Pretraining tetanic fimbrial stimulation impairs the expression but not the acquisition of contextual fear conditioning in mice

We recently reported that the pretraining induction of long-term potentiati on in the lateral septum by fimbrial tetanic stimulation altered contextual fear conditioning in mice. The aim of the present study was to examine at which stage of fear conditioning (i.e. either acquisition or expression) th is impairment takes place. Mice implanted with stimulating electrodes in th e fimbria and recording electrodes in the lateral septal were conditioned t o acquire fear towards a novel context using a footshock procedure. Twenty- four hours after conditioning, animals were re-exposed to the conditioning environment and the level of freezing behavior served as the measure of con ditioned fear. The level of fimbrial-lateral septal synaptic neurotransmiss ion was manipulated using either fimbrial tetanic stimulation (which induce d septal long-term potentiation) alone, or followed by fimbrial low-frequen cy stimulation producing depotentiation of the previously established long- term potentiation. The results showed that (i) septal long-term potentiatio n induced either prior to acquisition or only prior to retention testing im paired conditioned freezing; and (ii) the impairing effect of pretraining i nduction of long-term potentiation on conditioned freezing was not only abo lished by fimbrial low-frequency stimulation administered prior to retentio n testing but actually produced enhanced conditioned freezing with respect to controls. These data suggest that the level of fimbrial-lateral septal synaptic neuro transmission may influence the expression, but not the acquisition, of cont extual fear conditioning. (C) 1999 IBRO. Published by Elsevier Science Ltd.