Occlusal disharmony suppresses long-term potentiation in rat hippocampus

Objective: Hippocampal long-term potentiation (LTP) is a cellular mechanism underlying hippocampal memory. To evaluate how occlusal disharmony affects hippocampal learning and memory, we measured the hippocampal LTP of rats with occlusal disharmony and compared it to that of rats with no occlusal disharmony.

Methods: We used fourteen 10-week-old male Sprague-Dawley rats. We produced occlusal disharmony in half of them by attaching wires to molar teeth on both sides (occlusal disharmony group; OD). The surgical procedure for the remaining rats was identical to that of the OD rats except that we did not attach wires (sham-operated group; Sham). Following a 10-day post-surgical period during which rats had free access to water and food, we prepared hippocampal slices from the rats and recorded the field excitatory postsynaptic potential (fEPSP) in the CA1 region. We recorded stable baseline fEPSP activity every 30s for at least 10 min. LTP was induced using nine tetanic stimulation episodes (1s, 100 Hz) at 90s intervals. We measured the initial slope of each fEPSP and normalized it to the average value measured during the baseline period, and we used the average value of the slope of the fEPSPs recorded between 30 to 40 min after the end of the tetanic stimulation to statistically compare the LTP amplitudes of the two groups.

Results: The mean LTP amplitude for OD rats was 130±7% of baseline (n=7), a value that was significantly lower (p<0.001, Student t-test) than that for Sham rats (204±16% of baseline, n=7). This suppressed amplitude was associated with decrease in body weight of OD rats.

Conclusion: These results suggest that chronic occlusal disharmony is a stressor that reduces synaptic plasticity and impairs the learning and memory functions of the hippocampus. MEXT provided an Open Research Center subsidy (H18) to support this work.