LONG-TERM POTENTIATION IS ASSOCIATED WITH NEW EXCITATORY SPINE SYNAPSES ON RAT DENTATE GRANULE CELLS

To investigate possible morphological correlates to long-term potentia tion (LTP), three-dimensional reconstruction of serial electron microg raphs was employed. LTP was induced in the perforant path/dentate gran ule cell synapse in two rats. The surgically isolated contralateral si de served as control, along with two untreated animals. Longitudinally sectioned and transversally sectioned dendrites were sampled from the middle fifth of the molecular layer and all visibly connected spines were identified. A mixed, unbalanced, nested variance. component model was used to make a valid statistical comparison between the LTP and c ontrol groups. The spine density was higher in the experimental than i n the control groups. The changes were statistically significant in bo th the longitudinal and transverse sample. In addition, spines with a divided stem and two heads (bifurcating spines) were seen at a higher frequency in the LTP material compared with the contralateral material . From a subset of dendrites all connected spines were reconstructed a nd detailed measurements of head, neck, and PSD dimensions were made. We failed to find significant differences following LTP on either of t he dimensions measured. The results suggest that new spine synapses ar e formed following LTP, including some of the bifurcating type.