SYNAPTIC PHYSIOLOGY AND MITOCHONDRIAL-FUNCTION IN CRAYFISH TONIC AND PHASIC MOTOR-NEURONS

Phasic and tonic motor neurons of crustaceans differ strikingly in the ir junctional synaptic physiology. Tonic neurons generally produce sma ll excitatory postsynaptic potentials (EPSPs) that facilitate strongly as stimulation frequency is increased, and normally show no synaptic depression. In contrast, phasic neurons produce relatively large EPSPs with weak frequency facilitation and pronounced depression. We addres sed the hypothesis that mitochondrial function is an important determi nant of the features of synaptic transmission in these neurons. Mitoch ondrial fluorescence was measured with confocal microscopy in phasic a nd tonic axons and terminals of abdominal and leg muscles after exposu re to supravital mitochondrial fluorochromes, rhodamine-123 (Rh123) an d 4-diethylaminostyryl-N-methylpyridinium iodide (4-Di-2-Asp). Mitocho ndria of tonic axons and neuromuscular junctions had significantly hig her mean Rh123 and 4-Di-2-Asp fluorescence than in phasic neurons, ind icating more accumulation of the fluorochromes. Mitochondrial membrane potential, which is responsible for Rh123 uptake and is related to mi tochondrial oxidative activity (the production of ATP by oxidation of metabolic substrates), is likely higher in tonic axons. Electron micro scopy showed that tonic axons contain approximately fivefold more mito chondria per mu m(2) cross-sectional area than phasic axons. Neuromusc ular junctions of tonic axons also have a much higher mitochondrial co ntent than those of phasic axons. We tested the hypothesis that synapt ic fatigue resistance is dependent on mitochondrial function in crayfi sh motor axons. Impairment of mitochondrial function by uncouplers of oxidative phosphorylation, dinitrophenol or carbonyl cyanide m-chlorop henylhydrazone, or by the electron transport inhibitor sodium azide, l ed to marked synaptic depression of a tonic axon and accelerated depre ssion of a phasic axon during maintained stimulation. Iodoacetate, an inhibitor of glycolysis, and chloramphenicol, a mitochondrial protein synthesis inhibitor, had no significant effects on either mitochondria l fluorescence or synaptic depression in tonic or phasic axons. Collec tively, the results provide evidence that mitochondrial oxidative meta bolism is important for sustaining synaptic transmission during mainta ined stimulation of tonic and phasic motor neurons. Tonic neurons have a higher mitochondrial content and greater oxidative activity; these features are correlated with their greater resistance to synaptic depr ession. Conversely, phasic neurons have a lower mitochondrial content, less oxidative activity, and greater synaptic fatigability.