The role of morphine- and fentanyl-induced impairment of intestinal epithelial antibacterial activity in dysbiosis and its impact on the microbiota-gut-brain axis

Recent evidence suggests that chronic exposure to opioid analgesics such as morphine disrupts the intestinal epithelial layer and causes intestinal dysbiosis. Depleting gut bacteria can preclude the development of tolerance to opioid-induced antinociception, suggesting an important role of the gut-brain axis in mediating opioid effects. The mechanism underlying opioid-induced dysbiosis, however, remains unclear. Host-produced antimicrobial peptides (AMPs) are critical for the integrity of the intestinal epithelial barrier as they prevent the pathogenesis of the enteric microbiota. Here, we report that chronic morphine or fentanyl exposure reduces the antimicrobial activity in the ileum, resulting in changes in the composition of bacteria. Fecal samples from morphine-treated mice had increased levels of Akkermansia muciniphila with a shift in the abundance ratio of Firmicutes and Bacteroidetes. Fecal microbial transplant (FMT) from morphine-naïve mice or oral supplementation with butyrate restored (a) the antimicrobial activity, (b) the expression of the antimicrobial peptide, Reg3γ, (c) prevented the increase in intestinal permeability and (d) prevented the development of antinociceptive tolerance in morphine-dependent mice. Improved epithelial barrier function with FMT or butyrate prevented the enrichment of the mucin-degrading A. muciniphila in morphine-dependent mice. These data implicate impairment of the antimicrobial activity of the intestinal epithelium as a mechanism by which opioids disrupt the microbiota-gut-brain axis.

Endocannabinoid biosynthetic enzymes regulate pain response via LKB1-AMPK signaling

Diacylglycerol lipase-beta (DAGLβ) serves as a principal 2-arachidonoylglycerol (2-AG) biosynthetic enzyme regulating endocannabinoid and eicosanoid metabolism in immune cells including macrophages and dendritic cells. Genetic or pharmacological inactivation of DAGLβ ameliorates inflammation and hyper-nociception in preclinical models of pathogenic pain. These beneficial effects have been assigned principally to reductions in downstream proinflammatory lipid signaling, leaving alternative mechanisms of regulation largely underexplored. Here, we apply quantitative chemical- and phospho-proteomics to find that disruption of DAGLβ in primary macrophages leads to LKB1-AMPK signaling activation, resulting in reprogramming of the phosphoproteome and bioenergetics. Notably, AMPK inhibition reversed the antinociceptive effects of DAGLβ blockade, thereby directly supporting DAGLβ-AMPK crosstalk in vivo. Our findings uncover signaling between endocannabinoid biosynthetic enzymes and ancient energy-sensing kinases to mediate cell biological and pain responses.

Short-chain fatty acid, butyrate prevents morphine-and paclitaxel-induced nociceptive hypersensitivity

Nociceptive hypersensitivity is a significant side effect with the chronic administration of opioids as well as chemotherapeutics. Both opioid-induced hypersensitivity (OIH) and chemotherapy-induced hypersensitivity (CIH) are characterized by an increased sensitivity to painful stimuli which can significantly reduce the quality of life for individuals on either drug(s). Here we demonstrate the nociceptive hypersensitivity associated with repeated administration of morphine (opioid) and paclitaxel (chemotherapeutic) treatment can be reversed by oral supplementation with the short-chain fatty acid (SCFA) sodium butyrate (NaBut). In two separate mouse behavioral models for nociceptive hypersensitivity, we found that thermal hyperalgesia (for OIH) and cold allodynia (for CIH) were prevented by treatment with oral butyrate (p.o, b.i.d). Electrophysiological recordings of small diameter dorsal root ganglia (DRG) neurons from morphine and paclitaxel treated mice showed an increase in neuronal hyperexcitability in both drug models which was likewise prevented by oral butyrate treatment. Using colonic conditioned media obtained from excised colon segments we found that gut mediators of morphine treated mice can induce hyperexcitability in naïve DRG neurons, but such enhanced excitability is not present when animals are co-treated with NaBut suggesting gut derived mediators modulate neuronal hyperexcitability. In-vitro NaBut treatment did not prevent morphine-induced excitability, suggesting an indirect role of butyrate in modulating neuronal hypersensitivity. These data taken together suggest that gut derived mediators affect opioid and chemotherapeutic-induced neuronal hypersensitivity that is prevented by the SCFA butyrate.

Role of mu opioid receptor (MOR) agonist efficacy as a determinant of opioid antinociception in a novel assay of pain-depressed behavior in female and male mice

Introduction

Intermediate efficacy mu opioid receptor (MOR) agonists have potential to retain analgesic effectiveness while improving safety, but the optimal MOR efficacy for effective and safe opioid analgesia is unknown. Preclinical assays of pain-depressed behavior can assess effects of opioids and other candidate analgesics on pain-related behavioral depression, which is a common manifestation of clinically relevant pain and target of pain treatment. Accordingly, the present study goal was to validate a novel assay of pain-depressed locomotor behavior in mice and evaluate the role of MOR efficacy as a determinant of opioid analgesic effects and related safety measures.

Methods

Male and female ICR mice were tested in a locomotor chamber consisting of 2 compartments connected by a doorway that contained a 1-inch-tall barrier. Dependent measures during 15-min behavioral sessions included crosses between compartments (which required vertical activity to surmount the barrier) and total movement counts (which required horizontal activity to break photobeams in each compartment).

Results and Discussion

Intraperitoneal injection of lactic acid (IP acid) produced a concentration- and time-dependent depression of both endpoints. Optimal blockade of IP acid-induced behavioral depression with minimal motor impairment was achieved with intermediate-efficacy MOR treatments that also produced less gastrointestinal-transit inhibition and respiratory depression than the high-efficacy MOR agonist fentanyl. Sex differences in treatment effects were rare. Overall, these findings validate a novel procedure for evaluating opioids and other candidate analgesic effects on pain-related behavioral depression in mice and support continued research with intermediate-efficacy MOR agonists as a strategy to retain opioid analgesic effectiveness with improved safety.

Short-chain fatty acid, Butyrate prevents Morphine and Paclitaxel induced peripheral hypersensitivity

Peripheral hypersensitivity is a significant side effect with the chronic administration of opioids as well as chemotherapeutics. Both opioid-induced hypersensitivity (OIH) and chemotherapy induced hypersensitivity (CIH) are characterized by an increased sensitivity to painful stimuli which can significantly reduce the quality of life for individuals on either drug(s). Here we demonstrate the peripheral hypersensitivity associated with chronic morphine (opioid) and paclitaxel (chemotherapeutic) treatment can be reversed by oral supplementation with the short-chain fatty acid (SCFA) sodium butyrate. In two separate mouse behavioral models for peripheral hypersensitivity, we found that thermal hyperalgesia (for OIH) and cold allodynia (for CIH) were prevented by co-treatment with oral butyrate. Electrophysiological recordings of small diameter dorsal root ganglia (DRG) neurons from morphine and paclitaxel treated mice showed an increase in neuronal hyperexcitability in both drug models which was likewise prevented by oral butyrate treatment. Using colonic conditioned media obtained from excised colon segments we found that gut mediators of morphine treated mice can induce hyperexcitability in naïve DRG neurons, but such enhanced excitability is not present when animals are co-treated with butyrate suggesting gut derived mediators modulate neuronal hyperexcitability. In-vitro butyrate treatment did not prevent morphine induced excitability, suggesting an indirect role of sodium butyrate in modulating neuronal hypersensitivity. These data taken together suggest that gut derived mediators affect opioid and chemotherapeutic induced neuronal hypersensitivity that is prevented by the SCFA butyrate.

Opioid-like adverse effects of tianeptine in male rats and mice

RATIONALE

Tianeptine is a mu-opioid receptor (MOR) agonist with increasing reports of abuse in human populations. Preclinical data regarding the abuse potential and other opioid-like adverse effects of tianeptine at supratherapeutic doses are sparse.

