A Brief Overview of the Neuropharmacology of Opioid Addiction.

The world is in the midst of an opioid crisis. Nearly 92,000 persons in the U.S. alone died from illicit drugs and prescription opioids in 2020 [1]. This number does not include the countless other individuals who die as a result of the violent crime that accompanies the illicit drug trade. To address this crisis, we need to appreciate aspects of drug addiction. The goal of this brief review is to highlight some major facets of addiction neurobiology, focused on opioids, to provide a basic understanding of the research and terminology encountered in more detailed in-depth articles and discussions on addiction.

Novel variants of engineered water soluble mu opioid receptors with extensive mutations and removal of cysteines.

We have shown that water-soluble variants of the human mu opioid receptor (wsMOR) containing a reduced number of hydrophobic residues at the lipid-facing residues of the transmembrane (TM) helices can be expressed in E. coli. In this study, we tested the consequences of increasing the number of mutations on the surface of the transmembrane domain on the receptor's aqueous solubility and ligand binding properties, along with mutation of 11 cysteine residues regardless of their solvent exposure value and location in the protein. We computationally engineered 10 different variants of MOR, and tested four of them for expression in E. coli. We found that all four variants were successfully expressed and could be purified in high quantities. The variants have alpha helical structural content similar to that of the native MOR, and they also display binding affinities for the MOR antagonist (naltrexone) similar to the wsMOR variants we engineered previously that contained many fewer mutations. Furthermore, for these full-length variants, the helical content remains unchanged over a wide range of pH values (pH 6 ~ 9). This study demonstrates the flexibility and robustness of the water-soluble MOR variants with respect to additional designed mutations in the TM domain and changes in pH, whereupon the protein's structural integrity and its ligand binding affinity are maintained. These variants of the full-length MOR with less hydrophobic surface residues and less cysteines can be obtained in large amounts from expression in E. coli and can serve as novel tools to investigate structure-function relationships of the receptor.

Autophagy Inhibition Preserves Tight Junction of Human Cerebral Microvascular Endothelium Under Oxygen Glucose Deprivation.

AIMS: To investigate the role of autophagy in the tight junction of human brain endothelial cells during hypoxia and ischemia. BACKGROUND: Endothelial cells play an important role in the initiation, progression and recovery from ischemic stroke. The role of autophagy on human brain endothelial cells (HBECs) subjected to oxygen-glucose deprivation (OGD) is not fully elucidated. OBJECTIVE: The objective of this study was to investigate the effect of autophagy on HBECs during OGD. METHODS: HBECs were cultured in a 96-well plate and underwent 4 hours of OGD. For drug treatment, 3-Methyladenine (3-MA) (5mmol/L), an inhibitor of autophagy, was added at the start of OGD. Cell viability and cytotoxicity were tested by cell counting kit-8 (CCK-8) and lactate dehydrogenase (LDH) assays. Morphological changes in cells were examined by immunofluorescence microscopy. The protein expression of light chain 3 (LC3) was measured. Autophagosomes and endothelial cell tight junctions were observed using transmission electron microscopy. RESULTS: The results showed that OGD induced serious damage to HBECs. Cell viability was decreased significantly and LDH release increased significantly (p<0.05) following OGD. 3-MA protected HBECs from damage. Immunostaining further confirmed these results. Since 3-MA is an inhibitor of autophagy, we chose to examine alterations in the amount of LC3, a marker of autophagy. The ratios of LC3-II to LC3-I were significantly lower in the 3-MA treated OGD group than in the non-3-MA treated OGD group (p<0.05). Electron microscopy showed that 3-MA inhibited the formation of autophagolysosomes and revealed that the tight junction ultrastructure of HBECs, which was destroyed by OGD, was significantly protected by treatment with 3-MA. CONCLUSION: Autophagy is a key response to oxygen-glucose deprivation stress and its detrimental effects are closely related to the destruction of tight junctions of human brain endothelial cells. Strategies to inhibit autophagy could help to preserve tight junctions.

Molecular Interaction Between Butorphanol and κ-Opioid Receptor.

