Effects of ipsapirone, a 5-HT1A agonist, on sleep/wakefulness cycles: probable post-synaptic action

Sleep disturbances in autism spectrum disorder: Animal models, neural mechanisms, and therapeutics

Sleep is crucial for brain development. Sleep disturbances are prevalent in children with autism spectrum disorder (ASD). Strikingly, these sleep problems are positively correlated with the severity of ASD core symptoms such as deficits in social skills and stereotypic behavior, indicating that sleep problems and the behavioral characteristics of ASD may be related. In this review, we will discuss sleep disturbances in children with ASD and highlight mouse models to study sleep disturbances and behavioral phenotypes in ASD. In addition, we will review neuromodulators controlling sleep and wakefulness and how these neuromodulatory systems are disrupted in animal models and patients with ASD. Lastly, we will address how the therapeutic interventions for patients with ASD improve various aspects of sleep. Together, gaining mechanistic insights into the neural mechanisms underlying sleep disturbances in children with ASD will help us to develop better therapeutic interventions.

The GABAergic Gudden's dorsal tegmental nucleus: A new relay for serotonergic regulation of sleep-wake behavior in the mouse

Serotonin (5-HT) neurons are involved in wake promotion and exert a strong inhibitory influence on rapid eye movement (REM) sleep. Such effects have been ascribed, at least in part to the action of 5-HT at post-synaptic 5-HT_1A receptors (5-HT_1AR) in the brainstem, a major wake/REM sleep regulatory center. However, the neuroanatomical substrate through which 5-HT_1AR influence sleep remains elusive. We therefore investigated whether a brainstem structure containing a high density of 5-HT_1AR mRNA, the GABAergic Gudden's dorsal tegmental nucleus (DTg), may contribute to 5-HT-mediated regulatory mechanisms of sleep-wake stages. We first found that bilateral lesions of the DTg promote wake at the expense of sleep. In addition, using local microinjections into the DTg in freely moving mice, we showed that local activation of 5-HT_1AR by the prototypical agonist 8-OH-DPAT enhances wake and reduces deeply REM sleep duration. The specific involvement of 5-HT_1AR in the latter effects was further demonstrated by ex vivo extracellular recordings showing that the selective 5-HT_1AR antagonist WAY 100635 prevented DTg neuron inhibition by 8-OH-DPAT. We next found that GABAergic neurons of the ventral DTg exclusively targets glutamatergic neurons of the lateral mammillary nucleus (LM) in the posterior hypothalamus by means of anterograde and retrograde tracing techniques using cre driver mouse lines and a modified rabies virus. Altogether, our findings strongly support the idea that 5-HT-driven enhancement of wake results from 5-HT_1AR-mediated inhibition of DTg GABAergic neurons that would in turn disinhibit glutamatergic neurons in the mammillary bodies. We therefore propose a Raphe→DTg→LM pathway as a novel regulatory circuit underlying 5-HT modulation of arousal.

The serotonin hallucinogen 5-MeO-DMT alters cortico-thalamic activity in freely moving mice: Regionally-selective involvement of 5-HT1A and 5-HT2A receptors

5-MeO-DMT is a natural hallucinogen acting as serotonin 5-HT_1A/5-HT_2A receptor agonist. Its ability to evoke hallucinations could be used to study the neurobiology of psychotic symptoms and to identify new treatment targets. Moreover, recent studies revealed the therapeutic potential of serotonin hallucinogens in treating mood and anxiety disorders. Our previous results in anesthetized animals show that 5-MeO-DMT alters cortical activity via 5-HT_1A and 5-HT_2A receptors. Here, we examined 5-MeO-DMT effects on oscillatory activity in prefrontal (PFC) and visual (V1) cortices, and in mediodorsal thalamus (MD) of freely-moving wild-type (WT) and 5-HT_2A-R knockout (KO2A) mice. We performed local field potential multi-recordings evaluating the power at different frequency bands and coherence between areas. We also examined the prevention of 5-MeO-DMT effects by the 5-HT_1A-R antagonist WAY-100635. 5-MeO-DMT affected oscillatory activity more in cortical than in thalamic areas. More marked effects were observed in delta power in V1 of KO2A mice. 5-MeO-DMT increased beta band coherence between all examined areas. In KO2A mice, WAY100635 prevented most of 5-MeO-DMT effects on oscillatory activity. The present results indicate that hallucinatory activity of 5-MeO-DMT is likely mediated by simultaneous alteration of prefrontal and visual activities. The prevention of these effects by WAY-100635 in KO2A mice supports the potential usefulness of 5-HT_1A receptor antagonists to treat visual hallucinations. 5-MeO-DMT effects on PFC theta activity and cortico-thalamic coherence may be related to its antidepressant activity. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

NREM sleep hypersomnia and reduced sleep/wake continuity in a neuroendocrine mouse model of anxiety/depression based on chronic corticosterone administration

Sleep/wake disorders are frequently associated with anxiety and depression and to elevated levels of cortisol. Even though these alterations are increasingly sought in animal models, no study has investigated the specific effects of chronic corticosterone (CORT) administration on sleep. We characterized sleep/wake disorders in a neuroendocrine mouse model of anxiety/depression, based on chronic CORT administration in the drinking water (35 μg/ml for 4 weeks, "CORT model"). The CORT model was markedly affected during the dark phase by non-rapid eye movement sleep (NREM) increase without consistent alteration of rapid eye movement (REM) sleep. Total sleep duration (SD) and sleep efficiency (SE) increased concomitantly during both the 24h and the dark phase, due to the increase in the number of NREM sleep episodes without a change in their mean duration. Conversely, the total duration of wake decreased due to a decrease in the mean duration of wake episodes despite an increase in their number. These results reflect hypersomnia by intrusion of NREM sleep during the active period as well as a decrease in sleep/wake continuity. In addition, NREM sleep was lighter, with an increased electroencephalogram (EEG) theta activity. With regard to REM sleep, the number and the duration of episodes decreased, specifically during the first part of the light period. REM and NREM sleep changes correlated respectively with the anxiety and the anxiety/depressive-like phenotypes, supporting the notion that studying sleep could be of predictive value for altered emotional behavior. The chronic CORT model in mice that displays hallmark characteristics of anxiety and depression provides an insight into understanding the changes in overall sleep architecture that occur under pathological conditions.

