Seasonal Expression of NGF and Its Cognate Receptors in the Ovaries of Grey Squirrels (Sciurus carolinensis)

SIMPLE SUMMARY: Invasive alien species pose a significant threat to biodiversity, as once they have adapted to their new environment, they cause the reduction and even extinction of native species. In this framework, the American grey squirrel (Sciurus carolinensis) poses a serious threat to the European red species squirrel (Sciurus vulgaris), especially in the Umbria region of Italy. In fact, an invasive grey squirrel population has adapted well to the Umbrian territory, showing high reproductive success. In addition to its role in the development of the vertebrate nervous system, nerve growth factor (NGF) has recently been found to play an important role in reproduction. In order to investigate the reproductive physiology of female grey squirrels, the ovarian presence, distribution, and gene expression of NGF and its cognate receptors were evaluated during both breeding and nonbreeding seasons. The presence and gene expression of this system at the ovarian level, mainly during the breeding season, confirm the possible involvement of NGF and its receptors in the gonadal activity of this invasive grey squirrel population. ABSTRACT: The grey squirrel is an invasive alien species that seriously threatens the conservation of the native red squirrel species. With the aim of characterizing the reproductive physiology of this species due to its great reproductive success, the function of the ovarian nerve growth factor (NGF) system was analyzed in a grey squirrel population living in central Italy. During the breeding and nonbreeding seasons, the ovarian presence, distribution, and gene expression of NGF, neurotrophic tyrosine kinase receptor 1 (NTRK1), and nerve growth factor receptor (NGFR), as well as NGF plasma concentrations, were evaluated in female grey squirrels. NGF was found in the luteal cells and in the thecal and granulosa cells of follicles, while NTRK1 and NGFR were only observed in follicular thecal and granulosa cells. NGF and NGFR transcripts were almost two-fold greater during the breeding season, while no seasonal differences were observed in NTRK1 gene expression. During the breeding season, NGFR was more expressed than NTRK1. Moreover, no changes were observed in NGF plasma levels during the reproductive cycle. The NGF system seems to be involved in regulating the ovarian cycle mainly via local modulation of NGF/NGFR, thus playing a role in the reproductive physiology of this grey squirrel population.