Direct ovarian effects of a GnRH agonist in hypophysectomized immature rats: Inhibition of theca-interstitial luteinizing hormone receptor mRNA expression and induction of interstitial cell differentiation

The effect of a gonadotropin-releasing hormone (GnRH) agonist on luteinizin g hormone (LH) receptor mRNA expression was examined histologically in the ovaries of immature hypophysectomized (HPX) rats by in situ hybridization. In the ovaries of HPX rats treated with diethylstilbestrol (DES) and pregna nt mare serum gonadotropin (PMSG), LH receptor mRNA was expressed in the gr anulosa cells of mature follicles as well as the theca-interstitial cells. In DES-primed ovaries of rats treated with both GnRH agonist plus PMSG, man y follicles were luteinized without ovulation, and the signal of LH recepto r mRNA disappeared completely in the theca-interstitial cells as well as th e luteinized cells, but remained in the granulosa cells of unaffected matur e follicles. The complete suppression of the theca-interstitial LH receptor expression by GnRH agonist was also observed in HPX rats that received no other treatment. On the other hand, the coadministration of a GnRH antagoni st with PMSG resulted in the hyperstimulation of follicular growth, accompa nied by very strong expression of LH receptor mRNA in the granulosa cells a s well as the theca-interstitial cells. In addition, morphological changes in the ovarian interstitial cells were also induced by the administration o f GnRH agonist in HPX rats: loose connective tissue decreased and the inter stitial cell mass markedly increased. The increase of the interstitial cell s became more prominent when rats were treated with GnRH agonist and testos terone simultaneously. These results suggest that GnRH may be an important factor for modulating the interstitial cell function and differentiation in the rat ovary.