OCCURRENCE OF AN OVARIAN OPIOID SYSTEM IN OVIPAROUS VERTEBRATES: PROOPIOMELANOCORTIN mRNA EXPRESSION IN THE OVARY OF THE GREEN WATER FROG, RANA ESCULENTA by M. NABISSI1, A.M. POLZONETTI1*, O. CARNEVALI1, I. LIHRMANN2, F. CARTIER2 and H. VAUDRY2 (1Dipartimento di Biologia (MCA), Università di Camerino, via F. Camerini 2, 62032 Camerino (MC), Italy; 2 European Institute for PeptideResearch, Laboratory of Cellular and Molecular Neuroendocrinology, INSERM U413d, UA CNRS, University of Rouen, 76821 Mont-Saint- Aignan,France)) ABSTRACT The aim of the present study was to investigate whether the proopiomelanocortin (POMC) gene is expressed in the ovary of Ranaesculenta. Using a homologous probe, the presence of POMC mRNA could not be detected by Northern blot analysis in the ovary, while a positive signal was found in the pituitary gland. In contrast, after amplification of ovarian mRNA by the PCR technique, an intense signal of 480 bp was detected by Southern blotting, indicating that POMC is actually synthesized in the ovary. KEY WORDS: opioid peptides, ovarian function, POMC gene expression. POMC is a multifunctional precursor protein which generates several peptide hormones and neuropeptides including melanotropins and endorphins. In mammals, the expression of the POMC gene has been demonstrated not only in the pituitary gland but also in several periph- eral organs including the ovary (FACCHINETTI et al., 1995) and testis JEANNOTTF: et al., 1987). in the amphibians Xenopus laevis (MARTENS, 1986) and Rana ridibunda (HILARIO et al., 1990), the gene encoding POMC is actively expressed in melanotrope cells of the pars inter- media and corticotropc cells of the pars distalis. Concurrently, in the frog R. esculenta, processing products of POMC have also been identi- fied in non-pituitary tissues including brain and gonads (BENYAMINA et al., 1986; CARNEVALI et al., 1993; FACCHINETTI et al., 1992). In particu- lar, POMC-derived peptides have been localized by immu- nocytochemistry and characterized by chromatographic procedures in the frog ovary (GARNEVALC et al., 1993). Seasonal changes in POMC- derived peptides during the different phases of the reproductive cycle occur in the ovary of amphibians (Facchinetti et al., 1992) and reptiles (POI,ZONETTI et al., 1994a,b) suggesting that some of these peptides are