Cellular Mechanisms Underlying The Involvement Of Adrenomedullin Receptor Signaling In Morphine Tolerance-The Contribution To Glia Activation

Adrenomedullin (AM) is a member of the calcitonin gene related peptide (CGRP) family which is distributed in the CNS, including dorsal root ganglia (DRG) and superficial layers of the spinal cord. We have demonstrated that AM in the spinal dorsal horn and DRG are involved in the development of opioid tolerance. Co-administration of AM22-52, a selective AM receptor antagonist, has been demonstrated to improve the effectiveness of morphine and inhibit morphine-associated hyperalgesia. This effect is attributed to the modulation of pronociceptive mediators such as nNOS and CGRP, which are up-regulated by chronic use of morphine. These observations suggest that targeting AM receptors could be a promising approach to preserve the analgesic potency of opioids. The present study was designed to investigate the mechanisms underlying the contribution of AM to morphine tolerance.The results showed:(1) Co-administration of AM22-52(10nmol) inhibited morphine-induced increase in exciatatory amino acids in spinal dorsal horn and blood. This treatment also abolished morphine-induced in increase in IL-1β, IL-6mRNA levels in the spinal cord.(2) Treatment of cultured DRG with AM siRNA (50nM) inhibited increase in IL-1β and IL-6mRNA levels evoked by chronic morphine (3.3μM).(3) The level of IL-1β, IL-6and TNFa mRNA was increased in the spinal dorsal horn, but decreased in DRG following the7day-treatment with10μg AM1-50.(4) Co-administration of AM22-52inhibited morphine-induced increase in GFAP and OX-42expressions in the spinal dorsal horn while7day-treatment with i.t. AM also induced an increase in GFAP and OX-42expressions in the spinal dorsal horn.(5) OX-42is extensively co-localized with AM receptor components (CRLR and RAMP2) in dorsal horn.(6) Chronic AM1-50induced increase of phosphorylated p38protein. Pre-administration of minocycline inhibited hyperalgesia, level of phosphorylated p38protein and IL-1β, IL-6, TNF-a mRNA in dorsal horn induced by the chronic AM1-50.In the present study, we demonstrated that AM induced morphine tolerance by increased in exciatatory amino acids and IL-1β, IL-6and TNFa in spinal dorsal horn. Both astrocytes and microglia were activated by AM and increased the synthesis and release of cytokines such as IL-1β,IL-6and TNFα. Activation of p38MAPK in the spinal microglia was the mechanism underlying the contribution of AM to morphine tolerance.