Postoperative pain is a major morbidity of surgery. New therapies that treat both sensory and affective pain symptoms while preserving the respiratory drive are urgently needed. The development of novel analgesics is hindered, however, by a fundamental gap in understanding how pain is regulated in the brain. The long-term goal of this proposal is to understand the central regulation of postoperative pain. Glutamate signaling in the nucleus accumbens (NAc) is known to modulate pain symptoms. However, at the mechanistic level, the source of regulatory glutamate inputs to the NAc is not defined, and the ability to target isoxazolepropionic acid (AMPA) receptors, main receptors for glutamate, in the NAc to treat pain is not established. Furthermore, it is not known if glutamate signaling in the NAc is a common mechanism to control both acute and chronic postoperative pain. The overall objective of this application is to define the role of a-amino-3-hydroxy-5-methyl-4- glutamate signaling in the projection from the prefrontal cortex (PFC) to the NAc for the regulation of acute and chronic postoperative pain. The central hypothesis is that glutamate input from the PFC to the NAc decreases pain and that AMPAkines can enhance glutamate signaling in the NAc to treat postoperative pain. This hypothesis is supported by preliminary data showing that optogenetic activation of the PFC-NAc circuit inhibits persistent pain, and that systemic and intra-NAc delivery of AMPAkines relieves chronic postoperative pain.
In Aim 1, the analgesic effect of the glutamate projection from the PFC to the NAc will be defined in two rat models: the paw incision model for acute postoperative pain and the spared nerve injury model for persistent postoperative pain. Light-sensitive channel rhodopsins and halorhodopsins will be used to optogenetically activate and deactivate this glutamatergic projection respectively, using techniques established in preliminary experiments. Next, the impact of optogenetic modulation of the PFC-NAc circuit on sensory and affective pain symptoms will be quantified in detail using standard pain behavior assays.
In Aim 2, the analgesic efficacy and mechanism for AMPAkines will be established. AMPAkines are known to increase glutamate signaling in the medulla to treat opioid-induced hypoventilation. Thus, the pain-relieving property of clinically available AMPAkines will be determined, both as single agents and in combination with opioids, in postoperative pain models. Next, the ability of AMPAkines to increase AMPA receptor currents in the NAc will be quantified using electrophysiological recordings, and behavior tests will determine that AMPA receptors in the NAc specifically mediate the analgesic effects of AMPAkines. This project is innovative because it applies a new systems neuroscience approach to uncover a novel central pain-inhibitory mechanism. The work is significant because it identifies the PFC-NAc pain-inhibitory circuit as a potential target for neuromodulation therapies and, more importantly, it establishes AMPAkines as postoperative drugs that can treat both sensory and affective symptoms of pain while opposing opioid-induced hypoventilation, laying the groundwork for clinical trials.
Postoperative pain is a major public health issue, and affective pain symptoms such as depressed mood and respiratory suppression induced by opioid analgesics impair recovery. The proposed research is relevant to the NIH's mission because it will establish glutamate signaling in the brain circuit connecting the prefrontal cortex to the nucleus accumbens as a novel mechanism for the regulation of acute and persistent postoperative pain. This discovery will identify a potential anatomic target for neuromodulation therapies, and it can establish AMPAkines, enhancers of glutamate signaling, as a new class of postoperative analgesics that treat both sensory and affective symptoms of pain while supporting the respiratory drive.
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