RETRONASAL OR INTERNASAL OLFACTION CAN MEDIATE ODOR-GUIDED BEHAVIORS IN NEWBORN MICE

Studies of olfactory deprivation have most frequently used unilateral naris occlusion to effect deprivation. Recent psychophysical evidence suggests that adult rodents with either acute or chronic naris occlusi on show little decrement in olfactory ability. In this study the effec t of naris occlusion coupled with ipsilateral or contralateral olfacto ry bulb deafferentation on odor-guided behaviors was tested in neonata l mice. Animals that received bilateral bulbectomy or control manipula tion were also included. In Experiment 1, olfactory lesions were produ ced by bulb aspiration on the second day after birth (P2). Daily weigh t gain, a reliable measure of suckling success, was recorded until P21 . In Experiment 2, olfactory lesions consisted of bulb transection on P2. Daily weights were recorded until subjects were P10. Animals with bilateral bulbectomy had the highest mortality rate and slowest growth race. Both naris occlusion groups grew more slowly than controls but were not significantly different at P5 or P10. They diverged, thereaft er, such that at P20 the group with naris occlusion ipsilateral to bul bectomy was similar to controls, while the contralateral group was sim ilar to the bilateral bulbectomy group. In Experiment 2, the weights o f the naris occlusion groups diverged by the day after surgery, with t he contralateral group suffering the most arrested growth. Behavioral tests were combined from the two experiments for analysis. Subjects we re tested at P5 for their ability to find the nipple, at P8 for their ability to find the nest, and at P10 for unwashed vs. washed nipple pr eferences. Only the contralateral group had a significantly depressed ability to find the nipple, while all lesion groups had a significant but similar decline in nest-finding ability. Controls and both naris o cclusion groups showed significant preferences for unwashed nipples. T hese results suggest that while naris occlusion produces some deficits in olfactory ability it does not produce complete odor deprivation. I t is concluded that the olfactory epithelium in the occluded nasal fos sa may gain access to stimuli by a retronasal or internasal route.