Gregory Corder, Ph.D. - US grants

DESCRIPTION (provided by applicant): Peripheral tissue inflammation can lead to maladaptive plasticity in the spinal cord and brain which contributes to persistent pain. The intensity and quality of pain is determined by the net balance between the activities of pronociceptive systems with compensatory endogenous inhibitory systems, namely spinal opioid signaling. Interestingly, a small set of studies indicate that endogenous opioid inhibition f acute nociception persists even after the initial signs of hyperalgesia have subsided. For example, opioid receptor antagonists reinstate allodynia. This raises two intriguing questions. First, opioid receptor subtypes and neural circuits remain unclear. Second, does a latent nociceptive sensitization persist in the absence of overt behavioral signs of hypersensitivity? My preliminary data show that naltrexone, when intrathecally administered weeks to months after intraplantar CFA, reinstated behavioral signs of hypersensitivity and induced dorsal horn ERK phosphorylation. Both were blocked by spinal antagonism of NMDA receptors. My central hypothesis is that peripheral inflammation induces prolonged signaling of CNS opioidergic-circuits (Aim 1) that mask pronociceptive NMDA and AMPA signaling (Aim 2). This F31 proposal attempts to better characterize the mechanisms that underlie the latent sensitization that is masked by endogenous opioid activity. Aim 1 tests the hypothesis that spinal opioidergic signaling tonically masks nociceptive sensitization. Aim 1A investigates ¿-, ¿-, and ?-opioid receptor subtypes with the use of selective antagonists. Aim 1B and 1C test mechanisms of constitutive receptor signaling and tonic opioid release with the use of ex vivo spinal cord slice GTP ?S35 binding assay and intrathecal sequestering opioid peptide antiserum, respectively. Aim 1D tests the hypothesis that opioid receptor blockade disinhibits tonic afferent nociception. In Aim 2 I will test the idea that glutamatergic signaling drives the hypersensitivity that follows spinal opioid receptor blockade, with a focus on spinal NMDA (Aim 2A) and AMPA/kainate-receptors (Aim 2B). By better understanding long-lasting opioid antinociception, this project could pave the way for future strategies to enhance endogenous opioid analgesia in humans with chronic pain, and thus has the long-term potential to reveal novel targets to prevent the transition for acute to chronic pain. This F31 award will help me achieve my goals that will enable me to successfully compete for strong post-doctoral positions and ultimately, a successful career in research science and teaching beginning with a tenure-track position in a strong medical research university environment: PUBLIC HEALTH RELEVANCE: Sensitization of pain pathways, in the setting of tissue inflammation, leads to a state of hypersensitivity that is counteracted by compensatory endogenous inhibitory systems. This project aims to understand the mechanisms by which the spinal opioid system masks long-lasting glutamatergic sensitization. Therapeutic strategies designed to enhance this endogenous opioid signaling may reduce the occurrence of persistent pain.

? DESCRIPTION (provided by applicant): A major goal of translational pain research is to develop novel analgesic strategies that retain efficacy and do not pose unnecessary harm or discomfort to the patient being treated. However opioid drugs, such as morphine, remain the gold-standard for clinical pain relief but are severely limited by several detrimental sides effect including dependence, tolerance, and in select cases, a paradoxical pain state referred to as opioid- induced hyperalgesia (OIH). After millennia of documented opioid use, the specific neural circuits and synaptic mechanisms underlying the generation of analgesic tolerance and OIH are still disputed, and minimal gains have been made to improve opioid efficacy and safety. Opioid analgesia is produced via stimulation of mu opioid receptors (MORs) expressed along the nociceptive neural circuits, including TRPV1+ primary afferent nociceptors and numerous neurons in spinal cord and brain. The extent to which each of these MOR+ neuronal populations contributes to opioid analgesia, tolerance and OIH is currently not known, preventing the development of novel therapeutics to ameliorate opioid-based pain management. Prior electrophysiological studies found that brief opioid exposure is sufficient to generate long-term potentiation (LTP) between peripheral nociceptors and spinal cord neurons, signifying that this synapse might be a key neuronal substrate for morphine tolerance and OIH. The synaptic origin of opioid-induced LTP is presently unclear. Interestingly, previous studies indicated that ablation of TRPV1+ sensory neurons alone is sufficient to attenuate the development of morphine analgesic tolerance, OIH, and spinal LTP induction, suggesting that chronic opioid dosing may facilitate maladaptive plasticity in peripheral nociceptors that limit analgesic action. To further expand on this observation, we generated a conditional MOR knockout mouse line where MORs are absent in TRPV1+ afferents (MORTRPV1cKO), but intact in the spinal cord and brain. Strikingly, preliminary behavioral pharmacology studies show normal morphine analgesia but near total abrogation of tolerance and OIH. Here I propose to use an innovative combination of mouse genetics, optogentic-guided electrophysiology and behavioral pharmacology to test my central hypothesis that MORs in TRPV1+ nociceptors initiate the maladaptive plasticity underlying morphine analgesic tolerance and OIH. First, using light-assisted electrophysiological recordings in MORTRPV1cKO mice I will rigorously evaluate the effect of chronic morphine on synaptic transmission in peripheral afferent subpopulations. As a translational compliment study, I will test whether pharmacological blockade of MOR signaling in primary afferents is sufficient to abrogate morphine analgesic tolerance, OIH, and maladaptive synaptic plasticity. The cutting-edge methods and results obtained from these studies are expected to broadly impact our understanding of the neural circuits underlying morphine pathological sensory consequences and aim to develop novel therapeutic strategies to bolster opioid efficacy while minimizing side-effects.

