Behavioural Brain Research 151 (2004) 93–101 Research report Differential regulation of the expression of contextual freezing and fear-potentiated startle by 5-HT mechanisms of the median raphe nucleus R.C.B. Silva, Ana Carolina Gárgaro, M.L. Brandão ∗ Laboratório de Psicobiologia, FFCLRP, Campus USP, Av. Bandeirantes 3900, Ribeirão Preto 14049-901, SP, Brazil Received 15 January 2003; received in revised form 18 August 2003; accepted 18 August 2003 Abstract It has previously been shown that the median raphe nucleus (MR) is one of the main sources of projections to the septum and hippocampus. 5-HT projections from this nucleus to the hippocampus are implicated in the acquisition and expression of contextual fear (background stimuli), as assessed by freezing. It has also been reported that amygdala is involved in the acquisition of conditioned fear to foreground cues such as light, used as CS. As the MR projects to the hippocampus and amygdala, the role of this raphe nucleus in fear conditioning to contextual and classical fear conditioning remains to be elucidated. The present study examined the involvement of the MR serotonergic mechanisms in the expression of two distinct types of conditioned fear responses: contextual freezing and fear conditioning to explicit cue (light) measured in a fear-potentiated startle (FPS) procedure. Animals received MR electrolytic lesions of or microinjections of 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino tetralin) (1 g/0.2 l) into the MR, 1 or 7 days after two consecutive training sessions in which they received 10 pairings of the CS (light, 4 s)–US (foot-shocks 0.6 mA, 1 s) and were tested in a contextual fear paradigm and in a FPS procedure. The startle was clearly potentiated in the presence of light-CS in animals bearing lesions of or microinjected with 8-OH-DPAT into MR at 1 or 7 days post-training. However, animals bearing MR electrolytic lesions or microinjections of 8-OH-DPAT into the MR at 1 day, but not at 7 days post-training, showed a significant decrease in time spent in freezing than control ones. Thus, the memory for contextual conditioned fear seems to be formed during a time-window shorter than 1 week. As FPS may be produced in lesioned rats unable to freeze to fear contextual stimuli, dissociable systems seem to be recruited in each condition. Thus, the production of contextual freezing and fear-potentiated startle are conveyed by distinct 5-HT-mediated circuits of the MRN. © 2003 Elsevier B.V. All rights reserved. Keywords: Median raphe nucleus; Fear conditioning; Contextual freezing; Fear-potentiated startle 1. Introduction The median raphe (MR) nucleus is a major serotonin- containing cell group of the brainstem [19,23,37], and is one of the main sources of projections to the septum and hip- pocampus [7,8,39,40]. The hippocampus is widely believed to be essential for learning about the context in which condi- tioning occurs. This view is based primarily on evidence that lesions of the dorsal hippocampus after fear conditioning disrupt freezing to contextual cues. Moreover, the hippocam- pus is thought to have a temporary function in the storage of memory, because when the hippocampus is damaged, recent but not remote memories are impaired [15]. The production of retrograde amnesia in this condition suggests that with ∗ Corresponding author. Fax: +55-16-6330619. E-mail address: mbrandao@usp.br (M.L. Brandão). the passage of time memories are stabilized or consolidated elsewhere in the brain, for example, in the neocortex [24]. As a matter of fact, dorsal hippocampal lesions produce a severe deficit in recently, but not remotely, acquired con- textual fear without impairing memory of discrete training stimuli. In other words, lesions of this structure produce an anterograde and time-limited retrograde amnesia specific to contextual memory. Due to its anatomical connections with the hippocampus MR has also received attention in the research on neurochemical substrates of emotional states, particularly those related to fear conditioning responses [2–4,36]. The MR also sends projections to the amygdala [5,18]. In- deed, besides the well-known projections to the hippocam- pus and septum, fibers from the MR nucleus branch off as they course through the medial forebrain bundle and con- tinue through the internal capsule into the amygdala [32]. 0166-4328/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2003.08.015