INDOLEAMINES AND THE UV-LIGHT-SENSITIVE PHOTOPERIODIC RESPONSES OF THE MELANOCYTE NETWORK - A BIOLOGICAL CALENDAR

The pineal, serotoninergic and pigmented neurons are associated with l ight-dependent sleep/arousal, serving as a biological clock with a cir cadian rhythm. This rhythm is maintained by melatonin which serves to recognise the 'dark' phase. The neural network that responds to season al variations in day/night length has not been identified. The present study demonstrates that melanocytes in human skin respond to changes in the duration of UV exposure, and can serve as a biological calendar . These responses are mediated by two indoleamines, serotonin and mela tonin. Higher melatonin levels correspond to long nights and short day s (short UV pulse), while high serotonin levels in the presence of mel atonin reflect short nights and long days (long UV exposure). This res ponse recapitulates the sleep/arousal patterns in animals exposed to l arge variations in day/night cycle that cause changes in coat colour f rom pure white in winter to complete repigmentation in summer.