Effects of the serotonin 5-HT2A/2C receptor agonist DOI and of the selective 5-HT2A or 5-HT2C receptor antagonists EMD 281014 and SB-243213, respectively, on sleep and waking in the rat

Activation of ventral pallidum-projecting neurons in the nucleus accumbens via 5-HT2C receptor stimulation regulates motivation for wheel running in male mice

Rodents have a strong motivation for wheel running; however, the neural mechanisms that regulate their motivation remain unknown. We investigated the possible involvement of serotonin (5-HT) systems in regulating motivation for wheel running in male mice. Systemic administration of a 5-HT1A receptor antagonist (WAY100635) increased the number of wheel rotations, whereas administration of a 5-HT2A or 5-HT2C receptor antagonist (volinanserin or SB242084, respectively) decreased it. In the open field test, neither WAY100635 nor volinanserin affected locomotor activity, whereas SB242084 increased locomotor activity. To identify the brain regions on which these antagonists act, we locally injected these into the motivational circuitry, including the nucleus accumbens (NAc), dorsomedial striatum (DM-Str), and medial prefrontal cortex (mPFC). Injection of SB242084 into the NAc, but not the DM-Str or mPFC, reduced the number of wheel rotations without altering locomotor activity. The local administration of WAY100635 or volinanserin to these brain regions did not affect the number of wheel rotations. Immunohistochemical analyses revealed that wheel running increased the number of c-Fos-positive cells in the NAc medial shell (NAc-MS), which was reduced by systemic SB242084 administration. In vitro slice whole-cell recordings showed that bath application of the 5-HT2C receptor agonist lorcaserin increased the frequency of spontaneous excitatory and inhibitory postsynaptic currents in the ventral tegmental area (VTA)-projecting neurons, whereas it only increased the frequency of spontaneous excitatory postsynaptic currents in ventral pallidum (VP)-projecting neurons in the NAc-MS. These findings suggest that the activation of VP-projecting NAc-MS neurons via 5-HT2C receptor stimulation regulates motivation for wheel running.

GABALAGEN Facilitates Pentobarbital-Induced Sleep by Modulating the Serotonergic System in Rats

Gamma-aminobutyric acid (GABA) is one of the inhibitory neurotransmitters with beneficial effects including sedative properties. However, despite various clinical trials, scientific evidence regarding the impact on sleep of orally ingested GABA, whether natural or synthesized through biological pathways, is not clear. GABALAGEN (GBL) is the product of fermented collagen by Lactobacillus brevis BJ20 (L. brevis BJ20) and Lactobacillus plantarum BJ21 (L. plantarum BJ21), enriched with GABA and characterized by low molecular weight. The aim of this study was to investigate the effect of GBL on sleep improvement via a receptor binding assay in a pentobarbital-induced sleep-related rat model. We utilized a pentobarbital-induced sleep-related rat model to conduct this research. The present study investigated the sedative effects of GBL through electroencephalography (EEG) analysis in the pentobarbital-induced sleep animal model. Exploration of the neural basis of these positive effects involved evaluating orexin in the brain via immunohistochemical methods and 5-HT in the serum using an enzyme-linked immunosorbent assay (ELISA). Furthermore, we conducted a binding assay for 5-HT2C receptors, as these are considered pivotal targets in the mechanism of action for sleep aids. Diazepam (DZP) was used as a positive control to compare the efficacy of GBL. Results: In the binding assay, GBL displayed binding affinity to the 5-HT2C receptor (IC50 value, 5.911 µg/mL). Administration of a low dose of GBL (GBL_L; 100 mg/kg) increased non-rapid eye movement sleep time and decreased wake time based on EEG data in pentobarbital-induced rats. Administration of a high dose of GBL (GBL_H; 250 mg/kg) increased non-rapid eye movement sleep time. Additionally, GBL groups significantly increased concentration of the 5-HT level in the serum. GBL_H decreased orexin expression in the lateral hypothalamus. Conclusion: Overall, the sedative effect of GBL may be linked to the activation of serotonergic systems, as indicated by the heightened affinity of the 5-HT2C receptor binding and elevated levels of 5-HT observed in the serum. This suggests that GBL holds promise as a novel compound for inducing sleep in natural products.

5-HT receptors exert differential effects on seizure-induced respiratory arrest in DBA/1 mice