Role of the serotonergic system in alcohol dependence: from animal models to clinics

Alcohol dependence remains among the most common substance abuse problems worldwide, and compulsive alcohol consumption is a significant public health concern. Alcohol is an addictive drug that alters brain function through interactions with multiple neurotransmitter systems. These neurotransmitter systems mediate the reinforcing effects of alcohol. Specifically, the serotonergic system is important in mediating alcohol reward, preference, dependence, and craving. In this review chapter, we first discuss the serotonin system as it relates to alcoholism, and then outline interactions between this system and other neurotransmitter systems. We emphasize the serotonin transporter and its possible role in alcoholism, then present several serotonergic receptors and discuss their contribution to alcoholism, and finally assess the serotonin system as a target for pharmacotherapy, with an emphasis on current and potential treatments.

Ondansetron and fluoxetine reduce sleep apnea in mice lacking monoamine oxidase A

Prospective clinical trials addressing the role of serotonin (5-HT) in sleep apnea have indicated that the 5-HT uptake inhibitor fluoxetine is beneficial to some patients with obstructive apnea, whereas the 5-HT(3) receptor antagonist ondansetron seems of little value despite its efficacy in rat and dog models of sleep apnea (central and obstructive). Here, we examined the effect of these drugs in transgenic mice lacking monoamine oxidase A (Tg8), which exhibit approximately 3-fold higher rates of central sleep apnea than their wild-type counterparts (C3H), linked to their enhanced 5-HT levels. Acute ondansetron (2 mg kg(-1), intraperitoneal), acute fluoxetine (16 mg kg(-1)) and 13-day chronic fluoxetine (1 or 16 mg kg(-1)) decreased by approximately 80% the total (spontaneous and post-sigh) apnea index in Tg8 mice during non-rapid eye movement sleep, with no statistically significant effect on apnea in C3H mice. Our study shows that both drugs reduce the frequency of apneic episodes attributable to increased monoamine levels in this model of MAOA deficiency, and suggests that both may be effective in some patients with central sleep apneas.

5-HT7 receptor deletion enhances REM sleep suppression induced by selective serotonin reuptake inhibitors, but not by direct stimulation of 5-HT1A receptor

5-HT(7) receptors are involved in REM sleep and possibly in mood disorders. REM sleep suppression and antidepressant-like behavior is observed in 5-HT(7)(-/-) mice and in rats treated with 5-HT(7) receptor antagonists. We recently demonstrated that pharmacological blockade of 5-HT(7) receptors enhances REM sleep suppression and antidepressant-like behavior induced by citalopram in rodents. It has been hypothesized that the effect of citalopram on sleep is essentially mediated by the activation of 5-HT(1A) receptors. The present study investigates the impact of 5-HT(7) receptor gene deletion on the effect of various reuptake inhibitors on REM sleep and probes the role of 5-HT(1A) receptors in this response. Three SSRIs (citalopram, fluoxetine and paroxetine) but not the tricyclic antidepressant desipramine had a significantly stronger REM sleep suppressive effect in 5-HT(7)(-/-) mice compared to 5-HT(7)(+/+) mice. In contrast, REM sleep was similarly reduced in 5-HT(7)(+/+) mice and 5-HT(7)(-/-) mice after treatment with the 5-HT(1A) receptor agonist ipsapirone. Furthermore, both 5-HT(7)(+/+) and 5-HT(7)(-/-) mice displayed the same increase in REM sleep duration produced by the 5-HT(1A) receptor antagonist WAY-100635. These findings indicate that 5-HT(7) receptor deletion augments the effect of various SSRIs on REM sleep suppression and that this effect is distinct from those mediated via 5-HT(1A) receptors.

The involvement of serotonin receptors in suanzaorentang-induced sleep alteration

Sedative-hypnotic medications, including benzodiazepines and non-benzodiazepines, are usually prescribed for the insomniac patients; however, the addiction, dependence and adverse effects of those medications have drawn much attention. In contrast, suanzaorentang, a traditional Chinese herb remedy, has been efficiently used for insomnia relief in China, although its mechanism remains unclear. This study was designed to further elucidate the underlying mechanism of suanzaorentang on sleep regulation. One ingredient of suanzaorentang, zizyphi spinosi semen, exhibits binding affinity for serotonin (5-hydroxytryptamine, 5-HT) receptors, 5-HT(1A) and 5-HT(2), and for GABA receptors. Our previous results have implicated that GABA(A) receptors, but not GABA(B), mediate suanzaorentang-induced sleep alteration. In current study we further elucidated the involvement of serotonin. We found that high dose of suanzaorentang (4 g/kg/2 ml) significantly increased non-rapid eye movement sleep (NREMS) when comparing to that obtained after administering starch placebo, although placebo at dose of 4 g/kg also enhanced NREMS comparing with that obtained from baseline recording. Rapid eye movement sleep (REMS) was not altered. Administration of either 5-HT(1A) antagonist (NAN-190), 5-HT(2) antagonist (ketanserin) or 5-HT(3 )antagonist (3-(4-Allylpiperazin-1-yl)-2-quinoxalinecarbonitrile) blocked suanzaorentang-induced NREMS increase. These results implicate the hypnotic effect of suanzaorentang and its effects may be mediated through serotonergic activation, in addition to GABAergic system.

Homeostatic regulation of sleep in a genetic model of depression in the mouse: effects of muscarinic and 5-HT1A receptor activation

In depressed patients, sleep undergoes marked alterations, especially sleep onset insomnia, sleep fragmentation, and disturbances of the Rapid Eye Movement (REM) sleep. Abnormalities of rest-activity rhythms and of hypothalamic-pituitary-adrenocortical function have also been described in these patients. In the present study, we examined the presence of such abnormalities in a recently developed line of mice (Helpless mice-H) that exhibit depression-like behaviors in validated tests, compared to the nonhelpless (NH) line derived from the same colony. Experiments were essentially carried out in females for which previous studies showed marked differences between H and NH lines. Compared to NH mice, the H line exhibited (i) lower basal locomotor activity, (ii) sleep fragmentation, shift towards lighter sleep stages, and facilitation of REM sleep reflected by increased amounts and decreased latency, (iii) larger response to the REM sleep promoting effect of muscarinic receptor stimulation (by arecoline). In contrast, H and NH mice were equally responsive to the REM sleep inhibitory effect of 5-HT1A receptor stimulation (by 8-OH-DPAT). In addition, a deficiency in delta power enhancement after sleep deprivation was observed in the H group, and acute immobilization stress in this group failed to elicit a REM sleep rebound and was associated with a long-lasting raise in serum corticosterone levels. These results further validate H mice as a depression model and suggest they might be of particular interest for investigating the neurobiological mechanisms and possibly genetic substrates underlying sleep alterations associated with depression.

