Pineal photoreceptors rhythmically secrete melatonin.

Individual pineal cells secrete melatonin with a circadian period, reducing a vertebrate circadian system to the level of a single cell [Brain Res., 627 (1993) 141-146]. In the present study, dissociated pineal cells were identified as melatonin-secreting by a reverse hemolytic plaque assay (RHPA) and all melatonin-secreting cells were immuno-positive when analyzed for the photoreceptor protein S-antigen. The results are the first direct evidence that isolated photoreceptor cells secrete melatonin and taken together with our previous findings indicate that single pineal cells contain: (1) a circadian oscillator; (2) a photoreceptive capacity; and (3) the ability to secrete melatonin rhythmically.