BACKGROUND: The misuse of opioids stems, in part, from inadequate knowledge of molecular interactions between opioids and opioid receptors. It is still unclear why some opioids are far more addictive than others. The κ-opioid receptor (KOR) plays a critical role in modulating pain, addiction, and many other physiological and pathological processes. Butorphanol, an opioid analgesic, is a less addictive opioid with unique pharmacological profiles. In this study, we investigated the interaction between butorphanol and KOR to obtain insights into the safe usage of this medication. METHODS: We determined the binding affinity of butorphanol to KOR with a naltrexone competition study. Recombinant KORs expressed in mammalian cell membranes (Chem-1) were used for G-protein activation studies, and a human embryonic kidney-293 (HEK-293) cell line stably transfected with the human KOR was used for ß-arrestin study as previously described in the literature. The effects of butorphanol on KOR internalization were investigated using mouse neuroblastoma Neuro2A cells stably transfected with mKOR-tdTomato fusion protein (N2A-mKOR-tdT) cells overexpressing KOR. The active-state KOR crystal structure was used for docking calculation of butorphanol to characterize the ligand binding site. Salvinorin A, a full KOR agonist, was used as a control for comparison. RESULTS: The affinity of KOR for butorphanol is characterized by Kd of 0.1 ± 0.02 nM, about 20-fold higher compared with that of the µ-opioid receptor (MOR; 2.4 ± 1.2 nM). Our data indicate that butorphanol is more potent on KOR than on MOR. In addition, butorphanol acts as a partial agonist of KOR in the G-protein activation pathway and is a full agonist on the ß-arrestin recruitment pathway, similar to that of salvinorin A. The activation of the ß-arrestin pathway is further confirmed by KOR internalization. The in silico docking model indicates that both salvinorin A and butorphanol share the same binding cavity with the KOR full agonist MP1104. This cavity plays an important role in determining either agonist or antagonist effects of the ligand. CONCLUSIONS: In conclusion, butorphanol is a partial KOR agonist in the G-protein activation pathway and a potent KOR full agonist in the ß-arrestin recruitment pathway. The structure analysis offers insights into the molecular mechanism of KOR interaction and activation by butorphanol.

Plasma Exchange Therapy in Patients with Complex Regional Pain Syndrome.

BACKGROUND: Complex regional pain syndrome (CRPS) is a severe chronic pain condition that most often develops following trauma. Some investigators have postulated CRPS to be a post-traumatic neuralgia associated with distal degeneration of small-diameter peripheral axons. Intravenous immunoglobulin treatment (IVIG) has been shown to be efficacious in the treatment of painful polyneuropathies. Some CRPS patients have been reported to respond to IVIG. Based on a recent hypothesis proposing an autoimmune etiology for CRPS, we decided to offer plasma exchange therapy (PE) to CRPS patients with a clinical presentation suggestive of a small fiber neuropathy. OBJECTIVES: To evaluate the efficacy of PE in a group of CRPS patients with a clinical presentation suggestive of a small fiber neuropathy that were either non-responders or poor responders to their current treatment. STUDY DESIGN: This is a retrospective case series study of CRPS patients that met the Budapest diagnostic criteria for CRPS and received PE as treatment for their illness between September 2012 and June 2014. Approval for this review was granted by the Drexel University Institutional Review Board. SETTING: Drexel University College of Medicine pain clinic METHODS: Thirty-three CRPS patients that received PE treatment were retrospectively studied. The workup for these patients consisted of a complete medical and pain evaluation, the completion of the short-form McGill questionnaire, quantitative sensory testing (QST), and skin punch biopsy. The PE protocol was as follows: all patients had a series of PE therapies (range 5 to 11 with a mean of 7.2) performed over a 2 to 3 week period. Following the PE series, the patients had a pain evaluation and completed the short-form McGill questionnaire. Patients that responded to PE were offered maintenance therapy consisting of either weekly PE or other immune modulating agents. In these patients, their pain was evaluated during the maintenance phase. RESULTS: Thirty of the 33 patients demonstrated significant (P < 0.01) median pain reduction of 64% following the initial series of PE. Three patients demonstrated no improvement. Twenty-four patients are receiving maintenance therapy, the pain reduction in these patients following the initial PE series has been maintained with either weekly PE (n = 15), oral immune modulating agents (n = 8), or IVIG (n = 1). The remaining 6 patients are not receiving maintenance therapy and their pain has returned to pre-treatment levels. In addition, this study suggests that patients with the greatest loss of small fibers and the greatest temperature sensory deficits are most likey to benefit from PE therapy. LIMITATIONS: The major limitation of this study is its retrospective nature which includes non-randomization, non-blinding, and an uncontrolled design. CONCLUSIONS: This study shows that PE is effective in a subset of patients with severe long-standing CRPS and that the reduction in pain following the initial series of PE treatments can be maintained on a weekly PE schedule, IVIG, or with other immune modulating drugs. Large, randomized, placebo controlled studies may be required to confirm and expand these results. Such studies may lead to new therapies for this severe life-altering condition.

