Ontogenesis of opiate binding sites and radioimmunoassayable beta-endorphin and enkephalin in regions of rat brain.

The postnatal changes in the levels of radioimmunoassayable enkephalin and beta-endorphin, as well as the densities of [3H]methionine-enkephalin and [3H]naloxone binding sites in rat cerebellum, brainstem and whole forebrain were determined. The opiate peptides and the opiate binding sites reached their highest levels at the first week postpartum in the cerebellum, at the second week in the brainstem and at the third week in the whole forebrain. This finding is in line with the developmental profiles of other well-established neuronal pathways which also showed a caudal-to-rostral sequence of development. Moreover, there was a close relationship between the elevation and decline in the amounts of opiate binding sites and in the levels of opiate peptides in each brain region. These observations are consistent with other evidence which suggests that enkephalin and beta-endorphin are functioning as neurotransmitters or neuromodulators in the central nervous system.