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Differential regulation of the expression of contextual freezing and fear-potentiated startle by 5-HT mechanisms of the median raphe nucleus.
Behav Brain Res. 2004 May 05; 151(1-2):93-101.BB

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 microg/0.2 microl) 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, 1s) 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.

Authors+Show Affiliations

Laboratório de Psicobiologia, FFCLRP, Campus USP, Av. Bandeirantes 3900, Ribeirão Preto 14049-901, SP, Brazil.No affiliation info availableNo affiliation info available

Pub Type(s)

Comparative Study
Journal Article
Research Support, Non-U.S. Gov't

Language

eng

PubMed ID

15084425

Citation

Silva, R C B., et al. "Differential Regulation of the Expression of Contextual Freezing and Fear-potentiated Startle By 5-HT Mechanisms of the Median Raphe Nucleus." Behavioural Brain Research, vol. 151, no. 1-2, 2004, pp. 93-101.
Silva RC, Gárgaro AC, Brandão ML. Differential regulation of the expression of contextual freezing and fear-potentiated startle by 5-HT mechanisms of the median raphe nucleus. Behav Brain Res. 2004;151(1-2):93-101.
Silva, R. C., Gárgaro, A. C., & Brandão, M. L. (2004). Differential regulation of the expression of contextual freezing and fear-potentiated startle by 5-HT mechanisms of the median raphe nucleus. Behavioural Brain Research, 151(1-2), 93-101.
Silva RC, Gárgaro AC, Brandão ML. Differential Regulation of the Expression of Contextual Freezing and Fear-potentiated Startle By 5-HT Mechanisms of the Median Raphe Nucleus. Behav Brain Res. 2004 May 5;151(1-2):93-101. PubMed PMID: 15084425.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Differential regulation of the expression of contextual freezing and fear-potentiated startle by 5-HT mechanisms of the median raphe nucleus. AU - Silva,R C B, AU - Gárgaro,Ana Carolina, AU - Brandão,M L, PY - 2003/01/15/received PY - 2003/08/18/revised PY - 2003/08/18/accepted PY - 2004/4/16/pubmed PY - 2004/7/20/medline PY - 2004/4/16/entrez SP - 93 EP - 101 JF - Behavioural brain research JO - Behav Brain Res VL - 151 IS - 1-2 N2 - 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 microg/0.2 microl) 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, 1s) 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. SN - 0166-4328 UR - https://www.unboundmedicine.com/medline/citation/15084425/Differential_regulation_of_the_expression_of_contextual_freezing_and_fear_potentiated_startle_by_5_HT_mechanisms_of_the_median_raphe_nucleus_ L2 - https://linkinghub.elsevier.com/retrieve/pii/S0166432803003012 DB - PRIME DP - Unbound Medicine ER -