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The Journal of Neuroscience, April 13, 2005, 25(15):3824-3832; doi:10.1523/JNEUROSCI.5010-04.2005

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Cellular/Molecular
cAMP-Mediated Mechanisms for Pain Sensitization during Opioid Withdrawal

Bihua Bie, Yi Peng, Yong Zhang, and Zhizhong Z. Pan

Department of Anesthesiology and Department of Biochemistry and Molecular Biology, The University of Texas-M. D. Anderson Cancer Center, Houston, Texas 77030

Chronic opioid-induced drug dependence and withdrawal syndrome after opioid cessation remain a severe obstacle in clinical treatment of chronic pain and opioid drug addiction. One of the key symptoms during opioid withdrawal is a state of sensitized pain. The most significant molecular adaptation induced by chronic opioids in the brain is upregulation of the cAMP-signaling pathway. Although the cAMP system is known to have multiple effects on central neuron functions, how its upregulation mediates behavioral opioid dependence and withdrawal-induced pain in vivo remains unclear. In this study, we demonstrate that withdrawal from chronic morphine significantly upregulates the mRNA level of adenylyl cyclase (AC) VI and VIII isoforms and immunoreactivity of ACV/VI in the nucleus raphe magnus (NRM), a brainstem site critically involved in opioid modulation of pain. In cellular studies of NRM neurons containing µ-opioid receptors, we show that morphine withdrawal significantly increases glutamate synaptic transmission via a presynaptic mechanism mediated by an upregulated cAMP pathway. Morphine withdrawal also enhances the hyperpolarization-activated current in these neurons by increased intracellular cAMP. Both of the withdrawal-induced cAMP actions increase the excitability of these µ-receptor-containing neurons, which are thought to facilitate spinal pain transmission. Furthermore, in morphine-dependent rats in vivo, blocking the cAMP pathway significantly reduces withdrawal-induced pain sensitization. These results illustrate neurobiological mechanisms for the cAMP-mediated withdrawal pain and provide potential therapeutic targets for the treatment of opioid dependence and withdrawal-related problems.

Key words: glutamate EPSCs; I_h; adenylyl cyclase; nucleus raphe magnus; hyperalgesia; opioid dependence

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Received Sep 3, 2004; revised February 4, 2005; accepted March 1, 2005.

This article has been cited by other articles:

				W. Zhu, B. Bie, and Z. Z. Pan Involvement of Non-NMDA Glutamate Receptors in Central Amygdala in Synaptic Actions of Ethanol and Ethanol-Induced Reward Behavior J. Neurosci., January 10, 2007; 27(2): 289 - 298. [Abstract] [Full Text] [PDF]

				J. Ma, Y. Zhang, A. E. Kalyuzhny, and Z. Z. Pan Emergence of Functional {delta}-Opioid Receptors Induced by Long-Term Treatment with Morphine Mol. Pharmacol., April 1, 2006; 69(4): 1137 - 1145. [Abstract] [Full Text] [PDF]

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