Sequential changes in the synaptic structural profile following long-term potentiation in the rat dentate gyrus: I. The intermediate maintenance phase

Changes in synaptic structure have been reported following the induction of long-term potentiation (LTP). The structure of synapses during the interme diate maintenance of LTP has yet to be fully characterized in chronically i mplanted freely moving animals. The present study examined synapses in the middle third of the molecular layer (MML) of the rat dentate gyrus followin g repeated high frequency tetanization of the perforant path. Synapses from both 1) the ipsilateral inner third of the dentate molecular layer (IML), which was not directly stimulated during the induction of LTP, as well as 2 ) implanted, nonstimulated animals, served as controls. LTP was induced ove r a 4-h period, and the animals were sacrificed 24 h after the final stimul ation of the LTP group. Ultrastructural quantification included the total n umber of synapses, synaptic curvature, the presence of synaptic perforation s, and the maximum length of the synaptic contact. Although LTP was not ass ociated with an overall increase in synaptic number, there was a significan t increase in the proportion of presynaptically concave-shaped synapses. Fu rther, the concave synapses in the LTP tissue were found to be significantl y smaller than control concave synapses. There was also a significant incre ase in the number of perforated concave synapses which exceeded the overall increase in concave synapses, and occurred despite the lack of a general i ncrease in perforated synapses. It was concluded that this specific structu ral profile, observed at 24 h postinduction, may help support the potentiat ed response observed at this stage of LTP maintenance. Synapse 31:97-107, 1 999. (C) 1999 Wiley-Liss, Inc.