Effect of two weeks of continuous light on the development of the circadian melatonin rhythm in newborn lambs

Abstract

We have investigated the effect of exposure of newborn lambs to continuous light during the first 2 weeks after birth on the subsequent development of the phase and amplitude of the plasma melatonin rhythm during the next 8 weeks. During the first 2 weeks after birth, the mean plasma melatonin concentrations were significantly lower (P < 0.001) in lambs exposed to continuous light (58 +/- 15 pmol/l, n = 7) than in lambs exposed to LD 12:12 (i.e., "normal" lighting: 189 +/- 38 pmol/l, n = 9). Between 3 and 4 weeks of age, plasma melatonin concentrations during the daily light and dark periods were also significantly lower in the lambs exposed to continuous light during the first 2 weeks after birth. There was no effect of time of day on plasma melatonin in either group of lambs during the first 2 weeks after birth but between 3 and 10 weeks of age there was a significant (P < 0.001) effect of light and dark and time of day on plasma melatonin concentrations which was the same in both groups of lambs. Plasma melatonin concentrations were higher in the dark period than in the light period and between 2100 and 0700 than between 0900 and 1700 at all ages after week 2. We have therefore demonstrated that exposure of the lamb to 2 weeks of continuous light after birth results in a suppression of melatonin secretion that extends beyond the end of the exposure to continuous light.