Postnatal development of opioid systems in rat brain

Immunocytochemical and receptor autoradiographic techniques have been utilized to examine the postnatal development of opioid peptides and receptors. The pattern of met-enkephalin-like immunoreactivity was very similar at birth to the adult, increasing only in intensity throughout postnatal development. In contrast, β-endorphin-like immunoreactivity (BLI) in neonate was strikingly different from that of adult. In particular, cells and fibers were observed in germinal zones, structures present only in the neonate. The distribution of dynorphin-B-like immunoreactivity at birth was intermediate in its similarity to that of adult. Certain terminal fields were present, others developed postnatally, and others disappeared with age. In parallel studies, autoradiographic maps of μ, δ and ϰ opioid receptor subtypes were generated to determine whether any correspondences existed between the developmental distribution of opioids and their receptors. Although significant overlap existed, no simple one-to-one relationship was observed. The differential localization of peptides and receptors in neonatal and adult brain suggests that opioids may subserve distinct functions in the neonate. In particular, the presence of BLI in the germinal zones, where postnatal neurogenesis occurs, implicates opioid systems in regulation of neuronal cell division.