The characteristics of the melatonin secretory rhythm are not modified by the stage of pregnancy in ewes

Abstract

An experiment was designed to study if, in the same animals, characteristics of the plasma melatonin rhythm vary during pregnancy in ewes. Thirteen Ile-de-France ewes were maintained in natural photoperiod and measurements of the characteristics of the rhythm of melatonin secretion were determined during one estrous cycle and then during pregnancy. Duration and mean plasma concentrations of the nocturnal melatonin elevation were measured at four stages of pregnancy. Melatonin concentrations during the elevation were not significantly affected by the stage of the estrous cycle (mean +/- SEM: 227.8 +/- 34.9, 263.2 +/- 24.2 and 232.4 +/- 27.1 pg/mL for follicular phase, early luteal phase and late luteal phase, respectively), or by the stage of pregnancy (292.8 +/- 22.2, 268.7 +/- 21.7, 267.4 +/- 27.9 and 258.3 +/- 23.5 pg/mL for first, second, third and fourth month of pregnancy, respectively). A strong individual effect was detected (P < 0.01) for melatonin concentrations during elevation (range: 119.2 +/- 11.8 to 396.6 +/- 26.0 pg/mL of plasma). A highly significant correlation coefficient was observed within individuals for night melatonin concentrations between the different physiological stages. It was concluded that melatonin secretion is unaffected by the stage of pregnancy.