Induction of tyrosine hydroxylase expression in rat forebrain neurons.

Olfactory nerve input is required for the normal expression of tyrosine hydroxylase (TH) by dopaminergic neurons in the glomerular region of the rodent main olfactory bulb. To determine whether the olfactory nerve exerts a similar influence on neurons in other brain regions, we performed unilateral bulbectomies in rat pups on postnatal day 5-7 and examined the brains 2-6 months later, after the regenerated olfactory nerve had penetrated the forebrain. Tissue was stained for TH, dopamine beta-hydroxylase (DBH) and olfactory marker protein immunoreactivity. We observed novel TH-immunoreactivity in neurons located in those areas of the adult forebrain which received olfactory nerve fibers, particularly the rostral extension of the subependymal layer. Many of these neurons resembled the periglomerular cells of the olfactory bulb. No cell staining for DBH was observed in these areas, suggesting the possible dopaminergic phenotype of these neurons. Our data indicate that afferent regulation of neurotransmitter expression by the olfactory nerve is not limited to the cells of the olfactory bulb.