OBJECTIVES

The present study evaluated tianeptine in a rat model of abuse potential assessment and in mouse models of motor, gastrointestinal, and respiratory adverse effects.

METHODS

Abuse potential was assessed in adult male Sprague-Dawley rats using an intracranial self-stimulation (ICSS) procedure to determine effects of acute and repeated tianeptine on responding for electrical brain stimulation. Male ICR mice were used to determine the effects of tianeptine in assays of locomotor behavior and gastrointestinal motility. Male Swiss-Webster mice were monitored for respiratory changes using whole-body plethysmography.

RESULTS

In rats, acute tianeptine produced weak and delayed evidence for abuse-related ICSS facilitation at an intermediate dose (10 mg/kg, IP) and pronounced, naltrexone-preventable ICSS depression at a higher dose (32 mg/kg, IP). Repeated 7-day tianeptine (10 and 32 mg/kg/day, IP) produced no increase in abuse-related ICSS facilitation, only modest tolerance to ICSS depression, and no evidence of physical dependence. In mice, tianeptine produced dose-dependent, naltrexone-preventable locomotor activation. Tianeptine (100 mg/kg, SC) also significantly inhibited gastrointestinal motility and produced naloxone-reversible respiratory depression.

CONCLUSIONS

Tianeptine presents as a MOR agonist with resistance to tolerance and dependence in our ICSS assay in rats, and it has lower abuse potential by this metric than many commonly abused opioids. Nonetheless, tianeptine produces MOR agonist-like acute adverse effects that include motor impairment, constipation, and respiratory depression.

Chronic Morphine Induces IL-18 in Ileum Myenteric Plexus Neurons Through Mu-opioid Receptor Activation in Cholinergic and VIPergic Neurons

The gastrointestinal epithelium is critical for maintaining a symbiotic relationship with commensal microbiota. Chronic morphine exposure can compromise the gut epithelial barrier in mice and lead to dysbiosis. Recently, studies have implicated morphine-induced dysbiosis in the mechanism of antinociceptive tolerance and reward, suggesting the presence of a gut-brain axis in the pharmacological effects of morphine. However, the mechanism(s) underlying morphine-induced changes in the gut microbiome remains unclear. The pro-inflammatory cytokine, Interleukin-18 (IL-18), released by enteric neurons can modulate gut barrier function. Therefore, in the present study we investigated the effect of morphine on IL-18 expression in the mouse ileum. We observed that chronic morphine exposure in vivo induces IL-18 expression in the ileum myenteric plexus that is attenuated by naloxone. Given that mu-opioid receptors (MORs) are mainly expressed in enteric neurons, we also characterized morphine effects on the excitability of cholinergic (excitatory) and vasoactive intestinal peptide (VIP)-expressing (inhibitory) myenteric neurons. We found fundamental differences in the electrical properties of cholinergic and VIP neurons such that VIP neurons are more excitable than cholinergic neurons. Furthermore, MORs were primarily expressed in cholinergic neurons, although a subset of VIP neurons also expressed MORs and responded to morphine in electrophysiology experiments. In conclusion, these data show that morphine increases IL-18 in ileum myenteric plexus neurons via activation of MORs in a subset of cholinergic and VIP neurons. Thus, understanding the neurochemistry and electrophysiology of MOR-expressing enteric neurons can help to delineate mechanisms by which morphine perturbs the gut barrier.

Morphine Exacerbates Experimental Colitis-Induced Depression of Nesting in Mice

Opioids and non-steroidal anti-inflammatory drugs (NSAIDs) are excellent analgesics, but recent clinical evidence suggests that these drugs might worsen disease severity in Crohn's disease patients, limiting their clinical utility for treating Inflammatory Bowel Disease (IBD). One indicator of change in well-being from conditions such as IBD is behavioral depression and disruption to activities of daily living. Preclinical measures of behavioral depression can provide an indicator of changes in quality of life and subsequent modification by candidate analgesics. In mice, nesting is an adaptive unconditioned behavior that is susceptible to disruption by noxious stimuli, and some types of pain related nesting depression are responsive to opioid and NSAID analgesics. Here we show that a 2, 4, 6-trinitrobenzene sulfonic acid (TNBS) model of experimental colitis depresses nesting behavior in mice, and we evaluated effects of morphine, an opioid, and ketoprofen, a NSAID, on TNBS-induced nesting depression. In Swiss Webster mice, TNBS significantly reduced nesting that peaked on Day 3 and recovered in a time-dependent manner with complete recovery by Day 7. In the absence of colonic inflammation, daily treatment with morphine (1-10 mg/kg) did not decrease nesting except at 10mg/kg/day. However, in TNBS-treated mice 3.2 mg/kg/day morphine significantly exacerbated TNBS-induced nesting depression and delayed recovery. While 3.2 mg/kg/day morphine alone did not alter locomotor activity and TNBS-induced depression of locomotion recovered, the combination of TNBS and 3.2 mg/kg/day morphine significantly attenuated locomotion and prevented recovery. Daily treatment with 3.2 or 10 mg/kg ketoprofen in TNBS-treated mice did not prevent depression of nesting. These data suggest that opioid analgesics but not NSAIDS worsen colonic inflammation-induced behavioral depression. Furthermore, these findings highlight the importance of evaluating analgesic effects in models of colonic inflammation induced depression of behavior.

Sex-specific role for serotonin 5-HT2A receptor in modulation of opioid-induced antinociception and reward in mice

Opioids are among the most effective analgesics and the mainstay of pain management. However, concerns about safety and abuse liability have challenged their widespread use by the medical community. Opioid-sparing therapies include drugs that in combination with opioids have the ability to enhance analgesia while decreasing opioid requirement as well as their side effects. Sex differences in antinociceptive responses to opioids have received increasing attention in recent years. However, the molecular mechanisms underlying sex differences related to opioid-sparing adjuncts remain largely unexplored. Using warm water tail-withdrawal as a mouse model of acute thermal nociception, our data suggest that adjunctive administration of the serotonin 5-HT_2A receptor (5-HT_2AR) antagonist volinanserin dose-dependently enhanced potency of the opioid analgesic oxycodone in male, but not female, mice. This antinociceptive-like response induced by oxycodone was also augmented in 5-HT_2AR knockout (5-HT_2AR^-/-) male, but not female mice; an effect that was reversed by Cre-loxP-mediated selective expression of 5-HT_2AR in dorsal root ganglion (DRG) neurons of 5-HT_2AR^-/- littermates. Pharmacological inhibition with volinanserin or genetic deletion in 5-HT_2AR^-/- animals potentiated the ability of oxycodone to reduce DRG excitability in male mice. Adjunctive volinanserin did not affect oxycodone-induced conditioned place preference (CPP), whereas it reduced oxycodone-induced locomotor sensitization in male and female mice. Together, these results suggest that adjunctive volinanserin augments opioid-induced antinociception, but not abuse-related behavior, through a sex-specific signaling crosstalk mechanism that requires 5-HT_2AR expression in mouse DRG neurons. Ultimately, our results may pave the way for the clinical evaluation of volinanserin as a potential sex-specific opioid adjuvant.

