The Transition from Slow-wave Sleep to Paradoxical Sleep: Evolving Facts and Concepts of the Neurophysiological Processes Underlying the Intermediate Stage of Sleep

@

Neuroscience and Biobehavioral Reviews, Vol. 20, No, 3, pp. 367-387, 1996 Copyright 01996 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0149-7634/96 $32.00 + .XI

Pergamon

0149-7634(95)00055-0

The Transitionfrom Slow-wave Sleep to Paradoxical Sleep: Evolving Facts and Concepts of the Neurophysiological Processes Underlying the IntermediateStage of Sleep CLAUDE

GOTTESMANN

Laboratoire de Psychophysiologie, Facult&des Sciences, University de Nice-Sophia Antipolis, 06108 Nice cedex 2, France

GOTTESMANN, C. The transitionfrom slow-wave sleep to paradoxical sleep: Evolving facts and concepts of the neurophysiological processes underlying the intermediatestage of sleep. NEUROSCI BIOBEHAV REV 20(3)367-387,1996.— Paradoxical sleep in rats, cats and mice is usually preceded and sometimes followed by a short-lasting (a few seconds) electroencephalogram (EEG) stage characterized by high-amplitudespindles in the anterior cortex and low-frequencytheta rhythm in the dorsal hippocampus. The former is an index of advanced slow-wavesleep; the latter is an index of limbic activation since it occurs during active waking and paradoxical sleep. Barbiturates and benzodiazepines extend this intermediate stage at the expense of paradoxical sleep while concomitantly barbiturates suppress the pontine reticular activation characteristic of this sleep stage. During the intermediate stage, thalamocortical responsiveness and thalamic transmission level, which are controlled by brain stem activating influences, are the lowest of all sleep–wakingstages. The unusual EEG pattern of this stage is otherwise only observed in the acute intercollicular-transected preparation. Therefore, forebrain structures may be functionally briefly disconnected from the brain-stem during this short-lasting stage, which could possibly account for the mental content of a similar sleep period in humans. In spite of strong evidence in favour of this forebrain deafferentiation hypothesis, other data indicate that the IS is in some way linked either to slow-wavesleep or to paradoxical sleep. Copyright @ 1996Elsevier Science Ltd. Sleep mechanisms Intermediate stage Spindle Theta rhythm Central responsiveness Arousal threshold Hypnotics Serotonin Acetylcholine

have been generally overlooked, although they deserve study because they may help us to understand the functions of the preceding and succeeding stages. The aim of this review is to consider the work that has been carried out on the transition from SWS to PS. I shall take a historical approach in describing the development of ideas concerning the mechanisms and functions of this intermediate sleep stage.

INTRODUCTION

CLASSIFICATION GENERALLY begins with large entities, and progresses to finer distinctions and more entities. This has been the case with studies of the mechanisms involved in the sleep–waking cycle. When paradoxical sleep (PS) was first identified, and until quite recently (80), only three stages were distinguished in animals: waking, slow-wave sleep (SWS) and PS. In contrast, in human studies, five stages are distinguished (29). The two species which are studied most frequently, the cat and the rat, are small animals with relatively high metabolic rates and rapid alternation of stages, which are shorter in duration than for humans. It may be quite arbitrary to limit analysis to a simple distinction between three stages of the sleep-waking cycle. Until recently, the neurophysiological characteristics of the transitions between stages

IDENTIFICATION OF AN INTERMEDIATE STAGE BETWEEN SLOW-WAVE SLEEP AND PARADOXICAL SLEEP

The precise and complete description of the most important sleep-waking stages in the cat, initially described by Klaue (85), began with Dement’s (28) famous paper published in the EEG Journal. He described EEG patterns of waking, SWS and the 367

368

GOTT’ESMANN

FIG. 1. The intermediate stage (IS) of sleep in the rat. As slow-wavesleep (SWS) deepens, spindles occur in the anterior cortex. They progressively increase in amplitude becoming maximal prior to the cortical desynchronization of paradoxical sleep (PS) and persist, for a while, superimposed on the cortical activity of PS. Low-frequency theta rhythm already appears in the dorsal hippocampus during the high-amplitude cortical spindles. Only the end of PS phase is shown. Note the reappearance of the high amplitude cortical spindles prior to awakening while the frequency of the theta rhythm decreases. Calibration: 1 s, 200 pV. FF, fronto-frontal bihemispheric derivation; PP, parietal cortex; FRM, mesencephalic reticular formation; HPCd, dorsal hippocampus; EMG, dorsal neck muscles. (Reprinted from Ref. 47, with permission.)

“activated” stage of sleep, originally identified in humans by Aserinski and Kleitman (10), of rapid eye movement periods associated with subsequent dream recall. This stage was characterized by EEG lowvoltage activity, “complete absence of muscle potentials, twitching movements of the legs, ears and vibrissae and occasional tail movements. There were considerable movements of the eyeballs”. In the same paper, Demerit also found that the auditory arousal threshold was slightly higher during SWS than during

PS, which was confirmed recently (110). One year later, the important contribution of Jouvet and his group (73-77) began, with the demonstration of the crucial role played by the brain-stem [after the first incidental observations of Rioch (118)], and specifically, by the pontine reticular formation in the appearance of PS. In the rat, the first of these studies was performed by Michel et al. (104). Using a frontoparietal derivation and electromyographic electrodes (EMG) they

THE INTERMEDIATE

STAGE

369

NORMAL ANIMAL

UNDER PENTOBARBITAL

—I FIG. 2. Influence of barbiturates on the intermediate stage in the rat. (Top) A cortical spindle occurs during hippocampal theta rhythm prior to PS in the control condition. (Bottom) Under 15mg/kg of pentobarbital i.p., SWS patterns are not significantlymodified. Periodically, the rat enters the IS. However, instead of lasting for a few seconds the IS lasts up to several minutes and substitutes for PS. Calibration: 1 s, 100 pV. (Reprinted from Ref. 47, with permission.)

dissociated four sleep-waking stages: “hypervigilance”, with EEG theta rhythm and high muscular activity; waking with and without theta activity; SWS with slow waves and spindles; and PS, with occasional superimposed spindles. The next important paper on sleep in the rat was written by Roldan et al. (120). Using frontal cortex and dorsal hippocampus electrodes, but without EMG, these authors distinguished waking with theta rhythm; waking without theta rhythm; and slow-wave sleep, with “an acceleration of the irregular waves of high amplitude” 10s prior to PS (see legend to Fig. 3). The first paper to describe a transitional stage before and after PS was published 1 year later. We observed that PS in the Wistar rat was often preceded and sometimes followed by a short-lasting stage, characterized by high-amplitude spindles in the anterior cortex recorded monopolarly but still better bihemispherically, associated with theta activity in the dorsal hippocampus (Fig. 1) (47). This unusual EEG pattern association was remarkable since high-amplitude cortical spindles were considered to be an index of deep SWS, while theta activity was regarded as an index of central activation. Bremer (24) observed cortical spindles in acute intercollicular transected cats (cerveau iso16preparation), whereas theta activity occurs during active waking (64) and PS (70,81). Low or medium doses of pentobarbital (15-25 mg/kg i.p.) massively extended this intermediate stage(IS) at the expense of PS. The IS appeared periodically, as usual, but instead of being rapidly (a few seconds) replaced by PS patterns, it lasted up to several minutes, i.e., the usual duration of PS phases, but was suppressed by barbiturate (Fig. 2). As the barbiturate was progressively metabolized, the sustained highamplitude spindles gradually diminished in frequency of appearance while the cortical desynchronization reappeared.

In a short 1965 communication, Weiss and Adey (162) also mentioned the association of cortical spindles and hippocampal theta rhythm prior to the onset and at the end of PS. THE “INTERMEDIATE PHASE” OF

SLEEP

IN HUMANS

In 1986, a very interesting result was published in humans, and afterwards extended in 1968. Lairy (92) mentioned first an “intermediate phase” occurring before and sometimes just after PS. This short-lasting EEG stage (a few seconds to a few minutes) was characterized by the interspersed association of spindles and K complexes [an index of SWS (stage II)], and low-voltage activity without eye movements (an index of PS). Lairy also considered as intermediate phasis the desynchronized periods of 20s without eye movements appearing at the onset and end of PS. At the Bardolino meeting (1967), Lairy et al. (93) described the mental content associated with this sleep stage and modifications of this stage in mental illness. In the normal subject, it was difficult to establish a psychological contact with the subject behaviorally awakened from the intermediate phasis. Verbal reports did not reveal a visual content but, instead a “feeling of indefinable discomfort, anxious perplexity and harrowing worry”. Moreover, during brief reactive psychosisepisodes when patients were awakened from PS they momentarily showed no pathological symptoms [consistent with Snyder’s ethnologicaltheory of PS function in animals which predicted clear perception and cognition following PS (131)]. The troubles only reappeared after minutes when the waking state was definitively established. In contrast, these symptoms were amplified when the patients were awakened from the intermediate phasis. Lairy et al. (93) also found that this peculiar phasis of sleep was

370

GOTTESMANN

(i~ ( EQG —

–.

—

—

-.

--i-—-@r---+

90t4iNJlE3 NT&5 25rqkg PENIWWtBilAL

FIG. 3. Pontine reticular activity during the intermediate stage and paradoxical sleep in the rat. (Top) When the animal enters PS a strong tonic activation appears in the pontine reticular formation. This activation is further increased prior to and during the eye movement bursts. (Bottom) 90 min after 25 mg/kg of pentobarbital i.p., the pontine activation disappears during the high-amplitude cortical spindles of the prolonged IS. Note the appearance of pontine slow rhythmic activity. An isolated low-amplitude eye movement and a light hyperapnea occur during the transient residual pontine activation. Calibration 1s, 100pV. Cx monop, monopolar recording of frontal cortex; Pent, pens; EOG, electrooculogram. (Reprinted from Ref. 48, with permission.)

