Altered impulse activity modifies synaptic physiology and mitochondria in crayfish phasic motor neurons.

1. Crayfish phasic motor synapses produce large initial excitatory postsynaptic potentials (EPSPs) that fatigue rapidly during high-frequency stimulation. Periodic in vivo stimulation of an identified phasic abdominal extensor motor neuron (axon 3) induced long-term adaptation (LTA) of neuromuscular transmission: initial EPSP amplitude became smaller and synaptic depression was significantly reduced. We tested the hypothesis that activity-induced synaptic fatigue-resistance seen during LTA was dependent upon, or correlated with, mitochondrial oxidative competence. 2. Periodic unilateral conditioning stimulation of axon 3 entering 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 stimulation, were significantly larger than those measured with the same protocol from contralateral unstimulated axons. 3. During 5-Hz test stimulation of the conditioned axon 3 of segment 3, acute superfusion with 0.8 mM dinitrophenol or 20 mM sodium azide [inhibitors of oxidative adenosinetriphosphate (ATP) synthesis] produced increased synaptic depression. Drug-free saline superfusion of the conditioned axon 3 of segment 2 in these same animals did not affect the increased synaptic fatigue resistance seen in this segment. Thus both successful induction (in axon 3 of saline-perfused segment 2) and attenuation (in axon 3 of drug-perfused segment 3) of the increased synaptic stamina can be demonstrated with this twin-segment conditioning protocol. 4. Confocal microscopic imaging of mitochondrial rhodamine-123 (Rh123) fluorescence was used to assess relative oxidative competence of conditioned and unconditioned phasic axons. Conditioned phasic axons showed significantly higher mean mitochondrial Rh123 fluorescence than contralateral unstimulated axons. In the same preparations that showed increased postconditioning Rh123 fluorescence, the synaptic fatigue resistance measured from conditioned axon 3 was also significantly greater than that recorded from contralateral unstimulated axon 3. 5. Axotomy of the phasic extensor nerve root (containing axon 3), before in vivo conditioning stimulation of its decentralized segment, prevented induction of both the increased synaptic stamina in axon 3 and the enhanced mitochondrial fluorescence in decentralized motor axons of the nerve root. Hence, induction of both changes requires axonal transport of materials between the soma and the motor synapses of axon 3. 5. Axotomy of the phasic extensor nerve root (containing axon 3), before in vivo conditioning stimulation of its decentralized segment, Prevented induction of both the increased synaptic stamina in axon 3 and the enhanced mitochondrial fluorescence in decentralized motor axons of the nerve root Hence, induction of both changes requires axonal transport of materials between the soma and the motor synapses of axon 3 6. Because mitochondrial Rh123 fluorescence is primarily dependent upon the oxidative activity of these organelles, our findings suggest that conditioning stimulation of phasic extensor axon 3 increases its mitochondrial oxidative competence and that the enhanced synaptic stamina seen during LTA in axon 3 is correlated with, and dependent upon, oxidative activity.(ABSTRACT TRUNCATED AT 400 WORDS)