Endogenous Opioids at the Intersection of Opioid Addiction, Pain, and Depression: The Search for a Precision Medicine Approach

Modulation of endogenous opioid signaling by inhibitors of puromycin-sensitive aminopeptidase

The endogenous opioid system regulates pain through local release of neuropeptides and modulation of their action on opioid receptors. However, the effect of opioid peptides, the enkephalins, is short-lived due to their rapid hydrolysis by enkephalin-degrading enzymes. In turn, an innovative approach to the management of pain would be to increase the local concentration and prolong the stability of enkephalins by preventing their inactivation by neural enkephalinases such as puromycin-sensitive aminopeptidase (PSA). Our previous structure-activity relationship studies offered the S-diphenylmethyl cysteinyl derivative of puromycin (20) as a nanomolar inhibitor of PSA. This chemical class, however, suffered from undesirable metabolism to nephrotoxic puromycin aminonucleoside (PAN). To prevent such toxicity, we designed and synthesized 5'-chloro substituted derivatives. The compounds retained the PSA inhibitory potency of the corresponding 5'-hydroxy analogs and had improved selectivity toward PSA. In vivo treatment with the lead compound 19 caused significantly reduced pain response in antinociception assays, alone and in combination with Met-enkephalin. The analgesic effect was reversed by the opioid antagonist naloxone, suggesting the involvement of opioid receptors. Further, PSA inhibition by compound 19 in brain slices caused local increase in endogenous enkephalin levels, corroborating our rationale. Pharmacokinetic assessment of compound 19 showed desirable plasma stability and identified the cysteinyl sulfur as the principal site of metabolic liability. We gained additional insight into inhibitor-PSA interactions by molecular modeling, which underscored the importance of bulky aromatic amino acid in puromycin scaffold. The results of this study strongly support our rationale for the development of PSA inhibitors for effective pain management.

Similarities and differences in dynamic properties of brain networks between internet gaming disorder and tobacco use disorder

BACKGROUND

Internet gaming disorder (IGD) and tobacco use disorder (TUD) are two major addiction disorders that result in substantial financial loss. Identifying the similarities and differences between these two disorders is important to understand substance addiction and behavioral addiction. The current study was designed to compare these two disorders utilizing dynamic analysis.

METHOD

Resting-state data were collected from 35 individuals with IGD, 35 individuals with TUD and 35 healthy controls (HCs). Dynamic coactivation pattern analysis was employed to decipher their dynamic patterns.

RESULTS

IGD participants showed decreased coactivation patterns within the default mode network (DMN) and between the DMN and the salience network (SN). The SN showed reduced coactivation patterns with the executive control network (ECN) and DMN, and the ECN showed decreased coactivation patterns with the DMN. In the TUD group, the DMN exhibited decreased coactivation patterns with the SN, the SN exhibited reduced coactivation patterns with the DMN and ECN, and the ECN showed decreased coactivation patterns with the DMN and within the ECN. Furthermore, the triple network model was fitted to the dynamic properties of the two addiction disorders. Decoding analysis results indicated that addiction-related memory and memory retrieval displayed similar dysfunctions in both addictions.

CONCLUSION

The dynamic characteristics of IGD and TUD suggest that there are similarities in the dynamic features between the SN and DMN and differences in the dynamic features between the DMN and ECN. Our results revealed that the two addiction disorders have dissociable brain mechanisms, indicating that future studies should consider these two addiction disorders as having two separate mechanisms to achieve precise treatment for their individualized targets.

