III. State of sleep characterized by fast cortical activity paradoxical sleep
C. Structures and Mechanisms responsible for Paradoxical Sleep
The existence of tonic and phasic behavioral signs (disappearance of EMG activity on the neck, rapid eye movements) and of subcortical electric signs (monophasic pontine spikes), both of which are very specific for PS, has enabled us to outline withe relative precision the structures necessary and sufficient for the periodical triggering of this state of sleep (239,240).
The complete removal of the cerebellum (240) or of the anterior lobe (206) does not hinder the appearance of PS, which occurs in the normal way with the same behavioral signs seen in the intact animal. Thus the cerebellum, whose role in the disappearance of the postural tonus might be justly involved, is not necessary for the appearance of PS.
In the neodecorticate animal, PS is characterized by the appearance of the steady theta activity at the level of the ventral hippocampus, by monophasic pontine spikes, and by peripheral signs similar to those in the intact animal. The decrease of the muscular tone is total, and the bursts of eye movements are still present [though they have a pattern different from the normal animal (231)], as well as the other phasic muscular phenomena (clonic movements). The PS periodicity and duration are similar to those of the intact animal. Thus the neocortex does not play any part either in the triggering of PS, in the initial development of the hippocampal theta rythm, or in the most charactristic peripheral manifestation (240).
The removal of all neural structrures (including hypothalamus and hypophysis) rostral to the pons (242) does not prevent the periodical appearance of PS in the chronic pontine animal : it occurs with the regularity of a "biological clock", its mean duration is the same as in the intact animal (6 min.), and its circadian percentage is 10 % (slightly less than that in the intact animal). Paradoxical sleep is characterized by the sudden disappearance of muscular tone with atotal decrease of the EMG of the neck muscles, and by lateral ocular movements (which depend on the activity of the IVth nerve), an acceleration of the heart and respiratory rythms, and the presence of monophasic pontine spikes whose pattern and regional distribution are tha same as in the normal animal during this state. The structures responsible for the triggering of PS therefore must be located behind a prepontine transection.
On the contrary, in two animals, after a prebulbar transection from the posterior two-thirds of nucleus reticularis pontis caudalis to the rostral border of the trezoid body (240), it was impossible to observe a periodical disappearance of the muscular tone, and the EMG activity remained constant at the necck level during survival time (7 days). Thus, the bulbar inhibitory reticular formation (303) makes necessary a prebulbar mechanism, since by itself, it can not trigger the recurent inhibition of the tonus in these last preparations. The ocular and electric cortical aspects in these preparations are the same as in the MPP preparation [ a clear increase in the duration of cortical desynchronization, accompanied by ocular movements of observation (240)]. Moreover, there are periods of fast cerebral activity with spontaneous and short-lasting eye movements. These periods might be the expression of the rostral activity of PS, but this has not yet been proved. Yet the hippocampal activity in the dog is the same as in PS after midpontine transection (406).
The experiments of Rossi et al. also supports the hypothesis of the localization of the trigerring structures of the fast activity of PS at the level of the caudal part of nucleus reticularis pontis oralis (RPO) and of the rostral part of nucleus reticularis pontis caudalis (RPC), since a lateral hemisection located in front of these sructures suppresses or delays the homolaeral cortical desynchronization during PS, whereas an hemisection at a more caudal level does not involve an asymetry of the cortical desynchronization during this state (92,382,384).
The overall results of these experiments reveal that the structures sufficient fot the periodical appearance of the major behavioral and EEG signs of PS are located at the level of the pons.
Another series of experiments shows that some pontine structures are equally necessary.
The destruction of the nucleus centralis superior de Betcherew and of the medial part of nuclei RPO and RPC has no significant effect on either states of sleep. But the coagulation of mediolateral part of the caudal part of nucleus RPO and the rostral part of nucleus RPC suppresses PS in chronic cats, whereas they are not usually changes in slow sleep or arousal (239,240,242). Recent experiments are in favor of different pontine structures being responsible for both tonic and phasic components of PS.