Both clinical and animal studies demonstrated that seizure-induced respiratory arrest (S-IRA) contributes importantly to sudden unexpected death in epilepsy (SUDEP). It has been shown that enhancing serotonin (5-HT) function relieves S-IRA in animal models of SUDEP, including DBA/1 mice. Direct activation of 5-HT3 and 5-HT4 receptors suppresses S-IRA in DBA/1 mice, indicating that these receptors are involved in S-IRA. However, it remains unknown if other subtypes of 5-HT receptors are implicated in S-IRA in DBA/1 mice. In this study, we investigated the action of an agonist of the 5-HT1A (8-OH-DPAT), 5-HT2A (TCB-2), 5-HT2B (BW723C86), 5-HT2C (MK-212), 5-HT6 (WAY-208466) and 5-HT7 (LP-211) receptor on S-IRA in DBA/1 mice. An agonist of the 5-HT receptor or a vehicle was intraperitoneally administered 30 min prior to acoustic simulation, and the effect of each drug/vehicle on the incidence of S-IRA was videotaped for offline analysis. We found that the incidence of S-IRA was significantly reduced by TCB-2 at 10 mg/kg (30%, n = 10; p < 0.01, Fisher's exact test) but was not altered by other agonists compared with the corresponding vehicle controls in DBA/1 mice. Our data demonstrate that 5-HT2A receptors are implicated in S-IRA, and 5-HT1A, 5-HT2B, 5-HT2C, 5-HT6 and 5-HT7 receptors are not involved in S-IRA in DBA/1 mice.

LSD increases sleep duration the night after microdosing

Microdosing psychedelic drugs at a level below the threshold to induce hallucinations is an increasingly common lifestyle practice. However, the effects of microdosing on sleep have not been previously reported. Here, we report results from a Phase 1 randomized controlled trial in which 80 healthy adult male volunteers received a 6-week course of either LSD (10 µg) or placebo with doses self-administered every third day. Participants used a commercially available sleep/activity tracker for the duration of the trial. Data from 3231 nights of sleep showed that on the night after microdosing, participants in the LSD group slept an extra 24.3 min per night (95% Confidence Interval 10.3-38.3 min) compared to placebo-with no reductions of sleep observed on the dosing day itself. There were no changes in the proportion of time spent in various sleep stages or in participant physical activity. These results show a clear modification of the physiological sleep requirements in healthy male volunteers who microdose LSD. The clear, clinically significant changes in objective measurements of sleep observed are difficult to explain as a placebo effect. Trial registration: Australian New Zealand Clinical Trials Registry: A randomized, double-blind, placebo-controlled trial of repeated microdoses of lysergic acid diethylamide (LSD) in healthy volunteers; https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=381476 ; ACTRN12621000436875.

Herbal medicines for insomnia through regulating 5-hydroxytryptamine receptors: a systematic review

Insomnia is a common sleep disorder without effective therapy and can affect a person's life. The mechanism of the disease is not completely understood. Hence, there is a need to understand the targets related to insomnia, in order to develop innovative therapies and new compounds. Recently, increasing interest has been focused on complementary and alternative medicines for treating or preventing insomnia. Research into their molecular components has revealed that their sedative and sleep-promoting properties rely on the interactions with various neurotransmitter systems in the brain. In this review, the role of 5-hydroxytryptamine (5-HT) in insomnia development is summarized, while a systematic analysis of studies is conducted to assess the mechanisms of herbal medicines on different 5-HT receptors subtypes, in order to provide reference for subsequent research.

Psilocin acutely alters sleep-wake architecture and cortical brain activity in laboratory mice

Serotonergic psychedelic drugs, such as psilocin (4-hydroxy-N,N-dimethyltryptamine), profoundly alter the quality of consciousness through mechanisms which are incompletely understood. Growing evidence suggests that a single psychedelic experience can positively impact long-term psychological well-being, with relevance for the treatment of psychiatric disorders, including depression. A prominent factor associated with psychiatric disorders is disturbed sleep, and the sleep-wake cycle is implicated in the homeostatic regulation of neuronal activity and synaptic plasticity. However, it remains largely unknown to what extent psychedelic agents directly affect sleep, in terms of both acute arousal and homeostatic sleep regulation. Here, chronic electrophysiological recordings were obtained in mice to track sleep-wake architecture and cortical activity after psilocin injection. Administration of psilocin led to delayed REM sleep onset and reduced NREM sleep maintenance for up to approximately 3 h after dosing, and the acute EEG response was associated primarily with an enhanced oscillation around 4 Hz. No long-term changes in sleep-wake quantity were found. When combined with sleep deprivation, psilocin did not alter the dynamics of homeostatic sleep rebound during the subsequent recovery period, as reflected in both sleep amount and EEG slow-wave activity. However, psilocin decreased the recovery rate of sleep slow-wave activity following sleep deprivation in the local field potentials of electrodes targeting the medial prefrontal and surrounding cortex. It is concluded that psilocin affects both global vigilance state control and local sleep homeostasis, an effect which may be relevant for its antidepressant efficacy.

Cellular Effects of Rhynchophylline and Relevance to Sleep Regulation

Uncaria rhynchophylla is a plant highly used in the traditional Chinese and Japanese medicines. It has numerous health benefits, which are often attributed to its alkaloid components. Recent studies in humans show that drugs containing Uncaria ameliorate sleep quality and increase sleep time, both in physiological and pathological conditions. Rhynchophylline (Rhy) is one of the principal alkaloids in Uncaria species. Although treatment with Rhy alone has not been tested in humans, observations in rodents show that Rhy increases sleep time. However, the mechanisms by which Rhy could modulate sleep have not been comprehensively described. In this review, we are highlighting cellular pathways that are shown to be targeted by Rhy and which are also known for their implications in the regulation of wakefulness and sleep. We conclude that Rhy can impact sleep through mechanisms involving ion channels, N-methyl-d-aspartate (NMDA) receptors, tyrosine kinase receptors, extracellular signal-regulated kinases (ERK)/mitogen-activated protein kinases (MAPK), phosphoinositide 3-kinase (PI3K)/RAC serine/threonine-protein kinase (AKT), and nuclear factor-kappa B (NF-κB) pathways. In modulating multiple cellular responses, Rhy impacts neuronal communication in a way that could have substantial effects on sleep phenotypes. Thus, understanding the mechanisms of action of Rhy will have implications for sleep pharmacology.

