Browsing by Subject "opioid"

Now showing 1 - 6 of 6
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    Characterization of a novel opioid combination for the treatment of chronic pain
    (2020-08) Bruce, Daniel
    The peripherally-restricted combination of loperamide and oxymorphindole is a potent and efficacious preclinical analgesic treatment that relieves the behavioral hyperalgesia caused by inflammatory, post-operative and neuropathic pain states. The combination displayed analgesic synergy across all assays, pain conditions and species tested here, and was also effective in a non-evoked measure of spontaneous pain. From a clinical translation standpoint, the combination confers a protective phenotype relative to clinically approved opioids, demonstrated by the lack of constipation at therapeutic doses, no respiratory depression even at supratherapeutic doses, chronic therapeutic dosing that is devoid of analgesic tolerance, and significantly limited risk for self-administration. Mechanistically, the combination exerts its analgesic action by binding to opioid receptors¬—specifically MOR-DOR heteromers—on peripheral sensory afferent neurons, preferentially activating G protein-dependent signaling cascades to reduce neuronal excitability. Taken together, this thesis provides strong support for the continued investigation into peripheral opioid mechanisms and analgesic synergy as a path forward in our continued fight to develop better pharmacological pain treatment paradigms.
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    Inhibitory signaling and reward: the role of GIRK channels in the mesocorticolimbic dopamine system
    (2015-12) Kotecki, Lydia
    Drug abuse is a critical global problem and given the current lack of efficacious pharmacotherapies to treat addiction, it is vital that we gain a better understanding of the cellular and molecular targets of drugs of abuse, and how modulation of these targets leads to addiction. Although different abused drugs work through different mechanisms of action, addiction is thought to have a common pathway, the mesocorticolimbic system, which consists of the ventral tegmental area (VTA) and reciprocal connections with downstream targets. G protein-coupled receptors (GPCRs) and downstream effectors represent a major target for drug-induced changes in mesocorticolimbic signaling. G protein-gated inwardly rectifying K+ (GIRK) channels are a key downstream target of inhibitory GPCRs and have been shown to mediate the inhibitory effects of many neurotransmitters in the CNS. Dysregulation of GPCR-GIRK signaling has been identified in a number of disorders, including addiction. This dissertation focuses on the role of GIRK-dependent inhibitory signaling throughout the mesocorticolimbic system, and how this form of signaling contributes to the cellular and reward-related behavioral effect of drugs of abuse. The importance of GIRK-dependent signaling to drug addiction is supported by the fact that several drugs of abuse can produce adaptations to the GPCR-GIRK signaling cascade. GIRK-dependent signaling was also believed to be a key regulatory of opioid effects within the mesocorticolimbic system, thought to ultimately contribute to opioid reward. Here, we challenged the opioid signaling “dogma” and show that GIRK-dependent signaling in midbrain GABA neurons is not required for disinhibition of VTA DA neurons and subsequent opioid-induced motor-stimulation. Interestingly, it appears the unique GIRK2/GIRK3 channel found in VTA DA neurons can modulate the behavioral sensitivity to opioids. In addition, we found that this GIRK2/GIRK3 channel can also modulate cocaine locomotor sensitivity, suggesting it could be a shared mechanism of other drugs of abuse. Taken together, these findings demonstrate that GPCR-GIRK inhibitory signaling in VTA DA neurons is critical to the molecular and behavioral effects of drugs of abuse. These findings also suggest that the discovery of novel compounds that modulate GIRK channel function in a subunit-dependent manner could help us prevent and/or treat addiction.
