Two types of synaptic depression at synapses of a single crustacean motor axon

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Abstract
Depression of excitatory junctional potentials (EJP's) set up by the fast motor axon supplying the closer muscle in the walking legs of the shore crab, Pachygrapsus crassipes, was studied in different muscle fibers penetrated with intracellular electrodes. In some muscle fibers, a “low frequency” depression was seen: EJP's showed a pronounced depression at 0.5 to 1 Hz stimulation. In other muscle fibers, no depression was detected at 1 Hz, but pronounced depression appeared at 5 Hz (” high frequency depression"). Application of manganese, which reduced EJP amplitude, did not alter the time course or extent of low frequency depression, but reduced high frequency depression. Electron microscopy of individual muscle fibers showing high frequency depression or low frequency depression was achieved by isolating fibers marked by injection of dye through the recording microelectrode. After stimulation at 5 Hz to produce high frequency depression, a significant decrease in vesicles near synapses was observed. However, after stimulation at 1 Hz to produce low frequency depression, no demonstrable decrease in synaptic vesicles resulted. Evidence of membrane recycling was observed at synapses subjected to high frequency depression, but less evidence of recycling was seen at synapses subjected to low frequency depression. It was concluded that depletion of synaptic vesicles plays an important part in high frequency depression, but is probably of little importance in low frequency depression. The depressible synapses of the fast motor axon are physiologically adapted for brief periods of intense activity, and are probably used during the initial phase of escape responses.