Coadministration of indomethacin and minocycline attenuates established paclitaxel-induced neuropathic thermal hyperalgesia: Involvement of cannabinoid CB1 receptors

Licofelone, a Dual COX/LOX Inhibitor, Ameliorates Paclitaxel-Induced Mechanical Allodynia in Rats in a Cannabinoid Receptor-Dependent Manner

The use of paclitaxel as a chemotherapeutic drug is limited by the development of dose-dependent paclitaxel-induced neuropathic pain (PINP). Recently, we observed that the combination of indomethacin plus minocycline (IPM) attenuates PINP in a mouse model in a cannabinoid (CB) receptor-dependent manner. Indomethacin inhibits cyclooxygenase (COX) activity, and minocycline inhibits 5-lipoxygenase (5-LOX) activity. Male Sprague Dawley rats with paclitaxel-induced mechanical allodynia were treated with indomethacin, minocycline, IPM combination, licofelone (a dual COX/LOX inhibitor), or their vehicles. AM251, a CB1 receptor antagonist, and AM630, a CB2 receptor antagonist, were administered before the IPM combination or licofelone. Mechanical allodynia was measured using a dynamic plantar aesthesiometer. Molecular docking was performed using CB-Dock2. Licofelone and IPM combination had antiallodynic effects, which were significantly higher than either indomethacin or minocycline alone. AM251 and AM630 blocked the antiallodynic effects of IPM combination and licofelone. Molecular docking showed that licofelone binds to both CB1 and CB2 receptors with a high affinity similar to the phytocannabinoid 1-trans-delta-9-tetrahydrocannabinol and the synthetic cannabinoid WIN 55,212-2. Licofelone inhibits COX and LOX and/or directly interacts with CB receptors to produce antiallodynic effects in a rat model of PINP. The findings further suggest that licofelone could be a therapeutic agent for managing PINP.

An up-to-date view of paclitaxel-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN),referring to the damage to the peripheral nerves caused by exposure to a neurotoxic chemotherapeutic agent, is a common side effect amongst patients undergoing chemotherapy. Paclitaxel-induced peripheral neuropathy (PIPN) can lead to dose reduction or early cessation of chemotherapy, which is not conducive to patients'survival. Even after treatment is discontinued, PIPN symptoms carried a greater risk of worsening and plagued the patient's life, leading to long-term morbidity in survivors. Here, we summarize the research progress for clinical manifestations, risk factors, pathogenesis, prevention and treatment of PIPN, so as to embark on the path of preventing PIPN with prolongation of patient's life quality on a long-term basis.

Microglia and p38 MAPK Inhibitors Suppress Development of Mechanical Allodynia in Both Sexes in a Mouse Model of Antiretroviral-Induced Neuropathic Pain

Microglia activation in the spinal cord play a major role in the pathogenesis of neuropathic pain. The p38 mitogen-activated protein kinase (MAPK) regulates microglia activation. Previously, 2',3'-dideoxycytidine (ddC), a nucleoside reverse transcriptase inhibitor (NRTI), was found to induce mechanical allodynia and microglia activation in the spinal cords of male and female mice. In this study, we investigated the role of spinal microglia and p38 MAPK signaling in the development of mechanical allodynia using immunofluorescence staining and treatment with microglia and p38 MAPK inhibitors in both sexes. Male and female mice (BALB/c strain) treated intraperitoneally once daily with ddC 25 mg/kg for five consecutive days developed mechanical allodynia, assessed using the dynamic plantar aesthesiometer. Treatment with ddC increased microglia markers CD11b and ionized calcium-binding adapter molecule 1 (Iba1) staining intensity in male mice, while only CD11b was increased in female mice. Both sexes had increased phosphorylated p38 MAPK staining intensity. The administration of minocycline, an inhibitor of microglia activation, and adezmapimod, a selective p38 MAPK inhibitor, suppressed mechanical allodynia in both sexes at day 7 after ddC treatment. Therefore, microglia activation and p38 MAPK signaling are important for the development of antiretroviral drug-induced mechanical allodynia.

