Short-term variations of circulating melatonin in the ewe

Area of Science:

• Reproductive Endocrinology
• Chronobiology
• Animal Physiology

Background:

• Melatonin, a key hormone regulating circadian rhythms, is primarily produced by the pineal gland.
• Understanding melatonin secretion patterns is crucial for studying reproductive cycles and photoperiodism in animals.
• Previous studies have not fully elucidated the impact of sampling frequency and venous drainage on melatonin measurements.

Purpose of the Study:

• To investigate the effect of different blood sampling frequencies on melatonin levels in ewes.
• To assess potential stress responses (ACTH axis activation) due to frequent blood collection.
• To examine differences in melatonin levels between jugular veins, exploring potential vascular drainage effects.

Main Methods:

• Intact ewes were sampled at 0.5-, 2-, and 60-minute intervals during the dark phase.
• Blood samples were collected simultaneously from both jugular veins in one experiment.
• Plasma melatonin and cortisol levels were measured to analyze secretion patterns and stress indicators.

Main Results:

• Melatonin levels exhibited significant variation with intermittent peaks, reaching up to 365 pg/ml.
• The frequency of melatonin peaks increased proportionally with higher sampling frequencies.
• No correlation was found between plasma cortisol levels and melatonin peak frequency or amplitude.
• Higher melatonin levels were consistently observed in samples from the left jugular vein compared to the right.

Conclusions:

• Frequent blood sampling can reveal intermittent melatonin secretion patterns, influenced by sampling rate.
• The ACTH stress axis is not activated by frequent blood sampling in ewes under these conditions.
• Differential vascular drainage of the pineal gland to the jugular veins may explain observed asymmetry in melatonin levels.