Inhibition of shivering thermogenesis by centrally applied glucagon in muscovy ducklings.

Glucagon has marked thermogenic and lipolytic effects in birds but could also be involved in the central modulation of neural activity on the basis of the recently discovered glucagon receptors in several areas of the brain in ducklings. The aim of this work was to investigate the possible role of these receptors in the modulation of thermogenic processes. Glucagon was infused into the lateral ventricle of the brain in ducklings after an acute cold exposure (4 degrees C, 2 h) or at thermoneutrality (25 degrees C). Electromyographic (EMG) data were simultaneously recorded with electrodes implanted in the gastrocnemius muscle. Glucagon (10(-4) M) was infused at a rate of 8 microliters/min. When acutely exposed to cold, ducklings increased their metabolic rate by shivering thermogenesis. A significant decrease in shivering activity was elicited after 5 min of glucagon infusion. After 16 +/- 2 min of glucagon infusion, shivering was completely inhibited, corresponding to a total dose of 36 +/- 4 micrograms/kg. The suppression of shivering was accompanied by a diminution of metabolic rate (5.3 +/- 0.3 vs. 8.5 +/- 0.2 W/kg, P < 0.05). The values of metabolic rate obtained at 4 degrees C after glucagon infusion were not significantly different from those measured at 25 degrees C before glucagon infusion (6.4 +/- 0.3 W/kg, P > 0.05). The infusion of the same dose of glucagon did not induce any change in EMG activity and resting metabolic rate at 25 degrees C. These findings suggest that glucagon infused into the brain has no thermogenic effect but could be involved in the central control of somatic motricity. Although the origin and the mechanisms of action of the endogenous peptide still remain unknown, glucagon might have a role in the development of non shivering thermogenesis during prolonged cold exposure via an inhibition of shivering in birds.