and flexibility. Moreover, co-transmission allows subcortical structures to bi-directionally control postsynaptic neurons, thus helping to orchestrate several complex physiological functions such as sleep. Here, we present an in-depth review of co-transmission in hypothalamic WPNs and SPNs and discuss its functional significance in the sleep-wake network.

Enhanced histaminergic neurotransmission and sleep-wake alterations, a study in histamine H3-receptor knock-out mice

Long-term abolition of a brain arousal system impairs wakefulness (W), but little is known about the consequences of long-term enhancement. The brain histaminergic arousal system is under the negative control of H3-autoreceptors whose deletion results in permanent enhancement of histamine (HA) turnover. In order to determine the consequences of enhancement of the histaminergic system, we compared the cortical EEG and sleep-wake states of H3-receptor knockout (H3R-/-) and wild-type mouse littermates. We found that H3R-/-mice had rich phenotypes. On the one hand, they showed clear signs of enhanced HA neurotransmission and vigilance, i.e., a higher EEG θ power during spontaneous W and a greater extent of W or sleep restriction during behavioral tasks, including environmental change, locomotion, and motivation tests. On the other hand, during the baseline dark period, they displayed deficient W and signs of sleep deterioration, such as pronounced sleep fragmentation and reduced cortical slow activity during slow wave sleep (SWS), most likely due to a desensitization of postsynaptic histaminergic receptors as a result of constant HA release. Ciproxifan (H3-receptor inverse agonist) enhanced W in wild-type mice, but not in H3R-/-mice, indicating a functional deletion of H3-receptors, whereas triprolidine (postsynaptic H1-receptor antagonist) or α-fluoromethylhistidine (HA-synthesis inhibitor) caused a greater SWS increase in H3R-/- than in wild-type mice, consistent with enhanced HA neurotransmission. These sleep-wake characteristics and the obesity phenotypes previously reported in this animal model suggest that chronic enhancement of histaminergic neurotransmission eventually compromises the arousal system, leading to sleep-wake, behavioral, and metabolic disorders similar to those caused by voluntary sleep restriction in humans.

Neuropharmacology of Sleep and Wakefulness: 2012 Update

The development of sedative/hypnotic molecules has been empiric rather than rational. The empiric approach has produced clinically useful drugs but for no drug is the mechanism of action completely understood. All available sedative/hypnotic medications have unwanted side effects and none of these medications creates a sleep architecture that is identical to the architecture of naturally occurring sleep. This chapter reviews recent advances in research aiming to elucidate the neurochemical mechanisms regulating sleep and wakefulness. One promise of rational drug design is that understanding the mechanisms of sedative/hypnotic action will significantly enhance drug safety and efficacy.

Neuropharmacology of Sleep and Wakefulness

The development of sedative/hypnotic molecules has been empiric rather than rational. The empiric approach has produced clinically useful drugs but for no drug is the mechanism of action completely understood. All available sedative/hypnotic medications have unwanted side effects and none of these medications creates a sleep architecture that is identical to the architecture of naturally occurring sleep. This chapter reviews recent advances in research aiming to elucidate the neurochemical mechanisms regulating sleep and wakefulness. One promise of rational drug design is that understanding the mechanisms of sedative/hypnotic action will significantly enhance drug safety and efficacy.

Histamine in the regulation of wakefulness

The histaminergic system is exclusively localized within the posterior hypothalamus with projection to almost all the major regions of the central nervous system. Strong and consistent evidence exist to suggest that histamine, acting via H₁ and/or H₃ receptor has a pivotal role in the regulation of sleep-wakefulness. Administration of histamine or H₁ receptor agonists induces wakefulness, whereas administration of H₁ receptor antagonists promotes sleep. The H₃ receptor functions as an auto-receptor and regulates the synthesis and release of histamine. Activation of H₃ receptor reduces histamine release and promotes sleep. Conversely, blockade of H₃ receptor promotes wakefulness. Histamine release in the hypothalamus and other target regions is highest during wakefulness. The histaminergic neurons display maximal activity during the state of high vigilance, and cease their activity during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The cerebrospinal levels of histamine are reduced in diseased states where hypersomnolence is a major symptom. The histamine deficient L-histidine decarboxylase knockout (HDC KO) mice display sleep fragmentation and increased REM sleep during the light period along with profound wakefulness deficit at dark onset, and in novel environment. Similar results have been obtained when histamine neurons are lesioned. These studies strongly implicate the histaminergic neurons of the TMN to play a critical role in the maintenance of high vigilance state during wakefulness.

