Ontogeny of Melatonin Secretion and Functional Maturation of the Pineal Gland in the Embryonic Turkey (Meleagris gallopavo)

The pineal gland is a key component of the avian circadian system, responsible for melatonin (MLT) secretion that reflects and regulates daily rhythmicity. While the post-hatching turkey is known for its highly sensitive and efficient pineal system, the functional development of this gland during embryogenesis has not been described. To determine the measurable onset and regulation of MLT secretion, pineal glands from turkey embryos aged 22, 24, and 26 days (ED 22-26) were incubated in a superfusion culture under various light conditions and with or without norepinephrine (NE). MLT concentrations in the medium were measured by radioimmunoassay. Under the experimental conditions, MLT secretion was first reliably measurable at ED 22 and increased thereafter toward hatching. Under a 12L:12D cycle, pinealocytes exhibited a stable circadian rhythm with a fourfold amplitude between night and day. The glands adapted to a reversed light-dark cycle and responded strongly to a 3 h nocturnal light pulse with a 30% decrease in MLT levels. Rhythmic MLT secretion persisted under constant light or darkness, indicating an active endogenous oscillator. NE markedly suppressed MLT secretion in all embryos, confirming the early development of adrenergic regulation. Thus, by ED 22 the embryonic turkey pineal exhibits photosensitivity, endogenous rhythmicity, and NE responsiveness, although secretion at earlier stages cannot be excluded due to detection limits. These results fill a gap in understanding avian pineal ontogeny and establish the turkey embryo as a good model for studying the early development of circadian regulation in birds.