Tonic inhibition of muscular tonus. The bilateral destruction of a limited area situated in the rostral part of the mediolateral pontine tegmentum suppresses the occurrence of muscular atonia during PS (247) (whereas the phasic PGO activity still occurs). This area includes the nucleus locus coeruleus and a zone immediately medial and ventral to it. The destruction of a zone situated more caudally (anterior group of vestibular nuclei) or more laterally (nucleus parabrachialis-brachium conjunctivum) had no effect on the total atony of PS. Behavioral disturbances may occur in animals in which the dorsal part of the mediolateral pontine tegmentum is destroyed (247). After a period of slow sleep, a sudden increase in lateral geniculate spikes occurs, whereupon the cats may suddenly stand up and exhibit behavioral fear or rage. During these episodes, which occur periodically, there is an augmentation of muscular activity of the neck but the pupils remain fissurated, the nictitating membranes are relaxed, and the animal does not react to visual stimuli. The clear-cut dissociation between the ocular aspect of "deep sleep" and the behavior of rage has been compared to a "hallucinatory-like state." How the dorsal part of the mediolateral pontine tegmentum controls the supraspinal structures responsible for the tonic inhibition of muscular tonus is still obscure. It must be pointed out, however, that this region is very rich in noradrenergic neurons [group A6 of Dahlstrom and Fuxe (114)], which are mainly concentrated in the locus coeruleus and immediately ventral to it. This region is also particularly rich in mono-amino-oxidase ( 193) . These histochemical features favor a possible role of monoaminergic neurons in the control of postural atonia during PS but further experimental evidence is needed to support this hypothesis.
Phasic aspects of PS. The medial and descending vestibular nuclei are re sponsible for the burst of rapid eye movements, for the clonic jerks of the peripheral muscles, and for some phasic irregularity of the vegetative system (subite mydriasis or subite variation of blood pressure) during PS. Indeed, after bilateral destruction of these vestibular nuclei, there are only a few isolated eye movements without clonic jerks whereas there is still total atony of the neck muscles (322, 359).
The pontine structures that control the phasic PG0 activity during PS are less known. However, it has been recently shown that reserpine could induce electively, in the cat, PGO spikes similar to those of PS (119, 248). This activity can still be recorded in acute experiments under Flaxedil. In this condition neither any retropontine transection of the brain stem nor even total destruction of the vestibular nuclei can suppress the PGO activity induced by reserpine, whereas a total midpontine transection or the bilateral coagulation of the lateral pontine tegmentum (rostral to the vestibular nuclei and ventral to the locus coeruleus) will totally suppress the geniculo-occipItal. spikes (248). Thus the lateral pontine tegmentum is apparently necessary for the triggering of ascending extraretinal input to the lateral geniculate body.
The rhombencephalic localization of the structures responsible for the triggering of PS has made it possible to propose the name of "rhombencephalic phase of sleep" for this state of sleep (254). The mediolateral zones of the rhomben cephalon thus appear as triggering zones in relationship to some ascending and descending pathways, responsible for the electric cortical tonic and phasic phenomena and the behavioral tonic and phasic events specific to PS.
Ascending and descending structural organization in relationship with the pons
Ascending structures. The ascending structures responsible for the "cortical activation" in PS have not yet been delimited. We must discard the suggestion (241) that they follow the "limbic midbrain circuit," since coagulations at this level do not prevent cortical activation during PS (95, 206). These structures seem to be diffuse and nonspecific since neither the destruction of the rostral part of the reticular formation of the pons and of the specific pathways located in front of the rhombencephalic triggering zone nor the coagulation of the mesencephalic tegmentum suppresses this activation (91, 206). The destruction of the septum, on the contrary, suppresses the hippocampal theta rhythm in PS as well as during arousal (240, 288, 350, 351).
Cortical "activation" in PS. A total section of the brain stem at the level of the mesodiencephalic border (destroying the posterior diencephalon) suppresses the cortical activation during PS, whereas the pontine electrical signs and the peripheral signs of PS are still observed (206, 240, 252).
These results lead us to the conclusion that the cortical activation during PS cannot be attributed to a humoral action exerting a direct influence on the cortex (240). On the other hand, the number of experimental results obtained is still too small to enable us to know whether the cortical activation of PS is due to a neuronal, neurohumoral, or a humoral activation of the ARAS, or to other ex trareticular diencephalic structures. Moreover, the nature of the cortical activation must be considered from the point of view of two aspects
I) does the cortical ac tivation of PS reveal an increase or a decrease of the cortical "excitability," and
2) are its mechanisms different from cortical "arousal"?