Heterogeneity in Temporal Dynamics of Pain and Affect Among Individuals With Chronic Back Pain and Associations With Risk for Future Opioid-Related Problems

Chronic low back pain (CLBP) is a biopsychosocial phenomenon involving complex relationships between pain and psychosocial factors. In preregistered analyses, we examined dynamic relationships between pain and negative affect among individuals with CLBP (N = 87). We found that increased negative affect was concurrently and prospectively associated with increased pain for individuals on average. However, there was significant and meaningful between-persons variability in these effects such that risk for future opioid-related problems was positively associated with the within-persons correlation between pain and negative affect (β = 0.290, 95% credible interval [CI] = [0.071, 0.485]), the degree to which pain predicted increased negative affect (β = 0.439, 95% CI = [0.044, 0.717]), and the autoregressive effect of negative affect over 4-hr lags (β = 0.255, 95% CI = [0.007, 0.478]). These results suggest that variability in within-persons symptom dynamics may help identify chronic pain patients who are at greater risk of opioid-related problems.

Inhibition of the protective effects of preconditioning in ischemia-reperfusion injury by chronic methadone: the role of pAkt and pSTAT3

Cardiac ischemic preconditioning (Pre) reduces cardiac ischemia-reperfusion injury (IRI) by stimulating opioid receptors. Chronic use of opioids can alter the signaling pathways. We investigated the effects of chronic methadone use on IRI and Pre. The experiments were performed on isolated hearts of male Wistar rats in four groups: IRI, Methadone + IRI (M-IRI), Pre + IRI (Pre-IRI), Methadone + Pre + IRI (M-Pre-IRI). The infarct size (IS) in the Pre-IRI group was smaller than the IRI group (26.8% vs. 47.8%, P < 0.05). In the M-IRI and M-Pre-IRI groups, the infarct size was similar to the IRI group. Akt (Ak strain transforming) phosphorylation in the Pre-IRI, M-IRI, and M-Pre-IRI groups was significantly higher than in the IRI group (0.56 ± 0.15, 0.63 ± 0.20, and 0.93 ± 0.18 vs 0.28 ± 0.17 respectively). STAT3 (signal transducer and activator of transcription 3) phosphorylation in the Pre-IRI and M-Pre-IRI groups (1.38 ± 0.14 and 1.46 ± 0.33) was significantly higher than the IRI and M-IRI groups (0.99 ± 0.1 and 0.98 ± 0.2). Thus, chronic use of methadone not only has no protective effect against IRI but also destroys the protective effects of ischemic preconditioning. This may be due to the hyperactivation of Akt and changes in signaling pathways.

Endogenous opioids in the olfactory tubercle and their roles in olfaction and quality of life

Olfactory dysfunctions decrease daily quality of life (QOL) in part by reducing the pleasure of eating. Olfaction plays an essential role in flavor sensation and palatability. The decreased QOL due to olfactory dysfunction is speculated to result from abnormal neural activities in the olfactory and limbic areas of the brain, as well as peripheral odorant receptor dysfunctions. However, the specific underlying neurobiological mechanisms remain unclear. As the olfactory tubercle (OT) is one of the brain's regions with high expression of endogenous opioids, we hypothesize that the mechanism underlying the decrease in QOL due to olfactory dysfunction involves the reduction of neural activity in the OT and subsequent endogenous opioid release in specialized subregions. In this review, we provide an overview and recent updates on the OT, the endogenous opioid system, and the pleasure systems in the brain and then discuss our hypothesis. To facilitate the effective treatment of olfactory dysfunctions and decreased QOL, elucidation of the neurobiological mechanisms underlying the pleasure of eating through flavor sensation is crucial.

Incidence and risk factors for potential drug-drug interactions in outpatients receiving opioid analgesics

BACKGROUND

Opioids are the most frequently used drugs to treat pain in cancer patients. However opioid analgesics can cause adverse effects and potential drug-drug interaction.

RESEARCH DESIGN AND METHODS

This cross-sectional retrospective study analyzed pDDI in 1839 patients with opioid analgesics in a large comprehensive hospital in China from January 1 to 31 December 2022. Three drug interaction databases were used to screen for pDDI including Drugs (U.S.A.), Medscape (U.S.A.), and Drug Assistant of Dingxiangyuan (China).

