Modulation of brain catecholamine absorbing proteins by dopaminergic agents

Catecholamine absorbing proteins (CATNAPs) are localized in the brain and thus far have no known biochemical and pharmacological characteristics consistent with other receptor proteins or metabolic enzymes in the central nervous system. The oxidative metabolism of catecholamines in the brain, especially the catabolism of dopamine and its conjugation with metabolic brain proteins, results in the production of highly toxic free radicals. Since such processes are implicated in the pathophysiology of various neurodegenerative diseases, including parkinsonism, and since CATNAPs bind catecholamines with high affinity, there is a need to further investigate if these novel proteins could play a protective role against these harmful catecholamine metabolites. In this study, we demonstrate the purification, pharmacological characterization and modulation of CATNAPs, as the first steps necessary to elucidate the function of these proteins in the brain. First, CATNAPs were identified from tissues using [³H]N-n-propylnorapomorphine (a specific dopamine receptor agonist) and [¹²⁵I]6-hydroxy-5-iodo[N-(N-2,4-dinitro-phenyl)-aminopropyl]1,2,3,4 -tetrahydronaphthalene ([¹²⁵I]DATN; a highly specific ligand synthesized in our laboratory). Three proteins, with molecular masses of 47, 40 and 26 kDa, were identified and purified, which allowed for the subsequent production of antibodies against each of these CATNAPs. The effects of in vivo chronic administration of several dopaminergic agents on CATNAPs were also examined by Western immunoblotting. L-3,4-Dihydroxyphenylalanine (L-DOPA) treatment in rats resulted in the increase of all of the three proteins, as compared to controls. Treatment in rats with the dopamine depleting agent, reserpine, produced a significant decrease in all of the three CATNAPs. In addition, the effects of direct administration of apomorphine, dopamine, epinephrine, isopropylnorepinephrine, norepinephrine, N-n-propylnorapomorphine and 6-hydroxydopamine on CATNAP levels in rats were examined. Interestingly, we observed an increase (as compared to control) of the 47, 40 and 26 kDa proteins in animals treated with dopamine, norepinephrine, N-n-propylnorapomorphine and apomorphine. In contrast, animals treated with 6-hydroxydopamine showed significant decreases in the levels of all three proteins. It is evident that as the concentration of catecholamines increases, there is a corresponding increase in the levels of CATNAPs in the brain. These results clearly demonstrate the pharmacological modulation of CATNAPs by dopaminergic agents and suggest their possible role in the cytoprotection against damage caused by free radicals generated by oxidative stress.