Behavioural Brain Research 197 (2009) 24–30 Contents lists available at ScienceDirect Behavioural Brain Research journal homepage: www.elsevier.com/locate/bbr Research report Medial prefrontal cortex and nucleus accumbens core are involved in retrieval of the methamphetamine-associated memory Chih-Yuan Chiang a , Chianfang G. Cherng b , Yu-Ting Lai a , Hsin-Yi Fan a , Jia-Ying Chuang c , Gour-Shenq Kao c , Wan-Ting Chang a , Lung Yu a,c,∗ a Institute of Behavioral Medicine, National Cheng Kung University College of Medicine, Tainan 701, Taiwan b Department of Clinical Psychology, Chang Jung Christian University, Tainan 711, Taiwan c Institute of Basic Medical Sciences, National Cheng Kung University College of Medicine, Tainan 701, Taiwan article info Article history: Received 26 May 2008 Received in revised form 21 July 2008 Accepted 22 July 2008 Available online 5 August 2008 Keywords: Pavlovian conditioning Memory Methamphetamine Psychomotor stimulant Medial prefrontal cortex Nucleus accumbens core abstract Immunohistochemical Fos staining has proven to be a method to identify the neurons that are activated by stimulation. Although methamphetamine (MA)-conditioned place preference (CPP) memory was long- lasting, how this memory was established and retrieved remained unknown. We used the vehicle- and MA-conditioned environment (including cues and context) to reactivate the MA-CPP memory in mice. In the limbic system, Fos-positive neurons were examined following retrieval of the MA-CPP memory. We demonstrated that the current conditioning procedure produced reliable MA-CPP performance. More- over, enhanced Fos expressions were found in the medial prefrontal cortex and the core of the nucleus accumbens after reactivation of the MA-CPP memory. Furthermore, familiarity with the environmental cues/context was found to significantly enhance Fos expressions in dorsal striatum and dentate gyrus. Nucleus accumbens shell, basolateral or lateral amygdala, in this regard, did not seem to be involved in retrieval of the MA-CPP memory. These results, taken together, suggest that the medial prefrontal cor- tex and the core of the nucleus accumbens are anatomical substrates responsible for reactivation of the MA-CPP memory. © 2008 Elsevier B.V. All rights reserved. 1. Introduction c-fos is an immediate early gene that encodes a transcription fac- tor, Fos protein [13,26]. Intracellular Fos protein reaches its maximal level between 1 and 2 h following neuronal activation [17,33,40] and decays with a half-life less than 2 h [14,36]. Immunohistochemical staining for Fos in an appropriate time window, accordingly, has proven to be a method to identify the neurons involving stimulus processing. For example, many studies have shown that expo- sure to cues previously associated with drug administration may increase Fos expression in several nuclei of the limbic system [5,11,24,27,32,35]. Methamphetamine (MA)-conditioned place preference (CPP) can be established in rodent models by employing a Pavlovian con- ditioning protocol. In a previous study, we suggested that MA-CPP ought to be a long-term memory since MA-CPP performance was reliably observed in repeated memory tests over a long period ∗ Corresponding author at: Behavioral Neuropharmacology Laboratory, Institute of Behavioral Medicine, National Cheng Kung University College of Medicine, 1 Uni- versity Rd, Tainan 70101, Taiwan, ROC. Tel.: +886 6 2353535; fax: +886 6 2095616. E-mail address: lungyu@mail.ncku.edu.tw (L. Yu). of time [22]. Nonetheless, consolidation of the MA-CPP mem- ory seemed to be independent of de novo protein synthesis [18]. Lately, we reported that sensory status associated with MA and vehicle administrations played a pivotal role in acquisition of the MA-CPP memory [6], suggesting that sensory processing of the vehicle- and MA-associated context/cues determines acquisition of the MA-CPP memory. However, how the vehicle- and MA- associated context/cues was acquired and recalled still remained unknown. A recent study indicated that most of the activated neurons during the learning phase of a Pavlovian fear conditioning were reactivated at the retrieval phase of the memory [29]. More inter- estingly, numbers of the activated neurons at the retrieval phase were positively correlated with magnitude of the memory expres- sion [29]. Although these results cannot be used as a conclusive evidence for identifying the memory trace of the fear condi- tioning, the modified Fos-labeling method assists researchers to reveal the fact that memory performance is correlated with the number of activated neurons at retrieval phases. In this study, we employed an immunohistochemical Fos staining method to examine activated neurons in several nuclei of the limbic system following reactivation (retrieval) of the MA-CPP memory in a mouse model. 0166-4328/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2008.07.030