Early life blockade of 5-hydroxytryptamine 1A receptors normalizes sleep and depression-like behavior in adult knock-out mice lacking the serotonin transporter

In serotonin transporter knock-out (5-HTT-/-) mice, extracellular serotonin (5-HT) levels are markedly elevated in the brain, and rapid eye movement sleep (REMS) is enhanced compared with wild-type mice. We hypothesized that such sleep impairment at adulthood results from excessive serotonergic tone during early life. Thus, we assessed whether neonatal treatment with drugs capable of limiting the impact of 5-HT on the brain could normalize sleep patterns in 5-HTT-/- mutants. We found that treatments initiated at postnatal day 5 and continued for 2 weeks with the 5-HT synthesis inhibitor para-chlorophenylalanine, or for 4 weeks with the 5-HT(1A) receptor (5-HT(1A)R) antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635), induced total or partial recovery of REMS, respectively, in 5-HTT-/- mutants. Early life treatment with WAY 100635 also reversed the depression-like behavior otherwise observed in these mutants. Possible adaptive changes in 5-HT(1A)R after neonatal treatment with WAY 100635 were investigated by measuring 5-HT(1A) binding sites and 5-HT(1A) mRNA in various REMS- and/or depression-related brain areas, as well as 5-HT(1A)R-mediated hypothermia and inhibition of neuronal firing in the dorsal raphe nucleus. None of these characteristics were modified in parallel with REMS recovery, suggesting that 5-HT(1A)Rs involved in wild-type phenotype rescue in 5-HTT-/- mutants are located in other brain areas or in 5-HT(1A)R-unrelated circuits where they could be transiently expressed during development. The reversal of sleep alterations and depression-like behavior after early life blockade of 5-HT(1A)R in 5-HTT-/- mutants might open new perspectives regarding preventive care of sleep and mood disorders resulting from serotonin transporter impairments during development.

Role of alpha and beta adrenoceptors in locus coeruleus stimulation-induced reduction in rapid eye movement sleep in freely moving rats

Based on the results of independent studies the involvement of norepinephrine in REM sleep regulation was known. Isolated studies showed that the effect could be mediated through either one or more subtypes of adrenoceptors. Earlier we have reported that REM-OFF neurons continue firing during REM sleep deprivation and mild but continuous stimulation of locus coeruleus (LC) or picrotoxin injection into the LC, that did not allow the REM-OFF neurons in the LC to stop firing, reduced REM sleep. However, the mechanism of action and type of adrenoreceptors involved in REM sleep regulation were unknown. The possible mechanism of action has been investigated in this study. It was proposed that if LC stimulation-induced decrease in REM sleep was due to norepinephrine, adrenergic antagonist must prevent the effect. Therefore, in this study, the effects of alpha1, alpha2 and beta-antagonists, viz. prazosin, yohimbine and propranolol, respectively, and alpha2 agonist, clonidine, on LC stimulation-induced reduction in REM sleep were investigated. The results showed that stimulation of LC inhibited REM sleep by reducing the frequency of generation of REM sleep, although the duration per episode remained unaffected. This decrease in the frequency of REM sleep was blocked by beta-antagonist propranolol while the duration of REM sleep per episode was blocked by alpha1-antagonist, prazosin. Also, a critical level of norepinephrine in the system was required for the generation of REM sleep, however, a higher level may be inhibitory. Based on the results of this study and our earlier studies, an interaction between neurons, containing different neurotransmitters and their subtypes of receptors for LC-mediated regulation of REM sleep has been proposed.

Serotonergic regulation of the orexin/hypocretin neurons through the 5-HT1A receptor

Both orexin and serotonin (5-HT) have important roles in the regulation of sleep-wakefulness, as well as in feeding behavior. We examined the effects of 5-HT on orexin/hypocretin neurons, using hypothalamic slices prepared from orexin/enhanced green fluorescent protein (EGFP) transgenic mice in which EGFP is expressed exclusively in orexin neurons. Patch-clamp recording from EGFP-expressing cells showed that 5-HT hyperpolarized all orexin neurons in a concentration-dependent manner. The response was inhibited by the 5-HT1A receptor antagonist WAY100635. A 5-HT1A receptor agonist, 8-hydroxy-2-(dl-N-propyl-amino)tetralin, also evoked hyperpolarization on orexin neurons with potency comparable with 5-HT. A low concentration of Ba2+ (30 microM) inhibited 5-HT-induced hyperpolarization. Single-channel recording revealed that the conductance of 5-HT-induced channel activity was 33.8 pS, which is in good agreement with that of the G-protein-coupled inward rectifier potassium channel (GIRK). Moreover, 5-HT1A receptor-like immunoreactivity was observed on orexin neurons, and 5-HT transporter immunoreactive nerve endings are in close apposition to orexin neurons. Intracerebroventricular injection of the 5-HT1A receptor-selective antagonist WAY100635 (100 ng) increased locomotor activity during the latter half of dark phase in wild-type mice but not in orexin/ataxin-3 mice in which orexin neurons are specifically ablated, suggesting that activation of orexin neurons is necessary for the WAY100635-induced increase in locomotor activity. These results indicate that 5-HT hyperpolarizes orexin neurons through the 5-HT1A receptor and subsequent activation of the GIRK and that this inhibitory serotonergic input to the orexin neurons is likely to be important for the physiological regulation of this neuropeptide system.

