Changes in the number, size, and shape of synaptic vesicles in an experimental, projected cortical epileptic focus in the rat.

A projected epileptic focus was induced in the brains of experimental rats by applying penicillin G sodium to the contralateral hemisphere. Synapses of type I (after Gray) in the second cortical layer of the mirror area were analyzed morphometrically in the electron microscope 30 min after application of the epileptogenic drug. The number of agranular synaptic vesicles was counted per 0.01 micron2 over the active zone of the presynaptic membrane in an area 0.1 micron wide adjacent to the active zone and in an area of the same width 0.1 micron distant from the active zone. A statistically significant increase in the number of synaptic vesicles was found in the experimental animals as compared with the controls. The number of synaptic vesicles per 0.01 micron2 area was evaluated along the inactive presynaptic membrane, both in the area adjacent to the active zone of the presynaptic membrane and in other areas at successively greater distances from the active zone. The counts in some of these latter areas were statistically significantly higher in the experimental animals. The size and shape of synaptic vesicles were evaluated in three zones, each 0.1 micron wide, parallel to the membrane of the synaptic cleft. Zone I was directly adjacent to the membrane of the synaptic cleft; zone II was 0.1 micron distant; and zone III 0.1 micron distant from the membrane. The results show that the synaptic vesicles of both the experimental and control animals are significantly smaller in zone I than in the more distant zones. This trend is more marked in the controls than in the experimental animals. Histograms of vesical areas of zones II and III in the controls point to two populations of vesicles. The volumes of vesicles are approximately 10% greater in zone I of experimental animals than in the controls. The elongation of the vesicles in the three zones does not differ statistically either in controls or in experimental animals. Experimental animals, however, have statistically more rounded vesicles than the controls. Assuming that the synpatic vesicles contain the transmitters, our results might contribute to a better understanding of the increased excitability of the epileptic focus area.