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    Mechanisms Underlying Opioid Modulation of Gut Immunity
    (2014-08) Meng, Jingjing
    Opioids are used widely by clinicians due to their potent analgesic activities and sedative properties. However, opioid use or abuse is associated with multiple adverse gastrointestinal (GI) symptoms and higher susceptibility to infection caused by pathogens with gut origin. Both clinical and laboratory studies implied that opioids showed suppressive effects on gut immunity and predisposed critically sick patients to infections while the mechanism underlying this defect is still unknown. In the present study we investigated how opioids modulate gut epithelial barrier function and immune responses of gut associated lymphoid tissue (GALT). We demonstrated significant bacterial translocation from gut lumen to mesenteric lymph node (MLN) and liver following morphine treatment in wild-type (WT) animals that was significantly attenuated in Toll-like receptor (TLR2 and 4) knockout mice. We further observed significant disruption of tight junction protein organization only in the ileum but not in the colon of morphine treated WT animals. Inhibition of myosin light chain kinase (MLCK) blocked the effects of both morphine and TLR ligands, suggesting the role of MLCK in tight junction modulation by TLR. Additionally we determined the immune responses of GALT to polymicrobial sepsis in the presence and absence of opioids by using a murine cecal ligation and puncture model. The results showed that opioids accelerated the mortality rate of polymicrobial sepsis. During sepsis progression, morphine treatment altered gut microbiome and subsequently promoted gram-positive bacterial dissemination, which induced excess IL-17A production in a TLR2-dependent manner, resulting in increased gut permeability, sustained inflammation and higher mortality. This study improved our understanding of the role of morphine in modulating gut barrier functions and the roles of GALT in infection susceptibility, which may provide the potential therapeutic targets for novel drug development and lead to more powerful strategy to control or prevent severe infectious diseases like sepsis especially in the opioid using and abusing population.
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    Modulation of Adult Neurogenesis in Opioid Addiction
    (2016-05) ZHANG, YUE
    One of the major problems in treatment of opioid addiction is the repeated reuse and long-term memory of the drug-experience even after prolonged periods of abstinence. During the past decades, there has been enormous expansion in our understanding of how opioid drugs act on the nervous system. A complex brain network including the mesolimbic dopamine system, ventral striatum, extended amygdala, prefrontal cortex and hippocampus is suggested to be associated with the addiction cycle, in particular, the adult neurogenesis taken place in the dentate gyrus (DG) of the hippocampus has a functional implication in opioid addiction. It is intriguing to study the convergence between the modulation of adult neurogenesis and opioid addiction, since the adult-born granule cells were shown to play a role in neuroplasticity of hippocampus function and in the development and retention of drug-contextual memory. In the first part of my study, I attempted to define the temporal window of morphine’s inhibitory effect on adult neurogenesis with a transgenic mouse model. Four days of conditioned place preference (CPP) training with morphine significantly reduced the number of late stage progenitors and immature neurons in the sub-granular zone (SGZ) of mouse hippocampus but did not affect the number of early progenitor cells. The results from colocalization of cell-type selective markers suggested that under the condition of CPP training, morphine affects the transition of neural progenitor/stem cells differentiate into immature neurons. When the transcription factor neural differentiation1 (NeuroD1) was over-expressed in DG by stereotaxic injection of lentivirus, it rescued the loss of immature neurons and prolonged the extinction of morphine-trained CPP. Next, a synthetic small molecule KHS101 which was reported to increase NeuroD1 mRNA in cultured neural progenitors and induce neuronal differentiation in the DG of hippocampus, was utilized to mimic the effect of lentivirus-mediated NeuroD1 overexpression on morphine-primed CPP. The results indicated that subcutaneous injection of KHS101 before conditional training could enhance the retention of drug-related memory and prolong CPP extinction; while the same treatment after conditional training disrupted the drug-contextual associations and shortened CPP extinction. Such KHS101’s effect paralleled that observed when the over-expression of NeuroD1 was temporally controlled with an inducible tetracycline system. Furthermore, the KHS101’s effect could be abolished by the stereotaxic injected NeuroD1 shRNA lentivirus. These studies suggest that morphine decrease the total numbers of newborn neurons in the SGZ by interfering with neural progenitors’ differentiation via a mechanism involving NeuroD1. Since adult neurogenesis serves as an important form of neural plasticity, we assume that certain immature neurons contribute to the formation and consolidation of drug-contextual association memory, and NeuroD1 plays a key role during this process. Such assumption is supported by the observation that compounds such as KHS101 that could regulate NeuroD1 expression in the hippocampus possess the ability to manipulate the extinction of drug contextual memory. In conclusion, the regulation of NeuroD1 activity leads to modulation of adult neurogenesis, thus affecting the drug-association memory.
