Determination of Rhythmicity and Gestational Stage-Related Distribution of Blood Plasma Melatonin Concentrations in Donkey Mares.

Simple Summary: The domestic donkey (Equus asinus), like the horse (Equus caballus), is a short-day, seasonally polyestrous animal with an endogenous circannual reproduction rhythm. The main seasonal changes (winter–summer) significantly influence the development of the internal rhythm, which is directly controlled by melatonin. However, very little is known about the development of melatonin concentrations in donkey blood plasma. This study confirmed the circannual and circadian rhythms of the domestic donkey. It can be established that the lowest blood plasma melatonin concentration is typical in the Northern Hemisphere during the summer solstice. The significantly strongest melatonin production can be observed during the night of the winter solstice. The blood plasma melatonin concentration does not change as pregnancy progresses. The aim of this study is to obtain a more complete picture of blood plasma melatonin concentrations in the donkey mares. To this purpose, sampling and statistical processing were carried out in such a way that allowed the researchers to establish the annual and daily rhythms. Based on human observations, according to the hypothesis of the authors, the blood plasma melatonin concentration of pregnant individuals rises during the late gestational period, before parturition. To confirm this, the melatonin concentrations of pregnant and non-pregnant jennies were monitored and compared. In regard to the circannual rhythm, the significantly lowest midnight melatonin value (27.67 pg mL^–1) was typical for the summer solstice. Under consideration of circadian changes, a significantly strongest melatonin production (45.16 pg mL^–1) was observed on the night of the winter solstice (p < 0.001). Considering gestational age, the blood plasma melatonin concentration (around 38 pg mL^–1) does not change as gestation progresses (p = 0.136). The results obtained in this studied population of the domestic ass usefully expand the little knowledge previously gathered about the development of the blood plasma melatonin concentrations of this species. [ABSTRACT FROM AUTHOR]