Midazolam administration reverses thermal hyperalgesia and prevents gamma-aminobutyric acid transporter loss in a rodent model of neuropathic pain

Actions of remimazolam on inhibitory transmission of rat spinal dorsal horn neurons

Remimazolam is an ultra-short benzodiazepine that acts on the benzodiazepine site of γ-aminobutyric acid (GABA) receptors in the brain and induces sedation. Although GABA receptors are found localized in the spinal dorsal horn, no previous studies have reported the analgesic effects or investigated the cellular mechanisms of remimazolam on the spinal dorsal horn. Behavioral measures, immunohistochemistry, and in vitro whole-cell patch-clamp recordings of dorsal horn neurons were used to assess synaptic transmission. Intrathecal injection of remimazolam induced behavioral analgesia in inflammatory pain-induced mechanical allodynia (six rats/dose; p < 0.05). Immunohistochemical staining revealed that remimazolam suppressed spinal phosphorylated extracellular signal-regulated kinase activation (five rats/group, p < 0.05). In vitro whole-cell patch-clamp analysis demonstrated that remimazolam increased the frequency of GABAergic miniature inhibitory post-synaptic currents, prolonged the decay time (six rats; p < 0.05), and enhanced GABA currents induced by exogenous GABA (seven rats; p < 0.01). However, remimazolam did not affect miniature excitatory post-synaptic currents or amplitude of monosynaptic excitatory post-synaptic currents evoked by Aδ- and C-fiber stimulation (seven rats; p > 0.05). This study suggests that remimazolam induces analgesia by enhancing GABAergic inhibitory transmission in the spinal dorsal horn, suggesting its potential utility as a spinal analgesic for inflammatory pain.

Role of GABAergic system in the comorbidity of pain and depression

Patients with chronic pain often suffer with depressive symptoms, and these two conditions can be aggravated by each other over time, leading to an increase in symptom intensity and duration. The comorbidity of pain and depression poses a significant challenge to human health and quality of life, as it is often difficult to diagnose early and treat effectively. Therefore, exploring the molecular mechanisms underlying the comorbidity of chronic pain and depression is crucial to identifying new therapeutic targets for treatment. However, understanding the pathogenesis of comorbidity requires examining interactions among multiple factors, which calls for an integrative perspective. While several studies have explored the role of the GABAergic system in pain and depression, fewer have examined its interactions with other systems involved in their comorbidity. Here, we review the evidence that the role of GABAergic system in the comorbidity of chronic pain and depression, as well as the interactions between the GABAergic system and other secondary systems involved in pain and depression comorbidity, providing a comprehensive understanding of their intricate interplay.

The etiological contribution of GABAergic plasticity to the pathogenesis of neuropathic pain

Neuropathic pain developing after peripheral or central nerve injury is the result of pathological changes generated through complex mechanisms. Disruption in the homeostasis of excitatory and inhibitory neurons within the central nervous system is a crucial factor in the formation of hyperalgesia or allodynia occurring with neuropathic pain. The central GABAergic pathway has received attention for its extensive distribution and function in neural circuits, including the generation and development of neuropathic pain. GABAergic inhibitory changes that occur in the interneurons along descending modulatory and nociceptive pathways in the central nervous system are believed to generate neuronal plasticity, such as synaptic plasticity or functional plasticity of the related genes or proteins, that is the foundation of persistent neuropathic pain. The primary GABAergic plasticity observed in neuropathic pain includes GABAergic synapse homo- and heterosynaptic plasticity, decreased synthesis of GABA, down-expression of glutamic acid decarboxylase and GABA transporter, abnormal expression of NKCC1 or KCC2, and disturbed function of GABA receptors. In this review, we describe possible mechanisms associated with GABAergic plasticity, such as central sensitization and GABAergic interneuron apoptosis, and the epigenetic etiologies of GABAergic plasticity in neuropathic pain. Moreover, we summarize potential therapeutic targets of GABAergic plasticity that may allow for successful relief of hyperalgesia from nerve injury. Finally, we compare the effects of the GABAergic system in neuropathic pain to other types of chronic pain to understand the contribution of GABAergic plasticity to neuropathic pain.

