We tested the hypothesis that homosynaptic long-term depression (LTD) can be induced at the CA3-CA3 synapses in the adult, in vivo hippocampus while the CA3-CA1 synapses remain unchanged. Low-frequency conditioning stimulation of the contralateral fimbria significantly depressed the CA3 population response but did not change the simultaneously recorded CA3 response to angular bundle test stimulation. Similarly, in another group of animals, low-frequency conditioning stimulation of the contralateral fimbria depressed the CA3 synaptic response and left the collateral CA1 synaptic response unchanged. Among the possible explanations for this differential induction of homosynaptic LTD at the CA3-CA3 and CA3-CA1 synapses are differential control of intracellular calcium, differing levels of inhibition in these two regions, and the recency of 'natural' long-term potentiation in the two regions.
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