Fimbria-fornix lesions compromise the induction of long-term potentiation at the Schaffer collateral-CA1 synapse in the rat in vivo. . Although bilateral fimbria-fornix (FF) lesioning impairs spatial performance in animals, the literature is equivocal regarding its effects on hippocampal long-term potentiation (LTP). We examined the effects of FF lesioning on LTP induction in the Schaffer collateral-CA1 pathway in vivo with a protocol that delivered theta burst stimulation (TBS) trains of

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... sing length until a sufficient length was reached to induce LTP of the monosynaptic field excitatory postsynaptic potential (fEPSP). Experiments were performed in urethan-anesthetized Long-Evans rats either 4 or 12-16 wk after lesioning. In sham-operated controls, TBS trains ranging from 4 to 12 bursts were sufficient to induce robust LTP [170 Ϯ 10% (mean Ϯ SF) of control fEPSP slope; n ϭ 8]. Four-week post -FF-lesioned animals also displayed clear LTP (167 Ϯ 12% of control fEPSP slope; n ϭ 4) that did not differ from the shams (P Ͼ 0.05). In contrast, animals in the 12-to 16-wk post-lesion group showed a highly significant deficit in LTP induction (95 Ϯ 3% of control fEPSP slope; n ϭ 8; Յ28 burst TBS trains tested; P Ͻ 0.001 vs. sham-and 4-wk post-FF-lesion groups). Other quantitative measures of synaptic excitability (i.e., baseline fEPSP slope and input-output relation) did not differ between the sham-and the 12-to 16-wk post-FF-lesion groups. These results indicate that the FF lesion leads to an enduring defect in hippocampal long-term synaptic plasticity that may relate mechanistically to the cognitive deficits characterized in this model.