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    A Non-Canonical Role of Angiotensin-Converting Enzyme in Synaptic Plasticity
    (2022-06) Trieu, Brian
    A diverse repertoire of endogenous opioid peptides are found within the brain, but because they are often co-expressed and co-released with other peptides, their role in synaptic plasticity remains elusive. These neuropeptides can have profound control over synaptic transmission upon binding to opioid receptors, particularly within the nucleus accumbens where converging signals are integrated to drive motivated behaviors. Neuropeptide effects are often terminated by extracellular degradation, but the mechanisms underlying this are also poorly understood. The identification of specific endogenous opioid peptides and insight into their extracellular regulation can reveal under-appreciated mechanisms that influence opioid receptor signaling to modulate the elaborate neuronal connectivity within this region. The studies presented in this dissertation show that an unconventional and potent endogenous opioid called Met-Enkephalin-Arg-Phe (“MERF”), an enkephalin heptapeptide, dose-dependently inhibited excitatory synaptic transmission onto medium spiny neurons (MSNs) in mouse brain slices. Angiotensin-converting enzyme (ACE) classically regulates blood pressure in the periphery and was found to non-canonically degrade endogenous MERF thereby regulating its effect in the nucleus accumbens. Liquid chromatography-tandem mass spectrometry analysis showed that a class of cardiovascular medications called ACE inhibitors selectively preserved extracellular MERF without affecting conventional enkephalins. ACE inhibitors alone unveiled cell type-specific depression of glutamate release onto MSNs expressing the Drd1 dopamine receptor (D1-MSNs), but not onto those expressing the Drd2 receptor (D2-MSNs). Glutamatergic synaptic depression was mediated by MERF binding to presynaptic µ-opioid receptors and was absent after conditional genetic deletion of ACE. Fiber photometry recordings of D1-MSNs in vivo demonstrated decreased sensitivity to optogenetic stimulation of excitatory medial prefrontal cortex following systemic administration of the ACE inhibitor captopril. Furthermore, mice given captopril displayed attenuated fentanyl-induced place preference and increased social behavior with other mice. Collectively, this dissertation defines an endogenous mechanism of synaptic plasticity induced by MERF and gated by ACE. We interpret this to be preclinical evidence for a class of safe and efficacious cardiovascular medications that could be repositioned or redesigned to mitigate brain conditions with underlying aberrant striatal pathophysiology characterized by an imbalance of D1- to D2-MSN synaptic activity.
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    Sex differences in morphine reward and glutamatergic regulation of the nucleus accumbens
    (2021-12) Lopresti, Natalie
    Opioid substance use disorder (SUD) affects millions of Americans, and death rates are increasing with the introduction of more potent opioids. Sex differences in the analgesic properties of opioids are well established. Whether similar sex differences are observed in the addictive properties of opioids is less-well understood. Making comparisons across experiments is problematic as methodology vary. In this dissertation, we examined the effects of morphine in male, female, gonadectomized (GDX) male, and GDX female mice using conditioned place preference, a behavioral paradigm specifically looking at the rewarding effects of a drug. We found that GDX males had significantly higher reinstatement to re-exposure to morphine compared to intact males, whereas there were no differences between intact males and intact females. Replacement of testosterone to GDX males did not cause their reinstatement to resemble that of intact males, suggesting that circulating testosterone was not the cause of the difference in behavior. In a separate study we next examined potential sex differences to acute morphine exposure using transgenic mice that allowed us to visualize neurons that were active during exposure to drug. Using Fos-TRAP, we quantified active neurons in the nucleus accumbens (NAc), a region known to be involved in processing of reward. With this acute exposure, we see that the NAc shell (NAcSh) has significantly more active cells after acute morphine than our control saline mice. This importance of the NAcSh led us to finally examine excitatory inputs to the NAcSh and see how their activation impacts behavior in intra-cranial self-stimulation, specifically examining the medial prefrontal cortex (mPFC) and ventral hippocampus (vHPC). We find that female mice learn more quickly how to increase their time receiving mPFC stimulation compared to male mice. We also see that mPFC stimulation tends to cause animals to reside in the region where they receive stimulation, whereas vHPC stimulation causes animals to dart in and out of the stimulation region to increase stimulation time by bypassing the timeout period. Overall, these studies show us that, while there are no overt male / female differences in our CPP reward paradigm, we see male / GDX male differences that are not restored by addition of exogenous testosterone. We also showed that the NAcSh shows increased activation after acute morphine exposure, and that the two glutamatergic inputs we examined elicit differing strategies for increased stimulation of the region. Together, these findings show that further studying the sex differences in opioid reward and understanding how the NAcSh processes reward are important to help better treat SUD.