Influence of light environment and photoperiod on plasma melatonin and cortisol profiles in young domestic boars, comparing two commercial melatonin assays

Eighteen crossbred boars, three siblings from each of six litters, were dis tributed randomly among three groups after weaning. The 'Natural long-day'' group was housed in a standard room with windows, whereas the 'Artificial long-day' and 'Artificial short-day' groups were housed in light-sealed roo ms and under an artificial light regimen (1400 1x). In spring (16-17 hr of light) plasma levels of melatonin and cortisol were measured in samples col lected hourly for 24 hr. Two commercial melatonin radioimmunoassays with pr eassay diethyl ether extraction were compared. Only the assay from Buhlmann Laboratories AG showed low to undetectable melatonin levels during the lig ht-phase and was used for further analysis. Dark-phase melatonin concentrat ions were higher than light-phase melatonin concentrations (P < 0.001). Dar k-phase melatonin concentrations were higher in the 'Natural long-day' grou p than in the 'Artificial long-day' and the 'Artificial short-day' groups ( P < 0.001). Sire had a significant effect on dark-phase melatonin concentra tions (P < 0.01), but not on light-phase levels. Cortisol concentrations we re higher during the light-phase than during the dark phase, and light-phas e cortisol concentrations were higher in the 'Natural long-day' group than hi the 'Artificial long-day' and the 'Artificial short-day' groups (P < 0.0 1). This study showed that peripubertal boars express a typical circadian m elatonin rhythm under both artificial light regimens and in standard pig st able environment. Natural photoperiod and indoor lighting seem to interact in shaping the melatonin profile in standard stable environment. The great individual variation in the amplitude of the dark-phase melatonin levels co uld in this study be explained by the different sires. (C) 2000 Elsevier Sc ience Inc. All rights reserved.