The Cannabinoid Receptor Type 1 Positive Allosteric Modulator ZCZ011 Attenuates Naloxone-Precipitated Diarrhea and Weight Loss in Oxycodone-Dependent Mice

Opioid use disorder reflects a major public health crisis of morbidity and mortality in which opioid withdrawal often contributes to continued use. However, current medications that treat opioid withdrawal symptoms are limited by their abuse liability or lack of efficacy. Although cannabinoid 1 (CB₁) receptor agonists, including Δ⁹-tetrahydrocannabinol, ameliorate opioid withdrawal in both clinical and preclinical studies of opioid dependence, this strategy elicits cannabimimetic side effects as well as tolerance and dependence after repeated administration. Alternatively, CB₁ receptor positive allosteric modulators (PAMs) enhance CB₁ receptor signaling and show efficacy in rodent models of pain and cannabinoid dependence but lack cannabimimetic side effects. We hypothesize that the CB₁ receptor PAM ZCZ011 attenuates naloxone-precipitated withdrawal signs in opioid-dependent mice. Accordingly, male and female mice given an escalating dosing regimen of oxycodone, a widely prescribed opioid, and challenged with naloxone displayed withdrawal signs that included diarrhea, weight loss, jumping, paw flutters, and head shakes. ZCZ011 fully attenuated naloxone-precipitated withdrawal-induced diarrhea and weight loss and reduced paw flutters by approximately half, but its effects on head shakes were unreliable, and it did not affect jumping behavior. The antidiarrheal and anti-weight loss effects of ZCZ0111 were reversed by a CB₁ not a cannabinoid receptor type 2 receptor antagonist and were absent in CB₁ (-/-) mice, suggesting a necessary role of CB₁ receptors. Collectively, these results indicate that ZCZ011 completely blocked naloxone-precipitated diarrhea and weight loss in oxycodone-dependent mice and suggest that CB₁ receptor PAMs may offer a novel strategy to treat opioid dependence. SIGNIFICANCE STATEMENT: Opioid use disorder represents a serious public health crisis in which current medications used to treat withdrawal symptoms are limited by abuse liability and side effects. The CB₁ receptor positive allosteric modulator (PAM) ZCZ011, which lacks overt cannabimimetic behavioral effects, ameliorated naloxone-precipitated withdrawal signs through a CB₁ receptor mechanism of action in a mouse model of oxycodone dependence. These results suggest that CB₁ receptor PAMs may represent a viable strategy to treat opioid withdrawal.

Chemotherapy induced gastrointestinal toxicities

Chemotherapy-induced gastrointestinal dysfunction is a common occurrence associated with many different classes of chemotherapeutic agents. Gastrointestinal toxicity includes mucositis, diarrhea, and constipation, and can often be a dose-limiting complication, induce cessation of treatment and could be life threatening. The gastrointestinal epithelium is rich in rapidly dividing cells and hence is a prime target for chemotherapeutic drugs. The incidence of gastrointestinal toxicity, including diarrhea and mucositis, is extremely high for a wide array of chemotherapeutic and radiation regimens. In fact, 60%-100% of patients on high-dose chemotherapy suffer from gastrointestinal side effects. Unfortunately, treatment options are limited, and therapy is often restricted to palliative care. Therefore, there is a great unmet therapeutic need for preventing and treating chemotherapy-induced gastrointestinal toxicities in the clinic. In this review, we discuss our current understanding of the mechanisms underlying chemotherapy-induced diarrhea and mucositis, and emerging mechanisms involving the enteric nervous system, smooth muscle cells and enteric immune cells. Recent evidence has also implicated gut dysbiosis in the pathogenesis of not only chemotherapy-induced mucositis and diarrhea, but also chemotherapy-induced peripheral neuropathy. Oxidative stress induced by chemotherapeutic agents results in post-translational modification of ion channels altering neuronal excitability. Thus, investigating how chemotherapy-induced changes in the gut- microbiome axis may lead to gut-related toxicities will be critical in the discovery of new drug targets for mitigating adverse gastrointestinal effects associated with chemotherapy treatment.

The Guts of the Opioid Crisis

Bidirectional interactions of the gut epithelium with commensal bacteria are critical for maintaining homeostasis within the gut. Chronic opioid exposure perturbs gut homeostasis through a multitude of neuro-immune-epithelial mechanisms, resulting in the development of analgesic tolerance, a major underpinning of the current opioid crisis. Differences in molecular mechanisms of opioid tolerance between the enteric and central pain pathways pose a significant challenge for managing chronic pain without untoward gastrointestinal effects.

Role of β-arrestin-2 in short- and long-term opioid tolerance in the dorsal root ganglia

G-protein-biased agonists with reduced β-arrestin-2 activation are being investigated as safer alternatives to clinically-used opioids. β-arrestin-2 has been implicated in the mechanism of opioid-induced antinociceptive tolerance. Opioid-induced analgesic tolerance is classically considered as centrally-mediated, but recent reports implicate nociceptive dorsal root ganglia neurons as critical mediators in this process. Here, we investigated the role of β-arrestin-2 in the mechanism of opioid tolerance in dorsal root ganglia nociceptive neurons using β-arrestin-2 knockout mice and the G-protein-biased μ-opioid receptor agonist, TRV130. Whole-cell current-clamp electrophysiology experiments revealed that 15-18-h overnight exposure to 10 μM morphine in vitro induced acute tolerance in β-arrestin-2 wild-type but not knockout neurons. Furthermore, in wild-type neurons circumventing β-arrestin-2 activation by overnight treatment with 200 nM TRV130 attenuated tolerance. Similarly, acute morphine tolerance in vivo in β-arrestin-2 knockout mice was prevented in the warm-water tail-withdrawal assay. Treatment with 30 mg/kg TRV130 s.c. also inhibited acute antinociceptive tolerance in vivo in wild-type mice. Alternately, in β-arrestin-2 knockout neurons tolerance induced by 7-day in vivo exposure to 50 mg morphine pellet was conserved. Likewise, β-arrestin-2 deletion did not mitigate in vivo antinociceptive tolerance induced by 7-day exposure to 25 mg or 50 mg morphine pellet in both female or male mice, respectively. Consequently, these results indicated that β-arrestin-2 mediates acute but not chronic opioid tolerance in dorsal root ganglia neurons and to antinociception in vivo. This suggests that opioid-induced antinociceptive tolerance may develop even in the absence of β-arrestin-2 activation, and thus significantly affect the clinical utility of biased agonists.

Methylnaltrexone crosses the blood-brain barrier and attenuates centrally-mediated behavioral effects of morphine and oxycodone in mice

BACKGROUND

Antagonism of peripheral opioid receptors by methylnaltrexone (MNTX) was recently proposed as a potential mechanism to attenuate the development of opioid analgesic tolerance based on experiments conducted in mice. However, reports indicate that MNTX is demethylated to naltrexone (NTX) in mice, and NTX may subsequently cross the blood-brain barrier to antagonize centrally-mediated opioid effects. The goal of this study was to determine whether MNTX alters centrally-mediated behaviors elicited by the opioid analgesics, morphine and oxycodone, and to quantify concentrations of MNTX and NTX in blood and brain following their administration in mice.

METHODS

Combinations of MNTX and morphine were tested under acute and chronic conditions in thermal nociceptive assays. Effects of MNTX and NTX pretreatment were assessed in an oxycodone discrimination operant procedure. Blood and brain concentrations of these antagonists were quantified after their administration using liquid chromatography-mass spectrometry.

RESULTS

MNTX dose-dependently attenuated acute and chronic morphine antinociception. MNTX and NTX dose-dependently antagonized the discriminative stimulus effects of oxycodone. MNTX and NTX were detected in both blood and brain after administration of MNTX, confirming its demethylation and demonstrating that MNTX itself can cross the blood-brain barrier.

CONCLUSIONS

These results provide converging behavioral and analytical evidence that MNTX administration in mice attenuates centrally-mediated effects produced by opioid analgesics and results in functional concentrations of MNTX and NTX in blood and brain. Collectively, these findings indicate that MNTX cannot be administered systemically in mice for making inferences that its effects are peripherally restricted.

The "Culture" of Pain Control: A Review of Opioid-Induced Dysbiosis (OID) in Antinociceptive Tolerance

It is increasingly recognized that chronic opioid use leads to maladaptive changes in the composition and localization of gut bacteria. Recently, this "opioid-induced dysbiosis" (OID) has been linked to antinociceptive tolerance development in preclinical models and may therefore identify promising targets for new opioid-sparing strategies. Such developments are critical to curb dose escalations in the clinical setting and combat the ongoing opioid epidemic. In this article, we review the existing literature that pertains to OID, including the current evidence regarding its qualitative nature, influence on antinociceptive tolerance, and future prospects. PERSPECTIVE: This article reviews the current literature on OID of gut bacteria, including its qualitative nature, influence on antinociceptive tolerance, and future prospects. This work may help identify targets for new opioid-sparing strategies.

