Effects of chronic stress and exercise on age-related changes in end-plate architecture

The effects of chronic stress (foot shock) or stress coupled with exercise (treadmill running) on age-related changes in nerve terminal branch number per end plate were examined in three physiologically different muscles, the diaphragm and two hindlimb muscles, the extensor digitorum longus (EDL), and the soleus, from 24- to 25-mo rats. End-plate structure was examined by use of a silver-gold impregnation of the axon and its terminal arborization and was compared with similar data obtained from 10- and 25-mo sedentary control animals. Stress from foot shock resulted in decreases of 30 and 23%, which are significant at the 0.05 level, in the mean number of nerve terminal branches per end plate in the fast-twitch EDL and diaphragm of 25-mo rats, respectively; a similar, but less dramatic decline in branch number was observed in the end plates of the slow-twitch soleus. Exercise superimposed on foot shock counteracted the decrease in branch number resulting from foot shock alone in the hindlimb muscles; in contrast, terminal branch number within the diaphragm end plates declined an additional 29% in the exercised group. Exercise plus shocking eliminated the significant age-related decrease in branch number in the soleus between the 10- and 25-mo animals; however, it produced a significant decrease in the diaphragm of the 25-mo compared with the 10-mo animals. There was a significant increase in the occurrence of sprouting nerve terminals in the EDL and soleus of exercise-shocked animals compared with 25-mo controls, thus counteracting the age-related decrease observed in the 25-mo compared with 10-mo control animals.(ABSTRACT TRUNCATED AT 250 WORDS)