title: The effects of endomorphin-1 on conditioned defeat in Syrian hamsters (Mesocricetus auratus).
abstract: The present study examined the effect of endomorphin-1 (EM1), an endogenous opioid with a high affinity for the mu opiate receptor, on conditioned defeat. Conditioned defeat is a phenomenon in which hamsters that have been defeated subsequently fail to exhibit normal territorial aggression and instead display submissive/defensive behaviors even when paired with a non-aggressive intruder. In experiment 1, animals were placed in the home cage of a larger resident for 15 min and were defeated. After 24 h, animals received a 3-microl injection of EM1 (0.0, 0.3, 3.0, or 10 nmol) into the left lateral cerebral ventricle 5 min before a smaller non-aggressive intruder was placed in the home cage of the experimental animal. In experiment 2, animals were infused with EM1 immediately after the initial defeat and were paired with a non-aggressive intruder 24 h later as in experiment 1. EM1 reduced the duration of submissive/defensive behavior in experiment 1 (P<0.05) but not in experiment 2 (P>0.05). These data support the hypothesis that the highly selective mu receptor agonist endomorphin-1 modulates the expression of conditioned defeat, but provides no support for the hypothesis that endomorphin-1 modulates the consolidation of conditioned defeat.
authors.affiliation: Center for Behavioral Neuroscience, Department of Psychology, Georgia State University, Atlanta, GA 30303, USA.
authors.forename: R D
authors.initials: RD
authors.lastname: Whitten
authors.forename: A M
authors.initials: AM
authors.lastname: Jasnow
authors.forename: H E
authors.initials: HE
authors.lastname: Albers
authors.forename: S
authors.initials: S
authors.lastname: Martin-Schild
authors.forename: J E
authors.initials: JE
authors.lastname: Zadina
authors.forename: K L
authors.initials: KL
authors.lastname: Huhman
mesh_terms: D000818:Animals; D001007:Anxiety / Q000378:metabolism / Q000503:physiopathology; D001522:Behavior, Animal / Q000187:drug effects* / Q000502:physiology; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D003213:Conditioning, Psychological / Q000187:drug effects* / Q000502:physiology; D006224:Cricetinae; D003863:Depression / Q000378:metabolism / Q000503:physiopathology; D004305:Dose-Response Relationship, Drug; D005239:Fear / Q000187:drug effects* / Q000502:physiology; D007276:Injections, Intraventricular; D008297:Male; D008647:Mesocricetus; D009842:Oligopeptides / Q000378:metabolism / Q000494:pharmacology*; D017450:Receptors, Opioid, mu / Q000187:drug effects* / Q000378:metabolism; D013315:Stress, Psychological / Q000378:metabolism* / Q000503:physiopathology
chemical_list: D009842:Oligopeptides; D017450:Receptors, Opioid, mu; C105293:endomorphin 1