Light/darkness modulation of the hypothalamic-pituitary-ovarian axis in the plains vizcacha, Lagostomus maximus, a seasonal breeding species

Photoperiod is the main environmental signal that affects animal behavior and reproduction. Light stimulus is traduced by a neural pathway that modulates pineal gland melatonin release, which synchronizes physiologic functions with day duration, highly influencing seasonal reproduction. The plains vizcacha (Lagostomus maximus) is a Hystricomorph rodent with seasonal reproduction that inhabits the Neotropic in South America. The aim of this work was to elucidate the effect of light/darkness exposition on the reproductive hypothalamic-pituitary-ovarian (HPO) axis in the female plains vizcacha. During 15 days, animals were subjected to different light/darkness regimens (Control group, CTL: 12:12 h dark:light; Darkness group, DARK: continuous darkness; Light group, LIGHT: continuous light). The melatoninergic system and reproductive hormones were evaluated. Plasma melatonin levels significantly decreased in DARK whereas both melatonin receptors (MT₁ and MT₂) expression significantly increased in the hypothalamus and decreased in the pituitary gland, and only MT₁ expression increased in the ovaries. Continuous light did not induce significant variations in melatonin levels related to CTL, however, MTs expression changed at pituitary and ovary levels. Strikingly, both light/darkness regimens increased reproductive hormone expression. While darkness induced hypothalamic gonadotropin-releasing hormone (GnRH) expression and estradiol (E₂) secretion, light increased LH and progesterone (P₄) secretion. In conclusion, light availability may impact the reproductive axis of plains vizcacha inducing hormonal changes, with an organ-specific response, and sustaining HPO axis activity, thus ensuring reproduction. Environmental light and darkness, their availability and exposure length, could synchronize the reproductive axis in seasonal breeding species like the plains vizcacha. New & Noteworthy: Hypothalamic, pituitary, and ovarian variations were induced by continuous light or darkness in the plains vizcacha. Plasma melatonin decreased by continuous darkness-inducing hypothalamic, pituitary, and ovarian melatonin receptors variations. Fifteen days of continuous darkness induced GnRH, LH, and estradiol secretion, while 15 days of continuous light induced LH and P₄ secretion. Environmental light/darkness would synchronize the reproductive axis in seasonal breeding species like the plains vizcacha.