Neurotransmitters are known to play an important role in the development of the nervous system. We recently generated transgenic mice that ectopically express tyrosine hydroxylase (TH) and thereby produce dopamine (DA) de novo in pinealocytes of the pineal gland (PG). The transgenic PG also exhibited a dramatic decrease in TH-immunoreactive (IR) fibers putatively arising from the superior cervical ganglion (SCG) (Cho et al. [1996] Proc Natl Acad Sci USA 93:2862-2866). In the current study, however, we found that there was no reduction in the number of fibers immunostained for neurofilament protein or PGP9.5, markers known to be heavily localized in fibers, despite the reduction of TH fiber density. Therefore, we investigated whether the decreased TH-IR fiber density is the consequence of reduced sympathetic innervation, or a decrease in TH expression within innervating fibers. Immunohistochemical analysis comparing control and transgenic PG demonstrated no apparent differences in numbers of NPY- and aromatic-L-amino acid decarboxylase (AADC)-IR fibers, indicating that TH expression is decreased in a normal number of innervating fibers. Furthermore, presynaptic neurons in the transgenic SCG showed abnormal and heterogeneous TH immunoreactivity and reduced TH and norepinephrine transporter (NET) mRNA levels. These results show that ectopic DA production in the PG lowers TH and NET gene expression in the SCG without altering sympathetic innervation to the PG and suggest that the alteration of target neurotransmitter phenotype may influence gene expression of phenotype-specific proteins in projecting neurons.