Thermal pain in complex regional pain syndrome type I.

BACKGROUND: Quantitative sensory testing (QST), with thermal threshold determinations, is a routine part of the comprehensive clinical workup of patients suffering from chronic pain, especially those with Complex Regional Pain Syndrome seen at our outpatient pain clinic. This is done to quantitatively assess each patient's small fiber and sensory function in a controlled manner. Most patients have normal sensory detection thresholds, but there are large differences in thermal pain thresholds. Some patients display no thermal hyperalgesia, while other patients display severe thermal hyperalgesia when tested in all 4 limbs. OBJECTIVES: To ascertain the prevalence of thermal hyperalgesia in patients with complex regional pain syndrome type 1 (CRPS-I). STUDY DESIGN: This was a retrospective review of the results of QST performed on 105 patients as part of their clinical workup. SETTING: The outpatient clinic of the Department of Neurology at Drexel University College of Medicine. METHODS: All patients had a diagnosis of CRPS-I. Thermal quantitative sensory testing, including cool detection, warm detection, cold pain, and heat pain, was performed on 8 distal sites on each patient as part of a comprehensive clinical examination. RESULTS: With regards to thermal hyperalgesia, patients with CPRS-I appear to fall into distinct groups. One subgroup displays evidence of generalized cold and heat hyperalgesia, one subgroup displays evidence of generalized cold hyperalgesia only, one displays evidence of heat hyperalgesia only, and one subgroup does not display evidence of cold or heat hyperalgesia. LIMITATIONS: This study is based on retrospective information on a relatively small (105 patient records) number of patients. Since only patients with CRPS-I were included, the results are only applicable to this group. CONCLUSIONS: Thermal QST provides useful information about the sensory phenotype of individual patients. Subgrouping based on thermal hyperalgesia may be useful for future studies regarding prognosis, treatment selection, and efficacy.

Plasma amino acids changes in complex regional pain syndrome.

Complex regional pain syndrome (CRPS) is a severe chronic pain condition that most often develops following trauma. Blood samples were collected from 220 individuals, 160 CRPS subjects, and 60 healthy pain-free controls. Plasma amino acid levels were compared and contrasted between groups. L-Aspartate, L-glutamate, and L-ornithine were significantly increased, whereas L-tryptophan and L-arginine were significantly decreased in CRPS subjects as compared to controls. In addition, the L-kynurenine to L-tryptophan ratio demonstrated a significant increase, whereas the global arginine bioavailability ratio (GABR) was significantly decreased in the CRPS subjects. The CRPS subjects demonstrated a significant correlation between overall pain and the plasma levels of L-glutamate and the L-kynurenine to L-tryptophan ratio. CRPS subjects also showed a correlation between the decrease in plasma L-tryptophan and disease duration. This study shows that CRPS subjects exhibit significant changes in plasma levels of amino acids involved in glutamate receptor activation and in amino acids associated with immune function as compared to healthy pain-free controls. A better understanding of the role plasma amino acids play in the pathophysiology of CRPS may lead to novel treatments for this crippling condition.

Skin biopsy in complex regional pain syndrome: case series and literature review.