Rosuvastatin Synergistically Enhances the Antinociceptive Efficacy of Duloxetine in Paclitaxel-Induced Neuropathic Pain in Mice

Paclitaxel, a widely used cancer chemotherapeutic agent, has high incidence of neurotoxicity associated with the production of neuropathic pain, for which only duloxetine has shown significant but moderate analgesic effect. Since statins, classically used to reduce hypercholesterolemia, have shown antinociceptive effect in preclinical studies on neuropathic pain, we studied whether the antinociceptive efficacy of duloxetine could be synergistically potentiated by rosuvastatin in a model of paclitaxel-induced neuropathy in mice. The astrocytic and microglial responses in the spinal cord of paclitaxel-treated mice were also assessed by measuring GFAP and CD11b proteins, respectively. Paclitaxel treatment did not impair motor coordination and balance in rotarod testing. Rosuvastatin, duloxetine, and the rosuvastatin/duloxetine combination (combined at equieffective doses) dose-dependently decreased mechanical allodynia (ED₃₀, von Frey testing) and thermal hyperalgesia (ED₅₀, hot plate testing) in paclitaxel-treated mice. Isobolographic analysis showed a superadditive interaction for rosuvastatin and duloxetine, as both the ED₃₀ and ED₅₀ for the rosuvastatin/duloxetine combination contained only a quarter of each drug compared to the individual drugs. The rosuvastatin/duloxetine combination reversed paclitaxel-induced GFAP overexpression, indicating that such effects might depend in part on astrocyte inactivation. Results suggest that statins could be useful in synergistically enhancing the efficacy of duloxetine in some chemotherapy-induced neuropathic conditions.

Sex Differences in the Expression of Neuroimmune Molecules in the Spinal Cord of a Mouse Model of Antiretroviral-Induced Neuropathic Pain

Nucleoside reverse transcriptase inhibitors (NRTIs), drugs used to treat HIV infection, can cause neuropathic pain (NP) and neuroinflammation. An NRTI, 2′-3′-dideoxycytidine (ddC), was reported to induce mechanical allodynia and increase proinflammatory cytokines in the brains of female mice. In some models of NP, microglia activation is important for NP pathophysiology in male mice, while T cells are important in female mice. Age-matched female and male mice (BALB/c strain) treated intraperitoneally once daily with ddC for 5 days developed mechanical allodynia. Treatment with ddC increased Cd11b, H2-Aa, Cd3e, Mapk1, Il1b, Tnf, and Il10 mRNA levels in the spinal cords of female, but not male, mice, whereas there was no alteration found in Gfap and Mapk14 transcripts in both sexes on day 7 after ddC administration. The protein expression of CD11b and phospho-p38 MAPK was significantly increased in the spinal cords of ddC-treated female, but not male, mice, whereas Iba1 protein was elevated in ddC-treated male mice. There was no change in GFAP, CD3e, and phospho-p44/42 MAPK protein levels in both sexes. Thus, changes in neuroimmune cells and molecules in the spinal cords during ddC-induced neuroinflammation were sex-dependent, with female mice being more prone to neuroimmune changes than male mice.

Sex Differences in Taxane Toxicities

Simple Summary

Clinically observed sex differences in acute and long-term taxane chemotherapy-induced normal tissue toxicity are routinely documented but remain poorly understood despite the significant impact such toxicities have on treatment tolerance and quality of life outcomes in cancer survivors. This review draws from pre-clinical and clinical literature to highlight sex-specific mechanisms of action in taxane drug toxicity and proposes hypotheses for sex-specific clinical discrepancies in taxane-induced acute and long-term toxicities. To our knowledge, this is the first review exploring how sex as a biological variable impacts taxane-mediated mechanisms of action and clinical outcomes. In doing so, we have provided a novel framework to investigate and understand common sex differences observed in clinical and pre-clinical research.

Abstract

The taxane family of microtubule poisons and chemotherapeutics have been studied for over 50 years and are among the most frequently used antineoplastic agents today. Still, limited research exists characterizing taxane-induced sex-specific mechanisms of action and toxicities in cancer and non-cancerous tissue. Such research is important to advance cancer treatment outcomes as well as to address clinically observed sex-differences in short- and long-term taxane-induced toxicities that have disproportionate effects on female and male cancer patients. To gain more insight into these underlying differences between the sexes, the following review draws from pre-clinical and clinical paclitaxel and taxane oncology literature, examines sex-discrepancies, and highlights uncharacterized sex-dependent mechanisms of action and clinical outcomes. To our knowledge, this is the first literature review to provide a current overview of the basic and clinical sex dimorphisms of taxane-induced effects. Most importantly, we hope to provide a starting point for improving and advancing sex-specific personalized chemotherapy and cancer treatment strategies as well as to present a novel approach to review sex as a biological variable in basic and clinical biology.

