Freezing (%) 0 20 40 60 80 vmPFC-shams vmPFC-lesioned *** * ** PB1 PB2 PB3 PB4 Freezing (%) 0 20 40 60 80 Naïve Conditioned *** *** *** PB1 PB2 PB3 PB4 LESIONS OF THE VENTRAL MEDIAL PREFRONTAL CORTEX ENHANCED CONTEXTUAL FEAR GENERALIZATION IN C57BL6/J MICE. Julien Courtin, Cyril Herry Inserm U862, Neurocentre Magendie, Bordeaux, France 1 Introduction  Conditioned animals display fear generalization in comparison to naïve animals as a function of the similarity between the probe context and the conditioning context  Lesion of the vmPFC has no effect on contextual fear memories when the animals were tested in the original conditioning context .  vmPFC-lesioned animals display a significant enhancement of fear generalization in comparison to sham animals when tested in a probe context different from the original conditioning context.  These data suggest that the vmPFC prevents contextual fear overgeneralization During contextual fear conditioning an animal is exposed to a mild aversive footshock in an experimental context. After few pairings, the context becomes aversive and elicits fear responses. Although contextual fear is associated with an increase in neuronal activity in the ventral medial prefrontal cortex (vmPFC) [1], pre-training vmPFC lesions have no effect on contextual fear conditioning when the animals were tested in the conditioning context [2]. In contrast, vmPFC-lesioned animals display high contextual fear when tested in a context never associated with the footshock [3]. These data strongly suggest that the vmPFC is selectively engaged in situations requiring contextual discrimination. The present study evaluated this hypothesis in vmPFC- lesioned mice using a novel contextual fear generalization paradigm in which fear generalization is measured in three distinct probe contexts resembling more or less the original conditioning context. We found that pre-training vmPFC lesions are associated with an enhancement of contextual fear generalization in C57BL6/J mice. These data are in accordance with recent data suggesting that the vmPFC is selectively engaged in situation requiring contextual discrimination [3] and further suggest that the vmPFC plays a key role in preventing fear overgeneralization. 2 Behavioral protocol and histological analysis A) Behavioral apparatus and protocol A: Contextual fear conditioning protocol composed of a conditioning session (Contextual Fear Cond.) performed on day 1 and a probe test performed 24 hours following contextual fear conditioning (Contextual Fear Test). One week before contextual fear conditioning, some animals underwent a bilateral ventral medial prefrontal cortex (vmPFC) lesion using 1 mA anodal current. During contextual fear conditioning, non lesioned, vmPFC-lesioned and sham animals were submitted to two footshock exposure (0.9 mA, 1 s). Naïve unconditioned animals were only exposed to the context without footshocks. The next day, mice were exposed for 2 min in 4 different probe contexts resembling more or less the original conditioning context B: Schematic representation of vmPFC lesions covering the prelimbic (PL) and infralimbic (IL) but not cingulate (CG1) prefrontal cortices. Groups Contextual Fear Cond. Contextual Fear Test 0.9 mA, 1sec 2 x 24 h 3 Results 4 Conclusions 5 References [1] Beck CH, Fibiger HC. Conditioned fear-induced changes in behavior and in the expression of the immediate early gene c-fos: with and without diazepam pretreatment. J Neurosci., 1995, 15:709–720. [2] Morgan MA, Romanski LM, LeDoux JE. Extinction of emotional learning: Contribution of medial prefrontal cortex. Neuroscience Letters, 1993, 163: 109-113. 3] Antoniadis EA, McDonald R. Fornix, medial prefrontal cortex, nucleus accumbens, and mediodorsal thalamic nucleus: roles in a fear-based context discrimination task. Neurobiol Learn Mem., 2006, 85: 71−85. The authors declare no potential conflict of interest. 2 min probe context exposure Cg1 PL IL IL PL Cg1 Cg1 PL IL A: Freezing behavior during the 2 min exposure in the 4 probe contexts 24 hrs following fear conditioning for conditioned and naïve animals, *** p< 0.001. B: Freezing behavior during the 2 min exposure in the 4 probe contexts 24 hrs following fear conditioning for vmPFC-shams and vmPFC-lesioned animals, *** p<0.001, ** p<0.01, * p<0.05. C: Generalization index evaluated between conditioned vs naïve animals and vmPFC-shams vs vmPFC-lesioned animals. The generalization index was calculated by dividing the freezing value observed for each experimental group in PB1 to PB4 by the freezing value observed in PB1 for the conditioned or the vmPFC- shams animals. Naïve Contextual exposure Conditioned vmPFC shams vmPFC lesioned Probe context 1 (PB1) Probe context 2 (PB2) Probe context 3 (PB3) Probe context 4 (PB4) B) Histological analysis +1.98 mm from bregma +1.78 mm from bregma +1.54 mm from bregma A) Conditioned vs Naïve B) vmPFC-shams vs vmPFC-lesioned P.2.006 C) Generalization index Conditioned Naïve vmPFC-shams vmPFC-lesioned PB1 _ 0.19 _ 0.99 PB2 0.84 0.24 0.70 1.20 PB3 0.52 0.21 0.40 0.71 PB4 0.21 0.20 0.20 0.73