Intrathecal administration of adrenomedullin induces mechanical allodynia and neurochemical changes in spinal cord and DRG

This study investigated the effect of adrenomedullin (AM) on mechanical pain sensitivity and its possible mechanisms. Intrathecal injection of AM receptor agonist AM(1-50) (20 mu g) once per day briefly reduced mechanical pain threshold on days 1 and 2 but induced prolonged mechanical allodynia on day 3. However, AM(1-50) did not change mechanical pain sensation when the AM receptor antagonist AM(22-52) (20 mu g) was intrathecally co-administered. Daily administration of AM(1-50) (20 mu g) for 3 days increased expression of phosphorylated extra cellular signal-regulated protein kinase (pERK) and neuronal nitric oxide synthase (nNOS) in the spinal dorsal horn. The AM-induced increase in pERK and nNOS was inhibited by the co-administration of AM(22-52). The chronic administration of AM(1-50) also increased expression of microglial maker Iba1 and astrocytic marker GFAP (glial fibrillary acidic protein) in the spinal dorsal hom in an AM(22-52)-sensitive manner. Furthermore, the application of AM(1.50) (10 nM, 3 h) to dorsal root ganglion (DRG) explant cultures induced an increase in the expression of transient receptor potential vanilloid 1 (TRPV1). The treatment with AM(1-50) did not change TRPV1 expression in DRG in the presence of AM(22.52) (2 mu M). These results suggest that the increased AM bioactivity induced mechanical allodynia and may contribute to the mechanical pain hypersensitivity under pathological conditions. The mechanisms may involve the activation of ERK signaling pathway and spinal glia as well as the recruitment of nNOS and TRPV1 in the spinal dorsal horn or DRG. The present study indicates that inhibition of the activation AM receptor might provide a fruitful strategy to relieving chronic pain.