The Microglial Activation Inhibitor Minocycline, Used Alone and in Combination with Duloxetine, Attenuates Pain Caused by Oxaliplatin in Mice

The antitumor drug, oxaliplatin, induces neuropathic pain, which is resistant to available analgesics, and novel mechanism-based therapies are being evaluated for this debilitating condition. Since activated microglia, impaired serotonergic and noradrenergic neurotransmission and overexpressed sodium channels are implicated in oxaliplatin-induced pain, this in vivo study assessed the effect of minocycline, a microglial activation inhibitor used alone or in combination with ambroxol, a sodium channel blocker, or duloxetine, a serotonin and noradrenaline reuptake inhibitor, on oxaliplatin-induced tactile allodynia and cold hyperalgesia. To induce neuropathic pain, a single dose (10 mg/kg) of intraperitoneal oxaliplatin was used. The mechanical and cold pain thresholds were assessed using mouse von Frey and cold plate tests, respectively. On the day of oxaliplatin administration, only duloxetine (30 mg/kg) and minocycline (100 mg/kg) used alone attenuated both tactile allodynia and cold hyperalgesia 1 h and 6 h after administration. Minocycline (50 mg/kg), duloxetine (10 mg/kg) and combined minocycline + duloxetine influenced only tactile allodynia. Seven days after oxaliplatin, tactile allodynia (but not cold hyperalgesia) was attenuated by minocycline (100 mg/kg), duloxetine (30 mg/kg) and combined minocycline and duloxetine. These results indicate a potential usefulness of minocycline used alone or combination with duloxetine in the treatment of oxaliplatin-induced pain.

Targeting the endocannabinoid system for management of HIV-associated neuropathic pain: A systematic review

Human immunodeficiency virus (HIV) infection and antiretroviral therapy can independently induce HIV-associated neuropathic pain (HIV-NP). There is a dearth of drugs or therapeutic modalities that can alleviate HIV-NP. Smoked cannabis has been reported to improve pain measures in patients with neuropathic pain. Cannabis, phytocannabinoids, and the endocannabinoids such N-arachidonoylethanolamine (anandamide; AEA) and 2-arachidonoylglycerol (2-AG), produce some of their effects via cannabinoid receptors (CBRs). Endocannabinoids are degraded by various enzymes such as fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase. We searched PubMed, Google Scholar, clinicaltrials.gov and clinicaltrialsregister.eu using various key words and their combinations for published papers that studied HIV-NP and cannabis, cannabinoids, or endocannabinoids up to 27th December 2020. All original research articles that evaluated the efficacy of molecules that modulate the endocannabinoid system (ECS) for the prevention and/or treatment of pain in HIV-NP animal models and patients with HIV-NP were included. The PubMed search produced a total of 117 articles, whereas the Google Scholar search produced a total of 9467 articles. Amongst the 13 articles that fulfilled the inclusion criteria 11 articles were found in both searches whereas 2 articles were found in Google Scholar only. The clinicaltrials.gov and clinicaltrialsregister.eu searches produced five registered trials of which three were completed and with results. Ten preclinical studies found that the endocannabinoids (2-AG and AEA), synthetic mixed CB1R/CB2R agonist WIN 55,212-2, a CB2R-selective phytocannabinoid β-caryophyllene, synthetic CB2R-selective agonists (AM1710, JWH015, JWH133 and Gp1a, but not HU308); FAAH inhibitors (palmitoylallylamide, URB597 and PF-3845) and a drug combination of indomethacin plus minocycline, which produces its effects in a CBR-dependent manner, either prevented the development of and/or attenuated established HIV-NP. Two clinical trials demonstrated greater efficacy of smoked cannabis over placebo in alleviating HIV-NP, whereas another clinical trial demonstrated that cannabidivarin, a cannabinoid that does not activate CBRs, did not reduce HIV-NP. The available preclinical results suggest that targeting the ECS for prevention and treatment of HIV-NP is a plausible therapeutic option. Clinical evidence shows that smoked cannabis alleviates HIV-NP. Further research is needed to find out if non-psychoactive drugs that target the ECS and are delivered by other routes than smoking could be useful as treatment options for HIV-NP.

