OPIOID RECEPTOR GENE-EXPRESSION IN THE RAT-BRAIN DURING ONTOGENY, WITH SPECIAL REFERENCE TO THE MESOSTRIATAL SYSTEM - AN IN-SITU HYBRIDIZATION STUDY

The three main types of opioid receptors mu, delta and kappa are found in the central nervous system and periphery. In situ hybridization st udy was undertaken to determine the expression of mu, delta, kappa-opi oid receptors mRNAs in the brain during pre- and postnatal development , especially in the mesostriatal system. By G13, mu and kappa-opioid r eceptor mRNA were detectable in the telencephalon; mu-opioid receptor mRNA was found in the striatal neuroepithelium and cortical plate and kappa-opioid receptor mRNA in the corroidal fissure. By G15, kappa-opi oid receptor mRNA was detectable in the nucleus accumbens and dorsal s triatum, and in the substantia nigra and ventral tegmental area, sugge sting an early expression of the corresponding receptor on dopaminergi c terminal fibers. For the mu-opioid receptor mRNA in the striatum, pa tches appeared at G20. delta-Opioid receptor mRNA was first detected a t G21, in many areas including the accumbens nucleus and the dorsal st riatum. At P8, delta-opioid receptor mRNA was detected in large-sized cells of the striatum, possibly cholinergic, suggesting a possible mod ulation by opioids of the striatal cholinergic neurons. Our results de monstrate the early appearance of CL and kappa-opioid receptor mRNA (G 13) and the relatively late development of delta-opioid receptor mRNA (G21) in the brain. We also show a distinct pattern of expression for mu, delta and kappa-opioid receptor mRNAs in the mesostriatal system d uring the development. (C) 1998 Elsevier Science B.V. All rights reser ved.