Opioid Receptor Research Articles

Sex-dependent effects of acute stress and alcohol exposure during adolescence on mRNA expression of brain signaling systems involved in reward and stress responses in young adult rats

BackgroundAdolescent stress and alcohol exposure increase the risk of maladaptive behaviors and mental disorders in adulthood, with distinct sex-specific differences. Understanding the mechanisms underlying these early events is crucial for developing targeted prevention and treatment strategies.MethodsMale and female Wistar rats were exposed to acute restraint stress and intermittent alcohol during adolescence. We assessed lasting effects on plasma corticosterone (CORT) and adrenocorticotropic hormone (ACTH) levels, and mRNA expression of genes related to corticotropin releasing hormone (CRH), neuropeptide Y (NPY), corticoid, opioid, and arginine vasopressin systems in the amygdala and hypothalamus.ResultsThe main findings are as follows: (1) blood alcohol concentrations (BAC) increased after the final alcohol administration, but stressed males had lower BAC than non-stressed males; (2) Males gained significantly more weight than females; (3) Stressed females showed higher ACTH levels than non-stressed females, with no changes in males; (4) Stress increased CORT levels in males, while stressed, alcohol-treated females had lower CORT levels than non-stressed females; (5) CRH: Females had lower Crhr1 levels in the amygdala, while alcohol reduced Crhr2 levels in males but not females. Significant interactions among sex, stress, and alcohol were found in the hypothalamus, with distinct patterns between sexes; (6) NPY: In the amygdala, stress reduced Npy and Npy1r levels in males but increased them in females. Alcohol decreased Npy2r levels in males, with varied effects in females. Similar sex-specific patterns were observed in the hypothalamus; (7) Corticoid system: Stress and alcohol had complex, sex-dependent effects on Pomc, Nr3c1, and Nr3c2 in both brain regions; (8) Opioid receptors: Stress and alcohol blunted the elevated expression of Oprm1, Oprd1, and Oprk1 in the amygdala of males and the hypothalamus of females; (8) Vasopressin: Stress and alcohol interacted significantly to affect Avp and Avpr1a expression in the amygdala, with stronger effects in females. In the hypothalamus, alcohol increased Avp levels in females.ConclusionsThis study demonstrates that adolescent acute stress and alcohol exposure induce lasting, sex-specific alterations in systems involved in reward and stress responses. These findings emphasize the importance of considering sex differences in the prevention and management of HPA dysfunction and psychiatric disorders.

The Role of the Rat Prefrontal Cortex and Sex Differences in Decision-Making.

The prefrontal cortex is critical for decision-making across species, with its activity linked to choosing between options. Drift Diffusion Models (DDMs) are commonly employed to understand the neural computations underlying this behavior. Studies exploring the specific roles of regions of the rodent prefrontal cortex in controlling the decision process are limited. This study explored the role of the prelimbic cortex (PLC) in decision-making using a two-alternative forced-choice task. Rats first learned to report the location of a lateralized visual stimulus. The brightness of the stimulus indicated its reward value. Then, the rats learned to make choices between pairs of stimuli. Sex differences in learning were observed, with females responding faster and more selectively to high-value stimuli than males. DDM analysis found that males had decreased decision thresholds during initial learning, whereas females maintained a consistently higher drift rate. Pharmacological manipulations revealed that PLC inactivation reduced the decision threshold for all rats, indicating that less information was needed to make a choice in the absence of normal PLC processing. Mu opioid receptor stimulation of the PLC had the opposite effect, raising the decision threshold and reducing bias in the decision process towards high-value stimuli. These effects were observed without any impact on the rats' choice preferences. Our findings suggest that PLC has an inhibitory role in the decision process and regulates the amount of evidence that is required to make a choice. That is, PLC activity controls "when", but not "how", to act.Significance Statement This study reports causal evidence for a part of the rat prefrontal cortex, the prelimbic cortex, in controlling the amount of information needed to make a choice. Results were based on reversible inactivation using the GABA-A agonist muscimol and by stimulation of mu opioid receptors using intra-cortical infusions of the selective mu agonist DAMGO. We also found evidence for a sex difference in learning and performing a visually guided two-alternative forced-choice task. Drift Diffusion Models found that females had stable decision processes throughout learning, and showed a persistent bias against the lower value option. By contrast, males exhibited changes in their decision processes, notably reducing the amount of information needed to make choice over the period of early choice learning.