Taxane-induced neurotoxicity: Pathophysiology and therapeutic perspectives

Taxane-derived drugs are antineoplastic agents used for the treatment of highly common malignancies. Paclitaxel and docetaxel are the most commonly used taxanes; however, other drugs and formulations have been used, such as cabazitaxel and nab-paclitaxel. Taxane treatment is associated with neurotoxicity, a well-known and relevant side effect, very prevalent amongst patients undergoing chemotherapy. Painful peripheral neuropathy is the most dose-limiting side effect of taxanes, affecting up to 97% of paclitaxel-treated patients. Central neurotoxicity is an emerging side effect of taxanes and it is characterized by cognitive impairment and encephalopathy. Besides impairing compliance to chemotherapy treatment, taxane-induced neurotoxicity (TIN) can adversely affect the patient's life quality on a long-term basis. Despite the clinical relevance, not many reviews have comprehensively addressed taxane-induced neurotoxicity when they are used therapeutically. This article provides an up-to-date review on the pathophysiology of TIN and the novel potential therapies to prevent or treat this side effect.

β-Caryophyllene, a CB2-Receptor-Selective Phytocannabinoid, Suppresses Mechanical Allodynia in a Mouse Model of Antiretroviral-Induced Neuropathic Pain

Neuropathic pain associated with nucleoside reverse transcriptase inhibitors (NRTIs), therapeutic agents for human immunodeficiency virus (HIV), responds poorly to available drugs. Smoked cannabis was reported to relieve HIV-associated neuropathic pain in clinical trials. Some constituents of cannabis (Cannabis sativa) activate cannabinoid type 1 (CB1) and cannabinoid type 2 (CB2) receptors. However, activation of the CB1 receptor is associated with side effects such as psychosis and physical dependence. Therefore, we investigated the effect of β-caryophyllene (BCP), a CB2-selective phytocannabinoid, in a model of NRTI-induced neuropathic pain. Female BALB/c mice treated with 2′-3′-dideoxycytidine (ddC, zalcitabine), a NRTI, for 5 days developed mechanical allodynia, which was prevented by cotreatment with BCP, minocycline or pentoxifylline. A CB2 receptor antagonist (AM 630), but not a CB1 receptor antagonist (AM 251), antagonized BCP attenuation of established ddC-induced mechanical allodynia. β-Caryophyllene prevented the ddC-induced increase in cytokine (interleukin 1 beta, tumor necrosis factor alpha and interferon gamma) transcripts in the paw skin and brain, as well as the phosphorylation level of Erk1/2 in the brain. In conclusion, BCP prevents NRTI-induced mechanical allodynia, possibly via reducing the inflammatory response, and attenuates mechanical allodynia through CB2 receptor activation. Therefore, BCP could be useful for prevention and treatment of antiretroviral-induced neuropathic pain.

The effect of analgesics on stimulus evoked pain-like behaviour in animal models for chemotherapy induced peripheral neuropathy- a meta-analysis

Chemotherapy induced painful peripheral neuropathy (CIPN) is a common dose-limiting side effect of several chemotherapeutic agents. Despite large amounts of human and animal studies, there is no sufficiently effective pharmacological treatment for CIPN. Although reducing pain is often a focus of CIPN treatment, remarkably few analgesics have been tested for this indication in clinical trials. We conducted a systematic review and meta-analyses regarding the effects of analgesics on stimulus evoked pain-like behaviour during CIPN in animal models. This will form a scientific basis for the development of prospective human clinical trials. A comprehensive search identified forty-six studies. Risk of bias (RoB) analyses revealed that the design and conduct of the included experiments were poorly reported, and therefore RoB was unclear in most studies. Meta-analyses showed that administration of analgesics significantly increases pain threshold for mechanical (SMD: 1.68 [1.41; 1.82]) and cold (SMD: 1. 41 [0.99; 1.83]) evoked pain. Subgroup analyses revealed that dexmedetomidine, celecoxib, fentanyl, morphine, oxycodone and tramadol increased the pain threshold for mechanically evoked pain, and lidocaine and morphine for cold evoked pain. Altogether, this meta-analysis shows that there is ground to investigate the use of morphine in clinical trials. Lidocaine, dexmedetomidine, celecoxib, fentanyl, oxycodone and tramadol might be good alternatives, but more animal-based research is necessary.

