Figure 1. Experimental timeline.

Rats were first screened for reactivity to CO₂ (n=34) or Normoxic Air (n=22). Then,at least 5 days later, they were fear conditioned using 3 tone shock pairings. The next day, they received an extinction session (19 CSs). The day after extinction, they received a long-term memory test (LTM). At least 4 days after the LTM test, a subset of rats were tested in the elevated plus maze, and another subset were tested in the light-dark box. At least 6 days later, all rats received a CO₂ challenge, and were sacrificed one hour later for later immunohistochemistry processing.