Characteristics of Synapses in Area CA1 of Developing Rat Hippocampus

Dendritic spines support the vast majority of excitatory synapses in normal adult brains, but are absent or malformed in a number of cognitive disorders. Previous findings show that in area CA1 of rat hippocampus, dendritic spines are absent during the first postnatal week; the majority of synapses instead occur directly along dendritic shafts or on immature processes called filopodia.  Postnatal day 12 (P12), the age at which dendritic spines first naturally appear, coincides with the age at which acute hippocampal slices from Long-Evans rats first express enduring long-term potentiation (LTP), a cellular model of learning and memory. LTP is present at P10 if two bursts of high frequency stimulation, called theta bursts are given.  Little is known about synaptic composition between the ages P6 and P12. This study sought to discover if dendritic spines would be apparent after giving the P8 and P10 animals two theta burst stimulations, and sought to more closely examine the time course over which mature appearing synapses form. Male Long-Evans rats aged P8 and P10 were fixed via transcardial perfusion, and prepared for serial section electron microscopy.  Series were prepared from the stratum radiatum region of the area CA1 of rat hippocampi, and imaged via scanning electron microscope in transmission mode, and uploaded into Reconstruct software.  Synapses were sampled using the unbiased brick method.  The origins of the synapses, PSD size, dendrite size, and synapse orientation were recorded.  In the P8 series, most (68%) of the synapse origins were found on the dendritic shafts, 18% on filopodia, 0% on spines, and 14% on surface specializations. However, the P10 series showed different results, most notably a drastic increase in the amount of spines present.  In the P10 series, 48% of synapse origins were found on dendritic shafts, 25% on filopodia, 17% on spines, and 10% on surface specializations.  The P10 animal has protrusions from all four types of protrusion origins (filopodia, surface specializations, shaft, and spine).  This is a major difference when compared to the P8 animal.  The P8 animal did not have any synapses originating from spines; however, the P10 animal had 17% of its synapses originating from spines.  This is a major increase and supports the idea that spines need to be present for LTP to occur. The P8 animal did not have any spines and could not also sustain LTP.  The P10 animal did have a substantial amount of spines and also could sustain LTP after having received two theta bursts.