Multiple Opioid Receptors and the Control of Pain
Location(s): United States
The delta (DOR) and mu (MOR) subtypes of G protein-coupled opioid receptors are widely distributed in the central and peripheral nervous system. These receptors are targeted both by endogenous opioid peptides and by a host of exogenous opiate agonists, including morphine. Despite years of research, there are still numerous unanswered questions concerning the differential contribution of the DOR and MOR to the complex behaviors influenced by opioid agonists, including pain control, addiction and reward. In part, the lack of progress reflects the tools that are available to study the expression, interaction and function of the receptors in different CNS circuits. To this end, we have developed a delta opioid receptor (DOReGFP) reporter mouse that led to a complete reappraisal of the circuits that are influenced by agonists that act at the DOR. Contrary to the prevailing view, we find that the DOR and the MOR are expressed in non-overlapping subsets of dorsal root ganglia (DRG) "pain" transmission neurons (nociceptors) and regulate distinct pain modalities. The DOR is expressed in myelinated sensory neurons and in a subset of the non-peptidergic unmyelinated nociceptors and regulates mechanical pain and the mechanical hypersensitivity produced in the setting of injury. The MOR predominates in the peptidergic nociceptors, which express the capsaicin and heat responsive channel, TRPV1, and regulates heat pain. The present proposal builds upon these observations. Studies in Specific Aim 1 will use neuroanatomical methods to assess the extent to which segregation of the DOR and MOR also occurs in the spinal cord and brain, in the normal animal and in the setting of tissue or nerve injury. Specific Aim 2 uses a combination of behavioral, pharmacological and genetic methods to determine the differential contribution of the DRG and spinal cord neuron expression of the DOR and MOR to nociceptive processing and to the antinociceptive effects of opioid compounds. Finally, Specific Aim 3 will extend the pharmacological analysis to the differential contribution of DOR and MOR agonist action at supraspinal targets. Taken together these studies will not only provide new information as to the organization of endogenous opioid receptor systems, but will also assess the extent to which different modalities of pain can be controlled by opioid agonists that selectively target these receptors, in the peripheral and central nervous systems. Information derived from these studies will be an important contributor to the development of pain-relieving drugs with better side effect profiles.
Although it is well established that opioid receptors, which are widely distributed in the central and peripheral nervous system, mediate the pain relieving effects of drugs such as morphine, recent studies in our laboratory demonstrated that the mu and delta opioid receptor subtypes regulate, independently, distinct modalities of pain (e.g. thermal vs. mechanical and the mechanical hypersensitivity that occurs in the setting of injury). By examining the contribution of these receptor subtypes at all levels of the pain pathway, studies in this proposal will provide critical information for the development of new pain therapeutics, with improved side effect profiles.