Melatonin treatment during the breeding season increases testosterone in male green anole lizards (Anolis carolinensis)

Melatonin is a natural hormone that regulates seasonal behaviors in vertebrates by binding to its receptors (MT1 and MT2). Specifically, high levels of melatonin are associated with short photoperiods, often coinciding with the non-breeding season, meaning that melatonin may inhibit seasonal reproduction. Green anole lizards (Anolis carolinensis), have large, active gonads, increased levels of testosterone and estradiol, and increased reproductive behaviors during the breeding season. Previous studies have examined the role of melatonin in seasonal reproduction in this species, but it is unclear how melatonin receptors change seasonally or if melatonin treatment during the early breeding season influences reproduction. In Experiment 1, we measured MT1 and MT2 mRNA expression in the brains and gonads of unmanipulated anoles between breeding and non-breeding seasons. MT1 mRNA expression was significantly higher in the male brain during the breeding season compared to the non-breeding season, and MT1 mRNA levels were generally higher compared to MT2. This suggests that melatonin may regulate seasonal reproduction through MT1 in the brain, and higher levels during the breeding season may compensate for low seasonal levels of melatonin. In Experiment 2, anoles were treated with melatonin or a blank control for 10 weeks during the breeding season. In males, melatonin treatment increased testosterone levels. This suggests that rather than inhibiting reproduction, continuous high doses of melatonin may increase reproductive hormones during the breeding season. Our findings support the role of melatonin in modulating seasonal reproduction, but the exact mechanisms behind melatonin's stimulatory effect is unclear.