Induction of photosensitivity in neonatal rat pineal gland

Pineal glands removed from neonatal rats at 5, 7, and 9 days of age and exp lanted into short-term culture, synthesized melatonin when stimulated with norepinephrine (NE); their melatonin synthesis could not be suppressed with bright white light. Dispersed pineal cell cultures or pineal explants prep ared from 1-day-old neonates and held in culture for 7 or 9 days also synth esized melatonin when stimulated with NE, but in these cases melatonin synt hesis was significantly suppressed by light, demonstrating that the pineals had become photosensitive while in culture. The development of photosensit ivity in culture could be partially or completely abolished by the continuo us presence of 1 or 10 mu m of NE in the culture medium. The pineals of all nonmammalian vertebrates are photoreceptive, whereas those of mammals do n ot normally respond to light. We hypothesize that a mechanism to suppress p ineal photosensitivity by using NE released from sympathetic nerve endings evolved early in the history of mammals.