Interleukin-18 regulates motor activity, anxiety and spatial learning without affecting synaptic plasticity.

Expression of Schaffer collateral-CA1 long-term potentiation (LTP) and long-term depression (LTD) was not affected in hippocampal slices from wild-type mice pretreated with lipopolysaccharide (0.25mg/kg, i.p.), to increase interleukin-18 (IL-18) concentrations in the brain. For IL-18 knock-out (IL-18 KO) mice, the LTP was still expressed, the extent being similar to that for wild-type mice. In the open-field test to assess motor activity, rearing activity for IL-18 KO mice was significantly suppressed as compared with that for wild-type mice, without significant difference in the locomotion activity between two groups. In the passive avoidance test to assess fear memory, the retention latency for IL-18 KO mice was much shorter than for wild-type mice, without significant difference in the acquisition latency between two groups. In the water maze test, the acquisition latency for IL-18 KO mice significantly prolonged as compared with that for wild-type mice, without significant difference in the retention latency between two groups. For IL-18 KO mice, intraventricular injection with IL-18 for 4 days (total, 240 fg) prior to water maze task shortened the prolonged acquisition latency, reaching a level similar to that for wild-type mice. The results of the present study, thus, suggest that IL-18 is a critical regulator for exploratory activity, fear memory, and spatial learning.