Olfaction in utero: Behavioral studies of the mouse fetus

On the day before birth, mouse fetuses were tested for their behavioral responses to iso-amyl acetate and iso-valeric acid delivered into the nasal cavity in liquid phase. Pilot studies established that most subjects responded to most concentrations used with an increase in total behaviors and number of different behaviors displayed. When iso-amyl acetate was systematically delivered at three concentrations (10(-3) M, 10(-4) M and 10(-5) M) subjects showed significant responses to each. In contrast, responses were not evident when saline was the stimulus. The magnitude of the average behavioral response did not decline with decreasing concentration of odorant. In a second experiment, the ability of the fetus to discriminate between iso-amyl acetate and iso-valeric acid was studied. Results establish that the fetuses responded differently to the two odorants. Given the immaturity of the mouse's accessory olfactory system before birth and the observed responses to concentrations of odorants below the threshold of the trigeminal system, the results suggest that the mouse fetus has a functionally competent main olfactory system including the ability to discriminate between purified odorants. The results are discussed in terms of current models of chemosensory ontogeny.