Inhibition of steroidogenesis in frog testis "in vitro" by synthetic seminal plasma peptide

Seminal plasma peptide (pGlu-Val-Ala-Asp-Ser-Asp-Gln-Asn), synthesized following data obtained from biochemical and mass spectrometry analysis of a highly purified fraction from bovine seminal plasma, was characterized at paracrine level as an “in vitro” modulator of testosterone production in rat Leydig cells. 10-8 M synthetic peptide exerted the highest testosterone inhibition in rat Leydig cells stimulated with 50 mU of LH. The basal testosterone production (without LH stimulation) was not influenced by synthetic seminal plasma peptide. In Rana esculenta, spermatogenesis is well characterized at morphological, endocrine and paracrine levels. Spermatogonial multiplication occurs in late winter-early spring, while spermatocytes (I and II) and spermatids are rare or totally absent. Progression of spermatogenesis lasts until autumn with spermatids and appearance of spermatozoa. After the winter stasis, in the germinal compartment of the testis, only spermatogonia and spermatozoa are observed, due to the degeneration of the other stages. The aim of our research was to study the influence of synthetic seminal plasma peptide in amphibian testicular steroidogenesis during the annual reproductive cycle of the male frog, Rana esculenta. Male frogs were captured at different times throughout the year and testes were removed, placed in DMEM in presence of 10 mM Hepes, 0.1 U/ml penicillin G and 0.1 mg/ml streptomycin and incubated for 6 h at 25 °C with increasing doses of synthetic peptide. Testosterone levels in the incubation medium were measured by radioimmunoassay. Preliminary results showed inhibition of testosterone production by frog testis with the highest value (42%) in summer at a peptide concentration of 10-6 M, suggesting the influence of synthetic seminal plasma peptide in spermatocyte differentiation.