BACKGROUND: Accumulating experimental and clinical evidence supports the hypothesis that complex regional pain syndrome type I (CRPS-I) may be a small fiber neuropathy. OBJECTIVES: To evaluate the use of commercially available standard biopsy methods to detect intradermal axon pathology in CRPS-I, and to ascertain if these structural changes can explain quantitative sensory testing (QST) findings in CRPS-I. STUDY DESIGN: Retrospective review of charts and laboratory data. SETTING: Outpatient clinic METHODS: Skin biopsies from 43 patients with CRPS-I were stained with PGP 9.5, and epidermal nerve fiber density, sweat gland nerve fiber density and morphological abnormalities were evaluated. Thirty-five patients had quantitative sensory testing. RESULTS: Alterations in skin innervation were seen in approximately 20% of CRPS-I patients with commercial processing. There were no patient characteristics, including duration of disease, that predicted a decreased epidermal nerve fiber density (ENFD). There was no consistent relationship between QST changes and ENFD measured by standard commercial skin biopsy evaluation procedures. LIMITATIONS: Commercial processing of tissue does not utilize stereologic quantitative analysis of nerve fiber density. Biopsy material is utilized from a proximal and distal source only, and differences in denervation of a partial nerve territory may be missed. The functional attributes of small fibers cannot be assessed. CONCLUSIONS: The negative results indicate that CRPS-I may be associated with changes in the ultramicroscopic small fiber structure that cannot be visualized with commercially available techniques. Alternatively, functional rather than structural alterations of small fibers or pathological changes at a more proximal site such as the spinal cord or brain may be responsible for the syndrome.

Changes in plasma cytokines and their soluble receptors in complex regional pain syndrome.

UNLABELLED: Complex Regional Pain Syndrome (CRPS) is a chronic and often disabling pain disorder. There is evidence demonstrating that neurogenic inflammation and activation of the immune system play a significant role in the pathophysiology of CRPS. This study evaluated the plasma levels of cytokines, chemokines, and their soluble receptors in 148 subjects afflicted with CRPS and in 60 gender- and age-matched healthy controls. Significant changes in plasma cytokines, chemokines, and their soluble receptors were found in subjects with CRPS as compared with healthy controls. For most analytes, these changes resulted from a distinct subset of the CRPS subjects. When the plasma data from the CRPS subjects was subjected to cluster analysis, it revealed 2 clusters within the CRPS population. The category identified as most important for cluster separation by the clustering algorithm was TNFα. Cluster 1 consisted of 64% of CRPS subjects and demonstrated analyte values similar to the healthy control individuals. Cluster 2 consisted of 36% of the CRPS subjects and demonstrated significantly elevated levels of most analytes and in addition, it showed that the increased plasma analyte levels in this cluster were correlated with disease duration and severity. PERSPECTIVE: The identification of biomarkers that define disease subgroups can be of great value in the design of specific therapies and of great benefit to the design of clinical trials. It may also aid in advancing our understanding of the mechanisms involved in the pathophysiology of CRPS, which may lead to novel treatments for this very severe condition.

Laser Doppler imaging: usefulness in chronic pain medicine.

Sympathetic nervous system dysfunction is thought to be a factor in neuropathic pain conditions such as Complex Regional Pain Syndrome and in vascular conditions such as Raynaud's phenomenon. Laser Doppler fluxmetry has been used as a fast non-invasive method to quantify changes in skin capillary blood flow which reflect activation of sympathetically mediated vasoconstriction of the arterioles that supply the capillaries. Studies of dynamic change of skin capillary blood flow with sympathetic activation such as cold or inspiratory gasp have generally used single point laser Doppler systems where the probe is in contact with the skin. The results are a single line tracing representing the capillary flow at a single point on the skin a few millimeters in diameter. Laser Doppler imaging (moorLDI laser Doppler imager, Moor Instruments Ltd.) allows for non-contact recording of skin blood flow of an area as large as 50 centimeters square with a resolution of 256 by 256 pixels and 4 milliseconds per pixel. Most work with laser Doppler imaging has studied changes that occur between successive scans. We have found it useful to look at changes that occur during a scan. In this way we obtain data that is comparable to the time resolution of single point laser Doppler methods, but with the larger spatial information that is available with laser Doppler imaging. We present a small series of case reports in which inspiratory gasp during laser Doppler imaging was able to provide quick, useful and unequivocal clinical information regarding the status of regional bilateral skin capillary response to sympathetic activation. This may be useful for distinguishing sympathetically mediated from sympathetically independent pain. We believe the methods described may provide the basis for future quantitative studies similar to those that use single point laser Doppler methods.

