[3H]Tyramine binding: A comparison with neuronal [3H]-dopamine uptake and [3H]mazindol binding processes

³H]Spiperone (³H]SPI) binding sites in rat or bovine striata have been solubilized using CHAPS or digitonin detergents. Solubilized sites retained the binding characteristics of those in native membrane preparations. The same solubilized material, however, did not bind ³H]tyramine (³H]PTA), thus indicating that ³H]PTA binding sites and DA receptors are different chemico-physical entities. In membrane preparations or crude synaptosomes obtained from the c.striatum of neonatally-rendered hypothyroid rats, when central DA-pathways are impaired, both ³H]PTA binding and ³H]DA uptake processes were markedly decreased, with no effect on ³H]mazindol (³H]MAZ) binding, compared to euthyroids. Reserpine, a well-known inhibitor of DA-uptake into a variety of secretory vesicles, and a potent in vivo andin vitro inhibitor of ³H]PTA binding, did not affect the ³H]MAZ binding process. This further supported the suggestion that while ³H]PTA binding sites are almost totally associated with the vesicular transporter for DA, ³H]MAZ does label a site involved in the DA-translocation across the neuronal membrane. The latter process seems to be rather insensitive to thyroid hypofunction, when however the intracellular storage of DA might be consistently impaired. In conclusion, PTA might be well exploited as a marker of the DA vesicular transporter through its molecular characterization, whenever possible.Special issue dedicated to Dr. Paola S. Timiras