Bacterial peptidoglycan enhances sickness behaviour induced by bacterial lipopolysaccharide

Background Lipopolysaccharide (LPS) and peptidoglycan are microbial products recognized by Toll-like receptor-4 (TLR4) and nucleotide-binding oligomerization domain 1 (NOD1) and NOD2, respectively. LPS has been found to cause behavioural alterations indicative of sickness and depressed mood. The effect of peptidoglycan on exploratory and affective behaviour has not yet been explored, although it has been reported that it promotes sleep and anorexia. Since NOD1 and NOD2 are activated by two different peptidoglycan components, MurNAc-L-Ala-g-D-Glumeso-diaminopimelic acid (M-TriDAP) and muramyl dipeptide (MDP), respectively, the effects of both compounds, alone and in combination with LPS, on exploratory and affective behaviour were investigated.