Glutathione-independent prostaglandin D synthase [prostaglandin-H2 D-isomerase; (5Z,13E)-(15S)-9a,lla-epidioxy-15-hydroxyprosta-5,13-dienoate D-isomerase, EC 5.3.99.2] is an enzyme responsible for biosynthesis of prostaglandin D2 in the central nervous system. In situ hybridization with antisense RNA for the enzyme indicated that mRNA for the enzyme was predominantly expressed in the leptomeninges, choroid plexus, and oligodendrocytes of the adult rat brain. (2) is responsible for biosynthesis of PGD2 in the central nervous system (3), retina (4), and cochlea (5). Immunocytochemical studies with a polyclonal and two monoclonal anti-rat brain PGD synthase antibodies revealed that the major cellular location of the enzyme in the brain shifts postnatally from neurons of 1-to 2-week-old rats to oligodendrocytes in adult animals (6), suggesting that the enzyme may have distinct functions in those cells at each specific developmental stage and thus plays important roles in both maturation and maintenance of the central nervous system.We previously isolated cDNAs encoding this enzyme in the brain of rats (7) and humans (8). The homology search in data bases of protein primary structure (8-10) indicated that the enzyme is a member ofthe lipocalin superfamily consisting of a variety of secretory proteins involved in the transport of small hydrophobic molecules (11). PGD synthase is the only lipocalin known to be associated with enzyme activity. The gene structure of rat brain PGD synthase (12) is also remarkably conserved among the members of this protein family inThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.terms of the numbers and sizes of exons and phases and sites of exon/intron splicing.Inorganic quadrivalent selenocompounds, such as SeCL4 and Na2SeO3, have recently been found to inhibit rat brain PGD synthase activity without affecting the activities ofother enzymes in the arachidonate cascade (13). Those selenocompounds were subsequently shown to inhibit sleep in unrestrained rats, promoting physiological wakefulness, in a timeand dose-dependent manner, when administered into the third ventricle via a microdialysis probe (14). These results further support our hypothesis that PGD2 is involved in the promotion of sleep under physiological conditions and imply that the PGD synthase may be a key enzyme in regulation of sleep-wake activities.We report here, by using in situ hybridization with antisense RNA for the enzyme, that mRNA for glutathioneindependent PGD synthase is dominantly expressed in the leptomeninges and choroid plexus, as well as in oligodendrocytes, of the adult rat brain. These tissues and cells were also found to be enriched in both the immunoreactivity and the enzyme activity of PGD synthase, indicating that the major site of synthesis of the enzyme, and also of PGD2, is predominantly, if not exclusively, located in the leptomen...