Effects of selective activation of the 5-HT1B receptor with CP-94,253 on sleep and wakefulness in the rat

The role of monoaminergic neurons in modulating respiration during sleep and the connection with SUDEP

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death among epilepsy patients, occurring even more frequently in cases with anti-epileptic drug resistance. Despite some advancements in characterizing SUDEP, the underlying mechanism remains incompletely understood. This review summarizes the latest advances in our understanding of the pathogenic mechanisms of SUDEP, in order to identify possible targets for the development of new strategies to prevent SUDEP. Based on our previous research along with the current literature, we focus on the role of sleep-disordered breathing (SDB) and its related neural mechanisms to consider the possible roles of monoaminergic neurons in the modulation of respiration during sleep and the occurrence of SUDEP. Overall, this review suggests that targeting the monoaminergic neurons is a promising approach to preventing SUDEP. The proposed roles of SDB and related monoaminergic neural mechanisms in SUDEP provide new insights for explaining the pathogenesis of SUDEP.

Modulating role of serotonergic signaling in sleep and memory

Serotonin is an important neurotransmitter with various receptors and wide-range effects on physiological processes and cognitive functions including sleep, learning, and memory. In this review study, we aimed to discuss the role of serotonergic receptors in modulating sleep-wake cycle, and learning and memory function. Furthermore, we mentioned to sleep deprivation, its effects on memory function, and the potential interaction with serotonin. Although there are thousands of research articles focusing on the relationship between sleep and serotonin; however, the pattern of serotonergic function in sleep deprivation is inconsistent and it seems that serotonin has not a certain role in the effects of sleep deprivation on memory function. Also, we found that the injection type of serotonergic agents (systemic or local), the doses of these drugs (dose-dependent effects), and up- or down-regulation of serotonergic receptors during training with various memory tasks are important issues that can be involved in the effects of serotonergic signaling on sleep-wake cycle, memory function, and sleep deprivation-induced memory impairments. This comprehensive review was conducted in the PubMed, Scopus, and ScienceDirect databases in June and July 2021, by searching keywords sleep, sleep deprivation, memory, and serotonin.

Ablation of Central Serotonergic Neurons Decreased REM Sleep and Attenuated Arousal Response

Sleep/wake behavior is regulated by distinct groups of neurons, such as dopaminergic, noradrenergic, and orexinergic neurons. Although monoaminergic neurons are usually considered to be wake-promoting, the role of serotonergic neurons in sleep/wake behavior remains inconclusive because of the effect of serotonin (5-HT)-deficiency on brain development and the compensation for inborn 5-HT deficiency by other sleep/wake-regulating neurons. Here, we performed selective ablation of central 5-HT neurons in the newly developed Rosa-diphtheria toxin receptor (DTR)-tdTomato mouse line that was crossed with Pet1^Cre/+ mice to examine the role of 5-HT neurons in the sleep/wake behavior of adult mice. Intracerebroventricular administration of diphtheria toxin completely ablated tdTomato-positive cells in Pet1^Cre/+; Rosa-DTR-tdTomato mice. Electroencephalogram/electromyogram-based sleep/wake analysis demonstrated that central 5-HT neuron ablation in adult mice decreased the time spent in rapid eye movement (REM) sleep, which was associated with fewer transitions from non-REM (NREM) sleep to REM sleep than in control mice. Central 5-HT neuron-ablated mice showed attenuated wake response to a novel environment and increased theta power during wakefulness compared to control mice. The current findings indicated that adult 5-HT neurons work to support wakefulness and regulate REM sleep time through a biased transition from NREM sleep to REM sleep.

MDMA self-administration fails to alter the behavioral response to 5-HT(1A) and 5-HT(1B) agonists

RATIONALE

Regular use of the street drug, ecstasy, produces a number of cognitive and behavioral deficits. One possible mechanism for these deficits is functional changes in serotonin (5-HT) receptors as a consequence of prolonged 3,4 methylenedioxymethamphetamine (MDMA)-produced 5-HT release. Of particular interest are the 5-HT(1A) and 5-HT(1B) receptor subtypes since they have been implicated in several of the behaviors that have been shown to be impacted in ecstasy users and in animals exposed to MDMA.

