Circadian rhythms of serum melatonin and prolactin in the Rhesus monkey (Macaca mulatta)

The object of this study was to determine the normal circadian rhythm of serum melatonin and prolactin in mature female Rhesus monkeys. Animals were housed in a light: dark cycle of 14:10 (lights on at 0600) and bled over a 28 hr period at 0800, 1200, 1600, 2200, 0200, 0400, 0800 and 1200. Serum concentrations of melatonin and prolactin were determined by radioimmunoassay. The mean nighttime level of melatonin was three times higher than the mean daytime level of melatonin (P< 0.01). Only three of the eight monkeys, however, showed a distinctly nocturnal melatonin rise. Mean nighttime levels of prolactin were three times higher than mean daytime levels of prolactin (P < 0.01). Each monkey presented a distinct circadian rhythm of prolactin secretion. These data indicate that the Rhesus monkey has circadian rhythms of melatonin and prolactin secretion typical of other mammalian species. It is concluded that additional studies testing for possible interactions between melatonin and prolactin in the Rhesus monkey are warranted.