The current prescription opioid epidemic is characterized by high lifetime rates of relapse, often precipitated by the experience of pain. However, there is a dearth of preclinical studies examining the effect of pain on relapse into opioid use. Drug craving and relapse have been modeled in laboratory rats using the incubation of drug craving model, which refers to the time-dependent increase in drug seeking after withdrawal, a phenomenon that occurs in both rat models and humans. The objective of this proposal is to develop an animal model of prescription opioid craving and relapse and study the putative shared neurobiological mechanisms of prescription opioid relapse and pain. This proposal is focused on the central amygdala (CeA), a region involved in both the incubation of drug craving across drug classes and pain, and the neuropeptide Oxytocin (Oxt). This neuropeptide, classically associated with the hormonal regulation of reproductive functions and associated social bonding, also modulates neurotransmission in extrahypothalamic areas and decreases drug consumption, relapse and pain. Oxt receptors (OxtR) are expressed in the CeA and modulate local neurotransmission, but no studies have examined the role of CeA OxtR in drug craving and relapse with relation to pain. Our central hypothesis is that CeA OxtR signaling is a critical shared mechanism for incubation of oxycodone craving and pain. Here, we propose to develop a rat model of incubation of oxycodone craving and test the role of CeA OxtR in this model. Furthermore, given that the duration of pain in classical rodent models is not under experimenter control, we propose to develop a novel DREADD-based chemogenetic model of pain, in which repeated intermittent administration of clozapine n-oxide (CNO) will cause experimenter-controlled intermittent pain. We will test the role of CeA OxtR signaling in this novel model and determine the effect of intermittent pain on the incubation of oxycodone craving.
Lifetime relapse rates remain a major obstacle to addressing the current opioid epidemic, and former drug users often cite pain as a major reason for relapse1,2. Despite this clinical literature, preclinical rodent studies of prescription opioid relapse (e.g., oxycodone) are very limited15-17, and to our knowledge, a preclinical model examining the effect of pain on opioid relapse does not exist. We propose to initiate a new line of research to address this knowledge gap. Basic research focusing on opioid addiction and pain is significant from a public health perspective, as it will result in a better understanding of this major medical problem and may lead to the identification of novel approaches to prevent pain-induced opioid use and relapse. The central amygdala (CeA) is a region critical to both incubation of drug craving18-19 and pain modulation20-21. The CeA receives nociceptive input from spinal cord and hindbrain and projects to hypothalamic and extrahypothalamic regions, including regions involved in the descending pain modulation pathway. The CeA is critically involved in both the enhancement and reduction of pain, and can be activated following
hyperalgesic or hypoalgesic stimuli. Furthermore, the CeA has a high density of neuropeptides, which can modulate local neurotransmission to affect behavioral responses to noxious or painful stimuli33-33. There is a growing body of evidence that suggests targeting the brain Oxt system could provide a much-needed treatment breakthrough for addiction. A robust body of preclinical evidence (reviewed in27) and emerging clinical studies show that Oxt may have beneficial effects at each stage of the above-described addiction cycle, across all major classes of substances of abuse. Briefly, Oxt not only reduces drug consumption and drug reward, but also restores normal responding to at least some natural reinforcers, reduces heightened emotionality and stress reactivity during withdrawal and abstinence, and prevents reinstatement to drug seeking. OxtR are expressed widely throughout the central nervous system (CNS), including the CeA. Within the CeA, Oxt, synthetic Oxt or optogenetically induced local Oxt release was found to reduce behavioural fear responses in rats. Imaging studies have also linked the anxiolytic effects of intranasal Oxt in humans to actions in the amygdala. Furthermore, few studies demonstrated that oxytocin and its receptors are involved in nociceptive modulation27 possibly via a CeA action34.
Therefore, we propose to test the hypothesis that OxtR signaling in CeA is a critical shared mechanism of opioid relapse (Aim 1) and pain (Aim 2).
We also propose to develop a novel model of pain-induced oxycodone relapse that will allow for temporal control over pain onset and offset. These features are critical for establishing an animal model of pain-induced relapse (Aim 3). Pain can be induced in rodents by using classical models of inflammatory pain [e.g., the complete Freund¿s adjuvant model]. However, in this model and related pain models like the formalin test, the timing of pain onset/offset is not under experimenter control and animals learn to associate the injection site with pain. Recently, researchers have used optogenetic and chemogenetic technologies with a gene promoter for the sensory nociceptive receptor TRPv1, and retro-AAVs to inhibit nociceptive sensory afferents and reduce pain. However, these methodologies have yet to be adapted for the induction of pain, which would allow for a temporally controlled model of pain. We propose to develop a DREADD-mediated virus (AAV6.TRPv1.hM3Dq.mCherry) and inject this "retro-DREADD" virus in the rat's paw. This will result in transfection of nociceptive dorsal root ganglion neurons that project to the injected paw. Next, we will inject the designer drug clozapine n-oxide (CNO) to activate peripheral hM3Dq receptors and produce pain, allowing for temporal control of pain onset and offset. In this model system, repeated intermittent injection of CNO would induce intermittent pain to the rat. To sum up, we will use a combination of classical behavioral, physiological, and pharmacological techniques, and a novel viral method to investigate the putative shared neurobiological mechanisms of opioid relapse and pain.
We will also develop the first model of pain-induced opioid relapse and craving that will allow for mechanistic investigation of this important clinical problem. Using the chemogenetic-induced model of pain, we will determine the role of CeA OxtR signaling pain sensitivity (Aim 2). Finally, we will use this novel temporally controlled pain model to test the effect
of intermittent pain exposure on the magnitude of incubation of oxycodone craving (Aim 3). Importantly, though the focus of these aims is Oxt, we will examine the role of alternative molecular candidates within each aim, such that the success of a given aim or experiment is not dependent on another.