I) Though it is difficult to agree on the meaning of cortical "excitability," a substantial body of data is in favor of there being an increase in cortical "excitability" during PS (in comparison with slow sleep state)
b) increase in the background activity of the pyramidal tract (20);
e) increase in the cortical excitability by direct stimulation of the cortex, as tested by the appearance of peripheral responses (213) [on the other hand, responses of the flexor muscles to pyramidal tract stimulation decrease in PS (306)] ;
f) increase in the discharges elicited by local application of strychnine in the cortex to a rate similar to arousal, whereas these discharges decrease during slow sleep (341). It has not yet been established whether this increase in excitability is of a tonic nature (and would remain constant as long as the fast cortical activity would last) or of a phasic nature (in close linkage with motor phasic phenomena of PS).
2) Some results are in favor of a dissociation of the ascending "activating" mechanisms of PS and of arousal.
a) Some lesions located at the level of the diencephalon (subthalamic area, lateral hypothalamus, thalamectomy) involve a dissociation between the activations of wakefulness and those of PS, which implies the existence of different ascending pathways or mechanisms (218, 240, 337).
b) The unitary cortical activities of arousal and PS are different, at least in their patterns. It has been suggested that a difference of excitability of inhibitory interneurons could be the source of the unorganized discharges of pyramidal tract neurons (142).
c) The pattern of the evoked cortical responses during arousal is often different during PS (see above).
d) The presence of spontaneous spikes at the visual cortex (334) level (which does not exist during arousal) implies that some mechanisms different from those of arousal are working or that there is a new activity superimposed on arousal activity.
e) Last, the topography and the frequency of the hippocampal theta rhythm are different from those observed during wakefulness (240, 420) and the olfactory bulb activity is different from that of arousal (154, 263, 270).
Thus, all these data enable us to infer that the mechanisms of the neo- and paleocortical activation of PS are different from those of arousal.
Supraspinal influences on motoneurons. If the dorsal part of the mediolateral pon tine tegmentum appears to be necessary for the periodical atony of PS, other supraspinal structures exist in the lower brain stem that are under its control and can act on the spinal motoneurons. These structures have been the subject of a recent review (358); the hypothesis of an inhibition of the descending tonic activity exerted by the facilltating RF or by the nuclcus of Deiters is unlikely, since partial spinal transection that interrupts facilitatory influences (182) dose not prevent the inhibition of the spinal reflexes. On the other hand, there is no decrease in unit activity at the level of the nucleus of Deiters or of the superior vestibular nucleus in PS (53). That is why the hypothesis of an active intervention of the inhibitory bulbar activity seems the most likely (240). This hypothesis is supported by the results of Giaquinto et al. (181, 182), which, in interrupting the descending inhibitory reticulospinal pathways at the level of the ventrolateral quadrants of the spinal cord, suppress the characteristic inhibition of the monosynaptic spinal reflexes during PS. The distal jerks of the limbs, which generally occur at the acme of inhibition of the spinal reflexes (172) during the bursts of rapid eye movements, are accompanied by an increase in the activity at the level of the pyramidal tract (306). Yet they do not depend en tirely on the pyramidal bursts or on the gamma loop, for they still exist in the ani mal without its neocortex (240), with sensorimotor areas removed, with both pyramids destroyed, or after section of the posterior roots (306). These jerks are, on the contrary, strikingly reduced by section of the dorsolateral funiculi of the spinal cord (306).
To sum up, it seems that the inhibition of the muscular tonus, of the homo and heteronymous monosynaptic reflexes, of the polysynaptic spinal reflexes, and of the pyramidal motor responses can be attributed to an inhibitory supraspinal influence having its source, probably, at the level of the inhibitory bulbar reticular formation. This inhibition would exert an influence (pre- or postysnaptically) on the spinal reflex arcs. A detailed analysis of the descending influences on the spinal cord has been carried out with elegant techniques in chronic conditions. Two tests have been used by Pompeiano et al. (173-175) to evaluate the excitability of a population of spinal motoneurons during sleep
1) the recurlent discharge (RD) of the alpha-motoneurons, which can be recorded electromyographically in the deaflerented limbs;
2) the re sponse of a muscular nerve to direct stimulation of the motoneuronal pool after deafferentation of the limbs.
Both the RD of the alpha-motoneurons as well as the response of a muscular nerve to direct stimulations of the motoneuron pool are depressed throughout PS. This tonic reduction of the response of the motoneurons to antidromic or direct stimulation is due to hyperpolarization of the alpha-motoneurons produced by descending inhibitory volleys, and this postsynaptic inhibition is also responsible for the tonic depression of the homonymous monosynaptic reflexes tested in the same population of motoneurons during PS.
