ALTERED IMPULSE ACTIVITY MODIFIES SYNAPTIC PHYSIOLOGY AND MITOCHONDRIA IN CRAYFISH PHASIC MOTOR-NEURONS

1. Crayfish phasic motor synapses produce large initial excitatory pos tsynaptic potentials (EPSPs) that fatigue rapidly during high-frequenc y stimulation. Periodic in vivo stimulation of an identified phasic ab dominal extensor motor neuron (axon 3) induced long-term adaptation (L TA) of neuromuscular transmission: initial EPSP amplitude became small er and synaptic depression was significantly reduced. We tested the hy pothesis that activity-induced synaptic fatigue-resistance seen during LTA was dependent upon, or correlated with, mitochondrial oxidative c ompetence. 2. Periodic unilateral conditioning stimulation of axon 3 e ntering each of two adjacent homologous abdominal segments (segments 2 and 3) increased the synaptic stamina in both ''conditioned'' axons; mean final EPSP amplitudes, recorded after 20 min of 5-Hz test stimula tion, were significantly larger than those measured with the same prot ocol from contralateral unstimulated axons. 3. During 5-Hz test stimul ation of the conditioned axon 3 of segment 3, acute superfusion with 0 .8 mM dinitrophenol or 20 mM sodium azide [inhibitors of oxidative ade nosinetriphosphate (ATP) synthesis] produced increased synaptic depres sion. Drug-free saline superfusion of the conditioned axon 3 of segmen t 2 in these same animals did not affect the increased synaptic fatigu e resistance seen in this segment. Thus both successful induction (in axon 3 of saline-perfused segment 2) and attenuation (in axon 3 of dru g-perfused segment 3) of the increased synaptic stamina can be demonst rated with this twin-segment conditioning protocol. 4. Confocal micros copic imaging of mitochondrial rhodamine-123 (Rh123) fluorescence was used to assess relative oxidative competence of conditioned and uncond itioned phasic axons. Conditioned phasic axons showed significantly hi gher mean mitochondrial Rh123 fluorescence than contralateral unstimul ated axons. In the same preparations that showed increased postconditi oning Rh123 fluorescence, the synaptic fatigue resistance measured fro m conditioned axon 3 was also significantly greater than that recorded from contralateral unstimulated axon 3. 5. Axotomy of the phasic exte nsor nerve root (containing axon 3), before in vivo conditioning stimu lation of its decentralized segment, prevented induction of both the i ncreased synaptic stamina in axon 3 and the enhanced mitochondrial flu orescence in decentralized motor axons of the nerve root. Hence, induc tion of both changes requires axonal transport of materials between th e soma and the motor synapses of axon 3. 6. Because mitochondrial Rh12 3 fluorescence is primarily dependent upon the oxidative activity of t hese organelles, our findings suggest that conditioning stimulation of phasic extensor axon 3 increases its mitochondrial oxidative competen ce and that the enhanced synaptic stamina seen during LTA in axon 3 is correlated with, and dependent upon, oxidative activity. Standard ele ctrophysiological techniques and confocal imaging of mitochondrial Rh1 23 fluorescence can be used to correlate metabolic competence with spe cific aspects of synaptic plasticity in living, identifiable neurons i n situ.