Melatonin levels were estimated over a 24 h period in the pineal glands, retinas and Harderian glands of a diurnal rodent, the Richardson's ground squirrel (Spermophilus richardsonii). Animals were kept in light:dark cycles of 14:10 (lights on at 06.00 h) and killed at the following time points: 20.00, 24.00, 02.00, 04.00 and 08.00 h. Pineal levels of melatonin were low (< 300 pg/gland) during the day (08.00 and 20.00 h); however, by 4 h after lights out (at 24.00 h) the melatonin content of the pineals had increased by roughly 20-fold to nearly 6000 pg/gland. This represents the greatest nocturnal rise in pineal melatonin measured in any animal to date. At 02.00 and 04.00 h pineal melatonin levels were still greatly elevated over those measured during the period of light. The retinas and Harderian glands contain much smaller quantities of melatonin than do the pineal glands. Also, despite some variation over the 24 h period, no statistically significant rhythm in melatonin content could be documented in the latter two organs.
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