No abstract

The effects of experimentally produced hypoprolactinemia and hyperprolactinemia on tyrosine hydroxylase (TH) mRNA signal levels were examined in dopaminergic neurons ovariectomized rats. TH mRNA signal levels and relative TH quantity in the arcuate nuclei, zona incerta, and substantia nigra were evaluated by in situ hybridization and immunocytochemistry, respectively. The catalytic activity of TH in the stalk-median eminence (SME) was determined from the in vitro rate of 3,4-dihydroxyphenylalanine (DOPA) accumulation after inhibiting DOPA decarboxylase with brocresine. Chronic administration of bromocriptine (BROMO), a dopamine (DA) agonist, for 3 days reduced circulating rat PRL (rPRL) levels compared to those in the vehicle-treated controls. BROMO treatment decreased TH mRNA signal levels in the arcuate nuclei, the intensity of TH immunostaining in the arcuate-median eminence area, and the rate of DOPA accumulation in the SME. Concomitant administration of ovine PRL (oPRL) reversed the effects of BROMO on TH, resulting in markedly increased TH mRNA signal levels, intensity of TH immunostaining, and rate of DOPA accumulation. Treatment with oPRL by itself for 3 days increased TH mRNA signal levels in the arcuate nuclei and TH activity in the SME, compared to vehicle. Chronic treatment with haloperidol, a DA antagonist, increased circulating levels of endogenous rPRL and increased TH activity in the SME to values similar to those after oPRL treatment. However, in contrast to oPRL, mRNA levels in the arcuate nuclei of haloperidol-treated rats were similar to levels in vehicle-treated animals. To evaluate whether the effect of PRL on TH was species specific, oPRL or rPRL was continuously infused into the jugular vein using an osmotic minipump. TH mRNA levels in the arcuate nuclei were elevated above control levels by either oPRL or rPRL administration. TH mRNA levels in the DA perikarya located in the zona incerta and substantia nigra were not altered by treatment with a DA agonist, a DA antagonist, or PRL. These results indicate that hypoprolactinemia or hyperprolactinemia can selectively reduce or augment, respectively, TH mRNA levels in the tuberoinfundibular dopaminergic neurons. The alterations in TH mRNA content probably contribute to the decrease or increase in TH activity associated with hypoprolactinemia or hyperprolactinemia, respectively.

Estradiol (E₂) and progesterone (P₄) interact to influence tuberoinfundibular dopaminergic neuronal activity and contribute to the control of prolactin (PRL) release. This study examined tyrosine hydroxylase mRNA signal levels in the arcuate nucleus of the hypothalamus and tyrosine hydroxylase activity in the stalk-median eminence after 1 week of steroid treatment and related these to circulating PRL levels. Ovariectomized rats were untreated (control) or were implanted with E₂, P₄ or both E₂ and P₄ pellets and were sacrificed after 7 days at either 10:00 or 18:00 h. Some E₂+P₄-treated rats were injected with either RU 486 or its vehicle at 12-hour intervals for the last 3 of the 7 days of steroid treatment. Tyrosine hydroxylase mRNA signal levels in the arcuate nucleus were decreased by 70% at both 10:00 and 18:00 h in the E₂-treated rats compared to control rats. P₄ alone had no effect on tyrosine hydroxylase mRNA levels, but reversed the E₂-induced decrease so that mRNA levels in the E₂+P₄-treated rats were similar to control levels. The progesterone antagonist RU 486 blocked this effect of P₄, supporting the observation of decreased mRNA levels in E₂-treated rats. Steroid treatment had no effect on tyrosine hydroxylase mRNA levels in the medial zona incerta. Tyrosine hydroxylase activity in the stalk-median eminence was similar at 10:00 and 18:00 h in control rats, and was decreased by 25 and 36% at 10:00 and 18:00 h, respectively, in E₂-treated rats. P₄ alone had no effect on tyrosine hydroxylase activity, but reversed the E₂-induced decrease in enzyme activity to control levels at both 10:00 and 18:00 h. In contrast to the effect of RU 486 on tyrosine hydroxylase mRNA, tyrosine hydroxylase activity in E₂+P4-treated rats was not significantly altered by RU 486 at either 10:00 or 18:00 h. Circulating PRL levels were elevated in the E₂-treated and E₂+P₄-treated rats. A diurnal PRL rise was evident at 18:00 h in E₂-treated rats, but was abolished by concomitant treatment with P₄. The diurnal PRL surge was re-established in E₂+P4-treated rats after administration of RU 486, whereas basal circulating PRL levels were not altered by RU 486. These data indicate that P4 antagonizes the profound inhibitory effect or E₂ on tyrosine hydroxylase mRNA content in the tuberoinfundibular dopaminergic neurons. However, since tyrosine hydroxylase activity is only modestly reduced and exhibits a diurnal rhythm in E₂-treated rats, the regulation of enzyme activity likely is dependent on additional inputs. Although a decrease in tuberoinfundibular dopaminergic neuronal activity may contribute to elevated PRL levels in E₂-treated rats, it is likely not the primary drive for the diurnal E₂-induced PRL surge.

No abstract

This study examined the intracellular mechanisms for the regulation of tyrosine hydroxylase in the tuberoinfundibular dopaminergic neurons of cycling female rats. It also evaluated the hormonal influences that contribute to the control of this enzyme on proestrus. Tyrosine hydroxylase messenger RNA (mRNA) levels in the arcuate nucleus of the hypothalamus were assessed by in situ hybridization. Tyrosine hydroxylase activity in the stalk-median eminence was determined from the in vitro or in vivo rate of 3,4-dihydroxyphenylalanine (DOPA) accumulation after inhibiting DOPA decarboxylase with brocresine or m-hydroxybenzylhydrazine, respectively. Tyrosine hydroxylase mRNA levels and in vitro DOPA accumulation were similar on diestrous day 2 and proestrous mornings, but were reduced by 50% on estrus. Although circulating PRL concentrations were similar on the morning of each day of the estrous cycle, a broad preovulatory PRL surge was observed on the afternoon of proestrus. In vitro DOPA accumulation was similar at 1000 h before the PRL surge and at 1330 h during the peak phase of the PRL surge, but was reduced during the plateau phase of the PRL surge (1700 and 2200 h) coincident with the preovulatory progesterone rise and remained low on estrus. However, in vivo DOPA accumulation was transiently decreased only at 1700 h on proestrus. Tyrosine hydroxylase mRNA levels were similar at 1000, 1330, and 1700 h on proestrus, were reduced by 50% at 2200 h on proestrus subsequent to the decrease in enzyme activity, and remained low on the morning of estrus. Okadaic acid, a protein phosphatase-1 and -2A inhibitor, induced a similar increase in tyrosine hydroxylase activity in vitro at 1330 and 2200 h on proestrus and at 1100 h on estrus, indicating that tyrosine hydroxylase was capable of being activated in spite of decreased mRNA levels. Ovariectomy between 1100-1200 h on proestrus prevented the decrease in tyrosine hydroxylase mRNA levels and in vitro DOPA accumulation at 2200 h. The effects of ovariectomy were completely reversed by progesterone, whereas estradiol had no effect. Circulating PRL levels at 2200 h were suppressed to basal levels after ovariectomy, but were increased by progesterone treatment at 1530 h to levels similar to those in the plateau phase of the PRL surge in control rats. Administration of the progesterone antagonist RU486 at 1200 h on proestrus did not alter tyrosine hydroxylase activity, tyrosine hydroxylase mRNA levels, or circulating PRL concentrations at 2200 h.(ABSTRACT TRUNCATED AT 400 WORDS)