Transition from long-term depression to long-term potentiation as a function of stimulation frequency in the freely moving rat

The bioelectric phenomena of long-term potentiation (LTP) and long-term depression (LTD) are considered leading candidate mechanisms underlying the formation and storage of memory in the brain. To study the transition from LTD to LTP, we have recorded and analyzed evoked field potentials from the dentate gyrus using the population spike amplitude and the field EPSP slope measures following sustained stimulation (total of 900 pulses) of the lateral perforant path in both anesthetized and freely moving, behaving rats. Our results indicate that the strength and type (LTP or LTD) of synaptic plasticity within the dentate gyrus vary as a function of activation frequency: sustained low frequency stimulation (LFS) ranging from 1-30 Hz results in depression of activated synapses, while higher stimulation frequencies (i.e., 50-200 Hz) result in potentiation. Our findings are consistent with the neural network theory of the Hebbian synapse modification, as well as the Bienenstock-Cooper-Munro network learning rule