Short-term inhibitory effect of estradiol on tyrosine hydroxylase activity in tuberoinfundibular dopaminergic neurons in vitro.

The short-term inhibition by estradiol of tyrosine hydroxylase (TH) in tuberoinfundibular dopaminergic neurons was examined in vitro on hypothalamic slices from ovariectomized rats. TH activity (determined by L-3,4-dihydroxyphenylalanine accumulation in the median eminence after blockade of decarboxylase with NSD 1055) showed a 30-40% decrease within 1 h of incubation with estradiol. To determine whether a dephosphorylation process was involved in this decline in TH activity, we studied the sensitivity of the enzyme to dopamine (DA) feedback inhibition: In controls, we observed that two kinetically different forms of TH coexisted, with one exhibiting a Ki(DA) of 26.4 +/- 2 microM and the other being approximately 10-fold more sensitive to DA inhibition, with a Ki(DA) of 2.56 +/- 0.17 microM, likely corresponding to a phosphorylated and active form and to a nonphosphorylated and poorly active form, respectively. Conversely, after estradiol treatment all TH molecules exhibited the same Ki(DA) of 2.5 +/- 0.3 microM. This effect was stereospecific, because 17 alpha-estradiol could not promote it, whereas with 17 beta-estradiol, it could be observed at only 10(-11) M and after a short delay (30 min). Finally, this decrease in the Ki(DA) of the purported active form of TH could be prevented by okadaic acid (an inhibitor of protein phosphatases). These results suggest that estradiol can act directly on the mediobasal hypothalamus to trigger a rapid decline in TH activity and that this action may involve a decrease in TH phosphorylation.