How does long-term odor deprivation affect the olfactory capacity of adult mice?

Unilateral naris occlusion (UNO) has been the most common method of effecting stimulus deprivation in studies of olfactory plasticity. However, despite the large corpus on the effects of this manipulation, dating back to the 19th century, little is known about its behavioral sequela. Here we report the results of standard olfactory habituation and discrimination studies on adult mice that had undergone perinatal UNO followed by adult contralateral olfactory bulbectomy (bulb-x). The olfactory performance of UNO mice was compared to matched controls that had unilateral bulb-x but open nares. Both habituation and discrimination (operant) experiments employed a protocol in which after successful dishabituation or discrimination to dilute individual odors (A = 0.01% isoamyl acetate; B = 0.01% ethyl butyrate; each v/v in mineral oil), mice were challenged with a single odor versus a mixture comparison (A vs. A + B). In a series of tests the volume portion of Odor B in the mixture was systematically decreased until dishabituation or discrimination thresholds were reached. For the habituation experiment, UNOs (n = 10) and controls (n = 9) dishabituated to a 10% mixture of Odor B in Odor A after being habituated to A alone, while both groups failed to show differential responding to a 2% mixture of B in A. However, the UNO group's increased investigation durations for the 2% mixture approached significance (p < 0.06). A replication of this study (7 controls & 8 UNOs) confirmed that controls did not differentiate Odor A and a 2% mixture of B in A but UNOs did not (p < 0.05). For the discrimination experiment, 4 UNOs and 4 controls were shaped to dig in one of two containers of sand that contained the S+ odor (Odor B) to obtain sugar pellet rewards. As in the habituation experiment, UNOs displayed greater olfactory capacity than controls on this task. Controls and UNOs had an average mixture discrimination threshold of 1.6% (+/- 0.4) and 0.22% (+/- 0.102) respectively, a difference that was statistically significant (p < 0.02). Adult mice relying on an olfactory system deprived of odor by naris occlusion from near the time of birth display enhanced olfactory capacity compared to control mice. This counterintuitive result suggests that UNO is neither an absolute method of deprivation nor does it diminish olfactory capabilities. Enhanced olfactory capacity, as observed in the current study, that is a consequence of deprivation, is consistent with recent molecular and physiological evidence that stimulus deprivation triggers compensatory processes throughout the olfactory system.