Seizure-sensitive (SS) and seizure-resistant (SR) Mongolian gerbils were used for three experiments. In the first experiment, GABAergic neurons and terminals in the dentate gyrus were localized with GAD immunocytochemistry. GAD-positive puncta adjacent to cell bodies of GABAergic pyramidal basket cells were counted in light microscopic preparations. The pyramidal basket cells of SS gerbils displayed a significant threefold increase in the number of GAD-positive puncta associated with their cell bodies as compared to those from SR gerbils. These data indicate that the number of GABAergic synapses with pyramidal basket cell bodies in the dentate gyrus was greater in SS gerbils.
An electron microscopic (EM) analysis of GAD immunocytochemical preparations showed GAD-positive axon terminals forming symmetric synapses with GAD-positive basket cell bodies. However, numerous terminals forming symmetric axosomatic synapses with basket cells were not immunopositive, and other synapses formed by terminals were not classified becuase reaction product in the cell bodies obscured postsynaptic densities. Therefore, routine EM preparations were analyzed for symmetric and asymmetric axosomatic synapses on pyramidal basket cells and granule cells of SS and SR gerbils. The data obtained from these preparations showed that the pyramidal basket cells of SS gerbils had a selective increase in the number of symmetric synapses per 10 μm of soma as compared to those of the SR gerbils. In contrast, the granule cells did not show any significant difference in the number of either symmetric or asymmetric axosomatic synapses between SS and SR gerbils. These results indicate that pyramidal basket cell bodies of SS gerbils have more inhibitory synapses than do those of SR gerbils.
The third experiment used SS gerbils with lesions of the perforant pathway that stopped seizure activity (Ribak, C. E., and S. U. Khan (1987) The effects of knife cuts of hippocampal pathways on epileptic activity in the seizure-sensitive gerbil. Brain Res. 418:251–260). The percentage of axon terminal area occupied by synaptic vesicles and their packing density was determined in CA3 mossy fiber boutons and compared for lesioned and nonlesioned SS gerbils. The mossy fibers of nonlesioned SS gerbils showed a depletion of synaptic vesicles consistent with the previous results of Peterson et al. (Peterson, G. M., C. E. Ribak, and W. H. Oertel (1985) A regional increase in the number of hippocampal GABAergic neurons and terminals in the seizure-sensitive gerbil. Brain Res. 340:384–389). In contrast, lesioned SS gerbils showed mossy fiber boutons with a similar morphology to those of SR gerbils (i.e., lesioned SS gerbils had a significantly higher percentage of terminal area occupied by synaptic vesicles and a significantly greater packing density of vesicles than nonlesioned SS gerbils). It is interesting that a depletion of synaptic vesicles in granule cell axons of nonlesioned SS gerbils was found not only in the mossy fiber giant boutons, but also in the smaller terminals of their axon collaterals in the hilus of the dentate gyrus. The lesion study results suggest that intact perforant pathway fibers are necessary for the depletion of synaptic vesicles in mossy fibers of SS gerbils. Together, these data provide further support for differences in the synaptic organization of the dentate gyrus of SS gerbils and for the concept of disinhibition.