Experimental Colitis Enhances the Rate of Antinociceptive Tolerance to Morphine via Peripheral Opioid Receptors

Opioids are highly effective analgesics, however, their therapeutic use is limited by adverse effects that include respiratory depression, dependence, and tolerance. Inflammation has been implicated as a significant driver for the development of tolerance to opioids. Recent studies show that chronic morphine in mice results in gut microbial dysbiosis and inflammation in the colon. In the present study, we examined whether colonic inflammation results in tolerance to the antinociceptive effects of morphine. Colonic inflammation was induced in mice by intrarectal administration of 2,4,6-trinitro-benzene sulfonic acid. The development of antinociceptive tolerance was determined by warm-water tail-immersion assay in mice implanted with 25-, 50-, or 75-mg morphine pellet. Colonic inflammation significantly enhanced the rate at which tolerance developed in each cohort of chronic morphine-treated mice. At the lowest dose of morphine pellet (25 mg), antinociceptive tolerance only developed in the presence of colonic inflammation, whereas in 50- and 75-mg pelleted mice, tolerance developed faster in the inflamed animals than in the noninflamed mice. The enhanced antinociceptive tolerance was attenuated with daily administration of peripheral opioid receptor antagonist, 6β-N-heterocyclic-substituted naltrexamine derivative [17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(4'pyridyl)acetamido]morphinan (NAP)], irrespective of colonic inflammation. Collectively, these findings show that the rate of tolerance to morphine antinociception is exaggerated in the presence of colonic inflammation, and tolerance is prevented by a peripheral μ-opioid receptor antagonist. These studies suggest a peripheral component to the development of antinociceptive tolerance to opioids. Furthermore, peripherally selective opioid antagonists may be useful adjuncts in opioid-based pain management. SIGNIFICANCE STATEMENT: This study supports the notion that inflammation influences the development of antinociceptive tolerance to chronic morphine exposure. We found that, in the presence of colonic inflammation, the rate of development of tolerance to the antinociceptive effects of morphine increased. We also found that treatment with a peripheral opioid receptor antagonist prevented morphine antinociceptive tolerance. Increasing opioid intake during an inflammatory state would result in decreased analgesia and enhanced analgesic tolerance, which puts patients with inflammatory bowel diseases, inflammatory joint diseases, and sickle cell anemia at risk for heavy opioid use.

Gastrointestinal motility, dysbiosis and opioid-induced tolerance: is there a link?

Studies show that gut microbial dysbiosis induced by chronic opioid use is linked to central opioid tolerance. Here, we suggest that a persistent decrease in gastrointestinal motility by opioids is a primary cause of gut microbial dysbiosis and that improving gastrointestinal transit might be a strategy in preventing opioid analgesic tolerance.

Characterization of 17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(indole-7-carboxamido)morphinan (NAN) as a Novel Opioid Receptor Modulator for Opioid Use Disorder Treatment

The opioid crisis is a significant public health issue with more than 115 people dying from opioid overdose per day in the United States. The aim of the present study was to characterize the in vitro and in vivo pharmacological effects of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(indole-7-carboxamido)morphinan (NAN), a μ opioid receptor (MOR) ligand that may be a potential candidate for opioid use disorder treatment that produces less withdrawal signs than naltrexone. The efficacy of NAN was compared to varying efficacy ligands at the MOR, and determined at the δ opioid receptor (DOR) and κ opioid receptor (KOR). NAN was identified as a low efficacy partial agonist for G-protein activation at the MOR and DOR, but had relatively high efficacy at the KOR. In contrast to high efficacy MOR agonists, NAN did not induce MOR internalization, downregulation, or desensitization, but it antagonized agonist-induced MOR internalization and stimulation of intracellular Ca2+ release. Opioid withdrawal studies conducted using morphine-pelleted mice demonstrated that NAN precipitated significantly less withdrawal signs than naltrexone at similar doses. Furthermore, NAN failed to produce fentanyl-like discriminative stimulus effects in rats up to doses that produced dose- and time-dependent antagonism of fentanyl. Overall, these results provide converging lines of evidence that NAN functions mainly as a MOR antagonist and support further consideration of NAN as a candidate medication for opioid use disorder treatment.

Structure-Activity Relationship Studies of 6α- and 6β-Indolylacetamidonaltrexamine Derivatives as Bitopic Mu Opioid Receptor Modulators and Elaboration of the "Message-Address Concept" To Comprehend Their Functional Conversion

Structure-activity relationship (SAR) studies of numerous opioid ligands have shown that introduction of a methyl or ethyl group on the tertiary amino group at position 17 of the epoxymorphinan skeleton generally results in a mu opioid receptor (MOR) agonist while introduction of a cyclopropylmethyl group typically leads to an antagonist. Furthermore, it has been shown that introduction of heterocyclic ring systems at position 6 can favor antagonism. However, it was reported that 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(2'-indolyl)acetamido]morphinan (INTA), which bears a cyclopropylmethyl group at position 17 and an indole ring at position 6, acted as a MOR agonist. We herein report a SAR study on INTA with a series of its complementary derivatives to understand how introduction of an indole moiety with α or β linkage at position 6 of the epoxymorphinan skeleton may influence ligand function. Interestingly, one of INTA derivatives, compound 15 (NAN) was identified as a MOR antagonist both in vitro and in vivo. Molecular modeling studies revealed that INTA and NAN may interact with different domains of the MOR allosteric binding site. In addition, INTA may interact with W293 and N150 residues found in the orthosteric site to stabilize MOR activation conformation while NAN does not. These results suggest that INTA and NAN may be bitopic ligands and the type of allosteric interactions with the MOR influence their functional activity. These insights along with our enriched comprehension of the "message-address" concept will to benefit future ligand design.

Design, Synthesis, and Biological Evaluation of the Third Generation 17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(4'-pyridyl)carboxamido]morphinan (NAP) Derivatives as μ/κ Opioid Receptor Dual Selective Ligands

μ opioid receptor (MOR) agonists have been widely applied for treating moderate to severe pain. However, numerous adverse effects have been associated with their application, including opioid-induced constipation (OIC), respiratory depression, and addiction. On the basis of previous work in our laboratory, NAP, a 6β- N-4'-pyridyl substituted naltrexamine derivative, was identified as a peripheral MOR antagonist that may be used to treat OIC. To further explore its structure-activity relationship, a new series of NAP derivatives were designed, synthesized, and biologically evaluated. Among these derivatives, NFP and NYP significantly antagonized the antinociception effect of morphine. Whereas NAP acted mainly peripherally, its derivatives NFP and NYP actually can act centrally. Furthermore, NFP produced significantly lesser withdrawal symptoms than naloxone at similar doses. These results suggest that NFP has the potential to be a lead compound to treat opioid abuse and addiction.

Methylation Products of 6β- N-Heterocyclic Substituted Naltrexamine Derivatives as Potential Peripheral Opioid Receptor Modulators

Two 6β- N-heterocyclic naltrexamine derivatives, NAP and NMP, have been identified as peripherally selective mu opioid receptor (MOR) antagonists. To further enhance the peripheral selectivity of both compounds, the 17-amino group and the nitrogen atom of the pyridine ring in both NAP and NMP were methylated to obtain dMNAP and dMNMP, respectively. Compared with NAP and NMP, the binding affinities of dMNAP and dMNMP shifted to MOR and KOR (kappa opioid receptor) dual selective and they acted as moderate efficacy partial agonists. The results from radioligand binding studies were further confirmed by molecular docking studies. In vivo studies demonstrated that dMNAP and dMNMP did not produce antinociception nor did they antagonize morphine's antinociceptive activity, indicating that these compounds did not act on the central nervous system. Meanwhile, both dMNAP and dMNMP significantly slowed down fecal excretion, which indicated that they were peripherally acting opioid receptor agonists. All together, these results suggested that dMNAP and dMNMP acted as peripheral mu/kappa opioid receptor modulators and may be applicable in the treatment of diarrhea in patients with bowel dysfunction.