Increased GABAergic inhibitory function against ischemic long-term potentiation in the CA1 region of the hippocampus

Potentiation of N-methyl-D-aspartate receptor (NMDAR)-mediated excitatory synaptic plasticity around 1 h after brief exposure to anoxia/aglycemia is called ischemic long-term potentiation (iLTP), which is considered a pathological form of synaptic response during the early phase of ischemic stroke. It is known that GABAergic inhibitory transmission is also an important molecular process involved in synaptic plasticity and learning memory. However, whether GABAergic transmission is involved in iLTP and early-phase plasticity in ischemic stroke remains unknown. In this study, iLTP was found to be induced in the hippocampal Schaffer-collateral pathway by exposure to oxygen glucose deprivation (OGD). Western blot analysis was conducted to analyze excitatory synaptic receptors and inhibitory synaptic receptors following OGD. The β3 subunit of the GABA_A receptor (GABA_AR) was markedly reduced, whereas the GluN2B subunit of the NMDAR was increased in the hippocampal area in the OGD group. Using extracellular recording, we demonstrated that application of GABA_AR agonist midazolam could abolish the hippocampal iLTP. Moreover, midazolam had no significant effect on the increase in NMDAR subunit GluN2B, but ameliorated the reduction in the β3 subunit of GABA_AR after OGD. In summary, our results indicated that hippocampal GABA_AR reduction promoted synaptic potentiation after OGD. Activation of GABAergic inhibitory transmission function could inhibit iLTP; thus, modulation of GABAergic function is a protective treatment method in the acute phase of synaptic plasticity in ischemic stroke.

Reviewing the case for compromised spinal inhibition in neuropathic pain

A striking and debilitating property of the nervous system is that damage to this tissue can cause chronic intractable pain, which persists long after resolution of the initial insult. This neuropathic form of pain can arise from trauma to peripheral nerves, the spinal cord, or brain. It can also result from neuropathies associated with disease states such as diabetes, human immunodeficiency virus/AIDS, herpes, multiple sclerosis, cancer, and chemotherapy. Regardless of the origin, treatments for neuropathic pain remain inadequate. This continues to drive research into the underlying mechanisms. While the literature shows that dysfunction in numerous loci throughout the CNS can contribute to chronic pain, the spinal cord and in particular inhibitory signalling in this region have remained major research areas. This review focuses on local spinal inhibition provided by dorsal horn interneurons, and how such inhibition is disrupted during the development and maintenance of neuropathic pain.

Different effects of dexmedetomidine and midazolam on the expression of NR2B and GABAA-α1 following peripheral nerve injury in rats

Neuropathic pain is a complex, chronic pain condition and the treatment is a major clinical challenge. Recent studies have shown that two FDA approved drugs dexmedetomidine (DEX) and midazolam (MZL), may be useful in treating neuropathic pain, but the mechanism is not fully dementated. Here, we investigated the effects and mechanisms of DEX and MZL treatment in the peripheral nerve injury model. Intramuscular injection with DEX and MZL attenuated the development of mechanical allodynia and thermal hyperalgesia in rats with chronic constriction injury (CCI). Concurrently, the expression of NMDA receptor subunit 2B (NR2B), GABA (A) receptor subunit alpha1 (GABAA-α1), and Sonic Hedgehog (SHH) displayed different temporal patterns in the thalamus and the ipsilateral dorsal horn of the spinal cord after CCI. Such that (1) NR2B expression was decreased on day 1 and 14, whereas GABAA-α1 expression was increased on day 1 in the thalamus, and NR2B expression was decreased on day 1, whereas GABAA-α1 expression was increased on day 1 and day 30 in the ipsilateral spinal cord dorsal horn after DEX treatment. (2) NR2B expression was increased on day 1, then decreased on day 14 and returned to baseline on day30, whereas GABAA-α1 expression was no significant changes on day 1, 14, 30 in the thalamus, and NR2B expression was decreased on day 14 and 30, whereas GABAA-α1 expression was no changes on day 1 and 14 but increased on day 30 after MZL treatment. Furthermore, the mechanical allodynia was significantly attenuated after PUR administration. Meanwhile the expression of NR2B was significantly decreased, and the expression of GABAA-α1 was significantly increased, in the thalamus and in the ipsilateral spinal cord dorsal horn when detected on postoperative day 1, 7, and 14. Our findings indicate that DEX and MZL have different mechanisms in CCI rats, suggesting different strategies could be considered in managing neuropathic pain in different individuals. © 2018 IUBMB Life, 70(2):143-152, 2018.

