Essential role of interleukin-15 receptor in normal anxiety behavior

The interactions between the cytokine interleukin (IL)-15 and the classical neurotransmitter GABA have been shown in IL15Rα receptor knockout mice by observations of memory deficits and reduction of GABA. To test the hypothesis that IL15 affects anxiety-like behavior, knockout mice without IL15, IL15Rα, or the co-receptor IL2Rγ were subjected to open-field and elevated plus maze tests. All three strains showed reduction of anxiety, with greater changes in the IL15Rα knockout mice than in the IL15 or IL2Rγ knockout mice. This unexpected observation is opposite to the reported increase of anxiety in mice lacking other proinflammatory cytokines or their receptors. The reduced anxiety was not associated with changes in associated serum cytokines. However, Western blotting, qPCR, and immunohistochemistry all showed that IL15Rα knockout mice had mild microgliosis and astrogliosis in the hippocampus. To determine whether this gliosis plays a role in decreasing anxiety, IL15Rα knockout mice were treated with minocycline, but this did not cause a change in open field performance. To determine whether IL15 plays a direct role in anxiety, wildtype mice were treated with IL15 by intraperitoneal injection. This also failed to cause a change in open field behavior under the experimental conditions tested. Thus, IL15Rα is essential for normal anxiety-like behavior, but inhibition of gliosis in the fearless IL15Rα knockout mice or IL15 treatment of normal mice did not acutely modulate behavioral performance as tested.