Impact of day length on orexin A levels in blood, cerebrospinal fluid and various brain tissues in seasonally breeding sheep

Orexin A (OXA) is a key regulator of the sleep-wake cycle and energy homeostasis in mammals. In rodents, OXA synthesis follows a circadian rhythm positively associated with wakefulness. While similar research is lacking in seasonal animals, we previously demonstrated that OXA mediates photoperiodic effects on reproduction and metabolism in seasonally breeding sheep. This study investigated whether circadian and seasonal rhythms in OXA secretion occur in sheep and how they correspond to tissue-specific concentrations under different photoperiods. Ten adult ewes were kept under natural conditions during long-day (LD; May, nonbreeding season; ∼16 h daylight; n = 5) and short-day (SD; November, breeding season; ∼9 h daylight; n = 5) periods. The experiments were conducted separately for each season to distinguish photoperiodic effects. Plasma was collected over 24 hours; cerebrospinal fluid (CSF) and brain tissues including the choroid plexus (CP), pineal gland (PG), pituitary gland (PTG), and CA1 region of the hippocampus were collected at midday, when OXA peaks in diurnal species. OXA levels were significantly higher (P < 0.01) in LD compared to SD in the CP (95.83 %), CSF (140.00 %), CA1 (66.70 %), and plasma (38.46 %). Plasma OXA displayed circadian variation in both seasons; daytime levels were 41.38 % higher during SD (P < 0.01) and 17.28 % higher in LD (P > 0.05). No seasonal differences were found in PTG or PG. This study may reveal the role of OXA in mediating or modifying the effects of day length on seasonal physiological processes in sheep. For example, elevated OXA in spring and summer, when forage is abundant, may support energy demands for lactation and reserve accumulation in preparation for winter forage scarcity.