Similar effect on REM sleep but differential effect on slow wave sleep of the two 5-HT uptake inhibitors zimeldine and alaproclate in cats and rats

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.

Pre- and post-natal protein malnutrition alters the effect of rapid eye movements sleep-deprivation by the platform-technique upon the electrocorticogram of the circadian sleep-wake cycle and its frequency bands in the rat

Selective deprivation of paradoxical (or rapid eye movements) sleep (REMS) in protein malnourished young male rats, results in circadian and homeostatic alterations. By means of electrocorticographic recordings, we have examined the sleep-wake cycle as a functional maturity index, and its circadian and homeostatic mechanisms in prenatal (PM) and chronically (CM) protein malnourished young male rats. The effects of rapid eye movements sleep-deprivation (REMS-D), by the platform technique in a "conflict experiment" (i.e. recovery from REMS-D begun during the circadian phase of activity), revealed that in PM animals, wake (WAK) was increased significantly during recovery days 1 and 2 (RD1, RD2); and slow wave sleep (SWS) was reduced significantly during these days. Prenatal protein-malnutrition altered the phase of WAK and REMS rhythms, and the amplitude of SWS rhythm was decreased. The REMS compensatory increase after REMS-D (REMS rebound) was confined to the first 4-h block of the activity phase in all experimental groups and 24 h later another REMS rebound was displayed in PM animals. The paradoxical sleep-rebound in CM animals was significantly higher than control and PM rats and it was only shown at the first 4-h block after REMS-D. Before and after REMS-D the circadian distribution of both sleep states, and the electrocortical frequency bands showed different circadian phases at the same day-time in control, PM and CM rats. The aforementioned indicates that protein malnutrition exerts important effects on the circadian and homeostatic mechanisms driving sleep. Therefore, the temporal structure of the malnourished rats may not allow proper synchronization of some sleep parameters, particularly REMS, to the environmental time cues.

Fluoxetine decreases brain temperature and REM sleep in Syrian hamsters

The antidepressant drug, fluoxetine (FLX), a selective serotonin reuptake inhibitor, was administered to Syrian hamsters, and its acute and chronic effects on EEG sleep and hypothalamic temperature were recorded. Acute fluoxetine treatment at doses of 5, 10, 20 and 40 mg/kg decreased REM sleep and hypothalamic temperature in a dose-dependent manner. It increased NREM sleep, and, at doses of 20 and 40 mg/kg, it increased wakefulness. At 40 mg/kg, it decreased motor activity. During chronic treatment, tolerance developed to FLX's REM sleep-inhibiting effects, but tolerance did not develop to FLX's hypothalamic temperature-decreasing effects. Chronic FLX treatment produced circadian phase-dependent decreases in temperature beyond those that were observed during acute treatment. The apparent dissociation during chronic treatment between FLX's temperature-lowering effects and its REM-decreasing effects might be related to long-term changes in 5HT receptor function or FLX pharmacokinetics.

Homeostatic regulation of serotonergic function by the serotonin transporter as revealed by nonviral gene transfer

With the aim of exploring the relationship between the serotonin transporter (5-HTT or SERT) and the activity level of serotonin (5-HT) neurotransmission, in vivo expression of this protein was specifically altered using a nonviral DNA transfer method. Plasmids containing the entire coding sequence or a partial antisense sequence of the 5-HTT gene were complexed with the cationic polymer polyethylenimine and injected into the dorsal raphe nucleus of adult male rats. Significant increase or decrease in both [(3)H]citalopram binding and [(3)H]5-HT synaptosomal uptake were observed in various brain areas up to 2 weeks after a single administration of the sense plasmid or 7 d after injection of the short antisense plasmid, respectively. Such changes in 5-HTT expression were associated with functional alterations in 5-HT neurotransmission, as shown by the increased capacity of 5-HT(1A) receptor stimulation to enhance [(35)S]GTP-gamma-S binding onto the dorsal raphe nucleus in sections from rats injected with the sense plasmid. Conversely, both a decrease in 5-HT(1A)-mediated [(35)S]GTP-gamma-S binding and a reduced potency of the 5-HT(1A) receptor agonist ipsapirone to inhibit neuronal firing were observed in the dorsal raphe nucleus of antisense plasmid-injected rats. Furthermore, changes in brain 5-HT and/or 5-HIAA levels, and sleep wakefulness circadian rhythm in the latter animals demonstrated that altered expression of 5-HTT by recombinant plasmids has important functional consequences on central 5-HT neurotransmission in adult rats.

