Pineal photoreceptor cells: photoperiodic control of melatonin production after cell dissociation and culture.

Trout pineal cells were dissociated using a trypsin-DNase digestion technique. An enriched population of photoreceptor cells was selected from a Percoll gradient centrifugation. The ability of cultured photoreceptor cells (selected or not on a Percoll gradient) to produce melatonin rhythmically was investigated during seven 24 h light/dark cycles. During each cycle, trout pineal photoreceptor cells released low amounts of melatonin during daytime and high amounts during night-time. Under continuous darkness, melatonin release was continually high. The profile of its rhythm and that of the activity of the hydroxyindole-O-methyltransferase-the last enzyme of the melatonin biosynthetic pathway-depended on the substrates and on the culture media used. Some of them appear suitable for short- or long-term culture of photoreceptor cells permitting the study of their neuroendocrine properties.