Analysis of carbenoxolone by ultra-high-performance liquid chromatography tandem mass spectrometry in mouse brain and blood after systemic administration

Carbenoxolone is a derivative of glycyrrhetinic acid found in the root of Glycyrrhiza glabra, colloquially known as licorice. It has been used as a treatment for peptic and oral ulcers. In recent years, carbenoxolone has been utilized in basic research for its ability to block gap junctional communication. Better understanding the distribution of carbenoxolone after systemic administration can lead to a better understanding of its potential sites of action. Presented is an ultra high-performance liquid chromatography tandem mass spectrometer (UHPLC-MS/MS) method for the identification and quantification of carbenoxolone in mouse blood and brain tissue. Twenty mice were injected intraperitoneally with 25 mg/kg carbenoxolone and brain tissue and blood were collected for analysis. Blood concentrations (mean ± SD) at 15, 30, 60 and 120 min were determined to be (n = 5) 5394 ± 778, 2636 ± 836, 1564 ± 541 and 846 ± 252 ng/mL, respectively. Brain concentrations (mean ± SD) at 15, 30, 60 and 120 mins were determined to be (n = 5) 171 ± 62, 102 ± 35, 55 ± 10 and 27 ± 9 ng/g, respectively. The analysis of these specimens at the four different time points resulted in blood and brain half-lives in mice of ~43 and 41 min, respectively. The UHPLC-MS/MS method was determined to be sensitive and robust for quantification of carbenoxolone.

Tolerance to Morphine-Induced Inhibition of TTX-R Sodium Channels in Dorsal Root Ganglia Neurons Is Modulated by Gut-Derived Mediators

In the clinical setting, analgesic tolerance is a primary driver of diminished pain control and opioid dose escalations. Integral to this process are primary afferent sensory neurons, the first-order components of nociceptive sensation. Here, we characterize the factors modulating morphine action and tolerance in mouse small diameter dorsal root ganglia (DRG) neurons. We demonstrate that acute morphine inactivates tetrodotoxin-resistant (TTX-R) Na+ channels in these cells. Chronic exposure resulted in tolerance to this effect, which was prevented by treatment with oral vancomycin. Using colonic supernatants, we further show that mediators in the gut microenvironment of mice with chronic morphine exposure can induce tolerance and hyperexcitability in naive DRG neurons. Tolerance (but not hyperexcitability) in this paradigm was mitigated by oral vancomycin treatment. These findings collectively suggest that gastrointestinal microbiota modulate the development of morphine tolerance (but not hyperexcitability) in nociceptive primary afferent neurons, through a mechanism involving TTX-R Na+ channels.

CCR5 mediates HIV-1 Tat-induced neuroinflammation and influences morphine tolerance, dependence, and reward

The HIV-1 regulatory protein, trans-activator of transcription (Tat), interacts with opioids to potentiate neuroinflammation and neurodegeneration within the CNS. These effects may involve the C-C chemokine receptor type 5 (CCR5); however, the behavioral contribution of CCR5 on Tat/opioid interactions is not known. Using a transgenic murine model that expresses HIV-1 Tat protein in a GFAP-regulated, doxycycline-inducible manner, we assessed morphine tolerance, dependence, and reward. To assess the influence of CCR5 on these effects, mice were pretreated with oral vehicle or the CCR5 antagonist, maraviroc, prior to morphine administration. We found that HIV-1 Tat expression significantly attenuated the antinociceptive potency of acute morphine (2-64 mg/kg, i.p.) in non-tolerant mice. Consistent with this, Tat attenuated withdrawal symptoms among morphine-tolerant mice. Pretreatment with maraviroc blocked the effects of Tat, reinstating morphine potency in non-tolerant mice and restoring withdrawal symptomology in morphine-tolerant mice. Twenty-four hours following morphine administration, HIV-1 Tat significantly potentiated (∼3.5-fold) morphine-conditioned place preference and maraviroc further potentiated these effects (∼5.7-fold). Maraviroc exerted no measurable behavioral effects on its own. Protein array analyses revealed only minor changes to cytokine profiles when morphine was administered acutely or repeatedly; however, 24 h post morphine administration, the expression of several cytokines was greatly increased, including endogenous CCR5 chemokine ligands (CCL3, CCL4, and CCL5), as well as CCL2. Tat further elevated levels of several cytokines and maraviroc pretreatment attenuated these effects. These data demonstrate that CCR5 mediates key aspects of HIV-1 Tat-induced alterations in the antinociceptive potency and rewarding properties of opioids.

Ethanol Reversal of Oxycodone Tolerance in Dorsal Root Ganglia Neurons

Oxycodone is a semisynthetic opioid compound that is widely prescribed, used, and abused today, and has a well-established role in shaping the current opioid epidemic. Previously, we have shown that tolerance develops to the antinociceptive and respiratory depressive effects of oxycodone in mice, and that a moderate dose of acute ethanol or a protein kinase C (PKC) inhibitor reversed that tolerance. To investigate further if tolerance was occurring through neuronal mechanisms, our aims for this study were to assess the effects of acute and prolonged oxycodone in isolated dorsal root ganglia (DRG) neurons and to determine if this tolerance was reversed by either ethanol or a PKC inhibitor. We found that an acute exposure to 3 μM oxycodone reduced neuronal excitability, as measured by increased threshold potentials and reduced action potential amplitude, without eliciting measurable changes in resting membrane potential. Exposure to 10 μM oxycodone for 18-24 hours prevented oxycodone's effect on neuronal excitability, indicative of tolerance development. The development of opioid tolerance was mitigated in DRG neurons from β-arrestin 2 knockout mice. Oxycodone tolerance was reversed in isolated DRG neurons by the acute application of either ethanol (20 mM) or the PKC inhibitor, bisindolylmaleimide XI hydrochloride (Bis XI), when a challenge of 3 µM oxycodone significantly reduced neuronal excitability following prolonged exposure. Through these studies, we concluded that oxycodone acutely reduced neuronal excitability, tolerance developed to this effect, and reversal of that tolerance occurred at the level of a single neuron, suggesting that reversal of oxycodone tolerance by either ethanol or Bis XI involves cellular mechanisms.

Culture of Neurons and Smooth Muscle Cells from the Myenteric Plexus of Adult Mice

In the enteric nervous system, neurons make contact with smooth muscle cells to regulate gastrointestinal functions. Since neuronal cell alterations and intestinal motor dysfunctions are strictly linked, in vitro models based on the culture of neurons or smooth muscle cells are of great advantage to elucidate the functions of the enteric nervous system under physiological and pathological conditions. In this chapter, we provide the protocol for isolation of neurons and smooth muscle cells from the myenteric plexus of adult mice. The isolated cells are suitable for immunocytochemical applications or electrophysiological investigations and/or co-culturing experiments.

The gut-brain interaction in opioid tolerance

The prevailing opioid crisis has necessitated the need to understand mechanisms leading to addiction and tolerance, the major contributors to overdose and death and to develop strategies for developing drugs for pain treatment that lack abuse liability and side-effects. Opioids are commonly used for treatment of pain and symptoms of inflammatory bowel disease. The significant effect of opioids in the gut, both acute and chronic, includes persistent constipation and paradoxically may also worsen pain symptoms. Recent work has suggested a significant role of the gastrointestinal microbiome in behavioral responses to opioids, including the development of tolerance to its pain-relieving effects. In this review, we present current concepts of gut-brain interaction in analgesic tolerance to opioids and suggest that peripheral mechanisms emanating from the gut can profoundly affect central control of opioid function.

Sex Differences and Drug Dose Influence the Role of the α7 Nicotinic Acetylcholine Receptor in the Mouse Dextran Sodium Sulfate-Induced Colitis Model

Introduction

α7 nicotinic acetylcholine receptors (nAChRs) play an important role in vagus nerve-based cholinergic anti-inflammatory effects. This study was designed to assess the role of α7 nAChRs in dextran sodium sulfate (DSS)-induced colitis in male and female mouse. We first compared disease activity and pathogenesis of colitis in α7 knockout and wild-type mice. We then evaluated the effect of several α7 direct and indirect agonists on the severity of disease in the DSS-induced colitis.