THE INTERMEDIATE

STAGE

371

10

4 k@wJk

10

c

B

A 10

10*’

3Jqp#Jlu

29C

18A,

17

EOG

EMG

3

I

+Jf%rJ’

17*

‘

EOG ~

EOG

I

EMG ~

EM~ ~

I

FIG. 4. Anterior and uostenor cortical smndles durme sleeDm the rat (A) Anterior smndles recorded from Kreig’s (91) areas during the slow-wave~tage. Their ~mpli&rdedecreases as the recor~ings become progressively more posterior from the frontal cortex, eventually disappearing in the posterior 18a area. (B) Anterior spindles during the IS. The very large amplitude spindles recorded in anterior areas 10 and 3 diminish slightly in area 29c and are absent in area 17where theta rhythm appears. (C) Posterior spindles. During SWSa typical spindle occurs in areas 17 and 7. It is absent in anterior areas 10 and 3. Calibration: 1 s, 100pV. (Reprinted from 139,with permission.)

absent in dementia but extended at the expense of PS in patients experiencing brief reactive psychosis. Koresko et al. (90) had already described this increased EEG pattern during and at the end of PS in schizophrenia. An increase in intermediate phasis at the expense of PS was also later described in oligophrenia (65). The description of a transitional stage before and after PS in humans encouraged us to investigate further its mechanisms in animals. PONTINE PROCESS RELATED TO THE INTERMEDIATE STAGE AND PARADOXICAL SLEEP

In 1966 we studied the activity of the pontine reticular formation during the sleep–waking cycle in the rat (48). Multiunit recordings showed a strong pontine cell activation appearing specifically during PS (Fig. 3). This activation was so strong that it could be recorded with a preamplifier connected to an EEG ink recorder with a good pass-band (ECEM apparatus). It commenced during PS at the onset of EMG atonia, resembled high-amplitude EMG activity, and increased even more just before and during the rapid eye movement bursts. Although the hippocampal theta rhythm of IS was of a lower frequency than during PS, hypersynchronized theta rhythm accompanied periods

of pontine overactivation preceding the appearance of the eye movement bursts. Pentobarbital, at the same dose as used previously, suppressed the pontine activation, while high-amplitude spindles persisted in the cortex. Sometimes, residual activation occurred, particularly toward the end of action of pentobarbital. In that case, rare low-amplitude eye movements and a light hyperapnea could be seen (Fig. 3). At that time, the results were interpreted as follows (48–50).During waking, the reticular activating impulses ascending preferentially from the mesencephalic level induce cortical desynchronization (108) and theta rhythm in the dorsal hippocampus (64). During sleep onset these influences decrease, progressively releasing the EEG synchronizing processes manifested by the appearance of slow waves and spindles of increasing amplitude. Prior to PS these influences are suppressed, inducing a cerveau isoh$like preparation. When the pontine activation of PS appears, they inhibit the spindles of IS and induce cortical desynchronization and increase theta rhythm frequency. At the end of PS the pontine activation disappears first, and shortly afterwards induces the spindle-releasing processes before awakening or a return to SWS (which rarely occurred). The presence of low-frequency theta rhythm during IS under barbiturates, in spite of the abolition of pontine activation, was considered to be the result of a disinhibitory process. This was a new idea since, at that

372

time, it was thought that theta rhythm could only be generated through brain-stem activating processes (64). The pontine reticular activation during PS was later described with true microelectrodes by McCarley and Hobson (102) in the cat, and by Vertes in the rat (155).

GOTT’ESMANN C/s

3 h 122-

SPACE-TIME DISTRIBUTION AND CHARACTERISTICS OF CORTICAL SPINDLES IN THE RAT

A systematic analysis of the dorsal cortical areas was undertaken, so as to study the space–time distribution, the intrinsic characteristics and appearance modalities of the spindles in the rat (139). Monopolar recordings were taken from all of Kreig’s (91) areas and bihemispheric symmetrical recordings from frontal 10, parietal 3, occipital 17 and cingular 29c areas. Two kinds of spindles were identified, anterior and posterior ones. The anterior spindles (Fig. 4A) had the highest probability of occurrence in frontal area 10. This probability decreased the more posterior the cortical recordings. This rule applied to usual regular spindles occurring during the slow wave stage as well as to the sharper spindles of the IS (Fig. 4B). Spindles occurring during the slow-wave stage were of higher frequency, lower duration and lower amplitude than those of the IS (Fig. 5). The anterior spindles of the slow wave stage progressively increased in amplitude, reaching their maximal levels during the IS (Fig. 4B). This amplitude decreased from the anterior to the posterior regions of the cortex. The posterior spindles (Fig. 4C) had an opposite cortical distribution. They appeared preferentially at the cortical posterior region, decreased towards the anterior areas and were absent at the frontal level. Moreover, their intrinsic frequency was higher, their duration shorter and their amplitude lower than the anterior spindles, and they were more regular, without a pointed appearance. Finally, they were much less frequent than the anterior spindles. The modalities of appearance in each hemisphere showed that anterior and posterior spindles could appear simultaneously, even in phase, but could also appear independently. In a second study on spindles performed 7 years later (37), we first noticed that one (or more seldomly two) characteristic high amplitude spindles often appeared in the occipital cortex and dorsal hippocampus just before the appearance of the IS theta rhythm (Fig. 6, top). They seemed to be posterior spindles of SWS of maximal amplitude. Moreover, the posterior spindles of SWS had a mean amplitude of 263 pV on the occipital cortex, of 388 pV on the white matter of the neodecorticated rats and of 489 pV in the dorsal hippocampus. Therefore, these spindles appear to originate from the hippocampus and diffuse to the overlying posterior cortex, like the theta rhythm (44). Another finding was that unilateral decortication eliminated anterior spindles ipsilaterally and impaired their number, amplitude and duration on the intact side up to 1 month after the surgery. These results of anterior spindles were discussed in relation to their thalamic generating processes described by Andersen et al. (5).

B

s

18 .

b 1

1

c

Pi Soo.

il 23

0

40

Soo-

❑ ✻

200.

1

23

1

23

FIG. 5. Characteristics of the cortical spindles in the rat. (A) Intraspindle frequency. During the slow wav~stage the posterior spindles (1) have a significanthigher frequency than the anterior spindles (2) and those of the IS (3). (B) Spindle duration. The posterior spindles are shortest, whereas those of the IS are longest. The three spindle types have significantlydifferent mean durations. (C) Spindle amplitude. The spindles of the IS (3) are of a larger amplitude than the posterior spindles (1) and those of SWS(2). The amplitude of posterior spindles and anterior spindles of the slow-wave stage are also different (p = 0.05). (D) Spindle morphology. IS spindles very often have a pointed appearance, somewhat similar to paroxystic spike and waves.This is seldom the case for the anterior and posterior spindles occurring during SWS. (Reprinted from Ref. 139with permission.)

CENTRAL RESPONSIVENESS DURING THE INTERMEDIATE STAGE

To investigate the functional signification of the IS, the existence of which was subsequently confirmed by other authors (17,31,35,86,11 O,114*),we undertook the analysisof the central reactivity level of the somesthetic *It is worth mentioning that Trachsel et al. (144), with an electrode situated on the rat 3.5mm behind the bregma, found just before PS a transient important increase in power density in the 6.25–9.0Hz and 10.25–20Hz frequency bands (see Fig. 7). Since during the IS the theta rhythm frequency is about 5–8Hz and the cortical spindles about 7.5–14Hz, these authors also identified the EEG characteristics of this stage. A similar study by spectral analysis using a frontoparietal derivation was recently done by Benington et al. (17). The authors also detected the two patterns of the IS before PS, although their method probably would not always allow the spindles to be distinguished from the theta rhythm because of the overlapping of the frequency of these two activities.

THE INTERMEDIATE

STAGE

373

NORMAL

ANIMAL

FF

I

F Ret

I

““~

‘

EOG

—I

EMG ~

AFTER

NE MBUTAL i I t

““CERVEAU

IS OLE”

I

—I FIG. 6. The acute intercollicular transected rat. (Top) Entrance into PS. High amplitude cortical spindles occur during hippocampal theta rhythm. Note the two hippocampal spindles whichoccur just prior to the theta rhythm appearance. (Center) The same animal under a low dose of pentobarbital (25 mgkg i.p.). The IS substitutes for PS. (Bottom) The same animal after intercollicular transection. Regularly occurring cortical spindles appear during continuous low-frequency theta rhythm. Calibration: 1s, 200pV. (Reprinted from Ref. 51, with permission).

system; since the Wistar rat is albino, it has a modified visual system. We defined seven stages in the sleep-waking cycle (52,58): (1) waking with theta rhythm, corresponding to psychomotor active (149) and/or attentive waking (63); (2) waking without theta rhythm, during which the rat is quiet or displays automatic motor activities; (3) cortical slow waves characterizing the first stage of sleep; (4) spindles, which occurred as sleep deepens and progressively increase in amplitude and duration; (5) IS; (6) PS without eye movements; and (7) eye movement bursts of PS (Fig. 7).

Cortical responsiveness, studied by somesthetic radiations stimulation, did not yield any specific information about the IS since there was no significant variation in the amplitude of the postsynaptic positive wave 4 recorded in the somesthetic area SI (8) (Fig. 7,1). In contrast, the amplitude of the same component induced by lemniscal stimulation, which reflects the thalamocortical excitability, showed important variations during the sleep-waking cycle (36) (Fig. 7,2). Responsiveness was very low during waking with theta rhythm, became maximal during waking without theta activity,decreased progressively during sleep with slow

374

GO’ITESMANN

I

-i/$==‘$-J&J&

;&‘&

J)-,

FIG. 7. Central responsiveness during the sleep–waking cycle in the rat. The seven stages shown in their usual physiological sequence are: ,(1) waking with the theta rhythm, which corresponds to attentive and/or psychomotor active behavior (AW); (2) waking without theta rhythm corresponding to quiet waking or non-motivated motor activities (NA); (3) the first stage of sleep, characterized by slow waves of progressively increasing amplitude (SW); (4) spindles of increasing amplitude, which occur as sleep deepens (SP); (5) IS; (6) PS without eye movements (Pl); (7) eye-movement periods of PS (P2). Calibration: 1 s, 100I.LV.Fcx, frontal cortex recorded monopolarly; oc CX,occipital cortex. The cortical, thalamocortical and thalamic responsiveness of the somesthetic system corresponding to these stages is illustrated by five superimposed responses: (1) the SI primary cortex responsiveness, tested by the amplitude of the positive component 4 (shown by an arrow) induced by thalamocortical radiation stimulation, does not show any significant variation during the IS (calibration: 10ms, 500I.IV);(2) the S, cortical response induced by lemniscal stimulation (also quantified by the component 4 amplitude) is lowest during the IS but not very different from active waking (calibration: 10ms, 500pV); (3) the thalamic ventrobasal complex responsiveness to Iemniscal stimulation was quantified by the amplitude of the second (postsynaptic) component r, (shown by an arrow). It was the lowest of all slee~waking stages during the IS (calibration: 2 ms, 100pV); (4) the thalamic input was analysed by the first component amplitude of the Iemniscal response induced by ventrobasal stimulation. A significant presynaptic inhibition appeared only during waking with theta rhythm and during the eye movement periods of PS. The former had a cortical origin, the latter a pontine origin (see text). Calibration 1 ms, 25 I.IV.(Reprinted from Ref. 52, with permission.)

waves and spindles and became minimal during the IS before increasing massively during PS without eye movements. During the eye movement bursts the thalamocortical responsiveness decreased significantly. As there was no great difference between the responsiveness during active waking and during the IS, we studied the response induced in the thalamic ventrobasal complex using the same stimulus (Fig. 7,3). This tests the thalamic transmission level. Here, it became clear that the postsynaptic second component rl of the response, which reflected the thalamic transmission level, was of significantly lower amplitude during the IS than during all other sleep-waking

stages. To study the thalamic sensory input, the antidromic response induced in the lemniscus medialis by ventrobasal stimulation showed that the low transmission level during the IS was not related to presynaptic inhibition. Indeed, the response was not significantly increased in amplitude (Fig. 7,4). Incidentally, there was a presynaptic inhibition of cortical origin during waking with theta rhythm (39) and during the eye movement bursts of PS which had a pontine origin (96), since during these two stages there was a significant increase in the response amplitude. These results were important in confirming our hypothesis that the IS corresponded to a transient

THE INTERMEDIATE

375

STAGE

HYPOTHALAMIC

STIMULATION

FIG. 8. Responsiveness of the acute cerveau isoE rat. Medial posterior hypothalamus stimulation (100c/s) above a given threshold induces an increase in the continuous theta rhythm frequency, while the cortical spindles are not suppressed and their subsequent occurrence is not prevented. Similarly, olfactory stimulation increases the theta rhythm frequency without affecting the cortical recording (calibration: 1 s, 200 pV). Fl, left frontal cortex; Fr, right frontal cortex. (Reprinted from Ref. 51, with permission.)

