Tyrosine hydroxylase in rat auditory midbrain: distribution and changes following deafness

Tyrosine hydroxylase (TH), a key enzyme in the catecholaminergic pathway, allows for the differentiation of dopaminergic neurons. We previously showed decreases in TH gene expression in the rat inferior colliculus (IC) 3 and 21 days following deafness. In the present study, we characterized the normal distribution of TH as well as changes following deafness (bilateral cochlear ablation) in the IC and nuclei of the lateral lemniscus. Immunostaining was compared in three groups of rats: normal hearing (n=8), 21 day deaf (n=5) and 90 days following deafening (n=5). Many TH immunoreactive fibers and puncta were identified in the IC and nuclei of the lateral lemniscus of normal hearing animals and labeling was most dense in the external cortex of the IC. We also identified immunolabeling for fibers and puncta for another catecholaminergic enzyme, dopamine beta hydroxylase (DBH), but not phenylethanolamine-N-methyltranferase (PNMT). Neurons immunopositive for TH but not DBH or PNMT were observed in the dorsal cortex and dorsal horn of the central nucleus of the IC and ventral and intermediate lemniscus. In the central nucleus of the IC and dorsal lateral lemniscus many lightly labeled TH neurons were also DBH positive. Although the number of immunopositive cells in the IC and lemniscus declined 3 weeks and 3 months after deafening, the decline was not significant at three weeks in the VNLL nor after three months in the dorsal cortex. Immunolabeling for TH decreased significantly in IC and lemniscus 3 weeks and 3 months following deafening. These results suggest a role for dopaminergic neurons and fibers in deafness-related plasticity.