Chronobiology internationalJul 24, 2015

Melatonin changes daily movement patterns under constant light depending on early-life lighting conditions

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Mar 3, 2026

Abstract

Rats exposed to constant light during early development showed a more stable circadian rhythm compared to those raised in a light-dark cycle.

Prolonged exposure to constant light is associated with changes in circadian behavior in rats.
Melatonin treatment resulted in heightened reactivity in rats raised in a light-dark cycle compared to those raised in constant light.
Melatonin was able to shift the phase of the circadian rhythm in rats raised in a light-dark cycle.
No significant differences were observed in nitrite levels or catalase activity between the different groups, although both increased over time.
Depressive signs, indicated by reduced sucrose consumption, were observed in males raised in a light-dark cycle treated with melatonin.

Simplified

Early lighting conditions have been described to produce long-term effects on circadian behavior, which may also influence the response to agents acting on the circadian system. It has been suggested that melatonin (MEL) may act on the circadian pacemaker and as a scavenger of reactive oxygen and nitrogen species. Here, we studied the oxidative and behavioral changes caused by prolonged exposure to constant light (LL) in groups of rats that differed in MEL administration and in lighting conditions during suckling. The rats were exposed to either a light-dark cycle (LD) or LL. At 40 days old, rats were treated for 2 weeks with a daily subcutaneous injection of MEL (10 mg/kg body weight) or a vehicle at activity onset. Blood samples were taken before and after treatment, to determine catalase (CAT) activity and nitrite level in plasma. As expected, LL-reared rats showed a more stable motor activity circadian rhythm than LD rats. MEL treatment produced more reactivity in LD- than in LL rats, and was also able to alter the phase of the rhythm in LD rats. There were no significant differences in nitrite levels or CAT activity between the groups, although both variables increased with time. Finally, we also tested depressive signs by means of sucrose consumption, and anhedonia was found in LD males treated with MEL. The results suggest that the lighting conditions in early infancy are important for the long-term functionality of the circadian system, including rhythm manifestation, responses to MEL and mood alterations.

Melatonin changes daily movement patterns under constant light depending on early-life lighting conditions

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