Superfluous expression of tryptophan hydroxylase in the zona incerta dopaminergic neurones

We studied by double labelling the relationship between tryptophan hydroxylase-immunoreactive (TpOH-IR) neurones and A13 dopaminergic neurones in the zona incerta (ZI). TpOH-IR in the ZI was dramatically increased after colchicine treatment. While all of the TpOH-IR neurones exhibited also tyrosine hydroxylase-immunoreactivity (TH-IR), a marker of dopaminergic neurones, less than half (46%; n = 794) of the A13 TH-IR neurones in the ZI showed TpOH-IR. None of these neurones were immunoreactive, however, for serotonin either in intact animals or in animals pretreated with serotonin precursor L-tryptophan and the monoamine oxidase inhibitor pargyline. Thus, the neurones of the A13 dopaminergic group in the ZI expressing TpOH are unable to synthesize serotonin. These findings support the idea that neurones may superfluously express non-functional transmitter synthesizing enzymes, in this case TpOH, and suggest that expression of an enzyme synthesizing a certain transmitter does not necessarily confirm the corresponding transmitter phenotype.