Features of melatonin catabolism in chicks

Melatonin is a neurohormone that is involved in biorhythm synchronization that is produced by the pineal gland and retina. We have studied the production of the kynurenine products, N-acetyl-N-formyl-5-methoxykynurenamine (AFMK) and N-acetyl-5-methoxykynurenamine (AMK), during catabolism of melatonin in chicks (Gallus gallus). Analyses of blood serum samples, retinas, and pineal glands were performed using the HPLC method with fluorometric and electrochemical detection. Considerable amounts of melatonin were found in the pineal gland, retina, and blood serum, although endogenous AFMK and AMK were not found in these tissues. Nevertheless, after intraperitoneal injection of melatonin at a dose of 10 mg/kg, these compounds were found in the blood serum, and AMK was found in the retina. Analysis of AFMK and AMK pharmacodynamics in the blood serum showed that their peak concentrations were 4.8 ng/ml and 2 ng/ml, respectively; the peaks were observed within less than 5 minutes after melatonin injection for AFMK and in 20 minutes after melatonin injection for AMK. The amount of AMK in the retina also reached its peak (60 pg/retina) in 20 minutes. The concentration of both metabolites decreased to the detection limit within one hour. Thus, melatonin may be oxidized via the kynurenine pathway, and its oxidation products, AFMK and AMK, have short half lives. The total amount of both metabolites after melatonin injection was approximately 0.04% of its amount in the blood serum and retina, thus, this pathway of melatonin oxidation is not the major one.