Structural mechanisms of Long-term potentiation in the dentate gyrus of freely moving rats

Long-lasting enhancement of synaptic transmission, known as long-term potentiation (LTP), is involved in learning and memory and is associated with the function of tiny dendritic protrusions called spines. Importantly, our understanding of how these structures change to allow for the expression of LTP is incomplete. Previous studies conducted in the dentate gyrus (DG) have demonstrated that LTP is associated with increases in spine volume and synaptic area; however, controversy remains as to whether these changes are accompanied by alterations in the total number of spines on the dendritic shaft. Here, we have used in vivo electrophysiology, serial section electron microscopy, and a novel method of unbiased sampling to investigate how spines, synapses, and cellular organelles function to allow for the expression of LTP in the dentate gyrus of freely moving rats. We hypothesize that LTP would not be associated with significant changes in the density of spines along the dendritic shaft, but rather, expressed through increases in the volume and synaptic area of existing spines. Research has shown that an organelle known as the spine apparatus is involved with enlarging synapses. We hypothesized that spines containing a spine apparatus (SA) would show the largest increases in volume and synaptic area. LTP was induced by applying deltaburst stimulation to the medial perforant path projection to the middle molecular layer (MML) of the dentate gyrus of 2 adult, male rats. Thirty-minutes after the induction, the MML was processed for electron microscopy (EM) and the images were uploaded into Reconstruct, a software tool developed for the three-dimensional analysis of neuronal structures. Our results demonstrated that LTP was not associated with significant changes in spine density. Subsequent analysis revealed that average spine volume increased significantly 30 minutes following LTP. Marginal increases were also observed in average synapse area, however, this change did not reach significance. Overall, we found that the largest increases in spine volume and synaptic area were found on spines containing a SA. These effects were highly significant, and emphasize the importance of the spine apparatus in cellular models of learning and memory.