The sleep inducing brain lipid cis-oleamide (cOA) does not modulate serotonergic transmission in the CA1 pyramidal neurons of the hippocampus in vitro

cis-Oleamide (cOA) is a novel sleep inducing brain lipid with an unknown mechanism of action. High affinity interactions with metabotropic 5-HT receptors (2A/C and 1A subtypes) in frog oocytes and expression systems have been reported, but functional in vitro evidence for the modulatory effect is still lacking. Here, we addressed the ability of cOA to modulate 5-HT-induced cellular actions in the CA1 neurons of the rat hippocampal slice.5-HT (0.1-100 microM) concentration dependently reduced the amplitude of the evoked field population spike (fPS), and produced a hyperpolarising shift in the resting membrane potential (Vr) and a drop in input resistance (R in). The effects of a low dose of 5-HT (3.2 microM) on fPS, Vr and R in were reversed by the specific 5-HT(1A)-receptor antagonist WAY 100135 (10 microM). cOA (1 microM) failed to potentiate 5-HT1A receptor mediated effects on fPS, Vr or R in. High doses of 5-HT also recruited both 5-HT2 and 5-HT3 receptors, causing an increase in the rate and amplitude of sIPSCs. cOA (1 microM), in the presence of Y 25130, failed to potentiate the 5-HT2 receptor induced enhancement of sIPSCs. In summary, cis-oleamide failed to modulate metabotropic responses to exogenous 5-HT in this microelectrode study at concentrations well in excess of those reported to modulate 5-HT1A and 5-HT2A/C systems in earlier studies.

A serotonergic (5-HT2) receptor mechanism in the laterodorsal tegmental nucleus participates in regulating the pattern of rapid-eye-movement sleep occurrence in the rat

Serotonin [5-hydroxytryptamine (5-HT)] plays an inhibitory role in rapid-eye-movement (REM) sleep although the exact mechanism(s) and site(s) of action are not known. It is commonly assumed that 5-HT exerts its influence on REM sleep via input from the dorsal raphe nucleus (DRN) directly onto cholinergic neurons involved in the generation of REM sleep. 5-HT(2) receptor sites have been found on cholinergic neurons in the laterodorsal tegmental nucleus (LDT) and pedunculopontine tegmental nucleus (PPT). We locally microinjected the 5-HT(2) agonist DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl) and the 5-HT(2) antagonist, ketanserin, in LDT in rats to determine whether these receptor sites are involved in the regulation of behavioral states. DOI and ketanserin primarily affected REM sleep, by significantly decreasing or increasing, respectively, the number, but not the duration, of REM sleep episodes. DOI specifically decreased the occurrence of clusters of REM sleep episodes appearing at intervals less than or equal to 3 min (sequential episodes) without affecting single episodes separated by more than 3 min. An opposite effect of ketanserin on REM sleep clusters, although not statistically significant, was observed.

5-HT 1A/1B receptor-mediated effects of the selective serotonin reuptake inhibitor, citalopram, on sleep: studies in 5-HT 1A and 5-HT 1B knockout mice

Selective serotonin reuptake inhibitors (SSRIs) are extensively used for the treatment of depression. Aside from their antidepressant properties, they provoke a deficit in paradoxical sleep (PS) that is most probably mediated by the transporter blockade-induced increase in serotonin concentration in the extracellular space. Such an effect can be accounted for by the action of serotonin at various types of serotonergic receptors involved in PS regulation, among which the 5-HT(1A) and 5-HT(1B) types are the best candidates. According to this hypothesis, we examined the effects of citalopram, the most selective SSRI available to date, on sleep in the mouse after inactivation of 5-HT(1A) or 5-HT(1B) receptors, either by homologous recombination of their encoding genes, or pharmacological blockade with selective antagonists. For this purpose, sleep parameters of knockout mice that do not express these receptors and their wild-type counterparts were monitored during 8 h after injection of citalopram alone or in association with 5-HT(1A) or 5-HT(1B) receptor antagonists. Citalopram induced mainly a dose-dependent inhibition of PS during 2-6 h after injection, which was observed in wild-type and 5-HT(1B)-/- mice, but not in 5-HT(1A)-/- mutants. This PS inhibition was fully antagonized by pretreatment with the 5-HT(1A) antagonist WAY 100635, but only partially with the 5-HT(1B) antagonist GR 127935. These data indicate that the action of the SSRI citalopram on sleep in the mouse is essentially mediated by 5-HT(1A) receptors. Such a mechanism of action provides further support to the clinical strategy of antidepressant augmentation by 5-HT(1A) antagonists, because the latter would also counteract the direct sleep-inhibitory side-effects of SSRIs.

Involvement of 5-HT1A receptors in homeostatic and stress-induced adaptive regulations of paradoxical sleep: studies in 5-HT1A knock-out mice

For the last two decades, the involvement of 5-HT(1A) receptors in the regulation of vigilance states has been studied extensively thanks to pharmacological tools, but clear-cut conclusion has not been reached yet. By studying mutant mice that do not express this receptor type (5-HT(1A)-/-) and their wild-type 129/Sv counterparts, we herein demonstrate that 5-HT(1A) receptors play key roles in the control of spontaneous sleep-wakefulness cycles, as well as in homeostatic regulation and stress-induced adaptive changes of paradoxical sleep. Both strains of mice exhibited a diurnal sleep-wakefulness rhythm, but 5-HT(1A)-/- animals expressed higher amounts of paradoxical sleep than wild-type mice during both the light and the dark phases. In wild-type mice, pharmacological blockade of 5-HT(1A) receptors by WAY 100635 (0.5 mg/kg, i.p.) promoted paradoxical sleep, whereas the 5-HT(1A) agonist 8-OH-DPAT (0.25-1 mg/kg, s.c.) had an opposite effect. In contrast, none of the 5-HT(1A) receptor ligands affected sleep significantly in 5-HT(1A)-/- mice. However, 5-HT(1B) receptor stimulation by CP 94253 (1-3 mg/kg, i.p.) induced a reduction in paradoxical sleep in both strains, this effect being more pronounced in 5-HT(1A)-/- mutants. Finally, in contrast to wild-type mice, 5-HT(1A)-/- mutants did not exhibit any rebound of paradoxical sleep after either a 9 hr instrumental paradoxical sleep deprivation or a 90 min immobilization stress. Altogether, these data indicate that, in the mouse, 5-HT(1A) receptors participate in the spontaneous and homeostatic regulation, as well as in stress-induced adaptive changes of paradoxical sleep.

Serotonin and sleep

For 50 years, serotonin has been in the centre of the search for the mechanisms and control of sleep. Serotonergic neurotransmission is related to the behavioural state of the animal and plays an important role in modulation of the behavioural state, by interacting with other brain areas modulating circadian rhythm, sleep and waking. Serotonergic activity may be accompanied by waking or sleep depending on the brain area and receptor type involved in the response, on the current behavioural state and on the concomitant agonism/antagonism of other neurotransmitter systems.