The phasic inhibition of the monosynaptic reflexes that occurs during the bursts of eye movements, however, had no counterpart in enhanced depression of the response of the motoneurons to antidromic or direct stimulation.
It has also been shown that the phasic depression of spinal reflexes during the bursts of eye movement of PS is the result of presynaptic inhibition of the group 1a afferent pathway. During experiments in which the excitability of group 1a muscle afferents was tested in chronic cats following Wall's method, a phasic increase of the antidromic Ia volley appeared particularly when the bursts of ocular movements were rather intense. There was, however, no significant change in the amplitude of the group Ia antidromic volley during PS compared with slow sleep, when the bursts of eye movements were absent (323).
Thus it is likely that the increase in excitability observed during PS is due to supraspinal influences exerting a synaptic depolarizing action on terminals of the group 1a primary afferents. These influences are phasic in nature and appear to be related in time with the large bursts of eye movements. This phasic presynaptic inhibition may account for the striking depression of the homorlynlous monosynaptic reflex that occurs during the bursts of eye movements (358).
The complex organization of the neural structures involved in SP may be sum marized as follows. There are some structures at the level of the pontine reticular formation that appear necessary for the triggering of both descending and ascend ing tonic and phasic phenomena of PS.
The mediolateral pontine reticular formation is involved in the control of the bulbar inhibitory reticular formation, which acts tonically on the motoneurons through the ventrolateral funiculi of the spinal cord. The medial and descending vestibular nuclei are responsible for the burst of rapid eye movements and for most phasic vegetative and muscular events of PS. Phasic influences are transmitted through the dorsolateral funiculi of the spinal cord.
The fast cortical activity (and regular theta hippocampal rhythm) and the geniculo-occipItal. spikes are also dependent on the mediolateral part of the pontine tegmentum. The ascending pathways responsible for the tonic cortical activation appear to be diffuse at the level of the mesencephalon, whereas the hippocampal theta rhythm during PS is controlled by a pathway ascending through the septum.
A very discrete pathway, independent from the above ascending structures, situated in the dorsal part of the brain stem is responsible for the pontine ascending extra retinal input to the lateral geniculate body and occipItal. cortex.
Triggering mechanisms of paradoxical sleep
Central triggering. It is possible to trigger or "hasten" the appearance of PS im mediately or after a latency of a few seconds, in the intact animal, by a ihigh-fre quency stimulation (200-300 cycles/sec) of the pontine (240) mesencephalic RF (90, 152) or by a low-frequency stimulation of the hypothalamus (150, 264) and the hippocampus (152, 351). Induced PS resembles spontaneous PS in all points, including the decrease in blood pressure (90). This phenomenon appears only if the stimulation is made during slow sleep and several minutes after the end of a preceding PS episode, which implies the existence of a refractory phase (240). This triggering can also be realized in the mesencephalic or pontine animal (240) by stimulation of the pontine RF. Since PS is a spontaneous recurrent pherlomenon, relatively regular, some authors question whether this "central induction" is possible and assign it to chance in the rat and the rabbit (379, 380).
Chemical intracerebral stimulation (acetylcholine) at the level of the pontine RF (109, 177) and of the limbic midbrain circuit (199) can trigger PS too, but the necessary precedence of a phase of slow sleep does not enable us to learn whether this phenomenon would not have occurred spontaneously.
Reflex induction of PS. A direct triggering of PS cannot be obtairied during wakefulness in the normal cat by stimulation of extracerebral afferences. Yet, the stimulation of the Group II fibers of the cutaneous afferences during slow sleep in the intact cat can involve the phasic inhibition of the muscular tone of the neck [as well as a decrease in the monosynaptic contralateral reflex and polysynaptic flexion reflex (179, l80)], and in some cases this stimulation could "hasten" the appearance of PS (358, 360). On the contrary, in the chronic pontine cat, with a section behind the mesencephalic tegmentum, nociceptive stimulations (pinching of the ear) or proprioceptive stimulations (passive flexion of the legs or of the head) can trigger PS as a reflex. This phenomenon cannot be obtained in the anterior mesencephalic cat in which the same stimulations involve hypertonus (242). On the one hand, we must admit that the peripheral stimulations do not set off the inhibitory bulbar reticular formation directly, but rather pontine structures near (or the same as) the zone necessary to the triggering of PS, which is located at the dorsomediolateral part of the pontine RF. On the other hand, the integrity of the mesencephalic tegmentum suppresses the appearance of this phenomenon by a possible triggering of the descending facilitatory influences. Last, in the pontine animal, stimulations are efficient only if they occur l0-l5 min after the end of a period of PS (spontaneous or induced), otherwise they are ineffective or can involve a phasic or tonic reflex neck atony (lasting 2 or 3 min). The latter phenomenon similar to cataplexy resembles the "sudden posture collapse" described~ by Bard and Macht in the chronic pontine cat (27), but it is not accompanied by the appearance of monophasic pontine spikes, eye movements, nor vegetative shifts during genuine reflex periods of PS.
