In situ activity and phosphorylation of tyrosine hydroxylase in the median eminence

Intracellular activation and phosphorylation of tyrosine hydroxylase (TH; E.G. 1.14.16.2) in the median eminence of the rat brain were investigated. The in situ activity of TH was assayed by the accumulation of l-dihydroxyphenylalanine (DOPA) in the median eminence of hypothalamic fragments incubated in the presence of NSD 1015. When hypothalamic fragments were incubated with veratridine (0-10^-10 M), maximal stimulation of TH activity was observed at 10^-4 M. The mean concentration of DOPA in the median eminence of hypothalamic fragments incubated with 10^-4 M veratridine was 3 times that seen in its absence. Phosphorylation of TH in the median eminence was evaluated by autoradiographic quantification of [³²P]TH in ³²P-labelled median eminence tissue. The amount of [³²P]TH in ³²P-labelled median eminence incubated with 10^-4 M veratridine was 2 times that seen in the absence of veratridine. These data are consistent with the view that in the median eminence phosphorylation and activation of TH are linked events and that phosphorylation may be a means of regulating the biosynthesis of dopamine in this region of the brain.