RESULTS

The prevalence of pDDIs among 1839 patients was around 41.27% of 759 patients, and 564 patients (74.31%) with pDDIs were diagnosed with tumor. Further, the total of 275 various pDDIs combinations were identified. The combination of oxycodone with morphine had the most frequent occurrence of 229 times, and its adverse effects mainly related to exacerbate central respiratory depression. While, gender, tumor, number of diagnoses, and the variety of opioid analgesics used were independent risk factors for pDDIs.

CONCLUSIONS

Outpatients taking opioid analgesics had a higher incidence of pDDIs. As consequently, optimized monitoring and management of patients taking opioid analgesics is recommended in order to ensure patient medication safety.

Modulation of endogenous opioid signaling by inhibitors of puromycin sensitive aminopeptidase

The endogenous opioid system regulates pain through local release of neuropeptides and modulation of their action on opioid receptors. However, the effect of opioid peptides, the enkephalins, is short-lived due to their rapid hydrolysis by enkephalin-degrading enzymes. In turn, an innovative approach to the management of pain would be to increase the local concentration and prolong the stability of enkephalins by preventing their inactivation by neural enkephalinases such as puromycin sensitive aminopeptidase (PSA). Our previous structure-activity relationship studies offered the S-diphenylmethyl cysteinyl derivative of puromycin (20) as a nanomolar inhibitor of PSA. This chemical class, however, suffered from undesirable metabolism to nephrotoxic puromycin aminonucleoside (PAN). To prevent such toxicity, we designed and synthesized 5'-chloro substituted derivatives. The compounds retained the PSA inhibitory potency of the corresponding 5'-hydroxy analogs and had improved selectivity toward PSA. In vivo treatment with the lead compound 19 caused significantly reduced pain response in antinociception assays, alone and in combination with Met-enkephalin. The analgesic effect was reversed by the opioid antagonist naloxone, suggesting the involvement of opioid receptors. Further, PSA inhibition by compound 19 in brain slices caused local increase in endogenous enkephalin levels, corroborating our rationale. Pharmacokinetic assessment of compound 19 showed desirable plasma stability and identified the cysteinyl sulfur as the principal site of metabolic liability. We gained additional insight into inhibitor-PSA interactions by molecular modeling, which underscored the importance of bulky aromatic amino acid in puromycin scaffold. The results of this study strongly support our rationale for the development of PSA inhibitors for effective pain management.

Childhood abuse and craving in methamphetamine-dependent individuals: the mediating role of alexithymia

Individuals with a history of childhood abuse (CA, including neglect and abuse by caregivers before the age of 18 years) have more severe substance dependence problems than those without a history of childhood abuse. However, whether a history of CA exacerbates craving and the mechanism of this effect remain largely unknown. The aim of this study was to explore the role of alexithymia in the effects of CA on craving in a large sample of methamphetamine-dependent individuals based on latent vulnerability theory. A total of 324 methamphetamine-dependent individuals who met DSM-5 criteria for substance use disorder were recruited. CA, alexithymia, and craving data were collected from the Childhood Trauma Questionnaire, the Toronto Alexithymia Scale-20, and the Obsessive Compulsive Drug Use Scale, respectively. t tests and ANCOVA were conducted to compare variables between the CA and non-CA groups, while partial correlation and mediation analyses were conducted to examine the potential mediating role of alexithymia in the relationship between CA and craving. Abused methamphetamine-dependent individuals reported higher levels of craving and higher levels of alexithymia than those of non-abused methamphetamine-dependent individuals. Alexithymia partially mediated the link between CA and craving, especially the effect of CA on craving frequency was fully mediated by alexithymia. Our findings reveal that a history of childhood abuse has a lasting effect on craving in stimulant-dependent individuals, and alexithymia contributes to some extent to the severity of substance abuse problems in abused methamphetamine-dependent individuals.

