Background

To escape detection by predators, many mammalian species, including rodents, have developed innate fear responses triggered by odor stimuli that indicate the presence of predators (Takahashi et al., 2005). The predator’s odorous substance, such as excretion and fur particles, triggers anxiety in the rodent without direct contact and induces avoidance or freezing behavior, depending on the circumstances. For example, if a mouse is not able to run away from the source of the odor (e.g., confined to a small box), the mouse freezes. If the mouse can run away, they will avoid the source of the odor rather than freeze (Hacquemand et al., 2010; Johnston et al., 2012). TMT (2,4,5-trimethylthiazoline), a component of fox feces, is the most used synthesizable reagent for inducing innate fear in rodents (Vernet-Maury et al., 1984). Wallace et al. found that innate fear responses of rats can be quantified by measuring the freezing duration when the animals are exposed to TMT in a small confined space. They also found that innate fear responses to TMT do not induce conditioned learning. This finding indicates that different neural pathways are activated during TMT exposure from those activated during conventional footshock-induced fear responses. Lesion studies have shown that the regions associated with the innate fear responses include the medial/central nucleus of the amygdala and the bed nucleus of the stria terminalis (BNST) (Fendt et al., 2003; Müller and Fendt, 2006). Here, I present conventional methods for measuring TMT-induced fear responses in the mouse.