Paradoxical facilitatory effect of fornix lesions on acquisition of contextual fear conditioning in mice

The present study examined the effect of fornix lesions on freezing behavio r elicited by contextual and phasic conditioned stimuli. Male mice of the C 57Bl/6 strain received electrolytic lesions of the fornix. Ten days followi ng the lesion, they were submitted to acquisition of one-trial classical fe ar conditioning involving the pairing of an auditory conditioned stimulus ( CS) with a footshock unconditioned stimuli (US). Analysis of conditioned fe ar responses showed that fornix lesions enhanced the freezing response elic ited by exposure to the conditioning chamber 24 h after a single tone-shock pairing. In contrast, the two groups did not differ on their fear response s during the auditory cue test. Analysis of the time-course of freezing beh avior during re-exposure to the conditioning chamber suggests, however, tha t the observed fornix lesion-induced facilitation of freezing to the condit ioning chamber is more likely due to a facilitation of the processing of a simple (unimodal) rather than polymodal (contextual) CS-US association. (C) 2000 Elsevier Science B.V. All rights reserved.