ICV melatonin reduces acute stress responses in neonatal chicks

Melatonin is involved in a variety of biological functions including sleep and stress. Our previous study indicated that neonatal layer chicks were more susceptible to stress than broilers. However, it is not clear whether differences exist in melatonin concentrations between both types of chickens, nor is it known whether melatonin is directly involved in stress in neonatal chickens. In the present study we first compared melatonin concentrations in brain tissues (pineal gland, brain stem, telencephalon, and optic lobe) between neonatal broiler and layer chicks raised under either 12 h light:12 h dark cycle (lights on at 07:00 h) or continuous illumination. Although melatonin concentrations were much higher in broilers than layers at night under the alternative light-dark cycle, these differences disappeared under the 24 h illumination. We thus chose neonatal layers for a test system. We then investigated if intracerebroventricular (ICV) injection of melatonin modulated plasma corticosterone concentrations under continuous illumination. Neonatal layer chicks housed in groups were ICV injected (1) with melatonin (0, 0.116 and 1.16 μg) or with nothing as an intact control followed by isolation in an open-field environment for 10 min; and (2) were given one of the followings treatments: nothing (intact control), control (0 μg), corticotropin-releasing factor (CRF) (0.01 μg), melatonin (1.16 μg), or CRF (0.01 μg) + melatonin (1.16 μg). Ten minutes thereafter blood was collected via heart puncture to determine plasma corticosterone content. Isolation resulted in a significant increase in corticosterone concentration, and both doses of ICV melatonin completely suppressed this increase (P < 0.01). CRF injection resulted in a strong increase in plasma corticosterone concentrations (P < 0.01). Co-injection with melatonin attenuated the CRF-induced corticosterone elevation in plasma (P < 0.01). Our findings provide direct evidence that melatonin modulates the activity of the hypothalamo-pituitary-adrenal axis in chicks.