Pituitary prolactin biosynthesis is negatively regulated by hypothalamic dopamine through D2 receptors in pituitary lactotrophs, but little is known about the direct effect of dopamine on gonadotrophs. In this study, the clonal gonadotroph-derived cell line, αT3-1, was used to examine whether gene expression of the pituitary gonadotropin α subunit, stimulated with GnRH or pituitary adenylate cyclase-activating polypeptide (PACAP), was controlled by dopamine D2 receptor. Western blotting and reverse transcription-polymerase chain reaction analysis demonstrated the presence of dopamine D2 receptors in αT3-1 cells. Both GnRH and PACAP increased α subunit gene expression. GnRH-induced α subunit gene expression was not affected by quinpirol, a specific dopamine D2 receptor agonist. In contrast, PACAP-induced gene expression was significantly lower in the presence of quinpirol. The roles of extracellular signal-regulated kinase (ERK) and cAMP in the expression of the α subunit gene were examined. GnRH activated ERK, but PACAP did not, and the activation was not inhibited by quinpirol. GnRH-induced α subunit gene expression was completely inhibited by an ERK inhibitor, PD098059. Cyclic AMP accumulation in αT3-1 cells was increased by treatment with PACAP, and quinpirol inhibited this effect. GnRH did not affect cAMP production in these cells. These results suggest that in αT3-1 cells, dopamine D2 receptors negatively regulate pituitary α subunit gene expression in association with the cAMP-dependent pathway, but not with the ERK pathway.
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