Distinct neurochemical influences on fMRI response polarity in the striatum

The striatum, known as the input nucleus of the basal ganglia, is extensively studied for its diverse behavioral roles. However, the relationship between its neuronal and vascular activity, vital for interpreting functional magnetic resonance imaging (fMRI) signals, has not received comprehensive examination within the striatum. Here, we demonstrate that optogenetic stimulation of dorsal striatal neurons or their afferents from various cortical and subcortical regions induces negative striatal fMRI responses in rats, manifesting as vasoconstriction. These responses occur even with heightened striatal neuronal activity, confirmed by electrophysiology and fiber-photometry. In parallel, midbrain dopaminergic neuron optogenetic modulation, coupled with electrochemical measurements, establishes a link between striatal vasodilation and dopamine release. Intriguingly, in vivo intra-striatal pharmacological manipulations during optogenetic stimulation highlight a critical role of opioidergic signaling in generating striatal vasoconstriction. This observation is substantiated by detecting striatal vasoconstriction in brain slices after synthetic opioid application. In humans, manipulations aimed at increasing striatal neuronal activity likewise elicit negative striatal fMRI responses. Our results emphasize the necessity of considering vasoactive neurotransmission alongside neuronal activity when interpreting fMRI signal.

The Endogenous Opioid Met-Enkephalin Modulates Thalamo-Cortical Excitation Inhibition Balance in a Medial Thalamus-Anterior Cingulate Cortex Circuit

Activation of opioid receptors in the anterior cingulate cortex (ACC) mediates aspects of analgesia induced by both exogenous and endogenous opioids. We have previously shown that opioid signaling disrupts both afferent excitatory and indirect inhibitory synaptic transmission from the medial thalamus (MThal) to the ACC, but the effects of endogenous opioids within this circuit remain poorly understood. The goal of the current study was to understand how the endogenous opioid, [Met]5-enkephalin (ME), modulates thalamic-driven excitatory and inhibitory synaptic transmission onto layer V pyramidal neurons in the ACC. We used pharmacology, brain slice electrophysiology and optogenetic stimulation to study opioid-mediated modulation of optically evoked glutamatergic and GABAergic transmission. The results revealed that ME inhibited both AMPA-mediated excitatory and GABA-mediated inhibitory synaptic transmission in the ACC. However, inhibitory transmission was more potently inhibited than excitatory transmission by ME. This preferential reduction in GABAA-mediated synaptic transmission was primarily due to the activation of delta opioid receptors by ME and resulted in a net disinhibition of MThal-ACC excitatory pathway. These results suggest that moderate concentrations of ME can lead to net excitation of ACC circuitry and that analgesia may be associated with disinhibition rather than inhibition of ACC subcircuits.

Efficacy of dual enkephalinase inhibition in a preclinical migraine model is mediated by activation of peripheral delta opioid receptors

OBJECTIVE

The aim of this study was to evaluate whether elevating levels of enkephalin by inhibiting their degradation can attenuate stress-induced migraine-like behaviors in mice.

BACKGROUND

Previous studies in animals have suggested the delta opioid receptor (DOR) as a novel migraine target. The primary endogenous ligands for DOR are enkephalins and their levels can be increased by pharmacological inhibition of enkephalinases; however, it is not clear whether enkephalinase inhibition can be efficacious in preclinical migraine models through activation of DOR or whether other opioid receptors might be involved. Further, it is not clear whether opioid receptors in the central nervous system are necessary for these effects.

METHODS

This study used a model of repetitive restraint stress in mice that induces periorbital hypersensitivity and priming to the nitric oxide donor sodium nitroprusside (SNP; 0.1 mg/kg). Von Frey filaments were used to measure periorbital mechanical thresholds and grimace scores were evaluated by observing mouse facial features. Animals were treated with the dual enkephalinase inhibitor (DENKI) PL37.

RESULTS

On day two post-stress, PL37 given to mice via either intravenous injection (10 mg/kg) or oral gavage (20 mg/kg) significantly attenuated stress-induced periorbital hypersensitivity and facial grimace responses. Additionally, both intravenous (10 mg/kg) and oral gavage (20 mg/kg) of PL37 prior to SNP (0.1 mg/kg) administration on day 14 post-stress significantly reduced SNP-induced facial hypersensitivity. Injection of the DOR antagonist naltrindole (0.1 mg/kg) but not the mu-opioid receptor antagonist CTAP (1 mg/kg) prior to PL37 treatment blocked the effects. Finally, pretreatment of mice with the peripherally restricted opioid receptor antagonist naloxone methiodide (5 mg/kg) blocked the effects of PL37.

