A role of anterior cingulate cortex in the emergence of worker-parasite relationship.

We studied the brain mechanisms underlying action selection in a social dilemma setting in which individuals' effortful gains are unfairly distributed among group members. A stable "worker-parasite" relationship developed when three individually operant-conditioned rats were placed together in a Skinner box equipped with response lever and food dispenser on opposite sides. Specifically, one rat, the "worker," engaged in lever-pressing while the other two "parasitic" rats profited from the worker's effort by crowding the feeder in anticipation of food. Anatomically, c-Fos expression in the anterior cingulate cortex (ACC) was significantly higher in worker rats than in parasite rats. Functionally, ACC inactivation suppressed the worker's lever-press behavior drastically under social, but only mildly under individual, settings. Transcriptionally, GABAA receptor- and potassium channel-related messenger RNA expressions were reliably lower in the worker's, relative to parasite's, ACC. These findings indicate the requirement of ACC activation for the expression of exploitable, effortful behavior, which could be mediated by molecular pathways involving GABAA receptor/potassium channel proteins.

Brief stress impairs recognition memory through amygdalar activation in animals with medial prefrontal cortex lesions.

The medial prefrontal cortex (mPFC) is thought to exert inhibitory control over stress-induced activation of the amygdala and neurocognitive effects. As evidence to support this, we examined how exposure to either a brief or prolonged stress affected on amygdalar c-Fos levels and recognition memory of animals with mPFC chemical lesions. mPFC-lesioned and sham-operated animals were subjected to either a brief 20-min restraint+20 tailshocks or a prolonged 60-min restraint+60 tailshocks. Post-stress performances in the object recognition memory and c-Fos immunoreactivity in the amygdala were then assessed. In sham-operated animals, the object recognition memory was reliably impaired following the prolonged, but not following the brief stress exposure. On the other hand, in mPFC-lesioned animals, the brief stress significantly impaired recognition memory and enhanced c-Fos expression in the amygdala. Present findings of loss of mPFC activity exacerbating stress effects provide causal evidence that the mPFC exerts inhibitory control on stress.

Strain and sex differences in fear conditioning: 22 kHz ultrasonic vocalizations and freezing in rats

Strain and sex differences in fear conditioning were investigated in two commonly used laboratory rats: Sprague Dawleys and Long-Evans. Twenty-two kHz ultrasonic vocalization (USV) distress calls and freezing behavior were used to measure fear responses to contextual and auditory conditioned stimuli (CSs), which were previously paired with a footshock unconditioned stimulus (US). Both strain and sex had significant effects on USVs and freezing during training and subsequent context and tone tests. Overall, the male Sprague Dawley rats froze and emitted USVs more than the other groups. Additionally, levels of freezing and USVs were differentially influenced by the type of CS (context or tone). These results suggest that species-specific defense responses in laboratory rats are highly influenced by the strain and sex of the subject, and that these factors should be considered in future fear conditioning studies.