The effects of environmental illumination on the in vitro melatonin secretion from the embryonic and adult chicken pineal gland

The aim of this study was to monitor the changes in the pattern of the in vitro melatonin (MT) secretion under reversed illumination cycles with low intensity of light during photo phase. Although light is known to be one of the major synchronizing factors of the circadian MT rhythm in birds, there are no data available on the limits of direct light sensitivity of the avian pinealocytes. In our experiments, MT secretion from adult or from embryonic chicken pineal glands was monitored in a perifusion system for 4 days. The glands were repeatedly exposed to light with various intensities and duration. Reversed daily illumination inverted the in vitro MT rhythm even if the intensity of light was only 10 lux at the surface of the perifusion columns. MT release of embryonic pineal glands was also found to be sensitive to dim light. Our results showed that the chicken pineal gland is directly sensitive to light of low intensity. However, the various oscillator units in the gland may have different sensitivity to dim light. Light perception mechanism in the chicken pinealocytes is already fully developed on the 17th embryonic day.