Current understanding of the neurobiology of opioid use disorder: An overview

Neuroimaging of opioid effects in humans across conditions of acute administration, chronic pain therapy, and opioid use disorder

Evidence of central nervous system (CNS) exogenous opioid effects in humans has been primarily gained through neuroimaging of three participant populations: individuals after acute opioid administration, those with opioid use disorder (OUD), and those with chronic pain receiving opioid therapy. In both the brain and spinal cord, opioids alter processes of pain, cognition, and reward. Opioid-related CNS effects may persist and accumulate with longer opioid use duration. Meanwhile, opioid-induced benefits versus risks to brain health remain unclear. This review article highlights recent accumulating evidence for how exogenous opioids impact the CNS in humans. While investigation of CNS opioid effects has remained largely disparate across contexts of opioid acute administration, OUD, and chronic pain opioid therapy, integration across these contexts may enable advancement toward effective interventions.

Sex differences in oxycodone-taking behaviors are linked to disruptions in reward-guided, decision-making functions

Problematic opioid use that emerges in a subset of individuals may be due to pre-existing disruptions in the biobehavioral mechanisms that regulate drug use. The identity of these mechanisms is not known, but emerging evidence suggests that suboptimal decision-making that is observable prior to drug use may contribute to the pathology of addiction and, notably, serve as a powerful phenotype for interrogating biologically based differences in opiate-taking behaviors. The current study investigated the relationship between decision-making phenotypes and opioid-taking behaviors in male and female Long Evans rats. Adaptive decision-making processes were assessed using a probabilistic reversal-learning task and oxycodone- (or vehicle, as a control) taking behaviors assessed for 32 days using a saccharin fading procedure that promoted dynamic intake of oxycodone. Tests of motivation, extinction, and reinstatement were also performed. Computational analyses of decision-making and opioid-taking behaviors revealed that attenuated reward-guided decision-making was associated with greater self-administration of oxycodone and addiction-relevant behaviors. Moreover, pre-existing impairments in reward-guided decision-making observed in female rats was associated with greater oxycodone use and addiction-relevant behaviors when compared to males. These results provide new insights into the biobehavioral mechanisms that regulate opiate-taking behaviors and offer a novel phenotypic approach for interrogating sex differences in addiction susceptibility and opioid use disorders.

Neurocognitive Dysfunctions in People with Concurrent Cannabis Use and Opioid Dependence: A Cross-Sectional, Controlled Study

Cannabis and opioid co-dependence is independently associated with cognitive impairments. We examined neurocognitive dysfunctions in people with concurrent opioid dependence with cannabis dependence (OD+CD) or cannabis use (OD+CU) compared to those with only opioid dependence (OD) and healthy controls (HC). We selected adult participants, any sex, who met the diagnosis of OD (N = 268), OD+CU (N = 58), and OD + CD (N = 115). We recruited 68 education-matched HC. We administeredStandard progressive matrices (SPM), Wisconsin card sorting test (WCST), Iowa gambling task (IGT), Trail making tests A and B (TMT), and verbal and visual working memory 1-, 2-backtests. 496 (97.5%) were men, and 13 (2.5%) were women. In WCST, OD and OD+CD had significantly higher non-perseverative errors than HC. OD+CD group completed significantly lesser categories than HC. In verbal working memory 2-back, HC scored significantly fewer errors than OD and OD +CD. All patient groups, OD, OD+CU, and OD+CD, scored higher commission errors than HC in visual working memory 1-back. OD and OD+CD scored higher commission and total errors than the controls. OD+CU showed lesser error score than HC in TMT B. Cannabis and opioid co-dependence contribute to cognitive impairments, especially in working memory and executive functions.

[Functional impairments in large-scale brain projects in opioid addiction]

OBJECTIVE

To assess the functional state of large-scale resting networks of the brain in patients with opioid intoxication.

MATERIAL AND METHODS

Thirty-one male subjects, aged 27.4+5.1 years, were studied. Resting state functional MRI was performed in 12 patients with heroin intoxication aged 29.1+5.9 years. The control group consisted of 16 healthy volunteers without bad habits aged 26.2+4.2 years.

