Rhodopsin-like photosensitivity of isolated chicken pineal gland

The pineal gland of lower vertebrates such as fish, amphibians and reptiles is primarily a photoreceptor organ and its electrical activity changes in response to environmental lighting, whereas the mammalian pineal gland is a secretory organ and no longer responds to direct illumination. It is uncertain whether the avian pineal gland retains a photoreceptive capability, and also whether the gland is a photosensor involved in the photo-periodic control of reproductive systems and circadian rhythms in birds. Recently, I and other workers have shown that the circadian rhythm of serotonin N-acetyltransferase activity persisted in organ cultures of chicken pineals, indicating the presence of an endogenous circadian oscillator in the gland. Also, the observation that direct illumination of cultured chicken pineals suppressed the night-time increase of N-acetyl-transferase activity suggests that the gland contains a photoreceptor. I report here that the action spectrum of the photosensitivity of the isolated chicken pineal resembles the absorption spectrum of rhodopsin.