Summary Chronic pain is not merely a persistent sensory disorder, but a neurological disease of affective dysfunction that negatively impacts the mental state, professional goals, and personal relationships of over 100 million Americans. Emotionally-guided behaviors, such as avoiding pain and seeking pleasure, are derived from valence information generated by the limbic brain. The ability of valence circuits to categorize external and internal sensory information as either ?pleasant? or ?unpleasant? is essential for behavior selection, protective learning, and survival. However, miscoding of sensory information due to pathological plasticity within these valence circuits can produce unwanted psychological effects, including the suffering and depression associated with chronic pain. The amygdala is a brain region critical for processing emotional valence and influencing motivational drive. However, the functional relevance of amygdalar valence processing to the generation of hedonic perception and behavior-selection is defined primarily by its output connectivity with effector structures in limbic and cortical regions. Recent evidence proposes the existence of innate and distinct neuronal circuits for opposing positive and negative valence processing in the basolateral nucleus of the amygdala (BLA) that also diverge based on the downstream target structures, such as the nucleus accumbens (NAc). However the network-level interface between these opposing BLA valence circuits has been largely unexplored. Here, I propose to uncover the dynamic interactions of BLA valence circuits to determine their contribution to pain and hedonic affect, both locally within the BLA and at their long-range targets in the NAc. During the mentored K99 phase, my career development and training will be supervised by my co-mentors, Drs. Gregory Scherrer and Mark Schnitzer, with additional support from Drs. Robert Malenka, Sean Mackey, and Brian Kobilka. To investigate the neural network mechanisms driving pain unpleasantness and comorbid anhedonia, I will receive expert training in optogenetic-guided brain slice electrophysiology and time-lapse in vivo Ca2+ imaging in freely behaving mice to uncover the functional interactions of neural ensembles encoding nociceptive and appetitive sensory information throughout the development of chronic pain. During the independent R00 phase, I will determine whether BLA valence circuits that differently innervate the NAc define functionally and anatomically distinct ?hedonic zones? within opioidergic circuits. I will further investigate the relevance of these zones to behavior-selection and reinforcement during acute and chronic pain, and during drug use conditions. The advanced training I will receive during this K99/R00 award will lay the foundations for my future research program and NIH grant applications. This award will help me advance my own scientific capabilities, and bolster my career as a successful, independent research scientist and mentor. The successful completion of this work will also have important public health benefits as it will guide future efforts on novel analgesic strategies to reduce pain and lessen the need for prescription opioids.

PROJECT SUMMARY Opioid addiction is a chronic, progressive disorder that fuels the current US epidemic of opioid overdose deaths. Over the years, a tremendous amount of research effort has been devoted to understanding the biological roles of opioid receptors and developing newer generations of synthetic opioids to treat pain and combat opioid addiction. However, given the advancement of contemporary and novel neuroscience technologies, we have the tools to think beyond mu-opioid receptors (MORs) to develop improved OUD therapeutics. This proposal aims to investigate the architecture and function of endogenous MOR-expressing neural circuits in the brain and to determine how these circuits maintain cellular dependence and drive brain-wide maladaptive plasticity across different stages of the OUD cycle. In four complementary aims, we will first map the shifting structural and functional connectivity of opioidergic networks using viral-genetic and tissue clearing methods to identify monosynaptic inputs to withdrawal-active MOR-expressing cells and axonal output projections, as a function of opioid exposure and abstinence. We will then integrate these input/output maps with cell-type information and gene expression changes within dependence networks using hyper-multiplexed 3D in situ hybridizations to generate the anatomic localization of hundreds of dependence-related genes, targeted to cell types and retro- labeled connections. Finally, to reveal how MOR-expressing cells within core regions are modulated during opioid exposure in real-time, we will use miniature head-mounted microscopes to image the population activity? at cellular resolution?across weeks of opioid exposure and withdrawal. Our models will provide formal summaries of activity, connectivity, and gene expression as they evolve with repetitive opioid exposure and withdrawal, and our datasets will be made publicly available as they are generated. To bridge these experimental measurements and provide a common framework for our analyses, we will adopt Network Control Theory to identify brain nodes that drive the transition between opioid dependence states to identify potential candidates that disproportionately drive each state.