Sites of melatonin action in the brain of the house sparrow, Passer domesticus

The pineal gland and its hormone melatonin are important for the generation of circadian rhythms by passerine birds such as the house sparrow, Passer domesticus. The sites of melatonin action within the brain of this species were determined by employing two techniques. First, the distribution of 2[¹²⁵I]iodomelatonin (IMEL) binding was determined by in vitro incubation in IMEL, autoradiography, and computer image analysis. Data from these experiments indicated reversible IMEL binding in a wide array of cerebral structures primarily associated with vision. Second, the effects of exogenous melatonin on cerebral uptake of the metabolic marker 2‐deoxy[¹⁴C]glucose (2DG) were determined. Many of the same structures that bind IMEL also exhibited decreased 2DG uptake in response to melatonin administration, whereas structures that did not bind IMEL were unaffected by the hormone. The data suggest that much of the house sparrow's visual world is modulated on a circadian basis via the circadian secretion of melatonin. These observations are discussed in the context of avian circadian organization and the role it may play in the behavioral physiology of the bird.