Stress-induced activation of the immediate early gene Arc (activity-regulated cytoskeleton-associated protein) is restricted totelencephalicareasintheratbrain:relationshipto c-fos mRNA Sheila Ons, Octavi Martı ´andAntonioArmario Institut de Neurocie `ncies and Unitat de Fisiologia Animal, Facultat de Cie `ncies, Universitat Auto `noma de Barcelona, Bellaterra, Barcelona, Spain Abstract Arc is an effector immediate early gene whose expression is induced in situations of increased neuronal activity. However, there is no report on the influence of stress on Arc expression. Here, we compared the induction of both c-fos and Arc mRNAs in the brain of rats exposed to one of three different stressful situations: novel environment, forced swimming and immobilization. An absent or weak c-fos mRNA signal was observed in control rats, whereas those exposed to one of three stressors showed enhanced c-fos expression in a wide range of brain areas. Constitutive Arc expression was observed in some areas such as cortex, striatum, hippocam- pus, reticular thalamic nucleus and cerebellar cortex. In response to stressors, a strong induction of Arc was observed, but the pattern was different from that of c-fos. For instance, activation of Arc but not c-fos was observed in the nucleus accumbens after immobilization and in the hippocampus after novel environment. No Arc induction was observed in dien- cephalic and brainstem areas. The present data show that Arc has a neuroanatomically restricted pattern of induction in the brain after emotional stress. Telencephalic activation sug- gests that a more intense induction of synaptic plasticity is occurring in this area after exposure to emotional stressors. Keywords: Arc, c-fos, forced swimming, immobilization, novel environment, rat, stress. J. Neurochem. (2004) 89, 1111–1118. Dataobtainedfromvariousexperimentalapproachesindicate that exposure to stressful situations causes activation of numerous brain areas, likely to prepare the organisms for appropriately responding to the situation. Consequently, a widespread induction of the immediate early gene (IEG) c-fos,consideredasamarkerofneuronalactivation,hasbeen observedinthebraininresponsetoawiderangeofstressors (for reviews see Senba and Ueyama 1997; Kova ´cs 1998). Although other IEGs have been studied, and most of them are activated in the same conditions as c-fos (Senba and Ueyama1997),only zif268 hasbeenfoundtobeinducedin areas such as the hippocampal formation in conditions in whichweak c-fos inductionwasdetected(e.g.Cullinan et al. 1995). Very recently, attention has been paid to IEGs which do not act as trascription factors, but instead codify proteins directly involved in cellular function. One of the best characterized among those is Arc (activity-regulated cyto- skeleton-associated protein). In addition to being strongly induced by neuronal activity, this gene has other very interesting properties (Link et al.1995;Lyford et al. 1995): 1.itsmRNAisdistributedalongdendritesandthereforeit isinvolvedinlocalsynthesisofprotein 2.itcodifiesforaproteinsharinghomologywithspectrin, and is associated in vivo to F-actin, the main component of microfilaments. Because experience-induced brain changes appear to involve the development and/or reorganization of new synapses, Arc expression may be important for learning. In Received September 15, 2003; revised manuscript received November 28,2003;acceptedNovember28,2003. Address correspondence and reprint requests to Antonio Armario, UnitatdeFisiologiaAnimal,FacultatdeCie `ncies,UniversitatAuto `noma de Barcelona (08193) Bellaterra, Barcelona, Spain. E-mail: antonio.armario@uab.es Abbreviations used: BAS, basal; DEPC, diethylpyrocarbonate; DTT, dithiothreitol; FS, forced swimming; HB, hole board; IEG, immediate earlygene;IMO,immobilization;KPBS,potassiumphosphatebuffered saline; PFA, paraformaldehyde; SSC, saline-sodium citrate; TEA, triethanolamine. Journal of Neurochemistry ,2004, 89,1111–1118 doi:10.1111/j.1471-4159.2004.02396.x Ó 2004 International Society for Neurochemistry, J. Neurochem. (2004) 89,1111–1118 1111