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The effect of acetylcholine on the function and structure of the developing mammalian neuromuscular junction

1980; Elsevier BV; Volume: 5; Issue: 7 Linguagem: Inglês

10.1016/0306-4522(80)90209-2

1873-7544

Ronan O’Brien, Anna Östberg, Gerta Vrbovà,

Nerve injury and regeneration

Isolated soleus muscles from rats aged 9–12 days were exposed to acetylcholine for 2 h in normal Krebs solution. This treatment caused changes in the ultrastructural appearance of the neuro-muscular junction and a significant reduction of axon profiles per endplate. Nevertheless, most neuro-muscular junctions remained functional, since the ratio of the indirectly to directly elicited contraction was not reduced. If muscles were exposed to acetylcholine in Krebs solutions containing 12m M Ca2+ instead of the normal 1.9 m M, the ultrastructural changes produced by acetylcholine were more severe, and the number of axon terminals per endplate was further reduced so that many endplates became completely denervated. This was also reflected in the impaired function of the nerve-muscle preparation; the ratio of the indirectly to directly elicited contraction decreased and about 40% of the muscles fibres became functionally denervated. Addition of curare to the incubating medium prevented the functional deterioration of the preparation. Addition of the protease inhibitors leupeptin and pepstatin protected the nerve terminals from the damaging effects of acetylcholine in Krebs solution containing 12m M Ca2+ and the number of axon profiles per endplate remained normal. The functional deterioration was also much reduced when protease inhibitors were included in the incubation medium. These results suggest that acetylcholine causes the activation and release of proteolytic enzymes in developing muscles. The response is mediated by calcium and may have a role in the removal of superfluous nerve-muscle contacts during development.