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.

Citalopram: differential sleep/wake and EEG power spectrum effects after single dose and chronic administration

The sleep/wake effects of the selective serotonin re-uptake inhibitor citalopram were studied in both a single-dose study with three dose levels (0.5, 2.0 and 5.0 mg/kg), and a 5-week chronic administration study (15 mg/kg/24 h). Single doses of citalopram resulted in a dose-dependent inhibition of rapid eye movement (REM) sleep. After chronic citalopram treatment there was a sustained REM sleep inhibition. Single doses of citalopram resulted in only minor changes in non-REM (NREM) sleep as well as in NREM EEG power spectral density. Chronic administration resulted in a major shift from SWS-2 to SWS-1. The observed corresponding changes in EEG power density were regional. A 30 to 40 percent reduction of power density in the 0.5-15 Hz range in the fronto-parietal EEG derivation was seen for the whole 8-h registration period. In the fronto-frontal EEG derivation only minor changes were seen. A decreasing trend in NREM sleep power density between 0.5 and 7 Hz, usually seen during the course of the light period, was not observed in the chronic condition, but was seen in control and single-dose condition, suggesting altered diurnal distribution of slow wave activity in the chronic condition. The data indicate that acute and chronic administration of citalopram shows clear differences in sleep effect, which may be caused by alteration of serotonergic transmission, and may be related to the antidepressant effect.

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.

Serotonin uptake inhibitors: uses in clinical therapy and in laboratory research

Fluoxetine, zimelidine, sertraline, paroxetine, fluvoxamine, indalpine and citalopram are the selective inhibitors of serotonin uptake that have been most widely studied. Some of these compounds are or have been used clinically in the treatment of mental depression, obsessive-compulsive disorder and bulimia, and therapeutic benefit has been claimed in additional diseases as well. By blocking the membrane uptake carrier which transports serotonin from the extracellular space to inside the serotonin nerve terminals, these compounds increase extracellular concentrations of serotonin and amplify signals sent by serotonin neurons. Because serotonin neurons are widespread in the central nervous system, the functional consequences of blocking serotonin uptake are diverse, but are generally subtle. Animals treated with serotonin uptake inhibitors look normal in gross appearance, but effects such as reduced aggressive behavior, decreased food intake and altered food selection, analgesia, anticonvulsant activity, endocrine changes and neurochemical changes have been demonstrated and characterized. Serotonin uptake inhibitors have helped in revealing some dynamics of serotonin neurons; for example, when uptake is inhibited and extracellular serotonin concentration increases, presynaptic as well as postsynaptic receptors for serotonin are activated to a greater degree. A consequence of increased activation of autoreceptors on serotonin cell bodies and nerve terminals is a reduction in firing of serotonin neurons and a decrease in serotonin synthesis and release. The result is a limit on the degree to which extracellular serotonin and serotonergic neurotransmission are increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic treatment with two selective 5-HT2 antagonists on sleep in the rat

The effect of chronic administration of 2(2-dimethylaminoethylthio)-3-phenylquinoline (ICI-169,369) and 2(2-dimethylamino-2-methylpropylthio)-3-phenylquinoline (ICI-170,809), two selective 5-HT2 antagonists, on sleep was studied in rats. As previously shown, the acute effect of ICI-170,809 was to increase latency to rapid eye movement sleep (REMS), decrease the number of REM periods (REMPs), suppress the cumulative amount of REMS over 12 h, and increase the duration of REMPs in the first 6 h, while having no effect on non-REM sleep (NREMS). Administration of ICI-169,369 had similar effects except no change was seen in the duration of REMPs and cumulative REMS was suppressed for 24 h. When given 2 x daily for 5 days, tolerance to the REMS suppressant effects developed in both drugs. After discontinuation of treatment, a REMS rebound occurred after ICI-170,809, but not ICI-169,369. No significant effect on NREMS was seen after administration of ICI-170,809, whereas ICI-169,369 lowered 24-h cumulative NREMS on the fifth day of administration.