Impacts of endogenous opioid blockade and sex on working memory among ad-libitum and abstinent smokers and non-smokers

BackgroundResearch examining the effects of opioid receptor antagonists on working memory is limited by small sample sizes that are often comprised of only male participants. Moreover, they have yielded mixed findings, and no studies have examined such effects in the context of chronic nicotine use and temporary nicotine withdrawal. MethodsMale and female participants completed two lab visits at which they were administered either 50 mg naltrexone or an identical placebo (one at each lab using double-blind, counterbalanced administration). After a medication absorption period, performance on a mental arithmetic (working memory) task was assessed by number of correct responses, errors, and number of attempts. Participants were either non-smokers or regular smokers who were randomly assigned to use nicotine ad-libitum or to abstain from nicotine for 24 h prior to both lab visits. ResultsThe results indicate a significant main effect of opioid blockade on number of correct answers and accuracy rate during the mental arithmetic task, with poorer performance after opioid blockade compared to placebo. In addition, compared to male participants, female participants made fewer answer attempts, had fewer correct answers, and a lower accuracy rate. We did not find evidence of any differences based on smoking group (non-smoker, smoking abstinence, or ad-libitum smoking), nor were there any significant interaction effects. ConclusionsThe findings suggest a modulating role of the endogenous opioid system in working memory function. Further investigation is needed to help identify the underlying mechanisms explaining the effects of the endogenous opioid system on working memory.

Identification of pharmacological inducers of a reversible hypometabolic state for whole organ preservation

Drugs that induce reversible slowing of metabolic and physiological processes would have great value for organ preservation, especially for organs with high susceptibility to hypoxia-reperfusion injury, such as the heart. Using whole-organism screening of metabolism, mobility, and development in Xenopus, we identified an existing drug, SNC80, that rapidly and reversibly slows biochemical and metabolic activities while preserving cell and tissue viability. Although SNC80 was developed as a delta opioid receptor activator, we discovered that its ability to slow metabolism is independent of its opioid modulating activity as a novel SNC80 analog (WB3) with almost 1000 times less delta opioid receptor binding activity is equally active. Metabolic suppression was also achieved using SNC80 in microfluidic human organs-on-chips, as well as in explanted whole porcine hearts and limbs, demonstrating the cross-species relevance of this approach and potential clinical relevance for surgical transplantation. Pharmacological induction of physiological slowing in combination with organ perfusion transport systems may offer a new therapeutic approach for tissue and organ preservation for transplantation, trauma management, and enhancing patient survival in remote and low-resource locations.

Identification of pharmacological inducers of a reversible hypometabolic state for whole organ preservation.

Drugs that induce reversible slowing of metabolic and physiological processes would have great value for organ preservation, especially for organs with high susceptibility to hypoxia-reperfusion injury, such as the heart. Using whole-organism screening of metabolism, mobility, and development in Xenopus, we identified an existing drug, SNC80, that rapidly and reversibly slows biochemical and metabolic activities while preserving cell and tissue viability. Although SNC80 was developed as a delta opioid receptor activator, we discovered that its ability to slow metabolism is independent of its opioid modulating activity as a novel SNC80 analog (WB3) with almost 1000 times less delta opioid receptor binding activity is equally active. Metabolic suppression was also achieved using SNC80 in microfluidic human organs-on-chips, as well as in explanted whole porcine hearts and limbs, demonstrating the cross-species relevance of this approach and potential clinical relevance for surgical transplantation. Pharmacological induction of physiological slowing in combination with organ perfusion transport systems may offer a new therapeutic approach for tissue and organ preservation for transplantation, trauma management, and enhancing patient survival in remote and low-resource locations.

