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Neurotrophin‐3 is expressed in a discrete subset of olfactory receptor neurons in the mouse

In transgenic neurotrophin‐3 lacZ‐neo (NT‐3lacZneo) mice, in which the coding region for NT‐3 is replaced by Eschericia coli lacZ, the expression of β‐galactosidase faithfully mimics the expression of NT‐3 (Vigers AJ, Baquet ZC, Jones KR [2000], J Comp Neurol 416:398–416). During embryonic and early postnatal development, β‐galactosidase is detected in the olfactory system, beginning at embryonic day 11.5 in the nasal epithelium and at embryonic day 16.5 in the olfactory bulb. Levels of β‐galactosidase rise with age, reaching a peak during the second postnatal week, when β‐galactosidase reactivity is visible in up to 50% of the glomeruli. As the animal matures, the β‐galactosidase levels decline, but staining remains present in axons and cell bodies of a specific subset of olfactory receptor neurons (ORNs) projecting to a limited subset of glomeruli. The heavily labeled ORNs do not follow the typical OR expression zones in the epithelium but appear similar to the “patch” expression pattern of mOR37 receptors. The most heavily reactive glomeruli exhibit a striking reproducible pattern in the ventral olfactory bulb (OB). Some glomeruli of the OB contain calcitonin gene‐related peptide (CGRP)–immunoreactive fibers of the trigeminal nerve. However, double‐label immunocytochemistry for CGRP and β‐galactosidase rendered no correlation between trigeminal innervation and the degree of innervation by NT‐3–expressing ORNs. Thus, the timing and presence of β‐galactosidase in a subset of ORNs suggests that NT‐3 plays a role in synaptogenesis and/or synapse function in a specific subset of ORNs within the olfactory bulb. J. Comp. Neurol. 463:221–235, 2003. © 2003 Wiley‐Liss, Inc.