Mechanisms in the Photoperiodic Control of Reproduction in the Stickleback

Abstract

In sticklebacks, sexual maturation is stimulated by long photoperiods but not by short photoperiods, even at high temperatures. Extra-retinal photoreception can mediate this response, and appears to be more important thanretinal photoreception. Although plasma melatonin levels are high at night and low during the day, experiments using melatonin administration via the water indicate that melatonin is of no or little importance for the photoperiodic response. Androgens can be aromatised to estrogens in the stickleback brain. Treatment with aromatase inhibitors stimulates maturation of males also under short photoperiod, suggesting that aromatase is involved in the suppressive actions of short photoperiod. Expression of both follicle stimulating hormone (FSH)-β and luteinizing hormone (LH)-β is higher under long than under short photoperiod. FSH-β is controlled by a negative steroid feedback on the brain-pituitary-gonad axis under short photoperiod and by a positive steroid feedback under long photoperiod. It is suggested that the former can suppress reproduction under short photoperiod and the latter can stimulate breeding under long photoperiod.