Can the rat detect hypoxia in inspired air?

Rats reacted in immediate escape response to encounters with severe hypoxia. This led to a test of the hypothesis that mammals which live in closed spaces (burrows) should have a hypoxic alarm mechanism. A possible detection of hypoxia was studied. Rats were trained to thrust their heads into a compartment flushed by a gas mixture of high or low O2 (balance N2), and after a timed interval, to enter the compartment (on high O2) for a reward or to withdraw (on low O2) to avoid a punishment. Exposure times for the gas mixtures were 5, 3 or 1 sec. The initial conditions for high and low O2 mixtures were 30% and 3% for one series, and 21% and 3% for two other series. Successful discrimination between low and high O2 mixtures led to a stepwise reduction of the concentration difference between them. The rat could detect 21% as different from 30%, 13-17% as different from 21%, and 7-15% as different from 3%. From the short time needed for the learned response (1-3 sec), it is suggested that the rat's olfactory system or airway O2 receptors may be involved in the detection of oxygen.