AJP - Regulatory, Integrative and Comparative Physiology, Vol 247, Issue 6 1062-R1066, Copyright © 1984 by American Physiological Society
Plasma melatonin concentrations in hibernating marmots: absence of a plasma melatonin rhythm
G. L. Florant, M. L. Rivera, A. K. Lawrence and L. Tamarkin
Plasma melatonin concentrations were measured throughout bouts of
hibernation in marmots maintained in a short photoperiod (light-dark 8:16)
and ambient temperature of 5 or 15 degrees C. Melatonin concentration was
also measured in two animals maintained in constant darkness. As an animal
entered hibernation, plasma melatonin concentrations dropped to basal
levels when body temperature reached 25 degrees C, and they remained low
until arousal. During deep hibernation plasma melatonin values did not vary
significantly (P greater than 0.05) with respect to time of day or
different ambient temperatures. With nocturnal arousal plasma melatonin
levels were similar to euthermic night values. Lack of a plasma melatonin
rhythm during hibernation suggests that the pineal gland is not temperature
compensated during hibernation, and due to the low tissue temperature of
the pineal the circadian pacemaker driving melatonin secretion is incapable
of stimulating a rhythm.