Dorsal raphe serotonergic neurons promote arousal from isoflurane anesthesia

Aims

General anesthesia has been widely applied in surgical or nonsurgical medical procedures, but the mechanism behind remains elusive. Because of shared neural circuits of sleep and anesthesia, whether serotonergic system, which is highly implicated in modulation of sleep and wakefulness, regulates general anesthesia as well is worth investigating.

Methods

Immunostaining and fiber photometry were used to assess the neuronal activities. Electroencephalography spectra and burst‐suppression ratio (BSR) were used to measure anesthetic depth and loss or recovery of righting reflex to indicate the induction or emergence time of general anesthesia. Regulation of serotonergic system was achieved through optogenetic, chemogenetic, or pharmacological methods.

Results

We found that both Fos expression and calcium activity were significantly decreased during general anesthesia. Activation of 5‐HT neurons in the dorsal raphe nucleus (DRN) decreased the depth of anesthesia and facilitated the emergence from anesthesia, and inhibition deepened the anesthesia and prolonged the emergence time. Furthermore, agonism or antagonism of 5‐HT 1A or 2C receptors mimicked the effect of manipulating DRN serotonergic neurons.

Conclusion

Our results demonstrate that 5‐HT neurons in the DRN play a regulative role of general anesthesia, and activation of serotonergic neurons could facilitate emergence from general anesthesia partly through 5‐HT 1A and 2C receptors.

Analysis of the effects of a tricyclic antidepressant on secondary sleep disturbance induced by chronic pain in a preclinical model

Chronic pain and sleep have a bidirectional relationship that promotes a vicious circle making chronic pain more difficult to treat. Therefore, pain and sleep should be treated simultaneously. In our previous study, we suggested that hyperactivation of ascending serotonergic neurons could cause secondary sleep disturbance in chronic pain. This study aimed to demonstrate the effects of a tricyclic antidepressant (amitriptyline) and a selective 5-hydroxy-tryptamine 2A (5-HT_2A) antagonist (MDL 100907) that adjust serotonergic transmission, on secondary sleep disturbance induced in a preclinical chronic pain model. We produced a chronic neuropathic pain model by partial sciatic nerve ligation in mice, analyzed their electroencephalogram (EEG) and electromyogram (EMG) using the SleepSign software, and evaluated the sleep condition of the pain model mice after administration of amitriptyline or MDL 100907. Amitriptyline improved thermal hyperalgesia and the amount of sleep, especially non-REM sleep. Time change of normalized power density of δ wave in the nerve ligation group with amitriptyline administration showed a normal pattern that was similar to sham mice. In addition, MDL 100907 normalized sleep condition similar to amitriptyline, without improvement in pain threshold. In conclusion, amitriptyline could improve sleep quantity and quality impaired by chronic pain. 5-HT_2A receptor antagonism could partially contribute to this sleep improvement, but is not associated with pain relief.

Ibogaine Acute Administration in Rats Promotes Wakefulness, Long-Lasting REM Sleep Suppression, and a Distinctive Motor Profile

Ibogaine is a potent psychedelic alkaloid that has been the focus of intense research because of its intriguing anti-addictive properties. According to anecdotic reports, ibogaine has been originally classified as an oneirogenic psychedelic; i.e., induces a dream-like cognitive activity while awake. However, the effects of ibogaine administration on wakefulness (W) and sleep have not been thoroughly assessed. The main aim of our study was to characterize the acute effects of ibogaine administration on W and sleep. For this purpose, polysomnographic recordings on chronically prepared rats were performed in the light phase during 6 h. Animals were treated with ibogaine (20 and 40 mg/kg) or vehicle, immediately before the beginning of the recordings. Furthermore, in order to evaluate associated motor behaviors during the W period, a different group of animals was tested for 2 h after ibogaine treatment on an open field with video-tracking software. Compared to control, animals treated with ibogaine showed an increase in time spent in W. This effect was accompanied by a decrease in slow wave sleep (SWS) and rapid-eye movements (REM) sleep time. REM sleep latency was significantly increased in animals treated with the higher ibogaine dose. While the effects on W and SWS were observed during the first 2 h of recordings, the decrement in REM sleep time was observed throughout the recording time. Accordingly, ibogaine treatment with the lower dose promoted an increase on locomotion, while tremor and flat body posture were observed only with the higher dose in a time-dependent manner. In contrast, head shake response, a behavior which has been associated in rats with the 5HT_2A receptor activation by hallucinogens, was not modified. We conclude that ibogaine promotes a waking state that is accompanied by a robust and long-lasting REM sleep suppression. In addition, it produces a dose-dependent unusual motor profile along with other serotonin-related behaviors. Since ibogaine is metabolized to produce noribogaine, further experiments are needed to elucidate if the metabolite and/or the parent drug produced these effects.