RESULTS

In the group of opioids intoxication, there is a decrease in the functional activity of the salience network with the executive control network and the default mode network of the brain (p<0.05) compared to the control group. A positive correlation of functional connections is found between the anterior cingulate cortex and the medial prefrontal cortex (T=2.74; p=0.041), which is not recorded in the control group. The functional connections between the default mode network and executive control are more highly represented in opioid intoxication in comparison with the control group (medial prefrontal cortex - left posterior parietal cortex T=7.5; p=0.001; medial prefrontal cortex - right posterior parietal cortex T=3.71; p=0.014; posterior cingulate cortex - left posterior parietal cortex T=6.15; p=0.002; posterior cingulate cortex - right posterior parietal cortex T=3.25; p=0.023; posterior cingulate cortex - right dorsolateral prefrontal cortex T=2.83; p=0.037).

CONCLUSION

The results indicate that functional connections in large-scale resting networks are disrupted during opioid intoxication, which indicates a disturbance of the normal functional architectonics of the brain.

Comorbidities in older adults with opioid use disorders

BACKGROUND

There is a growing number of opioid use disorders (OUDs) and overdose deaths in older adults. In addition, older adults with OUD routinely receive lower-quality preventive and chronic care that result in poorer overall health.

PURPOSE

The purpose of this study was to identify older patients with OUD at the study site and determine the prevalence of medical and psychiatric comorbidities.

METHODOLOGY

This retrospective study used a computer algorithm employed by a network of Federally Qualified Health Center primary care clinics in lower New York state. With the approval from institutional review board, the IT department of the organization captured data about older adults with an OUD diagnosis. The IT department identified 664 patients and provided anonymized demographic and medical history data of these patients for analysis.

RESULTS

Patients with OUD had 2-3 times more medical and psychiatric co-occurring conditions than national sample of older adults with no OUD of similar age and income. The most frequent co-occurring medical conditions in the patients were heart or circulatory disorders, movement disorders, respiratory disorders, pain disorders, nutritional disorders, and metabolic disorders. The most common co-occurring psychiatric conditions were anxiety disorders, major depressive disorder, post-traumatic stress disorder (PTSD), and bipolar disorder.

CONCLUSIONS

Older adults with OUD often have complex health care needs that often include one or more chronic medical and psychiatric conditions.

IMPLICATIONS

Providers should be aware of the multiple needs of older adults with OUD so that appropriate and comprehensive care can be offered to the patients.

Cross-ancestry meta-analysis of opioid use disorder uncovers novel loci with predominant effects in brain regions associated with addiction

Despite an estimated heritability of ~50%, genome-wide association studies of opioid use disorder (OUD) have revealed few genome-wide significant loci. We conducted a cross-ancestry meta-analysis of OUD in the Million Veteran Program (N = 425,944). In addition to known exonic variants in OPRM1 and FURIN, we identified intronic variants in RABEPK, FBXW4, NCAM1 and KCNN1. A meta-analysis including other datasets identified a locus in TSNARE1. In total, we identified 14 loci for OUD, 12 of which are novel. Significant genetic correlations were identified for 127 traits, including psychiatric disorders and other substance use-related traits. The only significantly enriched cell-type group was CNS, with gene expression enrichment in brain regions previously associated with substance use disorders. These findings increase our understanding of the biological basis of OUD and provide further evidence that it is a brain disease, which may help to reduce stigma and inform efforts to address the opioid epidemic.

Reward Responsiveness in Patients with Opioid Use Disorder on Opioid Agonist Treatment: Role of Comorbid Chronic Pain

OBJECTIVE

Evidence suggests that blunted reward responsiveness may account for poor clinical outcomes in both opioid use disorder (OUD) and chronic pain. Understanding how individuals with OUD and comorbid chronic pain (OUD+CP) respond to rewards is, therefore, of clinical interest because it may reveal a potential point of behavioral intervention.

METHODS

Patients with OUD (n = 28) and OUD+CP (n = 19) on opioid agonist treatment were compared on: 1) the Probabilistic Reward Task (an objective behavioral measure of reward response bias) and 2) ecological momentary assessment of affective responses to pleasurable events.