Sleep and waking following microdialysis perfusion of the selective 5-HT1A receptor antagonist p-MPPI into the dorsal raphe nucleus in the freely moving rat

The aim of this study was to examine the involvement of the dorsal raphe nucleus (DRN) presynaptic serotonergic 5-HT1A autoreceptors on sleep and waking parameters, in particular rapid eye movement (REM) sleep. In a previous study, the systemic administration of the selective 5-HT1A receptor antagonist p-MPPI reduced REM sleep in a dose-dependent manner suggesting a blockade of the 5-HT1A autoreceptors. In the present study, a blockade by microdialysis perfusion of 10 microM and 100 microM of p-MPPI for 7 h into the DRN in freely behaving rats influenced vigilance state only to a small extent. The administration of 10 microM of p-MPPI induced a reduction of total REM sleep mainly due to a suppression of REM sleep during the third 2 h period of the recording of sleep and waking. Perfusion of 100 microM of p-MPPI decreased total transition type sleep (TRANS) but the effect on REM sleep did not reach significance. There was no change in waking or slow wave sleep (SWS) following any of the doses. The data suggest that 5-HT1A receptor-mediated mechanisms in the DRN may be only moderately important in the serotonergic modulation of REM sleep.

Muscarinic and PACAP receptor interactions at pontine level in the rat: significance for REM sleep regulation

Cholinergic and PACAPergic systems within the oral pontine reticular nucleus (PnO) play a critical role in REM sleep generation in rats. In this present work, we have investigated whether REM sleep enhancement induced by carbachol (a cholinergic agonist) or PACAP, depends on an interaction between muscarinic and PACAP receptors. This hypothesis was tested by recording sleep-wake cycles in freely moving rats injected into the PnO with PACAP in combination with the muscarinic receptor antagonist atropine, or with carbachol in combination with the PACAP receptor antagonist PACAP6-27. When administered alone, PACAP (3 pmol) or carbachol (110 pmol) induced an enhancement of REM sleep during 8 h (+61%, n = 8; +70%, n = 5), which was totally prevented by infusion of atropine (290 pmol) for PACAP, or of PACAP6-27 (3 pmol) for carbachol. Quantitative autoradiographic studies indicated that (i) PACAP (10-9-10-7 M) induced in the PnO an increase (+35%) of the specific binding of the muscarinic antagonist [3H]quinuclidinyl benzylate, which could be completely prevented by PACAP6-27 (IC50 = 8 x 10-8 M) and (ii) both carbachol and PACAP enhanced [35S]GTP-gamma-S binding in a concentration-dependent manner in the PnO. The maximal increase due to carbachol was significantly higher in the presence (+126%) than in the absence (+102%) of PACAP (0.1 microM). These data showed that interactions between muscarinic and PACAP receptors do exist within the PnO and play a role in the local mechanisms of REM sleep control in the rat.

Sleep-wake effects following the selective 5-HT(1A) receptor antagonist p-MPPI in the freely moving rat

The 5-HT(1A) receptors appear to play an important role in the serotonergic modulation of sleep and waking. Both presynaptic somatodendritic 5-HT(1A) autoreceptors and postsynaptic 5-HT(1A) heteroreceptors may be involved. The present study addressed the question of whether the selective 5-HT(1A) receptor antagonist 4-(2'-methoxy-phenyl)-1-[2'-(n-2"-pyridinyl)-p-iodobenzamido]-ethy l-p iperazine (p-MPPI) affected sleep and waking and whether such an effect would be dose-related. Polygraphic recording of sleep and waking in freely moving rats was employed following control injection and three doses of p-MPPI (1, 5 and 10 mg/kg i.p. in a balanced order design. Waking was increased and deep slow wave sleep decreased, while rapid eye movement (REM) sleep was suppressed over the first 6 h following injection, compared to after control injection. REM sleep was also suppressed following 10 mg/kg i.p. of p-MPPI as compared to following 1 mg/kg i.p. of p-MPPI. The interpretation of the effects is complex and the effects are not easily compatible with a simple model for serotonergic sleep-waking modulation. However, the REM sleep reduction probably reflects p-MPPIs ability to block the presynaptic 5-HT(1A) autoreceptors, increasing the firing activity in the serotonergic neurones and possibly inhibiting serotonin sensitive REM sleep active neurones.

Serotonin and the sleep/wake cycle: special emphasis on microdialysis studies

Several areas in the brainstem and forebrain are important for the modulation and expression of the sleep/wake cycle. Even if the first observations of biochemical events in relation to sleep were made only 40 years ago, it is now well established that several neurotransmitters, neuropeptides, and neurohormones are involved in the modulation of the sleep/wake cycle. Serotonin has been known for many years to play a role in the modulation of sleep, however, it is still very controversial how and where serotonin may operate this modulation. Early studies suggested that serotonin is necessary to obtain and maintain behavioral sleep (permissive role on sleep). However, more recent microdialysis experiments provide evidence that the level of serotonin during W is higher in most cortical and subcortical areas receiving serotonergic projections. In this view the level of extracellular serotonin would be consistent with the pattern of discharge of the DRN serotonergic neurons which show the highest firing rate during W, followed by a decrease in slow wave sleep and by virtual electrical silence during REM sleep. This suggests that during waking serotonin may complement the action of noradrenaline and acetylcholine in promoting cortical responsiveness and participate to the inhibition of REM-sleep effector neurons in the brainstem (inhibitory role on REM sleep). The apparent inconsistency between an inhibitory and a facilitatory role played by serotonin on sleep has at least two possible explanations. On the one hand serotonergic modulation on the sleep/wake cycle takes place through a multitude of post-synaptic receptors which mediate different or even opposite responses; on the other hand the achievement of a behavioral state depends on the complex interaction between the serotonergic and other neurotransmitter systems. The main aim of this commentary is to review the role of brain serotonin in relation to the sleep/wake cycle. In particular we highlight the importance of microdialysis for on-line monitoring of the level of serotonin in different areas of the brain across the sleep/wake cycle.