Midazolam and ropivacaine act synergistically to inhibit bone cancer pain with different mechanisms in rats

Analgesic strategy of a single drug analgesia in bone cancer pain (BCP) has shifted to combined analgesia with different drugs which have different mechanism. After tumor cell inculation, the activation of signal transducer and activator of transcription (STAT3) and extracellular signal-regulated kinase (ERK) signaling pathway are involved in the development and maintenance of BCP, whereas a decrease in the expression of spinal STAT3 and ERK through using their specific blocker, lead to attenuation of BCP. Hence, in this study, we clarified that intrathecal (i.t.) injection of midazolam (MZL) and ropivacaine (Ropi) induces synergistic analgesia on BCP and is accompanied with different mechanisms of these analgesic effect. Hargreaves heat test was used to detect the analgesic effect of single dose of i.t. MZL, Ropi and their combination on the BCP rats. At consecutive daily administration experiment, thermal hyperalgesia was recorded, and immunohistochemical staining was used to detect the expression of c-Fos, spinal glial fibrillary acidic protein (GFAP) and ionized calcium binding adapter molecule-1 (IBA-1). Then, western blot analysis was used to examine spinal TSPO, GFAP, IBA-1, pERK/ERK and pSTAT3/STAT3 levels on day 14 after tumor cell inoculation. i.t. MZL or Ropi showed a short-term analgesia dose-dependently, and MZL displayed better effect on inhibition of pSTAT3 expression than pERK, but Ropi was just the reverse, then consecutive daily administrations of their combination acted synergistically to attenuate thermal hyperalgesia with downregulated spinal 'neuron-astrocytic activation' in the BCP rats. i.t. co-delivery of MZL and Ropi shows synergistic analgesia on the BCP with the inhibition of spinal 'neuron-astrocytic activation'. Spinal different signaling pathway inhibition for MZL and Ropi may be involved in this process.

Evaluation of heat hyperalgesia and anxiety like-behaviors in a rat model of orofacial cancer

Pain and anxiety are commonly experienced by cancer patients and both significantly impair their quality of life. Some authors claim that there is a relationship between pain and anxiety, while others suggest that there is not a direct association. In any case, there is indeed a consensus that anxiety impairs the pain condition beyond be under diagnosed and undertreated in cancer pain patients. Herein we investigated if rats presenting heat hyperalgesia induced by orofacial cancer cell inoculation would display anxiety-like behaviors. In addition, we evaluated if pain blockade would result in alleviation of anxiety behaviors, as well as, if blockade of anxiety would result in pain relief. Orofacial cancer was induced in male Wistar rats by inoculation of Walker-256 cells into the right vibrissal pad. Heat facial hyperalgesia was assessed on day 6 after the inoculation, and on this time point rats were submitted to the elevated plus maze and the light-dark transition tests. The influence of lidocaine and midazolam on heat hyperalgesia and anxiety-like behaviors was assessed. The peak of facial heat hyperalgesia was detected 6 days after cancer cells inoculation, and at this time point, rats exhibited increased anxiety-like behaviors. Local treatment with lidocaine (2%/50μL) caused a marked reduction of heat hyperalgesia, but failed to affect the anxiety-like behaviors, while midazolam (0.5mg/kg, i.p.) treatment failed to change the heat threshold, but induced an anxiolytic-like effect. Altogether, our data demonstrated that rats with orofacial cancer present pain- and anxiety-like behaviors, but brief heat hyperalgesia relief does not affect the anxiety-like behaviors, and vice-versa, in our experimental conditions.