Methods

Male and female adult mice were administered 2.5% DSS solution freely in the drinking water for 7 consecutive days and the colitis severity (disease activity index) was evaluated as well as colon length, colon histology, and levels of tumor necrosis factor-alpha colonic levels.

Results

Male, but not female, α7 knockout mice displayed a significantly increased colitis severity and higher tumor necrosis factor-alpha levels as compared with their littermate wild-type mice. Moreover, pretreatment with selective α7 ligands PHA-543613, choline, and PNU-120596 decreased colitis severity in male but not female mice. The anti-colitis effects of these α7 compounds dissipated when administered at higher doses.

Conclusions

Our results suggest the presence of a α7-dependent anti-colitis endogenous tone in male mice. Finally, our results show for the first time that female mice are less sensitive to the anti-colitis activity of α7 agonists. Ovarian hormones may play a key role in the sex difference effect of α7 nAChRs modulation of colitis in the mouse.

Implications

Our collective results suggest that targeting α7 nAChRs could represent a viable therapeutic approach for intestinal inflammation diseases such as ulcerative colitis with the consideration of sex differences.

Reversal of oxycodone and hydrocodone tolerance by diazepam

The Centers for Disease Control has declared opioid abuse to be an epidemic. Overdose deaths are largely assumed to be the result of excessive opioid consumption. In many of these cases, however, opioid abusers are often polydrug abusers. Benzodiazepines are one of the most commonly co-abused substances and pose a significant risk to opioid users. In 2016, the FDA required boxed warnings - the FDA's strongest warning - for prescription opioid analgesics and benzodiazepines about the serious risks associated with using these medications at the same time. The point of our studies was to evaluate the interactions between these two classes of drugs. We investigated whether diazepam adds to the depressant effects of opioids or do they alter the levels of tolerance to opioids. In the present study, we have found that the antinociceptive tolerance that developed to repeated administration of oxycodone was reversed by an acute dose of diazepam. Antinociceptive tolerance to hydrocodone was also reversed by acute injection of diazepam; however, a fourfold higher dose of diazepam was required when compared to reversal of oxycodone-induced tolerance. These doses of diazepam did not potentiate the acute antinociceptive effect of either opioid. The same dose of diazepam that reversed oxycodone antinociceptive tolerance also reversed oxycodone locomotor tolerance while having no potentiating effects. These studies show that diazepam does not potentiate the acute effect of prescription opioids but reverses the tolerance developed after chronic administration of the drugs.

Ethanol Reversal of Tolerance to the Antinociceptive Effects of Oxycodone and Hydrocodone

This study compared the development of tolerance to two orally bioavailable prescription opioids, oxycodone and hydrocodone, to that of morphine, and the reversal of this tolerance by ethanol. Oxycodone (s.c.) was significantly more potent in the mouse tail-withdrawal assay than either morphine or hydrocodone. Oxycodone was also significantly more potent in this assay than hydrocodone when administered orally. Tolerance was seen following chronic subcutaneous administration of each of the three drugs and by the chronic administration of oral oxycodone, but not following the chronic oral administration of hydrocodone. Ethanol (1 g/kg i.p.) significantly reversed the tolerance to the subcutaneous administration of each of the three opioids that developed when given 30 minutes prior to challenge doses. It took twice as much ethanol, when given orally, to reverse the tolerance to oxycodone. We investigated whether the observed tolerance to oxycodone and its reversal by ethanol were due to biodispositional changes or reflected a true neuronal tolerance. As expected, a relationship between brain oxycodone concentrations and activity in the tail-immersion test existed following administration of acute oral oxycodone. Following chronic treatment, brain oxycodone concentrations were significantly lower than acute concentrations. Oral ethanol (2 g/kg) reversed the tolerance to chronic oxycodone, but did not alter brain concentrations of either acute or chronic oxycodone. These studies show that there is a metabolic component of tolerance to oxycodone; however, the reversal of that tolerance by ethanol is not due to an alteration of the biodisposition of oxycodone, but rather is neuronal in nature.

Connexin-purinergic signaling in enteric glia mediates the prolonged effect of morphine on constipation

Morphine is one of the most widely used drugs for the treatment of pain. However, side effects, including persistent constipation and antinociceptive tolerance, limit its clinical efficacy. Prolonged morphine treatment results in a "leaky" gut, predisposing to colonic inflammation that is facilitated by microbial dysbiosis and associated bacterial translocation. In this study, we examined the role of enteric glia in mediating this secondary inflammatory response to prolonged treatment with morphine. We found that purinergic P2X receptor activity was significantly enhanced in enteric glia that were isolated from mice with long-term morphine treatment (in vivo) but not upon direct exposure of glia to morphine (in vitro). LPS, a major bacterial product, also increased ATP-induced currents, as well as expression of P2X4, P2X7, IL6, IL-1β mRNA in enteric glia. LPS increased connexin43 (Cx43) expression and enhanced ATP release from enteric glia cells. LPS-induced P2X currents and proinflammatory cytokine mRNA expression were blocked by the Cx43 blockers Gap26 and carbenoxolone. Likewise, colonic inflammation related to prolonged exposure to morphine was significantly attenuated by carbenoxolone (25 mg/kg). Carbenoxolone also prevented gut wall disruption and significantly reduced morphine-induced constipation. These findings imply that enteric glia activation is a significant modulator of morphine-related inflammation and constipation.-Bhave, S., Gade, A., Kang, M., Hauser, K. F., Dewey, W. L., Akbarali, H. I. Connexin-purinergic signaling in enteric glia mediates the prolonged effect of morphine on constipation.

Nanoconjugated NAP as a Potent and Periphery Selective Mu Opioid Receptor Modulator To Treat Opioid-Induced Constipation

Opioids are the mainstay for cancer and noncancer pain management. However, their use is often associated with multiple adverse effects. Among them, the most common and persistent one is probably opioid-induced constipation (OIC). Periphery selective opioid antagonists may alleviate the symptoms of OIC without compromising the analgesic effects of opioids. Recently our laboratories have identified one novel lead compound, 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(4'-pyridyl)acetamido]morphinan (NAP), as a peripherally selective mu opioid receptor ligand carrying subnanomolar affinity to the mu opioid receptor and over 100-folds of selectivity over both the delta and kappa opioid receptors, with reasonable oral availability and half-life, and potential to treat OIC. Nanoparticle-based drug delivery systems are now widely considered due to their technological advantages such as good stability, high carrier capacity, low therapeutic side effects, etc. Herein we report nanoparticle supported NAP as a potential candidate for OIC treatment with improved peripheral selectivity over the original lead compound NAP.

6β-N-Heterocyclic Substituted Naltrexamine Derivative BNAP: A Peripherally Selective Mixed MOR/KOR Ligand

The 6β-N-heterocyclic naltrexamine derivative, NAP, has been demonstrated to be a peripherally selective mu opioid receptor modulator. To further improve peripheral selectivity of this highly potent ligand, its pyridal ring was quaterinized with benzyl bromide to produce BNAP. In radioligand binding assay, the Ki of BNAP for MOR was 0.76 ± 0.09 nM and was >900-fold more selective for MOR than DOR. The Ki for KOR was 3.46 ± 0.05 nM. In [(35)S]GTPγS ligand stimulated assay, BNAP showed low agonist efficacy with 14.6% of the maximum response of DAMGO with an EC50 of 4.84 ± 0.6 nM. However, unlike its parent compound NAP, BNAP displayed partial agonist activity at KOR with % maximum response at 45.9 ± 1.7% of U50,488H. BNAP did not reverse morphine-induced antinociception when administered subcutaneously but did antagonize when administered intracerebroventricularly. BNAP antagonized morphine-induced contractions of the circular muscle in mice colon. BNAP inhibition of field-stimulated contractions in longitudinal muscle strips for the guinea-pig ileum were also blocked by nor-BNI, a kappa opioid receptor antagonist. BNAP induced inhibition of acetic acid induced abdominal stretching in chronic morphine treated mice. These findings suggest that BNAP is a dual MOR antagonist/KOR agonist and may have functional use in irritable bowel patients.

