Decline in basic fibroblast growth factor (FGF-2) mRNA expression in female rat hypothalamus at puberty.

There is a growing acceptance of the importance of hypothalamic growth factors in the control of sexual maturation. Basic fibroblast growth factor (bFGF, FGF-2), a potent mitogen and neurotropic factor for brain cells in vitro, including hypothalamic cells, is widely expressed in the post-natal CNS but its physiological functions there are largely unknown. Previously, studies of FGF-2 mRNA regulation in vivo have been hampered by the low levels of FGF-2 mRNA present in post-natal tissues. We have applied a sensitive semi-quantitative procedure based on reverse transcription followed by polymerase chain reaction amplification (RT-PCR) to detect and estimate relative amounts of mRNAs encoding FGF-2 and its receptor in the hypothalamic-hypophyseal axis in individual female rats undergoing sexual maturation. FGF receptor and FGF-2 mRNAs were detectable in all brain regions examined. Injections of the glutamate agonist N-methyl-D-aspartic acid (NMDA) or pregnant mare's serum gonadotropin (PMSG) were used to advance the onset of puberty in immature female rats, and the levels of FGF-2 and FGF receptor mRNA in MBH and cortex were examined. Daily injections of NMDA (20 mg/kg) from day 24-28 resulted in advancement of first ovulation and vaginal opening (VO) in 5 of 9 treated rats. None (0/4) of the saline treated controls achieved first ovulation during the course of the experiment. Expression of FGF-2 mRNA in the medial-basal hypothalamus of the NMDA-treated VO animals, but not nonVO animals, was significantly (P<0.05) reduced by 50% vs saline-treated nonVO controls. There was no effect of NMDA on FGF-2 expression in cerebral cortex of VO Vs nonVO animals. FGF receptor mRNA levels were unaffected by NMDA treatment. To assess the possibility that the decline in hypothalamic FGF-2 mRNA levels was related to puberty and not just to an effect of NMDA, pregnant mare's serum gonadotropin was used to induce first ovulation and vaginal opening. Injection of PMSG to immature female rats on day 26 resulted in precocious first ovulation on day 29. This was accompanied by a significant 40% reduction in the steady-state level of FGF-2 mRNA in the medial basal hypothalamus compared to saline treated controls. As with NMDA treatment, PMSG did not affect FGF-2 mRNA abundance in the cortex, nor the FGF receptor mRNA in MBH or cortex. Immunohistochemical detection of FGF-2 protein in the arcuate nucleus revealed that FGF-2 immunoreactivity was also significantly modified in peri-ovulatory NMDA-treated animals. FGF-2 immunoreactivity in NMDA treated rats was significantly elevated at day 29 (the day of ovulation), but significantly inhibited by day 33. These findings suggest that alterations in the level of FGF-2 mRNA in the hypothalamus may be associated with first ovulation and the onset of sexual maturation in the female rat.