Comparison of ontogenies of opioid receptors in rat, guinea pig and rabbit CNS by autoradiography

The ontogeny of opioid receptors in rat CNS from E12 to adult at 12 ages, in guinea pig from E30 to adult at 12 ages and, in rabbit from E16 to adult at 13 ages were studied by autoradiography and gray scale measurement. y-binding sites labeled with [3H]ohmefentanyl or [3H](D-Ala2, MePhe4, Gly-ol5)enkephalin usually appeared earlier than h-binding sites labeled with [3H](D-Ala2, D-Leu6)enkephalin (V-binding was suppressed) and K-binding sites labeled with [3H]etorphine (lL- and h-binding were suppressed). Corpus striatum and spinal cord were the regions where opioid receptors appeared earlier with their densities higher than other regions. There were apparent species differences of the receptor subtypes and receptor distributions in hippocampal formation and in cerebellum. Cerebral cortex was the area in which opioid receptors reached the adult level later than other regions.