DEVELOPMENT OF LUTEINIZING-HORMONE ACTION IN THE PERINATAL RAT OVARY

The expression of LH receptor (LHR) mRNA was studied in fetal rat gona ds using polymerase chain reaction multiplication of reverse-transcrib ed mRNA. A primer pair corresponding to the extracellular domain of th e receptor revealed the expression of LHR mRNA in fetal ovaries and te stes as early as embryonic Day 13.5, the earliest age studied. The loc alization of LHR mRNA was examined in the perinatal rat ovary using in situ hybridization with two antisense probes, one encoding the extrac ellular and the other encoding the transmembrane domains of LHR. At th e age of 4 days, only the extracellular LHR probe gave specific signal over ovarian stromal and follicular cells, excluding the ova. Three d ays later, similar distribution of specific hybridization was observed with both probes. In the 10- and 30-day-old rat ovaries, clear expres sion of LHR mRNA was found to be with both probes in theca cells. Gona dotropin-stimulated production of cAMP was studied in cultures of disp ersed perinatal rat ovarian cells. When 5-day-old ovarian cells were c ultured for 3 days in vitro and then stimulated by either hCG (0.1 mg/ L) or FSH (1 mg/L) for 6 h, cAMP production was enhanced only in cells stimulated by FSH. In a similar experiment with 7-day-old ovarian cel ls, cAMP production was stimulated by both FSH and hCG. Stimulation wi th hCG (0.1 mg/L) during the 3-day culture caused homologous desensiti zation of cAMP production, but stimulation with FSH (0.1 mg/L) had no such effect. The desensitization of the LHR was also investigated by t reating neonatal rats in vivo with a high dose of hCG (600 IU/kg BW s. c. as a single injection on Day 7) or with a dosage of recombinant hum an (rec) FSH on Days 3-9 (0.3 IU s.c. twice daily). Thereafter, at the age of 10 days, the ovaries were incubated either with recFSH (200 IU /L) or hCG (CR-121; 0.1 mg/L) for 1 h. Homologous desensitization of c AMP production by hCG was observed, but the FSH-mediated cAMP producti on was not affected. The hCG-induced steroidogenesis (progesterone and testosterone production) was not desensitized. In conclusion, these f indings indicate that 1) the expression of the mRNA encoding the extra cellular domain of LHR, i.e., truncated receptor, occurs in fetal rat gonads as early as embryonic Day 13.5; 2) the expression of the trunca ted LHR mRNA occurs uniformly in the differentiating ovarian cells bef ore the appearance of the functional theca cell layer; 3) full-length LHR message appears in the developing ovary concomitantly with appeara nce of differentiated theca cells; 4) homologous desensitization of cA MP output by hCG, without steroidogenic desensitization, is present in perinatal rat ovaries; and 5) no FSH-evoked desensitization of cAMP p roduction occurs in perinatal ovaries.