PINEAL AND THYROID FUNCTIONS IN NEWBORN SEALS

Daily variations of pineal and plasma melatonin and plasma thyroid hor mones were measured in harp seals (Phoca groenlandica), grey seals (Ha lichoerus grypus), and hooded seals (Cystophora cristata), ranging in age from newborn to 14 days. In newborn harp seals the mean mass of th e pineal gland was 273 mg (+/-45 SEM, n = 11), containing 49 ng (media n) melatonin. In newborn, 4- and 10-day-old grey seals, the pineal mas s was similar, weighing on average 337 mg (+/-74, n = 6) and containin g 90 ng melatonin. Two newborn hooded seal pups had pineals weighing 5 20 and 1289 mg, with 254 and 7600 ng melatonin, respectively. There we re no day-night differences in the pineal contents of melatonin or in the number of pineal beta-adrenergic receptors measured in newborn har p seals, and, in newborn, 4- and 10-day-old grey seals, there were no day-night or age differences in pineal melatonin content. Plasma melat onin levels were 10 times higher in newborn seals than in two 10-day-o ld grey seals and one 14-day-old harp seal pup. In ah seal pups, the l evels exhibited a 24-hr rhythmicity, with increasing night- and decrea sing daytime concentrations. Plasma levels of thyroxine (T-4) and trii odothyronine (T-3) were generally higher in newborn seals than in 10-a nd 14-day-old seals or in adult females. There was no apparent 24-hr r hythmicity, but the thyroid hormone levels generally declined througho ut each sampling sequence. High pineal and thyroid activities may play a thermoregulatory role in newborn seals, but the results do not indi cate a stimulatory action of melatonin in the peripheral conversion of T-4 to T-3. It is speculated that the large and active pineal gland, particularly in newborn seals, may be related to aspects of their divi ng habit. (C) 1995 Academic Press, Inc.