Transition from neurogenic to myogenic receptivity for thyrotropin-releasing hormone (TRH) in the duodenum of the neonatal rat.

A tetrodotoxin- and hexamethonium-sensitive response to thyrotropin-releasing hormone (TRH), an in vitro contraction, first appeared in the duodenum of the 3 day old rat, was increased by day 16, decreased there-after and was extinguished after weaning. A tetrodotoxin-resistant response to TRH, a relaxation, appeared at day 13 and became distinct after weaning. The development of neurogenic receptivity for TRH precedes the appearance of myogenic receptivity in the rat duodenum.