Indomethacin Enhances Type 1 Cannabinoid Receptor Signaling

In addition to its known actions as a non-selective cyclooxygenase (COX) 1 and 2 inhibitor, we hypothesized that indomethacin can act as an allosteric modulator of the type 1 cannabinoid receptor (CB1R) because of its shared structural features with the known allosteric modulators of CB1R. Indomethacin enhanced the binding of [³H]CP55940 to hCB1R and enhanced AEA-dependent [³⁵S]GTPγS binding to hCB1R in Chinese hamster ovary (CHO) cell membranes. Indomethacin (1 μM) also enhanced CP55940-dependent βarrestin1 recruitment, cAMP inhibition, ERK1/2 and PLCβ3 phosphorylation in HEK293A cells expressing hCB1R, but not in cells expressing hCB2R. Finally, indomethacin enhanced the magnitude and duration of CP55940-induced hypolocomotion, immobility, hypothermia, and anti-nociception in C57BL/6J mice. Together, these data support the hypothesis that indomethacin acted as a positive allosteric modulator of hCB1R. The identification of structural and functional features shared amongst allosteric modulators of CB1R may lead to the development of novel compounds designed for greater CB1R or COX selectivity and compounds designed to modulate both the prostaglandin and endocannabinoid systems.

Indomethacin plus minocycline coadministration relieves chemotherapy and antiretroviral drug-induced neuropathic pain in a cannabinoid receptors-dependent manner

Neuropathic pain sometimes occurs during chemotherapy with paclitaxel or HIV/AIDS antiretroviral therapy with nucleoside reverse transcriptase inhibitors (NRTIs). We previously reported that coadministration of indomethacin plus minocycline (IPM) was antihyperalgesic in a cannabinoid type 1 (CB1) receptor-dependent manner in a mouse model of paclitaxel-induced neuropathic pain. We evaluated if IPM combination has antihyperalgesic and antiallodynic activities in animal models of paclitaxel or NRTI (ddC, zalcitabine)-induced neuropathic pain, and whether antagonists of CB1, CB2 receptors or G protein-coupled receptor 55 (GPR55) can inhibit these activities of IPM. IPM produced antihyperalgesic and antiallodynic effects against paclitaxel and ddC-induced thermal hyperalgesia and mechanical allodynia. WIN 55,212-2, a cannabinoid receptor agonist, also had antihyperalgesic activity. The antihyperalgesic and antiallodynic activities of IPM were antagonized by a CB1 receptor antagonist AM251 and a CB2 receptor antagonist AM630, but not a GPR55 antagonist ML193. IPM had no effects on the mean time spent on the rotarod, whereas WIN 55,212-2 reduced it in a dose-dependent manner. These results show that IPM at a fixed ratio produces antihyperalgesic and antiallodynic effects in mice models of both paclitaxel and NRTI-induced neuropathic pain which is dependent on both CB1 and CB2 receptors, without causing the typical cannabinoid receptor agonist-induced motor impairment.

Targeting the Endocannabinoid System for Prevention or Treatment of Chemotherapy-Induced Neuropathic Pain: Studies in Animal Models

There is a scarcity of drugs to either prevent or properly manage chemotherapy-induced neuropathic pain (CINP). Cannabis or cannabinoids have been reported to improve pain measures in patients with neuropathic pain. For this review, a search was done in PubMed for papers that examined the expression of and/or evaluated the use of cannabinoids or drugs that prevent or treat established CINP in a CB receptor-dependent manner in animal models. Twenty-eight articles that fulfilled the inclusion and exclusion criteria established were analysed. Studies suggest there is a specific deficiency of endocannabinoids in the periphery during CINP. Inhibitors of FAAH and MGL, enzymes that degrade the endocannabinoids, CB receptor agonists, desipramine, and coadministered indomethacin plus minocycline were found to either prevent the development and/or attenuate established CINP in a CB receptor-dependent manner. The studies analysed suggest that targeting the endocannabinoid system for prevention and treatment of CINP is a plausible therapeutic option. Almost 90% of the studies on animal models of CINP analysed utilised male rodents. Taking into consideration clinical and experimental findings that show gender differences in the mechanisms involved in pain including CINP and in response to analgesics, it is imperative that future studies on CINP utilise more female models.

