Region‐specific consequences of PCD gene expression in the olfactory system

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Abstract
These studies investigated the response of olfactory bulb juxtaglomerular dopamine neurons to the loss of mitral cells in 6–7–month‐old Purkinje cell degeneration (PCD) mice. Previous studies in normal mice, with tyrosine hydroxylase (TH) enzyme as a marker, demonstrated that following peripheral olfactory afferent denervation in TH activity and immunoreactivity. These intrinsic dopamine neurons also receive afferent input via dendrodendritic contacts with mitral cells. In contrast to the deficits produced by peripheral denervation, following mitral cell degeneration in homozygous recessive PCD mice, TH activity and immunoreactivity were unaltered as compared to normal heterozygous littermates. Moreover, TH activity in the substantia nigra also was unchanged, thus suggesting that the dopamine phenotype is resistant to the influences of the pcd gene. Despite the absence of a well‐defined effect of the pcd gene on neurons earing the TH phenotype, the expression of this mutation within the olfactory system is not limited to mitral cell degeneration. The current studies also demonstrate the absence of the anterior commissure, especially pars anterior, in homozygous recessive PCD mice at 6–7 months postnatal. Whether or not the loss of the anterior commissure is a primary effect or one that is secondary to mitral cell degeneration, this structural alteration provides evidence that the pcd gene exerts more widespread effects within the olfactory system than previously appreciated. The neuronal specificity of those effects remains apparent as indicated by the lack of change in TH expression.

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