Ultrastructural correlates of vesicular docking in the rat dentate gyrus

To determine the extent to which CA1 synapses are typical of those found in other regions of the hippocampal formation, we have carried out a quantitative analysis of synapses in the middle molecular layer of the rat dentate gyrus, reconstructed from serial electron microscopy, and have compared these data with previous observations from CA1. In general, the morphology of synapses in areas CA1 and the dentate agree, other than an increased density of multisynaptic boutons. Thus, it seems that either area may form an equally effective model for the function of individual synapses in the hippocampal formation. In addition, the current study examines presynaptic curvature, which recent mathematical models have suggested may have profound effects on synaptic transmission. When synapses of distinct curvature profiles (i.e., presynaptically concave, convex, and flat) are examined, the average characteristics of these three synapse populations are distinct. In general, concave synapses have a greater number of morphologically docked vesicles, and thus, likely a greater probability of release. This, however, seems to be accounted for by the fact that these synapses are larger--the spatial density of docked vesicles remains identical across these curvature profiles. This study provides crucial data for further modeling of individual synapse function.