OBJECTIVES

This study aimed to determine the effect of extensive MDMA self-administration on behavioral responses to the 5-HT(1A) agonist, 8-hydroxy-2-(n-dipropylamino)tetralin (8-OH-DPAT), and the 5-HT(1B/1A) agonist, RU 24969.

METHODS

Male Sprague-Dawley rats self-administered a total of 350 mg/kg MDMA, or vehicle, over 20-58 daily self-administration sessions. Two days after the last self-administration session, the hyperactive response to 8-OH-DPAT (0.03-1.0 mg/kg) or the adipsic response to RU 24969 (0.3-3.0 mg/kg) were assessed.

RESULTS

8-OH-DPAT dose dependently increased horizontal activity, but this response was not altered by MDMA self-administration. The dose-response curve for RU 24969-produced adipsia was also not altered by MDMA self-administration.

CONCLUSIONS

Cognitive and behavioral deficits produced by repeated exposure to MDMA self-administration are not likely due to alterations in 5-HT(1A) or 5-HT(1B) receptor mechanisms.

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.

Differentiated effects of the multimodal antidepressant vortioxetine on sleep architecture: Part 2, pharmacological interactions in rodents suggest a role of serotonin-3 receptor antagonism

Antidepressants often disrupt sleep. Vortioxetine, a multimodal antidepressant acting through serotonin (5-HT) transporter (SERT) inhibition, 5-HT3, 5-HT7 and 5-HT1D receptor antagonism, 5-HT1B receptor partial agonism, and 5-HT1A receptor agonism, had fewer incidences of sleep-related adverse events reported in depressed patients. In the accompanying paper a polysomnographic electroencephalography (sleep-EEG) study of vortioxetine and paroxetine in healthy subjects indicated that at low/intermediate levels of SERT occupancy, vortioxetine affected rapid eye movement (REM) sleep differently than paroxetine. Here we investigated clinically meaningful doses (80-90% SERT occupancy) of vortioxetine and paroxetine on sleep-EEG in rats to further elucidate the serotoninergic receptor mechanisms mediating this difference. Cortical EEG, electromyography (EMG), and locomotion were recorded telemetrically for 10 days, following an acute dose, from rats receiving vortioxetine-infused chow or paroxetine-infused water and respective controls. Sleep stages were manually scored into active wake, quiet wake, and non-REM or REM sleep. Acute paroxetine or vortioxetine delayed REM onset latency (ROL) and decreased REM episodes. After repeated administration, vortioxetine yielded normal sleep-wake rhythms while paroxetine continued to suppress REM. Paroxetine, unlike vortioxetine, increased transitions from non-REM to wake, suggesting fragmented sleep. Next, we investigated the role of 5-HT3 receptors in eliciting these differences. The 5-HT3 receptor antagonist ondansetron significantly reduced paroxetine's acute effects on ROL, while the 5-HT3 receptor agonist SR57227A significantly increased vortioxetine's acute effect on ROL. Overall, our data are consistent with the clinical findings that vortioxetine impacts REM sleep differently than paroxetine, and suggests a role for 5-HT3 receptor antagonism in mitigating these differences.