cerveau iso16 preparation, since it was already well known then that brain-stem ascending activation controls the thalamic transmission level (34,132). THE ACUTE CERVEAU ISOLE RAT

To verify the hypothesis that the IS corresponds to a transient cerveau iso16preparation, we made bilateral intercollicular transections in the acute rat (60). Twenty-four animals were anesthetized with either ether or pentobarbital. In 13 animals the transection was made in chronic preparations which have been previously recorded for several days in the normal date and under administration of a dose of 15–25mg/kg i.p. of pentobarbital. As soon as the anesthetic was elimi-

nated the rats showed a strong myosis and the classical cortical spindles of the recently transected cerveau iso16 preparation (13–15, 24,112,160) associated with continuous or almost continuous low-frequency theta rhythm (4-6 c/s) (Fig. 6, bottom). This unusual association of cortical and hippocampal patterns lasted for many hours until the experiment was terminated by a barbiturate overdose. In the discussion we pointed out that the theta rhythm of the cerveau iso16preparation, like that of the IS, could only appear through a brainstem disinhibitory process. We also mentioned that in the cat, Tokizane (143) already showed theta rhythm associated with spindles in the acute cerveau iso16 preparation in his important paper (in a figure without a legend) and that Olmstead and Villablanca (112)

376

observed an “excess of theta rhythm” in the chronic cerveau iso16preparation immediately after recovery from surgery. In addition, we emphasized that during the IS, as in the cerveau iso16preparation, the brainstem ascending activation from the reticular core and inhibition based on the evoked potentials study of Demetrescu et al. (30), and on the serotoninergic (103) and noradrenergic (27) influences are strongly lowered or suppressed. Indeed, the monoaminergic neurons decrease their firing rates during SWS and rarify or suppress their firing during the transition into PS. This silence of monoaminergic neurons was later confirmed by several authors (11,67,116,117).We concluded by reiterating the psychophysiologicalconclusions of our thesis that the particular mental content of LairY’s intermediate phase of sleep might be related to the transient lowering of the global brain stem ascending influences, “which could lead to unusual and no longer controlled higher nervous processes”. We also noted that “Finally, during PS the postulated coexistence of strong activating influences and the low level of ascending inhibitory ones acting on the phylogenetic recent structures may contribute to explain the differences of mental processes in waking and sleep” (60). The continuous or almost continuous low-frequency theta rhythm and cortical spindles of the acute cerveau isol~preparation bore a striking resemblance to the IS. Did this theta rhythm reflect a true activation of the hippocampus, while the cortex was functionally disconnected? We decided to measure the responsiveness of the acute cerveau iso16preparation to central stimulation of the medial posterior hypothalamus and peripheral olfactory stimulation of expired air of an experimenter blown through a plastic tube on to the rat’s nostrils (146). Above a given threshold, the central stimulation (200 c/s, 100 ps) induced an increase of theta rhythm frequency, which was voltage dependent (Fig. 8). The mean frequency increased from 4.2 c/s to 6 c/s (p < 0.001) during the 2s stimulations, but this increase was tonic since 15s after the end of stimulation the frequency remained higher than baseline at 4.5 c/s (p < 0.005). The olfactory stimulus also increased the theta rhythm frequency (Fig. 8) from 4.5 c/s before stimulation to 6.6 c/s (p< 0.001). This increase was essentially phasic, but sometimes outlasted the olfactory stimulus, but to a lesser degree than after hypothalamic stimulation. However, the central and peripheral stimulations neither suppressed nor prevented the spontaneous appearance of cortical spindles, which clearly showed that the cortex was comatose while the dorsal hippocampus was highly reactive. This theta rhythm (like that of the IS) occurred through a brain-stem disinhibitory process, and other experimental data showed that electrolytic elimination of the medial raphe nucleus induced high levels of theta rhythm (100). We pointed out that during PS, pontine reticular activation reinforces the anterior pacemaker, increasing the frequency of the IS theta rhythm. We noted some interesting species differences. Olfactory stimulation could desynchronize the cortex of the acute cerveau iso16cat (6) but not in the rabbit, as shown by Arduini and Pompeiano (7) of pentobarbital. As soon as the anesthetic was elimi-

GOTTESMANN

licular transected animal. Using the same cat preparation, Tokizane (143) could not desynchronize the cortex by medial posterior hypothalamic stimulation. Recently, Sakai et al. (124) showed that cholinergic stimulation of the posterolateral hypothalamus induces cortical desynchronization and hippocampal theta rhythm in the cat cerveau iso16preparation. THE INTERMEDIATE STAGE IN THE CAT

Although the neurophysiological processes underlying the transition from SWS to PS were elucidated in the rat, did the IS also exist in other species, i.e., is it a general sleep phenomenon or is it restricted to the rat? To answer this question we studied the cat. Peculiar patterns occurring before entrances into PS had already been observed by other investigators. Thomas and Benoit (141,142)were the first to describe the sommeil phasique h ondes lentes (SWS with PGO waves) characterized by cortical slow waves, absence of eye movements, persistence of EMG and pontogeniculo-occipital waves (PGO). The latter was an index of PS (19,68,73,105). Two years later, Ursin (145), without recording PGO waves, described a short-lasting transition period prior to PS during which cortical slow waves disappeared that were replaced by spindles. Sometimes, similar patterns were also observed during paradoxical sleep. In 1974, Puizilloux et al. (l15a) also observed SWS with PGO waves prior to PS and easily induced this stage by vago-aortic low voltage stimulation in the acute enc6phale isoh$cat. We implanted electrodes in the frontal, occipital and somesthesic SI cortex, dorsal hippocampus, lateral geniculate nucleus and ventrobasal complex of cats. Electrodes were also implanted on each side of one orbit and into the dorsal neck muscles (55). We were able to dissociate nine functionally successivestages of sleep-waking: (1) waking with theta rhythm; (2) waking without theta rhythm; (3) behaviorally motionless waking with cortical low-voltage synchronized activity (122); (4) drowsiness with low-amplitude spindles; (5) slow-waveactivity;(6) spindles of progressively increasing amplitude; (7) IS patterns; (8) PS without eye movements; and (9) PS with eye movements (Fig. 9). The IS, clearly characterized by cortical spindles and low-frequency theta rhythm (3.8 c/s)—which increased during PS (4.8 c/s, p < 0.025)— occurred during the sommeil phasique ii ondes lentes. Occasionally, spikes occurred during the IS without any change in the cortical and hippocampal patterns. The IS occurred clearly in Z7Y0 of cases, in abortive form in 307. and was absent in 43%. It appeared clearly at the end of PS in 4°L of cases, in abortive form in 1lYo, and was absent in 85Y0.Its duration was shorter than in rats (l–3 s). Pentobarbital (10 mg/kg i.p.) massivelyincreased the IS at the expense of pS as the rat. We also studied the thalamocortical responsiveness of the somesthetic system. The amplitude of the postsynaptic positive wave 4 of the evoked potential induced in the cortical SI area by ventrobasal stimulation varied during the sleep-waking cycle exactly as in the rat, being the lowest of all stages a- legend) aiid-that @mitead and Villabianca (112)

THE INTERMEDIATE

377

STAGE

NA

MW

Sw

OR

SP

— —

yw’w M

—

Ml!

Is

P2

—

w —

‘l-

-’

!“”+ ‘o-

.. &

/’- +-

FIG. 9. The intermediate stage in the cat. (Top) During the sleep-waking cycle the cat exhibits two more stages than the rat: (1) motionless waking with cortical synchronized activity (MW); and (2) drowsiness with low-amplitude spindles (DR) (calibration: 1s, 100pV). (Bottom) As in the rat, the somesthetic thalamocortical responsiveness during the IS is the lowest of all sleep-waking stages. Average of 16 responses (calibration: 5 ms, 10 wV).S, Cx, somesthetic S,, cortical area; LGB, lateral geniculate nucleus. (Reprinted from Ref. 55, with permission.)

TABLE 1 NORMAL RAT AND CAT COMPARISON OF EEG ACTIVITIES DURING THE INTERMEDIATE STAGE (A), UNDER BARBITURATES (B) AND IN THE ACUTE CERVEAU ISOLE PREPARATION (C)

Duration Occurrence frequency Frontal spindles Hippocampal theta rhythm

Rat

Cat

Before PS [ * After PS Mean amplitude Mean intrinsic frequency { Mean duration Mean intrinsic frequency

1–5s 7570 15% 497 pv 10.3Cls 2.8 S 6.3 C/S

1–3 s 27Y0 470 125 pV 13.0Cls 2.0s 3.8 Ck

Mean amplitude Mean intrisic frequency Mean duration [ Mean number per minute Mean intrinsic frequency

1.5 min 519 pv 10.4Cls 2.9 S 21 5.7 Cls

9.0 min 194 pv 12.2Cls 2.2 s 12 3.3 Cls

Mean amplitude Mean intrinsic frequency Mean duration [ Mean number per minute Mean intrinsic frequency

Several hours 507 l.lv 8.6 Cls 2.2 s 6 4.3 Cls

Several hours 150 pv 12.5 C/S 2.0 s 8 3.5 Cls

●

Duration Frontal spindles Hippocampal theta rhythm Duration Frontal spindles Hippocampal theta rhythm

Reprinted from Ref. 53, with permission.

378

GOTTESMANN

FIG. 10.The intermediate stage in the mouse. A monotonous rather irregular low-frequencytheta rhythm occurs about 20 prior to PS during cortical slow waves interspersed with spindles. Just before entrance into PS a high-amplitude spindle appears during the theta rhythm. Calibration: 1 s, 100wV.(Reprinted from Ref. 45, with permission.)

amplitude between this stage and PS without eye movements. Finally, we made intercollicular transections under fluothan anesthesia on these cats. These acute cerveau isol~ preparations showed continuous or almost continuous low-frequency (3.5 c/s) theta rhythm associated with cortical spindles, similar to patterns we had previously shown in true acute animals (54,61). Subsequently, to compare more precisely the results obtained from rats and cats, we quantitatively analysed the EEG characteristics under normal conditions, barbiturates and after intercollicular transection (53) (Table 1). The results confirmed that the IS appears before and after PS more often in rats than in cats, and is longer lasting in the rat, with higher amplitude spindles and higher frequency of the theta rhythm. Similar differences between species were present under barbiturates, which lowered the theta rhythm frequency in both species. Finally, after intercollicular transection, the mean intrinsic frequency of the cortical spindles decreased only in rats, while that of the theta rhythm was decreased in both species. We concluded that “these various data suggest that during the IS (as in the acute cerveau isok$rat and cat) the anterior studied structures (neocortex, dorsal hippocampus, ventrobasal complex) seem to be released from brain stem and peripheral influences, leading to a large but transient intracerebral deafferentation. From a teleological point of view it is understandable that such a crucial stage is very shortlasting since it is vital for species survival” (53). Some years later we studied (45) the sleep–waking behavior of mice (Eialb C strain) (Fig. 10) which had previously been studied by others (148,154,163). Among 208 episodes of PS, the IS, prior to PS, appeared in all cases, contrary to our findings in rats and cats. However, after PS ended, only two IS episodes occurred. Paradoxical sleep was preceded by slow waves interspersed with spindles which became of high amplitude just before the onset of PS. The theta

rhythm frequency appeared 16.4s before the cortical desynchronization and the PS phases lasted 85s. The theta rhythm frequency was 5.9 c/s during the IS vs 7.5 cls during PS and 6.7 cls during waking. Differences between all were significant. The only difference between the IS in this mouse strain and that in rats and cats, was that the disinhibitory influences of the median raphe nucleus on the theta rhythm pacemaker occurred earlier in mice. Consequently, since the IS occurs in rats, cats, mice and probably in humans, this supports the existence of this sleep stage in mammals in general. THE TRIGGER ZONE OF THE INTERMEDIATE STAGE THETA RHYTHM

