Neonatal Sprague-Dawley rats administered the fragment of adrenocorticotropic hormone ACTH/MSH 4-10 (10 micrograms/kg/daily, SC) postnatally, show marked differences in the plasticity of the functional and morphological parameters of their neuromuscular system. Initial contraction durations of the immature fast muscle, extensor digitorum longus (EDL), are shorter than saline-treated controls indicating accelerated development. Qualitative studies of the developing EDL neuromuscular junctions as viewed by the scanning electron microscope and quantitative analysis permitted by light microscopy confirms that ACTH/MSH 4-10 affects the maturation of the endplate region. Motor behavior of rat pups demonstrates an age-related difference in the susceptibility to this peptide fragment; one week old neonates showing no response to ACTH/MSH 4-10, two week old pups showing an increase in motor activity. The results indicate that while the developing neuromuscular system is sensitive to the input of ACTH/MSH peptide treatment, this susceptibility is age-related.

Experiments were designed to assess the effects of chronic maximum 'smoking dose' nicotine administration on neonatal motor behavior and neuromuscular structural maturation. Thus, Sprague-Dawley rat neonates received daily subcutaneous injections of nicotine (0.05 mg/kg/day) from postnatal days 1 to 14. Motor behavior (ambulations, stop and go movements) was not affected, but a marked increase in grasping duration resulted, indicating increased neuromuscular maturation. Concomitantly, neuromuscular morphological and dynamic contractile changes were observed in the in situ extensor digitorum longus (EDL) muscle. The EDL muscle showed accelerated morphological dimensions of the endplate at postnatal day 7 and a more rapid twitch at 2 weeks of age. The results suggest that this maximum 'smoking dose' of nicotine induces time-dependent maturational changes in the neuromuscular model.