Complex circadian regulation of pineal melatonin and wheel-running in Syrian hamsters.

Circadian regulation of pineal melatonin content was studied in Syrian hamsters (Mesocricetus auratus), especially melatonin peak width and the temporal correlation to wheel-running activity. Melatonin was measured by radioimmunoassay in glands removed at different circadian times with respect to activity onset (= CT 12). Pineal melatonin peak width (h; for mean > or = 125 pg/gland) and activity duration (alpha) were both 4-5 h longer after 12 or 27 weeks than after 5 or 6 days in continuous darkness (DD). Increased peak width was associated with a delay in the morning decline (M) of melatonin to baseline, correlated with a similar delay in wheel-running offset. In contrast, the evening rise (E) in melatonin occurred at approximately the same circadian phase regardless of the length of DD. Fifteen min light pulses produced similar phase-shifts in melatonin and activity. In a phase advance shift, M advanced at once, while E advanced only after several days of adjustment. Independent timing of shifts in the E and M components of the melatonin rhythm suggest that these events are controlled separately by at least two circadian oscillators whose mutual phase relationship determines melatonin peak width. This two-oscillator control of melatonin peak width is integral to the circadian mechanism of hamster photoperiodic time measurement.