The Journal of Neurophysiology Vol. 81 No. 2 February 1999, pp. 498-506
Copyright ©1999 by the American Physiological Society

Role of Mitochondrial Dysfunction in the Ca²⁺-Induced Decline of Transmitter Release at K⁺-Depolarized Motor Neuron Terminals

 ¹Department of Anatomy and Neurobiology, College of Medicine, University of Vermont, Burlington, Vermont 05405; and  ²Department of Biology, Ithaca College, Ithaca, New York 14850

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 (MEPC) frequency that occurs with nerve-muscle preparations maintained in a 35 mM potassium propionate (35 mM KP) solution containing elevated calcium. When the 35 mM KP contained control Ca²⁺ (1 mM), the MEPC frequency increased 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 s¹ but then gradually fell approaching frequencies <50 s¹. A progressive swelling and eventual distortion of mitochondria within the twitch motor neuron terminals occurred during prolonged exposure to 35 mM KP with elevated Ca²⁺. After ~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 elevated Ca²⁺ was less when 15 mM glucose was present or when preparations were pretreated with 10 µM oligomycin and then bathed in the 35 mM KP with glucose. When glucose was present, with or without oligomycin pretreatment, a greater percentage of twitch terminals accumulated FM1-43. However, the mitochondria in these preparations were still greatly swollen and distorted. We propose that prolonged depolarization of twitch motor neuron terminals by 35 mM KP with elevated Ca²⁺ produced a Ca²⁺-induced decrease in mitochondrial ATP production. Under these conditions, the cytosolic ATP/ADP ratio was decreased thereby compromising both transmitter release and refilling of recycled synaptic vesicles. The addition of glucose stimulated glycolysis which contributed to the maintenance of required ATP levels.

0022-3077/99 $5.00 Copyright © 1999 The American Physiological Society