Evidence for Protein Kinase C Involvement in the Short‐Term Activation by Prolactin of Tyrosine Hydroxylase in Tuberoinfundibular Dopaminergic Neurons

The mechanism of the short‐term activation by prolactin (PRL) of tyrosine hydroxylase (TH) in tuberoinfundibular dopaminergic neurons was examined in vitro on hypothalamic slices from ovariectomized rats. TH activity (determined by 3,4‐dihydroxyphenylalanine accumulation in the median eminence after blockade of decarboxylase with NSD 1055) showed a dose‐dependent increase within 2 h of incubation of the hypothalamic slices with PRL. To determine whether a phosphorylation process was involved in this increase in TH activity, we studied the sensitivity of the enzyme to dopamine (DA) feedback inhibition. In control median eminences, two kinetically different forms of TH coexisted, one exhibiting a K 1(DA) value of 29.92 ± 0.49 μ M , the other being × 15‐fold more sensitive to DA inhibition with a K 1(DA) of 1.96 ± 0.09 μ M , likely corresponding to a phosphorylated and active form and to a nonphosphorylated and less active form, respectively. After PRL treatment, the TH form of low K 1(DA) remained unaffected, whereas the K 1(DA) of the purported active form of TH increased to 62.6 ± 0.8 μ M , suggesting an increase in the enzyme phosphorylation. This increase in the K I(DA) of TH was selectively prevented by GF 109203X, a potent and selective inhibitor of protein kinase C, but not by a specific inhibitor of protein kinase A or calmodulin. Finally, this action of PRL could be mimicked by 12‐ O ‐tetradecan‐oylphorbol 13‐acetate (a direct activator of protein kinase C). These results suggest that PRL, at the median eminence level, activates TH by increasing the enzyme phosphorylation and that this action may involve an activation of protein kinase C.