Although the generalization of the IS in mammal species was demonstrated and its neurophysiological background well elucidated, one fact remained unsolved. The continuous or almost continuous lowfrequency theta rhythm in the acute intercollicular transected rat and cat, and during the IS under barbiturates, must be triggered by structures in front of the cerveau iso16 transection. Nevertheless, it is possible that, in the intercollicular transected animal, the small part of the mesencephalic reticular formation in front of the transection could have induced this monotonous theta rhythm. In the same way, an ongoing small portion of the brain-stem activating influences of PS (48,49,102,155)could induce the theta rhythm of the IS in the normal animal and a residual one in the rat and cat under barbiturates. To elucidate this point we made precollicular transections in the acute rat and cat. In the rat the transection induced continuous slow waves interspersed with spindles in the cortex and massive amounts of low-frequency (4.5 c/s) theta rhythm in the dorsal hippocampus (Fig. 11A) (56). The mean value of occurrence of theta was 34’Yoof the recording time but it attained more than 70Y0in several

THE INTERMEDIATE

STAGE

379

A FCx##(#$J@4 ~’w~ HFC-WMm~

B

18v

— FIG. 11. The EEG activity of the acute precollicular transected rat. (A) The transected preparation displays cortical slow waves, interspersed with spindles and continuous low-frequency theta rhythm, in the dorsal hippocampus. (B) Medial posterior hypothalamus stimulation (250 c/s) induces the theta rhythm when the limbic rhythm is absent. (C) The same stimulation increases the theta rhythm frequency when it is present. Calibration: 1 s, 200pV. (Reprinted from Ref. 56, with permission.)

animals. Medial posterior hypothalamic stimulation (200c/s, 300 pV) was able to induce the limbic theta rhythm when it was absent (Fig. 1lB), and when it was present increased its frequency up to 7.3 c/s, while the frontal cortex remained synchronized (Fig. 1lC). In the cat, we confirmed previous findings of Kawamura and Domino (82) of cortical slow waves interspersed with spindles but no spontaneous theta rhythm (62). However, it was previously shown that the limbic rhythm could easily be induced by medial posterior hypothalamic stimulation (82). Therefore, the trigger zone of the theta rhythm was situated in the front of the mesencephalon. At that time, Bland (20) concluded that the hippocampal theta rhythm generators (21,166)were under the influence of one (95,113) or possibly two (106) septal pacemakers. If so, would either the septum or hippocampus generate the theta rhythm shown in slice preparations (89)? We considered another hypothesis. Theta rhythms are induced by the medial posterior hypothalamus stimulation in the high brain-stem acute-transected animal (56,82,143,146).To investigate whether this level is a trigger zone for the theta rhythm observed in the brainstem-transected. animal and during the IS, the acute high amphtude just betore the onset ot YS. “l’hetheta

stimulation in the chronic animal (4) because a descending process cannot be supported. However, since the experiment utilized acute preparations the stimulation could have acted on transected passing fibers arising from the brain-stem. Therefore, we made acute transections passing through the posterior pole of the superior colliculus and ending at the mid-hypothalamic level, thereby disconnecting the posterior hypothalamus from the forebrain (46). After elimination of the anesthetic (ether or thiopental) the cortical EEG showed cortical slow waves without spindles that probably result from injury to the thalamic reticular nucleus (134). Most importantly, no theta rhythm appeared. A few seconds of theta observed during a 7h recording session could have either issued from the hippocampus (89) or been triggered by the entorhinal cortex, which is linked to hippocampus CAI area (167), and whose cells fire at theta rhythm frequencies (l). We concluded that these results were in agreement with the assertion of Wilson et al. (165) that the medial posterior hypothalamus is a trigger zone for the theta rhythm. Our experimental data strongly indicated not only that the theta rhythm of the IS should occur throu activation of the posterior hypothalamic trigger recorPmg time out n attamea more tnan iu~Om several

380 could have the same origin. We hypothesized that the pontine-activating influences of this stage of sleep (48,49,102,155)would be solelyresponsible for the slight tonic increase in theta rhythm frequency, still physically increased during the eye movement bursts. Our reasoning was based on the decreased frequency of the PS theta rhythm followingelectrolytic lesions of the posterior hypothalamus (119). Because these lesions also destroy cells and passing fibers we suggested that it would be interesting to use ibotenic acid, which would only destroy the cell bodies of the postulated theta rhythm trigger zone (51). If our working hypothesis is correct, the theta rhythm of the IS should disappear while that of PS should decrease in frequency. It should also be pointed out that we did not know at this time that Alonso and Llinas (2) had reported in a short abstract that in vitro medial posterior hypothalamus neurons fire at theta rhythm frequency and subsequently concluded that “the theta rhythm is boosted by the medial mammillary bodies” (3). THE INTERMEDIATE STAGE IN A RAT MODEL OF ABSENCE EPILEPSY

We have been studying the effects of anti-epileptic in WAG/Rij rats (41), which show genetically induced spike-wave discharges during the transitions from quiet waking to SWS (152). These rats had a longer duration of the IS (8.7s vs 2.9s, p < 0.01) with, per phase, an increased number of cortical spindles (5.0 vs 1.8,p < 0.01) of higher amplitude (699 pV vs 308 pV, p < 0.02) than Wistar rats (40). Moreover, the IS occurred much more often prior to PS in WAG/Rij rats, such that PS without prior IS occurred in 2.2910of cases vs 32.1~0 (p < 0.001) in Wistar rats. The WAG/Rij rats were less likely to enter PS. The IS was followed by PS in 25.4% of cases vs 53.8% (p< 0.05) of cases in Wistar rats. Because the number of PS episodes was lower (1.0 vs 2.5 per h, p < 0.02) the total amount of PS was also decreased (2.OYOof the total recording period vs 5. Q’%0, p < 0.05).Therefore, this rat strain seemed a good model for studies of the IS, in which we also showed that riluzole (at 4 mg/kg i.p.), a compound which inhibits excitatory amino acid transmission (16), reduced the frequency of spike-wave discharges and enhanced SWS at the expense of waking, without modifying the IS and PS (121).

GO’ITESMANN

SWS control ‘c’ ~ HPC EOG EMG~

SWS under Triazolam

IS under Triazolam

drugs

INFLUENCE OF SECOND AND THIRD HYPNOTIC GENERATION COMPOUNDS ON THE INTERMEDIATE STAGE

We had already shown that barbiturates extend the IS at the expense of PS in rats (47) and cats (55). We subsequently studied the effects on sleep stages of benzodiazepines (the next important class of hypnotics), and of zolpidem [an imidazopyridine (32)] and zopiclone [a cyclopyrrolone (78)], two new generation non-benzodiazepine hypnotics. We injected three benzodiazepines i.p.: two with a short half-life, midazolam (1.0 and 3.0 mg/kg) and triazolam (0.3 and 1.0 mg/kg); and one with a long halflife (diazepam 1.0 and 3.0 mg/kg) in Wistar and

I I

FIG. 12. Influence of benzodiazepines on the intermediate stage in the rat. During SWS under 3 mgkg of triazolam i.p., characteristic spindles of high frequency and low amplitude are superimposed on the slow waves. When the IS periodically occurs (here 110min after injection) it substitutes for PS. Calibration: 1 s, 200pV. (Reprinted from Ref. 42, with permission.)

WAG/Rij rats (Fig. 12) (42). The principle resul~was that under the three benzodiazepines, as under barbiturates, the duration of the IS phases was massively increased and they substituted for PS. In addition, all doses of the three compounds increased the intrinsic frequency of cortical spindles during the slow-wave and intermediate stages and high doses decreased their amplitude. However, their amplitude was much higher during the IS than during slow-wave stages. The theta rhythm frequency during the IS remained unchanged, but was decreased during PS by all doses of the three compounds. After PS had reappeared, the frequency of eye movements remained depressed by diazepam and triazolam, but not by midazolam.

THE INTERMEDIATE

STAGE

The effects of 2.5, 5.0 and 7.5 mg/kg i.p. zolpidem were quite different (59). The 2.5 mg/kg dose (already hypnotic) only decreased PS during the first 2 h of the 6 h recordings. The 5 mg/kg dose very slightly increased the total duration and number of IS episodes, but significantly increased the duration and number of those not followed by PS. It also increased the latency of PS appearance so that its total duration was decreased during the first 3 h. The high dose of 7.5 mg/kg decreased the total amount of PS without affecting the theta rhythm frequency of the IS, PS or the frequency of eye movements during PS. Thus, this new compound, which binds GABAA receptors containing the VI subunit (115), did not increase the IS at the expense of PS and, therefore, better preserves the normal physiology of sleep. Zopiclone was administered i.p. at 2.5, 5.0 and 7.5 mg/kg. The latencies of occurrence of the IS and PS were increased dose dependently as under zolpidem, so that their total amounts were decreased during the 6-h recordings. Like zolpidem (but unlike barbiturates and benzodiazepines), zopiclone never substituted for PS by the IS (Gauthier, Arnaud and Gottesmann, in preparation). THE RELATIVE PROXIMITY OF THE INTERMEDIATE STAGE TO SLOW-WAVE AND PARADOXICAL SLEEP STAGES

Reticular Formation Arousal Threshold

Although the IS has specific EEG characteristics, distinct from those of slow-wave and PS stages, we investigated its functional properties that could be linked with the previous or following stages. We began by studying the mesencephalic arousal threshold (114). It was already well known that this is much higher during PS than during SWS (18,48,120).The arousal threshold to stimulation (100 c/s, 100 ps for 2s) during the IS (77 PA) was slightly higher than during wellestablished slow-wave stages (61 PA, p < 0.03) but much lower than during PS (132 pA, p < 0.001). We also confirmed that the spindles during the IS were of a higher amplitude (559 pV vs 369 jIV, p < 0.05) and longer duration (1.88s vs 0.99s, p < 0.01) than during the slow-wave stage. In addition, the theta rhythm was of a lower frequency than during PS without eye movement bursts (7.20 c/s vs 7.84 c/s, p < 0.01). We concluded that the IS truly constitutes a specific sleep stage. In support of this conclusion we presented the following evidence: the thalamic reticular nucleus which is implicated in cortical spindling (133), has the highest firing rates of all sleep-waking stages during the transition from SWS to PS (99), which could be responsible for high-amplitude spindles and low thalamic responsiveness of the IS since these influences extend to the ventrobasal complex. Conversely, based on the reticular arousal thresholds, the IS seems functionally nearer to the slow wave stage than to PS. We hypothesized that the ascending brain-stemactivating influences of waking, which decrease during SWS, decrease further during the IS, while the pontine influences inducing the high arousal threshold and