Major clinical evidence on the use of low-dose naltrexone in the treatment of cancer: a systematic review

Introduction: Opioid receptors are groups of receptors (γ-, κ-, δ-, and ζ-opioid receptors) that are widely distributed in nerve cells in the brain, spinal cord, and digestive tract. Naltrexone is a type of general opioid receptor antagonist. It has been used to treat chronic pain syndrome, autoimmune diseases, and cancer at a dose of 5 mg/day, which is generally called low-dose naltrexone (LDN). Objective: It was to analyze the pharmacological functions of low-dose naltrexone, especially in anti-inflammation and immunoregulation, and its therapeutic potential against cancer. Methods: The research and development of the work were carried out from June to July 2024 in the Scopus, PubMed, Science Direct, and Scielo databases, using scientific articles from the last 15 years, following the PRISMA rules. The quality of the studies was based on the GRADE and AMSTAR2 instruments, and the risk of bias was assessed using the Cochrane instrument (Funnel Plot). Results and Conclusion: Fifteen studies were included in the systematic review out of 30. Most studies showed homogeneity in their results, with X2=94.5%>50%. Low-dose naltrexone has immunomodulatory and therapeutic effects. Low-dose naltrexone regulates the production of inflammatory cytokines, influencing the level of endogenous opioid peptides in the body. Furthermore, low-dose naltrexone has an antitumor effect and can modulate the neuroblastoma tumor response, delaying the onset and reducing the incidence rate of tumors, significantly decreasing tumor volume and weight, and DNA synthesis in cancer.

Examination of the antiallodynic effect of rosmarinic acid in neuropathic pain and possible mechanisms of action

This study aimed to explore the potential antiallodynic effects of rosmarinic acid, a natural antioxidant with a demonstrated safety profile across a broad dose range. Using a chronic constriction injury-induced neuropathic pain model, the impact of rosmarinic acid on allodynia was investigated. Furthermore, the involvement of adrenergic and opioidergic mechanisms in its activity was assessed. To evaluate rosmarinic acid’s efficacy, doses of 10, 20, and 40 mg/kg were administered and the electronic von Frey test was utilized along with an activity cage apparatus. % MPE values were calculated to gauge the extent of pain relief. Mechanistic insights were obtained by pretreating animals with the β-adrenergic receptor antagonist propranolol, the α1-adrenergic receptor antagonist prazosin, α2-adrenergic receptor antagonist yohimbine, and the opioid receptor antagonist naloxone. Rosmarinic acid demonstrated a statistically significant antiallodynic effect that was independent of locomotor activity. This effect was noteworthy as it resembled both the level and duration of relief provided by pregabalin. Additionally, the %MPE value of the group treated with 40 mg/kg rosmarinic acid showed a significant difference compared to the value of the pregabalin-treated group (P<0.001). Pre-administration of the antagonists revealed that the antiallodynic activity was shown to be mediated by the stimulation of opioid and adrenergic receptors, with a primary contribution from α2-adrenergic receptor stimulation. Our findings suggest that rosmarinic acid may hold promise as a potential therapeutic agent for neuropathic pain. By elucidating the involvement of adrenergic and opioidergic mechanisms, we have provided valuable preclinical data that could inform novel treatment approaches.

Oxa-Iboga alkaloids lack cardiac risk and disrupt opioid use in animal models

Ibogaine and its main metabolite noribogaine provide important molecular prototypes for markedly different treatment of substance use disorders and co-morbid mental health illnesses. However, these compounds present a cardiac safety risk and a highly complex molecular mechanism. We introduce a class of iboga alkaloids – termed oxa-iboga – defined as benzofuran-containing iboga analogs and created via structural editing of the iboga skeleton. The oxa-iboga compounds lack the proarrhythmic adverse effects of ibogaine and noribogaine in primary human cardiomyocytes and show superior efficacy in animal models of opioid use disorder in male rats. They act as potent kappa opioid receptor agonists in vitro and in vivo, but exhibit atypical behavioral features compared to standard kappa opioid agonists. Oxa-noribogaine induces long-lasting suppression of morphine, heroin, and fentanyl intake after a single dose or a short treatment regimen, reversal of persistent opioid-induced hyperalgesia, and suppression of opioid drug seeking in rodent relapse models. As such, oxa-iboga compounds represent mechanistically distinct iboga analogs with therapeutic potential.

Peripheral opioid receptor antagonism alleviates fentanyl-induced cardiorespiratory depression and is devoid of aversive effects.