Role for serotonin2A (5-HT2A) and 2C (5-HT2C) receptors in experimental absence seizures

Absence seizures (ASs) are the hallmark of childhood/juvenile absence epilepsy. Monotherapy with first-line anti-absence drugs only controls ASs in 50% of patients, indicating the need for novel therapeutic targets. Since serotonin family-2 receptors (5-HT₂Rs) are known to modulate neuronal activity in the cortico-thalamo-cortical loop, the main network involved in AS generation, we investigated the effect of selective 5-HT_2AR and 5-HT_2CR ligands on ASs in the Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a well established polygenic rat model of these non-convulsive seizures. GAERS rats were implanted with fronto-parietal EEG electrodes under general anesthesia, and their ASs were later recorded under freely moving conditions before and after intraperitoneal administration of various 5-HT_2AR and 5-HT_2CR ligands. The 5-HT_2A agonist TCB-2 dose-dependently decreased the total time spent in ASs, an effect that was blocked by the selective 5-HT_2A antagonist MDL11,939. Both MDL11,939 and another selective 5-HT_2A antagonist (M100,907) increased the length of individual seizures when injected alone. The 5-HT_2C agonists lorcaserin and CP-809,101 dose-dependently suppressed ASs, an effect blocked by the selective 5-HT_2C antagonist SB 242984. In summary, 5-HT_2ARs and 5-HT_2CRs negatively control the expression of experimental ASs, indicating that selective agonists at these 5-HT₂R subtypes might be potential novel anti-absence drugs.

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5-HT2AR activation decreases absence seizures in GAERS.

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5-HT2CR activation decreases absence seizures in GAERS.

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5-HT2AR blockade increases absence seizures in GAERS.

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5-HT2CR blockade does not affect absence seizures in GAERS.

Psychedelics

Psychedelics (serotonergic hallucinogens) are powerful psychoactive substances that alter perception and mood and affect numerous cognitive processes. They are generally considered physiologically safe and do not lead to dependence or addiction. Their origin predates written history, and they were employed by early cultures in many sociocultural and ritual contexts. After the virtually contemporaneous discovery of (5R,8R)-(+)-lysergic acid-N,N-diethylamide (LSD)-25 and the identification of serotonin in the brain, early research focused intensively on the possibility that LSD and other psychedelics had a serotonergic basis for their action. Today there is a consensus that psychedelics are agonists or partial agonists at brain serotonin 5-hydroxytryptamine 2A receptors, with particular importance on those expressed on apical dendrites of neocortical pyramidal cells in layer V. Several useful rodent models have been developed over the years to help unravel the neurochemical correlates of serotonin 5-hydroxytryptamine 2A receptor activation in the brain, and a variety of imaging techniques have been employed to identify key brain areas that are directly affected by psychedelics. Recent and exciting developments in the field have occurred in clinical research, where several double-blind placebo-controlled phase 2 studies of psilocybin-assisted psychotherapy in patients with cancer-related psychosocial distress have demonstrated unprecedented positive relief of anxiety and depression. Two small pilot studies of psilocybin-assisted psychotherapy also have shown positive benefit in treating both alcohol and nicotine addiction. Recently, blood oxygen level-dependent functional magnetic resonance imaging and magnetoencephalography have been employed for in vivo brain imaging in humans after administration of a psychedelic, and results indicate that intravenously administered psilocybin and LSD produce decreases in oscillatory power in areas of the brain's default mode network.

Insomnia Caused by Serotonin Depletion is Due to Hypothermia

STUDY OBJECTIVE

Serotonin (5-hydroxytryptamine, 5-HT) neurons are now thought to promote wakefulness. Early experiments using the tryptophan hydroxylase inhibitor para-chlorophenylalanine (PCPA) had led to the opposite conclusion, that 5-HT causes sleep, but those studies were subsequently contradicted by electrophysiological and behavioral data. Here we tested the hypothesis that the difference in conclusions was due to failure of early PCPA experiments to control for the recently recognized role of 5-HT in thermoregulation.

DESIGN

Adult male C57BL/6N mice were treated with PCPA (800 mg/kg intraperitoneally for 5 d; n = 15) or saline (n = 15), and housed at 20 °C (normal room temperature) or at 33 °C (thermoneutral for mice) for 24 h. In a separate set of experiments, mice were exposed to 4 °C for 4 h to characterize their ability to thermoregulate.

MEASUREMENTS AND RESULTS

PCPA treatment reduced brain 5-HT to less than 12% of that of controls. PCPA-treated mice housed at 20 °C spent significantly more time awake than controls. However, core body temperature decreased from 36.5 °C to 35.1 °C. When housed at 33 °C, body temperature remained normal, and total sleep duration, sleep architecture, and time in each vigilance state were the same as controls. When challenged with 4 °C, PCPA-treated mice experienced a precipitous drop in body temperature, whereas control mice maintained a normal body temperature.