381

strong inhibition of the masseteric and other reflexes during PS (164) are either still absent or only just beginning to occur. We also mentioned the difference already indicated with the peripheral arousal threshold. Indeed, recently Neckelmann and Ursin (110) showed that the acoustical arousal threshold is higher in rats during slow-wave and intermediate stages than during PS. However, as indicated by these authors, peripheral stimulation is “particularly dependent on the experimental design (novelty of stimulus, degree of meaningfulness of stimulus, increase in stimulus strength, criteria for response)”. Study of Serotoninergic 5-HT1 and 5-HTICReceptor Influences on the Intermediate Stage

It is known that serotonin is implicated in sleep–waking mechanisms (72). Since the discovery of several serotonin receptor subtypes (23), it was shown that ritanserin, a 5-HTZ,IC specific antagonist, increases deep SWS (SWSZ)and decreases PS (33). Therefore, we tested whether ritanserin would increase the IS at the expense of PS (130). It was administered i.p. at 0.63 and 2.5 mglkg and the seven sleep-waking stages were scored by our computer program (38). We confirmed that ritanserin increased the slow-wave stage and decreased PS, but it did not affect the spindle stage or the IS. Therefore, these two serotonergic receptors are not involved in the IS. Furthermore, our results demonstrated a functional dissociation between the IS and PS. Effects of Atropine on the Intermediate Stage

We previously described how barbiturates extend the IS at the expense of PS (47) and also suppress the pontine reticular formation activation of PS (48,50). Cholinomimetic compounds injected at this level induce PS-like patterns (43,127), as do anticholinesterasic ones (12), whereas muscarinic antagonists decrease PS (71). Therefore, we hypothesized that atropine could, like barbiturates, extend the IS at the expense of PS by suppressing the activation of the pontine cholinoceptive neurons. We used three i.p. doses: 5, 10 and 20 mg/kg in rats. The latencies of occurrence of the IS and PS were increased and the amounts were dose-dependently decreased for several hours (Fig. 13), principally because of their decreased frequency. The theta rhythm intrinsic frequency was significantly increased by all doses for several hours during the IS and PS without eye movement bursts [type 2 theta (151)], as well as during the eye movement bursts and active waking [type 1 theta (9)]. It is worth mentioning that Usui and Iwahara (147) have already shown that under atropine there is an increase in theta rhythm frequency during the rapid eye movements of PS. These results showed that, contrary to our hypothesis, even though the pontine release of acetylcholine begins to increase prior to PS (88), atropine did not extend the IS at the expense of PS, and these two stages involve a common muscarinic process. Furthermore, the new-found and very unexpected

382

GOTTESMANN IS: TOTAL AMOUNT (5mgKg) 1

b’

~~

H44UJ

840

a

40

Sm 20 100 0

1

HOURS

234S6

0

12346

6

❑ ~

~ IW

HOURS

CONTROL ATROPINE

PS: TOTAL AMOUNTJlOmg/Kg)

IS: TOTAL AMOUNT (lOmg/Kg)

u 8

Xm

P’” 400

40

300

40 mo 20

100 0

0

1234S

6

1

HOURS

IS: TOTAL AMOUNT (20mg/Kg)

6

234S

HOURS

PS: TOTAL AMOUNT (20mg/Kg)

a“ 5 ~40

i: &j 4W

40

mo am

20 fm o

1234S

s

HOURS

o 12346

6

HOURS

FIG. 13. Influence of atropine on the intermediate stage and paradoxical sleep in the rat. At 5, 10 and 20 mg/kg, atropine decreases the amounts of the two stages of sleep. The IS is not extended at the expense of PS. (Reprinted from Ref. 9, with permission.)

increase in theta rhythm frequency under atropine was problematic, i.e., during the previously described atropine-sensitive and atropine-resistant theta rhythm (150). We speculated that the transient blockade of muscarinic receptors by atropine could potentate cholinergic theta-inducing processes, which could result in an increase in theta rhythm frequency as soon as these receptors progressively become free for access of acetylcholine. However, this would not be the case for PS mechanisms, since there is no rebound of PS following atropine administration.

brain stem transections of the pens at the middle level of the caudal reticular nucleus in rats previously recorded in chronic conditions (57). The transected animals were kept alive up to 9 days. Although waking and SWS patterns persisted, PS was absent, which agrees with earlier findings of Siegel et al. (129) and Vanni-Mercier et al. (153). However, more relevant to this review was the elimination of the IS by the transections, so that there seems to be a link between the IS and PS. CONCLUSIONS

Effects of Low Brain-stem Transections on the Intermediate Stage

In spite of the importance of the pens in PS generating processes (26,69,71,128) several authors have shown that medullary processes are involved in this sleep stage (111,123,140,153,161).Therefore, we made

Neurophysiological characteristics of the IS differ from those of the slow-wave stage (even when accompanied by spindles) and of PS. The anterior cortex is invaded by spindles of higher amplitude and longer duration, and which often have a pointed aspect, which is not the case for spindles

THE INTERMEDIATE

STAGE

interspersed with slow waves. These peculiar spindles, which sometimes resemble the spike-wave discharges of the animal model of absence epilepsy (152), are probably related to the high GABAergic discharge of the thalamic reticular nucleus, which is specific to this sleep–waking stage (99). If these neurons, which control the spindle-generating processes (133,134)are disinhibited during the IS, then the brain-stem influences, which directly (135) or indirectly (25) inhibit the thalamic reticular nucleus are either operating at their lowest level, or are suppressed. This inference is verified by the effect of barbiturates, which suppress the pontine activation of PS, and the intercollicular and precollicular transections, which induce high-amplitude spindles in the rat and cat. Incidentally, a relationship has been postulated between spindles and the spikewave discharges of the animal model of absence epilepsy (97,136). It was proposed that the hyperpolarization of the ventrobasal neurons would be higher during spike-wave discharges than during spindles. However, it should be mentioned that in the rat, the spike-wave discharges preferentially occur during the transition from quiet waking to slow-wave sleep (94,152), whereas the high-amplitude spindles appear during advanced SWS prior to PS. The thalamic transmission level in the rat and thalamocortical reactivity in the cat also indicate the uniqueness of the IS. Both measures progressively decrease during the slow-wave and spindle stages and reach the minima of all sleep-waking stages. The large increase in responsiveness during PS, even outside eye movement bursts, points to the functional specificityof this sleep stage. The intense neural activity of the thalamic reticular nucleus (22,99) is certainly responsible for the hyperpolarization (i.e., inhibition) of the ventrobasal postsynaptic neurons, since there presynaptic inhibition is absent. The reticular arousal threshold also shows that the IS differs functionally from both slow-wave stage with spindles and PS, but to a large extent more from the latter. The hippocampal activity also has specificproperties during the IS. During SWStheta rhythms are absent, and during PS they have a significantlyhigher frequency, even during the absence of eye movement bursts. Here too, the effects of barbiturates (and benzodiazepines) and high brain-stem acute transections clearly show that the IS theta rhythm occurs through a disinhibitory process.As shown by mid-hypothalamictransections,the posterior hypothalamus is a trigger zone for the IS theta rhythm and probably, at least partly, for the basic theta rhythm of PS. This is probably due to the median raphe nucleus (100) a serotoninergic structure controlled by glutamatergic processes (83), which becomes silent or ahnost silent before and during PS (116). It is worth mentioning that several authors independently reached similar conclusions about the role of the posterior hypothalamus.As mentioned earlier, Alonso and Llinas’ (2,3) finding that posterior hypothalamic cells fire at theta rhythm frequencies was confirmed by Kirk and Naughton (84) and Kocsis and Vertes (87). Moreover, Vertes (157) described important projections from the supramammillarynucleus to the hippocampus.Kirk and

383

Naughton (84) observed that neuronal firing at theta rhythm frequencies persisted in the supramammillary nucleus after medial septum transient inhibition by procaine, and suggestedthat it has a more important role than the medial septum for the theta-generating processes. The former would be responsible for the transformation of the continuous activation from the brain-stem into theta rhythm pulses. Our finding, that without any brain-stem influencesthe posterior hypothalamus generates more theta rhythms than in the intact rat and cat, is important in this respect. The supramammillarynucleusis not only an amplifier,but also a fundamental generator of theta rhythm. However, during waking and PS, the reticularis pontis oralis and the pedonculopontinenuclei (156,158)seem to modulate the hypothalamictrigger. During waking their activationhas to overcome the inhibitory influences of the median raphe nucleus, as recently confirmed (159). All the findings reviewed to this point are consistent with the hypothesis that during the IS the forebrain is functionally disconnected from the brain-stem. However, there are some contradictory results. First, some PGO spikes begin to occur prior to PS in the cat. These waves result from phasic ascending activating influences, since forebrain-evoked potentials to central stimuli are concomitantly increased in amplitude (125,142).However, the cortical and hippocampal EEG field activities remain unchanged when these spikes occur during the brief IS included in the sommeil phasique ~ ondes lentes. Therefore, it appears that these phasic phenomena do not tonically change the state of forebrain structures. In other respects, cortical neurons (101) like those of the thalamus centralis lateralis (98) and bulbar (137), pontine (66) and mesopontine (135) brain-stem begin to increase their firing rates prior to PS cortical patterns, while acetylcholine release at the pontine reticular level (88) and arterial blood pressure (126) controlled by structures situated in front of the pontine level (79) also begin to increase at the same time. Similarly, in the cerebromotor system “during the transition from quiet sleep to active sleep the depolarizing potential (induced in the trigeminal motoneurons by pontine reticularis oralis nucleus stimulation) gradually became reduced in amplitude and was superseded by a hyperpolarizing potential” (109). Lumbar motoneurons also begin to be hyperpolarized prior to PS (107). All of these results were obtained from cats. However, the results of three of our own studies on rats are somewhat inconsistent with the general findings. First, atropine increased the latency of occurrence of both the IS and PS, which suggests a common muscarinic support. However, this finding should be interpreted with caution since recent results show that it could be related to hypothermia (138). Second, during PS-specificdeprivation the IS never substituted for PS (36a). Third, the caudo-pontine chronic transected rat displayed waking and SWS EEG patterns without any paradoxical or IS sleep (57). Consequently, although the forebrain spontaneous and evoked field activities of the IS suggest a functional brief disconnection from the brain-stem, the basic generating processes of the IS and PS seem to be related in some way.

384

GO’ITESMANN

Therefore, in spite of active research of the brain processes underlying the transition period between SWS and PS, its neurophysiological significance has not been entirely elucidated. Further studies involving hypothalamic neurotoxic lesions and neurochemical analysis should improve our understanding of the functional significance of the IS and its possible corresponding stage in humans.

ACKNOWLEDGEMENTS I thank the members of the team for the excellent atmosphere in which the experiments were performed and discussed,and Ralph Berger and Steven J. Cooper for helpful comments and improvement of the English. (DRET grant no. 93-164).

