Mammalian neuromuscular development accelerated with early but slowed with late gestational administration of ACTH...

The neuropeptide ACTH 4-10, a nonsteroidogenic fragment of adrenocorticotropic hormone, has two distinct and opposite effects on developing nerve and muscle. Muscle is positively influenced by ACTH during the first part of gestation (G days 3-12) before innervation occurs. Subsequent effects on innervation are largely depressive and exerted only during G13-21. Treatment during G3-12 increases twitch amplitude, rise time and speed of contraction of directly and indirectly stimulated extensor digitorum longus (EDL) muscle of two wk old rats. Treatment during G13-21 slows contractions of indirectly stimulated EDL, whereas treatment throughout gestation (G3-G21) shows little effect. Thus, ACTH first accelerates muscle development then modulates this development through neuronal depression.