CONCLUSIONS

These results indicate that early experiments using para-chlorophenylalanine that led to the conclusion that 5-hydroxytryptamine (5-HT) causes sleep were likely confounded by hypothermia. Temperature controls should be considered in experiments using 5-HT depletion.

5-HT2A receptor activation is necessary for CO2-induced arousal

Hypercapnia-induced arousal from sleep is an important protective mechanism pertinent to a number of diseases. Most notably among these are the sudden infant death syndrome, obstructive sleep apnea and sudden unexpected death in epilepsy. Serotonin (5-HT) plays a significant role in hypercapnia-induced arousal. The mechanism of 5-HT's role in this protective response is unknown. Here we sought to identify the specific 5-HT receptor subtype(s) involved in this response. Wild-type mice were pretreated with antagonists against 5-HT receptor subtypes, as well as antagonists against adrenergic, cholinergic, histaminergic, dopaminergic, and orexinergic receptors before challenge with inspired CO2 or hypoxia. Antagonists of 5-HT(2A) receptors dose-dependently blocked CO2-induced arousal. The 5-HT(2C) receptor antagonist, RS-102221, and the 5-HT1A receptor agonist, 8-OH-DPAT, attenuated but did not completely block CO2-induced arousal. Blockade of non-5-HT receptors did not affect CO2-induced arousal. None of these drugs had any effect on hypoxia-induced arousal. 5-HT2 receptor agonists were given to mice in which 5-HT neurons had been genetically eliminated during embryonic life (Lmx1b(f/f/p)) and which are known to lack CO2-induced arousal. Application of agonists to 5-HT(2A), but not 5-HT(2C), receptors, dose-dependently restored CO2-induced arousal in these mice. These data identify the 5-HT(2A) receptor as an important mediator of CO2-induced arousal and suggest that, while 5-HT neurons can be independently activated to drive CO2-induced arousal, in the absence of 5-HT neurons and endogenous 5-HT, 5-HT receptor activation can act in a permissive fashion to facilitate CO2-induced arousal via another as yet unidentified chemosensor system.

Analysis of sleep disorders under pain using an optogenetic tool: possible involvement of the activation of dorsal raphe nucleus-serotonergic neurons

Background

Several etiological reports have shown that chronic pain significantly interferes with sleep. Inadequate sleep due to chronic pain may contribute to the stressful negative consequences of living with pain. However, the neurophysiological mechanism by which chronic pain affects sleep-arousal patterns is as yet unknown. Although serotonin (5-HT) was proposed to be responsible for sleep regulation, whether the activity of 5-HTergic neurons in the dorsal raphe nucleus (DRN) is affected by chronic pain has been studied only infrequently. On the other hand, the recent development of optogenetic tools has provided a valuable opportunity to regulate the activity in genetically targeted neural populations with high spatial and temporal precision. In the present study, we investigated whether chronic pain could induce sleep dysregulation while changing the activity of DRN-5-HTergic neurons. Furthermore, we sought to physiologically activate the DRN with channelrhodopsin-2 (ChR2) to identify a causal role for the DRN-5-HT system in promoting and maintaining wakefulness using optogenetics.

Results

We produced a sciatic nerve ligation model by tying a tight ligature around approximately one-third to one-half the diameter of the sciatic nerve. In mice with nerve ligation, we confirmed an increase in wakefulness and a decrease in non-rapid eye movement (NREM) sleep as monitored by electroencephalogram (EEG). Microinjection of the retrograde tracer fluoro-gold (FG) into the prefrontal cortex (PFC) revealed several retrogradely labeled-cells in the DRN. The key finding of the present study was that the levels of 5-HT released in the PFC by the electrical stimulation of DRN neurons were significantly increased in mice with sciatic nerve ligation. Using optogenetic tools in mice, we found a causal relationship among DRN neuron firing, cortical activity and sleep-to-wake transitions. In particular, the activation of DRN-5-HTergic neurons produced a significant increase in wakefulness and a significant decrease in NREM sleep. The duration of NREM sleep episodes was significantly decreased during photostimulation in these mice.

Conclusions

These results suggest that neuropathic pain accelerates the activity of DRN-5-HTergic neurons. Although further loss-of-function experiments are required, we hypothesize that this activation in DRN neurons may, at least in part, correlate with sleep dysregulation under a neuropathic pain-like state.

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.

