Artifical light at night triggers slight transcriptomic effects on melatonin signaling but not synthesis in tadpoles of two anuran species.

The worldwide expansion of artificial light at night (ALAN) is acknowledged as a threat to biodiversity through alterations of the natural photoperiod triggering the disruption of physiological functions. In vertebrates, melatonin production during the dark phase can be decreased or suppressed by nocturnal light as shown in many taxa. But the effect of ALAN at low intensity mimicking light pollution in peri-urban area has never been investigated in amphibians. We filled this gap by studying the impact of low ALAN levels on the expression of genes related to melatonin synthesis and signaling in two anurans (agile frog, Rana dalmatina, and common toad, Bufo bufo). Circadian expression of genes encoding enzymes catalyzing melatonin synthesis (aralkylamine N-acetyltransferase, AANAT and acetylserotonin O-methyltransferase, ASMT) or melatonin receptors (Mel1a, Mel1b and Mel1c) was investigated using RT-qPCR after 23 days of nocturnal exposure to control (< 0.01 lx) or low ALAN (3 lx). We showed that the relative abundance of most transcripts was low in late afternoon and early evening (06 pm and 08 pm) and increased throughout the night in R. dalmatina. However, a clear and ample nocturnal pattern of target gene expression was not detected in control tadpoles of both species. Surprisingly, a low ALAN level had little influence on the relative expression of most melatonin-related genes. Only Mel1c expression in R. dalmatina and Mel1b expression in B. bufo were affected by ALAN. This target gene approach provides experimental evidence that melatonin signaling pathway was slightly affected by low ALAN level in anuran tadpoles.