Increased site-specific phosphorylation of tyrosine hydroxylase accompanies stimulation of enzymatic activity induced by cessation of dopamine neuronal activity. Mol Pharmacol
ABSTRACT 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 ␥-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 GBL treatment (750 mg/kg, 35 min) significantly increased phosphorylation at Ser19 (ϩ40%) and Ser40 (ϩ217%) without altering Ser31 phosphorylation. Concomitantly, GBL 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 (ϩ89%) and Ser40 (ϩ158%) but not at Ser31; dopa levels were increased accordingly (ϩ191%). Kinetic analysis of TH activity established that GBL treatment and hemitransection primarily decreased the K m for the cofactor tetrahydrobiopterin (3-fold). The effects of GBL and hemitransection were abolished or attenuated by pretreatment with the DA agonist R-(Ϫ)-N-n-propylnorapomorphine (NPA; 30 g/ kg, 40 min), presumably via stimulation of inhibitory presynaptic DA autoreceptors. NPA dose-response curves for reversal of GBL-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. It is now well established that short-term regulation of tyrosine hydroxylase (TH) is accomplished by dynamic changes in the phosphorylation state of the enzyme Electrical stimulation of the medial forebrain bundle (containing the dopaminergic afferents to the forebrain) is known to elicit activation of striatal T
