Morphological Correlates Of Synaptic Plasticity After Long Term Potentiation In The Rat Hippocampus

Changes in synapse and neuronal morphology have been reported in the rat hippocampal formation after the induction of long-term potentiation (LTP) of the perforant path, although few studies have investigated such parameters in the maintenance phase of L-LTP. Moreover, the results of investigations of synaptic and neuronal morphometry changes after LTP have varied and this could be due to the methods of analysis employed, the choice of stimulation protocol and or whether an in vitro or in vivo study. This in vivo investigation applied unbiased stereological methods to examine the morphology and morphometry of perforant path-granule cell synapses, in the dentate gyrus, after the induction of LTP. Two controls were employed, the contralateral hemisphere of each animal and the inner molecular layer, where the medial perforant path has little synaptic input. Many previous studies of the first 6Omin post tetanisation have used high frequency stimulation (HFS) to induce LTP however, in this study – to determine whether changes in morphology were due to LTP per se - potentiation was induced by theta burst stimulation (TBS). 45min after the induction of LTP there were no significant differences, between hemispheres, in the mean numerical density (Nv) of axodendritic or axospinous asymmetric synapses, or the mean number of synapses per neuron in the middle molecular layer (MML) of the dentate gyrus. There were no significant differences, between potentiated and non-potentiated tissue, in the Nvs of those asymmetric synapses with perforated or concave profiles. Neither were significant differences following LTP demonstrated in the size of the postsynaptic densities of these synaptic subtypes or the volume density of apposition zone (AZ) area (Sv) of individual, or all, asymmetric axospinous synapses. However, there was a trend towards larger perforated synapses in the potentiated hemisphere and, in both hemispheres, concave and perforated synapses were larger than average. In the inner molecular layer (IML), there were no differences except for a significant decrease in the total AZ volume density in the potentiated hemisphere. This would suggest that any morphological modifications taking place in the induction phase of L-LTP may be restricted to a fraction of synapses in the MML, although perforated synapses appear to be involved. The second part of this study examined morphological correlates 24h after the induction of LTP with TBS and HFS. In the MML after induction of LTP with TBS there were significant increases in the Nv of asymmetric axodendritic synapses and the mean number of axodendritic synapses per neuron. There was an increase in the Nv of axospinous synapses and in the mean number of axospinous synapses per neuron that was not significant. This was reflected in significant increases in the total AZ Sv and in the frequency of macular synapses in the potentiated hemisphere. 24h post tetanisation with HFS, there was a significant difference in the Nv of axospinous synapses in the MML of the potentiated compared to the contralateral hemisphere. There were also significant differences in the frequency of synapses with perforated and concave profiles. There were no significant differences in synaptic morphometric parameters, between hemispheres, in the IML after either of the stimulating regimes. Results from the three animals in each group showing the greatest degree of potentiation, were pooled and demonstrated significant differences in the Nv and mean number of axospinous synapses per neuron. There was also a significant difference in the number of synapses with concave profiles but this was replicated in the IML. The effects of these morphological changes, after LTP induction, on the cellular mechanisms involved and on synaptic efficacy are discussed, and possible reasons for the variable pattern of morphology after different stimulating protocols is considered.