Locus coeruleus (LC) noradrenergic neurons are active during wakefulness, slow their discharge rate during slow wave sleep, and stop firing during paradoxical sleep (PS). A large body of data indicates that their inactivation during PS is due to a tonic GABAergic inhibition. To localize the neurons responsible for such inhibition, we first examined the distribution of retrogradely and Fos double-immunostained neurons following cholera toxin b subunit (CTb) injection in the LC of control rats, rats selectively deprived of PS for 3 days, and rats allowed to recover for 3 hours from such deprivation. We found a significant number of CTb/Fos double-labeled cells only in the recovery group. The largest number of CTb/Fos double-labeled cells was found in the dorsal paragigantocellular reticular nucleus (DPGi). It indeed contained 19% of the CTb/Fos double-labeled neurons, whereas the ventrolateral periaqueductal gray (vlPAG) contained 18.3% of these neurons, the lateral paragigantocellular reticular nucleus (LPGi) 15%, the lateral hypothalamic area 9%, the lateral PAG 6.7%, and the rostral PAG 6%. In addition, CTb/Fos double-labeled cells constituted 43% of all the singly CTb-labeled cells counted in the DPGi compared with 29% for the LPGi, 18% for the rostral PAG, and 10% or less for the other structures. Although all these populations of CTb/Fos double-labeled neurons could be GABAergic and tonically inhibit LC neurons during PS, our results indicate that neurons from the DPGi constitute the best candidate for this role.
Copyright 2006 Wiley-Liss, Inc.