A method of achieving physiological plasma levels of melatonin in the chicken by oral administration

In avian species it has been difficult to elucidate the precise role of melatonin in the control of reproductive cycles. We have investigated ways of administering melatonin to immature chickens and laying hens to achieve physiological levels and patterns in blood simulating either short or long photoperiods. Melatonin was administered orally using different doses and various ways of applying melatonin to the feed. For subcutaneous injections, melatonin was suspended in propylene glycol or grape seed oil. Melatonin always appeared in the first blood samples taken within an hour of administration. When melatonin was absorbed into feed pellets or whole wheat, a high initial plasma concentration was reached, followed by a rapid decrease over the ensuing 2-3 hr, but was still detectable as long as 24 hr after administration. For example, doses of 300 microg/kg produced 15 nM, which is more than ten times higher than the nocturnal peak concentration. When melatonin was absorbed into cracked wheat grains that were subsequently washed with ethanol, the initial transitory peak was eliminated, levels in plasma were sustained for at least 12 hr in the normal nocturnal range (750 pM), and no melatonin (< 60 pM) was present 18 hr later. When injected (2 microg/bird), concentrations peaked (610 pM) within 30 min and decreased rapidly over the next 2-3 hr. It was concluded that melatonin-treated, ethanol-rinsed cracked wheat grains can be used to experimentally mimic long-night plasma melatonin patterns. Injections may be useful for mimicking the melatonin patterns of very short nights in chickens experiencing constant light.