Circadian modulation of fos responses to odor of the red fox, a rodent predator, in the rat olfactory system.

We have previously shown that neuronal responses to a biologically neutral odor, cedar wood oil, in the olfactory system are greater in the subjective night compared to subjective day. In the present study, we confirm these results and extend them to a biologically relevant odor, the urine of the red fox, a rodent predator. Fos induced by exposure of rats to fox urine or a neutral odor, mineral oil, was markedly enhanced during the subjective night compared to subjective day in the main olfactory bulb, primary olfactory cortex, and other structures related to olfaction. These results show that neuronal responses to an ethologically relevant odor follow a circadian rhythm similar to biologically neutral odors. Fos responses induced by fox urine were observed to be of greater magnitude than a neutral odor in brain areas involved in fear responses, suggesting that fox urine activates fear circuitry.