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.

Brain structures and mechanisms involved in the control of cortical activation and wakefulness, with emphasis on the posterior hypothalamus and histaminergic neurons

Wakefulness is a functional brain state that allows the performance of several "high brain functions", such as diverse behavioural, cognitive and emotional activities. Present knowledge at the whole animal or cellular level suggests that the maintenance of the cerebral cortex in this highly complex state necessitates the convergent and divergent activity of an ascending network within a large reticular zone, extending from the medulla to the forebrain and involving four major subcortical structures (the thalamus, basal forebrain, posterior hypothalamus and brainstem monoaminergic nuclei), their integral interconnections and several neurotransmitters, such as glutamate, acetylcholine, histamine and noradrenaline. In this mini-review, the importance of the thalamus, basal forebrain and brainstem monoaminergic neurons in wake control is briefly summarized, before turning our attention to the posterior hypothalamus and histaminergic neurons, which have been far less studied. Classical and recent experimental data are summarized, supporting the hypothesis that (1) the posterior hypothalamus constitutes one of the brain ascending activating systems and plays an important role in waking; (2) this function is mediated, in part, by histaminergic neurons, which constitute one of the excitatory sources for cortical activation during waking; (3) the mechanisms of histaminergic arousal involve both the ascending and descending projections of histaminergic neurons and their interactions with diverse neuronal populations, such as neurons in the pre-optic area and cholinergic neurons; and (4) other widespread-projecting neurons in the posterior hypothalamus also contribute to the tonic cortical activation during wakefulness and/or paradoxical sleep.

Acute wake-promoting actions of JNJ-5207852, a novel, diamine-based H3 antagonist

1 1-[4-(3-piperidin-1-yl-propoxy)-benzyl]-piperidine (JNJ-5207852) is a novel, non-imidazole histamine H3 receptor antagonist, with high affinity at the rat (pKi=8.9) and human (pKi=9.24) H3 receptor. JNJ-5207852 is selective for the H3 receptor, with negligible binding to other receptors, transporters and ion channels at 1 microm. 2 JNJ-5207852 readily penetrates the brain tissue after subcutaneous (s.c.) administration, as determined by ex vivo autoradiography (ED50 of 0.13 mg kg(-1) in mice). In vitro autoradiography with 3H-JNJ-5207852 in mouse brain slices shows a binding pattern identical to that of 3H-R-alpha-methylhistamine, with high specific binding in the cortex, striatum and hypothalamus. No specific binding of 3H-JNJ-5207852 was observed in brains of H3 receptor knockout mice. 3 In mice and rats, JNJ-5207852 (1-10 mg kg(-1) s.c.) increases time spent awake and decreases REM sleep and slow-wave sleep, but fails to have an effect on wakefulness or sleep in H3 receptor knockout mice. No rebound hypersomnolence, as measured by slow-wave delta power, is observed. The wake-promoting effects of this H3 receptor antagonist are not associated with hypermotility. 4 A 4-week daily treatment of mice with JNJ-5207852 (10 mg kg(-1) i.p.) did not lead to a change in body weight, possibly due to the compound being a neutral antagonist at the H3 receptor. 5 JNJ-5207852 is extensively absorbed after oral administration and reaches high brain levels. 6 The data indicate that JNJ-5207852 is a novel, potent and selective H3 antagonist with good in vitro and in vivo efficacy, and confirm the wake-promoting effects of H3 receptor antagonists.

