Fluvoxamine Exerts Sigma-1R to Rescue Autophagy via Pom121-Mediated Nucleocytoplasmic Transport of TFEB

Expansion of the GGGGCC-RNA repeat is a known cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), which currently have no cure. Recent studies have indicated the activation of Sigma-1 receptor plays an important role in providing neuroprotection, especially in ALS and Alzheimer's disease. Nevertheless, the mechanisms underlying Sigma-1R activation and its effect on (G4C2)n-RNA-induced cell death remain unclear. In this study, we demonstrated that fluvoxamine is a Sigma-1R agonist that can increase chaperone activity and stabilize the protein expression of Pom121 in (G4C2)31-RNA-expressing NSC34 cells, leading to increased colocalization at the nuclear envelope. Interestingly, fluvoxamine treatment increased Pom121 protein expression without affecting transcription. In C9orf72-ALS, the nuclear translocation of TFEB autophagy factor decreased owing to nucleocytoplasmic transport defects. Our results showed that pretreatment of NSC34 cells with fluvoxamine promoted the shuttling of TFEB into the nucleus and elevated the expression of LC3-II compared to the overexpression of (G4C2)31-RNA alone. Additionally, even when used alone, fluvoxamine increases Pom121 expression and TFEB translocation. To summarize, fluvoxamine may act as a promising repurposed medicine for patients with C9orf72-ALS, as it stabilizes the nucleoporin Pom121 and promotes the translocation of TFEB in (G4C2)31-RNA-expressing NSC34 cells.

Astrocytic Cebpd Regulates Pentraxin 3 Expression to Promote Fibrotic Scar Formation After Spinal Cord Injury

Astroglial-fibrotic scars resulted from spinal cord injury affect motor and sensory function, leading to paralysis. In particular, the fibrotic scar is a main barrier that disrupts neuronal regeneration after spinal cord injury. However, the association between astrocytes and fibrotic scar formation is not yet understood. We have previously demonstrated that the transcriptional factor Cebpd contributes to astrogliosis, which promotes glial scar formation after spinal cord injury. Herein, we show that fibrotic scar formation was decreased in the epicenter region in Cebpd^-/- mice after contusive spinal cord injury and astrocytic Cebpd promoted fibroblast migration through secretion of Ptx3. Furthermore, the expression of Mmp3 was increased under recombinant protein Ptx3 treatment in fibroblasts by observing microarray data, resulting in fibroblast migration. In addition, regulation of Mmp3 occurs through the NFκB signaling pathway by using an irreversible inhibitor of IκBα phosphorylation in pretreated fibroblasts. Of note, we used the synthetic peptide RI37, which blocks fibroblast migration and decreases fibroblast Mmp3 expression in IL-1β-treated astrocyte conditioned media. Collectively, our data suggest that fibroblast migration can be affected by astrocytic Cebpd through the Ptx3/NFκB/Mmp3 axis pathway and that the RI37 peptide may act as a therapeutic medicine to inhibit fibrotic scar formation after spinal cord injury.

Nucleoporin POM121 signals TFEB-mediated autophagy via activation of SIGMAR1/sigma-1 receptor chaperone by pridopidine

Macroautophagy/autophagy is an essential process for cellular survival and is implicated in many diseases. A critical step in autophagy is the transport of the transcription factor TFEB from the cytosol into the nucleus, through the nuclear pore (NP) by KPNB1/importinβ1. In the C9orf72 subtype of amyotrophic lateral sclerosis-frontotemporal lobar degeneration (ALS-FTD), the hexanucleotide (G4C2)RNA expansion (HRE) disrupts the nucleocytoplasmic transport of TFEB, compromising autophagy. Here we show that a molecular chaperone, the SIGMAR1/Sigma-1 receptor (sigma non-opioid intracellular receptor 1), facilitates TFEB transport into the nucleus by chaperoning the NP protein (i.e., nucleoporin) POM121 which recruits KPNB1. In NSC34 cells, HRE reduces TFEB transport by interfering with the association between SIGMAR1 and POM121, resulting in reduced nuclear levels of TFEB, KPNB1, and the autophagy marker LC3-II. Overexpression of SIGMAR1 or POM121, or treatment with the highly selective and potent SIGMAR1 agonist pridopidine, currently in phase 2/3 clinical trials for ALS and Huntington disease, rescues all of these deficits. Our results implicate nucleoporin POM121 not merely as a structural nucleoporin, but also as a chaperone-operated signaling molecule enabling TFEB-mediated autophagy. Our data suggest the use of SIGMAR1 agonists, such as pridopidine, for therapeutic development of diseases in which autophagy is impaired.Abbreviations: ALS-FTD, amyotrophic lateral sclerosis-frontotemporal dementiaC9ALS-FTD, C9orf72 subtype of amyotrophic lateral sclerosis-frontotemporal dementiaCS, citrate synthaseER, endoplasmic reticulumGSS, glutathione synthetaseHRE, hexanucleotide repeat expansionHSPA5/BiP, heat shock protein 5LAMP1, lysosomal-associated membrane protein 1MAM, mitochondria-associated endoplasmic reticulum membraneMAP1LC3/LC3, microtubule-associated protein 1 light chain 3NP, nuclear poreNSC34, mouse motor neuron-like hybrid cell lineNUPs, nucleoporinsPOM121, nuclear pore membrane protein 121SIGMAR1/Sigma-1R, sigma non-opioid intracellular receptor 1TFEB, transcription factor EBTMEM97/Sigma-2R, transmembrane protein 97.

