An investigation of the age-related deficits in the febrile response of the rabbit

The febrile response of the New Zealand White rabbit in animals less than 1 yr old was compared with that in 3-yr-old animals. A reduced febrile response to both endotoxin and live bacteria injected intravenously was observed in the older group of animals. Peripheral vasoconstriction was observed, suggesting the drive to increase body temperature remained. Plasma catecholamines increased significantly in both groups of animals during fever. However, significantly greater increases in plasma epinephrine were observed in the older animals. A significant deficit in catecholamine-induced thermogenesis was observed in the older group of rabbits. This deficit alone does not explain the reduced febrile response, as beta-adrenergic blockade does not suppress the febrile response of young animals. Thus it is suggested that the primary deficit resulting in a reduced febrile response in the 3-yr-old rabbits is due to other age-related changes in the thermoregulatory system.