Fear-like biochemical and behavioral responses in rats to the predator odor, TMT, are dependent on the exposure environment

Several laboratories have reported that exposure to predator odor can result in stress-like effects in rodents. While some laboratories have reported fear-like alterations in behavior, other laboratories, including our own, have failed to consistently observe fearful behaviors in rats exposed to the predator odor TMT. One potential contributing factor to this discrepancy is the handling of the rat and its test environment. In the current report, we examine biochemical, endocrinological, and behavioral effects of TMT in two distinct open fields: one small, familiar, and dimly lit, while the other was large, novel, and brightly lit. Only exposure to TMT in the large, novel open field resulted in fearful behavior; however, no increase in dopamine turnover was noted compared to no odor and control odor rats. As expected, the different open fields resulted in some biochemical and behavioral differences, including more horizontal locomotion and less grooming, higher serum corticosterone, and increased dopamine turnover in the ventral prefrontal cortex in the large open field. Finally, compared to the same open field controls, TMT exposure elevated rat serum corticosterone levels in both open fields and dopamine turnover in the dorsal and ventral medial prefrontal cortex and amygdala of rats only in the small, familiar open field. These results indicate that the TMT-induced biochemical activation of may occur without detectable fearful behaviors and may indicate a mechanism that prepares the animal for the expression of a fearful response if additional provocative stimuli are present.