MELATONIN RHYTHM ONSET IN THE ADULT SIBERIAN HAMSTER - INFLUENCE OF PHOTOPERIOD BUT NOT 60-HZ MAGNETIC-FIELD EXPOSURE ON MELATONIN CONTENT IN THE PINEAL-GLAND AND IN CIRCULATION

To determine the relationship between pineal melatonin production and its appearance in circulation, the rising phase of the pineal and seru m melatonin rhythm was studied in the adult Siberian hamster. Melatoni n concentrations increased in the pineal gland and in serum at 1.50 an d 1.75 h, respectively, relative to lights off in long days (16 h of l ight/day) and at 2.00 and 2.75 h, respectively, in short days (10 h of light/day). Thus, a photoperiod-dependent melatonin rise in circulati on lagged production by the pineal gland by 0.50 h-a delay of 0.75 h i n short-day hamsters versus 0.25 h in long-day hamsters. Following ini tiation of this rise, concentrations that were typical of the nighttim e peak were achieved within 2 h of melatonin rhythm onset, regardless of photoperiod. To determine whether clock control of the rising phase of the melatonin rhythm, in the absence of photoperiod cues, may be d isrupted by perturbations in the ambient magnetic field, hamsters in c onstant darkness were acutely exposed to a 1-Gauss, 60-Hz magnetic fie ld for 15 min or were daily exposed to this treatment for 14 or 21 day s. Neither the melatonin rise in pineal content or circulation during subjective night was affected by acute or chronic magnetic field expos ures; testes regression similarly occurred in sham and daily magnetic field-exposed hamsters in constant darkness. These findings indicate t hat magnetic field exposures are unlikely to serve as a zeitgeber for the circadian mechanism that controls onset of the melatonin rhythm; r ather, photoperiod is a predominant cue that may differentially regula te the rising phase of melatonin production in the pineal gland and co ncentration in circulation.