Synchronization of circadian rhythms of house sparrows by oral melatonin: Effects of changing period

House sparrows (Passer domesticus) whose circadian rhythms of locomotor activity and feeding had been abolished by pinealectomy were held in constant dim light and periodically exposed to melatonin in the drinking water. By alternating 8 h of melatonin water with variable phases of tap water, rhythms with periods (T) ranging from 21 to 27 h were produced. When melatonin was administered in rhythms with periods of 23, 24, and 25 h, feeding and locomotion behavior of most birds were rhythmic and synchronized with the exogenous melatonin rhythm. The rest phase coincided approximately with the phase of melatonin availability. Under melatonin cycles < 23 h and > 25 h, fewer birds had synchronized rhythms. Nonsynchronized birds were either arrhythmic or they expressed free-running rhythms. Under melatonin rhythms with periods between 23 and 26 h, the phase-angle difference between defined phases of the behavioral rhythms and the melatonin rhythm became more positive with increasing T. These data are consistent with the hypothesis (a) that periodic exogenous melatonin can substitute, at least to a certain degree, for the endogenous plasma melatonin rhythm normally resulting from the periodic melatonin secretion by the pineal gland, and (b) that this melatonin rhythm acts on another oscillator, possibly the SCN, as part of the overall circadian pacemaking system.