RESULTS

Both the OUD and the OUD+CP groups evidenced an increase in reward response bias in the Probabilistic Reward Task. The rate of change in response bias across blocks was statistically significant in the OUD group (B = 0.06, standard error [SE] = 0.02, t = 3.92, P < 0.001, 95% confidence interval [CI]: 0.03 to 0.09) but not in the OUD+CP group (B = 0.03, SE = 0.02, t = 1.90, P = 0.07, 95% CI: -0.002 to 0.07). However, groups did not significantly differ in the rate of change in response bias across blocks (B = 0.03, SE = 0.02, t = 1.21, P = 0.23, 95% CI: -0.02 to 0.07). Groups did not significantly differ on state measures of reward responsiveness (P's ≥0.50).

CONCLUSIONS

Overall, findings across objective and subjective measures were mixed, necessitating follow-up with a larger sample. The results suggest that although there is a reward response bias in patients with OUD+CP treated with opioid agonist treatment relative to patients with OUD without CP, it is modest and does not appear to translate into patients' responses to rewarding events as they unfold in daily life.

The Role of Mu-Opioids for Reward and Threat Processing in Humans: Bridging the Gap from Preclinical to Clinical Opioid Drug Studies

PURPOSE OF REVIEW

Opioid receptors are widely expressed in the human brain. A number of features commonly associated with drug use disorder, such as difficulties in emotional learning, emotion regulation and anhedonia, have been linked to endogenous opioid signalling. Whereas chronic substance use and misuse are thought to alter the function of the mu-opioid system, the specific mechanisms are not well understood. We argue that understanding exogenous and endogenous opioid effects in the healthy human brain is an essential foundation for bridging preclinical and clinical findings related to opioid misuse. Here, we will examine psychopharmacological evidence to outline the role of the mu-opioid receptor (MOR) system in the processing of threat and reward, and discuss how disruption of these processes by chronic opioid use might alter emotional learning and reward responsiveness.

RECENT FINDINGS

In healthy people, studies using opioid antagonist drugs indicate that the brain's endogenous opioids downregulate fear reactivity and upregulate learning from safety. At the same time, endogenous opioids increase the liking of and motivation to engage with high reward value cues. Studies of acute opioid agonist effects indicate that with non-sedative doses, drugs such as morphine and buprenorphine can mimic endogenous opioid effects on liking and wanting. Disruption of endogenous opioid signalling due to prolonged opioid exposure is associated with some degree of anhedonia to non-drug rewards; however, new results leave open the possibility that this is not directly opioid-mediated.

SUMMARY

The available human psychopharmacological evidence indicates that the healthy mu-opioid system contributes to the regulation of reward and threat processing. Overall, endogenous opioids can subtly increase liking and wanting responses to a wide variety of rewards, from sweet tastes to feelings of being connected to close others. For threat-related processing, human evidence suggests that endogenous opioids inhibit fear conditioning and reduce the sensitivity to aversive stimuli, although inconsistencies remain. The size of effects reported in healthy humans are however modest, clearly indicating that MORs play out their role in close concert with other neurotransmitter systems. Relevant candidate systems for future research include dopamine, serotonin and endocannabinoid signalling. Nevertheless, it is possible that endogenous opioid fine-tuning of reward and threat processing, when unbalanced by e.g. opioid misuse, could over time develop into symptoms associated with opioid use disorder, such as anhedonia and depression/anxiety.

Decision making of individuals with heroin addiction receiving opioid maintenance treatment compared to early abstinent users

BACKGROUND AND AIMS

Individuals with heroin addiction are prone to dysfunctional decision-making. They frequently choose the short-term rewarding option of drug intake despite experiencing long-term negative consequences. Opioid maintenance treatment (OMT) is the most common treatment of heroin addiction.

METHODS

In this study, 38 individuals in an early stage of abstinence from heroin addiction (ESA-HA individuals) at the end of inpatient detoxification treatment and 41 individuals in long-term OMT were examined. Decision-making was assessed by (I) a modified version of the Iowa Gambling Task (IGT) with drug-related stimuli focusing on decision-making under ambiguity and (II) the Game of Dice Task (GDT) assessing decision-making under objective risk.

RESULTS

OMT-individuals showed significantly better performance in the IGT than the ESA-HA-individuals. They also showed significantly less craving under exposure of drug-related pictures. In the GDT, OMT-individuals showed significantly less risky decision-making than ESA-HA-individuals.

CONCLUSION

The results suggest that patients receiving OMT show better functional decision-making and lower craving reactions. It could be assumed that the effectiveness of OMT in preventing relapse is linked to better decision-making and lower craving among these patients.