Serotonin conflict in sleep-feeding

Short sleep duration has been suggested to be a risk factor for weight gain and adiposity. Serotonin (5-HT) substantially contributes to the regulation of sleep and feeding behavior. Although 5-HT predominately promotes waking and satiety, the effects of 5-HT depend on 5-HT receptor function. The 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C, 5-HT6, and 5-HT7 receptors reportedly contribute to sleep-waking regulation, whereas the 5-HT1B and 5-HT2C receptors contribute to the regulation of satiety. The 5-HT1B and 2C receptors may therefore be involved in the regulation of sleep-feeding. In genetic studies, 5-HT1B receptor mutant mice display greater amounts of rapid eye movement sleep (REMS) than wild-type mice, while displaying no effects on waking or slow wave sleep (SWS). On the other hand, 5-HT2C receptor mutant mice exhibit increased wakefulness and decreased SWS, without any effect on REMS. Moreover, the 5-HT2C receptor mutants display leptin-independent hyperphagia, leading to a middle-aged onset of obesity, whereas 5-HT1B receptor mutants do not display any effect on food intake. Thus, the genetic deletion of 5-HT2C receptors results in sleep loss-associated hyperphagia, leading to the late onset of obesity. This is a quite different pattern of sleep-feeding behavior than is observed in disturbed leptin signaling, which displays an increase in sleep-associated hyperphagia. In pharmacologic studies, 5-HT1B and 5-HT2C receptors upregulate wakefulness and downregulate SWS, REMS, and food intake. These findings suggest that 5-HT1B/2C receptor stimulation induces sleep loss-associated anorexia. Thus, the central 5-HT regulation of sleep-feeding can be dissociated. Functional hypothalamic proopiomelanocortin and orexin activities may contribute to the dissociated 5-HT regulation.

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.

MDMA treatment 6 months earlier attenuates the effects of CP-94,253, a 5-HT1B receptor agonist, on motor control but not sleep inhibition

The possible long-term effects of the recreational drug "ecstasy" (3,4-methylenedioxymethamphetamine, MDMA) on the function of 5-hydroxytryptamine-1B (5-HT(1B)) receptor in sleep and motor control were investigated using a selective 5-HT(1B) receptor agonist, 5-propoxy-3-(1,2,3,6-tetrahydro-4-pyrinzidyl)-1H-pyrrolo([3,2-b])pyridine hydrochloride (CP-94,253; 5 mg/kg). CP-94,253 or vehicle was administered to freely moving rats pre-treated with MDMA (15 mg/kg) or vehicle 6 months earlier, and polygraphic recording for 24 h and motor activity measurements were performed. Active wake (AW), passive wake (PW), light slow wave sleep (SWS-1), deep slow wave sleep (SWS-2), paradoxical sleep (PS), and diurnal rhythm were analyzed for the whole period. In additional, the EEG power spectrum was calculated for the second hour after the acute treatment for AW, PW, SWS-1, and SWS-2. 5-HT transporter (5-HTT) immunohistochemistry was measured in brain areas related to sleep and motor control 6 months after MDMA treatment. CP-94,253 increased AW and PW, decreased SWS-2 and PS, and altered parameters of diurnal rhythm in control animals. CP-94,253 decreased the EEG power spectra at higher frequencies. The effects of CP-94,253 on AW and diurnal rhythm were reduced or eliminated in MDMA-treated animals. MDMA treatment decreased 5-HTT fibre density in posterior hypothalamus, tuberomammillary nucleus, caudate putamen and ventrolateral striatum. These data suggest that long-term changes in 5-HT(1B) receptor function occur after serotonergic damage caused by a single dose of MDMA.

Differential effects of the 5-HT1A receptor agonist flesinoxan given locally or systemically on REM sleep in the rat