The use of ketamine in complex regional pain syndrome: possible mechanisms.

Complex regional pain syndrome is a neuropathic pain syndrome that is characterized by: severe pain beyond the area of injury; autonomic dysregulation; neurogenic edema; movement disorder; and atrophy and dystrophy. Ketamine is an open-channel NMDA blocker that only acts on those receptors whose Mg(2+) block has been lifted. It is effective in the treatment of the syndrome when standard treatments have failed. Different protocols are utilized depending on the severity of illness. There have been no serious ketamine-induced complications from these protocols, owing to careful psychological screening and the liberal use of midazolam and lorazepam to counter any psychomimetic effects and clonidine to block possible 'Olney's' lesions.

Syncope in complex regional pain syndrome.

BACKGROUND: Complex regional pain syndrome (CRPS) is a debilitating pain syndrome characterized clinically by severe pain along with signs and symptoms of autonomic dysfunction. Presyncope and syncope are common in these patients. The purpose of this study was to investigate the cause of these symptoms in these patients. HYPOTHESIS: Patients with CRPS are more prone to presyncope and syncope as evidenced by head-up tilt table testing (HUTT). METHODS: Patients with CRPS underwent a complete cardiac evaluation that included a 12-lead electrocardiogram, 2-dimensional echocardiography, 24-hour Holter monitoring, and HUTT. RESULTS: Seventy-four patients with CRPS were evaluated. Eight CRPS patients (10%) could not complete a HUTT due to pain. Of the 66 CRPS patients who completed a HUTT, 15 (37.9%) reported pretest symptoms of presyncope or syncope. Twenty-eight CRPS patients (42.4%) had a positive HUTT. CRPS patients with lower limb involvement were more likely to have vasovagal syncope or orthostasis on HUTT than those with upper extremity or total body involvement (12 of 18 [67%] vs 16 of 48 [33%]; P = 0.015). CONCLUSIONS: Syncope is common in patients with CRPS, especially with lower limb involvement. Autonomic dysregulation of the lower extremities leads to impaired sympathetic vasoconstriction and venous pooling, which can predispose these patients to syncope. Physician awareness of this syndrome will lead to improved recognition and treatment of their symptoms of presyncope or syncope.

Outpatient intravenous ketamine for the treatment of complex regional pain syndrome: a double-blind placebo controlled study.

Complex regional pain syndrome (CRPS) is a severe chronic pain condition that most often develops following trauma. The pathophysiology of CRPS is not known but both clinical and experimental evidence demonstrate the important of the NMDA receptor and glial activation in its induction and maintenance. Ketamine is the most potent clinically available safe NMDA antagonist that has a well established role in the treatment of acute and chronic pain. This randomized double-blind placebo controlled trial was designed to evaluate the effectiveness of intravenous ketamine in the treatment of CRPS. Before treatment, after informed consent was obtained, each subject was randomized into a ketamine or a placebo infusion group. Study subjects were evaluated for at least 2 weeks prior to treatment and for 3 months following treatment. All subjects were infused intravenously with normal saline with or without ketamine for 4h (25ml/h) daily for 10 days. The maximum ketamine infusion rate was 0.35mg/kg/h, not to exceed 25mg/h over a 4h period. Subjects in both the ketamine and placebo groups were administered clonidine and versed. This study showed that intravenous ketamine administered in an outpatient setting resulted in statistically significant (p<0.05) reductions in many pain parameters. It also showed that subjects in our placebo group demonstrated no treatment effect in any parameter. The results of this study warrant a larger randomized placebo controlled trial using higher doses of ketamine and a longer follow-up period.

