Exogenous melatonin reduces the resynchronization time after phase shifts of a nonphotic zeitgeber in the house sparrow (Passer domesticus)

Continuous melatonin administration via silastic implants accelerates the r esynchronization of the circadian locomotor activity rhythm in house sparro ws (Passer domesticus) after exposure to phase shifts of a weak light-dark cycle. Constant melatonin might induce this effect either by increasing the sensitivity of the visual system to a light zeitgeber or by reducing the d egree of self-sustainment of the circadian pacemaker. To distinguish betwee n these two possible mechanisms, two groups of house sparrows, one carrying melatonin implants and the other empty implants, were kept in constant dim light and subjected to advance and delay shifts of a 12-h feeding phase. T he resynchronization times of their circadian feeding rhythm following the pi-case shifts were significantly shorter when the birds carried melatonin implants than when they carried empty implants. In a second experiment, mel atonin-implanted and control birds were released into food ad libitum condi tions 2 days after either a delay or an advance phase shift. The number of hours by which the activity rhythms had been shifted on the second day in f ood ad libitum conditions was assessed. Melatonin-implanted house sparrows had significantly larger phase shifts in their circadian feeding rhythm tha n control birds. This is in accordance with the first experiment since a la rger phase shift at a given time reflects accelerated resynchronization. Ad ditionally, the second experiment also excludes any possible masking effect s of the nonphotic zeitgeber. In conclusion, constant melatonin accelerates resynchronization even after phase shifts of a nonphotic zeitgeber, indica ting that constant high levels of melatonin can reduce the degree of self-s ustainment of the circadian pacemaker independent of any effects on the pho toreceptive system.