RAPID DOWN-REGULATION OF TYROSINE-HYDROXYLASE EXPRESSION IN THE OLFACTORY-BULB OF NARIS-OCCLUDED ADULT-RATS

In most sensory systems, afferent innervation regulates morphological and biochemical characteristics of target cells for a limited time dur ing development. Sensory deprivation experiments in adult rats also ha ve suggested a critical period for afferent influences on olfactory bu lb structure and function. Previous odorant deprivation studies that e mployed unilateral naris closure in neonatal rats demonstrated down-re gulation of the catecholamine biosynthetic enzyme tyrosine hydroxylase (TH) in dopamine neurons intrinsic to the olfactory bulb. Accompanyin g the altered biochemical parameters was a decrease in bulb size. To d istinguish between deprivation-induced alterations in TH expression se condary to developmental sequelae and those occurring in mature neuron s, the consequences of unilateral naris closure were assessed in young adult rats. In agreement with previous studies significant postnatal increases occurred in TH expression and total protein, an indication o f bulb size. At 30 days post-closure, total protein was unaltered in t he ipsilateral olfactory bulb but showed a small (12.9%), significant decline at 60 days.:In contrast to the limited morphological consequen ces of odor deprivation, profound reductions occurred in TH expression . TH activity ipsilateral to the closure decreased significantly by 14 days post-closure and remained depressed for up to 6 months. In paral lel with enzyme activity, TH immunoreactivity did not decline in the f irst few days post-closure. In situ hybridization revealed that TH mRN A levels decreased rapidly, i.e., by 2 days post-closure, reached a na dir at 1 month, and remained depressed for at least 6 months. The capa city of odor deprivation in the adult rat olfactory system to down-reg ulate TH expression suggests that this phenotypic alteration occurs in dependently of a presumed critical period. (C) 1996 Wiley-Liss, Inc.