CONCLUSIONS

These data demonstrate that inhibiting enkephalinases, and thus protecting enkephalins from degradation, attenuates stress-induced migraine-like behavior via activation of peripheral DOR. Peripheral targeting of endogenous opioid signaling may be an effective therapeutic strategy for migraine.

Mindfulness-Oriented Recovery Enhancement remediates anhedonia in chronic opioid use by enhancing neurophysiological responses during savoring of natural rewards

BACKGROUND

Neuropsychopharmacologic effects of long-term opioid therapy (LTOT) in the context of chronic pain may result in subjective anhedonia coupled with decreased attention to natural rewards. Yet, there are no known efficacious treatments for anhedonia and reward deficits associated with chronic opioid use. Mindfulness-Oriented Recovery Enhancement (MORE), a novel behavioral intervention combining training in mindfulness with savoring of natural rewards, may hold promise for treating anhedonia in LTOT.

METHODS

Veterans receiving LTOT (N = 63) for chronic pain were randomized to 8 weeks of MORE or a supportive group (SG) psychotherapy control. Before and after the 8-week treatment groups, we assessed the effects of MORE on the late positive potential (LPP) of the electroencephalogram and skin conductance level (SCL) during viewing and up-regulating responses (i.e. savoring) to natural reward cues. We then examined whether these neurophysiological effects were associated with reductions in subjective anhedonia by 4-month follow-up.

RESULTS

Patients treated with MORE demonstrated significantly increased LPP and SCL to natural reward cues and greater decreases in subjective anhedonia relative to those in the SG. The effect of MORE on reducing anhedonia was statistically mediated by increases in LPP response during savoring.

CONCLUSIONS

MORE enhances motivated attention to natural reward cues among chronic pain patients on LTOT, as evidenced by increased electrocortical and sympathetic nervous system responses. Given neurophysiological evidence of clinical target engagement, MORE may be an efficacious treatment for anhedonia among chronic opioid users, people with chronic pain, and those at risk for opioid use disorder.

Effect of Selective Lesions of Nucleus Accumbens µ-Opioid Receptor-Expressing Cells on Heroin Self-Administration in Male and Female Rats: A Study with Novel Oprm1-Cre Knock-in Rats

The brain µ-opioid receptor (MOR) is critical for the analgesic, rewarding, and addictive effects of opioid drugs. However, in rat models of opioid-related behaviors, the circuit mechanisms of MOR-expressing cells are less known because of a lack of genetic tools to selectively manipulate them. We introduce a CRISPR-based Oprm1-Cre knock-in transgenic rat that provides cell type-specific genetic access to MOR-expressing cells. After performing anatomic and behavioral validation experiments, we used the Oprm1-Cre knock-in rats to study the involvement of NAc MOR-expressing cells in heroin self-administration in male and female rats. Using RNAscope, autoradiography, and FISH chain reaction (HCR-FISH), we found no differences in Oprm1 expression in NAc, dorsal striatum, and dorsal hippocampus, or MOR receptor density (except dorsal striatum) or function between Oprm1-Cre knock-in rats and wildtype littermates. HCR-FISH assay showed that iCre is highly coexpressed with Oprm1 (95%-98%). There were no genotype differences in pain responses, morphine analgesia and tolerance, heroin self-administration, and relapse-related behaviors. We used the Cre-dependent vector AAV1-EF1a-Flex-taCasp3-TEVP to lesion NAc MOR-expressing cells. We found that the lesions decreased acquisition of heroin self-administration in male Oprm1-Cre rats and had a stronger inhibitory effect on the effort to self-administer heroin in female Oprm1-Cre rats. The validation of an Oprm1-Cre knock-in rat enables new strategies for understanding the role of MOR-expressing cells in rat models of opioid addiction, pain-related behaviors, and other opioid-mediated functions. Our initial mechanistic study indicates that lesioning NAc MOR-expressing cells had different effects on heroin self-administration in male and female rats.SIGNIFICANCE STATEMENT The brain µ-opioid receptor (MOR) is critical for the analgesic, rewarding, and addictive effects of opioid drugs. However, in rat models of opioid-related behaviors, the circuit mechanisms of MOR-expressing cells are less known because of a lack of genetic tools to selectively manipulate them. We introduce a CRISPR-based Oprm1-Cre knock-in transgenic rat that provides cell type-specific genetic access to brain MOR-expressing cells. After performing anatomical and behavioral validation experiments, we used the Oprm1-Cre knock-in rats to show that lesioning NAc MOR-expressing cells had different effects on heroin self-administration in males and females. The new Oprm1-Cre rats can be used to study the role of brain MOR-expressing cells in animal models of opioid addiction, pain-related behaviors, and other opioid-mediated functions.