Studies on sleep/wake effects of serotonin reuptake inhibitors and receptor subtype involvement

Studies with the serotonin uptake inhibitors zimeldine and alaproclate show biphasic effects on the sleep/wake axis in rats and cats. Zimeldine induced an initial waking response succeeded by a small SWS-2 increase in rats. The waking increase was not blocked by the 5-HT2 antagonist ritanserin nor by the putative 5-HT1A antagonist (-)-alprenolol. In cats, zimeldine induced initial behavioural changes which were succeeded by a large SWS-2 increase. Alaproclate gave similar initial responses as zimeldine in both species, and was succeeded by a moderate sleep increase in cats but not in rats. The complex sleep/wake effects following the serotonin uptake inhibitors may result from simultaneous induction of incompatible serotonergic effects.

Behavioral, sleep-waking and EEG power spectral effects following the two specific 5-HT uptake inhibitors zimeldine and alaproclate in cats

Sleep, waking and EEG power spectra were studied in cats for 15 h following peroral administration of placebo or 10 mg/kg and 20 mg/kg of the 5-HT reuptake inhibitors zimeldine and alaproclate. Behavior was also observed during the initial period following drug administration. Both drugs had effects on motor behavior and initiated hallucinatory like behavior. Zimeldine increased latency to stable sleep and to SWS-2. Alaproclate increased latency to SWS-1. Both drugs increased SWS (NREM sleep) and particularly SWS-2. REM sleep latency was increased and REM sleep was reduced following both drugs. EEG slow wave activity was increased following zimeldine. It is concluded that the 5-HT stimulation caused by the drugs yields complex effects on the sleep-waking axis, both sleep incompatible and sleep promoting effects.

Sleep, depression, and suicide

In a retrospective study of the electroencephalographic (EEG) sleep of major depressives with and without a history of suicide attempts, suicide attempters had longer sleep latency, lower sleep efficiency, and fewer late-night delta wave counts than normal controls. Nonattempters, compared to attempters, had less rapid eye movement (REM) time and activity in period 2, but more delta wave counts in non-REM period 4. Although both attempters and nonattempters were like controls in regard to REM period 2, patients with suicide attempts had altered intranight temporal distribution of phasic REM activity, with increased REM activity (by both visual and automated scoring) in REM sleep period 2 (significant group x period interaction). These findings, which may be more traitlike or persistent than state-related, are discussed in the context of current theories on the role of serotonin in the regulation of sleep and in suicidal behavior.

Effects of zimeldine, a selective 5-HT reuptake inhibitor, combined with ritanserin, a selective 5-HT2 antagonist, on waking and sleep stages in rats

Sleep and waking in rats were studied 8 h following administration of a selective 5-hydroxytryptamine (5-HT) reuptake inhibitor (zimeldine), a selective 5-HT2 antagonist (ritanserin) and a combination of ritanserin and zimeldine. Consistent with earlier findings, zimeldine gave a biphasic effect on sleep and waking. Waking was increased the first 3 h, followed by an increase in deep slow wave sleep (SWS-2), maximal in hours 4 and 5. Ritanserin gave an increase in SWS-2 that was spread out over the recording period. Ritanserin + zimeldine also gave a biphasic effect as zimeldine did, and the initial increase in waking and the following increase in SWS-2 tended to be stronger. Thus, ritanserin did not block the initial waking effect seen after zimeldine administration, indicating that this waking effect was not due to 5-HT2 stimulation. The increase in SWS-2 seemed to reflect an addition of the increases following the zimeldine and ritanserin alone conditions. This suggests that the increase in SWS-2 seen after 5-HT reuptake inhibition and 5-HT2 blockade are independent phenomena. Zimeldine alone, ritanserin alone and the combination all gave a clear reduction of rapid eye movement sleep.

