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MILK TRANSPORT OF [3H]MELATONIN TO SUCKLING RATS

Milk transport of melatonin (MEL), the putative antigonadotropic pineal hormone, was investigated to determine if milk could provide suckling rats with an exogenous source of MEL. First, we established that [3H]MEL in the maternal circulation was rapidly transferred into lactating mammary tissue but not stored there in significant amounts. In the second phase of study we found that milk in the stomachs of rats suckling these mothers contained [3H]MEL. To study the fate and tissue distribution of [3H]MEL from the stomach, suckling rats were gavage fed 0.2 nmol of [3H]MEL; 15 and 60 min later [3H]MEL was recovered from 7 tissues and plasma. GI absorption of [3H]MEL produced sustained plasma (1 nM) and tissue concentrations. Brain contained 0.3% of the [3H]MEL administered; liver and kidney contained 1.0 and 0.2% respectively. The tissue distribution of [3H]MEL after gavage was similar to that found in adult rats after injections. Finally, we found that in vitro [3H]MEL metabolism to water-soluble products could not be detected in lactating mammary tissue, was very low in neonatal stomach, and highest in neonatal liver (5 nmol/g/hr). Our finding that milk can transport MEL to suckling rats is important because endogenous MEL production does not occur in the rat prior to the second week of life. Thus, maternal MEL transported via milk could influence reproductive physiology in the neonate, perhaps through the inhibitory effects of MEL on LHRH-regulated secretion of LH by the neonatal rat pituitary gland (Science 191; 301, 1976).