Sleep in transgenic and gene-knockout mice for lipocalin-type prostaglandin D synthase

Prostaglandin (PG) D₂ is a potent sleep-inducing substance in mammals. We investigated the effects of overproduction and deficiency of endogenous PGD₂ on sleep regulation by using transgenic (TG) mice overexpressing human lipocalin-type PGD synthase (L-PGDS) gene and gene-knockout (KO) mice produced by a null mutation of the L-PGDS gene, respectively. The circadian rhythm of non-rapid eye movement (NREM) and REM sleep was almost unchanged among wild-type (WT), TG, and KO mice. However, we found that noxious stimulation by tail clipping (Tc) of TG, but not of WT, mice transiently (∼5 h) increased the amount of NREM sleep without affecting REM sleep. This increase was coupled with elevation of the PGD₂ content in the brain, suggesting that the overproduction of PGD₂ in the brain by Tc induced NREM sleep in TG mice. We also found that sleep deprivation (SD) for 6 h induced rebound of NREM sleep in WT, but not in KO, mice, and of REM sleep in both of these mice, suggesting that the L-PGDS gene is involved in the homeostasis of NREM sleep after SD. Therefore, PGD₂ endogenously produced by L-PGDS in the brain is considered to regulate NREM sleep after noxious stimulation or SD.