Alpha peak activity in resting-state EEG is associated with depressive score

Introduction

Depression is a serious psychiatric disorder characterized by prolonged sadness, loss of interest or pleasure. The dominant alpha peak activity in resting-state EEG is suggested to be an intrinsic neural marker for diagnosis of mental disorders.

Methods

To investigate an association between alpha peak activity and depression severity, the present study recorded resting-state EEG (EGI 128 channels, off-line average reference, source reconstruction by a distributed inverse method with the sLORETA normalization, parcellation of 68 Desikan-Killiany regions) from 155 patients with depression (42 males, mean age 35 years) and acquired patients' scores of Self-Rating Depression Scales. We measured both the alpha peak amplitude that is more related to synchronous neural discharging and the alpha peak frequency that is more associated with brain metabolism.

Results

The results showed that over widely distributed brain regions, individual patients' alpha peak amplitudes were negatively correlated with their depressive scores, and individual patients' alpha peak frequencies were positively correlated with their depressive scores.

Discussion

These results reveal that alpha peak amplitude and frequency are associated with self-rating depressive score in different manners, and the finding suggests the potential of alpha peak activity in resting-state EEG acting as an important neural factor in evaluation of depression severity in supplement to diagnosis.

Endogenous opioid systems alterations in pain and opioid use disorder

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

Substance use and common contributors to morbidity: A genetics perspective

Excessive substance use and substance use disorders (SUDs) are common, serious and relapsing medical conditions. They frequently co-occur with other diseases that are leading contributors to disability worldwide. While heavy substance use may potentiate the course of some of these illnesses, there is accumulating evidence suggesting common genetic architectures. In this narrative review, we focus on four heritable medical conditions - cardiometabolic disease, chronic pain, depression and COVID-19, which are commonly overlapping with, but not necessarily a direct consequence of, SUDs. We find persuasive evidence of underlying genetic liability that predisposes to both SUDs and chronic pain, depression, and COVID-19. For cardiometabolic disease, there is greater support for a potential causal influence of problematic substance use. Our review encourages de-stigmatization of SUDs and the assessment of substance use in clinical settings. We assert that identifying shared pathways of risk has high translational potential, allowing tailoring of treatments for multiple medical conditions.

Funding

SSR acknowledges T29KT0526, T32IR5226 and DP1DA054394; RLK acknowledges AA028292; AA acknowledges DA054869 & K02DA032573. The funders had no role in the conceptualization or writing of the paper.

