Oral administration of melatonin to old ring doves (Streptopelia risoria) increases plasma levels of melatonin and heterophil phagocytic activity

We analyzed the effect of oral melatonin (23 μg/0.1 ml/animal/d; 1 h before dark on 12 consecutive days) in old birds, in natural photoperiods, on the hormone's plasma levels, and phagocytosis. Blood collections were performed daily at 2:00 AM and 4:00 PM until 5 days after the treatment. From day 1, the melatonin levels were significantly higher than basal levels at both times. Values at 2:00 AM were significantly higher than the 4:00 PM values. After treatment, the melatonin levels declined, returning from day 14 to basal values at both hours. At 2:00 AM. phagocytosis was significantly greater than that obtained at 4:00 PM and greater than basal values. The 4:00 PM values were only significantly greater than basal on days 6 and 8, parallel to a decline in Superoxide anion levels, which were lowest at 2:00 AM. Melatonin administered to old ring doves increases the differences between nocturnal and diurnal plasma levels, and, in parallel, increases phagocytosis and reduces Superoxide radical levels in heterophils.