FREQUENCY-DEPENDENCE OF LONG-TERM POTENTIATION AND DEPRESSION IN THE DENTATE GYRUS OF THE FREELY MOVING RAT

Long-term potentiation (LTP) or long-term depression (LTD) can be prod uced in the dentate gyrus (DG) of the hippocampus with high- or low-fr equency stimulation trains, respectively. Although LTP can be elicited in a variety of preparations, we know of no reports of LTD unaccompan ied by seizure activity in the awake rat. In this experiment, test pul ses at alternating high (95% of maximum response) and moderate (50-75% of maximum) intensities were presented at 0.05 Hz to the perforant pa th of freely moving rats in order to assess changes in DG population s pike amplitude. Trains were delivered at 10-min intervals, and intrafr ain frequency was adjusted either upward from 3 Hz or downward from 40 0 Hz until all subjects had received three consecutive tetani at each of 3, 6, 12.5, 25, 50, 100, 200, and 400 Hz. Potentiation was observed at high frequencies regardless of whether ascending (ASC) or descendi ng (DES) test order was used. Depression occurred at low frequencies, but only in ASC rats. The LTD observed in this preparation was not ver y robust and was clearly seen only when moderate-intensity test pulses were used. The threshold frequency (at which depression gives way to potentiation) was approximately 6-9 Hz for DES rats but was 100-120 Hz for ASC animals. Prior stimulation therefore affected the response to subsequent trains. These results are generally consistent with the hy pothesis of a variable threshold for LTP induction. Our findings can a lso be explained by postulating a wide ''labile range'' at moderate fr equencies within which no plastic changes occur. (C) 1996 Wiley-Liss, Inc.