|
|||||||||
|
5. Physiological role of the GABA-ergic inputs to the LCOur electrophysiological data indicates the existence of a tonic GABA-ergic input on the LC cells during W, SWS and PS. Further, we found that when the same neuron was recorded during short successive periods of SWS and W during the bicuculline effect, its increases in discharge rate was not statistically different between W and SWS. These results highly suggest that GABA release is responsible for the inactivation of LC noradrenergic neurons during SWS. Unfortunately, due to the smaller number of LC cells recorded during PS, we were not able to make the same comparison between SWS and PS. However, with the microdialysis technique, Nitz and Siegel (1997) recently found an increase of the GABA release in the cat LC during SWS and moreover PS as compared to waking values. Based on these and our results, we therefore suggest that during W, the LC cells are under a tonic GABA-ergic inhibition which increases during SWS and even more during PS and is at least partly responsible for the inactivation of these neurons during these states. Our results indicate that the LC receives GABA-ergic inputs from neurons located in a large number of distant regions from the forebrain to the medulla (Fig. 6). We also observed a substantial number of GAD+/CTb+ neurons in the pontine and mesencephalic periaqueductal gray. These results indicate that the GABA innervation of this nucleus arises from multiple distant GABA-ergic groups in addition to local GABAergic groups. Such result contrast with the classical concept that GABA is mainly contained in interneurons. They suggest that the NA neurons of the LC could be inhibited by multiple populations of GABA-ergic neurons located in different structures and raises the question of the functional role of such complexity. One possibility is that only some of these GABA-ergic afferents are destined to the noradrenergic neurons of the LC. This seems unlikely because the LC in rats contains nearly exclusively noradrenergic cells. Another possibility is that some of these afferents are postsynaptic and others presynaptic, but the more likely explanation is that each of these afferents is active only under specific physiological conditions. Based on physiological and electrophysiological data (see above), we expect that one or several of these GABA-ergic afferents are "turned on" specifically during SWS and others during PS and are responsible for the progressive decrease of activity of noradrenergic neurons during these sleep states. The most likely candidate for the inhibition of the noradrenergic neurons during SWS is the lateral preoptic area. Indeed, lesion of this structure in cats and rats induces an insomnia while its stimulation induced SWS (Asala et al., 1990; Lucas and Sterman, 1975; McGinty and Sterman, 1968; Sallanon et al., 1989; John et al., 1994; Sterman and Clemente, 1962). Neurons increasing their activity during SWS have been recorded in this area (Kaitin, 1984; Szymusiak and McGinty, 1986, Koyama and Hayaishi, 1994). Moreover, C-Fos positive cells were observed in the lateral preoptic area after long periods of SWS (Sherin et al., 1996) and it has been further shown that these neurons are in part GABA and galanin positive and project to the tuberomamillary nucleus which contains waking active neurons presumably histaminergic (Vanni-Mercier et al., 1984). From these and our results, we can therefore propose that GABA-ergic neurons in the lateral preoptic area increase their firing just before the onset and during SWS and induce SWS via their inhibitory projections to waking-inducing structures (tuberomamillary nucleus, DRN and LC among others). Besides the lateral preoptic area, the posterior hypothalamic areas, the periaqueductal gray and the dorsal paragigantocellular nucleus provide substantial GABA inputs to the LC. The strong GABA-ergic projection from the lateral hypothalamic area is rather puzzling. Indeed, since the initial demonstration that the lesion of the posterior hypothalamus induces somnolence (Von Economo, 1926), the histaminergic neurons of the posterior hypothalamus have been more specifically implicated in waking (Lin et al., 1996). These neurons are located in the tuberomamillary nucleus ventral and caudal to the GABA-ergic neurons from the lateral hypothalamic area projecting to the LC. Nevertheless, muscimol injections in cats in the lateral hypothalamic area in addition to those in the tuberomamillary nucleus induced hypersomnia (Lin et al., 1989). Additional experiments are therefore needed to determine whether the lateral hypothalamic area and its GABA-ergic neurons play a role in vigilance via their projections to the LC. The GABAergic afferents responsible for the inhibition of noradrenergic neurons during PS should be located in the brainstem. Indeed, it is well known that PS-like episodes occur in pontine or decerebrate cats (Jouvet, 1972). Moreover, it has recently been shown that, in decerebrate animals, PS episodes induced by carbachol injections in the pons are still associated with a cessation of activity of serotonergic neurons of the raphe obscurus and pallidus nuclei (Woch et al., 1996). In the brainstem, we observed GABA-ergic projections to the LC from the periaqueductal gray and the dorsal paragigantocellular nucleus. In agreement with this last result, local application of bicuculline blocked the dorsal paragigantocellular-evoked inhibition of LC neurons (Ennis and Aston-Jones, 1989). The GABAergic afferents from the periaqueductal gray and the dorsal paragigantocellular nucleus could therefore be responsible for the inhibition of noradrenergic neurons of the LC during PS. The hypothesis that this inhibition is arising from neurons located in the periaqueductal gray is further supported by two recent studies. Indeed, Yamuy et al showed that after a long period of PS induced by pontine injection of carbachol, a large number of C-Fos positive cells are visible in the DRN and lateral to it (Yamuy et al., 1995). Moreover, Maloney and Jones (1997) observed after a PS rebound induced by deprivation an increase in c-Fos+/GAD+ neurons in the periaqueductal gray and the lateral tegmental nucleus. Finally, it must be precised that although a number of arguments are in favor of a role of GABA in the inhibition of noradrenergic neurons of the LC during SWS and PS, other inhibitory neuroactive substances might also participate in this inhibition. Indeed, it has for example been shown that LC cells are inhibited by local application of enkephalin (Bird and Kuhar, 1977, Guyenet and Aghajanian, 1979). Moreover, it has been shown that adenosine release increases during wakefulness and decreases during SWS (Porkka-Heiskanen et al., 1997) and adenosine application is inhibitory on LC noradrenergic cells (Pan et al., 1995; Shefner and Chiu, 1986). The removal during SWS or PS of some tonic excitatory inputs may be present during W (like acetylcholine or glutamate) might also participate in the decrease of activity of LC neurons during sleep. |