URINARY 6-SULFATOXYMELATONIN PROFILES IN MALE DJUNGARIAN HAMSTERS (PHODOPUS-SUNGORUS) RESPONDING AND NOT RESPONDING TO SHORT-DAY PHOTOPERIODS - POSSIBLE ROLE OF ELEVATED DAYTIME LEVELS

The lack of endocrine and physiological responses of some Djungarian h amsters (Phodopus sungorus) to the transition from long to short photo periods (L:D 16:8 --> L:D 8:16) has been known for a long time but is not yet understood. We investigated the role of melatonin synthesis in this context because melatonin, as part of the circadian system, may play a role in non-responsiveness. In ten responding and ten non-respo nding male hamsters, the urinary 24 hr 6-sulfatoxymelatonin (aMT6s) pr ofiles under L:D 8:16 and L:D 16:8 were measured. Both short day respo nding and non-responding hamsters showed diurnal aMT6s excretion rhyth ms. Whereas responders reacted to the transition L:D 16:8 --> L:D 8:16 with a marked elevation of aMT6s excretion, in non-responders no adju stment of the melatonin rhythm to the change of the photoperiod was se en. Furthermore, under L:D 16:8 the daytime levels of aMT6s were signi ficantly (P<0.001) lower in responders compared to non-responders wher eas under L:D 8:16 these levels were higher (P<0.01). It is speculated that high daytime levels of aMT6s under long-day photoperiods in nonr esponders result in down-regulation of melatonin receptors of the nucl eus suprachiasmaticus, the pacemaker for the pineal gland, leading to a lack of response to the transition to short-day photoperiods.