Postranslational Modification of Ion Channels in Colonic Inflammation

Voltage-gated ion channels are key regulators of cell excitability. There is significant evidence that these channels are subject to modulation by redox status of the cells. Here we review the post-translational modifications of ion channels that occur in colonic inflammation. The redox mechanisms involve tyrosine nitration, covalent modification of cysteine residues and sulfhydration by hydrogen sulfide in experimental colitis. In the setting of colonic inflammation, modifications of cysteine and tyrosine are likely to occur at several sites within the same channel complex. In this review we describe alterations in channel function due to specific modifications of tyrosine and cysteine residues by reactive nitrogen, oxygen and hydrogen-sulfide resulting in altered motility.

Molecular physiology of enteric opioid receptors

Opioid drugs have powerful antidiarrheal effects and many patients taking these drugs for chronic pain relief experience chronic constipation that can progress to opioid-induced bowel dysfunction. Three classes of opioid receptors are expressed by enteric neurons: μ-, δ-, and κ-opioid receptors (MOR, DOR, and KOR). MOR and DOR couple to inhibition of adenylate cylase and nerve terminal Ca(2+) channels and activation of K(+) channels. These effects reduce neuronal activity and neurotransmitter release. KOR couples to inhibition of Ca(2+) channels and inhibition of neurotransmitter release. In the human gastrointestinal tract, MOR, DOR, and KOR link to inhibition of acetylcholine release from enteric interneurons and purine/nitric oxide release from inhibitory motorneurons. These actions inhibit propulsive motility. MOR and DOR also link to inhibition of submucosal secretomotor neurons, reducing active Cl(-) secretion and passive water movement into the colonic lumen. These effects account for the constipation caused by opioid receptor agonists. Tolerance develops to the analgesic effects of opioid receptor agonists but not to the constipating actions. This may be due to differential β-arrestin-2-dependent opioid receptor desensitization and internalization in enteric nerves in the colon compared with the small intestine and in neuronal pain pathways. Further studies of differential opioid receptor desensitization and tolerance in subsets of enteric neurons may identify new drugs or other treatment strategies of opioid-induced bowel dysfunction.

Enhanced Sensitivity of α3β4 Nicotinic Receptors in Enteric Neurons after Long-Term Morphine: Implication for Opioid-Induced Constipation

Opioid-induced constipation is a major side effect that persists with long-term opioid use. Previous studies demonstrated that nicotine-induced contractions are enhanced after long-term morphine exposure in guinea pig ileum. In the present study, we examined whether the increased sensitivity to nicotine could be observed in single enteric neurons after long-term morphine exposure, determined the subunits in mouse enteric neurons, and examined the effect of nicotine in reversing opioid-induced constipation. Nicotine (0.03-1 mM) dose-dependently induced inward currents from a holding potential of -60 mV in isolated single enteric neurons from the mouse ileum. The amplitude of the currents, but not the potency to nicotine, was significantly increased in neurons receiving long-term (16-24 h) but not short-term (10 min) exposure to morphine. Quantitative mRNA analysis showed that nicotinic acetylcholine receptor (nAChR) subunit expression in the mouse ileum was α3 ≥ β2 > β4 > α5 > α4 > β3 > α6. Nicotine-induced currents were obtained in neurons from α7, β2, α5, and α6 knockout mice. The currents were, however, inhibited by mecamylamine (10 μM) and the α3β4 blocker α-conotoxin AuIB (3 μM), suggesting that nicotine-induced currents were mediated by the α3β4 subtype of nAChRs on enteric neurons. Conversely, NS3861, a partial agonist at α3β4 nAChR, enhanced fecal pellet expulsion in a dose-dependent manner in mice that received long-term, but not short-term, morphine treatment. Overall, our findings suggest that the efficacy of nAChR agonists on enteric neurons is enhanced after long-term morphine exposure, and activation of the α3β4 subtype of nAChR reverses chronic, but not acute, morphine-induced constipation.

17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-(4'-pyridylcarboxamido)morphinan (NAP) Modulating the Mu Opioid Receptor in a Biased Fashion

Mounting evidence has suggested that G protein-coupled receptors can be stabilized in multiple conformations in response to distinct ligands, which exert discrete functions through selective activation of various downstream signaling events. In accordance with this concept, we report biased signaling of one C6-heterocyclic substituted naltrexamine derivative, namely, 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-(4'-pyridylcarboxamido)morphinan (NAP) at the mu opioid receptor (MOR). NAP acted as a low efficacy MOR partial agonist in the G protein-mediated [(35)S]GTPγS binding assay, whereas it did not significantly induce calcium flux or β-arrestin2 recruitment. In contrast, it potently blocked MOR full agonist-induced β-arrestin2 recruitment and translocation. Additionally, NAP dose-dependently antagonized MOR full agonist-induced intracellular calcium flux and β-arrestin2 recruitment. Further results in an isolated organ bath preparation confirmed that NAP reversed the morphine-induced reduction in colon motility. Ligand docking and dynamics simulation studies of NAP at the MOR provided more supporting evidence for biased signaling of NAP at an atomic level. Due to the fact that NAP is MOR selective and preferentially distributed peripherally upon systemic administration while β-arrestin2 is reportedly required for impairment of intestinal motility by morphine, biased antagonism of β-arrestin2 recruitment by NAP further supports its utility as a treatment for opioid-induced constipation.

Sensitization of enteric neurons to morphine by HIV-1 Tat protein

BACKGROUND

Gastrointestinal (GI) dysfunction is a major cause of morbidity in acquired immunodeficiency syndrome (AIDS). HIV-1-induced neuropathogenesis is significantly enhanced by opiate abuse, which increases proinflammatory chemokine/cytokine release, the production of reactive species, glial reactivity, and neuronal injury in the central nervous system. Despite marked interactions in the gut, little is known about the effects of HIV-1 in combination with opiate use on the enteric nervous system.

METHODS

To explore HIV-opiate interactions in myenteric neurons, the effects of Tat ± morphine (0.03, 0.3, and 3 μM) were examined in isolated neurons from doxycycline- (DOX-) inducible HIV-1 Tat(1-86) transgenic mice or following in vitro Tat 100 nM exposure (>6 h).

KEY RESULTS

Current clamp recordings demonstrated increased neuronal excitability in neurons of inducible Tat(+) mice (Tat+/DOX) compared to control Tat-/DOX mice. In neurons from Tat+/DOX, but not from Tat-/DOX mice, 0.03 μM morphine significantly reduced neuronal excitability, fast transient and late long-lasting sodium currents. There was a significant leftward shift in V(0.5) of inactivation following exposure to 0.03 μM morphine, with a 50% decrease in availability of sodium channels at -100 mV. Similar effects were noted with in vitro Tat exposure in the presence of 0.3 μM morphine. Additionally, GI motility was significantly more sensitive to morphine in Tat(+) mice than Tat(-) mice.

CONCLUSIONS & INFERENCES

Overall, these data suggest that the sensitivity of enteric neurons to morphine is enhanced in the presence of Tat. Opiates and HIV-1 may uniquely interact to exacerbate the deleterious effects of HIV-1-infection and opiate exposure on GI function.