Circadian rhythms in behavior and clock gene expressions in the brain of mice lacking histidine decarboxylase

To clarify functional roles of histamine in the circadian clock system, circadian rhythms of behavior and clock gene expression in the brain were examined in the mouse lacking histidine decarboxylase (HDC-/- mouse). Wheel-running and spontaneous locomotion were recorded under light-dark cycle (LD) and constant darkness (DD). mPer1, mPer2 and mBMAL1 mRNA expression rhythms under LD and DD were measured in the suprachiasmatic nucleus (SCN), cerebral cortex and striatum by in situ hybridization. The activity levels under LD and DD in the HDC-/- mice were lower than that in the wild type regardless of activity types (wheel-running and spontaneous locomotion). The free-running period under DD was significantly longer in the HDC-/- mice than in the wild type. The 24-h profiles of mPer1, mPer2 and mBMAL1 mRNA expressions in the SCN were not different between the two genotypes. By contrast, the mPer1 and mPer2 mRNA rhythms in the other brain areas such as the cortex and striatum were significantly disrupted in the HDC-/- mice. These results suggest that histamine is involved in the circadian system especially in the output pathway or feedback route from behavior to the pacemaker in the SCN, and that mPer genes in the brain areas outside the SCN play an important role in the expression of behavioral rhythm.

Anatomical, physiological, and pharmacological characteristics of histidine decarboxylase knock-out mice: evidence for the role of brain histamine in behavioral and sleep-wake control

The hypothesis that histaminergic neurons are involved in brain arousal is supported by many studies. However, the effects of the selective long-term abolition of histaminergic neurons on the sleep-wake cycle, indispensable in determining their functions, remain unknown. We have compared brain histamine(HA)-immunoreactivity and the cortical-EEG and sleep-wake cycle under baseline conditions or after behavioral or pharmacological stimuli in wild-type (WT) and knock-out mice lacking the histidine decarboxylase gene (HDC-/-). HDC-/-mice showed an increase in paradoxical sleep, a decrease in cortical EEG power in theta-rhythm during waking (W), and a decreased EEG slow wave sleep/W power ratio. Although no major difference was noted in the daily amount of spontaneous W, HDC-/-mice showed a deficit of W at lights-off and signs of somnolence, as demonstrated by a decreased sleep latencies after various behavioral stimuli, e.g., WT-mice placed in a new environment remained highly awake for 2-3 hr, whereas HDC-/-mice fell asleep after a few minutes. These effects are likely to be attributable to lack of HDC and thus of HA. In WT mice, indeed, intraperitoneal injection of alpha-fluoromethylhistidine (HDC-inhibitor) caused a decrease in W, whereas injection of ciproxifan (HA-H3 receptor antagonist) elicited W. Both injections had no effect in HDC-/-mice. Moreover, PCR and immunohistochemistry confirmed the absence of the HDC gene and brain HA-immunoreactive neurons in the HDC-/-mice. These data indicate that disruption of HA-synthesis causes permanent changes in the cortical-EEG and sleep-wake cycle and that, at moments when high vigilance is required (lights off, environmental change em leader ), mice lacking brain HA are unable to remain awake, a prerequisite condition for responding to behavioral and cognitive challenges. We suggest that histaminergic neurons also play a key role in maintaining the brain in an awake state faced with behavioral challenges.