In this study, we compare the central versus peripheral components underlying fentanyl-induced cardiorespiratory depression to prevent overdose deaths. Our data indicates that these effects are, at least partially, due to the activation of mu opioid receptors present in peripheral sites. These findings provide insight into peripheral contributions to fentanyl-induced overdoses and could potentially lead to the development of treatments selectively targeting the peripheral system, sparing individuals from the CNS-driven acute opioid withdrawal generally observed with the use of naloxone.

Opioid drug seeking after early-life adversity: a role for delta opioid receptors

Opioid use disorder (OUD) is associated with a history of early-life adversity (ELA), an association that is particularly strong in women. In a rodent model, we previously found that ELA enhances risk for opioid addiction selectively in females, but the mechanisms for this effect are unclear. Here, we show that ELA robustly alters cFos responses to opioid drugs in females’ nucleus accumbens (NAc) and basolateral amygdala (BLA), but not elsewhere. We further identify delta opioid receptors (DOR), which mature in the first week of life and thus later than kappa or mu opioid receptors, as a potential mediator of ELA's impacts on reward circuit functions. Accordingly, DOR mRNA in NAc was persistently reduced in adult females with ELA history. Moreover, pharmacological stimulation of NAc DORs increased opioid demand in control females (recapitulating the ELA phenotype), while blocking DORs in ELA females conversely reduced high-effort drug consumption, simulating the control rearing phenotype. These findings support a role for NAc DORs in mediating ELA-induced opioid vulnerability. In contrast, BLA neurons expressing DOR protein do not overlap heroin- responsive cells in ELA rats, arguing against a direct relationship of BLA DORs to heroin's addiction-relevant actions in the brain. Together, these results suggest a novel and selective role for NAc DORs in contributing to enduring, ELA-provoked vulnerability to OUD.

Whole brain modelling for simulating pharmacological interventions on patients with disorders of consciousness

Disorders of consciousness (DoC) represent a challenging and complex group of neurological conditions characterised by profound disturbances in consciousness. The current range of treatments for DoC is limited. This has sparked growing interest in developing new treatments, including the use of psychedelic drugs. Nevertheless, clinical investigations and the mechanisms behind them are methodologically and ethically constrained. To tackle these limitations, we combined biologically plausible whole-brain models with deep learning techniques to characterise the low-dimensional space of DoC patients. We investigated the effects of model pharmacological interventions by including the whole-brain dynamical consequences of the enhanced neuromodulatory level of different neurotransmitters, and providing geometrical interpretation in the low-dimensional space. Our findings show that serotonergic and opioid receptors effectively shifted the DoC models towards a dynamical behaviour associated with a healthier state, and that these improvements correlated with the mean density of the activated receptors throughout the brain. These findings mark an important step towards the development of treatments not only for DoC but also for a broader spectrum of brain diseases. Our method offers a promising avenue for exploring the therapeutic potential of pharmacological interventions within the ethical and methodological confines of clinical research.

Cyclic Glycopeptide Analogs of Endomorphin-1 Provide Highly Effective Antinociception in Male and Female Mice.

Opioids acting at the mu opioid receptor (MOR) remain the most effective treatment for moderate to severe pain, but their use is limited by serious side effects. We have shown that a cyclized analog of endomorphin-1 provided pain relief comparable to that of morphine with reduction or absence of several side effects, including abuse liability. Glycosylation can promote penetration of cellular barriers. Here we developed cyclic glycopeptide endomorphin (glycoEM) analogs as drug candidates for potent and long-lasting analgesia. The analogs were assessed in receptor binding and functional assays and for blood-brain barrier penetration by microdialysis and MS. Two of the analogs showed MOR selectivity and more potent and longer lasting antinociception than morphine in male and female mice. Comparable antinociception occurred at A2 doses 5-fold lower (20-fold on a molar basis) than morphine doses. The results support further study of the glycoEMs for clinical application.

Exploring the Confluence of Animal Medicine and its Implications for Human Health: A Systematic Literature Review.

