Previously we showed that long-term depression (a lasting diminishment in synaptic strength) can be reliably induced in the hippocampal dentate gyrus of the adult rat in vivo. In the present study, we designed experiments to further characterize the frequency-dependence of synaptic plasticity at two stages of development: preweaning stage and adulthood. To this end, we examined frequency-dependent transition from homosynaptic longterm depression (LTD) to long-term potentiation (LTP) in the lateral perforant pathway/dentate gyrus synapse in the adult (90 days of age) and preweaning (15 days of age) freely moving rat. Dentate evoked field potentials were recorded and analyzed using the population spike amplitude (PSA) measure following sustained stimulation (900 pulses) of the lateral perforant pathway at various frequencies. Our results indicate that both the strength and direction (LTP or LTD) of synaptic plasticity in the lateral perforant path/dentate gyrus synapse vary as a function of frequency. In addition, a significant development-related shift in the transition frequency was observed in younger animals compared to adult animals. These findings provide evidence for the presence of a synapse modification threshold mechanism within the hippocampal formation that may play a role in learning and memory.
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