Inhibition of β-ARK1 Ameliorates Morphine-induced Tolerance and Hyperalgesia Via Modulating the Activity of Spinal NMDA Receptors

Dysregulation of Vesicular Glutamate Transporter VGluT2 via BDNF/TrkB Pathway Contributes to Morphine Tolerance in Mice

Morphine is widely used in the treatment of moderate to severe pain. Long-term use of morphine leads to various adverse effects, such as tolerance and hyperalgesia. Vesicular glutamate transporter 2 (VGluT2) accumulates glutamate into synaptic vesicles and plays multiple roles in the central nervous system. However, the specific role of VGluT2 in morphine tolerance has not been fully elucidated. Here, we investigated the regulatory role of VGluT2 in morphine tolerance and assessed the potential role of the brain-derived neurotrophic factor (BDNF)/tyrosine kinase B (TrkB) pathway in VGluT2 mediated morphine antinociceptive tolerance in mice. In the present study, we found that VGluT2 is upregulated in the spinal cord after the development of morphine tolerance. Furthermore, inhibition of VGluT2 with its antagonist (Chicago sky blue 6 B, CSB6B) or knockdown of VGluT2 by lentivirus restored the analgesic effect of morphine, suppressed the activation of astrocytes and microglia, and decreased glial-derived pro-inflammatory cytokines. Overexpression of VGluT2 by lentivirus facilitated morphine tolerance and mechanical hyperalgesia. In addition, we found the expression of BDNF is correlated with VGluT2 expression in the spinal cord after chronic morphine administration. Intrathecal injection of the BDNF/TrkB pathway antagonist K252a attenuated the development of morphine tolerance and decreased the expression of VGluT2 in the spinal cord, which suggested the BDNF/TrkB pathway participates in the regulation of VGluT2 in morphine tolerance. This study elucidates the functional capability of VGluT2 in modulating morphine tolerance and identifies a novel mechanism and promising therapeutic target for morphine tolerance.

PCC-0105002, a novel small molecule inhibitor of PSD95-nNOS protein-protein interactions, attenuates neuropathic pain and corrects motor disorder associated with neuropathic pain model

In view of postsynaptic density 95kDA (PSD95) tethers neuronal NO synthase (nNOS) to N-methyl-d-aspartate receptor (NMDAR), the PSD95-nNOS complex represents a therapeutic target of neuropathic pain. This study therefore sought to explore the ability of PCC-0105002, a novel PSD95-nNOS small molecule inhibitor, to alter pain sensitivity in rodent neuropathic pain models. Firstly, the IC50 of PCC-0105002 for PSD95 and NOS1 binding activity was determined using an Alpha Screen assay kit. Then, we examined the effects of PCC-0105002 in the mouse formalin test and in the rat spinal nerve ligation (SNL) model, and explored the ability of PCC-0105002 to mediate analgesia and to effect motor coordination in a rota-rod test. Moreover, the mechanisms whereby PCC-0105002 mediates analgesia was explored via western blotting, Golgi staining, and co-immunoprecipitation experiments in dorsal horn. The outcomes indicated that PCC-0105002 exhibited dose-dependent attenuation of phase II pain-associated behaviors in the formalin test. The result indicated that PCC-0105002 disrupted the PSD95-nNOS interaction with IC50 of 1.408 μM. In the SNL model, PCC-0105002 suppressed mechanical allodynia, thermal hyperalgesia, and abnormal dorsal horn wide dynamic range neuron discharge. PCC-0105002 mediated an analgesic effect comparable to that of MK-801, while it was better able to enhance motor coordination as compared with MK-801. Moreover, PCC-0105002 altered signaling downstream of NMDAR and thus functionally and structurally attenuating synaptic plasticity through respective regulation of the NR2B/GluR1/CaMKIIα and Rac1/RhoA pathways. These findings suggest that the novel PSD95-nNOS inhibitor PCC-0105002 is an effective agent for alleviating neuropathic pain, and that it produces fewer motor coordination-associated side effects than do NMDAR antagonists.

Hydrogen enriched saline alleviates morphine tolerance via inhibiting neuroinflammation, GLT-1, GS nitration and NMDA receptor trafficking and functioning in the spinal cord of rats

The development and maintenance of morphine tolerance showed association with neuroinflammation and dysfunction of central glutamatergic system (such as nitration of glutamate transporter). Recent evidence indicated that hydrogen could reduce the levels of neuroinflammation and oxidative stress, but its role in morphine tolerance has not been studied. The rats were intrathecally administered with morphine (10 μg/10 μL each time, twice/day for 5 days). Hydrogen enriched saline (HS) or saline was given intraperitoneally at 1, 3 and 10 mL/kg for 10 min before each dose of morphine administration. The tail-flick latency, mechanical threshold and thermal latency were assessed one day (baseline) before and daily for up to 5 days during morphine injection. The pro-inflammatory cytokine expressions [tumor necrosis factor-alpha (TNF-α), interleukin-1β (IL-1β), IL-6)] (by western blotting), astrocyte activation (by immunofluorescence and western blotting), and nitration of glutamate transporter-1 (GLT-1) and glutamine synthetase (GS) (by immunoprecipitation), membrane and total expression of N-methyl-d-aspartic acid (NMDA) receptor NR1 and NR2B subunits were carried out in the spinal dorsal horns. Chronic morphine administration induced antinociceptive tolerance, and together led to increased TNF-α, IL-1β and IL-6 expression, astrocyte activation, GLT-1 and GS nitration, increased membrane and total NR1, NR2B expression. Injection of HS attenuated morphine tolerance in a dose-dependent manner, decreased proinflammatory cytokine expression, inhibited astrocyte activation, decreased GLT-1 and GS nitration, and inhibited membrane trafficking of NMDA receptor. Our result showed that hydrogen pretreatment prevented morphine tolerance by reducing neuroinflammation, GLT-1, GS nitration, NMDA receptor trafficking in the spinal dorsal horn. Pretreatment with hydrogen might be considered as a novel therapeutic strategy for the prevention of morphine tolerance.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

