Abstract
The influence of fasting and winter-acclimatization (cold and short-day acclimatization) on mouse plasma leptin, ghrelin, growth hormone (GH) and melatonin concentrations was determined from blood samples taken at mid-day and midnight. A 16-h fast decreased the plasma leptin but almost doubled the plasma ghrelin concentrations which contribute to energy saving, appetite stimulation and, in the case of leptin, to inhibition of reproduction. Winter-acclimatization did not affect plasma ghrelin concentrations, whereas leptin decreased to the same level as in fasting. The low leptin concentrations possibly enable an increased caloric intake for the purpose of thermogenesis. Fasting and winter-acclimatization seemed to abolish the diurnal leptin rhythm, but had no effect on that of ghrelin. Plasma melatonin concentration correlated negatively with ghrelin, suggesting a possible role for melatonin in the regulation of ghrelin concentration. SNS-activity and insulin appear to be the main regulators of leptin plasma concentration in the mouse, rather than melatonin as in some seasonal mammals. Interestingly, endogenous ghrelin did not stimulate GH secretion, which is a well-documented reaction to exogenous ghrelin injections.
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