The possibility of triggering PS through reflex pathways thus raises the problem of a physiological triggering by extracerebral afferents or by a muscular factor (294). In fact, neither the section of the posterior roots of Cl to C7, nor the total section of the spinal cord in Dl, nor the section of both vagi and of the buffer nerves, nor the removal of the cervical sympathetic nerves prevents the appearance of PS in the intact animal (233, 242), and therefore we can reject the hypothesis of an exclusive triggering of PS by any afferent nervous pathways of extracerebral origin.
Humoral influences. The peculiar conditions required for the appearance of PS in the rabbit have made it difficult to recognize it. Its very existence has even been refuted by some authors (55). In fact, it is now admitted (17, 149, l50, 263) that PS corresponds to what has been formerly described in the female rabbit in terms of "hyperarousal" (394) or "afterreaction" of EEG (395).
Paradoxical sleep appears spontaneously too in the male rabbit but it requires long habituation of the animal to the environment (150) for it to become apparent. In the female rabbit, PS shows close relationships with hormonal changes. Copulation is usually followed (after a latency of l0-l5 min) by a period of PS, whereas PS is followed by a complex olfactory-buccal-anal-genItal.-sexual behavior (149 151, 153, 263, 394). The injection of placentary or pituitary gonadotrophins, of LH and mainly LTH, of ADH, oxytocin, and epinephrine encourages the appearance of PS (150, 394, 395). Moreover, an injection in situ of LH at the level of the nucleus reticularis pontis caudalis can cause the appearance of PS (l50). But the injection of testosterone in castrated females or of synthetic progestative drugs (inhibiting ovulation) would suppress, for I or 2 days, spontaneous PS or PS in duced by central stimulation (394). As a matter of fact, PS can be induced in the rabbit by a low-frequency stimulation of various structures having close relation ships with the hypothalamus (hypothalamus, olfactory bulb, septum, hippocampus, amygdala) (l50, 151, 264, 430). These stimulations are able to exert an influence by triggering hypothalamic neurohormones or pituitary hormones (395), and this raised the problem that a suprapontine command (or modulation) of PS triggering may exist. Yet, the hypothalamo-pituitary complex does not seem necessary to the appearance of PS, at least not in the cat. The complete removal of the hypothalamus and hypophysis (in the pontine cat) is still compatible with the appearance of PS during the first 5 days after the operation (242). Its percentage steadily decreases down to the death of the animals on the 6th or 7th day. On the contrary, an injection of ACTH and ADH restores the normal periodicity of PS. Thus the pituitary hormones (which completely disappear from the blood within a few hours after ablation of the hypothalamus and hypophysis) do not seem to be necessary for the appearance of PS. They thus intervene only in an indirect way to allow survival of the animals by restoring an ionic blood equilibrium that would otherwise be disturbed.
The influence of the blood osmolarity on the periodicity of PS is very important in the pontine animal (with a hypothalamic island disconnected from the pons by a barrier in acrylic resin) (242). The blood hyperosmolarity, through injection of a hypertonic saline or water deprivation, elevates the percentage of PS by about 100-200 %. The hypoosmolarity induced by a water surcharge ( 10 % of the animal's weight) suppresses the appearance of PS for 6-12 hr. The interactions between the blood osmolarity and the brain (415) and the Na and K contents of the neuroglia (whose role in the cerebral water-ion pool is known) have led to the suggestion of a possible intervention of the glial cells in PS triggering (242).
Selective deprivation of paradoxical sleep
The first attempt to suppress PS electively was made in man by Dement (123) by awakening the subjects immediately after the onset of PS. A progressive in crease in PS "attempts" was observed and an augmentation of PS was found dur ing the nights after deprivation. The same results have been obtained in animals.
