Effects of low-dose lipopolysaccharide and age on spatial learning in different Morris water maze protocols

OBJECTIVES Animals administered lipopolysaccharide exhibit dose-related sickness behaviors (decreased food intake, weight loss, and cognitive changes). While research has demonstrated that spatial learning is impaired following a lipopolysaccharide immune challenge, the results differ depending on the methodology used to evaluate spatial learning. Additionally, few studies have evaluated the effects of low-dose lipopolysaccharide on spatial learning. Therefore, we assessed spatial learning, food intake, and weight changes in adult and aged rats after a low-dose lipopolysaccharide immune challenge in the Morris water maze using two water temperatures. METHODS Adult (5-6 months) and aged (22 months) male Brown-Norway rats were administered either 50 or 100 μg/kg lipopolysaccharide or saline, and then tested in the Morris water maze for 5 days, rested for 7 days, and later underwent 2 days of retention tests. Probe trials were conducted at the end of initial and retention testing. RESULTS Low-dose lipopolysaccharide administration did not result in food intake or weight changes. While the aged experimental group took longest to improve directional heading error in both cold and warm water, heading error was greater in cold water. Behavioral testing revealed an apparent age and water temperature effect on swim time. Retention and probe trial results showed that aged experimental animals had the worst performance in cold water. CONCLUSION We conclude that while low-dose lipopolysaccharide did not result in typical sickness behaviors (decreased food intake or weight), spatial learning and memory were impaired in the aged experimental group. These results have important implications for the care of elderly individuals experiencing mild to moderate infections.