Norepinephrine and REM sleep
Pierre-Hervé Luppi , Christelle Peyron, Claire Rampon, Damien Gervasoni, Bruno Barbagli, Romuald Boissard and Patrice Fort
Rapid Eye Movement Sleep pp. 107-122B.N. Mallick, S. Inoue (Editors) Narosa Publishing House 1999

Table of contents

1. Introduction

2. Effect of the application of gaba and glycine antagonists on the activity of the rat locus coeruleus neurons during sleep

3. Glycinergic and gaba-ergic afferent projections to the locus coeruleus

4. Physiological role of the glycinergic inputs to the LC

5. Physiological role of the gaba-ergic inputs to the LC

6. Conclusions and new hypothesis

1. Introduction

Using transection and lesions, Jouvet and coworkers (Review in Jouvet, 1972) demonstrated that the structures responsible for the genesis of paradoxical sleep (PS) are localized in the pontine reticular formation. The subsequent identification in this region of noradrenergic neurons in the nucleus locus coeruleus (LC) (Shimizu et al., 1959; Dahlström and Fuxe, 1964) and pharmacological results showing that manipulation of the noradrenaline modify the occurrence of PS led Jouvet to hypothesize that the LC noradrenergic neurons might be responsible for the genesis of PS. Supporting this idea, Jouvet and Delorme (1965) found that electrolytic lesions involving the noradrenergic locus coeruleus (LC) localized in the cat dorsolateral tegmentum are followed by a loss of PS and/or the muscular atonia that accompanies it.

However, subsequent lesions, pharmacological and electrophysiological studies showed that the region ventral to the LC rather than the LC and its noradrenergic neurons contains the neurons responsible for the genesis of PS. Indeed, lesions more limited to the noradrenergic neurons (Henley and Morrison, 1974; Jones et al., 1977) did not have a profound effect on PS. In addition, pharmacological depletion of the catecholamines with a-methyl-para-tyrosine (AMPT) increases PS and reduces waking. (King and Jewet, 1971). Moreover, 6-hydroxydopamine-induced lesions of the ascending noradrenergic bundle induced a significant reduction in waking but not of PS (Lidbrink, 1974). In addition, electrophysiological recordings have shown that the noradrenergic neurons of the LC are active during waking, decrease their activity during slow wave sleep and cease firing during PS. (Hobson et al., 1975, Aston-Jones and Bloom, 1981). Altogether these data indicates that the LC noradrenergic neurons by means of their widespread projections throughout the entire brain constitutes one of the structures involved in the control of wakefulness (Jouvet, 1972, Hobson et al., 1975). It has further been shown in rats and cats that LC noradrenergic cells display a burst of activity when the animals are subjected to new stimuli. The LC neurons might therefore be more specifically implicated in arousal and vigilance that in the tonic maintenance of waking. This view is supported by results showing that lesions of LC or the noradrenergic bundle do not induce a long lasting decrease of wakefulness (Lidbrink, 1974).