Stress reveals a specific behavioral phenotype for opioid abuse susceptibility

Susceptibility to stress has long been considered important for the development of substance use disorders. Nonetheless, behavioral and physiological responses to stress are highly variable, making it difficult to identify the individuals who are most likely to abuse drugs. In the present study, we employed a comprehensive battery of tests for negative valence behaviors and nociception to identify individuals predisposed to opioid seeking following oral opioid self-administration. Furthermore, we examined how this profile was affected by a history of stress. We observed that mice receiving foot shock stress failed to exhibit a preference for sucrose, showed increased immobility in the forced swim task, and exhibited mechanical hypersensitivity when compared to controls. When considering these behaviors in light of future fentanyl-seeking responses, we observed that heightened mechanical sensitivity corresponded to higher opioid preference in mice with a history of stress, but not controls. Moreover, we were surprised to discover that paradoxically high sucrose preferences predicted fentanyl preference in shock mice, while signs of anhedonia predicted fentanyl preference in controls. Taken together, these results indicate that stress can act as a physiological modulator, shifting profiles of opioid abuse susceptibility depending on an individual's history.

Risk for opioid misuse in chronic pain patients is associated with endogenous opioid system dysregulation

µ-Opioid receptors (MOR) are a major target of endogenous and exogenous opioids, including opioid pain medications. The µ-opioid neurotransmitter system is heavily implicated in the pathophysiology of chronic pain and opioid use disorder and, as such, central measures of µ-opioid system functioning are increasingly being considered as putative biomarkers for risk to misuse opioids. To explore the relationship between MOR system function and risk for opioid misuse, 28 subjects with chronic nonspecific back pain completed a clinically validated measure of opioid misuse risk, the Pain Medication Questionnaire (PMQ), and were subsequently separated into high (PMQ > 21) and low (PMQ ≤ 21) opioid misuse risk groups. Chronic pain patients along with 15 control participants underwent two separate [¹¹C]-carfentanil positron emission tomography scans to explore MOR functional measures: one at baseline and one during a sustained pain-stress challenge, with the difference between the two providing an indirect measure of stress-induced endogenous opioid release. We found that chronic pain participants at high risk for opioid misuse displayed higher baseline MOR availability within the right amygdala relative to those at low risk. By contrast, patients at low risk for opioid misuse showed less pain-induced activation of MOR-mediated, endogenous opioid neurotransmission in the nucleus accumbens. This study links human in vivo MOR system functional measures to the development of addictive disorders and provides novel evidence that MORs and µ-opioid system responsivity may underlie risk to misuse opioids among chronic pain patients.

Ocular surface complications in diabetes: The interrelationship between insulin and enkephalin

Diabetes is a multi-faceted disorder with increasing prevalence and rising healthcare costs. The burden of diabetes is increased because of associated complications affecting nearly all organs including the eye. The underlying pathophysiology for the onset of these ocular surface disorders is not well known. Enkephalins are endogenous opioids that originate in the brain and have numerous actions in the human body. Opioid growth factor (OGF), chemically termed [Met5]-enkephalin, binds to a novel, nuclear-associated receptor and mediates cellular homeostasis. Serum OGF levels are elevated in diabetic individuals and rodent models of diabetes. Sustained blockade of the OGF receptor (OGFr) with opioid receptor antagonists, such as naltrexone (NTX), reverses many complications of diabetes in the animal model, including delayed cutaneous wound healing, dry eye, altered corneal surface sensitivity, and keratopathy. The increased enkephalin levels observed in diabetes suggest a relationship between endogenous opioid peptides and the pathophysiology of diabetes. It is common for diabetic patients to undergo insulin therapy to restore normal blood glucose levels. However, this restoration does not alter OGF serum levels nor ameliorate ocular surface complications in the animal model of diabetes. Moreover, sex differences in the prevalence of diabetes, response to insulin therapy, and abnormalities in the OGF-OGFr axis have been reported. This review highlights current knowledge on the dysregulation of the OGF-OGFr pathway and possible relationships of insulin and enkephalins to the development of ocular surface defects in diabetes. It proposes that this dysregulation is a fundamental mechanism for the pathobiology of diabetic complications.

