Increase in the relative proportion of perforated axospinous synapses following hippocampal kindling is specific for the synaptic field of stimulated axons.

A comparative analysis of axospinous synapses was performed in the middle (MML) and inner (IML) molecular layer of the hippocampal dentate gyrus of rats kindled via medial perforant path stimulation and sacrificed 4 weeks after reaching a criterion of 5 generalized seizures. The MML was a directly stimulated structure, while the IML was not. Both are immediately adjacent synaptic fields likely to be equally susceptible to any generalized effects of convulsions and hypoxia. In these two subdivisions of the molecular layer, the so-called perforated and non-perforated synapses, distinguished respectively by a discontinuous or continuous postsynaptic density, were differentially quantified. In the MML, the ratio of perforated to non-perforated synapses was found to be markedly increased in kindled rats relative to controls. In the IML, however, no change in this ratio was detected following kindling. Thus, the shift in the relative preponderance of perforated synapses over non-perforated ones is not a consequence of generalized phenomena accompanying the kindling process.