The expression of c-Fos and colocalisation of c-Fos and glucocorticoid receptors in brain structures of low and high anxiety rats subjected to extinction trials and re-learning of a conditioned fear response Małgorzata Lehner a, * , Aleksandra Wisłowska-Stanek b , Ewa Taracha a , Piotr Maciejak a,b , Janusz Szyndler b , Anna Skórzewska a , Danuta Turzyn ´ ska a , Alicja Sobolewska a , Adam Hamed b , Andrzej Bidzin ´ ski a , Adam Płaz ´ nik a,b a Department of Neurochemistry, Institute of Psychiatry and Neurology, 9 Sobieskiego Street, 02-957 Warsaw, Poland b Department of Experimental and Clinical Pharmacology, Medical University, 26/28 Krakowskie Przedmies ´cie Street, 00-927 Warsaw, Poland article info Article history: Received 17 April 2009 Revised 16 June 2009 Accepted 3 July 2009 Available online 9 July 2009 Keywords: Conditioned freezing Re-learning Colocalisation c-Fos/GRs-ir Prefrontal cortex Dentate gyrus Basolateral amygdala Rats abstract We designed an animal model to examine the mechanisms of differences in individual responses to aver- sive stimuli. We used the rat freezing response in the context fear test as a discriminating variable: low responders (LR) were defined as rats with a duration of freezing response one standard error or more below the mean value, and high responders (HR) were defined as rats with a duration of freezing response one standard error or more above the mean value. We sought to determine the colocalisation of c-Fos and glucocorticoid receptors-immunoreactivity (GR-ir) in HR and LR rats subjected to condi- tioned fear training, two extinction sessions and re-learning of a conditioned fear. We found that HR ani- mals showed a marked decrease in conditioned fear in the course of two extinction sessions (16 days) in comparison with the control and LR groups. The LR group exhibited higher activity in the cortical M2 and prelimbic areas (c-Fos) and had an increased number of cells co-expressing c-Fos and GR-ir in the M2 and medial orbital cortex after re-learning a contextual fear. HR rats showed increased expression of c-Fos, GR-ir and c-Fos/GR-ir colocalised neurons in the basolateral amygdala and enhanced c-Fos and GR-ir in the dentate gyrus (DG) in comparison with LR animals. Our data indicate that recovery of a context- related behaviour upon re-learning of contextual fear is accompanied in HR animals by a selective increase in c-Fos expression and GRs-ir in the DG area of the hippocampus. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Extinction of classical fear conditioning is thought to involve activity-dependent potentiation of synaptic transmission in the medial prefrontal cortex, resulting in the inhibition of amygdala- dependent fear responses (Herry & Mons, 2004; Quirk, Garcia, & Gonzáles-Lima, 2006). The impaired extinction of fear memories; i.e., delayed extinction or the failure to extinguish acquired fear and anxiety responses, is crucial for anxiety disorders (Lang et al., 2009). Extinction involves the formation of new memories that inhibit without erasing the original conditioning trace and is known to be particularly resistant to pharmacotherapy. Human imaging studies have implicated the amygdala, prefrontal cortex and hippocampus in the extinction and re-learning of stimulus- reinforcement associations. Specifically, phobic patients displayed increased activation in the amygdala as well as decreased activa- tion in the medial orbitofrontal cortex during the first episode of exposure therapy. Exposure therapy-related reductions in anxiety symptoms consisted of increased medial orbitofrontal cortex activ- ity and activation decreases in the amygdala (Schienle, Schäfer, Hermann, Rohrmann, & Vaiti, 2007). There is strong experimental evidence that similar processes may occur in an animal model. For example, Muigg et al. (2008) examined Wistar rat lines selectively bred for high (HAB) and low (LAB) anxiety-related behaviour in a classical fear conditioning task utilising freezing responses as a measure of fear. In the extinc- tion trial, HAB rats showed a marked deficit in the attenuation of freezing responses to repeated auditory conditioned stimulus pre- sentations and an attenuated c-Fos response in the infralimbic and cingulate cortices but had an increased c-Fos response in the med- ial part of the central amygdala. In recent years, we designed another animal model to examine the neurochemical background of differences in the individual re- sponses to conditioned aversive stimuli using the strength of a rat conditioned freezing response (the context fear test) as a discrim- inating variable (Lehner, Taracha, Turzyn ´ ska, et al., 2008; Lehner, Taracha, Skórzewska, et al., 2008; Lehner et al., 2009). It was shown 1074-7427/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.nlm.2009.07.002 * Corresponding author. Fax: +48 22 4582771/+48 22 4582595. E-mail address: mlehner@ipin.edu.pl (M. Lehner). Neurobiology of Learning and Memory 92 (2009) 535–543 Contents lists available at ScienceDirect Neurobiology of Learning and Memory journal homepage: www.elsevier.com/locate/ynlme