The physiology of brain histamine

Histamine-releasing neurons are located exclusively in the TM of the hypothalamus, from where they project to practically all brain regions, with ventral areas (hypothalamus, basal forebrain, amygdala) receiving a particularly strong innervation. The intrinsic electrophysiological properties of TM neurons (slow spontaneous firing, broad action potentials, deep after hyperpolarisations, etc.) are extremely similar to other aminergic neurons. Their firing rate varies across the sleep-wake cycle, being highest during waking and lowest during rapid-eye movement sleep. In contrast to other aminergic neurons somatodendritic autoreceptors (H3) do not activate an inwardly rectifying potassium channel but instead control firing by inhibiting voltage-dependent calcium channels. Histamine release is enhanced under extreme conditions such as dehydration or hypoglycemia or by a variety of stressors. Histamine activates four types of receptors. H1 receptors are mainly postsynaptically located and are coupled positively to phospholipase C. High densities are found especially in the hypothalamus and other limbic regions. Activation of these receptors causes large depolarisations via blockade of a leak potassium conductance, activation of a non-specific cation channel or activation of a sodium-calcium exchanger. H2 receptors are also mainly postsynaptically located and are coupled positively to adenylyl cyclase. High densities are found in hippocampus, amygdala and basal ganglia. Activation of these receptors also leads to mainly excitatory effects through blockade of calcium-dependent potassium channels and modulation of the hyperpolarisation-activated cation channel. H3 receptors are exclusively presynaptically located and are negatively coupled to adenylyl cyclase. High densities are found in the basal ganglia. These receptors mediated presynaptic inhibition of histamine release and the release of other neurotransmitters, most likely via inhibition of presynaptic calcium channels. Finally, histamine modulates the glutamate NMDA receptor via an action at the polyamine binding site. The central histamine system is involved in many central nervous system functions: arousal; anxiety; activation of the sympathetic nervous system; the stress-related release of hormones from the pituitary and of central aminergic neurotransmitters; antinociception; water retention and suppression of eating. A role for the neuronal histamine system as a danger response system is proposed.

Innervation of histaminergic tuberomammillary neurons by GABAergic and galaninergic neurons in the ventrolateral preoptic nucleus of the rat

The tuberomammillary nucleus (TMN) is the major source of histaminergic innervation of the mammalian brain and is thought to play a major role in regulating wake-sleep states. We recently found that sleep-active neurons in the ventrolateral preoptic nucleus (VLPO) provide a major input to the TMN, but the specificity of this projection and the neurotransmitters involved remain unknown. In this study, we examined the relationship of VLPO efferents to the TMN using both retrograde and anterograde tracing, combined with immunocytochemistry. We found that the descending projection from the VLPO selectively targets the cell bodies and proximal dendrites of the histaminergic TMN. In addition, VLPO axons could be traced into the brainstem, where they provided terminals in the the serotoninergic dorsal and median raphe nuclei, and the core of the noradrenergic locus coeruleus. Approximately 80% of the VLPO neurons that were retrogradely labeled by tracer injections including the TMN were immunoreactive either for galanin or for glutamic acid decarboxylase (GAD), the synthetic enzyme for GABA. Virtually all of the galaninergic neurons in the VLPO were also GAD positive. Our results indicate that the VLPO may provide inhibitory GABAergic and galaninergic inputs to the cell bodies and proximal dendrites of the TMN and other components of the ascending monoaminergic arousal system. Because these cell groups are simultaneously inhibited during sleep, the VLPO sleep-active neurons may play a key role in silencing the ascending monoaminergic arousal system during sleep.

The selective histamine H1-receptor agonist 2-(3-trifluoromethylphenyl)histamine increases waking in the rat

The effects of the selective histamine H1-receptor agonist, 2-(3-trifluoromethylphenyl)histamine, were studied in rats implanted with electrodes for chronic sleep recordings. 2-(3-Trifluoromethylphenyl)histamine (80-120 micrograms) injected into the left lateral ventricle increased wakefulness, whereas slow wave sleep was reduced. Pretreatment with pyrilamine (2.0 mg/kg) prevented the effects of the H1-receptor agonist on wakefulness and slow wave sleep. Our results further support the involvement of histamine in the modulation of the waking state.

