ACTIVITY OF PHASIC MOTOR-NEURONS PARTIALLY TRANSFORMS THE NEURONAL AND MUSCLE PHENOTYPE TO A TONIC-LIKE STATE

We present a model preparation, the crayfish, to investigate chronic s timulation effects in muscle fiber type and neuronal conversion from f ast to slow. The results show a presynaptic alteration in transmitter release after 1 week of stimulation at 5 Hz for a 2-h daily regime. Wi th the same stimulation paradigm, the muscle proteins displayed on a p olyacrylamide gel only start to show changes after 3 weeks. The origin al phasic motoneurons within 1 week display an enhanced ability to res ist synaptic depression, as do tonic motoneurons. The results show tha t identified phasic motoneurons and muscle fibers in the crayfish can be transformed to a toniclike state, and that the nerve terminals conv ert prior to the muscle fibers. Electrophysiological clinical measures indicating a change in transmitter release properties may not necessa rily mean that the muscle fibers have fully adapted for long-lasting e ffects. This preparation allows stimulation conditions to be examined with ease. (C) 1998 John Wiley & Sons, Inc.