Twenty-four-hour rhythm patterns of plasma melatonin in short-day and long-day breeders maintained under natural environmental conditions

Photoperiodic treatments have been of practical interest in controlling seasonal reproduction in sheep, goats and horses. Melatonin is the principal mediator of the environmental photoperiodic message. To investigate the intra- and inter-subject variability of melatonin 24 h rhythm, ten female Italian Saddle horses (8-10 yrs old, mean body weight 525 +/- 30 kg), ten female Sarda breed sheep (2-3 yrs old, mean body weight 40.5 +/- 2.8 kg) and ten female Sarda breed goats (3-4 yrs old, mean body weight 38.9 +/- 4.1 kg), housed individually in a 4 x 4 m soundproof box equipped with 50 x 100 cm opening windows, were subjected to a natural photoperiod of the vernal equinox (sunrise 06:00 h; sunset 18:00 h). Blood samples were collected from each animal, every 3 h over a 48 h period starting at 00:00 h of day 1 and ending at 00:00 h of day 3. Plasma melatonin concentrations were determined by direct radioimmunoassay (MelatoninDirect RIA, Labor Diagnostika Nord GmbH, Nordhorn, Germany). The application of single cosinor method substantiated a circadian rhythm of melatonin with a nocturnal peak in all studied species. The application of two-way ANOVA on the rhythmic parameters indicated statistically significant differences between the three species in all of the cosinor analysis-derived parameters of MESOR, amplitude, acrophase and robustness of rhythm. Analyses of intra- and inter-subject variability indicate that organization of the melatonin 24 h rhythm is characterized by great accuracy of control within and between the individuals of a breed. In conclusion, features of the 24 h rhythm of melatonin among species; however, the 24 h rhythmicity of melatonin each species showed high stability within the various subjects and within the same subject. These findings must be taken into consideration when applying photoperiod and melatonin treatments for breeding purposes.