Postnatal expression of opioid genes in rat brain.

Developmentally regulated expression of opioid genes in rat brain was studied by Northern blot hybridization, using cloned rat genes as probes. Total RNA was isolated from the rat medulla oblongata, hypothalamus and striatum at 11 stages of development from birth to postnatal day 28. The steady-state levels of opioid precursor mRNAs in the above tissues were determined by hybridization to 32P-labeled opioid DNA probes and densitometric scanning of the appropriate bands. Analysis of the relative steady-state concentrations of opioid mRNAs has shown that the tissue distribution and size of these mRNAs during postnatal development of the CNS are similar to the pattern described in the adult. However, up to 50-fold changes in the relative mRNA concentrations were observed. These changes are tissue-specific, characterized by a peak within the postnatal stage of development, and are followed by a build-up of mRNA levels characteristic of the adult. Moreover, comparison of the changes in mRNA levels of proenkephalin to those of prodynorphin at their main site of synthesis in the brain, namely the striatum, revealed striking similarities in pattern. The association of this phenomenon with a possible role of opioid genes in development is suggested.