Chronic pain represents one of the most significant societal burdens in terms of the number of Americans affected, its impact on the health care system and lost productivity. Classical opiates, such as morphine, remain the "gold standard" of care for the management of moderate to severe post-operative and cancer pain as well as for the treatment of chronic non-malignant and inflammatory pain. However, the long-term use of mu opioid receptor (MOP) agonists such as morphine, in the setting of chronic pain, is limited by the development of tolerance and physical dependence. Opiate tolerance is the gradual loss of drug potency or efficacy, and reduced duration of action. The opioid receptor subfamilies include mu, delta, and kappa opioid receptors (MOP, DOP, and KOP). While it is clear that morphine-induced analgesia is mediated by MOP activation, the role of DOP in analgesia remains unclear. It has been reported that morphine-induced analgesic tolerance in acute pain is reduced upon administration of DOP antagonists and in mice lacking functional DOP. Thus, it seems that MOP-DOP interactions play an important role in modulating morphine-induced analgesic tolerance. Surprisingly, there are no studies regarding the role of DOP or MOP-DOP interactions in the development of morphine-induced analgesic tolerance in chronic pain. We have recently reported that pretreatment with the DOP2 antagonist, naltriben, disrupts morphine conditioned place preference and that this effect is associated with an increase in the levels of DOP dimer at the synapse. In addition, our preliminary studies show that morphine tolerance in the presence of chronic inflammatory pain is associated with an increased expression of the DOP at the synapse and with increased levels of the MOP-DOP heteromer in the spinal cord dorsal horn. Based on these data, we propose that pretreatment with DOP antagonists or disruption of the MOP-DOP heteromer will result in an attenuation of the analgesic tolerance that develops after repeated morphine injections during chronic pain. We also propose that morphine-induced analgesic tolerance is mediated by increased DOP function and MOP-DOP heteromer abundance, which in turn reduces MOP-mediated inhibition of excitatory transmission. This results in a loss of opiate analgesic potential, at the primary afferent, spinal cord and at brain sites implicated in opioid control of nociception such as the midbrain periaqueductal gray (PAG).
In Specific Aim 1 we will conduct behavioral and biochemical analyses to investigate the role of MOP-DOP interactions in the attenuation of morphine-induced analgesic tolerance during chronic inflammatory pain.
In Specific Aim 2 using in vitro recordings, we will characterize the role of DOP and MOP-DOP interactions in the control of excitatory transmission during morphine-induced analgesic tolerance in the presence of chronic inflammatory pain. The outcomes of the present studies will have a sustained, powerful impact on the fields of the biology and pharmacology of opioid receptors with the prospects of novel, safer and more effective pharmacotherapeutic strategies for the treatment of chronic pain.
Morphine and related opiates are commonly used in the clinical management of various types of pain, both acute and chronic pain patients. Opiate use for chronic pain is limited, however, by a rapid development of opiate tolerance and physical dependence. Many attempts to develop a better opiate have been based on a flawed or incomplete hypothesis of tolerance development and dependence liability. The proposed investigations will provide a new approach to tackle these side effects and will expedite further efforts to exploit the interactions between opioid receptors at the molecular, pharmacological and behavioral levels. Thus, the outcomes of the present studies will have a sustained, powerful impact on the fields of the biology and pharmacology of opioid receptors with the prospects of novel, safer and more effective pharmacotherapeutic strategies for the treatment of chronic pain.