Nitric oxide synthase (NOS) I during postnatal development in rat and mouse skeletal muscle

Previous studies on adult rat and mouse skeletal muscles have shown the spatial association of nitric oxide synthase (NOS) I to the dystrophin complex (DC) in the sarcolemma of type II fibers and, in combination with the NMDA receptor-1 (NMDAR-1), an accumulation of the enzyme at the neuromuscular junctions (NMJ) of this fiber type. Using immunohistochemistry, enzyme histochemistry and alpha-bungarotoxin labeling we report here temporal relationships of NOS I, members of the DC, other components of the cortical cytoskeleton in the junctional and non-junctional sarcolemma as well as of molecules involved in NMJ transmission of either type I or II myofibers especially in head and neck muscles during postnatal rat and mouse development. Fiber typing was performed by specific anti-myosin antibodies. Beginning with postnatal day (PD) 1 in both fiber types dystrophin, dystrophin-associated glycoproteins (DAG), beta-dystroglycan, alpha-sarcoglycan (adhalin) and spectrin were present in the junctional and extrajunctional sarcolemma, while utrophin, acetylcholinesterase, alpha-bungarotoxin labeled acetylcholine receptors were concentrated in the NMJ of both fiber types. NOS I activity and immunoreactivity were only found in the NMJ region of type II fibers, where NMDAR-1 appeared around PD 15. Primarily in the tongue there was no strict correlation between muscle fiber type and NOS I behaviour during early postnatal development, and muscle fibers not reactive for myosin antibodies against both fiber types were negative or positive for NOS I but always positive for the other molecules either in both the junctional and extrajunctional sarcolemma or in the NMJ only; later all muscle fibers of the tongue were of type II and NOS I-positive. Maturation of enzyme activities, immunoreactivities and AChR intensity depended on the respective muscle and can last until PD 50; in the tongue and neck muscles they appeared to increase approximately until PD 20 or 25. In conclusion, in type II fibers of rat and mouse skeletal muscle all molecules with the exception of NMDAR-1 and relevant for NOS I targeting and positioning as well as function inside and outside the NMJ are already present at birth, but their concentrations and/or activities increase postnatally, and the adult situation appears to be reached between the third and seventh week of postnatal life. Therefore, initial interactions between NOS I and the other molecules necessary for the formation of the NOS I-DC in and on the way to the sarcolemma presumably take place before birth.