In the pontine cat, the suppression of PS by an electric shock makes it reappear after shorter and shorter intervals, so that after a few hours it becomes almost im possible to awake the animal, which immediately collapses into the state of PS after the shock (242). Thus, there is a state of "a need for PS" and it seems to be the expression of a quite active mechanism situated in the lower brain stem. The same phenomenon has been obtained in the intact cat with the same technique (343).
In the intact cat, the selective deprivation of PS is also achieved by placing the animal under conditions in which it cannot completely relax its muscular tone (isolated on a small stand in a bath) (237, 429). Under these conditions, behavioral and EEG slow sleep can persist in a normal way (50 % of the day) without PS appearing. Some vegetative electrical and behavioral changes have been noticed during PS deprivation. There is a permanent increase in the heart rate (237, 429), a facilitation of the recovery cycle of click-evoked responses at the cortical level (132), and a diminution of the seizure threshold for electroshock (105). Disturbances of sexual behavior (hypersexuality) have also been noticed in male cats or rats (132, 429). These results suggest that the excitability of the nervous system may be increased when PS is electively suppressed. During recovery sleep (after suppression for several days) an important and durable increase of PS percentage is observed (up to 60 % of total sleep). This increase may persist for several days and is proportional to the duration of deprivation (it lasts for a duration equal to half the deprivation time). It proceeds mainly from an increase of the frequency of PS and not from the increase of its mean duration. At the onset of the recoveries, PS may occur immediately after wakefulness, without any intermediary phase of slow sleep. The phenomenon evokes cataplectic states, thus showing that PS can sometimes occur without a preceding slow sleep state. The elective suppression of PS has also been obtained in the rabbit by subjecting it to continuous intensive noises. Return to silence is followed by a "rebound" recovery of PS (269). The results of elective PS deprivation contrast with the effect of total sleep deprivation. In such case the recovery sleep is different. There is first an increase of SWS, which is only secondarily followed by an increase of PS (429). Similar results have been obtained in man (439).
These experiments demonstrate that the essential mechanisms expressing the "need" for PS exist in the lower brain stem. The long-lasting recovery process after PS deprivation evokes the eventual accumulation of a neurohumoral or metabolic by-product during deprivation and meets Pieron's theory of hypnotoxins (356). The increase of this neurohumoral agent appears to be responsible for a state of increased excitability of the nervous system. The recovery after PS deprivation necessitates a long-lasting increase of PS, suggesting that the elimina tion of the specific (and unknown) neurohumoral agent during PS is effectuated through some autoregulating mechanism whose frequency (but not duration) is increased.
The literature concerning the pharmacology of sleep is so important that a whole review could be devoted to it (see the most recent bibliography in ref. 187). For this reason we shall limit our review mostly to the pharmacological influences on PS.
The active mechanism responsible for PS proves to be relatively resistant to certain factors. A decrease in rectal temperature down to 30 C during artificial hypothermia does not prevent the recurrent appearance of PS in the pontine animal (240). PentobarbItal. (25-30 mg/kg) does not suppress the recurrent appear ance of PS in the neodecorticate animal (290). It is easy to detect in these preparations thanks to the appearance of pontine spikes, grouped in periods of 6 min, that clearly stand out against the ever-fast and low-amplitude basic activity of the brain stem. In the intact animal, the cortical and subcortical spindles during narcosis under Nembutal (30-35 mg/kg) have long delayed the conspicuousness of PS; yet it still appears (234). It is evinced by typical pontine and geniculate spikes grouped in regular periods of 6 min, without cortical activation or REM at the onset, whereas these signs gradually reappear at the end of the narcosis. It is interesting to notice that some increase in pontine and geniculate spikes is also observed, after selective PS deprivation, during recovery under Nembutal narcosis (234).
Among the numerous drugs used for understanding the mechanisms of sleep this review is limited to three groups whose action is especially interesting.