The Role of Regulatory Transporters in Neuropathic Pain

Neuropathic pain arises from an injury or disease of the somatosensory nervous system rather than stimulation of pain receptors. As a result, the fine balance between excitation and inhibition is perturbed leading to hyperalgesia and allodynia. Various neuropathic pain models provide considerable evidence that changes in the glutamatergic, GABAergic, and monoaminergic systems. Neurotransmitter reuptake transporter proteins have the potential to change the temporal and spatial profile of various neurotransmitters throughout the nervous system. This, in turn, can affect the downstream effects of these neurotransmitters and hence modulate pain. This chapter explores various reuptake transporter systems and implicates their role in pain processing. Understanding the transporter systems will enhance drug discovery targeting different facets of neuropathic pain.

The efficacy of different doses of Midazolam added to Lidocaine for upper extremity Bier block on the sensory and motor block characteristics and postoperative pain

Objective:

This study was designed to evaluate the effect of different doses of midazolam on anesthesia and analgesia quality when added to lidocaine during the intravenous regional anesthesia (IVRA).

Methods:

One hundred and forty patients underwent hand surgery were randomly allocated into four groups to receive 3 mg/kg lidocaine 2% diluted with saline to a total volume of 40 mL in the control Group L-C (n = 35), 30 μg/kg midazolam plus 3 mg/kg lidocaine 2% diluted with saline to a total volume of 40 mL in the midazolam Group L-M₁ (n = 35), 40 μg/kg midazolam plus 3 mg/kg 2% lidocaine diluted with saline to a total volume of 40 mL in the midazolam Group L-M₂ (n = 35), and 50 μg/kg midazolam plus 3 mg/kg lidocaine 2% diluted with saline to a total volume of 40 mL in the midazolam Group L-M₃ (n = 35). Sensory and motor block and recovery times, tourniquet pain, intra-operative analgesic requirement, and visual analog scale (VAS) scores were recorded.

Findings:

Onset time of sensory and motor block in L-M₃ Group was shorter than the L-M₂ and L-M₁ and L-C Groups (P < 0.001). Furthermore, prolonged sensory (P = 0.005) and motor recovery time (P = 0.001) in L-M₃ were longer than the other groups. Intra-operative VAS score and intra-operative fentanyl consumption in L-M₃ were lower than the other groups (P < 0.001). The numbers of patients needed to pethidine in Group L-M₃ were significantly less compared with the other groups (P = 0.035). VAS scores were significantly lower in Group L-M₃ in different time intervals in the postoperative period compared with the other groups (P < 0.001).

Conclusion:

Addition of 50 μg/kg midazolam for IVRA (Group L-M₃) enhanced intra-operative analgesia and improved anesthesia quality better than other groups receiving lower midazolam doses as well as a control group.

Glycine transporter GlyT1, but not GlyT2, is expressed in rat dorsal root ganglion--Possible implications for neuropathic pain

Glycinergic inhibitory neurotransmission plays a pivotal role in the development of neuropathic pain. The glycine concentration in the synaptic cleft is controlled by the glycine transporters GlyT1 and GlyT2. GlyT1 is expressed throughout the central nervous system, while GlyT2 is exclusively located in glycinergic neurons. Aim of the present study was to investigate whether GlyTs are also expressed in the peripheral sensory nervous system and whether their expression is modulated in experimental neuropathic pain. Neuropathic pain was induced in male Wistar rats by Chronic Constriction Injury (CCI) and verified by assessment of mechanical allodynia (von Frey method). Expression patterns of GlyTs and the glycine binding subunit NR1 of the N-methyl-d-aspartate (NMDA) receptor in the spinal cord and dorsal root ganglia (DRG) were analyzed by Western blot analysis, PCR and immunohistochemistry. While both GlyT1 and GlyT2 were detected in the spinal cord, only GlyT1, but not GlyT2, was detected in DRG. Immunofluorescence revealed a strictly neuronal localization of GlyT1 and a co-localization of GlyT1 and NR1 in DRG. Compared to sham procedure, spinal cord and DRG expression of GlyT1 was not altered and NR1 was unchanged in DRG 12 days after CCI. GlyT1, but not GlyT2, is expressed in the peripheral sensory nervous system. The co-expression of GlyT1 and NMDA receptors in DRG suggests that GlyT1 regulates glycine concentration at the glycine binding site of the NMDA receptor. Differential regulation of GlyT1 expression in the spinal cord or DRG, however, does not seem to be associated with the development of neuropathic pain.

