LOW-FREQUENCY TRAINS OF PAIRED STIMULI INDUCE LONG-TERM DEPRESSION INAREA CA1 BUT NOT IN DENTATE GYRUS OF THE INTACT RAT

We have examined the efficacy of a recently introduced protocol for in ducing homosynaptic long-term depression (LTD) in area CA1 of the anes thetized rat (Thiels et al. [1994] J Neurophysiol 72:3009-3116.). In a rea CA1 of the awake animal, this protocol, consisting of 200 pairs of pulses delivered at 0.5 Hz, with an interpulse interval of 25 ms, con sistently produced LTD, provided the initial pulse was sufficiently st rong to produce significant paired-pulse depression of the evoked resp onse. We extended these experiments to the dentate gyrus, using either paired pulses given to the perforant path in the awake adult rat, or, in the anesthetized adult, a two-pathway pairing procedure, in which the first pulse was delivered to the commissural input to the dentate gyrus and the second to the perforant path. In both cases, the first p ulse led to substantial suppression of the response evoked by the seco nd pulse. With neither protocol, however, was there any evidence for L TD or depotentiation. Paired-pulse stimulation of the perforant path o f young rats (10-11 days) also failed to induce LTD or depotentiation of the population excitatory postsynaptic potential (EPSP). Thus, the dentate gyrus in the intact animal appears to be less susceptible to L TD and depotentiation than area CA1, a conclusion consistent with prev ious experiments in which we found that stimulation at 1-5 Hz produced LTD/depotentiation in area CA1 of young (but not adult) rats in vivo but was ineffective at any age in the dentate gyrus. Our results do no t rule out the possibility that other, untested protocols may produce homosynaptic LTD and/or depotentiation in the dentate gyrus in vivo. ( C) 1996 Wiley-Liss, Inc.