A quantitative analysis of ultrastructural changes induced by electrical stimulation of identified spinal cord axons in the larval lamprey.

Microelectrodes filled with horseradish peroxidase (HRP) were used to label single identified giant axons in the isolated lamprey spinal cord. Subsequent to the iontophoretic injection of HRP, the spinal cord was stimulated at repetition rates of 20-30/s and the activity in labelled axons monitored. Immediately following failure of the action potential, the spinal cord was fixed by immersion and processed for light and electron microscopy. Electron micrographs were taken of synaptic contacts made by the labelled axons. Several quantitative measures were made from each synapse using a digitizing tablet interfaced with a digital computer. These measures included vesicle number (VN), vesicle area (VA), length of differentiated membrane (DM), vesicle density (VD = VN/VA), vesicle frequency (VF = VN/DM), and a relative measure of the amount of vesicle membrane added to the axolemma during the stimulation period, the curvature ratio (CR). Measures from 106 stimulated synapses were compared with 134 synapses from injected but unstimulated giant axons. The results from these experiments suggest that measurable ultrastructural changes occur during transmitter release at identified C.N.S. synapses, which are consistent with the hypothesis of synaptic vesicle recycling.