Increased site-specific phosphorylation of tyrosine hydroxylase accompanies stimulation of enzymatic activity induced by cessation of dopamine neuronal activity

Activation of striatal dopamine (DA) neurons by neuroleptic treatment or by electrical stimulation of the nigrostriatal pathway increases the activity of tyrosine hydroxylase (TH). The increase is mediated by phosphorylation of the enzyme. However, abolition of DA neuronal activity [by gamma-butyrolactone (GBL) treatment or transection of the nigrostriatal pathway] also increases TH activity. Quantitative blot immunolabeling experiments using site- and phosphorylation state-specific antibodies to TH demonstrated that GEL treatment (750 mg/kg, 35 min) significantly increased phosphorylation at Ser19 (+40%) and Ser40 (+217%) without altering Ser31 phosphorylation. Concomitantly, GEL treatment [along with the 3,4-dihydroxyphenylalanine (dopa) decarboxylase inhibitor NSD-1015, 100 mg/kg, 30 min] increased in vivo striatal dopa accumulation and in vitro TH activity 3-fold. Likewise, cerebral hemitransection of the nigrostriatal pathway significantly increased phosphorylation of TH at Ser19 (+89%) and Ser40 (+158%) but not at Ser31; dopa levels were increased accordingly (+191%). Kinetic analysis of TM activity established that GEL treatment and hemitransection primarily decreased the K-m for the cofactor tetrahydrobiopterin (3-fold). The effects of GEL and hemitransection were abolished or attenuated by pretreatment with the DA agonist R-(-)-N-n-propylnorapomorphine (NPA; 30 mu g/ kg, 40 min), presumably via stimulation of inhibitory presynaptic DA autoreceptors, NPA dose-response curves for reversal of GEL-induced dopa accumulation and Ser40 phosphorylation were identical; however, only the highest dose of NPA reversed the small and variable increase in Ser19 phosphorylation, Thus, TH activity seems to be regulated by phosphorylation in both hyper- and hypoactive striatal DA neurons; in the latter case, activation seems to be caused by selective phosphorylation of Ser40.