Opioid-induced hyperalgesia and tolerance: understanding opioid side effects

Exploring the Relationship Between Endogenous Pain Modulation, Pain Intensity, and Depression in Patients Using Opioids for Chronic Low Back Pain

INTRODUCTION

Endogenous pain modulatory processes appear to play an important role in shaping pain-related outcomes, but we know relatively little about the influence of psychosocial factors on those pain modulatory processes. The primary objective of this study was to explore associations between endogenous pain modulation (ie, conditioned pain modulation, CPM; temporal summation, TS), chronic pain, and negative affective factors (ie, depression, anxiety symptoms) in a sample of participants with chronic low back pain (CLBP) treated with long-term daily opioids.

METHODS

Adults with opioid-treated CLBP (N=107) completed questionnaires assessing pain, pain symptoms, and psychological measures. CPM and TS were evaluated as predictors of pain intensity ratings (Brief Pain Inventory), with depression scores (Hospital Anxiety and Depression Scale, depression subscale) examined as potential moderators of those associations.

RESULTS

Moderation analyses demonstrated associations between CPM and back pain intensity ratings, moderated by depression symptom scores (B=-0.002, SE=0.0008, P<0.01) when controlling for daily opioid dose, with participants with higher depression scores showing a relatively stronger link between lower CPM and increased pain intensity ratings. Significant associations were observed between depression, pain intensity, and CPM-derived outcomes.

CONCLUSION

Our findings suggest that reduced pain-inhibitory capacity is associated with elevated self-reported pain intensity in adults with opioid-treated CLBP, particularly among those with higher severity of depression symptoms.

Opioid-Induced Hyperalgesia in a Cancer Patient on High-Dose Methadone Maintenance Therapy: A Case for Subspecialty Opioid Use Disorder Primary Care

As opioid use disorder (OUD) reaches epidemic levels in the United States, medication-assisted treatment (MAT) plays a central role in its treatment. Methadone, a long-acting mu-opioid receptor agonist has been shown to be effective in managing OUD. It is also known that chronic opioid therapy may have the paradoxical effect of increased sensitivity to pain, a phenomenon called opioid-induced hyperalgesia (OIH). This presents a conundrum when a patient such as ours, on MAT presents with acute pain and OIH. This case report illustrates the current challenges health care providers encounter when treating patients on chronic MAT for non-opioid-related conditions. As this patient population ages, these encounters will become more common. These patients will need appropriate health care screening and chronic care management. This case serves two purposes; to highlight the difficulty in treating acute pain in patients on long-term high-dose methadone coupled with the missed opportunity for primary care for OUD patient population, and proposes that education reforms in this area be implemented now. 

Impaired frontostriatal functional connectivity among chronic opioid using pain patients is associated with dysregulated affect

Preclinical studies have shown effects of chronic exposure to addictive drugs on glutamatergic-mediated neuroplasticity in frontostriatal circuitry. These initial findings have been paralleled by human functional magnetic resonance imaging (fMRI) research demonstrating weaker frontostriatal resting-state functional connectivity (rsFC) among individuals with psychostimulant use disorders. However, there is a dearth of human imaging literature describing associations between long-term prescription opioid use, frontostriatal rsFC, and brain morphology among chronic pain patients. We hypothesized that prescription opioid users with chronic pain, as compared with healthy control subjects, would evidence weaker frontostriatal rsFC coupled with less frontostriatal gray matter volume (GMV). Further, those opioid use-related deficits in frontostriatal circuitry would be associated with negative affect and drug misuse. Prescription opioid users with chronic pain (n = 31) and drug-free healthy controls (n = 30) underwent a high-resolution anatomical and an eyes-closed resting-state functional scan. The opioid group, relative to controls, exhibited weaker frontostriatal rsFC, and less frontostriatal GMV in both L.NAc and L.vmPFC. Frontostriatal rsFC partially mediated group differences in negative affect. Within opioid users, L.NAc GMV predicted opioid misuse severity. The current study revealed that prescription opioid use in the context of chronic pain is associated with functional and structural abnormalities in frontostriatal circuitry. These results suggest that opioid use-related abnormalities in frontostriatal circuitry may undergird disturbances in affect that may contribute to the ongoing maintenance of opioid use and misuse. These findings warrant further examination of interventions to treat opioid pathophysiology in frontostriatal circuitry over the course of treatment.

Endomorphin-2 Inhibition of Substance P Signaling within Lamina I of the Spinal Cord Is Impaired in Diabetic Neuropathic Pain Rats

Opiate analgesia in the spinal cord is impaired in diabetic neuropathic pain (DNP), but until now the reason is unknown. We hypothesized that it resulted from a decreased inhibition of substance P (SP) signaling within the dorsal horn of the spinal cord. To investigate this possibility, we evaluated the effects of endomorphin-2 (EM2), an endogenous ligand of the μ-opioid receptor (MOR), on SP release within lamina I of the spinal dorsal horn (SDH) in rats with DNP. We established the DNP rat model and compared the analgesic efficacy of EM2 between inflammation pain and DNP rat models. Behavioral results suggested that the analgesic efficacy of EM2 was compromised in the condition of painful diabetic neuropathy. Then, we measured presynaptic SP release induced by different stimulating modalities via neurokinin-1 receptor (NK1R) internalization. Although there was no significant change in basal and evoked SP release between control and DNP rats, EM2 failed to inhibit SP release by noxious mechanical and thermal stimuli in DNP but not in control and inflammation pain model. We also observed that EM2 decreased the number of FOS-positive neurons within lamina I of the SDH but did not change the amount of FOS/NK1R double-labeled neurons. Finally, we identified a remarkable decrease in MORs within the primary afferent fibers and dorsal root ganglion (DRG) neurons by Western blot (WB) and immunohistochemistry (IHC). Taken together, these data suggest that reduced presynaptic MOR expression might account for the loss of the inhibitory effect of EM2 on SP signaling, which might be one of the neurobiological foundations for decreased opioid efficacy in the treatment of DNP.