The role of the opioid peptide dynorphin during the seasonal and gonadotropin-induced ovarian recrudescence in the gecko

The aim of the current investigation was to elucidate the influence of the opioid peptide dynorphin-A (DYN) on the reproductive axis during breeding and non-breeding phases of the ovarian cycle in the gecko Hemidactylus frenatus. During the recrudescence phase, administration of a high dose of DYN (10 µg/0.1 ml saline) caused a significant reduction in the numbers of oogonia and primary oocytes in the germinal bed, compared to those of the initial controls or experimental controls. Administration of a low (2 µg DYN/0.1 ml saline) or high dose of DYN did not affect the follicular development up to stage IV, but there were no stage V (vitellogenic) follicles in the ovary in contrast to their presence in the experimental controls. Furthermore, there was a significant reduction in gonadotropin-releasing hormone-immunoreactive (GnRH-ir) content in the median eminence (ME) and pars distalis of the pituitary gland (PD) in low or high doses of DYN-treated lizards. During the regression phase, treatment with follicle-stimulating hormone (FSH) resulted in the appearance of stage IV and V follicles, in contrast to their absence in initial controls and treatment controls. However, treatment with 10 µg DYN + FSH did not promote the development of these follicles. In addition, in vitro treatment of DYN significantly inhibited ovarian levels of estradiol. Collectively, these findings reveal an inhibitory influence of DYN on the seasonal ovarian recrudescence, possibly mediated through the suppression of GnRH release into the ME and PD and directly at the level of the ovary by impairment in steroidogenesis and vitellogenic follicular growth in the gecko.