ABSTRACT
To determine the physiological role of the ovaries in regulation of LH subunit gene expression, levels of cytoplasmic mRNA were measured in a cDNA-RNA dot-blot hybridization assay. An increase (twofold) in α mRNA was first detected 8 days after ovariectomy and then remained stable for 4 weeks. In contrast, LH-β mRNA increased by 60–79% within 12 h of removing the ovaries and then rose progressively to six times the intact values at 3 and 4 weeks. Increases in LH-β mRNA were always greater than those of α mRNA. Oestradiol, and oestradiol plus progesterone, but not progesterone alone, prevented the rise in α and LH-β mRNA 10 days after ovariectomy.
Three days after ovariectomy, α mRNA, but not LH-β mRNA, was suppressed to below intact control values by oestradiol and oestradiol plus progesterone, indicating greater sensitivity of α mRNA to oestradiol inhibition at this stage. A single injection of oestradiol (1 μg s.c.) to rats ovariectomized 14 days previously transiently suppressed α and LH-β mRNA levels and serum LH concentrations in parallel for 1–8 h, after which high preinjection values were restored. However, pituitary LH content remained suppressed after LH mRNA levels had returned to the control values of ovariectomized rats.
In most instances there was a qualitative positive correlation between changes in α and LH-β mRNA, pituitary LH content and serum LH concentrations. LH content reflected LH-β mRNA changes more closely than those of α mRNA. However, in oestradiol-treated rats ovariectomized 10 days previously, LH content remained increased despite normalization of the LH-β and α mRNA levels, suggesting differential sensitivity to oestradiol of the gene expression and translational processes. Thus divergence of pre- and post-translational regulation of LH biosynthesis was demonstrated. These results imply an important physiological role for female sex hormones in the control of LH gene expression and LH biosynthesis.
Prolactin mRNA fell by 30–50% for the first 2 weeks after ovariectomy, but by 3 and 4 weeks values were similar to those of intact controls. Serum and pituitary prolactin levels were reduced by 50% or more at all time-points, despite normalization of mRNA. Treatment of ovariectomized rats for 10 days with oestradiol and progesterone, either alone or combined, reversed the fall in prolactin mRNA and serum and pituitary prolactin levels. These changes in prolactin gene expression and synthesis were opposite to those of LH subunits in response to the same in-vivo hormone manipulations.
Growth hormone mRNA levels were unchanged by ovariectomy, oestradiol or progesterone treatment. Levels of TSH-β mRNA increased slightly (maximum up to 50%) after ovariectomy, but were unaltered by oestradiol and progesterone treatment for 10 days. These results support the view that α mRNA changes, resulting from ovariectomy, oestradiol and progesterone treatment, occur in gonadotrophs and not thyrotrophs, which also express the α subunit gene.
Is the primitive regulation of pituitary prolactin (tPRL177 and tPRL188) secretion and gene expression in the euryhaline tilapia (Oreochromis mossambicus) hypothalamic or environmental?