REFERENCES 1. Alonso, A.; Garcia Austt, E. Neuronal sources of theta rhythm in the entorhinal cortex of the rat. IL Phase relations between unit discharges and theta field potentials. Exp. Brain Res. 67:502-509;1987. 2. Alonso, A.; Llinas, R. Voltage-dependent calcium conductance and mammillary body neurons autorhythmicity: An in vitro study. Soc. Neurosci. Abstr. 14:900;1988. 3. Alonso, A.; Llinas, R. Electrophysiology of the mammillary complex in vitro. II. Medial mammillary neurons. J. Neurophysiol. 68:1321-1331;1992. 4. Anchel, H.; Lindsley, D. B. Differentiation of two reticulo hypothalamic systems regulating hippocampal activity. Electroenceph. Clin. Neurophysiol. 32:209-226;1972. 5. Andersen, P.; Andersson, S. A.; Lomo, T. Some factors involved in the thalamic control of spontaneous barbiturate spindles. J. Physiol. London 192:257-281;1967. 6. Arduini, A.; Moruzzi, G. Olfactory arousal reactions in the cerveau iso16cat. Electroenceph. Clin. Neurophysiol. 2:243–250; 1953. 7. Arduini, A.; Pompeiano, O. Attivita elettrica di singole units dell’ippocampo e sue modificazioni con stimoli afferenti. Archs Sci. Biol. 39:397406; 1955. 8. Arnaud, C.; Gandolfo, G; Gottesmann, C. The reactivity of the somesthetic S1 cortex during sleep and waking in the rat. Brain Res. Bull. 4:735-740;1979. 9. Arnaud, C.; Gauthier, P.; Gottesmann, C. Atropine effects on the intermediate stage and paradoxical sleep in rats. Psychopharmacology116:304308; 1994. 10. Aserinski, E.; Kleitman, N. Regularly occurring periods of eye motility and concomitant phenomena during sleep. Science 118:273-274;1953. 11. Aston-Jones, G.; Bloom, F. E. Activity of norepinephrinecontaining locus coeruleus neurons in behaving rats anticipates fluctuations in the sleep-waking cycle. J. Neurosci. 1:876886; 1981. 12. Baghdoyan, H. A.; Monaco, A. P.; Rodrigo-Angulo, M. L.; Assens, F.; McCarley, R. W.; Hobson, J. A. Micro-injection of neostigmine in the pontine reticular formation of cats enhance desynchronized sleep signs. J. Pharmacol. Exp. Ther. 231:173-180;1984. 13. Batsel, H. L. Electroencephalographic synchronization and desynchronization in the chronic cerveau isol~ of the dog. E1ectroenceph. C1in.Neurophysiol. 12:421430; 1960. 14. Batsel, H. L. Spontaneous desynchronization in the chronic cat “cerveau isoh$”.Arch. Ital. Biol. 102:547–566;1964. 15. Belardetti, F.; Borgia, R.; Mancia, M. Proencephalic mechanisms of ECOG desynchronization in cerveau iso16 cats. Electroenceph. Clin. Neurophysiol. 42:213-225;1977. 16. Benavides, J.; Camerlin, J. C.; Mitrani, N.; Flamand, F.; Uzan, A.; Legrand, J. J.; Gueremy, C.; Le Fur, G. 2-Amino-6-trifluoro methoxy benzothiazole, a possible antagonist of excitatory amino acid neurotransmission. II. Biochemical properties. Neuropharmacology 24:1085-1090;1985. 17. Benington, J. H.; Kodali, S. K.; Heller, H. C. Scoring transitions to REM sleep in rats based on the phenomena of pre-REM sleep: An improved analysis of sleep structure. Sleep 17:2%36; 1994. 18. Benoit, O.; Bloch, V. Seuil d’eveil r6ticulaire et sommeil profond chez le chat. J. Physiol. Paris 52:17–18;1960. 19. Bizzi, E.; Brooks, D. C. Functional connections between pontine reticular formation and lateral geniculate nucleus during deep sleep. Arch. Ital. Biol. 101:666-680;1963.

20. Bland, B. H. The physiologyand pharmacology of hippocampal formation theta rhythms. Prog. Neurobiol. 26:1-54;1986. 21. Bland, B. H.; Whishaw, I. Q. Generators and topography of hippocampal theta (RSA) in the anaesthetized and freely moving rat. Brain Res. 118:259–280;1976. 22. Block, F. Stimulation of N-methykr-aspartate receptors in the rat nucleus reticularis thalami suppresses somatosensory evoked potentials. Brain Res. 636:143-146;1994. 23. Bradley, P. B.; Engel, G.; Feniak, W.; Fozard, J. R.; Humphrey, P. P. A.; Middelmiss,D. N.; Mylecharane, E. J.; Richardson, B. P.; Saxena, P. R. Proposals for the classification and nomenclature of functional receptors for 5-hydroxytryptamine. Neuropharmacology 25:563-576;1986. 24. Bremer, F. Cerveau “isolt” et physiologic du sommeil. C. R. Soc. Biol. Paris 118:1235-1241;1935. 25. Buzsaki, G.; Bickford, R. G.; Ponomareff, G.; Thai, L. J.; Mandel, R.; Cage, F. H. Nucleus basalis and thalamic control of neocortical activity in the freely moving rat. J. Neurosci. 8:4007-4026;1988. 26. Carli, G.; Zanchetti, A. A study of pontine lesions suppressing deep sleep in the cat. Arch. Ital. Biol. 103:751-788;1965. 27. Chu, N.; Bloom, F. E. Norepinephrine-containing neurons: Changes in spontaneous discharge patterns during sleep and waking. Science 179:90%910;1973. 28. Demerit, W. The occurrence of low voltage, fast electroencephalogram patterns during behavioral sleep in the cat. Electroenceph. Clin. Neurophysiol. 10:291–296;1958. 29. Demerit, W.; Kleitman, N. Cyclicvariations in EEG during sleep and their relation to eye movements, body motility and dreaming, Electroenceph. Clin. Neurophysiol. 9:673+90; 1957. 30. Demetrescu, M.; Demetrescu, M.; Iosif, G. Diffuse regulation of visual thalamocortical responsiveness during sleep and wakefulness. Electroenceph. Clin. Neurophysiol. 20:450469; 1966. 31. Depoortere, H.; Loew, D. M. Motor phenomena during sleep in the rat: a means of improvingthe analysis of the sleep-wakefulness cycle. In: Jovanovic, U., ed. The nature of sleep. Stuttgart: Gustav-Fischer; 1973:101-104. 32. Depoortere, H.; Zivkovic, B.; Lloyd, K. G.; Sanger, D. J.; Perrault, G.; Langer, S. Z.; Bartholini, G. Zolpidem, a novel non benzodiazepine hypnotic. I. Neuropharmacological and behavioral effects. J. Pharmacol. Exp. Ther. 237:649+58; 1986. 33. Dugovic, C.; Wauquier, A.; Leysen, J. E.; Marrannes, R.; Janssen, P. A. J. Functional role of 5-HT2 receptors in the regulation of sleep and wakefulness in the rat. Psychopharmacology97:436-442;1989. 34. Dumont, S.;Dell, P. Facilitationssp6cifiqueset non sp&ifiquesdes r6ponsesvisuellescorticales.J. Physiol.Paris 50:261–264;1958. 35. Gaillard, J. M.; Martinoli, R.; Kafi, S. Automatic analysis of states of vigilance in the rat. In: Koella, W. P.; Levin, P., eds. Sleep 1976.Base]: Karger; 1977:455457. 36. Gandolfo.,.G.: Arnaud.,,C.: Gottesmann. C. Transmission in the ventrobasal complexof rats during the sleep-wakingcycle. Brain Res. Bull. 5:553-562;1980. 36a. Gandolfo. G.: Gauthier. P.: Amaud. C.: Gottesmann. C. Influence ‘of paradoxical sleep deprivation on the intermediate stage of sleep in the rat. Neurosci. Res. in press. 37. Gandolfo, G.; Glin, L.; Gottesmann, C. Study of sleep spindles in the rat: A new improvement. Acta Neurobiol. Exp. 45:151-162;1985. 38. Gandolfo, G.; Glin, L.; Lacoste, G.; Rodi, M.; Gottesmann, C. Automatic sleep–waking scoring in the rat on microcomputer Apple II. Int. J. Biomed. Comput. 23:83-95;1988.

THE INTERMEDIATE

STAGE

39. Gandolfo, G.; Gottesmann, C. Transmission in the ventrobasal complex of thalamus during rapid sleep and wakefulness in the homolaterally neodecorticated rat. Acta Neurobiol. Exp. 42:443-455;1982. 40. Gandolfo, G.; Romettino, S.; Gottesmann, C.; van Luijtelaar, G.; Coenen, A. Genetically epileptic rats show a pronounced intermediate stage of sleep. Physiol. Behav. 47:213–215;1990. 41. Gandolfo, G.; Romettino, S.; Gottesmann, C.; van Luijtelaar, G.; Coenen, A.; Bidard, J. B.; Lazdunski, M. K+ channel openers decrease seizures in genetically epileptic rats. Eur. J. Pharmacol. 167:181–183;1989. 42. Gandolfo, G.; Scherschlicht, R.; Gottesmann, C. Benzodiazepines promote the intermediate stage at the expense of paradoxical sleep in the rat. Pharmacol. Biochem. Behav. 49:921–927;1994. 43. George, R.; Haslett, W. L.; Jenden, D. J. A cholinergic mechanism in the brainstem reticular formation; Induction of paradoxical sleep. Int. J. Neuropharmacol. 3:541–552;1964. 44. Gerbrandt, L. K.; Lawrence, J. C.; Eckart, M. J.; Lloyd, R. L. Origin of the neocortically monitored theta rhythm in the curarized rat. Electroenceph.Clin.Neurophysiol.45:454467;1978. 45. Glin, L.; Arnaud, C.; Berracochea, D.; Galey, D.; Jaffard, R.; Gottesmann, C. The intermediate stage of sleep in mice. Physiol. Behav. 50:951-953;1991. 46. Glin, L.; Zernicki, B.; Gottesmann, C. Hippocampal and cortical EEG activity in rats with transected hypothalamus. Brain Res. Bull. 27:637-640;1991. 47. Gottesmann, C. Donn6es sur l’activit6 corticale au cours du sommeil profond chez le rat. C. R. Soc. Biol. Paris 158:1829-1834;1964. 48. Gottesmann, C. Recherche sur la psychophysiologiedu sommeil chez Ie rat. Paris: Presses du Palais Royal; 1967. 49. Gottesmann, C. Donn6es r6centes sur les activit6s 61ectrophysiologiques phasiqu~s. Psychol. Fran&.12:1–16;1967. 50. Gottesmann, C. Etude sur Ies activite%61ectrophysiologiques phasiques chez le rat. Physiol. Behav. 4:495-504;1969. 51. Gottesmann, C. Theta rhythm: The brain stem involvement. Neurosci. Biobehav. Rev. 16:25-30;1992. 52. Gottesmann, C. Detection of seven sleep-waking stages in the rat. Neurosci. Biobehav. Rev. 16:31–38;1992. 53. Gottesmann, C.; Gandolfo, G. A massive but short-lasting forebrain deafferentation during sleep in the rat and cat. Arch. Ital. Biol. 124:257–269. 54. Gottesmann, C.; Gandolfo, G.; Zernicki, B. Hippocampal theta rhythm in the acute cerveau isol~ cat. Acta Neurobiol. Exp. 41:251-255;1981. 55. Gottesmann, C.; Gandolfo, G.; Zernicki, B. Intermediate stage of sleep in the cat. J. Physiol. Paris 79:365–372;1984. 56. Gottesmann, C.; Gandolfo, G.; Zernicki, B. Hippocampal theta activity in the acute precollicular rat. Brain Res. Bull. 22:959-962;1989. 57. Gottesmann, C.; Gandolfo, G.; Zernicki, B. The chronic rat preparations with brain stem transected at the caudopontine level. Brain Res. Bull. 36:573-580;1995.. 58. Gottesmann, C.; Juan de Mendoza, J. L.; Lacoste, G.; Lallement, B.; Rodrigues, L.; Tasset, M. Etude de I’analyse et la quantification automatiques des diff6rents 6tats de veille et de sommeil chez le rat. C. R. Acad. Sci. 272:301–302;1971. 59. Gottesmann, C.; Trefouret, S.; Depoortere, H. Influence of zolpidem, a novel hypnotic, on the intermediate-stage and paradoxical sleep in the rat. Pharmacol. Biochem. Behav. 47:359-362;1994. 60. Gottesmann, C.; User, P.; Gioanni, H. Sleep: A physiological “cerveau isol&”stage? Waking sleep 4:111–117;1980. 61. Gottesmann, C.; Zernicki, B.; Gandolfo, G. Hippocampal theta activity in the acute cerveau isoh$cat. Acta Neurobiol. Exp. 41:251-255;1981. 62. Gottesmann, C.; Zernicki, B.; Kraszewski, K. EEG activity in the acute precollicular cat. Arch. Ital. Biol. 126:193–197;1988. 63. Grastyan, E.; Lissak, K.; Madarsz, I.; Donhoffer, H. D. Hippocampal electrical activity during the development of conditional reflexes. Electroenceph. Clin. Neurophysiol. 11:409-430;1959. 64. Green, J. D.; Arduini, A. Hippocampal electrical activity in arousal. J. Neurophysiol. 17:532–557;1954. 65. Grubar, J. C. Sleep and mental deficiency. Rev. EEG Neurophysiol. 13:107-114;1983.