Long-term enhancement of REM sleep by the pituitary adenylyl cyclase-activating polypeptide (PACAP) in the pontine reticular formation of the rat

In rats, rapid eye movement (REM) sleep can be elicited by microinjection of vasoactive intestinal polypeptide (VIP) into the oral pontine reticular nucleus (PnO). In the present study, we investigated whether this area could also be a REM-promoting target for a peptide closely related to VIP: the pituitary adenylyl cyclase-activating polypeptide (PACAP). When administered into the posterior part of the PnO, but not in nearby areas, of freely moving chronically implanted rats, PACAP-27 and PACAP-38 (0.3 and 3 pmol) induced a marked enhancement (60-85% over baseline) of REM sleep for 8 h that could be prevented by prior infusion of the antagonist PACAP-(6-27) (3 pmol) into the same site. Moreover, injections of PACAP into the centre of the posterior PnO resulted in REM sleep enhancement which could last for up to 11 consecutive days. Quantitative autoradiography using [125I]PACAP-27 revealed the presence in the PnO of specific binding sites with high affinity for PACAP-27 and PACAP-38 (IC50 = 2.4 and 3.2 nM, respectively), but very low affinity for VIP (IC50 > 1 microM). These data suggest that PACAP within the PnO may play a key role in REM sleep regulation, and provide evidence for long-term (several days) mechanisms involved in such a control. PAC1 receptors which have a much higher affinity for PACAP than for VIP might mediate this long-term action of PACAP on REM sleep.

Key role of 5-HT1B receptors in the regulation of paradoxical sleep as evidenced in 5-HT1B knock-out mice

The involvement of 5-HT1B receptors in the regulation of vigilance states was assessed by investigating the spontaneous sleep-waking cycles and the effects of 5-HT receptor ligands on sleep in knock-out (5-HT1B-/-) mice that do not express this receptor type. Both 5-HT1B-/- and wild-type 129/Sv mice exhibited a clear-cut diurnal sleep-wakefulness rhythm, but knock-out animals were characterized by higher amounts of paradoxical sleep and lower amounts of slow-wave sleep during the light phase and by a lack of paradoxical sleep rebound after deprivation. In wild-type mice, the 5-HT1B agonists CP 94253 (1-10 mg/kg, i.p.) and RU 24969 (0.25-2.0 mg/kg, i.p.) induced a dose-dependent reduction of paradoxical sleep during the 2-6 hr after injection, whereas the 5-HT1B/1D antagonist GR 127935 (0.1-1.0 mg/kg, i.p.) enhanced paradoxical sleep. In addition, pretreatment with GR 127935, but not with the 5-HT1A antagonist WAY 100635, prevented the effects of both 5-HT1B agonists. In contrast, none of the 5-HT1B receptor ligands, at the same doses as those used in wild-type mice, had any effect on sleep in 5-HT1B-/- mutants. Finally, the 5-HT1A agonist 8-OH-DPAT (0.2-1.2 mg/kg, s.c.) induced in both strains a reduction in the amount of paradoxical sleep. Altogether, these data indicate that 5-HT1B receptors participate in the regulation of paradoxical sleep in the mouse.

Rapid eye movement sleep induction by vasoactive intestinal peptide infused into the oral pontine tegmentum of the rat may involve muscarinic receptors

In rats, rapid eye movement sleep can be induced by microinjection of either the cholinergic agonist carbachol or the neuropeptide vasoactive intestinal peptide into the oral pontine reticular nucleus. Possible involvement of cholinergic mechanisms in the effect of vasoactive intestinal peptide was investigated using muscarinic receptor ligands. Sleep-waking cycles were analysed after infusion into the oral pontine reticular nucleus of vasoactive intestinal peptide (10 ng in 0.1 microl), carbachol (20 ng), atropine (200 ng) and pirenzepine (50, 100 ng), performed separately or in combination at 15-min intervals. The increase in rapid eye movement sleep due to the combined infusion of vasoactive intestinal peptide and carbachol (+58.7+/-4.6% for 8 h, P<0.05) was not significantly different from that induced by each compound separately. The enhancement of rapid eye movement sleep by vasoactive intestinal peptide was totally prevented by infusion of atropine, but not pirenzepine, a relatively selective M1 antagonist. On their own, none of the latter two compounds affected the sleep-waking cycle. Quantitative autoradiographic studies using [3H]quinuclidinyl benzylate (1 nM) and pirenzepine (0.5 microM) indicated that muscarinic receptors correspond to pirenzepine-insensitive binding sites in the oral pontine reticular nucleus. In vitro, vasoactive intestinal peptide (1-100 nM) significantly increased (+30-40%) the specific binding of [3H]quinuclidinyl benzylate to the oral pontine reticular nucleus in rat brain sections. This effect appeared to be due to an increased density, with no change in affinity, of pirenzepine-insensitive binding sites in this area. These data suggest that pirenzepine-insensitive muscarinic binding sites are involved in the induction of rapid eye movement sleep by vasoactive intestinal peptide at the pontine level in the rat.

Changes in sleep and wakefulness following 5-HT1A ligands given systemically and locally in different brain regions

Serotonin (5-HT) has been implicated in the regulation of vigilance, but whether 5-HT is important for sleep or waking processes remains controversial. This review addresses the role of 5-HT1A receptors in sleep and wakefulness. Systemic administration of 5-HT1A agonists consistently increases wakefulness, whereas slow wave sleep (SWS) and REM (rapid-eye movement) sleep are reduced. However, systemic 5-HT1A agonists also produce a delayed increase in deep slow wave sleep, or an increase in slow wave activity. Intrathecal administration of a selective 5-HT1A agonist produces an increase in SWS, whereas wakefulness is reduced, presumably by stimulating 5-HT1A receptors located presynaptically on primary afferents in the spinal cord. Microinjection of serotonin into the region of the cholinergic basalis neurons produces an increase in slow wave activity, presumably by stimulating 5-HT1A receptors. Microdialysis perfusion of a selective 5-HT1A agonist into the dorsal Raphe nucleus causes an increase in REM sleep, whereas the other sleep/wake stages are unaltered. The REM sleep increase is likely due to a decrease in 5-HT neuronal activity, and thereby reduced 5-HT neurotransmission in projection areas, e.g. the laterodorsal and pedunculopontine tegmental nuclei. Direct injection of a selective 5-HT1A agonist into the pedunculopontine tegmental nuclei reduces REM sleep, consistent with such a hypothesis. These complex sleep/wake data of 5-HT1A ligands suggest that 5-HT1A receptor activation may increase waking, increase slow wave sleep or increase REM sleep depending on where the 5-HT1A receptors are located within the central nervous system.

