Retinal illumination phase shifts the circadian rhythm of serotonin N-acetyltransferase activity in the chicken pineal gland.

The pineal gland of birds, in contrast to its mammalian counterpart, is a directly photosensitive organ. It has recently been demonstrated that light also acting via the retina acutely suppresses melatonin synthesis in the chicken pineal gland. The present study was aimed to investigate whether retinal illumination alone was capable of resetting the biological oscillator generating the circadian rhythm of pineal serotonin N-acetyltransferase (AA-NAT) activity in the chicken. Ocular exposure of chickens to 6 h low intensity white light (4 lux) potently suppressed AA-NAT activity (the penultimate and key regulatory enzyme in the melatonin biosynthetic pathway) in the pineal gland. In addition, this light pulse produced phase-dependent shifts in the circadian AA-NAT rhythm. Exposure to light early in the subjective night (circadian time (CT) 12-18) caused a phase delay in the circadian rhythm of pineal AA-NAT activity by 3.5+/-0.4 h compared to non-exposed controls. When the light pulse was applied during the second half of the subjective night (CT18-24), it produced a large phase advance of the circadian rhythm of pineal AA-NAT activity by 10.9+/-0.4 h. The advancing effect of light was more pronounced than the phase-delaying effect. Our results suggest that in the chicken retinally perceived light provides a powerful and important signal for synchronization of circadian rhythmicity in the pineal gland.