Number of synaptic vesicles in rat cortex immediately after cessation of the self-sustained afterdischarge during kindling.
The sensorimotor area of rat cerebral cortex was subjected to repetitive electrical stimulation at 10-min intervals; this led to progressive lengthening of self-sustained afterdischarges (SSADs). At 50-60 s after cessation of the third SSAD the brains of the animals were perfused transaortally with fixing fluid, and a quantitative electron microscopic analysis of type I synapses in the second cortical layer of the sensorimotor area of the contralateral hemisphere was carried out. In a zone 0.1 micron wide adjacent to the presynaptic membrane in the active zone, the number of synaptic vesicles (converted to the number per 0.01 micron2 area) in the experimental animals fell by 64.6%. In a zone of the same width 0.1 micron distant from the presynaptic membrane, it fell by 67.6%. The mean absolute synaptic vesicle count per presynaptic bag section fell by 66.6%. The number of synapses showing signs of exocytosis rose in the experimental animals from 3.8 to 5.9%, while the number with clearly demonstrable exocytosis rose from 0.7 to 11.4%. The proportion of synapses with signs of (clearly demonstrable) endocytosis rose from 2.8% (2.1%) in the controls to 7.1% (3.7%) in the experimental rats. In the experimental and control animals we demonstrated complex vesicle formation in 4.3 and 3.8%, respectively, of the synapses and dense-cored vesicles in 7.7 and 2.8%, respectively, of the synapses. We consider changes in the number and formation of synaptic vesicles to be signs of exhaustion of the synapses caused by the previous epileptic seizure, which simultaneously activates mechanisms of vesicle formation.