Sleep/waking effects of a selective 5-HT1A receptor agonist given systemically as well as perfused in the dorsal raphe nucleus in rats

Sleep/waking stages and behavior were studied following the selective 5-HT1A agonist 8-OH-DPAT given subcutaneously (s.c.) (0.010-0.375 mg/kg) as well as perfused continuously (10 microM) for 6 h into the dorsal raphe nucleus (DRN) using microdialysis. Given systemically, 8-OH-DPAT at 0.375 mg/kg s.c. induced 5-HT behavioral syndrome, increased waking to 149% and reduced slow wave sleep (SWS) to 86%, transition to 76% and rapid eye movement (REM) sleep to 73%. The effect on deep SWS (SWS-2) was biphasic, with an increase after 2 h. 8-OH-DPAT at 0.010 mg/kg did not have any vigilance effects. 8-OH-DPAT perfusion in DRN produced a fourfold increase in REM sleep compared to perfusion of artificial cerebrospinal fluid. This is consistent with the hypothesis that reduced 5-HT neurotransmission following 5-HT1A autoreceptor stimulation will disinhibit cholinergic REM-promoting mesopontine neurons and thereby lead to a REM sleep increase. The other sleep/waking stages were not significantly affected by 8-OH-DPAT perfusion in DRN.

Human sleep EEG following the 5-HT1A antagonist pindolol: possible disinhibition of raphe neuron activity

The sleep electroencephalogram (EEG) was used to assay central effects of pindolol (10 and 30 mg p.o.), a mixed beta(1/2)-adrenoceptor/5-hydroxytryptamine (5-HT)(1A/1B) receptor blocker, in humans. Compared to placebo, pindolol produced a dose-related suppression of rapid-eye-movement (REM) sleep, including a prolongation of REM latency, and a decrease of REM time and REM density. At the higher dose, it also reduced EEG spectral power during non-REM sleep in portions of the delta, theta, and alpha frequencies (1.125-5.125 Hz, 7.125-9.625 Hz). By contrast, betaxolol (20 mg p.o.), a selective beta1-antagonist devoid of serotonergic affinity, affected neither REM sleep nor EEG power. REM sleep is, in part, under the inhibitory control of serotonergic neurons projecting from the dorsal raphe nucleus to pontine cholinergic/cholinoceptive cells. The EEG power spectrum induced by pindolol tended to be opposite to what has previously been reported for ipsapirone, a 5-HT1A agonist. Therefore, the present data, tentatively, are consistent with the contention that pindolol inhibits, possibly selectively, somatodendritic 5-HT1A autoreceptors in humans and may antagonize self-inhibition of midbrain raphe nuclei 5-HT neurons.

Vasoactive intestinal polypeptide microinjections into the oral pontine tegmentum enhance rapid eye movement sleep in the rat

Rapid eye movement sleep can be elicited in the rat by microinjection of the cholinergic agonist carbachol into the oral pontine reticular nucleus. Intracerebroventricular administration, during the light period, of vasoactive intestinal peptide enhances rapid eye movement sleep in several species. Since this peptide is co-localized with acetylcholine in many neurons in the central nervous system, it was assumed that the oral pontine tegmentum could also be one target for vasoactive intestinal peptide to induce rapid eye movement sleep. This hypothesis was tested by recording the sleep-wakefulness cycle in freely-moving rats injected with vasoactive intestinal peptide or its fragments (1-12 and 10-28) directly into the oral pontine reticular nucleus. when administered into the posterior part of this nucleus, vasoactive intestinal peptide at 1 and 10 ng (in 0.1 microliter of saline), but not its fragments, induced a 2-fold enhancement of rapid eye movement sleep during 4 h, at the expense of wakefulness. At the dose of 10 ng, a significant increase in rapid eye movement sleep persisted for up to 8 h. Moreover, when the peptide was injected into the centre of the positive zone, rapid eye movement sleep was enhanced during three to eight consecutive days. These data provide the first evidence that rapid eye movement sleep can be elicited at both short- and long-term by a single intracerebral microinjection of vasoactive intestinal peptide. Peptidergic mechanisms, possibly in association with cholinergic mechanisms, within the caudal part of the oral pontine reticular nucleus may play a critical role in the long-term regulation of rapid eye movement sleep in rats.

The 5-HT1A agonist ipsapirone enhances EEG slow wave activity in human sleep and produces a power spectrum similar to 5-HT2 blockade

The REM sleep-suppressing effect of postsynaptic 5-HT1A stimulation has been well established. Here we investigate the effects of the 5-HT1A agonist ipsapirone (10 and 20 mg) on sleep EEG power spectra during non-REM sleep in nine healthy humans. At the lower dose, slow wave activity (SWA; EEG power in the delta (1-4.5 Hz) range) was significantly enhanced. At the higher dose, where side-effects occurred, the enhancement in SWA was not significant. The spectral profile was characterized by a bimodal increase of power in the lower delta and in the theta (5-8 Hz) frequencies, and by troughs at 4 Hz and at 11 Hz, a pattern compellingly similar to that reported for a 5-HT2 antagonist (seganserin). We propose that the spectral data following the lower ipsapirone dose reflect a net decrease of neuronal activity at 5-HT2 receptors, mediated through stimulation of somatodendritic autoreceptors in the raphe nuclei (presynaptic) and/or through stimulation of postsynaptic 5-HT1A receptors colocalized with 5-HT2 receptors. The spectral non-REM sleep EEG profile might be used to investigate central 5-HT function in humans.

Sleep and EEG power spectrum effects of the 5-HT1A antagonist NAN-190 alone and in combination with citalopram

The sleep and waking and EEG power spectrum effects of the putative 5-HT1A antagonist NAN-190 (0.5 mg/kg, i.p.) were studied alone and in co-administration with the selective serotonin re-uptake inhibitor citalopram (5.0 mg/kg, i.p.) in the rat. Citalopram, as in a prior dose-response study, reduced REM sleep. In addition, a slight increase in NREM sleep was observed. Citalopram reduced NREM fronto-parietal (FP) EEG power density in the 5-20 Hz range. When administered alone, NAN-190 suppressed REM sleep in the first 2 h, and reduced SWS-2 in the first 4 after administration. NAN-190 also suppressed selectively NREM sleep slow-wave activity in both fronto-frontal (FF) and FP EEG power spectrum. When administered in combination with citalopram, an attenuation of the power density reduction in the 7-15 Hz range in the FF EEG of citalopram alone, was observed. However, the EEG power spectral density and REM sleep suppressive effects of NAN-190 were both augmented. The results are compatible with the notion that serotonin is involved in the modulation of the slow wave activity in the EEG during NREM sleep. The results are cordant with other data suggesting that postsynaptic 5-HT1A stimulation might increase slow wave activity in the NREM EEG, and that serotonergic stimulation of other receptor subtypes (possibly 5-HT2) may decrease slow wave activity in the NREM EEG.