Antiallodynic effect of β-caryophyllene on paclitaxel-induced peripheral neuropathy in mice

Painful peripheral neuropathy is a common side effect of paclitaxel (PTX). The use of analgesics is an important component for management of PTX-induced peripheral neuropathy (PINP). However, currently employed analgesics have several side effects and are poorly effective. β-caryophyllene (BCP), a dietary selective CB₂ agonist, has shown analgesic effect in neuropathic pain models, but its role in chemotherapy-induced neuropathic pain has not yet been investigated. Herein, we used the mouse model of PINP to show the therapeutic effects of BCP in this neuropathy. Male Swiss mice receiving PTX (2 mg kg^-1, ip, four alternate days) were treated with BCP (25 mg kg^-1, po, twice a day) either during or after PTX administration. Some groups were also pretreated with AM630 (CB₂ antagonist, 3 mg kg^-1, ip) or AM251 (CB₁ antagonist, 1 mg kg^-1, ip). Spinal cord samples were collected in different time points to perform immunohistochemical analysis. BCP attenuated the established mechanical allodynia induced by PTX (p < 0.0001) in a CB₂-dependent manner. Of note, when given concomitantly with PTX, BCP was able to attenuate the development of PINP (p < 0.0001). Spinal cord immunohistochemistry revealed that preventive treatment with BCP reduced p38 MAPK and NF-κB activation, as well as the increased Iba-1 and IL-1β immunoreactivity promoted by PTX. Our findings show that BCP effectively attenuated PINP, possibly through CB₂-activation in the CNS and posterior inhibition of p38 MAPK/NF-κB activation and cytokine release. Taken together, our results suggest that BCP could be used to attenuate the establishment and/or treat PINP.

Core-shell nanocarriers with high paclitaxel loading for passive and active targeting

Rapid blood clearance and premature burst release are inherent drawbacks of conventional nanoparticles, resulting in poor tumor selectivity. iRGD peptide is widely recognized as an efficient cell membrane penetration peptide homing to αVβ3 integrins. Herein, core-shell nanocapsules (NCs) and iRGD-modified NCs (iRGD-NCs) with high drug payload for paclitaxel (PTX) were prepared to enhance the antitumor activities of chemotherapy agents with poor water solubility. Improved in vitro and in vivo tumor targeting and penetration were observed with NCs and iRGD-NCs; the latter exhibited better antitumor activity because iRGD enhanced the accumulation and penetration of NCs in tumors. The NCs were cytocompatible, histocompatible, and non-toxic to other healthy tissues. The endocytosis of NCs was mediated by lipid rafts in an energy-dependent manner, leading to better cytotoxicity of PTX against cancer cells. In contrast with commercial product, PTX-loaded NCs (PTX-NCs) increased area under concentration-time curve (AUC) by about 4-fold, prolonged mean resident time (MRT) by more than 8-fold and reduced the elimination rate constant by greater than 68-fold. In conclusion, the present nanocarriers with high drug-loading capacity represent an efficient tumor-targeting drug delivery system with promising potential for cancer therapy.

Minocycline enhances inhibitory transmission to substantia gelatinosa neurons of the rat spinal dorsal horn

Minocycline, a second-generation tetracycline, is well known for its antibiotic, anti-inflammatory, and antinociceptive effects. Modulation of synaptic transmission is one of the analgesic mechanisms of minocycline. Although it has been reported that minocycline may suppress excitatory glutamatergic synaptic transmission, it remains unclear whether it could affect inhibitory synaptic transmission, which also plays a key role in modulating pain signaling. To examine the effect of minocycline on synaptic transmission in rat spinal substantia gelatinosa (SG) neurons, we recorded spontaneous inhibitory postsynaptic currents (sIPSCs) using whole-cell patch-clamp recording at a holding potential of 0 mV. Bath application of minocycline significantly increased the frequency but not the amplitude of sIPSCs in a reversible and concentration-dependent manner with an EC50 of 85. The enhancement of inhibitory synaptic transmission produced by minocycline was not affected by the glutamate receptor antagonists CNQX and D-APV or by the voltage-gated sodium channel blocker tetrodotoxin (TTX). Moreover, the potency of minocycline for facilitating sIPSC frequency was the same in both glycinergic and GABAergic sIPSCs without changing their decay phases. However, the facilitatory effect of minocycline on sIPSCs was eliminated in a Ca(2+)-free Krebs solution or by co-administration with calcium channel blockers. In summary, our data demonstrate that baseline inhibitory synaptic transmission in SG neurons is markedly enhanced by minocycline. This may function to decrease the excitability of SG neurons, thus leading to a modulation of nociceptive transmission.