Evidence has been presented in several previous reports that typical uterine oestrus is not always associated with vaginal oestrus [Freed and Soskin, 1937; Freed, Mesirow, and Soskin, 1937, 1938]. Examinations of uterus and vagina simultaneously, under various experimental conditions, have revealed that in the presence of a fully cornified vaginal smear the uterine response to ovarian oestrogens may be absent or 'partial' (i.e. gross uterine atrophy with proliferated endometrial epithelium or gross uterine hypertrophy with infantile endometrial epithelium). To account for the reactions described, we have postulated that the rat ovary elaborates various 'factors' or incomplete oestrogens which, operating together, are responsible for the complete oestrous phenomenon. From this viewpoint the 'oestrous' picture is not a simple reaction to a single oestrogenic hormone, but is a composite of several partial oestrous responses induced by these factors. Since the uterine response during vaginal oestrus cannot be predicted, we have concluded that

Leucine-rich repeat-containing G-protein-coupled receptor 8 (LGR8, or RXFP2) is a member of the type C leucine-rich repeat-containing G protein-coupled receptor family, and its endogenous ligand is insulin-like peptide-3 (INSL3). Although LGR8 expression has been demonstrated in various human tissues, including testis, ovary, brain and kidney, the precise roles of this receptor in many of these tissues are unknown. In an effort to better understand INSL3–LGR8 systems in the rat, we cloned the full-length Lgr8 cDNA and investigated the presence and cellular localization of Lgr8 mRNA expression in adult and developing rat kidney. On the basis of these findings, we investigated the presence and distribution of renal ¹²⁵I-labelled human INSL3-binding sites and the nature of INSL3–LGR8 signalling in cultured renal cells. Thus, using in situ hybridization histochemistry, cells expressing Lgr8 mRNA were observed in glomeruli of renal cortex from adult rats and were tentatively identified as mesangial cells. Quantitative, real-time PCR analysis of the developmental profile of Lgr8 mRNA expression in kidney revealed highest relative levels at late stage gestation (embryonic day 18), with a sharp decrease after birth and lowest levels in the adult. During development, silver grains associated with Lgr8 mRNA hybridization were observed overlying putative mesangial cells in mature glomeruli, with little or no signal associated with less-mature glomeruli. In adult and developing kidney, specific ¹²⁵I-INSL3-binding sites were associated with glomeruli throughout the renal cortex. In primary cultures of glomerular cells, synthetic human INSL3 specifically and dose-dependently inhibited cell proliferation over a 48 h period, further suggesting the presence of functional LGR8 (receptors) on these cells (mesangial and others). These findings suggest INSL3–LGR8 signalling may be involved in the genesis and/or developmental maturation of renal glomeruli and possibly in regulating mesangial cell density in adult rat kidney.