Sleep and waking in 5,7-DHT-lesioned or (-)-pindolol-pretreated rats after administration of buspirone, ipsapirone, or gepirone

The effects of partial 5-HT1A receptor agonists buspirone (0.010-4.0 mg/kg), ipsapirone (0.010-6.0 mg/kg), and gepirone (0.025-4.0 mg/kg) on sleep and waking were studied in vehicle-treated and 5,7-dihydroxytryptamine (5,7-DHT)-injected rats. 5,7-DHT-treated animals showed a marked and significant serotonin and 5-HIAA depletion in the raphe regions of the pons and upper brain stem, cerebral cortex, hippocampus, and striatum. Subcutaneous administration of the partial agonists to both the vehicle-infused and the 5,7-DHT-treated animals significantly increased waking (W) and reduced light sleep (LS), slow-wave sleep (SWS), and REM sleep (REMS). Pretreatment with (-)pindolol (2.0 mg/kg) reversed the effects of buspirone and gepirone on W and non-REM sleep (LS + SWS) whereas REMS remained suppressed. (-)-Pindolol failed to reverse the effects of ipsapirone on sleep and W. The present results tend to indicate that increased W after acute administration of buspirone, ipsapirone, or gepirone depends upon the activation of postsynaptic 5-HT1A receptors. The well-known anxiolytic action observed after chronic administration of the azapirones seems to be related to mechanisms other that these involved in their stimulant effect.

Effects of sertindole on sleep-wake states, electroencephalogram, behavioral patterns, and epileptic activity of rats

In this study we addressed the effects of the 5-HT2 receptor antagonist sertindole in rats. The compound was administered in doses of 0.08, 0.32, and 1.28 mg/kg, whereas a control group received the solvent. The effects of sertindole on sleep-wake states, behavioral patterns, and background electroencephalogram were studied. Following injection of drug or solvent, we recorded the electroencephalogram and electromyogram for two periods of 4 h in the dark period of the light-dark cycle on 2 successive days. On the 1st day sertindole induced a significant increase in deep slow-wave sleep, but only with a dose of 0.32 mg/kg. Furthermore, a decrease in REM sleep in all three drug groups was established. The suppression of REM sleep was still present on the 2nd day. Sertindole also induced a decrease in alternation between behavioral patterns on the 1st day. There were no significant changes in the spectral content of the background electroencephalogram. In a parallel experiment it appeared that sertindole had no main effects on epileptic spike-wave discharges. This was established with a dose of 1.28 mg/kg sertindole in rats with absence seizures. These findings suggest that sertindole, similar to other compounds modulating 5-HT2 receptors, influences sleep-wake states in rats by decreasing REM sleep and mildly increasing deep slow-wave sleep, whereas behavioral variation is slightly diminished, with no effects on the background EEG and almost no effects on spike-wave discharges.

Lesion of descending 5-HT pathways increases zimeldine-induced waking in rats

Sleep, waking, and EEG power spectra were investigated in rats with spinal 5,6-dihydroxytryptamine (5,6-DHT) lesions, following 20 mg/kg zimeldine or vehicle IP injections. 5,6-DHT selectively lesioned the descending serotonergic pathways. Lesion alone did not change sleep and waking stages compared to baseline, except for a reduction in REM sleep. Consistent with earlier findings, zimeldine in nonlesioned rats increased waking the first 2 h of recording. Zimeldine treatment in lesioned rats gave a significant additional 50% increase in waking the first 2 h and a corresponding decrease in total slow wave sleep, suggesting a potentiation of these effects. Zimeldine gave no significant changes in waking EEG power spectral density. Lesion gave a tendency to reduction between 4.0 and 15.5 Hz compared with baseline, and between 10.0 and 16.5 compared to the independent control group. In both comparisons, the combined treatment strengthened this effect, again suggesting a potentiating effect of lesion. In sleep, zimeldine reduced power over the whole spectrum (0.5-20.0 Hz), less in the lower frequencies than in the higher frequencies.

The influence of ipsapirone, a 5-HT1A agonist, on sleep patterns of healthy subjects

Ipsapirone is a new pyrimidinylpiperazine ligand specific for 5-HT1A receptors, with potential therapeutic use in affective disorders. Because 5-HT is involved in the regulation of sleep, we investigated the effect of ipsapirone hydrochloride on sleep patterns in 18 normal, healthy subjects of both sexes. Compared to placebo, ipsapirone 5 mg administered by mouth three times daily for 14 days decreased rapid eye movement (REM) sleep duration and, by the tenth day of treatment, began to reduce slow wave sleep (SWS) duration. The decrease in REM sleep occurred in the first 3 h of sleep. The latency to REM sleep was increased from the first night following ipsapirone administration, remained increased throughout the 14 days of administration, and fell to equal latency on placebo immediately administration ended. Subjective assessments of sleep revealed no differences between ipsapirone and placebo. Our experiments confirm a role of 5-HT1A receptors in sleep. The effects of ipsapirone on the sleep patterns of patients with affective disorders still need to be determined.

Inhibition of REM sleep by ipsapirone, a 5HT1A agonist, in normal volunteers

In order to test the hypothesis that serotonergic mechanisms inhibit REM sleep via a 5HT1A receptor, we administered placebo and ipsapirone (10 and 20 mg by mouth 15 min before bedtime) to ten normal volunteers in a double blind fashion. Ipsapirone is a relatively selective 5HT1A receptor agonist. As predicted, ipsapirone prolonged REM latency and Mean Latency to Eye Movements (M-LEM), a measure of time between onset of REM sleep and the first eye movement of the REM period, and REM% at both doses compared with placebo. It also reduced sleep efficiency and total REM sleep time at the highest dose. These results support the hypothesis that systemic stimulation of 5HT1A receptors prolong REM latency and inhibit REM sleep.