IN-SITU HYBRIDIZATION OF LUTEINIZING-HORMONE HUMAN CHORIONIC-GONADOTROPIN RECEPTOR MESSENGER-RIBONUCLEIC-ACID DURING HORMONE-INDUCED DOWN-REGULATION AND THE SUBSEQUENT RECOVERY IN RAT CORPUS-LUTEUM

Previous studies have shown that injection of a pharmacological dose o f hCG to rats primed with PMSG/hCG results in a loss of hCG binding in the luteinized ovary, which is closely coupled with the loss of LH/hC G receptor (LH/hCG R) messenger RNA (mRNA). The time course of down-re gulation of the receptor mRNA reveals that mRNA totally disappears 24 h after the hormone injection, but fully recovers by 72 h. The purpose of this study was to determine by in situ hybridization whether the r ecovery of the receptor mRNA occurs in preexisting or newly formed cor pora lutea. Twenty-one-day-old female rats were treated with 50 IU PMS G, followed 56 h later by a single injection of hCG. On day 5 after th e hCG injection, one group of rats was treated with a desensitizing do se of hCG, and a control group received saline. The ovaries were colle cted 6, 12, 24, 48, 72, and 96 h after the treatments and were process ed for in situ hybridization using S-35 antisense RNA synthesized from a 750-mer LH/hCG-R complementary DNA. In control ovaries, heavily lab eled LH/hCG-R mRNA-containing cells were observed in the numerous corp ora lutea. Ribonuclease pretreatment of the sections eliminated the si gnal, and no specific hybridization was observed when sense strand pro be was used. No hybridization to the granulosa cells was seen. Some hy bridization occurred to the theca interna, but the intensity was lower than that in the corpora lutea. Time-course studies showed a marked d ecline in the hCG-R mRNA signals in corpora lutea as early as 6 h afte r hCG injection, with a maximum loss of receptor mRNA by 24 h. After 4 8 h, hCG-R mRNA reappeared in preexisting corpora lutea, with the inte nsity of the hybridization signal equaling that in corpora lutea of co ntrols. New corpora lutes could not be identified at any time after in jection of the down-regulating dose of hCG. As down-regulated receptor mRNA recovered in preexisting, not new, corpora lutes, hormone-induce d loss of luteal cell receptors would appear to be reversible.