Serotonin control of sleep-wake behavior

Based on electrophysiological, neurochemical, genetic and neuropharmacological approaches, it is currently accepted that serotonin (5-HT) functions predominantly to promote wakefulness (W) and to inhibit REM (rapid eye movement) sleep (REMS). Yet, under certain circumstances the neurotransmitter contributes to the increase in sleep propensity. Most of the serotonergic innervation of the cerebral cortex, amygdala, basal forebrain (BFB), thalamus, preoptic and hypothalamic areas, raphe nuclei, locus coeruleus and pontine reticular formation comes from the dorsal raphe nucleus (DRN). The 5-HT receptors can be classified into at least seven classes, designated 5-HT(1-7). The 5-HT(1A) and 5-HT(1B) receptor subtypes are linked to the inhibition of adenylate cyclase, and their activation evokes a membrane hyperpolarization. The actions of the 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptor subtypes are mediated by the activation of phospholipase C, with a resulting depolarization of the host cell. The 5-HT(3) receptor directly activates a 5-HT-gated cation channel which leads to the depolarization of monoaminergic, aminoacidergic and cholinergic cells. The primary signal transduction pathway of 5-HT(6) and 5-HT(7) receptors is the stimulation of adenylate cyclase which results in the depolarization of the follower neurons. Mutant mice that do not express 5-HT(1A) or 5-HT(1B) receptor exhibit greater amounts of REMS than their wild-type counterparts, which could be related to the absence of a postsynaptic inhibitory effect on REM-on neurons of the laterodorsal and pedunculopontine tegmental nuclei (LDT/PPT). 5-HT(2A) and 5-HT(2C) receptor knock-out mice show a significant increase of W and a reduction of slow wave sleep (SWS) which has been ascribed to the increase of catecholaminergic neurotransmission involving mainly the noradrenergic and dopaminergic systems. Sleep variables have been characterized, in addition, in 5-HT(7) receptor knock-out mice; the mutants spend less time in REMS that their wild-type counterparts. Direct infusion of the 5-HT(1A) receptor agonists 8-OH-DPAT and flesinoxan into the DRN significantly enhances REMS in the rat. In contrast, microinjection of the 5-HT(1B) (CP-94253), 5-HT(2A/2C) (DOI), 5-HT(3) (m-chlorophenylbiguanide) and 5-HT(7) (LP-44) receptor agonists into the DRN induces a significant reduction of REMS. Systemic injection of full agonists at postsynaptic 5-HT(1A) (8-OH-DPAT, flesinoxan), 5-HT(1B) (CGS 12066B, CP-94235), 5-HT(2C) (RO 60-0175), 5-HT(2A/2C) (DOI, DOM), 5-HT(3) (m-chlorophenylbiguanide) and 5-HT(7) (LP-211) receptors increases W and reduces SWS and REMS. Of note, systemic administration of the 5-HT(2A/2C) receptor antagonists ritanserin, ketanserin, ICI-170,809 or sertindole at the beginning of the light period has been shown to induce a significant increase of SWS and a reduction of REMS in the rat. Wakefulness was also diminished in most of these studies. Similar effects have been described following the injection of the selective 5-HT(2A) receptor antagonists volinanserin and pruvanserin and of the 5-HT(2A) receptor inverse agonist nelotanserin in rodents. In addition, the effects of these compounds have been studied on the sleep electroencephalogram of subjects with normal sleep. Their administration was followed by an increase of SWS and, in most instances, a reduction of REMS. The administration of ritanserin to poor sleepers, patients with chronic primary insomnia and psychiatric patients with a generalized anxiety disorder or a mood disorder caused a significant increase in SWS. The 5-HT(2A) receptor inverse agonist APD-125 induced also an increase of SWS in patients with chronic primary insomnia. It is known that during the administration of benzodiazepine (BZD) hypnotics to patients with insomnia there is a further reduction of SWS and REMS, whereas both variables tend to remain decreased during the use of non-BZD derivatives (zolpidem, zopiclone, eszopiclone, zaleplon). Thus, the association of 5-HT(2A) antagonists or 5-HT(2A) inverse agonists with BZD and non-BZD hypnotics could be a valid alternative to normalize SWS in patients with primary or comorbid insomnia.

Role of the 5-HT7 receptor in the central nervous system: from current status to future perspectives

Pharmacological and genetic tools targeting the 5-hydroxytryptamine (5-HT)7 receptor in preclinical animal models have implicated this receptor in diverse (patho)physiological processes of the central nervous system (CNS). Some data obtained with 5-HT7 receptor knockout mice, selective antagonists, and, to a lesser extent, agonists, however, are quite contradictory. In this review, we not only discuss in detail the role of the 5-HT7 receptor in the CNS but also propose some hypothetical models, which could explain the observed inconsistencies. These models are based on two novel concepts within the field of G protein-coupled receptors (GPCR), namely biphasic signaling and G protein-independent signaling, which both have been shown to be mediated by GPCR dimerization. This led us to suggest that the 5-HT7 receptor could reside in different dimeric contexts and initiate different signaling pathways, depending on the neuronal circuitry and/or brain region. In conclusion, we highlight GPCR dimerization and G protein-independent signaling as two promising future directions in 5-HT7 receptor research, which ultimately might lead to the development of more efficient dimer- and/or pathway-specific therapeutics.

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.