Short-chain fatty acids. The artificial induction of PS by gamma-butyrolactone (GBL) or gamma-hydroxybutyrate of sodium (GNA) was first recognized by Jouvet et al. (242, 245). In the intact animal this drug (50-60 mg/kg) causes the appearance of a sleep similar to slow sleep in its EEG characteristics, which is always followed almost immediately by PS. Higher doses (above l00 mg/kg) provoke a state of narcosis with an electrical activity different from that of slow sleep (spikes and slow waves) and PS does not appear. But in the decorticate or pontine animal, the injection of GBL even in 100-mg/kg doses induces the appearance of PS, which always occurs in periods of 6 min mean duration. But it reappears more often and its percentage rises from 200 to 300 % after the injection. GBL is unable to induce PS m animals with lesions of the pons (which do not show spontaneous PS) (245). The intimate mechanisms of action of GBL or of GNA are still unknown (it is still under discussion whether they are normal components of the brain or not) (38, 48, 49, 160, 184, 185, 287). However, it has been shown recently that other short-chain fatty acids can also induce both states of sleep in normal cats or, electively, PS in mesencephalic cats (310). Sodium butyrate, isobutyrate, isovalerate, or caproate and a-hydroxyisobutyrate were effective, whereas sodium propionate, acetoacetate, and ,alpha-hydroxybutyrate were not effective in producing PS.
Cholinergic and anticholinergic drugs. In normal conditions, atropine sulfate (1-2 mg/kg) suppresses PS for several hours both in normal or pontine cats, whereas eserine increases the duration of PS in pontine cats (240). However, after selective PS deprivation, when the "need for PS" is enhanced, atropine does not suppress the immediate appearance of behavioral slow wave sleep followed by the usual increase of PGO activity during recovery sleep. Thus the preliminary phenomena of PS (phasic PGO activity) are not suppressed by anticholinergic drugs. On the contrary, the final steps of PS (fast EEG activity and total disappearance of EMG) are more sensitive to atropine, and they reoccur only after several hours (428) . Thus, if it is possible that some cholinergic mechanisms could play a role in the tonic phenomena of PS (low-voltage fast cortical activity and postural atonia), these mechanisms are not involved either in the production of slow wave sleep or in the triggering of the phasic EEG phenomena of PS.
Drugs acting on brain monoamine level. Whereas the facilitatory effect of short chain fatty acids or the suppressor effect of atropine on both states of sleep is short, the effect of drugs acting on brain monoamines is usually long (several days), and only the method of continuous polygraphic recordings associated with selective deprivation of PS has made the study of their action possible.
INCREASE IN BRAIN MONOAMINES.
I) Injection of precursors. The injection of s-hy droxytryptophan (5-HTP) (30-50 mg/kg), which is the precursor of serotonin (5-HT), provokes in normal cats an increase of slow wave sleep for 5-6 hr, whereas PS is totally suppressed during this period. The immediate and elective suppressor effect of 5-HTP on PS is also observed if 5-HTP is injected at the beginning of recovery sleep after PS deprivation. But the administration of 3,4-dihydroxyphenyl alanine (DOPA) (30-50 mg/kg), which is a precursor of the catecholamines, induces an increase in the waking state with an almost total disappearance of SWS and PS for 6 hr.
2) Blockage of metabolism of monoamine. Another way to increase the monoamine level is to block the mono-amino-oxidases (MAO) with an inhibitor. Potent MAO inhibitors (Nialamide, Pargyline, Ipronazide) increase mostly the 5-HT brain level in the cat. These drugs increase SWS and electively suppress PS (256). The suppressor effect of Nialamide is quite dramatic; after a single injection PS is totally suppressed for 3-4 days and SWS is increased, and a normal level of PS is only reached after 1 week. The same suppressor effect is also observed after PS deprivation. The very potent and elective suppressor effect of MAO inhibitors on PS has led to the hypothesis that the transition from SWS to PS may necessitate the inter vention of MAO (243).
RELEASE OF BRAIN MONOAMINES ACTION OF RESERPINE. The decrease in brain 5-HT and catecholamines after reserpine is a well-established fact (118, 309) even if the intimate mechanism of action of this drug is still unknown. In the cat, reserpine (at a unique dose of 0.5 mg/kg) induces a very peculiar alteration of the sleep states, which can be summarized as follows
1) it suppresses SWS for 12-14 hr;
2) it totally suppresses PS for 22-24 hr, and the control level of PS is reached only after 5-6 days of recovery;
3) it electively triggers a permanent PGO activity that lasts for 50-60 hr.
About 40-60 min after a single injection of reserpine, after a brief period of agitation, the cat shows the well-known reserpine syndrome with a fissurated myosis, but may still react to loud noise or painful stimuli. At the same time there is a fast cortical activity with some discrete low-voltage spindling at 11 cycles/sec lasting continuously for 10-12 hr. Periods of SWS with high-voltage spindles reap pear after 12 hr and a normal level of SWS is reached on the following day.
