Hormonal control of tyrosine hydroxylase in the median eminence: demonstration of a central role for the pituitary gland

In intact male rats the concentration of dopamine in hypophysial portal plasma of animals treated simultaneously with estradiol and progesterone was twice that of animals treated with the solvent vehicle. Treatment with estradiol or progesterone alone had no effect on dopamine in portal plasma. The rate of synthesis of dihydroxyphenylalanine (DOPA), the precursor of dopamine, in tuberoinfundibular dopaminergic (TID) neurites in the median eminence (ME) was 15 +/- 1.0 (mean +/- SE) pmol DOPA/ME.h in estradiol-progesterone-treated animals compared to 3.2 +/- 0.02 in vehicle-treated controls. Treatment with estradiol or progesterone alone gave a result similar to that seen in controls. In hypophysectomized animals treated with estradiol and progesterone, DOPA synthesis in the ME was greatly attenuated compared to that in intact rats. The in situ activity of tyrosine hydroxylase (TH; expressed as moles of DOPA per mol TH/h) in the ME was 178 +/- 16.5 in estradiol-progesterone-treated intact rats, but was 27 +/- 2.4, 52 +/- 4.2, and 35 +/- 2.5 in animals treated with the solvent vehicle, estradiol, and progesterone, respectively. In hypophysectomized rats the in situ activity of TH in the ME of animals treated with estradiol and progesterone was 53 +/- 8.4, which was significantly (P less than 0.01) less than that in similarly treated intact animals. The circulating PRL level in vehicle-treated animals was 35 +/- 4.6 ng/ml compared to 121 +/- 16 in estradiol-treated animals and 133 +/- 12.2 in estradiol- and progesterone-treated rats, indicating that the difference in the effects of estradiol and estradiol-progesterone on dopamine release, DOPA synthesis, and in situ TH activity was not solely due to a difference in circulating PRL levels. Maintenance for 7 days of anterior pituitary tissue as a graft in a lateral ventricle of intact rats resulted in a 2-fold increase in the synthesis of DOPA and TH activity in the ME compared to that in animals with liver implants. Results obtained in hypophysectomized animals with implants were similar to those in intact animals. The concentrations of PRL in cerebrospinal fluid of intact rats and hypophysectomized rats with anterior pituitary implants in the lateral ventricles were 96 +/- 32 and 127 +/- 35 ng/ml, respectively, which was significantly (P less than 0.001) greater than those in animals with liver implants. We suggest that a factor of pituitary origin stimulates TH activity in TID neurons. This stimulation may be due to PRL, but the existence of another stimulatory substance secreted by pituitary cells cannot be excluded.