From Pharmacology to Physiology: Endocrine Functions of μ-Opioid Receptor Networks

The steady rise in opioid users and abusers has uncovered multiple detrimental health consequences of perturbed opioid receptor signaling, thereby creating the need to better understand the biology of these systems. Among endogenous opioid networks, μ-receptors have received special attention due to their unprecedented biological complexity and broad implications in homeostatic functions. Here, we review the origin, molecular biology, and physiology of endogenous opioids with a special focus on μ-opioid receptor networks within the endocrine system. Moreover, we summarize the current evidence supporting an involvement of the latter in regulating distinct endocrine functions. Finally, we combine these insights to present an integrated perspective on μ-opioid receptor biology and provide an outlook on future studies and unresolved questions in this field.

Endogenous Opioid Peptides and Alternatively Spliced Mu Opioid Receptor Seven Transmembrane Carboxyl-Terminal Variants

There exist three main types of endogenous opioid peptides, enkephalins, dynorphins and β-endorphin, all of which are derived from their precursors. These endogenous opioid peptides act through opioid receptors, including mu opioid receptor (MOR), delta opioid receptor (DOR) and kappa opioid receptor (KOR), and play important roles not only in analgesia, but also many other biological processes such as reward, stress response, feeding and emotion. The MOR gene, OPRM1, undergoes extensive alternative pre-mRNA splicing, generating multiple splice variants or isoforms. One type of these splice variants, the full-length 7 transmembrane (TM) Carboxyl (C)-terminal variants, has the same receptor structures but contains different intracellular C-terminal tails. The pharmacological functions of several endogenous opioid peptides through the mouse, rat and human OPRM1 7TM C-terminal variants have been considerably investigated together with various mu opioid ligands. The current review focuses on the studies of these endogenous opioid peptides and summarizes the results from early pharmacological studies, including receptor binding affinity and G protein activation, and recent studies of β-arrestin2 recruitment and biased signaling, aiming to provide new insights into the mechanisms and functions of endogenous opioid peptides, which are mediated through the OPRM1 7TM C-terminal splice variants.

Naltrexone modulates contextual processing in depression

Context, the information surrounding an experience, can significantly alter the meaning and the affective responses to events. Yet the biological mechanisms through which context modulate experiences are not entirely understood. Here, we hypothesized that the µ-opioid system-extensively implicated in placebo effects, a clinical phenomenon thought to rely on contextual processing-modulates the effects of contextual information on emotional attributions in patients with depression. To test this hypothesis, 20 unmedicated patients with depression completed a randomized, double-blind, placebo-controlled, crossover study of one dose of 50 mg of naltrexone, or placebo immediately before completing two sessions of the Contextual Framing fMRI task. This task captures effects of valenced contextual cues (pleasant vs. unpleasant) on emotional attribution (the rating of subtle emotional faces: fearful, neutral, or happy). Behaviorally, we found that emotional attribution was significantly moderated by the interaction between contextual cues and subtle emotional faces, such that participants' ratings of valenced faces (fearful and happy), compared to neutral, were more negative during unpleasant, compared to pleasant context cues. At a neural level, context-induced blood-oxygen-level-dependent responses in the ventromedial prefrontal cortex, the dorsal anterior cingulate, the dorsolateral prefrontal cortex, and the lateral orbitofrontal cortex, significantly moderated the effects of context on emotional attribution, and were blunted by naltrexone. Furthermore, the effects of naltrexone on emotional attribution were partially abolished in more severely depressed patients. Our results provide insights into the molecular alterations underlying context representation in patients with depression, providing pivotal early data for future treatment studies.

Editorial: 50 Years of Opioid Research and the International Narcotics Research Conference

In the past 50 years, scientists have made considerable strides toward understanding how opioids act. This special issue of Molecular Pharmacology celebrates these 50 years of opioid research and the role that the International Narcotics Research Conference has played in driving this research, by bringing together review and original research articles that present historical highlights, the current state of the art, and perspectives on the future of opioid research. SIGNIFICANCE STATEMENT: Opioids have been used for thousands of years to manage pain and cause euphoria, but their use has been highly limited due to serious side effects. Deciphering the mechanisms of how opioids mediate beneficial and adverse physiological outcomes is essential for developing better treatments for pain and for opioid addiction.