Acetylcholine receptor binding properties at the rat neuromuscular junction during aging

Specific binding characteristics of acetylcholine receptors at the diaphragm neuromuscular junction of rats aged 10 (mature adult) and 28 (aged) months were assayed by measuring 125I-alpha-bungarotoxin binding. Maximal binding to intact tissue samples was greater in the older rats; this could be attributed to an age-related increase in terminal branching. The toxin concentration at which half-maximal binding occurred increased in the older rats. Binding kinetics were assayed in finely minced tissue samples, and the association rate constant was observed to decrease in the 28-month animals. Retardation of the initial rate of toxin binding by d-tubocurarine (dTC) in minced tissue was described by a two-component nonlinear Hofstee plot; IC50 values (7.1-7.2 microM and 39.0-46.5 nM) were about the same for both age groups, but there was a significant shift toward the low-affinity values in the aged rats. Rhodamine-conjugated alpha-bungarotoxin was used to visualize receptor localization. There were no major changes in receptor distribution, and nerve terminals were consistently associated with receptors and vice versa. The data indicate a shift toward lower binding affinity during aging, which may involve changes either in one of the two toxin-binding sites on individual receptors, in dTC blocking of the channel moiety, or in receptor types.