Histamine does not play an essential role in electrocortical activation during waking behavior

Intraperitoneal injection of alpha-fluoromethylhistidine (alpha-FMH; 200 mg/kg), a specific inhibitor of histidine decarboxylase produced a severe depletion of neocortical and hippocampal histamine 3 h later as determined by a radioenzymatic assay. This treatment had no obvious effect on either low voltage fast activity (LVFA) in the neocortex or on rhythmical slow activity (RSA) in the hippocampus during an 8 h recording period during the rats' light cycle. Scopolamine-sensitive LVFA, scopolamine-resistant LVFA and scopolamine-resistant hippocampal RSA all appeared unaffected. This suggests that any contribution histamine makes to electrocortical activation is probably indirect, acting via other transmitter systems.

The effect of REM sleep deprivation on histamine concentrations in different brain areas

Rats were deprived of REM sleep (REMS) for 72 h with the platform method and decapitated in the morning immediately after the deprivation or in the afternoon after having been allowed 5 hours of rebound sleep. The histamine concentrations of the anterior and posterior hypothalamus, the cortex, the hippocampus and the pineal gland were measured, as well as the tele-methylhistamine concentrations of the anterior and posterior hypothalamus. Histamine concentrations were no different after REMS deprivation compared to large platform or dry cage controls, but in the anterior hypothalamus histamine levels increased during rebound sleep only in the REMS deprived rats. tele-Methylhistamine/histamine ratios were higher after 72 h of both REMS deprivation and the large platform treatment compared to dry cage controls, indicating increased histamine utilization during the platform treatment procedure.

Involvement of histamine in the control of the waking state

Available evidence indicates that histamine (HA) plays a neuroregulatory role in the waking state. Support for this proposal is provided by electrophysiological, lesion and pharmacological studies, as well as by fluctuations of HA levels according to a circadian pattern. Thus, 1) HA-containing neurons unit activity changes dramatically as a function of behavioral state across the sleep-wakefulness continuum, from 2.3 spikes/sec during active waking to virtual silence during slow wave sleep and REM sleep; 2) HA levels reach a minimum during the dark phase followed by an increase during the light period in rats kept under controlled environmental conditions; in addition histidine decarboxylase and HA-N-methyl-transferase activities are higher during darkness; 3) lesions or cooling of the posterior hypothalamus in the area where HA-immunoreactive neurons are located gives rise to a state of somnolence or hypersomnia; 4) 2-thiazolylethylamine, the predominantly H1-receptor agonist and thioperamide, the H3-receptor antagonist increase wakefulness in laboratory animals, while the HA synthesis inhibitor a-fluoromethylhistidine, the H1-receptor antagonists mepyramine, diphenhydramine and chlorpheniramine, and the H3-receptor agonist (R)-a-Me-histamine produce opposite effects.

Effects of the histamine H3 receptor ligands thioperamide and (R)-alpha-methylhistamine on histidine decarboxylase activity of mouse brain

The effects of the histamine H3 receptor ligands thioperamide and (R)-alpha-methylhistamine on the histidine decarboxylase (HDC) activity and histamine content of mouse brain were examined. Thioperamide, a histamine H3 antagonist, significantly increased the HDC activity in the brain of ddY, W/Wv and ICR mice 2-6 hr after its intraperitoneal (i.p.) injection. On the other hand, (R)-alpha-methylhistamine, a histamine H3-receptor agonist, caused no significant change in the HDC activity. The whole brain histamine content of ddY mice decreased significantly to 60-70% of the control level 2-8 hr after injection of thioperamide (25 mg/kg, i.p.), but then increased to 90% of the control level 10 hr after the injection. These in vivo results showed that blockade of the presynaptic histamine H3-receptor, which causes release of presynaptic histamine, increased the HDC activity.