385 66. Ito, K.; McCarley, R. W. Alterations in membrane potential and excitability of cat medial pontine reticular formation neurons during naturally occurring slee~wake states. Brain Res. 292:169-175;1984. 67. Jacobs, B. L.; Heym, J.; Trulson, M. E. Behavioral and physiological correlates of brain serotoninergic unit activity. J. Physiol. Paris 77:431-436;1981. 68. Jeannerod, M. Organisation de l’activit6 61ectriquephasique du sommeil paradoxal. Lyon: LMD; 1963. 69. Jones, B. E. Elimination of paradoxical sleep by lesions of the pontine gigantocellular tegmental field in thecat~Neurosci. Lett. 13:285-293;1979. 70. Jouvet, M. Telencephalic and rhombencephalic sleep in the cat. In: The nature of sleep. London: Ciba Foundation Symposium, Churchill; 1961:188-206. 71. Jouvet, M. Recherches sur Ies structures nerveuses et les m6canismes responsables des diff6rentes phases du sommeil physiologique. Arch. Ital. Biol. 100:125-206;1962. 72. Jouvet, M. The role of monoamine and acetylcholine-containing neurons in the regulation of sleep-waking cycle. In: Ergibnisse der Physiologic.Berlin: Springer; 1972:166-307. 73. Jouvet, M.; Michel, F. Correlations &lectromyographiquesdu sommeil chez Ie Chat d6cortiqu4 et m6senc6phaliquechronique. C. R. Soc. Biol. Paris 153:422425; 1959. 74. Jouvet, M.; Michel, F. Mise en 6vidence d’un centre hypnique au niveau du rhombenc6phale chez le Chat. C. R. Acad. Sci. Paris 251:1188-1190;1960. 75. Jouvet, M.; Michel, F. D&clenchementde la “phase paradoxale” du sommeil par stimulation du tronc c6r6bral chez le chat intact et m6senc6phalique chronique. C. R. Soc. Biol. Paris 154:636+tl; 1960. 76. Jouvet, M.; Michel, F.; Courjon, J. Sur un stade d’activit~ 61ectriquec6r6brale rapide au cours du sommeil physiologique. C. R. Soc. Biol. Paris 153:1024-1028;1959. 77. Jouvet, M.; Mounier, D. Effets des 16sionsde la formation r6ticu16e pontique sur le sommeil du chat. C. R. Soc. Biol. Paris 154:2301-2305;1960. 78. Julou, L.; Bardone, M. C.; Blanchard, J. C.; Garret, C.; Stutzmann, J. M. Pharmacological studies of zopiclone. Pharmacology 27:46-58;1983. 79. Kanamori, N.; Sakai, K.; Sei, H.; Salvert, D.; Vanni-Mercier, G.; Yamamoto, M.; Jouvet, M. Power spectral analysis of blood pressure fluctuations during sleep in normal and decerebrate cats. Arch. Ital. Biol. 132:105–115;1994. 80. Karasinski, P.; Stinus, L.; Robert, C.; Limoge, A. Real time sleep-wake scoring in the rat using a single EEG channel. Sleep 17:113-119;1994. 81. Karmos, G.; Grastyan, E. An electrophysiologicalstudy of sleep. Electroenceph. Clin. Neurophysiol. 12:933;1960. 82. Kawamura, H.; Domino, E. F. Hippocampal slow “arousal” wave activation in the rostral midbrain transected cat. Electroenceph. Clin. Neurophysiol. 25:471-480;1968. 83. Kinney, G. G.; Kocsis, B.; Vertes, R. P. Injections of excitatory amino acid antagonists into the median raphe nucleus produce hippocampal theta rhythm in the urethane-anesthetized rat. Brain Res. 654:96-104;1994. 84. Kirk, I.; McNaughton,N. Supramammillarycell firing and hippocampal rhythmical slow activity. Neuroreport 2:723–725;1991. 85. Klaue, R. Die bioelectrische Tatigkeit der Grosshirnrinde in normalen Schlaf und in der Narkose durch Schlafmittel. J. Psycho].Neurol. Leipzig 47:510-531;1937. 86. Kleinlogen, H. Analysis of the vigilance stages in the rat by fast Fourier transformation. Neuropsychobiology23:197-204; 87. Kocsis, B.; Vertes, R. P. Characterization of neurons of the supramammillary nucleus and mammillary body discharge rhythmically with the hippocampal theta rhythm in the rat. J. Neurosci. 14:7040-7052;1994. 88. Kodoma, T.; Takahashi, Y.; Honda, Y. Enhancement of acetylcholine release during paradoxical sleep in the dorsal tegmental field of the cat brain stem. Neurosci. Lett. 114:277-282;1990. 89. Konopacki, J.; MacIver, M. B.; Bland, B. H.; Roth, S. H. Theta in hippocampal slices: Relation to synaptic responses of dentate neurons. Brain Res. Bull. 18:25–27;1987. 90. Koresko, R. L.; Snyder, F.; Feinberg, L “Dream time” in hallucinating and non-hallucinating schizophrenic patients. Nature 199:1118-1119;1963.

386 91. Kreig, W. J. S. Connections of cerebral cortex. I. The albino rat. A topography of the cortical areas. J. Comp. Neurol. 84:221-275;1946. 92. Lairy, G. C. Donn6es r6centes sur la physiologic et la physiopathologiede l’activit6onirique. Donn6es chez le malade mental. In: Lopez-Ibor, J. J., ed. IV World Congress on Psychiatry, vol. 1. Madrid: Excerpta Medica, 1966:189-197. 93. Lairy, G. C.; Barros-Ferreira, M.; Goldsteinas, L. Donn4es r6centes sur la physiologic et la physiopathologie de l’activit~ onirique. In: Gastaud, H.; Lugaresi, E.; Berti Ceroni, G.; Coccagna, G., eds. The abnormalities of sleep in man, Bologna: Aulo Gaggi; 1968:275-283. 94. Lannes, B.; Micheletti, G.; Vergnes, M.; Marescaux, C.; Depaulis, A.; Warter, J. M. Relationship between spike-wave discharges and vigilance levels in rats with spontaneous petit real-like epilepsy. Neurosci. Lett. 94:187–191;1988. 95. Macadar, O.; Roig, J. A.; Monti, J. M.; Budelli, R. The functional relationship between septal and hippocampal theta rhythm. Physiol. Behav. 5:1443–1449;1970. 96. Marchiafava, P. L.; Pompeiano, O. Enhanced excitability in intra geniculate optic tract endings produced by vestibular volleys. Arch. Ital. Biol. 104:459-479;1966. 97. Marini, G.; Giglio, R.; Macchi, G.; Mancia, M. Normal and abnormal EEG rhythmicity and rodent nucleus reticularis thalami. J. Sleep Res. 3(Suppl. 1):158;1994. 98. Mariotti, M.; Zhou, G. C.; Serpico, S. Unitary activity in centralis Iateralis thalamic nucleus during transitions from slowwave sleep to REM. In: Chase, M. H.; Parmeggiani, P. L.; O’Connor, C., eds. Sleep research. Los Angeles: Brain Information Service; 1991:47. 99. Marks, G. A.; Roffwarg, H. P. Spontaneous activity in the thalamic reticular nucleus during the sleep-wake cycle of the free moving rat. Brain Res. 623:241-248;1993. 100. Maru, E. L.; Takahashi, K.; Iwahara, S. Effects of median raphe nucleus lesions on hippocampal EEG in the freely moving rat. Brain Res. 163:223-234;1979. 101. McCarley, R. W.; Hobson, J. A. Cortical unit activity in desynchronized sleep. Science 167:901–903;1970. 102. McCarley, R. W.; Hobson, J. A. Single neuron activity in cat giganto cellular tegmental field: selectivity of discharge in desynchronized sleep. Science 174:125CL1252;1971. 103. McGinty, D. J.; Harper, R. M.; Fairbanks, M. K. Neuronal unit activity and the control of sleep states. In: Weitzman, E. D., ed. Advances in sleep research, vol. 1. New York: Spectrum; 1974:173-176. 104. Michel, F.; Klein, M.; Jouvet, D.; Valatx, J. L. Etude polygraphique du sommeil chez le rat. C. R. Soc. Biol. Paris 155:2389-2392;1961. 105. Mikiten, T.; Niebyl, P.; Hendley, C. EEG desynchronization during behavioral sleep associated with spike discharges from the thalamus of the cat. Fed. Proc. 20:327;1961. 106. Monmaur, P.; Houcine, O.; Delacour, J. Experimental dissociation between wakefulness and paradoxical sleep hippocampal theta. Physiol. Behav. 23:471-479;1979. 107. Morales, F.; Chase, M. H. Intracellular recording of lumbar motoneuron membrane potential during sleep and wakefulness. Exp. Neurol. 62:821-827;1978. 108. Moruzzi, G.; Magoun, H. W. Brain stem reticular formation and activation of the EEG. Electroenceph. Clin. Neurophysiol. 1:455473; 1949. 109. Nakamura, Y.; Goldberg, L. J.; Chandler, S. H.; Chase, M. H. Intracellular analysis of trigeminal motoneurons activity during sleep in the cat. Science 199:204-207;1978. 110. Neckelmann, D.; Ursin, R. Sleep stages and EEG power spectrum in relation to acoustical stimulus threshold in the rat. Sleep 16:467477; 1993. 111. Nosjean, A.; Arluison, M.; Laguzzi,R. F. Increase of paradoxical sleep after destructionof serotoninergicinnervationof the nucleus tractus solitaries of the rat. Neuroscience23:469481;1987. 112. Olmstead, C. E.; Villablanca, J. R. Hippocampal theta rhythm persists in the permanently isolated forebrain of the cat. Brain Res. Bull. 2:93-100;1977. 113. Petsche, H.; Stumpf, C.; Gogolak, G. The significance of the rabbit’s septum as a relay station between the midbrain and the hippocampus. I. The control of hippocampus arousal activity by the septum cells. Electroenceph. Clin. Neurophysiol.14:202–211; 1962.

