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 micro, delta and kappa are found in the central nervous system and periphery. In situ hybridization study was undertaken to determine the expression of mu, delta, kappa-opioid receptors mRNAs in the brain during pre- and postnatal development, especially in the mesostriatal system. By G13, mu and kappa-opioid receptor 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-opioid receptor mRNA was detectable in the nucleus accumbens and dorsal striatum, and in the substantia nigra and ventral tegmental area, suggesting an early expression of the corresponding receptor on dopaminergic terminal fibers. For the mu-opioid receptor mRNA in the striatum, patches appeared at G20. Delta-opioid receptor mRNA was first detected at G21, in many areas including the accumbens nucleus and the dorsal striatum. At P8, delta-opioid receptor mRNA was detected in large-sized cells of the striatum, possibly cholinergic, suggesting a possible modulation by opioids of the striatal cholinergic neurons. Our results demonstrate the early appearance of mu and kappa-opioid receptor mRNA (G13) 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 during the development.