The abuse of veterinary drugs has emerged as a concerning trend, with global fatalities on the rise. Our understanding of this phenomenon remains limited. This study aims to identify the veterinary drugs being misused, the reasons behind their misuse, and how they are obtained. Utilising PubMed, Scopus, and Web of Science databases, along with related grey literature, we applied the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) framework for data collection. Screening and cross-referencing yielded 66 relevant articles, encompassing case reports, surveys, reports, and systemic literature reviews. The analysis identified 28 distinct veterinary drugs being misused in humans, primarily falling into categories, e.g., α-2- and β-2-adrenergic receptor agonists, GABAergic receptor modulators, opioid receptor agonists, nonsteroidal anti-inflammatory drugs (NSAIDs), and N-methyl-D-aspartate (NMDA) receptor antagonists. These drugs were used for various purposes, including recreational use, weight loss, bodybuilding, pain relief, and self-medication for stress-related symptoms. Routes of administration predominantly included parenteral, oral, and inhalation methods. Veterinary workers/assistants and individuals connected to animals were identified as contributors to the misuse of these medications. Motivations for their utilisation ranged from affordability and accessibility to the ease of obtaining multiple prescriptions from various veterinary sources, often in conjunction with other illicit substances. Dependence and addiction were common outcomes of the misuse of veterinary medicines by humans. Overall, this systematic review underscores the increasing popularity of veterinary prescription drug misuse despite being under-reported with limited available data. Healthcare professionals are urged to remain vigilant to potential overdose events involving these medications.

Effect of opioid receptor antagonist on mitigating tumor necrosis factor-like weak inducer of apoptosis (TWEAK)-induced apoptolysis in pemphigus pathogenesis

Pemphigus is a severe autoimmune blistering disease characterized by acantholysis triggered by autoantibodies against desmoglein 1 and 3 (DSG1/3). Apoptosis plays a pivotal role in facilitating acantholysis, yet the precise underlying mechanism remains obscure. Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) is known to promote apoptosis and disrupt cell junctions, although its involvement in pemphigus pathogenesis remains ambiguous. Our study observed decreased DSG1/3 expression alongside increased TWEAK/fibroblast growth factor-inducible 14 (Fn14) expression and keratinocyte apoptosis in both lesional and perilesional skin. In vitro experiments revealed that TWEAK-stimulated keratinocytes exhibited enhanced apoptosis, STAT1 phosphorylation, and reduced intercellular DSG1/3 expression. Notably, bulk-RNA sequencing unveiled that CASPASE-3 was responsible for mediating the DSG1/3 depletion, as confirmed by direct interaction with DSG1/3 in a co-immunoprecipitation assay. Naloxone, known for preserving cellular adhesion and preventing cell death, effectively reduced apoptosis and restored DSG1/3 levels in TWEAK-stimulated keratinocytes. The anti-apoptotic properties of naloxone were further validated in a murine pemphigus model. Our findings elucidate that TWEAK facilitates keratinocyte apoptosis by augmenting caspase-3 activity, leading to DSG1/3 depletion and apoptosis in pemphigus. Importantly, naloxone can counter TWEAK-induced apoptosis in pemphigus pathogenesis, offering a potential therapeutic intervention.

Effect of chronic delivery of the NOP/MOR partial agonist AT-201 and NOP antagonist J-113397 on heroin relapse in a rat model of opioid maintenance.

The opioid crisis persists despite availability of effective opioid agonist maintenance treatments (methadone and buprenorphine). Thus, there is a need to advance novel medications for the treatment of opioid use and relapse. We recently modeled maintenance treatment in rats and found that chronic delivery of buprenorphine and the mu opioid receptor (MOR) partial agonist TRV130 decreases relapse to oxycodone seeking and taking. In contrast, chronic delivery of the buprenorphine analog BU08028 had mixed effects on different heroin relapse-related measures. Here, we tested the effect of the mixed nociceptin (NOP) receptor/MOR partial agonist AT-201 and the NOP receptor antagonist J-113397 on different heroin relapse-related measures. We trained male and female rats to self-administer heroin (6-h/d, 14-d) in context A and then implanted osmotic minipumps containing AT-201 (0, 3.8, or 12mg/kg/d) or J-113397 (0, 12.6, or 40mg/kg/d). Next, we tested the effect of chronic delivery of the compounds on (1) incubation of heroin seeking in a non-drug context B, (2) extinction responding reinforced by heroin-associated discrete cues in context B, (3) context A-induced reinstatement of heroin seeking, and (4) reacquisition of heroin self-administration in context A. In females, AT-201 modestly increased reacquisition of heroin self-administration and J-113397 modestly decreased incubation of heroin seeking. The compounds had no effect on the other relapse-related measures in females, and no effect on any of the measures in males. The NOP/MOR partial agonist AT-201 and the NOP antagonist J-113397 did not mimic buprenorphine's inhibitory effects on relapse in a rat model of opioid maintenance treatment.

