SUPERFLUOUS EXPRESSION OF TRYPTOPHAN-HYDROXYLASE IN THE ZONA INCERTA DOPAMINERGIC-NEURONS

WE studied by double labelling the relationship between tryptophan hyd roxylase-immunoreactive (TpOH-IR) neurones and A13 dopaminergic neuron es in the zona incerta (ZH). TpOH-IR in the ZI was dramatically increa sed after colchicine treatment. While all of the TpOH-IR neurones exhi bited 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 immunor eactive, however, for serotonin either in intact animals or in animals pretreated with serotonin precursor L-tryptophan and the monoamine ox idase inhibitor pargyline. Thus, the neurones of the A13 dopaminergic group in the ZI expressing TpOH are unable to synthesize serotonin. Th ese findings support the idea that neurones may superfluously express nonfunctional transmitter synthesizing enzymes, in this case TpOH, and suggest that expression of an enzyme synthesizing a certain transmitt er does not necessarily confirm the corresponding transmitter phenotyp e.