Melatonin modulates corticosterone-induced effects on steroidogenesis and reproductive-linked processes in male tree sparrow (passer montanus).

In birds, stressors increase corticosterone, a glucocorticoid, causing physiological, reproductive, and behavioural changes. Besides, melatonin, a neurohormone produced by the pineal gland, regulates circadian rhythms and seasonal reproduction and exhibits antioxidant properties. In the present study, photorefractory adult male tree sparrows were used. Body mass, bill colour, and testicular volume were recorded. Later, birds (n = 6/group) were divided into three groups and were treated for 4 weeks under 8L:16D light conditions: Group 1 received ethanolic saline (control), group 2 received corticosterone (2 µg/100 µl), and group 3 received corticosterone (2 µg/100 µl) plus melatonin (10 µg/100 µl), with melatonin administered 30 min before lights-off. After 4 weeks of treatment, the birds were subsequently shifted to 16L:8D for 30 days. At the experiment's end, samples were collected mid-light phase, and brain and testis tissues were stored at -80 °C for gene expression analysis. The study reveals no variation in the body mass and bill colour; however, testicular volume significantly increased across groups. Corticosterone treatment downregulated hypothalamic reproductive transcripts in tree sparrows, while the corticosterone plus melatonin and control groups showed upregulation of these genes. Furthermore, steroidogenic genes were downregulated in the testis of the corticosterone group compared with the other groups. Similarly, epigenetic genes (Hdac1, Hdac3, and Dnmt1) were upregulated in the hypothalamus in the corticosterone group; however, no changes were observed in the testis. These findings indicate corticosterone negatively impacts reproductive and steroidogenic activity in tree sparrows, but melatonin mitigates these effects, enhancing reproductive-linked steroidogenesis. This suggests a protective role of melatonin in counteracting corticosterone-induced reproductive suppression in this species.