Identification of c[D-Trp-Phe-β-Ala-β-Ala], the First κ-Opioid Receptor-Specific Negative Allosteric Modulator.

Recently, the fungus secondary metabolite cyclotetrapetide c[Trp-Phe-D-Pro-Phe] (CJ-15,208) and its derivatives deserved some attention for their unusual structure and distinctive in vitro and in vivo activity. These tryptophan-containing noncationic opioid peptides can be truly regarded as versatile picklocks capable of activating all opioid receptors. Intriguingly, minimal modification of the potent κ-opioid receptor (KOR) agonist c[D-Trp-Phe-Gly-β-Ala] (3) yielded c[D-Trp-Phe-β-Ala-β-Ala] (11), the first KOR-specific negative allosteric modulator (NAM) reported to-date. KOR exerts control over numerous functions in the central nervous system, including pain, depression, stress, mood, and reward. Hence, this KOR-selective NAM looks promising for modulating the KOR in addiction and neuropsychiatric disorders.

The Role of the Mu Opioid Receptors of the Medial Prefrontal Cortex in the Modulation of Analgesia Induced by Acute Restraint Stress in Male Mice.

Mu opioid receptors (MORs) represent a vital mechanism related to the modulation of stress-induced analgesia (SIA). Previous studies have reported on the gamma-aminobutyric acid (GABA)ergic "disinhibition" mechanisms of MORs on the descending pain modulatory pathway of SIA induced in the midbrain. However, the role of the MORs expressed in the medial prefrontal cortex (mPFC), one of the main cortical areas participating in pain modulation, in SIA remains completely unknown. In this study, we investigated the contributions of MORs expressed on glutamatergic (MORGlut) and GABAergic (MORGABA) neurons of the medial prefrontal cortex (mPFC), as well as the functional role and activity of neurons projecting from the mPFC to the periaqueductal gray (PAG) region, in male mice. We achieved this through a combination of hot-plate tests, c-fos staining, and 1 h acute restraint stress exposure tests. The results showed that our acute restraint stress protocol produced mPFC MOR-dependent SIA effects. In particular, MORGABA was found to play a major role in modulating the effects of SIA, whereas MORGlut seemed to be unconnected to the process. We also found that mPFC-PAG projections were efficiently activated and played key roles in the effects of SIA, and their activation was mediated by MORGABA to a large extent. These results indicated that the activation of mPFC MORGABA due to restraint stress was able to activate mPFC-PAG projections in a potential "disinhibition" pathway that produced analgesic effects. These findings provide a potential theoretical basis for pain treatment or drug screening targeting the mPFC.

Mitragynine and morphine produce dose-dependent bimodal action on food but not water intake in rats.

Kratom (Mitragyna speciosa), containing the primary alkaloid mitragynine, has emerged as an alternative self-treatment for opioid use disorder. Mitragynine binds numerous receptor types, including opioid receptors, which are known to modulate food consumption. However, the ability of acute mitragynine to modulate food consumption remains unknown. The current study assessed the effects of acute mitragynine or morphine administration on unconditioned food and water intake in 16 Sprague-Dawley rats. Food and water intake changes were monitored in response to morphine, mitragynine (1.78-56 mg/kg ip), saline, or vehicle controls for 12 h, starting at the onset of the dark cycle. Naltrexone pretreatment was used to examine pharmacological specificity. Both morphine and mitragynine demonstrated a biphasic food intake dose-effect, with low doses (5.6 mg/kg) increasing and high doses (56 mg/kg) decreasing food intake. All morphine doses reduced water intake; however, only the highest dose of mitragynine (56 mg/kg) reduced water intake. Naltrexone attenuated both stimulatory and inhibitory effects of morphine on food intake, but only the stimulatory effect of mitragynine. In conclusion, low doses of mitragynine stimulate food intake via opioid-related pathways, while high doses likely recruit other targets.NEW & NOTEWORTHY This study reveals that morphine and the kratom alkaloid mitragynine produce dose-dependent effects on feeding in rats. Low doses stimulate food intake via opioid pathways, while high doses decrease consumption through nonopioid mechanisms. Morphine potently suppresses water intake at all doses, whereas only high doses of mitragynine reduce drinking. These findings provide novel insights into the complex opioid and nonopioid mechanisms underlying the effects of mitragynine on ingestive behaviors.

