onwards (Figure 4C,D), as well as in the olfactory tubercle [Mollard et al., 2000; Ruberte et al., 1993]. Rarb transcripts are also detected in the choroid plexuses and the developing meninges, which also express the RA-synthesizing enzyme Raldh2 [Niederreither et al., 1997]. Among Rxrs, Rxra and Rxrb are relatively ubiquitously expressed in the developing mouse [Dolle et al., 1994] and rat [Zetterstrom et al., 1999] brain, whereas Rxrg appears in specific brain regions at newborn stages [Zetterstrom et al., 1999]. Specific distributions of RARs and RXRs have also been reported, both at mRNA and protein levels, in the adult mouse brain [Krezel et al., 1999]. In particular, RARβ expression persists in the caudate-putamen and nucleus accumbens, where it is coexpressed with RXRγ, and both RARα and RARγ are coexpressed in the hippocampal fields, where RARβ is also detected. Through the analysis of viable Rar/Rxr compound mutant mice, it has been possible to characterize some of the corresponding functions in locomotor control [Krezel et al., 1998] or learning and memory [Chiang et al., 1998; Wietrzych et al., 2005].