Effects of L-tryptophan and other amino acids on electroencephalographic sleep in the rat

Electroencephalographic sleep was quantitated in adult male Sprague-Dawley rats following single injections of the methylesters of tryptophan, valine or alanine. The amino acids were administered at the onset of the daily light period (09.00 h); electrographic data were collected for the succeeding 6-h period. Saline served as the injection control, and fluoxetine, a serotonin-reuptake blocker, as a positive control. The injection of tryptophan methylester (125 mg/kg) caused a delay in rapid eye movement (REM) sleep onset, and significantly reduced the amount of REM sleep during the first 2 h postinjection. Tryptophan produced no effect on sleep onset, nor did it influence total sleep time. Fluoxetine (2.5 mg/kg) produced similar effects, as previously observed. The methylesters of valine and alanine were without effect on REM sleep, when injected at a molar dose equivalent to that for tryptophan. No consistent effects of any of the test substances were noted on non-REM (NREM) sleep or waking time, or on any of the other sleep indices quantitated. Together, the data indicate that tryptophan selectively reduces REM sleep; the effect is not due to a non-specific action of amino acids or their methylesters. The effect on REM sleep may be the consequence of a tryptophan-induced stimulation of 5-HT synthesis and release, since it is like that produced by fluoxetine, a drug that enhances transmission across serotonin synapses.

Effects of indole-pyruvic acid on sleep and food intake in the rat

Indole-pyruvic acid was studied for its short- and long-term effects on electroencephalographic sleep and on food intake in rats implanted with cortical and muscular electrodes. Following a single injection, indole-pyruvic acid (10-50 mg kg-1 i.p.) reduced by 16-23 min (range) the latency of the first slow-wave episode in a dose-related fashion and produced a significant increase in slow-wave sleep time (12-40%) in doses of 10-30 mg kg-1. Rapid eye movement sleep latency and rapid eye movement sleep time were increased (by 23-37 min) and reduced (57-71%) respectively. The effects of indole-pyruvic acid on slow-wave sleep time were still present after 3, 7 and 14 days of chronic administration (10 mg kg-1 day-1), whereas tolerance to the effect of indole-pyruvic acid on rapid eye movement sleep was observed. Daily food consumption was reduced (20-28%) by acute administration of indole-pyruvic acid (15-30 mg kg-1 i.p.), but tolerance developed after 5 days of repeated injections. These findings are in accordance with previous evidence suggesting that indole-pyruvic acid effects may be related to the activation of central serotonin neurons, which are involved in the inhibitory control of sleep and food intake.

Chronic zimeldine administration to cats: sustained increase of serotonergic effect as measured with sleep parameters

Sleep effects of oral administration of the serotonin uptake inhibitor zimeldine to cats on 15 consecutive days was measured. On administration days 8 and 15 REM sleep and PGO activity were significantly reduced. The amount of SWS-2 was not changed for the whole group, but a subgroup with low baseline values showed an increase during zimeldine administration. A REM sleep rebound was observed on withdrawal day 2, and NREM sleep PGO activity was increased on withdrawal day 5. The results indicate an increased serotonergic inhibition of PGO wave activity and REM sleep during chronic administration of zimeldine.

Increased waking as well as increased synchronization following administration of selective 5-HT uptake inhibitors to rats

