Counteracting the effects of artificial light at night: The role of melatonin in brain plasticity and nighttime activity in a songbird.

Melatonin is important for synchronizing biological rhythms and regulating behavioral, physiological, and neurobiological circadian processes. Therefore, it is a good candidate for investigating the consequences of artificial light at night (ALAN), which is a major global pollutant and reduces nighttime melatonin levels. The negative effects of ALAN on health and biodiversity are thoroughly studied, however less is known about its effects on adult brain plasticity (ABP). This study investigated the role of melatonin in mediating the effects of ALAN on ABP and behavior in zebra finches (Taeniopygia guttata). Birds were exposed to control (dark nights) or ALAN (5 lx) conditions and treated with melatonin via a skin cream. We measured nighttime locomotor activity (NMA), plasma melatonin levels, and ABP parameters: cell proliferation in the ventricular zone, neuronal recruitment, apoptosis, and total neuronal densities in three forebrain regions: nidopallium caudale (NC), medial striatum (MSt), and hippocampus (HC). ALAN alone reduced nighttime melatonin, increased NMA, and increased cell proliferation, neuronal recruitment, and apoptosis in the MSt and NC but not the HC, which appeared more resilient. However, when melatonin treatments were combined with ALAN, melatonin generally counteracted the effects of ALAN on NMA and ABP, except for the HC, which remained more resilient even under the combined treatments. Our findings suggest that melatonin can counteract ALAN-induced changes in ABP and behavior. Further research (e.g. with wild species and lifelong ALAN-exposed birds) is needed to evaluate these results in an ecological context, which can be used for nature conservation efforts.