The effects of flesinoxan, a selective 5-HT1A receptor agonist on spontaneous sleep, were studied in adult rats implanted for chronic sleep recordings. Flesinoxan was administered systemically or infused directly into the dorsal raphe nucleus, the left laterodorsal tegmental nucleus or the medial pontine reticular formation. Systemic administration of flesinoxan (0.03 and/or 0.06 micromol/kg) significantly increased wakefulness and sleep latencies, and reduced rapid eye movement (REM) sleep and the number of REM periods, during the first and/or second 2-h period after treatment. Direct infusion of the 5-HT1A receptor agonist (0.06 and/or 0.12 nmol) into the dorsal raphe nucleus induced a significant increment of REM sleep and augmented the number of REM periods during the second and/or third 2-h period of recording. Microinjection of flesinoxan (0.03, 0.06 and/or 0.12 nmol) into the laterodorsal tegmental nucleus reduced REM sleep and the number of REM periods, and augmented REM sleep latency during the first, second and/or third 2-h recording period. Finally, direct infusion of flesinoxan (0.48 nmol) into the medial pontine reticular formation decreased REM sleep and the number of REM periods, and increased REM sleep latency during the first and second 2 h of recording. Our results indicate that the 5-HT1A receptor is involved in the inhibitory effect of serotonin on brainstem structures that act to promote and to induce REM sleep.

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.

Increase of waking and reduction of NREM and REM sleep after nitric oxide synthase inhibition: prevention with GABAA or adenosine A1 receptor agonists

The effect of N(G)-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of enzyme nitric oxide synthase (NOS), on spontaneous sleep during the light period, was studied in adult rats implanted for chronic sleep recordings. L-NAME was injected by subcutaneous (s.c.) route or was infused directly into the dorsal raphe nucleus (DRN). Subcutaneous (46.0--185.0 micromol/kg) administration of L-NAME increased waking (W), slow wave sleep (SWS) and rapid-eye-movement sleep (REMS) latency, whereas SWS, REMS and the number of REM periods were reduced. Direct application of L-NAME into the DRN (0.37--1.1 micromol) induced an increment of W and a reduction of SWS and REMS. Values corresponding to SWS and REMS latency, and the number of REM periods remained within control levels. Subcutaneous administration of the GABA(A) receptor agonist muscimol (1.7--3.5 micromol/kg) or the adenosine A(1) receptor agonist L-PIA [L(-)N(6)-(2-phenylisopropyl)adenosine] (0.1--0.3 micromol/kg) induced slight but inconsistent changes of W, light sleep (LS), SWS and REMS that did not attain significance. Pretreatment with muscimol (1.7--3.5 micromol/kg, s.c.) or L-PIA (0.1--0.3 micromol/kg, s.c.) antagonized the increase of W and reduction of SWS and REMS induced by s.c. (92.0 micromol/kg) or intra-DRN (0.74 micromol) administration of L-NAME. However, neither muscimol nor L-PIA prevented the increase of REMS latency induced by L-NAME 92.0 micromol/kg, s.c. Our findings tend to indicate that the change of behavioral state observed after systemic or intra-DRN administration of L-NAME is partly related to the reduction of GABA and adenosine at critical sites in the CNS.

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.

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.

Role of nitric oxide in sleep regulation: effects of L-NAME, an inhibitor of nitric oxide synthase, on sleep in rats

The effect of N(G)-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of enzyme nitric oxide synthase (NOS), on spontaneous sleep during the light period, was studied in adult rats implanted for chronic sleep recordings. L-NAME was injected by subcutaneous (s.c.) or intracerebroventricular (i.c.v.) routes or was infused directly into the dorsal raphe nuclei (DRN). Subcutaneous (1.25-5.0 mg/kg) or i.c.v. (0.25-1.0 mg) administration of L-NAME increased waking (W) and reduced slow wave sleep (SWS) and rapid-eye-movement sleep (REMS) during the first 3 h of recording. On the other hand, direct application of L-NAME into the DRN (50.0-150.0 microg) induced an increment of W and a reduction of SWS without suppressing REMS. Values of W and SWS were significantly different compared with those of controls during the 6-h recording period. The effects of L-NAME observed after s.c. or i.c.v. administration confirm previous studies in rabbits and rats, in which the NOS inhibitor reduced sleep and increased W in a dose-dependent manner. It is possible that REMS suppression after L-NAME could be related to a reduction of acetylcholine release in areas critical for REMS promotion. A decrease in gamma-aminobutyric acid (GABA) release after nitric oxide synthesis inhibition could play a role in the reduction of SWS.