Acetylcholine receptors at mature and aged mouse neuromuscular junctions.

The density and distribution of junctional and perijunctional ACh receptors (AChR) were studied in young (8-12 months) and old (24-25 months) C57 mice to determine: (1) if increased amplitude of spontaneous postsynaptic potentials previously reported in old C57 muscle was due to increased junctional AChR; (2) if increased extrajunctional AChR would be found in association with previously reported nerve terminal complexity; and (3) if extrajunctional AChR was present as in disused or denervated muscle. Microdissection of individual muscle fibers combined with I125-alpha-bungarotoxin labeling, gamma counting, measurement of surface area, cholinesterase stains, and autoradiography were used to obtain the results. In both young and old mice there was a sharp gradient in AChR between the end-plate and the perijunctional region. End-plate AChR densities and total AChR per end-plate were the same at old and young end-plates, as were perijunctional values. Thus, neither end-plate nor extrajunctional AChR density changes with age. An increased mepp amplitude reported previously in old CB57 animals must be due to other factors. The perijunctional AChR in old mice show no changes characteristic of disuse or denervation, or those which might give rise to the observed nerve terminal complexity.