Long-term maintenance of presynaptic function in the absence of target muscle fibers.

Here we have investigated the role of the muscle fiber in the maintenance of presynaptic function at frog motor nerve terminals. Muscle fibers were selectively damaged and prevented from regenerating while leaving the motor innervation intact. Presynaptic activity of the resulting target-deprived nerve terminals was assayed using the fluorescent dye, FM1-43. FM1-43 stains nerve terminals in an activity-dependent fashion in that it incorporates into synaptic vesicles and can be released upon stimulation. We demonstrate that nerve terminals permanently deprived of target for 1 to 5 months maintain the ability to release and recycle synaptic vesicles in response to stimulation. The level of presynaptic function of target-deprived nerve terminals was not detectably different from that of nerve terminals in contact with muscle fibers. The remarkable maintenance of presynaptic function in these target-deprived nerve terminals indicates that the molecular machinery required for vesicular release is maintained in a functional state for long periods without target muscle fibers.