Signaling pathway of morphine induced acute thermal hyperalgesia in mice

Systemic administration of morphine induced a hyperalgesic response in the hot plate test, at an extremely low dose (1-10 mu g/kg). We have examined in vivo whether morphine, at an extremely low dose, induces acute central hypernociception following activation of the opioid receptor-mediated PLC/PKC inositol-lipid signaling pathway. The PLC inhibitor U73122 and the PKC blocker, calphostin C, dose dependently prevented the thermal hypernociception induced by morphine. This effect was also prevented by pretreatment with aODN against PLCP3 at 2 nmol/mouse and PKC gamma at 2-3 nmol/mouse. Low dose morphine hyperalgesia was dose dependently reversed by selective NMDA antagonist MK801 and ketamine. This study demonstrates the presence of a nociceptive PLC beta(3)/PKC gamma/NMDA pathway stimulated by low concentrations of morphine, through mu OR1, receptor, in mouse brain. This signaling pathway appears to play an opposing role in morphine analgesia. When mice were treated with a morphine analgesic dose (7 mg/kg), the downregulation of PLC beta(3) or PKC gamma at the same aODN doses used for the prevention of the hyperalgesic effect induced, respectively, a 46% and 67% potentiation in analgesic response. Experimental and clinical studies suggest that opioid may activate pronociceptive systems, leading to pain hypersensitivity and short-term tolerance, a phenomenon encountered in postoperative pain management by acute opioid administration. The clinical management of pain by morphine may be revisited in light of the identification of the signaling molecules of the hyperalgesic pathway. (c) 2006 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.