Low-frequency trains of paired stimuli induce long-term depression in area CA1 but not in dentate gyrus of the intact rat.

We have examined the efficacy of a recently introduced protocol for inducing homosynaptic long-term depression (LTD) in area CA1 of the anesthetized rat (Thiels et al. [1994] J Neurophysiol 72:3009-3116.). In area 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, consistently produced LTD, provided the initial pulse was sufficiently strong to produce significant paired-pulse depression of the evoked response. 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 pulse led to substantial suppression of the response evoked by the second pulse. With neither protocol, however, was there any evidence for LTD or depotentiation. Paired-pulse stimulation of the perforant path of 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 LTD and depotentiation than area CA1, a conclusion consistent with previous 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 not rule out the possibility that other, untested protocols may produce homosynaptic LTD and/or depotentiation in the dentate gyrus in vivo.