TISSUE-SPECIFIC REGULATION OF GONADOTROPIN-RELEASING-HORMONE (GNRH) RECEPTOR GENE-EXPRESSION DURING THE PERIOVULATORY PERIOD

We have used the reverse transcription-polymerase chain reaction (RT-PCR) to synthesize, clone and sequence a partial cDNA, potentially encoding the ovarian GnRH receptor. A 703 basepair PCR-product generated from the rat ovary was, upon sequencing found identical to the recently reported cDNA encoding GnRH receptors in rat pituitary. Northern blot analysis of total RNA from various tissues revealed a major transcript of 4.4 kilobases (kb), as well as a less abundant, smaller transcript of 1.2-1.5 kb in rat pituitary, ovary and testis but no expression was seen in the placenta. The relative abundance of GnRH receptor mRNA in pooled ovaries obtained from rats during different days of the estrous cycle did not change. The levels of GnRH receptor mRNA were also examined in PMSG/hCG primed immature rats, killed prior to hCG injection, at ovulation or 33-36 h post-ovulation. No change in ovarian GnRH receptor gene expression was seen 48 h after PMSG injection. However, a marked decline to levels only 30% (P<0.05, n = 8) of controls was detected during ovulation, 12 h following hCG-injection. This decline is transient, since postovulatory levels were elevated to those seen prior to hCG injection. The GnRH mRNA expression is differentially regulated in ovaries and pituitary, since PMSG injection decreased the pituitary transcript as detected by a quantitative RT-PCR method, whereas hCG treatment did not affect the levels measured. The demonstration of tissue-specific regulation of GnRH receptor gene regulation adds further support to the extra-pituitary actions of GnRH as an autocrine/paracrine factor involved in ovulation.