Hobson et al (1975) in the first report on the activity of LC neurons across the sleep-waking stages proposed that the LC cells might gate the appearance of PS. In fact, they first found neurons in the gigantocellular tegmental field (FTG) with a dramatic and state-specific discharge rate increases during PS (McCarley and Hobson, 1971). These findings led them to postulate that these FTG neurons may be a critical part of an executive system for PS. They further made the hypothesis that the periodic activation of these cells might be due to decreased activity of an inhibitory population. Supporting this proposal, they later made the first demonstration that LC neurons are nearly silent during PS and have therefore an inverse discharge rate selectivity of FTG cells (Hobson et al., 1975). The LC neurons were named "REM-off" or "PS-off" cells. Although this denomination is not inaccurate, it does not reflect that the main role of noradrenergic neurons of the LC seems to be related to wakefulness. Indeed, the inhibition of PS by stimulation of the noradrenergic system could be secondary to the induction of wakefulness rather that due to a direct inhibition of the PS generating systems. However, supporting the hypothesis that such inhibition occurs, Sakai and Koyama (1996) recently reported that cholinoceptive PS-on cells localized in the peri-LCa are inhibited by noradrenaline. Furthermore, Sakai and Onoe (unpublished observations) and Tononi et al. (1989) showed that application of a2 adrenergic agonists in the periLCa region suppressed PS. In conclusion, they are some arguments that PS-off noradrenergic cells of the LC could inhibit the PS-on cells. The classical "reciprocal interactions" models also implied that the cessation of firing of LC noradrenergic neurons during PS is the result of active PS-specific inhibitory processes, originating from the pontine neurons responsible for the PS onset and maintenance (PS-on cells)(Hobson et al., 1975; Sakai, 1985). These neurons were first thought to be the PS-on neurons presumed cholinergic localized in the dorsal pons (laterodorsal tegmental, pedunculopontine and peri-LCa nuclei). It has later been suggested that they might use GABA or glycine rather than acetylcholine as an inhibitory neurotransmitter (Luppi et al., 1991; Jones, 1991). Indeed, acetylcholine excites LC noradrenergic neurons (Guyenet and Aghajanian; 1979; Koyama and Kayama, 1993). In contrast, in anesthetized rats, iontophoretic application of GABA or glycine (GLY) strongly inhibits LC neurons and co-iontophoresis of bicuculline or strychnine, respectively GABAA and GLY antagonists, antagonizes this effect (Luppi et al., 1991; Ennis and Aston-Jones, 1989; Gallager and Aghajanian, 1976; Gallager, 1978). Furthermore, in vitro studies on slices using focal stimulation and bath-applied bicuculline or strychnine revealed GABA- and GLY- mediated IPSP's in LC neurons (Cherubini et al., 1988; Williams et al., 1991; Osmanovic and Shefner, 1990; Pan and Williams, 1989). In agreement with these results, GABA-and GLY-immunoreactive varicose fibers and GABAA and GLY receptors have been found in the rat LC (Luppi et al, 1991; Jones, 1991; Wang et al., 1992; Luque et al., 1994; Zarbin et al., 1981). Supporting the hypothesis that glycinergic neurons could be responsible for the inhibition of monoaminergic neurons during PS, it has been shown that glycine was responsible for the inhibition of the motoneurons during PS (Chase et al. 1989). In contrast, rather supporting a role for GABA, microinjections of picrotoxin (a GABAA antagonist) in rats LC significantly reduced the duration of PS episodes (Kaur et al., 1997).

In addition to a near cessation of activity during PS, the noradrenergic neurons greatly reduce their activity during SWS. This reduction implies either that a deactivation or an active inhibition takes place at the transition between W and SWS and during SWS. The classical view is that the GABA transmission is increased during SWS as supported by the strong hypnotic properties of the benzodiazepines acting on the GABAA receptors (Reviews in Mendelson, 1992, Gaillard, 1994). Moreover, Nitz and Siegel (1997) recently found an increase of the amount in GABA in the cat's LC during SWS compared to W. A GABA inhibition might therefore likely be responsible for the decrease of activity of noradrenergic neurons of the LC during SWS. Another possibility is that the LC neurons are deactivated during SWS. Against this hypothesis, it has been proposed that the spontaneous tonic firing of LC neurons during W, which is close to that observed in anesthetized rats is mainly due to their intrinsic pacemaker properties, as revealed by intracellular recordings in slices (Aghajanian and Vandermaelen, 1982, Williams et al., 1984) and in cultures of LC cells (Masuko et al., 1986). This implies that at least during quiet W the LC noradrenergic cells are not tonically activated by excitatory influences.

hypothesis : 1) we applied bicuculline or strychnine on LC noradrenergic cells during SWS, PS and W using a new method which allows extracellular single-unit recordings of neurons, combined with iontophoresis in the head-restrained awake rat (Darracq et al., 1996) (Fig. 1) and 2) we localized in rats the glycinergic and GABA-ergic neurons potentially responsible for the inhibition of the monoaminergic neurons of the LC, combining injections of the retrograde tracer cholera-toxin B subunit (CTb) in LC and immunohistochemistry of GLY or GAD (GABA enzyme of synthesis).

Fig. 1

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