Sleep and waking stages and EEG power spectra were investigated in rats following saline injections and injection of 10 and 20 mg/kg zimeldine or 10 and 20 mg/kg alaproclate, both selective 5-HT reuptake inhibitors. Following zimeldine there was a biphasic effect on sleep and waking, waking being increased during the first 2 1/2 h of recording, while slow wave sleep (SWS), in particular highly synchronized SWS-2 with high slow wave activity, was increased during the second 2 1/2 h recording period. Analysis of EEG power spectra indicated that the amount of synchronized slow wave activity was also increased within the sleep that occurred during the waking-dominated initial 2 1/2 h period. These data suggest simultaneous appearance of increased waking and increased synchronization following general serotonergic stimulation. They are interpreted as due to effects on different regions of the serotonergic system or on different serotonergic receptors. Consistent with earlier findings, zimeldine also suppressed rapid eye movement (REM) sleep. Following alaproclate, a clear waking effect was present, but only a weak synchronizing effect was seen. This is consistent with data on regional differences in uptake inhibition for zimeldine and alaproclate. Alaproclate also reduced REM sleep. Zimeldine or alaproclate was also administered to rats that had reduced sleep following pretreatment with a moderate dose of parachlorophenylalanine (PCPA). None of the drugs increased waking any further, but the PCPA-pretreated animals that received zimeldine had increased SWS-2, indicating that the SWS-2 increase following zimeldine alone was not a rebound effect.

Effects of ritanserin on sleep disturbances of dysthymic patients

Ritanserin, a selective and potent serotonin-2 antagonist, is effective in the treatment of a variety of syndromes related to anxiety and depression, including dysthymic disorder. In animals and healthy volunteers, ritanserin specifically increases slow-wave sleep and the hypothesis arises that this effect on sleep may contribute to its therapeutic properties. Therefore, we studied the effects of ritanserin on sleep in a group of dysthymic patients (DSM-III). Polygraphic recording as well as subjective evaluations of the quality of sleep were performed before and at the end of a 4-week period of double-blind medication with either ritanserin (10 mg o.d. in the morning) or placebo. At baseline, patients showed at fragmented and superficial sleep, with low amounts of slow wave sleep. Ritanserin significantly increased Slow Wave Sleep and changed the frequency and distribution of some stage transitions during the night. No other sleep parameters were modified by ritanserin treatment.

Short-term effects of fluoxetine and trifluoromethylphenylpiperazine on electroencephalographic sleep in the rat

Fluoxetine and trifluoromethylphenylpiperazine (TFMPP) were studied for their short-term effects on electroencephalographic sleep in male rats. Following single injection, each drug produced a sizeable, dose-related suppression of rapid-eye-movement (REM) sleep that persisted for 4-5 h (fluoxetine, 0.625-5 mg/kg; TFMPP, 0.10-1.25 mg/kg). TFMPP also consistently increased non-REM (NREM) sleep during the second hour after drug injection, though this effect was not dose-related (it was seen at all doses tested). Fluoxetine produced small effects on NREM sleep that varied non-systematically with dose and time after drug injection. TFMPP, but not fluoxetine, also increased at all doses the number of delta waves per minute of NREM sleep in the second hour. A structural analog of TFMPP that is inactive at serotonin (5-HT) receptors [4-(m-trifluoromethylphenyl)piperadine; LY97117] was also tested, and found to be devoid of effects on NREM and REM sleep. Both fluoxetine (a 5-HT reuptake blocker) and TFMPP (a 5-HT agonist) enhance transmission across 5-HT synapses, though by different mechanisms. Because they have the common effect of suppressing REM sleep, and in a dose-related manner, the data support the notion that 5-HT neurons in the brain, when active, can suppress REM sleep.

The effects of zimeldine and alaproclate combined with a small dose of 5-HTP on waking and sleep stages in cats

Sleep and waking stages in cats were studied 8 h following administration of zimeldine and alaproclate, in combination with saline or 5-hydroxy-1-tryptophan (5-HTP). Both drugs in combination with saline reduced rapid eye movement sleep and ponto-geniculo-occipital wave activity, and the effects were potentiated with 5-HTP. After administration of zimeldine in combination with 5-HTP there was an increase in synchronized waking (W-2), followed by an increase in slow wave sleep (SWS), at first SWS-1 with spindles and then highly synchronized SWS-2. The changes were interpreted as reflecting a serotonergic deactivating effect expressed by an electroencephalographic synchronizing effect. This is consistent with earlier studies following serotonin depletion and serotonin precursor loading. After alaproclate in combination with 5-HTP there were changes in W-2 and SWS-1 suggestive of the same process but much less pronounced. The difference between the two serotonin uptake inhibitors is interpreted as being due to regional differences in their uptake inhibition.