Different Sickness Responses in Adult and Aged Rats Following Lipopolysaccharide Administration

Immune challenges result in sickness responses such as decreased activity, fever, and spatial learning deficits. While these responses occur simultaneously, they are not usually evaluated concurrently or for an extended time. The purpose of this study was to examine how an immune challenge affected activity and temperature responses in animals tested concurrently in the Morris water maze (MWM) over 5 days and how aging interacts with such responses. An accepted model of aging, adult (n = 10; 5–6 months) and aged (n = 7; 22 months) male Brown-Norway rats were implanted with a telemetry device (Mini Mitter, Oakmont, PA) to continuously monitor temperature and activity following an immune challenge. These animals were injected with either 250 μg/kg lipopolysaccharide (LPS) or 0.9% sodium chloride and then assessed in the MWM for 5 days. Temperature responses varied by age. Initial temperatures decreased in both experimental groups followed by an increase (fever) in the adult group, while the temperatures of the aged animals remained decreased. Although both age groups were sedentary at baseline, activity decreased after LPS only in the adult group. An LPS immune challenge resulted in age-dependent temperature and activity changes. There was an absence of fever and no effect on activity in aged LPS-treated animals. These results may suggest the need to assess a broader spectrum of sickness responses when monitoring elderly individuals for infection and not rely on the presence of fever. Activity may not be a sensitive indicator of sickness in some aging models