Atp1a2 and Kcnj9 are candidate genes underlying sensitivity to oxycodone-induced locomotor activation and withdrawal-induced anxiety-like behaviors in C57BL/6 substrains.

Opioid use disorder is heritable, yet its genetic etiology is largely unknown. C57BL/6J and C57BL/6NJ mouse substrains exhibit phenotypic diversity in the context of limited genetic diversity which together can facilitate genetic discovery. Here, we found C57BL/6NJ mice were less sensitive to oxycodone (OXY)-induced locomotor activation versus C57BL/6J mice in a conditioned place preference paradigm. Narrow-sense heritability was estimated at 0.22-0.31, implicating suitability for genetic analysis. Quantitative trait locus (QTL) mapping in an F2 cross identified a chromosome 1 QTL explaining 7-12% of the variance in OXY locomotion and anxiety-like withdrawal in the elevated plus maze. A second QTL for EPM withdrawal behavior on chromosome 5 near Gabra2 (alpha-2 subunit of GABA-A receptor) explained 9% of the variance. To narrow the chromosome 1 locus, we generated recombinant lines spanning 163-181 Mb, captured the QTL for OXY locomotor traits and withdrawal, and fine-mapped a 2.45-Mb region (170.16-172.61 Mb). Transcriptome analysis identified five, localized striatal cis-eQTL transcripts and two were confirmed at the protein level (KCNJ9, ATP1A2). Kcnj9 codes for a potassium channel (GIRK3) that is a major effector of mu opioid receptor signaling. Atp1a2 codes for a subunit of a Na+/K+ ATPase enzyme that regulates neuronal excitability and shows functional adaptations following chronic opioid administration. To summarize, we identified two candidate genes underlying the physiological and behavioral properties of opioids, with direct preclinical relevance to investigators employing these widely used substrains and clinical relevance to human genetic studies of opioid use disorder.

Pregnancy ameliorates neuropathic pain through suppression of microglia and upregulation of the δ-opioid receptor in the anterior cingulate cortex in late-pregnant mice.

Pregnancy-induced analgesia develops in late pregnancy, but its mechanisms are unclear. The anterior cingulate cortex (ACC) plays a key role in the pathogenesis of neuropathic pain. The authors hypothesized that pregnancy-induced analgesia ameliorates neuropathic pain by suppressing activation of microglia and the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and by upregulating opioid receptors in the ACC in late-pregnant mice. Neuropathic pain was induced in non-pregnant (NP) or pregnant (P) C57BL/6JJmsSlc female mice by partial sciatic nerve ligation (PSNL). The nociceptive response was evaluated by mechanical allodynia and activation of microglia in the ACC was evaluated by immunohistochemistry. The expressions of phosphorylated AMPA receptors and opioid receptors in the ACC were evaluated by immunoblotting. In von Frey reflex tests, NP-PSNL-treated mice showed a lower 50% paw-withdrawal threshold than NP-Naïve mice on experimental day 9. No difference in 50% paw-withdrawal threshold was found among the NP-Naïve, NP-Sham, P-Sham, and P-PSNL-treated mice. The number of microglia in the ACC was significantly increased in NP-PSNL-treated mice compared to NP-Sham mice. Immunoblotting showed significantly increased expression of phosphorylated AMPA receptor subunit GluR1 at Ser831 in NP-PSNL-treated mice compared to NP-Sham mice. Immunoblotting also showed significantly increased δ-opioid receptor in the ACC in P-Sham and P-PSNL-treated mice compared to NP-Sham mice. Pregnancy-induced analgesia ameliorated neuropathic pain by suppressing activation of microglia and the expression of phosphorylated AMPA receptor subunit GluR1 at Ser831, and by upregulation of the δ-opioid receptor in the ACC in late-pregnant mice.