Blocking the GABA transporter GAT-1 ameliorates spinal GABAergic disinhibition and neuropathic pain induced by paclitaxel

Paclitaxel is a chemotherapeutic agent widely used for treating carcinomas. Patients receiving paclitaxel often develop neuropathic pain and have a reduced quality of life which hinders the use of this life-saving drug. In this study, we determined the role of GABA transporters in the genesis of paclitaxel-induced neuropathic pain using behavioral tests, electrophysiology, and biochemical techniques. We found that tonic GABA receptor activities in the spinal dorsal horn were reduced in rats with neuropathic pain induced by paclitaxel. In normal controls, tonic GABA receptor activities were mainly controlled by the GABA transporter GAT-1 but not GAT-3. In the spinal dorsal horn, GAT-1 was expressed at presynaptic terminals and astrocytes while GAT-3 was only expressed in astrocytes. In rats with paclitaxel-induced neuropathic pain, the protein expression of GAT-1 was increased while GAT-3 was decreased. This was concurrently associated with an increase in global GABA uptake. The paclitaxel-induced attenuation of GABAergic tonic inhibition was ameliorated by blocking GAT-1 but not GAT-3 transporters. Paclitaxel-induced neuropathic pain was significantly attenuated by the intrathecal injection of a GAT-1 inhibitor. These findings suggest that targeting GAT-1 transporters for reversing disinhibition in the spinal dorsal horn may be a useful approach for treating paclitaxel-induced neuropathic pain. Patients receiving paclitaxel for cancer therapy often develop neuropathic pain and have a reduced quality of life. In this study, we demonstrated that animals treated with paclitaxel develop neuropathic pain, have enhancements of GABA transporter-1 protein expression and global GABA uptake, as well as suppression of GABAergic tonic inhibition in the spinal dorsal horn. Pharmacological inhibition of GABA transporter-1 ameliorates the paclitaxel-induced suppression of GABAergic tonic inhibition and neuropathic pain. Thus, targeting GAT-1 transporters for reversing GABAergic disinhibition in the spinal dorsal horn could be a useful approach for treating paclitaxel-induced neuropathic pain.

Comprehensive analysis of the GABAergic system gene expression profile in the anterior cingulate cortex of mice with Paclitaxel-induced neuropathic pain

The supraspinal pathophysiology of the painful neuropathy induced by paclitaxel, a chemotherapeutic agent, is not well understood. The γ-aminobutyric acid (GABA) neurotransmitter system has been implicated in the pathogenesis of neuropathic pain. Gene expression of GABAergic system molecules was examined in the anterior cingulate cortex (ACC) of mice brains, by real-time PCR, during paclitaxel-induced neuropathic pain, because this area is involved in pain perception and modulation that might contribute to neuropathic pain. Paclitaxel treatment resulted in thermal hyperalgesia and in increased GABA transporter-1 (GAT-1) mRNA expression, but not that of other GABA transporters or GABA(A) ergic enzymes in the ACC compared to vehicle treatment. Among the 18 GABA(A) receptor subunits analyzed, only β2, β3, δ, and γ2 had increased mRNA levels, and for the receptor subunit, only GABA(B2) had increased mRNA levels in the ACC of paclitaxel-treated mice, whereas the rest of the GABA receptor subunits were not altered. The mRNA expression of GABAA receptor subunits α6, θ, π, ρ1, ρ2, and ρ3 were not detected in the ACC. In conclusion, these data show that during paclitaxel-induced neuropathic pain there is significant increase in GAT-1 expression in the ACC. GAT-1 is the main transporter of GABA from the synapse, and thus its increased expression possibly results in less GABA at the synapse and dysregulation of the GABAergic system. GAT-1 is a potential therapeutic target for managing paclitaxel-induced neuropathic pain.

