The Role of Melatonin on Cortisol Level and Anxiety-like Behaviour Induced by Light at Night in Sprague Dawley Rats

Abstract

Circadian rhythm disruption due to artificial light at night (LAN) is increasingly recognized as a factor for neuroendocrine and behavioral disturbance. This study investigated the effects of varying nocturnal light intensities on plasma cortisol levels and anxiety-like behavior in male Sprague Dawley rats and evaluated the potential neuroprotective role of melatonin. Twenty-five rats were divided into five groups: Control (dark, 0 lux), Dim light, Dim light with melatonin (DLAN + Mel), Continuous Light Exposure (CLE), and CLE with melatonin (CLE + Mel). Melatonin was administration orally at 10 mg/kg body weight daily for five weeks. Cortisol levels were measured using ELISA, and anxiety-like behavior was assessed via the Open Field Test (OFT). Exposure to dim and continuous light at night elevated plasma cortisol concentrations compared to the control, with the CLE group showing the highest levels. Melatonin administration significantly reduced cortisol levels in the CLE + Mel group (p < 0.05) relative to CLE alone. Behavioral analysis showed that light-exposed rats exhibited increased anxiety-like behaviors, indicated by reduced center entries and center time. Melatonin treatment improved exploratory behavior and decreased anxiety-like responses. These findings suggest that higher-intensity nocturnal light exposure exacerbates stress hormone secretion and anxiety, while melatonin mitigates these adverse effects by modulating HPA axis activity and stabilizing circadian rhythm.