At the onset of the behavioral changes induced by reserpine (40-60 min after the injection) some clusters of PGO spikes appear. They are first isolated and may be separated by intervals of some minutes. During the 2nd hr after injection they become permanent, with a frequency of 40-60 min. They are usually unique, but frequently bursts of 6-8 monophasic spikes, entirely similar to those of PS, appear. This PGO activity is similar in all respects to the PGO activity of PS. It can also be recorded at the same level of the pons, mesencephalon, lateral geniculate, and occipItal. cortex. It is accompanied by very discrete lateral eye movements, small twitches of the vibrissae, ears, and even "fingers." But there is no decrease in the EMG of the neck, and the behavioral aspect of the cat is quite different from that of sleep. This PGO activity is not suppressed by external stimuli and may increase during the period of agitation. It lasts permanently for 40 60 hr with the same frequency and disappears slowly.
When the first brief episode of behavioral and polygraphic PS appears (after 22-24 hr), this activity becomes more irregular and clusters of monophasic spikes increase. The decrease in PS lasts for 4-5 days and the normal level of 15 % PS is usually reached on the 6th day. Thus reserpine has the unique property of electively and permanently inducing the PGO activity without affecting the other tonic components of PS. Since reserpine is known to depress the level of both 5-HT and catecholamines in the brain, the precursors of both amines were injected after this drug in order to increase them selectively.
1) Reserpine + 5-HTP. 5-HTP (30-50 mg/kg) injected 2-3 hr after reserpine immediately (within 1or 2 min) and totally suppresses the PGO activity induced by reserpine in 4-6 hr; and induces EEG and behavioral signs of SWSÑsuppres sion of fast cortical activity and appearance of high-voltage spindles (periods of SWS reappear for 4-6 hr, after which the usual fast cortical activity and PGO activity of reserpine reappear).
2) Reserpine + DOPA. DOPA (30-50 mg/kg) injected 2-3 hr after reserpine increases (after 10-15 min) the frequency of PGO activity by about 30-50 % for 4-6 hr; and induces (after a latency of 50-60 min) brief periods of SWS that are often followed by behavioral and polygraphic PS (with total extinction of the neck activity and clusters of rapid eye movements). Thus during the 5-6 hr that follow DOPA injection, two or three periods of PS may appear (and an almost normal level of SWS and PS is reached during this short period). When the effect of DOPA has disappeared (after 5-6 hr), the PGO activity returns to the base-line level (40-60/min), and the reserpine syndrome reappears.
Thus the association of reserpine with precursors of both monoamines shows the importance of serotonergic mechanisms in the occurrence of SWS, whereas the induction of the tonic phenomena of PS when DOPA is injected after reserpine suggests that a catecholaminergic mechanism may be involved in the total atony of PS. A more detailed analysis of drugs acting on monoamines in relationship with sleep mechanisms has been published recently (243, 244).
The actual state of the neuropharmacological approach on sleep mechanisms could be summarized as follows.
There is no unique and continuous hypnogenic mechanism presiding over the periodical succession of the states of sleep since it is possible, by altering the brain monoamine level, to increase SWS and to suppress PS. If there were a single sleep mechanism for both states of sleep, PS, which is considered deep sleep, would also have been increased (or at least would not have been suppressed).
Increase of the brain 5-HT level led to an increase in SWS (and a parallel decrease of PS).
Some metabolic step requiring MAO appears to determine the transition from SWS to PS.
The release of monoamines (whether 5-HT or catecholamines or both) at monoaminergic terminals induced by reserpine triggers electively the appearance of the phasic EEG components of PS.
The total atony of PS appears to be dependent on cholinergic mechanisms (since it is suppressed by atropine) and to necessitate also a catecholaminergic mechanism since DOPA is able to induce normal PS after reserpine.
Thus, the neuropharmacological approach to a sleep mechanism, still at its beginning, has revealed at least two different mechanisms in the process governing the transition from SWS to PS.
The first mechanism (the appearance of PGO activity, which always heralds the occurrence of PS) is totally suppressed by an MAO inhibitor (but not by atropine) whereas the second mechanism (which triggers total atony of PS) is dependent on both cholinergic and catecholaminergic mechanisms. The mode of action of short-chain fatty acids able to act rapidly on these different mechanisms is still totally unknown.