Light- and temperature-dependence of the melatonin secretion rhythm in thepineal organ of the lamprey, Lampetra japonica

To identify the characteristics of the oscillator located in the pineal org an, we examined the effects of temperature and light on melatonin secretion rhythm using pineal organs in cultures. At 20 degreesC, the melatonin rhyt hm was obvious: low secretion during the daytime and high during the nightt ime. When the temperature was lowered from 20 to 10 degreesC, the melatonin rhythm disappeared. When the temperature was returned from 10 to 20 degree sC, the rhythm quickly reappeared. The plasma melatonin level was measured in living lampreys kept at 7 degreesC to establish the melatonin profile at low temperature in vivo: secretion was not significantly different between daytime and nighttime, Under continuous light conditions, the melatonin el evation normally seen during the subjective night became obscure after 72 h . When the LD cycle was shifted by 6 h (phase-advanced or phase-delayed), t he melatonin rhythm shifted to remain in the same phase relation to the LD cycle. This re-synchronization took several LD cycles. The results indicate that, in cultures, the melatonin secretion rhythm in the pineal organ of t he lamprey is both light- and temperature-sensitive, and that in vivo, the melatonin rhythm is not the critical factor maintaining the locomotor activ ity rhythm of the lamprey, The role of the pineal organ and melatonin in th e circadian organization of the lamprey is discussed.