Atypical chest pain: evidence of intercostobrachial nerve sensitization in Complex Regional Pain Syndrome.

BACKGROUND: Atypical chest pain is a common complaint among Complex Regional Pain Syndrome (CRPS) patients with brachial plexus involvement. Anatomically, the intercostobrachial nerve (ICBN) is connected to the brachial plexus and innervates the axilla, medial arm and anterior chest wall. By connecting to the brachial plexus, the ICBN could become sensitized by CRPS spread and become a source of atypical chest pain. OBJECTIVE: To evaluate the sensitivity of chest areas in CRPS patients and normal controls. DESIGN: Prospective investigation of pressure algometry in chest areas to determine chest wall sensitivity. METHODS: CRPS patients and normal controls volunteered to participate in our study. Each individual was examined to meet inclusion criteria. Patients' report of chest pain history was collected from every participant. Pressure algometry was used to measure pressure sensitivity in the axilla, anterior axillary line second intercostal space, mid-clavicular third rib, mid-clavicular tenth rib, and midsternal. Each of these measurements were compared to an intra-participant abdominal measure to control for an individuals generalized sensitivity. The ratios of chest wall sensitivities were compared between CRPS patients and normal controls. RESULTS: A history of chest pain was reported by a majority (94%) of CRPS patients and a minority (19%) of normal controls. CRPS patients reported lifting their arm as a major initiating factor for chest pain. To pressure algometry, the ratios of CRPS patients were significantly greater than control subjects (p< 0.02 throughout), indicating increased chest wall sensitivity. LIMITATIONS: This study is limited by the relatively small number of patients (n=35) and controls (n=21) used. CONCLUSION: The results of this study support the idea that chest pain is greater in CRPS patients than normal controls. The ICBN could be the source of this sensitization by CRPS spread from the brachial plexus.

Efficacy of 5-day continuous lidocaine infusion for the treatment of refractory complex regional pain syndrome.

OBJECTIVE: Chronic regional pain syndrome (CRPS) is a severe pain condition that usually results from an injury or surgical procedure. The pain in CRPS often spreads from the site of injury, and with time becomes refractory to conventional therapy. The present study was undertaken to evaluate the effects of 5-day continuous intravenous lidocaine treatment in patients afflicted with CRPS. METHODS: Intravenous lidocaine was administered in an escalating dose schedule to 49 severely affected CRPS patients in a monitored setting over 5 days. Evaluation of pain parameters and other signs and symptoms of CRPS were obtained during the infusion and at 1, 3, and 6 months following therapy. RESULTS: The majority of patients demonstrated a significant decrease in pain parameters and other symptoms and signs of CRPS. The pain reduction lasted an average of 3 months. Lidocaine may be particularly effective for thermal and mechanical allodynia. Less clinically significant effects were documented on the motor aspects of the syndrome. DISCUSSION: Intravenous lidocaine administration titrated to 5 mg/L demonstrated: 1) a significant decrease in mechanical and thermal allodynia for three months, 2) lessened associated inflammatory components of CRPS, and 3) only minimal side effects and no severe complications.

Brachial plexus traction injury: quantification of sensory abnormalities.

OBJECTIVE: Stretch injury to the brachial plexus may occur following traumatic flexion-extension of the cervical spine often seen in motor vehicle accidents or falls. Radiologic and conventional nerve conduction studies are negative in many cases. The present study was undertaken in an attempt to simplify, standardize, and quantify the positive and negative sensory abnormalities that are most often seen during the clinical examination. METHODS: Quantitation of thresholds for thermal detection and pain, vibration, pressure pain and elevated arm stress test was performed in a series of 38 patients with the clinical picture of brachial plexus traction injury as well as a group of age and sex matched control subjects. RESULTS: Significant decreases in all evoked pain thresholds, except for heat pain, along with increases in sensory detection thresholds were found in the patient group compared with controls. DISCUSSION: Quantification of sensory findings may greatly facilitate and substantiate the diagnosis of this type of injury. The data are consistent with the hypothesis that brachial plexus traction injury causes dysfunction of small sensory fiber systems and results in a form of neuropathic pain.