Gonadotrophin-releasing hormone regulation of gonadotrophin subunit gene expression: studies in tri-iodothyronine-suppressed rats
Authors: Haisenleder D. J. , Ortolano G. A. , Dalkin A. C. , Paul S. J. , Chin W. W. , Marshall J. C. 
Published: December 2008

We have previously shown that a pulsatile gonadotrophin-releasing hormone (GnRH) stimulus can increase steady-state levels of α and LH-β subunit mRNAs in the male rat pituitary. Since α subunit is produced in both thyrotroph and gonadotroph cells, the effect of GnRH specifically on gonadotroph α gene expression is uncertain. To address this tissue, adult male rats were given injections of tri-iodothyronine (T3; 20 μg/100 g body wt, i.p.) daily for 8 days (day 8 = day of death) in order to decrease thyrotroph α mRNA levels (+ T3 group). Saline injections (i.p.) were given to control animals (− T3 group). Three days before GnRH administration, the animals were castrated and testosterone implants inserted s.c., to inhibit endogenous GnRH secretion. GnRH pulses (25 ng/pulse; 30-min interval) were given to freely moving animals (saline pulses to controls) via an atrial cannula for 12, 24 or 48 h. Serum LH and FSH were measured before and 20 min after the last GnRH pulse. Pituitary RNA was extracted and α, LH-β, FSH-β and prolactin mRNA levels were determined by dot-blot hybridization using 32P-labelled cDNA probes.

Castration and testosterone replacement reduced α and LH-β mRNA levels by 30 and 40% respectively, compared with levels in untreated intact males, but did not decrease FSH-β concentrations. T3 administration further decreased α mRNA to 30% of values seen in intact males, but LH-β mRNA levels were unchanged. FSH-β mRNA concentrations were decreased by 23% in T3-treated rats (P < 0·05 vs intact controls). In −T3 rats, 12 h of GnRH pulses increased FSH-β mRNA levels (twofold) vs saline-pulsed controls, but significant increases in α or LH-β mRNA levels were not seen until after 24 h of GnRH pulses. In the +T3 group, an increase in α mRNA was observed earlier, after 12 h of GnRH pulses. After 24 and 48 h of GnRH, the increments in α and LH-β were of similar magnitude in both the +T3 and − T3 groups (4–5 and 3–4 fold increases in α and LH-β respectively; P < 0·05 vs saline-pulsed controls). In contrast, the stimulatory effect of GnRH on FSH-β mRNA was lost in + T3 animals after 48 h of pulses.

In order to examine whether this loss in FSH-β mRNA responsiveness to GnRH was related to an inhibitory interaction of T3 in the presence of testosterone, a second study was conducted in castrated animals. The results showed that α mRNA levels were decreased by 33% in +T3 compared with −T3 castrated animals (P < 0·05), but LH-β and FSH-β mRNAs were unaffected by T3 administration. In castrated animals given GnRH pulses, T3 inhibited subunit mRNA responses and this effect was most marked for FSH-β mRNA. In contrast, prolactin mRNA levels were significantly higher (P < 0·05) in all +T3 experimental groups compared with their −T3 controls. These data indicate that T3 can inhibit FSH-β mRNA responses to pulsatile GnRH and that this action occurs in the absence of testosterone.

Journal of Endocrinology (1989) 122, 117–125