GOTTESMANN 114. Piallat, B.; Gottesmann, C. The reticular arousal threshold during the transition from slow wave sleep to paradoxical sleep in the rat. Physiol. Behav. 58:199–202;1995. 115. Puia, G.; Vicini, S.; Seeburg, P. H.; Costa, E. Influence of recombinant y-aminobutyric acid~ receptor subunit composition on the action of allosteric modulators of y-aminobutyric acid-gated Cl- currents. Mol. Pharmacol. 39:691-696; 1991. l15a. Puizilloux,J. J.; Ternaux, J. P.; Foutz, A. S.; Fernandez, G. Les stades de sommeil de la preparation “encephale isol&”. Declenchement des pointes ponto-ghiculo-occipitales et du sommeil phasique ?i ondes lentes. Electroenceph. Clin. Neurophysiol. 37:561–576;1974. 116. Rasmussen, K.; Heym, J.; Jacobs, B. L. Activity of serotonincontaining neurons in nucleus centralis superior of freely moving cats. Exp. Neurol. 83:302–317;1984. 117. Rasmussen, K.; Morilak, D. A.; Jacobs, B. L. Single unit activity of locus coeruleus neurons in the freely moving cat. 1. During naturalistic behaviors and in response to simple and complex stimuli. Brain Res. 371:324-334;1986. 118. Rioch, Mck D. Discussion. In: Delafresnay, J. F., ed. Brain mechanisms and consciousness. Oxford: Blackwell; 1954:133-134. 119. Robinson, T. E.; Whishaw, I. Q. Effects of posterior hypothalamic lesions on voluntary behavior and hippocampal electroencephalograms in the rat. J. Comp. Physiol. Psychol. 86:76%786;1974. 120. Roldan, E.; Weiss, T.; Fifkova, E. Excitability changes during the sleep cycle of the rat. Electroenceph. Clin. Neurophysiol. 15:775-785;1963. 121. Romettino, S.; Lazdunski, M.; Gottesmann, C. Anticonvulsant and sleepwalking influences of riluzole in a rat model of absence epilepsy. Eur. J. Pharmacol. 199:371–373;1991. 122. Rougeul, A.; Letalle, A.; Courvoisier, J. Activit4 rythmique du cortex somesthe%iqueprimaire en relation avec l’immobilit~ chez le chat libre 6veil16.Electroenceph. Clin. Neurophysiol. 33:23-39;1972. 123. Sakai, K. Anatomical and physiological basis of paradoxical sleep. In: McGinty, D. J.; Drucker-Colin, R. A.; Morrison, A.; Parmaggiani, C., eds. Brain mechanisms of sleep. New York: Raven Press; 1985:111-137. 124. Sakai, K.; El Mansari, M.; Lin, J. S.; Zhang, J. G.; VanniMercier, G. The posterior hypothalamus in the regulation of wakefulness and paradoxical sleep. In: Mancia, M.; Marine, G., eds. The diencephalon and sleep. New York: Raven Press; 1990:171-198. 125. Satoh, T. Direct cortical response and PGO spike during paradoxical sleep of the cat. Brain Res. 28:576-578;1971. 126. Sei, H.; Sakai, K.; Kanamori, N.; Salvert, D.; Vanni-Mercier, G.; Jouvet, M. Long-term variations in arterial blood pressure during sleep in freely moving cats. Physiol. Behav. 55:673<79; 1994. 127. Shiromani, P. J.; McGinty, D. J. Pontine neuronal response to local cholinergic infusion: Relation to REM sleep. Brain Res. 386:2&31;1986. 128. Siegel,J. M.; Nienhuis, R.; Tomaszewski,K. S. REM sleep signs rostral to chronic transections at the pontomedullary junction. Neurosci. Lett. 45:241-246;1984. 129. Siegel, J. M.; Nienhuis, R.; Tomaszewski, K. S.; Wheeler, R. Behavioral states after brainstem transection at the medullary level. Soc. Neurosci. Abstr. 7:233;1981. 130. Silhol, S.; Glin, L.; Gottesmann, C. Study of the 5-HTZantagonist ritanserin on sleep-waking cycle in the rat. Pharmacol. Biochem. Behav. 41:241-243;1991. 131. Snyder, F. Quelques hypothi%esau sujet de la contribution du sommeil avec movements oculaires rapides ~ la survivance des mammif?xes.R&e et conscience, Paris, PUF; 1968:37–50. 132. Steriade, M. Ascending control of thalamic and cortical responsiveness. Int. Rev. Neurobiol. 12:87–144;1970. 133. Steriade, M.; Deschenes, M.; Domich, L.; Mulle, C. Abolition of spindle oscillations in thalamic neurons disconnected from nucleus reticularis thalami. J. Neurophysiol.54:1473–1497;1985. 134. Steriade, M.; Domich, L.; Oakson, G.; Deschenes, M. The deafferented reticular thalamic nucleus generates spindle rhythmicity. J. Neurophysiol. 57:260–273;1987. 135. Steriade, M.; McCarley, R. W. Brainstem control of wakefulness and sleep. New York: Plenum Press; 1990.

THE INTERMEDIATE

STAGE

136. Steriade, M.; McCormick, D. A.; Sejnowski, T. J. Thalamocortical oscillations in the sleeping and aroused brain. Science 262:679485; 1993. 137. Steriade, M.; Sakai, K.; Jouvet, M. Bulbo thalamic neurons related to thalamocortical activation processes during paradoxical sleep. Exp. Brain Res. 54:463-475;1984. 138. Szymusiak,R.; Danowski, J.; McGinty, D. REM sleep-suppressing effects of atropine in cats vary with environmental temperature. Brain Res. 636:115–118;1994. 139. Terrier, G.; Gottesmann, C. Study of cortical spindles during sleep in the rat. Brain Res. Bull. 3:701–706;1978. 140. Thakkar, M.; Mallick, B. N. Effect of rapid eye movement sleep deprivation on rat brain monoamine oxydase. Neuroscience 55:677483; 1993. 141. Thomas, J.; Benoit, O. H6t6rog4n6it4 du sommeil 2r ondes lentes. J. Physiol. Paris 58:623-624;1966. 142. Thomas, J.; Benoit, O. Individualisation d’un sommeil h ondes lentes et activit6 phasique. Brain Res. 5:221-235;1967. 143. Tokizane, T. Sleep mechanisms: Hypothalamiccontrol of cortical activity. In: Aspects anatomo-fonctionnels de la physiologic du sommeil. Paris: CNRS; 1965:151-186. 144. Trachsel, L.; Tobler, L; Borbely, A. A. Electroencephalogram analysis of non-rapid eye movement sleep in rats. Am. J. Physiol. 24:R27-R37; 1988. 145. Ursin, R. The two stages of slow wave sleep in the cat and their relation from REM sleep. Brain Res. 11:347–356;1968. 146. User, P.; Gottesmann, C. The central responsiveness of the acute cerveau isol~ rat. Brain Res. Bull. 8:15–21;1982. 147. Usui, S.; Iwahara, S. Effects of atropine upon the hippocampal electrical activity in rats with special reference to paradoxical sleep. Electroenceph. Clin. Neurophysiol. 42:510-517;1977. 148. Valatx, J. L.; Bugat, R. Facteurs g&s6tiquesclans la d&erminisme du cycle veille-sommeil chez la souris. Brain Res. 69:315-330;1974. 149. Vanderwolf, C. H. Hippocampal electrical activity and voluntary movement in the rat. Electroenceph. Clin. Neurophysiol. 26:407-418;1969. 150. Vanderwolf, C. H. Cerebral activity and behavior: Control by central cholinergic and serotoninergic systems. Int. Rev. Neurobiol. 30:225-340;1988. 151. Vanderwolf, C. H.; Robinson, T. E. Reticulo-cortical activity and behavior: A critique of the arousal theory and a new synthesis. Behav. Brain Sci. 4:459–514;1981. 152. Van Luijtelaar, E. L. J. M.; Coenen, A. M. L. Two types of electrocortical paroxysms in an inbred strain of rats. Neurosci.

387 Lett. 70:393-397;1986. 153. Vanni-Mercier, G.; Sakai, K.; Lin, J. S.; Jouvet, M. Carbachol microinjections in the mediodorsal pontine tegmentum are unable to induce paradoxical sleep after caudal pontine and prebulbar transections in the cat. Neurosci. Lett. 130:41-45; 1991. 154. Van Twyver, H. Sleep patterns in five rodent species. Physiol. Behav. 4:901-905;1969. 155. Vertes, R. P. Selective firing of rat pontine gigantocellular neurons during movements and REM sleep. Brain Res. 128:146-152;1977. 156. Vertes, R. P. An analysis of ascending brain stem systems involved in hippocampal synchronization and desynchronization. J. Neurophysiol. 46:l14&l159; 1981. 157. Vertes, R. P. PHA-L analysis of projections from the supramammillary nucleus in the rat. J. Comp. Neurol. 326:595<22; 1992. 158. Vertes, R. P.; Colom, L. V.; Fortin, W. J.; Bland, B. H. Brainstem sites for the carbachol elicitation of the hippocampal theta rhythm in the rat. Exp. Brain Res. 96:419429; 1993. 159. Vertes, R. P.; Kinney, G. G.; Kocsis, B.; Fortin, W. J. Pharmacologicalsuppressionof the median raphe nucleuswith serotonin (1.) agonists 8-OH-DPAT and buspirone, produces hippocampal theta rhythm in the rat. Neuroscience 60:441-451;1994. 160. Villablanca, J. The electrocorticogran in the chronic cerveau isolg cat. Electroenceph. Clin. Neurophysiol. 19:576-586;1965. 161. Webster, H. H.; Friedman, L.; Jones, B. E. Modification of paradoxical sleep followingtransections of the reticular formation at the pontomedullary junction. Sleep 9:1–23;1986. 162. Weiss, T.; Adey, W. R. Excitability changes during paradoxical sleep in the rat. Experiential21:1-4; 1965. 163. Weiss, T.; Fifkova, E. Sleep cycles in mice. Physiol. Bohemoslov. 13:242-245;1964. 164. Wills, N.; Chase, M. H. Brain stem control of masseteric reflex activity during sleep and wakefulness. Mesencephalon Pens Exp. Neurol. 64:98-117;1979. 165. Wilson, C. L.; Motter, B. C.; Lindsley, D. B. Influences of hypothalamic stimuli upon septal and hippocampal electrical activity in the cat. Brain Res. 107:55+58;1976. 166. Winson, J. Hippocampal theta rhythm. I. Depth profiles in the curarized rat. Brain Res. 103:57–70;1976. 167. Yeckel, M. F.; Berger, T. W. Feedforward excitation of the hippocampus by afferents from the entorhinal cortex: Redefinition of the role of the trisynaptic pathway. Proc. Natl Acad. Sci. USA 87:5832-5836;1986.