Circadian and daily rhythms of melatonin in the blood and pineal gland of free-running and entrained Syrian hamsters.

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 (< or = 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.(ABSTRACT TRUNCATED AT 250 WORDS)