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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

Accumbens. --- Activation. --- Activity. --- Amygdala. --- Anxiety-like behavior. --- Anxiety. --- Area. --- Behavior. --- Behavioral-responses. --- Butyric acid. --- Cortex. --- Defensive behavior. --- Double dissociation. --- Emotional responses. --- Enkephalin. --- Experience. --- Exposure. --- Expression. --- Extended amygdala. --- Fos-related antigen. --- Freezing. --- Gene. --- Genes. --- Individual-differences. --- Infralimbic cortex. --- Infralimbic. --- Level. --- Light. --- Long-term. --- Male-rats. --- Mice. --- Neuronal. --- Neurons. --- Nucleus accumbens. --- Nucleus-accumbens. --- Nucleus. --- Odor. --- Posttraumatic-stress-disorder. --- Predator odor. --- Predator. --- Prelimbic. --- Response. --- Responses. --- Self-stimulation. --- Stressor. --- Test. --- Time. --- Unconditioned fear. --- Ventral tegmental area.