Role of mitochondrial dysfunction in the Ca2+-induced decline of transmitter release at K+-depolarized motor neuron terminals

The present study tested whether a Ca2+-induced disruption of mitochondrial function was responsible for the decline in miniature endplate current (ME PC) frequency that occurs with nerve-muscle preparations maintained in a 35 mM potassium propionate (35 mM KP) solution containing elevated calcium. W hen the 35 mM KP contained control Ca2+ (1 mM), the MEPC frequency increase d and remained elevated for many hours, and the mitochondria within twitch motor neuron terminals were similar in appearance to those in unstimulated terminals. All nerve terminals accumulated FM1-43 when the dye was present for the final 6 min of a 300-min exposure to 35 mM KP with control Ca2+. In contrast, when Ca2+ was increased to 3.6 mM in the 35 mM KP solution, the MEPC frequency initially reached frequencies >350 s(-1) but then gradually fell approaching frequencies <50 s(-1). A progressive swelling and eventual distortion of mitochondria within the twitch motor neuron terminals occurr ed during prolonged exposure to 35 mM KP with elevated Ca2+. After similar to 300 min in 35 mM KP with elevated Ca2+, only 58% of the twitch terminals accumulated FM1-43. The decline in MEPC frequency in 35 mM KP with elevate d Ca2+ was less when 15 mM glucose was present or when preparations were pr etreated with 10 mu M oligomycin and then bathed in the 35 mM KP with gluco se. When glucose was present, with or without oligomycin pretreatment, a gr eater percentage of twitch terminals accumulated FM1-43. However, the mitoc hondria in these preparations were still greatly swollen and distorted. We propose that prolonged depolarization of twitch motor neuron terminals by 3 5 mM KP with elevated Ca2+ produced a Ca2+-induced decrease in mitochondria l ATP production. Under these conditions, the cytosolic ATP/ADP ratio was d ecreased thereby compromising both transmitter release and refilling of rec ycled synaptic vesicles. The addition of glucose stimulated glycolysis whic h contributed to the maintenance of required ATP levels.