Midazolam exacerbates morphine tolerance and morphine-induced hyperactive behaviors in young rats with burn injury

Midazolam and morphine are often used in pediatric intensive care unit (ICU) for analgesia and sedation. However, how these two drugs interact behaviorally remains unclear. Here, we examined whether (1) co-administration of midazolam with morphine would exacerbate morphine tolerance and morphine-induced hyperactive behaviors, and (2) protein kinase C (PKC) would contribute to these behavioral changes. Male rats of 3-4 weeks old were exposed to a hindpaw burn injury. In Experiment 1, burn-injured young rats received once daily saline or morphine (10mg/kg, subcutaneous, s.c.), followed 30min later by either saline or midazolam (2mg/kg, intraperitoneal, i.p.), for 14 days beginning 3 days after burn injury. In Experiment 2, young rats with burn injury were administered with morphine (10mg/kg, s.c.), midazolam (2mg/kg, i.p.), and chelerythrine chloride (a non-specific PKC inhibitor, 10nmol, intrathecal) for 14 days. For both experiments, cumulative morphine anti-nociceptive dose-response (ED50) was tested and hyperactive behaviors such as jumping and scratching were recorded. Following 2 weeks of each treatment, ED50 dose was significantly increased in rats receiving morphine alone as compared with rats receiving saline or midazolam alone. The ED50 dose was further increased in rats receiving both morphine and midazolam. Co-administration of morphine and midazolam also exacerbated morphine-induced hyperactive behaviors. Expression of the NR1 subunit of the N-methyl-d-aspartate (NMDA) receptor and PKCγ in the spinal cord dorsal horn (immunohistochemistry; Western blot) was upregulated in burn-injured young rats receiving morphine alone or in combination with midazolam, and chelerythrine prevented the development of morphine tolerance. These results indicate that midazolam exacerbated morphine tolerance through a spinal NMDA/PKC-mediated mechanism.

Quality of lidocaine analgesia with and without midazolam for intravenous regional anesthesia

PURPOSE

Midazolam has analgesic effects mediated by gamma aminobutyric acid-A receptors. This study was designed to evaluate the effect of midazolam on anesthesia and analgesia quality when added to lidocaine for intravenous regional anesthesia (IVRA).

METHODS

Forty patients undergoing hand surgery were randomly assigned to two groups to receive IVRA. The control group received 3 mg/kg lidocaine 2% w/v diluted with saline to a total volume of 40 ml, and the midazolam group received an additional 50 μg/kg midazolam. Sensory and motor block onset and recovery times, tourniquet pain, intraoperative analgesic requirements, sedation, and anesthesia quality were recorded. Postoperative pain and sedation scores, time to first analgesic requirements, analgesic use in the first 24 h, and side effects were noted.

RESULTS

Sensory and motor block onset and recovery times did not differ significantly between groups. Tourniquet pain scores were lower at 10, 15, 20, and 30 min (P < 0.0001) in the midazolam group. Three (15%) patients in the midazolam group required fentanyl for tourniquet pain compared with thirteen (65%) patients in the control group (P = 0.02). Patients in both groups received fentanyl once. Midazolam group showed that significantly less patients required diclofenac for postoperative analgesia (P < 0.01) and analgesic-free period during first postoperative 24 h was significantly longer (726.8 ± 662.8 min vs. 91.0 ± 35.9 min, P < 0.0001). Postoperative pain scores were lower (P < 0.0001) and sedation scores higher (P < 0.05) for the first 2 h in the midazolam group.

CONCLUSION

Addition of midazolam to lidocaine for IVRA improves anesthesia quality and enhances intraoperative and postoperative analgesia without causing side effects.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.