Re-exposure and environmental enrichment reveal NPY-Y1 as a possible target for post-traumatic stress disorder Hendrikus Hendriksen * , Diewertje I. Bink, Eileen G. Daniels, Rahul Pandit, Carole Piriou, Roderick Slieker, Koen G.C. Westphal, Berend Olivier, Ronald S. Oosting Division of Pharmacology, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands article info Article history: Received 1 November 2011 Received in revised form 16 May 2012 Accepted 18 May 2012 Keywords: Infusion Extinction Glutamate receptors Amygdala Neuropeptides abstract Exposure-based cognitive behavioral therapy in post-traumatic stress disorder (PTSD) patients relieves symptoms caused by fear association as well as symptoms that are not the result of associative learning. We used the inescapable foot shock model (IFS), an animal model for PTSD, to study the possible involvement of glutamate receptors, the corticotropin-releasing factor (CRF) system, and the neuropeptide Y (NPY) system in the reduction of stress sensitization following repeated re-exposure to the conditioning context. Starting one week after the IFS procedure, the rats were repeatedly re-exposed tothe shock environment. Stress sensitivity was measured in a modified open field test (sudden silence was used as a stressor). Selected mRNAs (GluN1, -2A-C, GluA1-4, GluK1-5, CRF, CRF-R1, NPY, NPY-Y1) were quantified in the amygdala. Repeated re-exposure (RE) to the IFS context reduced both trauma-associated anxiety (to the IFS context) and the enhanced stress sensitivity (in the open field). Changes in glutamate receptor subunits (GluN1, GluN2A-B, GluA1, GluA4, GluK3, GluK4) were detected in the amygdala that were normalized by RE. However, infusion of the AMPA/kainate antagonist NBQX in the BLA (basolateral amygdala) did not improve the anxious behavior. RE normalized IFS-induced increases in CRF-R1 mRNA and increased NPY- Y1 mRNA expression in the amygdala. Previously, and repeated here, we showed that environmental enrichment (EE) enhances recovery from IFS. EE led to similar changes in CRF-R1 and NPY-Y1 expression as RE did. Importantly, administration of [Leu31, Pro34]-NPY (Y1 agonist) in the BLA normalized the enhanced sensitivity to stress after IFS. Our data suggest that the NPY-Y1 receptor in the amygdala may serve as a therapeutic target for the treatment of PTSD. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Post-traumatic stress disorder (PTSD) patients show symptoms, such as intrusive memories, avoidance, and an exaggerated response to cues that have just a slight resemblance to the trauma (Hackmann et al., 2004; Reynolds and Brewin, 1998). These symp- toms are considered consequences of associative learning, such as classical conditioning, operant conditioning, (re)consolidation, and generalization (Grillon et al., 1996; Solomon et al.,1987). In addition, PTSD also leads to symptoms that are caused by the trauma but are not the result of associative fear learning. These symptoms are rather the result of fear incubation (Boyd, 1981; Eysenck, 1968; Pickens et al., 2009b) (resulting in enhanced reaction to situational reminders over time without further conditioning) or sensitization (Post et al., 1995; Stam, 2007; Stam et al., 2000) (resulting in an enhanced sensitivity towards (neutral) stressful stimuli that have no association with the trauma). These forms of non-associative learning can lead to persistent symptoms of anxiety and hyper- arousal that may include hypervigilance, exaggerated startle response, and emotional numbing (Pamplona et al., 2010; Siegmund and Wotjak, 2007a, b; Stam et al., 2000). On top of these symptoms there is a high comorbidity with depression (77%) and generalized anxiety (34%) (Kean et al., 2007). Exposure-based behavioral therapy is used to acquire extinction for trauma- associated cues, but it also relieves the non-associated symptoms (Moulds and Nixon, 2006; Paunovic and Ost, 2001; Powers et al., 2010; Yehuda, 2002). The extinction process has been extensively studied both in animals and in humans (Bouton and King, 1983; Myers and Davis, 2007; Quirk et al., 2010). These studies have shown, for instance, that glutamatergic N-methyl D-aspartate (NMDA) and a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the amygdala play an important role in * Corresponding author. Tel.: þ31 6 20250539. E-mail addresses: h.hendriksen@uu.nl, erik.hendriksen@gmail.com (H. Hendriksen). Contents lists available at SciVerse ScienceDirect Neuropharmacology journal homepage: www.elsevier.com/locate/neuropharm 0028-3908/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropharm.2012.05.028 Neuropharmacology 63 (2012) 733e742