5-HT(2A) and 5-HT(2C) receptor stimulation are differentially involved in the cortical dopamine efflux-Studied in 5-HT(2A) and 5-HT(2C) genetic mutant mice

Both 5-HT(2A) and 5-HT(2C) receptors modulate cortical dopamine efflux, but in opposite directions. We have now compared the ability of the three 5-HT(2A/2C) receptor agonists, DOI (R(-)-2,5-dimethoxy-4-iodoamphetamine), mCPP (meta-chlorophenylpiperazine) and MK-212 (6-Chloro-2-(piperazinyl) pyrazine), to modulate cortical dopamine efflux in 5-HT(2A) and 5-HT(2C) genetic mutant mice. In the 5-HT(2A) mice, the preferential 5-HT(2A) receptor agonist DOI (2.5mg/kg, s.c.) induced a slight but significant increase in cortical dopamine efflux only in the wild type (WT) mice; MK-212 (2.5mg/kg) reduced dopamine efflux in both WT and receptor knockout (KO) mice; moreover, MCPP, 2.5mg/kg, had no effect in either types. In 5-HT(2C) mice, DOI increased dopamine efflux in both types; while MK-212 decreased dopamine efflux in the WT, but not the receptor KO mice. These results provide new evidence that 5-HT(2A) receptor stimulation enhances and 5-HT(2C) receptor stimulation inhibits cortical dopamine efflux, and suggest the effects of DOI, MK-212 and mCPP on the cortical dopamine efflux are due to their different abilities on 5-HT(2A) and 5-HT(2C) receptors stimulation. Of these three agents, only DOI, the more selective 5-HT(2A) receptor agonist, is hallucinogenic. The absence of hallucinations with mCPP may be due to its relatively more potent 5-HT(2C) receptor agonist effect, inhibiting the ability of mCPP to enhance dopamine efflux in cortical and perhaps limbic regions as well. The present data provide additional evidence that hallucinations are due, in part, to 5-HT(2A) rather than 5-HT(2C) receptor stimulation. These findings suggest that 5-HT(2C) receptor agonists may be useful as antipsychotics, consistent with previous suggestions.

Synthesis and in vivo evaluation of phenethylpiperazine amides: selective 5-hydroxytryptamine(2A) receptor antagonists for the treatment of insomnia

Recent developments in sleep research suggest that antagonism of the serotonin 5-HT(2A) receptor may improve sleep maintenance insomnia. We herein report the discovery of a series of potent and selective serotonin 5-HT(2A) receptor antagonists based on a phenethylpiperazine amide core structure. When tested in a rat sleep pharmacology model, these compounds increased both sleep consolidation and deep sleep. Within this series of compounds, an improvement in the metabolic stability of early leads was achieved by introducing a carbonyl group into the phenethylpiperazine linker. Of note, compounds 14 and 27 exhibited potent 5-HT(2A) receptor binding affinity, high selectivity over the 5-HT(2C) receptor, favorable CNS partitioning, and good pharmacokinetic and early safety profiles. In vivo, these two compounds showed dose-dependent, statistically significant improvements on deep sleep (delta power) and sleep consolidation at doses as low as 0.1 mg/kg.

Role of 5-HT2A receptor antagonists in the treatment of insomnia

Insomnia encompasses a difficulty in falling asleep (sleep-onset insomnia) and/or a difficulty in staying asleep (SMI). Several selective serotonin-2A (5-HT2A) receptor antagonists have been in development as potential treatments for SMI. However, none have shown a sufficiently robust benefit-to-risk ratio, and none have reached market approval. We review the role of the 5-HT2A mechanism in sleep, the preclinical and clinical data supporting a role for 5-HT2A receptor antagonism in improving sleep maintenance, and the status of 5-HT2A receptor antagonists in clinical development. Overall, the polysomnography data strongly support an increase in slow-wave sleep and a decrease in waking after sleep onset following treatment with 5-HT2A receptor antagonists, although it has been more difficult to show subjective improvements in sleep with these agents. The incidence and prevalence of SMI, whether primary or secondary to psychiatric, neurologic, or other medical conditions, will increase as our population ages. There will be an increased need for safe and efficacious treatments of insomnia characterized by difficulty maintaining sleep, and there remains much promise for 5-HT2A receptor antagonism to play a role in these future treatments.

Nelotanserin, a novel selective human 5-hydroxytryptamine2A inverse agonist for the treatment of insomnia

5-Hydroxytryptamine (5-HT)(2A) receptor inverse agonists are promising therapeutic agents for the treatment of sleep maintenance insomnias. Among these agents is nelotanserin, a potent, selective 5-HT(2A) inverse agonist. Both radioligand binding and functional inositol phosphate accumulation assays suggest that nelotanserin has low nanomolar potency on the 5-HT(2A) receptor with at least 30- and 5000-fold selectivity compared with 5-HT(2C) and 5-HT(2B) receptors, respectively. Nelotanserin dosed orally prevented (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 5-HT(2A) agonist)-induced hypolocomotion, increased sleep consolidation, and increased total nonrapid eye movement sleep time and deep sleep, the latter marked by increases in electroencephalogram (EEG) delta power. These effects on rat sleep were maintained after repeated subchronic dosing. In healthy human volunteers, nelotanserin was rapidly absorbed after oral administration and achieved maximum concentrations 1 h later. EEG effects occurred within 2 to 4 h after dosing, and were consistent with vigilance-lowering. A dose response of nelotanserin was assessed in a postnap insomnia model in healthy subjects. All doses (up to 40 mg) of nelotanserin significantly improved measures of sleep consolidation, including decreases in the number of stage shifts, number of awakenings after sleep onset, microarousal index, and number of sleep bouts, concomitant with increases in sleep bout duration. Nelotanserin did not affect total sleep time, or sleep onset latency. Furthermore, subjective pharmacodynamic effects observed the morning after dosing were minimal and had no functional consequences on psychomotor skills or memory. These studies point to an efficacy and safety profile for nelotanserin that might be ideally suited for the treatment of sleep maintenance insomnias.

