Brief exposure to predator odor and resultant anxiety enhances mesocorticolimbic activity and enkephalin expression in CD‐1 mice

Abstract

The present study assessed alterations in mesolimbic enkephalin (ENK) mRNA levels after predator [2,5-dihydro-2,4,5-trimethylethiazoline (TMT)] and non-predator (butyric acid) odor encounter and/or light-dark (LD) testing in CD-1 mice immediately, 24, 48 and 168 h after the initial odor encounter and/or LD testing. The nucleus accumbens, ventral tegmental area, basolateral (BLA), central (CEA) and medial amygdaloid nuclei, prelimbic and infralimbic cortex were assessed for fos-related antigen (FRA) and/or ENK mRNA as well as neuronal activation of ENK neurons (FRA/ENK). Mice exposed to TMT displayed enhanced freezing and spent less time in the light of the immediate LD test relative to saline- or butyric acid-treated mice. Among mice exposed to TMT, LD anxiety-like behavior was associated with increased FRA in the prelimbic cortex and accumbal shell and decreased ENK-positive neurons in the accumbal core. Mice displaying high TMT-induced LD anxiety exhibited increased ENK-positive neurons in the BLA, CEA and medial amygdaloid nuclei relative to mice that displayed low anxiety-like behavior in the LD test after TMT exposure. In the BLA and CEA, 'high-anxiety' mice also displayed increased FRA/ENK after TMT exposure and LD testing. In contrast to neural cell counts, the level of ENK transcript was decreased in the BLA and CEA of 'high-anxiety' mice after TMT exposure and LD testing. These data suggest that increased FRA may regulate stressor-responsive genes and mediate long-term behavioral changes. Indeed, increased ENK availability in mesolimbic sites may promote behavioral responses that detract from the aversiveness of the stressor experience.

Nucleus accumbens D2- and D1-receptor expressing medium spiny neurons are selectively activated by morphine withdrawal and acute morphine, respectively

“…Withdrawn mice were then kept in separate cages and perfused two hours after the injection. To avoid the possibility of novelty or stress-induced induction of c-Fos in NAc, opioid withdrawal signs were not explicitly scored in separate observation chambers in the present study (Hebb et al, 2004). Nonetheless, withdrawal signs appeared comparable to those reported previously (Hacker et al, 2006;Bagley et al, 2011).…”

Moderate Stress-Induced Social Bonding and Oxytocin Signaling are Disrupted by Predator Odor in Male Rats

“…Exposure to fox urine increases anxiety (Hebb et al, 2004) and disrupts memory function (Morrow et al, 2000) in rodents and therefore represents a likely model for a more threatening stressor than acute immobilization alone. It is also commonly used to model PTSD-like symptoms in rodents (Janitzky et al, 2015) and may therefore represent a good stimulus for rejection of social support.…”

The Role of Amygdalar Mu-Opioid Receptors in Anxiety-Related Responses in Two Rat Models

“…Since opioid systems in the amygdala modulate nociceptive responses (Kang et al, 1999), this test was modified from that described in Wilson et al (2004) by substituting a noxious (predator) odor rather than a shock probe. Predator odor can induce a similar burying behavior as that seen after shock probe exposure (see Hebb et al, 2004). A Plexiglas chamber (45 Â 30 Â 44 cm) filled up to a depth of 5 cm with fresh pine bedding, with a piece of ferret-scented towel (5 cm 2 ) placed 2 cm above bedding, was used for this analysis.…”

“…Large numbers of enkephalin-immunoreactive neurons are observed in the amygdala (Roberts, 1992;Veinante et al, 1997;Fallon and Leslie, 1986;Gray et al, 1984;Harlan et al, 1987), and the central nucleus of the amygdala (CEA) receives enkephalin afferents from the bed nucleus of the stria terminalis (BNST) as well as other amygdalar nuclei (Poulin et al, 2006). The number of enkephalin neurons and enkephalin mRNA expression show differences between high-anxiety and low-anxiety mice, enkephalin mRNA expression in the amygdala is altered by exposure to predator odor, and pre-proenkephalin-knockout mice show increased levels of anxiety-like behaviors (Konig et al, 1996;Hebb et al, 2004;Filliol et al, 2000). Enkephalins interact with both mu-opioid receptors (MORs) and delta-opioid receptors (DORs), both of which are seen in the amygdala (Mansour et al, 1993(Mansour et al, , 1994a(Mansour et al, , b, 1987(Mansour et al, , 1995Poulin et al, 2006;Wilson et al, 2002;Goodman et al, 1980) and could mediate the effects of amygdalar enkephalin in anxiety or fear responses.…”