Ontogenesis of enkephalin and humoral endorphin in the rat brain.

Radioimmunoassay and column chromatography techniques were used to study the postnatal development of two different opioid ligands, humoral endorphin and enkephalin in the rat brain. Similar patterns were observed for both male and female animals during the period examined (from birth to the seventh week of life). Humoral endorphin content of the developing rat brain was found to increase in parallel to enkephalin, exhibiting a 'lag' period of 2 weeks. The most dramatic increase in opioid levels was detected during the third week of life; this stage was followed by a gradual change up to the adult levels.