The degree of potentiation is associated with synaptic number during the maintenance of long-term potentiation in the rat dentate gyrus

There is a considerable degree of variation in the amount of potentiation induced in different animals following the induction of long-term potentiation (LTP). This variation provided us with the opportunity to determine what types of synaptic changes were dependent upon the degree of induced potentiation. To examine possible 'degree of potentiation' effects on synapses, we conducted a multiple regression analysis examining the relationship between the degree of potentiation in LTP animals and a series of synaptic structural measures. We examined synapses in the middle third of the molecular layer (MML) of the rat dentate gyrus following repeated high frequency tetanization of the perforant path. LTP was induced over a 4 h period, and the animals were sacrificed 24 h after the final stimulation. Synapses from the ipsilateral inner third of the dentate molecular layer (IML) and from implanted only animals were also examined for comparison. Ultrastructural quantification included the total number of synapses per neuron, synaptic curvature, the presence of synaptic perforations, and the maximum length of the synaptic apposition. The only structural change that was significantly associated with the degree of potentiation was a positive correlation between the degree of LTP and the number of synapses per neuron. Therefore, synaptic number, while not appearing to be significantly associated with the induction of LTP, appears to be important for the degree of LTP expressed.