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 mel atonin in the control of reproductive cycles. We have investigated ways of administering melatonin to immature chickens and laying hens to achieve phy siological levels and patterns in blood simulating either short or long pho toperiods. Melatonin was administered orally using different doses and vari ous ways of applying melatonin to the feed. For subcutaneous injections, me latonin was suspended in propylene glycol or grape seed oil. Melatonin alwa ys appeared in the first blood samples taken within an hour of administrati on. When melatonin was absorbed into feed pellets or whole wheat. a high in itial plasma concentration was reached, followed by a rapid decrease over t he ensuing 2-3 hr, but was still detectable as long as 24 hr after administ ration. For example, doses of 300 mu g/kg produced 15 nM, which is more tha n ten times higher than the nocturnal peak concentration. When melatonin wa s absorbed into cracked wheat grains that were subsequently washed with eth anol, the initial transitory peak was eliminated, levels in plasma were sus tained for at least 12 hr in the normal nocturnal range (750 pM), and no me latonin ( < 60 pM) was present 18 hr later. When injected (2 mu g/bird), co ncentrations peaked (610 pM) within 30 min and decreased rapidly over the n ext 2-3 hr. It was concluded that melatonin-treated, ethanol-rinsed cracked wheat grains can be used to experimentally mimic long-night plasma melaton in patterns. Injections mag be useful for mimicking the melatonin patterns of very short nights in chickens experiencing constant light.