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-butyrol actone (GBL) treatment or transection of the nigrostriatal pathway] also in creases TH activity. Quantitative blot immunolabeling experiments using sit e- and phosphorylation state-specific antibodies to TH demonstrated that GE L treatment (750 mg/kg, 35 min) significantly increased phosphorylation at Ser19 (+40%) and Ser40 (+217%) without altering Ser31 phosphorylation. Conc omitantly, GEL treatment [along with the 3,4-dihydroxyphenylalanine (dopa) decarboxylase inhibitor NSD-1015, 100 mg/kg, 30 min] increased in vivo stri atal dopa accumulation and in vitro TH activity 3-fold. Likewise, cerebral hemitransection of the nigrostriatal pathway significantly increased phosph orylation of TH at Ser19 (+89%) and Ser40 (+158%) but not at Ser31; dopa le vels were increased accordingly (+191%). Kinetic analysis of TM activity es tablished that GEL treatment and hemitransection primarily decreased the K- m for the cofactor tetrahydrobiopterin (3-fold). The effects of GEL and hem itransection were abolished or attenuated by pretreatment with the DA agoni st R-(-)-N-n-propylnorapomorphine (NPA; 30 mu g/ kg, 40 min), presumably vi a stimulation of inhibitory presynaptic DA autoreceptors, NPA dose-response curves for reversal of GEL-induced dopa accumulation and Ser40 phosphoryla tion were identical; however, only the highest dose of NPA reversed the sma ll and variable increase in Ser19 phosphorylation, Thus, TH activity seems to be regulated by phosphorylation in both hyper- and hypoactive striatal D A neurons; in the latter case, activation seems to be caused by selective p hosphorylation of Ser40.