ORCID
https://orcid.org/0000-0002-1674-7761
Date of Award
9-11-2023
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Pain is a complex experience characterized by sensory stimulus perception and higher order emotional processing. Acute pain is imperative to survival until it becomes maladaptive upon the transition from acute to chronic. Chronic pain is the most common cause of long-term disability with an estimated 20.9% of U.S. adults having chronic pain in 2021 and an estimated $560 billion each year in direct medical costs, lost productivity, and disability programs. Despite this immense impact, there are insufficient therapeutic options for treating chronic pain and those that do improve patient conditions become less effective over time. Therapeutic discovery in this area has been challenging due to reproducibility and translational issues in the preclinical study of pain. Here we identify these specific issues and propose new methods to overcome challenges to therapeutic development while also identifying a novel potential target of chronic neuropathic pain. Neuropathic pain causes both sensory and emotional maladaptation. Preclinical animal studies of neuropathic pain-induced negative affect could result in novel insights into the mechanisms of chronic pain. Modeling pain-induced negative affect, however, is variable across research groups and conditions. The same injury may or may not produce robust negative affective behavioral responses across different species, strains, and laboratories. We found no significant effect of spared nerve injury across a variety of approach-avoidance, hedonic choice, and coping strategy assays. We hypothesized these inconsistencies may stem in part from the short test duration of these assays. To test this hypothesis, we used the homecage-based Feeding Experimentation Device version 3 to conduct 12-hour, overnight progressive ratio testing to determine whether mice with chronic spared nerve injury had decreased motivation to earn palatable food rewards. Our data demonstrate that despite equivalent task learning, spared nerve injury mice are less motivated to work for a sugar pellet than sham controls. Further, when we normalized behavioral responses across all the behavioral assays we tested, we found that a combined normalized behavioral score is predictive of injury-state and significantly correlates with mechanical thresholds. Together these results suggest that homecage-based operant behaviors may be useful for modeling nerve injury-induced negative affect and that valuable pain-related information can arise from amalgamative data analyses across behavioral assays - even when individual inferential statistics do not demonstrate significant mean differences. Most animal models of neuropathic pain use targeted nerve injuries quantified with reflexive measures in response to an applied noxious stimulus. However, reflexive quantification is not representative of spontaneous pain which may be more associated with human chronic pain conditions. With this in mind, we attempted to develop and objective pain quantification tool using a deep learning-based markerless pose estimation tool to quantify spontaneous limb position in C57BL/6J mice during tail suspension following either SNI or sham surgery. Using this granular detail, we identified the expected flail foot-like impairment, but we also found SNI mice hold their injured limb closer to the body midline compared to shams. Together these results suggest SNI causes previously undescribed phenotypes unrelated to altered sensation that are likely underappreciated while interpreting preclinical pain research outcomes. The locus coeruleus (LC) provides the largest noradrenergic innervation to the central nervous system and is a major node in pain neural circuitry. The LC responds to noxious stimuli with a shift from low tonic to phasic activation and stimulation of the LC has been shown to be antinociceptive. Paradoxically, however, here we show that optogenetic inhibition of LC activity is also acutely antinociceptive - a finding opposite of the canonical role of LC neurons in nociceptive processing. To determine the mechanistic underpinnings of this novel phenomenon, we focused on the dense concentration of mu opioid receptors (MORs) in the LC. Intra-LC MORs have been implicated in several biological processes including neuropathic pain and stress. Literature suggests intra-LC MORs may be particularly critical for LC-mediated changes in nociception. Pharmacological activation of these MORs has been shown to dose-dependently inhibit LC neuronal activity and promote antinociception. To determine the role of intra-LC MORs in nociceptive processing we bred a MOR conditional knockout (cKO) mouse line. In these mice, MORs are deleted in cells where MORs coexpress with the critical enzyme required to produce norepinephrine (dopamine beta hydroxylase). This conditional deletion of MORs in dopamine beta hydroxylase cells resulted in a decrease in nociceptive thresholds at baseline. To determine whether intra-LC MORs are sufficient to return lower withdrawal thresholds in MOR cKO mice back to baseline, we cell type-selectively rescued MOR function in these mice using a photoactivatable MOR, opto-MOR. Rescue of mu opioid receptor function, via stereotaxic cre-dependent delivery of human MOR into the LC, in mice experiencing chronic neuropathic pain also resulted in reversal of a hypersensitive state. Together, our results suggest the LC controls basal nociception in a MOR-dependent manner that becomes highly dynamic in a neuropathic pain state. Altogether, results from our studies have championed reexamination of common approaches to studying and quantifying pain preclinically with a hope to improve viability of chronic pain targets identified in preclinical studies like the intra-LC MORs identified here.
Language
English (en)
Chair and Committee
Jordan McCall
Recommended Citation
Norris, Makenzie, "The Painful Problem: Improving Preclinical Pain Assessment to Probe Locus Coeruleus Noradrenergic Adaptations During Chronic Pain" (2023). Arts & Sciences Electronic Theses and Dissertations. 3168.
https://openscholarship.wustl.edu/art_sci_etds/3168