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

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.

Odor-induced variation in anxiety-like behavior in mice is associated with discrete and differential effects on mesocorticolimbic cholecystokinin mRNA expression.

The present investigation assessed alterations in mesocorticolimbic cholecystokinin (CCK) mRNA following novel predator and non-predator odor exposure and light-dark testing in CD-1 mice. In brief, acute exposure of CD-1 mice to the predator odor, 2,5-dihydro-2, 4,5-trimethylthiazoline (TMT; the major component of the anal gland secretions of the red fox), or the control odor, butyric acid (BA), suppressed rearing behavior during odor presentation, subsequently induced anxiety in the light dark test, and was associated with increased mesocorticolimbic CCK mRNA relative to saline treated mice. Only mice exposed to TMT displayed elevated freezing behaviors during odor treatment. In the light-dark test, mice exposed to either BA or TMT took longer to reenter the light section of the apparatus and spent less cumulative time in the light relative to mice exposed to saline. The decreased time spent in the light as well as light dark transitions were exaggerated among mice exposed to fox odor. Odor presentation was associated with increased CCK mRNA in mesocorticolimbic sites. Butyric acid was associated with enhanced CCK gene expression in the VTA, while both BA and TMT were associated with increased medial prefrontal cortex (mPFC) CCK mRNA levels. Increased CCK mRNA within the VTA and mPFC was evident among mice despite testing in the light-dark box. In contrast, basolateral nucleus of the amygdala (BLA) CCK mRNA was enhanced following odor exposure among mice in the light dark test relative only to saline treated mice which demonstrated a natural decrease in BLA CCK mRNA following the light dark test. The differential pattern of CCK mRNA associated with discrete psychogenic stressor manipulations and the provocation of anxiety-like behavior associated with such experiences is discussed.