Early degeneration and sprouting at the rat neuromuscular junction following acrylamide administration

Prolonged acrylamide administration produces motor nerve-terminal branch degeneration and impairs axonal outgrowth following nerve crush. It is unclear how early terminal branch degeneration is initiated and whether there is a compensatory regenerative response at the neuromuscular junction (NMJ). A modified Pestronk and Drachman silver-acetylcholinesterase strain was used to carry out a detailed morphometric analysis of the NMJ in soleus and lumbrical muscles. Rats were given 3, 5, or 10 doses of acrylamide, 35 mg/kg/day, by intraperitoneal injection, 5 days/week, and killed 4, 7, or 14 days after the first dose, respectively. Degenerating terminal branches were evident in soleus NMJ after only three doses of acrylamide. Diminished synaptic vesicle content, neurofilament accumulations and tubulo-vesicular profiles were evident after three doses. At later time points, degenerating terminals contained few synaptic vesicles and were engorged with neurofilaments. Endplate lengthening, indicative of denervation supersensitivity, accompanied degeneration. Terminal sprouting proliferated after 3 and 5 doses but was less prominent after 10 doses. Although similar changes occurred in the lumbrical muscle, they were not initiated until after 5 doses. These experiments reveal that pathological changes in terminal branches commence earlier and after a lower cumulative dose of acrylamide than previously reported and suggest that acrylamide exerts a primary effect at motor nerve-terminal branches. Early, vigorous terminal sprouting indicates that acrylamide does not prevent the initiation of regeneration, but with prolonged treatment does cause degeneration of maturing sprouts.