WITHDRAWN: PCC-0105002, a novel small molecule inhibitor of PSD95-nNOS protein-protein interactions, attenuates neuropathic pain and corrects motor coordination-associated side effects in neuropathic pain model

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α2δ-1-Bound N-Methyl-D-aspartate Receptors Mediate Morphine-induced Hyperalgesia and Analgesic Tolerance by Potentiating Glutamatergic Input in Rodents

BACKGROUND

Chronic use of μ-opioid receptor agonists paradoxically causes both hyperalgesia and the loss of analgesic efficacy. Opioid treatment increases presynaptic N-methyl-D-aspartate receptor activity to potentiate nociceptive input to spinal dorsal horn neurons. However, the mechanism responsible for this opioid-induced activation of presynaptic N-methyl-D-aspartate receptors remains unclear. α2δ-1, formerly known as a calcium channel subunit, interacts with N-methyl-D-aspartate receptors and is primarily expressed at presynaptic terminals. This study tested the hypothesis that α2δ-1-bound N-methyl-D-aspartate receptors contribute to presynaptic N-methyl-D-aspartate receptor hyperactivity associated with opioid-induced hyperalgesia and analgesic tolerance.

METHODS

Rats (5 mg/kg) and wild-type and α2δ-1-knockout mice (10 mg/kg) were treated intraperitoneally with morphine twice/day for 8 consecutive days, and nociceptive thresholds were examined. Presynaptic N-methyl-D-aspartate receptor activity was recorded in spinal cord slices. Coimmunoprecipitation was performed to examine protein-protein interactions.

RESULTS

Chronic morphine treatment in rats increased α2δ-1 protein amounts in the dorsal root ganglion and spinal cord. Chronic morphine exposure also increased the physical interaction between α2δ-1 and N-methyl-D-aspartate receptors by 1.5 ± 0.3 fold (means ± SD, P = 0.009, n = 6) and the prevalence of α2δ-1-bound N-methyl-D-aspartate receptors at spinal cord synapses. Inhibiting α2δ-1 with gabapentin or genetic knockout of α2δ-1 abolished the increase in presynaptic N-methyl-D-aspartate receptor activity in the spinal dorsal horn induced by morphine treatment. Furthermore, uncoupling the α2δ-1-N-methyl-D-aspartate receptor interaction with an α2δ-1 C terminus-interfering peptide fully reversed morphine-induced tonic activation of N-methyl-D-aspartate receptors at the central terminal of primary afferents. Finally, intraperitoneal injection of gabapentin or intrathecal injection of an α2δ-1 C terminus-interfering peptide or α2δ-1 genetic knockout abolished the mechanical and thermal hyperalgesia induced by chronic morphine exposure and largely preserved morphine's analgesic effect during 8 days of morphine treatment.

CONCLUSIONS

α2δ-1-Bound N-methyl-D-aspartate receptors contribute to opioid-induced hyperalgesia and tolerance by augmenting presynaptic N-methyl-D-aspartate receptor expression and activity at the spinal cord level.

NR2B receptor- and calpain-mediated KCC2 cleavage resulted in cognitive deficiency exposure to isoflurane

BACKGROUND

During brain development, volatile anesthetic can rapidly interfere with physiologic patterns of dendritic development and synaptogenesis and impair the formation of precise neuronal circuits. KCC2 plays vital roles in spine development and synaptogenesis through its Cl- transport function and structural interactions with the spine cytoskeleton protein 4.1 N. The aim of this study was to dissect the mechanism of volatile anesthetics, which impair dendritic development and synaptogenesis via mediation of KCC2 cleavage.

METHODS

Westernblotting was employed to assess the expression change of NR2B, NR2A, calpain-1, calpain-2, KCC2, and 4.1 N protein of rat (PND 5). Co-immunoprecipitation was applied to demonstrate the interaction between KCC2 and 4.1 N protein. Long-term cognitive deficiency was assessed by MWM. Lentivirus-calpain-2 was administered by hippocampus stereotaxic injection.

RESULTS

There was a significant increase in the level of NR2B instead of NR2A exposure to isoflurane. Calpain-2 was excessively activated via NR2B after 6 h of isoflurane exposure. The expression of plasmalemmal KCC2 and 4.1 N protein was significantly decreased treated with isoflurane. The isoflurane group showed longer traveled distance, prolonged escape latency, less time spent in the target quadrant, and decreased platform crossings. Pretreatment with ifenprodil and downregulated calpain-2 expression significantly alleviated these neurotoxicity responses and cognitive deficiency after isoflurane exposure.

CONCLUSIONS

A significant increase in NR2B, excessive activation of calpain-2 and increased cleavage of plasmalemmal KCC2, are involved in isoflurane-induced neurotoxicity and long-term cognitive deficiency. Blocking NR2B and calpain-2 activity significantly attenuated these responses. The KCC2 cleavage mediated by NR2B and calpain-2 is a major determinant of isoflurane-induced long-term cognitive deficiency.