Influence of the novel antidepressant and melatonin agonist/serotonin2C receptor antagonist, agomelatine, on the rat sleep-wake cycle architecture

RATIONALE

The novel antidepressant, agomelatine, behaves as an agonist at melatonin MT(1) and MT(2) receptors and as an antagonist at serotonin (5-HT)(2C) receptors. In animal models and clinical trials, agomelatine displays antidepressant properties and re-synchronizes disrupted circadian rhythms.

OBJECTIVES

The objectives of this study were to compare the influence of agomelatine upon sleep-wake states to the selective melatonin agonists, melatonin and ramelteon, and to the selective 5-HT(2C) receptor antagonist, S32006.

METHODS

Rats were administered with vehicle, agomelatine, ramelteon, melatonin, or S32006, at the onset of either dark or light periods. Polygraphic recordings were performed and changes determined over 24 h, i.e., number and duration of sleep-wake episodes, latencies to rapid eye movement (REM) and slow-wave (SWS) sleep, power band spectra of the electroencephalogram (EEG), and circadian changes.

RESULTS

Administered at light phase onset, no changes were induced by agomelatine. In contrast, administered shortly before dark phase, agomelatine (10 and 40 mg/kg, per os) enhanced duration of REM and SWS sleep and decreased wake state for 3 h. Melatonin (10 mg/kg, per os) induced a transient enhancement in REM sleep followed by a reduction in REM and SWS sleep and an increase in waking. Ramelteon (10 mg/kg, per os) provoked a transient increase in REM sleep. Finally, S32006 (10 mg/kg, intraperitoneally), administered at dark phase onset, mimicked the increased SWS provoked by agomelatine, yet diminished REM sleep.

CONCLUSIONS

Agomelatine possesses a distinctive EEG profile compared with melatonin, ramelteon, and S32006, possibly reflecting co-joint agonist and antagonist properties at MT(1)/MT(2) and 5-HT(2C) receptors, respectively.

APD125, a selective serotonin 5-HT(2A) receptor inverse agonist, significantly improves sleep maintenance in primary insomnia

INTRODUCTION

Insomnia is a condition affecting 10% to 15% of the adult population and is characterized by difficulty falling asleep, difficulty staying asleep, or nonrestorative sleep, accompanied by daytime impairment or distress. This study evaluates APD125, a selective inverse agonist of the 5-HT(2A) receptor, for treatment of chronic insomnia, with particular emphasis on sleep maintenance. In phase 1 studies, APD125 improved sleep maintenance and was well tolerated.

METHODOLOGY

Adult subjects (n=173) with DSM-IV defined primary insomnia were randomized into a multicenter, double-blind, placebo-controlled, 3-way crossover study to compare 2 doses of APD125 (10 mg and 40 mg) with placebo. Each treatment period was 7 days with a 7- to 9-day washout period between treatments. Polysomnographic recordings were performed at the initial 2 screening nights and at nights (N) 1/2 and N 6/7 of each treatment period.

RESULTS

APD125 was associated with significant improvements in key sleep maintenance parameters measured by PSG. Wake time after sleep onset decreased (SEM) by 52.5 (3.2) min (10 mg) and 53.5 (3.5) min (40 mg) from baseline to N 1/2 vs. 37.8 (3.4) min for placebo, (P < 0.0001 for both doses vs. placebo), and by 51.7 (3.4) min (P = 0.01) and 48.0 (3.6) min (P = 0.2) at N 6/7 vs. 44.0 (3.8) min for placebo. Significant APD125 effects on wake time during sleep were also seen (P < 0.0001 N 1/2, P < 0.001 N 6/7). The number of arousals and number of awakenings decreased significantly with APD125 treatment compared to placebo. Slow wave sleep showed a statistically significant dose-dependent increase. There was no significant decrease in latency to persistent sleep. No serious adverse events were reported, and no meaningful differences in adverse event profiles were observed between either dose of APD125 and placebo. APD125 was not associated with next-day psychomotor impairment as measured by Digit Span, Digit Symbol Copy, and Digit Symbol Coding Tests.

CONCLUSIONS

APD125 produced statistically significant improvements in objective parameters of sleep maintenance and sleep consolidation and was well tolerated in adults with primary chronic insomnia.