The occurrence and physiological functions of melatonin in the most primitive eumetazoans, the planarians.

Asexual planarians of the species Dugesia dorotocephala exhibit a distinct circadian rhythm of fissoning (asexual reproduction) under the influence of normal photoperiod; fissioning occurs only at night. This rhythm is broken down by continuous illumination, continuous darkness or by decapitation. The fissioning rate increases when planarians are exposed to light for less than 1 hour/day or when they are decapitated. Fissioning of decapitated planarians is suppressed by continuous treatment with melatonin, whereas fissioning resumes when these are returned to normal culture water. Interestingly, fissioning occurs at night when decapitates are treated with melatonin in the daytime, while it is observed in the daytime with night-time melatonin treatment. Endogenous melatonin was detected by HPLC and RIA. The endogenous melatonin level is always higher in those heads collected in the scotophase than in those collected in the photophase. A type of neurosecretory cell, which may synthesize melatonin, is found in the assembly of photoreceptor cells. [ABSTRACT FROM AUTHOR]