Interaction between hydrogen sulfide-induced sulfhydration and tyrosine nitration in the KATP channel complex

Hydrogen sulfide (H₂S) is an endogenous gaseous mediator affecting many physiological and pathophysiological conditions. Enhanced expression of H2S and reactive nitrogen/oxygen species (RNS/ROS) during inflammation alters cellular excitability via modulation of ion channel function. Sulfhydration of cysteine residues and tyrosine nitration are the posttranslational modifications induced by H₂S and RNS, respectively. The objective of this study was to define the interaction between tyrosine nitration and cysteine sulfhydration within the ATP-sensitive K(+) (KATP) channel complex, a significant target in experimental colitis. A modified biotin switch assay was performed to determine sulfhydration of the KATP channel subunits, Kir6.1, sulphonylurea 2B (SUR2B), and nitrotyrosine measured by immunoblot. NaHS (a donor of H₂S) significantly enhanced sulfhydration of SUR2B but not Kir6.1 subunit. 3-Morpholinosydnonimine (SIN-1) (a donor of peroxynitrite) induced nitration of Kir6.1 subunit but not SUR2B. Pretreatment with NaHS reduced the nitration of Kir6.1 by SIN-1 in Chinese hamster ovary cells cotransfected with the two subunits, as well as in enteric glia. Two specific mutations within SUR2B, C24S, and C1455S prevented sulfhydration by NaHS, and these mutations prevented NaHS-induced reduction in tyrosine nitration of Kir6.1. NaHS also reversed peroxynitrite-induced inhibition of smooth muscle contraction. These studies suggest that posttranslational modifications of the two subunits of the KATP channel interact to alter channel function. The studies described herein demonstrate a unique mechanism by which sulfhydration of one subunit modifies tyrosine nitration of another subunit within the same channel complex. This interaction provides a mechanistic insight on the protective effects of H₂S in inflammation.

Morphine dependence in single enteric neurons from the mouse colon requires deletion of β-arrestin2

Chronic administration of morphine results in the development of tolerance to the analgesic effects and to inhibition of upper gastrointestinal motility but not to colonic motility, resulting in persistent constipation. In this study we examined the effect of chronic morphine in myenteric neurons from the adult mouse colon. Similar to the ileum, distinct neuronal populations exhibiting afterhyperpolarization (AHP)-positive and AHP-negative neurons were identified in the colon. Acute morphine (3 μM) decreased the number of action potentials, and increased the threshold for action potential generation indicative of reduced excitability in AHP-positive neurons. In neurons from the ileum of mice that were rendered antinociceptive tolerant by morphine-pellet implantation for 5 days, the opioid antagonist naloxone precipitated withdrawal as evidenced by increased neuronal excitability. Overnight incubation of ileum neurons with morphine also resulted in enhanced excitability to naloxone. Colonic neurons exposed to long-term morphine, remained unresponsive to naloxone suggesting that precipitated withdrawal does not occur in colonic neurons. However, morphine-treated colonic neurons from β-arrestin2 knockout mice demonstrated increased excitability upon treatment with naloxone as assessed by change in rheobase, number of action potentials and input resistance. These data suggest that similar to the ileum, acute exposure to morphine in colonic neurons results in reduced excitability due to inhibition of sodium currents. However, unlike the ileum, dependence to chronic exposure of morphine develops in colonic neurons from the β-arrestin2 knockout mice. These studies corroborate the in-vivo findings of the differential role of neuronal β-arrestin2 in the development of morphine tolerance/dependence in the ileum and colon.

Increased PDE5 activity and decreased Rho kinase and PKC activities in colonic muscle from caveolin-1-/- mice impair the peristaltic reflex and propulsion

Caveolae are specialized regions of the plasma membrane that concentrate receptors and associated signaling molecules critical in regulation of cellular response to transmitters and hormones. We have determined the effects of caveolin-1 (Cav-1) deletion, caveolin-1 siRNA, and caveolar disruption in mice on the signaling pathways that mediate contraction and relaxation in colonic smooth muscle and on the components of the peristaltic reflex in isolated tissue and propulsion in intact colonic segments. In Cav-1-/- mice, both relaxation and contraction were decreased in smooth muscle cells and muscle strips, as well as during both phases of the peristaltic reflex and colonic propulsion. The decrease in relaxation in response to the nitric oxide (NO) donor was accompanied by a decrease in cGMP levels and an increase in phosphodiesterase 5 (PDE5) activity. Relaxation by a PDE5-resistant cGMP analog was not affected in smooth muscle of Cav-1-/- mice, suggesting that inhibition of relaxation was due to augmentation of PDE5 activity. Similar effects on relaxation, PDE5 and cGMP were obtained in muscle cells upon disruption of caveolae by methyl-β-cyclodextrin or suppression of Cav-1. Sustained contraction mediated via inhibition of myosin light chain phosphatase (MLCP) activity is regulated by Rho kinase and PKC via phosphorylation of two endogenous inhibitors of MLCP: myosin phosphatase-targeting subunit (MYPT1) and 17-kDa PKC-potentiated protein phosphatase 1 inhibitor protein (CPI-17), respectively. The activity of both enzymes and phosphorylation of MYPT1 and CPI-17 were decreased in smooth muscle from Cav-1-/- mice. We conclude that the integrity of caveolae is essential for contractile and relaxant activity in colonic smooth muscle and the maintenance of neuromuscular function at organ level.

An in-vitro preparation of isolated enteric neurons and glia from the myenteric plexus of the adult mouse

The enteric nervous system is a vast network of neurons and glia running the length of the gastrointestinal tract that functionally controls gastrointestinal motility. A procedure for the isolation and culture of a mixed population of neurons and glia from the myenteric plexus is described. The primary cultures can be maintained for over 7 days, with connections developing among the neurons and glia. The longitudinal muscle strip with the attached myenteric plexus is stripped from the underlying circular muscle of the mouse ileum or colon and subjected to enzymatic digestion. In sterile conditions, the isolated neuronal and glia population are preserved within the pellet following centrifugation and plated on coverslips. Within 24-48 hr, neurite outgrowth occurs and neurons can be identified by pan-neuronal markers. After two days in culture, isolated neurons fire action potentials as observed by patch clamp studies. Furthermore, enteric glia can also be identified by GFAP staining. A network of neurons and glia in close apposition forms within 5-7 days. Enteric neurons can be individually and directly studied using methods such as immunohistochemistry, electrophysiology, calcium imaging, and single-cell PCR. Furthermore, this procedure can be performed in genetically modified animals. This methodology is simple to perform and inexpensive. Overall, this protocol exposes the components of the enteric nervous system in an easily manipulated manner so that we may better discover the functionality of the ENS in normal and disease states.

Sepiapterin ameliorates chemically induced murine colitis and azoxymethane-induced colon cancer

The effects of modulating tetrahydrobiopterin (BH4) levels with a metabolic precursor, sepiapterin (SP), on dextran sodium sulfate (DSS)-induced colitis and azoxymethane (AOM)-induced colorectal cancer were studied. SP in the drinking water blocks DSS-induced colitis measured as decreased disease activity index (DAI), morphologic criteria, and recovery of Ca(2+)-induced contractility responses lost as a consequence of DSS treatment. SP reduces inflammatory responses measured as the decreased number of infiltrating inflammatory macrophages and neutrophils and decreased expression of proinflammatory cytokines interleukin 1β (IL-1β), IL-6, and IL-17A. High-performance liquid chromatography analyses of colonic BH4 and its oxidized derivative 7,8-dihydrobiopterin (BH2) are inconclusive although there was a trend for lower BH4:BH2 with DSS treatment that was reversed with SP. Reduction of colonic cGMP levels by DSS was reversed with SP by a mechanism sensitive to 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a specific inhibitor of the NO-sensitive soluble guanylate cyclase (sGC). ODQ abrogates the protective effects of SP on colitis. This plus the finding that SP reduces DSS-enhanced protein Tyr nitration are consistent with DSS-induced uncoupling of NOS. The results agree with previous studies that demonstrated inactivation of sGC in DSS-treated animals as being important in recruitment of inflammatory cells and in altered cholinergic signaling and colon motility. SP also reduces the number of colon tumors in AOM/DSS-treated mice from 7 to 1 per unit colon length. Thus, pharmacologic modulation of BH4 with currently available drugs may provide a mechanism for alleviating some forms of colitis and potentially minimizing the potential for colorectal cancer in patients with colitis.