Effects of selective activation or blockade of the histamine H3 receptor on sleep and wakefulness

The effects of the histamine H3 receptor agonist, (R)-alpha-methylhistamine were compared with those of the histamine H3 antagonist, thioperamide, in rats implanted with electrodes for chronic sleep recordings. (R)-alpha-Methylhistamine (1.0-4.0 micrograms) injected bilaterally into the premammillary area where histamine immunoreactive neurons have been detected increased slow wave sleep, whereas wakefulness and REM sleep were decreased. No significant effects were observed when (R)-alpha-methylhistamine (1.0-8.0 mg/kg) was administered i.p. Thioperamide (1.0-4.0 mg/kg i.p.) increased wakefulness and decreased slow wave sleep and REM sleep. Pretreatment with thioperamide (4.0 mg/kg) prevented the effects of (R)-alpha-methylhistamine (2.0 micrograms) on slow wave sleep and wakefulness. Our results further support an active role for histamine in the control of the waking state.

Effect of histamine depletion on the circadian amplitude of the sleep-wakefulness cycle

Behavioral states of rats were automatically classified with a newly developed computer program into three sleep stages (awake, slow-wave sleep and REM sleep) from continuous long-term EEG and EMG recordings for several circadian cycles under entrained circumstances (L:D = 12:12). Histamine was depleted by 100 mg/kg intraperitoneal administration of a specific inhibitor of its synthesis, alpha-fluoromethylhistidine, in the mid-light period. This treatment had no effect on the amount of each sleep stage in the total 24-h period or in the light period, but caused significant increases in slow-wave sleep and REM sleep in the dark period. Equivalent decrease in the awake stage during the dark period was also observed. As a result, histamine depletion decreased the light:dark ratio of slow-wave sleep. These findings suggest that decrease of the histamine content of the brain attenuated the circadian amplitude of sleep-wakefulness by suppressing the surge of wakefulness during the dark period. From these results, histamine is suggested to modulate the circadian amplitude of the sleep-wakefulness cycle.

Histaminergic neuron system: morphological features and possible functions

The histaminergic neuron systems in rat brain have been identified by immunocytochemical techniques using antibodies against histidine decarboxylase or histamine itself. Here, the details of the distribution of the histaminergic neuron networks are presented. Judging from the widespread distribution of the nervous system, it is postulated that the histaminergic neuron system is involved in various brain functions. Some functions, including the circadian rhythms, sleep-arousal cycles, drinking, feeding, thermoregulation, and neuroendocrine controls which were elucidated by administration of alpha-fluoromethylhistidine, a suicide substrate for histidine decarboxylase, are discussed here, although the true functions are still under investigations.

Sleep variables are unaltered by zolantidine in rats: are histamine H2-receptors not involved in sleep regulation?

The effects of the H1-receptor antagonist diphenhydramine and the brain-penetrating H2-receptor antagonist zolantidine were studied in rats implanted for chronic sleep recordings. Diphenhydramine (1.0-4.0 mg/kg) significantly increased slow wave sleep and decreased wakefulness. Zolantidine (0.25-8.0 mg/kg) had no significant effects on any of the sleep parameters examined. One possibility is that zolantidine did not enter the brain in sufficient concentration to produce significant changes on sleep and wakefulness. Another possibility is that blockade of H2-receptor involved parts of the brain other than those implicated in the sleep-wake cycle. The feasibility remains that H2-receptors are not involved in sleep regulation. The absence of selective, brain-penetrating H2-receptor agonists precludes a more detailed analysis of the role of this subtype of receptor in the control of sleep and wakefulness.

Involvement of histaminergic neurons in arousal mechanisms demonstrated with H3-receptor ligands in the cat

The effects of histamine H3-receptor ligands on sleep-waking parameters were studied in freely moving cats. Oral administration of (R)alpha-methylhistamine (alpha MHA), a H3-agonist, caused a significant increase in deep slow wave sleep while that of thioperamide, a H3-antagonist, enhanced wakefulness in a marked and dose-dependent manner. The arousal effects of thioperamide were prevented by pretreatment with alpha MHA or mepyramine, a H1-receptor antagonist. The findings support the hypothesis that the histaminergic neurons are critically involved in arousal mechanisms and suggest that H3-receptors play an active part in these mechanisms by regulating histamine transmission.