Postnatal change in receptivity for methionine5-enkephalin in rat duodenum: transition from neurogenic to myogenic receptivity.

The study concerned the postnatal ontogenesis of the response to methionine5-enkephalin (ENK) in the duodenum of developing rats. Sustained relaxation was produced by ENK in the isolated duodenum of 8 day old rats. The response was blocked by tetrodotoxin or naloxone. The response increased by day 18, decreased thereafter and was extinguished after 40 days. On the other hand, a tetrodotoxin resistant response to ENK, transient relaxation, appeared at day 20 and was augmented thereafter. The development of neurogenic receptivity for ENK preceded the appearance of myogenic receptivity in the rat duodenum.