Neuropeptide Y Y2 receptors in acute and chronic pain and itch.

Pain and itch are regulated by a diverse array of neuropeptides and their receptors in superficial laminae of the spinal cord dorsal horn (DH). Neuropeptide Y (NPY) is normally expressed on DH neurons but not sensory neurons. By contrast, the Npy2r receptor (Y2) is expressed on the central and peripheral terminals of sensory neurons but not on DH neurons. Neurophysiological slice recordings indicate that Y2-selective agonists inhibits spinal neurotransmitter release from sensory neurons. However, behavioral pharmacology studies indicate that Y2 agonists exert minimal changes in nociception, even after injury. Additional discrepancies in the behavioral actions of the Y2-antagonist BIIE0246 - reports of either pronociception or antinociception - have now been resolved. In the normal state, spinally-directed (intrathecal) administration of BIIE0246 elicits ongoing nociception, hypersensitivity to sensory stimulation, and aversion. Conversely, in the setting of nerve injury and inflammation, intrathecal BIIE024 reduced not only mechanical and thermal hypersensitivity, but also a measure of the affective dimension of pain (conditioned place preference). When administered in chronic pain models of latent sensitization, BIIE0246 produced a profound reinstatement of pain-like behaviors. We propose that tissue or nerve injury induces a G protein switch in the action of NPY-Y2 signaling from antinociception in the naïve state to the inhibition of mechanical and heat hyperalgesia in the injured state, and then a switch back to antinociception to keep LS in a state of remission. This model clarifies the pharmacotherapeutic potential of Y2 research, pointing to the development of a new non-opioid pharmacotherapy for chronic pain using Y2 antagonists in patients who do not develop LS.