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The aim of this study was to develop a radioimmunoassay for the measurement of endogenous circulating melatonin concentrations in the Syrian hamster, and then to determine the effect of various photic manipulations upon this endocrine signal. In experiment 1, pineal-intact or pinealectomized adult male Syrian hamsters, housed under a long photoperiod (LD; 16 h light:8 h darkness) for 2 weeks, were then either maintained on LD or transferred to a short photoperiod (SD; 8 h light:16 h darkness) for a further 8 weeks. The profile of serum melatonin concentrations was determined from blood samples taken by cardiac puncture at intervals over a 24-h period. Radioimmunoassay revealed that daytime concentrations were at or below the limit of sensitivity of the assay ( 50 pmol/l). Under both photoperiods, the concentration of melatonin in the serum of pineal-intact animals rose 4–5 h after the onset of darkness, and the peak amplitude of the melatonin rhythm was not significantly different between the SD- or LD-housed animals (200–250 pmol/l). Premature exposure of animals to light during the dark phase of LD caused a precipitous decline in circulating concentrations to daytime values within 15 min and they remained there for several hours. In animals which experienced an uninterrupted night on either LD or SD, the most striking difference in the rhythm of endogenous melatonin secretion was the duration. Animals housed under LD had high levels until the start of the light period, a peak duration of 3·7 h. In contrast, animals housed under SD exhibited a peak duration of 10 h, levels falling 1·5 h before the start of the light period. The nocturnal increase in serum melatonin concentration was abolished in pinealectomized animals. Serum levels in these animals were not significantly different from those observed in pineal-intact animals during the light phase, being at, or very close to, the limit of sensitivity of the assay.

In experiment 2, animals were housed under LD prior to transfer to continuous darkness for 10 days, during which time their locomotor activity rhythms were recorded. Animals were then chronically cannulated and serial blood samples were removed from the jugular vein at hourly intervals, starting 48 h after surgery. A pronounced circadian rhythm in plasma melatonin concentrations was observed, with levels rising significantly 3 h after the onset of activity and falling 10 h later. During subjective day, levels were at or below the limit of sensitivity of the assay. At the end of the study, pineal melatonin was also measured at intervals across the circadian cycle. This revealed a very close correlation between the plasma and pineal concentrations of melatonin, the duration and phase of both nocturnal peaks being equivalent. These results confirm that the duration of the nocturnal secretion of melatonin varies in proportion to the length of the dark phase, that this rhythm is endogenously driven, and that peak physiological levels of melatonin in this species are of the order of 200 pmol/l.

Journal of Endocrinology (1993) 136, 65–73

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