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-P CR) to synthesize, clone and sequence a partial cDNA, potentially enco ding the ovarian GnRH receptor. A 703 basepair PCR-product generated f rom 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 transcrip t of 4.4 kilobases (kb), as well as a less abundant, smaller transcrip t of 1.2-1.5 kb in rat pituitary, ovary and testis but no expression w as seen in the placenta. The relative abundance of GnRH receptor mRNA in pooled ovaries obtained from rats during different days of the estr ous cycle did not change. The levels of GnRH receptor mRNA were also e xamined in PMSG/hCG primed immature rats, killed prior to hCG injectio n, at ovulation or 33-36 h post-ovulation. No change in ovarian GnRH r eceptor 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 det ected during ovulation, 12 h following hCG-injection. This decline is transient, since postovulatory levels were elevated to those seen prio r to hCG injection. The GnRH mRNA expression is differentially regulat ed in ovaries and pituitary, since PMSG injection decreased the pituit ary transcript as detected by a quantitative RT-PCR method, whereas hC G treatment did not affect the levels measured. The demonstration of t issue-specific regulation of GnRH receptor gene regulation adds furthe r support to the extra-pituitary actions of GnRH as an autocrine/parac rine factor involved in ovulation.