Development of radioimmunoassayable beta-endorphin and methionine-enkephalin binding sites in regions of rat brain.

The development profiles of [3H]methionine-enkephalin ([3H]Met-enk) binding sites and radioimmunoassayable (RIA) beta-endorphin in regions of rat brain were determined. The amount [3H]Met-enk bound reached its maximum in the 1st week after birth in the cerebellum, in the brainstem at the 2nd week, and in the whole forebrain at the 3rd week, that of RIA beta-endorphin reached its highest level at day 2 postpartum (p.p.) in the cerebellum. at days 7-15 p.p. in the whole forebrain, and at days 17-21 p.p. in the brainstem. These findings suggest that both the development of RIA beta-endorphin and [3H]met-enk sites in rat brain follow a caudal to rostral sequence. Also, the close interrelationship between the elevation and decline in the amount of [3H]Met-enk bound and RIA beta-endorphin levels in each brain region suggests that these two components are important entities of the central nervous system.