Role of retinoids in the CNS: differential expression of retinoid binding proteins and receptors and evidence for presence of retinoic acid

Abstract

Retinoic acid (RA), a retinoid metabolite, acts as a gene regulator via ligand‐activated transcription factors, known as retinoic acid receptors (RARs) and retinoid X receptors (RXRs), both existing in three different subtypes, α, β and γ. In the intracellular regulation of retinoids, four binding proteins have been implicated: cellular retinol binding protein (CRBP) types I and II and cellular retinoic acid binding protein (CRABP) types I and II. We have used in situ hybridization to localize mRNA species encoding CRBP‐ and CRABP I and II as well as all the different nuclear receptors in the developing and adult rat and mouse central nervous system (CNS), an assay to investigate the possible presence of RA, and immunohistochemistry to also analyse CRBP I‐ and CRABP Iimmunoreactivity (IR). RXRβ is found in most areas while RARα and ‐β and RXRα and ‐γ show much more restricted patterns of expression. RARα is found in cortex and hippocampus and RARβ and RXRγ are both highly expressed in the dopamine‐innervated areas caudate/putamen, nucleus accumbens and olfactory tubercle. RARγ could not be detected in any part of the CNS. Using an in vitro reporter assay, we found high levels of RA in the developing striatum. The caudate/putamen of the developing brain showed strong CRBP I‐IR in a compartmentalized manner, while at the same time containing many evenly distributed CRABP I‐IR neurons. The CRBP I‐ and CRABP I‐IR patterns were closely paralleled by the presence of the corresponding transcripts. The specific expression pattern of retinoid‐binding proteins and nuclear retinoid receptors as well as the presence of RA in striatum suggests that retinoids are important in many brain structures and emphasizes a role for retinoids in gene regulatory events in postnatal and adult striatum.