Knocking Out Sigma-1 Receptors Reveals Diverse Health Problems

Sigma-1 receptor (Sig-1R) is a protein present in several organs such as brain, lung, and heart. In a cell, Sig-1R is mainly located across the membranes of the endoplasmic reticulum and more specifically at the mitochondria-associated membranes. Despite numerous studies showing that Sig-1R could be targeted to rescue several cellular mechanisms in different pathological conditions, less is known about its fundamental relevance. In this review, we report results from various studies and focus on the importance of Sig-1R in physiological conditions by comparing Sig-1R KO mice to wild-type mice in order to investigate the fundamental functions of Sig-1R. We note that the Sig-1R deletion induces cognitive, psychiatric, and motor dysfunctions, but also alters metabolism of heart. Finally, taken together, observations from different experiments demonstrate that those dysfunctions are correlated to poor regulation of ER and mitochondria metabolism altered by stress, which could occur with aging.

Genomic Action of Sigma-1 Receptor Chaperone Relates to Neuropathic Pain

Sigma-1 receptors (Sig-1Rs) are endoplasmic reticulum (ER) chaperones implicated in neuropathic pain. Here we examine if the Sig-1R may relate to neuropathic pain at the level of dorsal root ganglia (DRG). We focus on the neuronal excitability of DRG in a "spare nerve injury" (SNI) model of neuropathic pain in rats and find that Sig-1Rs likely contribute to the genesis of DRG neuronal excitability by decreasing the protein level of voltage-gated Cav2.2 as a translational inhibitor of mRNA. Specifically, during SNI, Sig-1Rs translocate from ER to the nuclear envelope via a trafficking protein Sec61β. At the nucleus, the Sig-1R interacts with cFos and binds to the promoter of 4E-BP1, leading to an upregulation of 4E-BP1 that binds and prevents eIF4E from initiating the mRNA translation for Cav2.2. Interestingly, in Sig-1R knockout HEK cells, Cav2.2 is upregulated. In accordance with those findings, we find that intra-DRG injection of Sig-1R agonist (+)pentazocine increases frequency of action potentials via regulation of voltage-gated Ca2+ channels. Conversely, intra-DRG injection of Sig-1R antagonist BD1047 attenuates neuropathic pain. Hence, we discover that the Sig-1R chaperone causes neuropathic pain indirectly as a translational inhibitor.

Sigma-1 receptor chaperones rescue nucleocytoplasmic transport deficit seen in cellular and Drosophila ALS/FTD models

In a subgroup of patients with amyotrophic lateral sclerosis (ALS)/Frontotemporal dementia (FTD), the (G4C2)-RNA repeat expansion from C9orf72 chromosome binds to the Ran-activating protein (RanGAP) at the nuclear pore, resulting in nucleocytoplasmic transport deficit and accumulation of Ran in the cytosol. Here, we found that the sigma-1 receptor (Sig-1R), a molecular chaperone, reverses the pathological effects of (G4C2)-RNA repeats in cell lines and in Drosophila. The Sig-1R colocalizes with RanGAP and nuclear pore proteins (Nups) and stabilizes the latter. Interestingly, Sig-1Rs directly bind (G4C2)-RNA repeats. Overexpression of Sig-1Rs rescues, whereas the Sig-1R knockout exacerbates, the (G4C2)-RNA repeats-induced aberrant cytoplasmic accumulation of Ran. In Drosophila, Sig-1R (but not the Sig-1R-E102Q mutant) overexpression reverses eye necrosis, climbing deficit, and firing discharge caused by (G4C2)-RNA repeats. These results on a molecular chaperone at the nuclear pore suggest that Sig-1Rs may benefit patients with C9orf72 ALS/FTD by chaperoning the nuclear pore assembly and sponging away deleterious (G4C2)-RNA repeats.

Increased thrombospondin-4 after nerve injury mediates disruption of intracellular calcium signaling in primary sensory neurons

Painful nerve injury disrupts Ca²⁺ signaling in primary sensory neurons by elevating plasma membrane Ca²⁺-ATPase (PMCA) function and depressing sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA) function, which decreases endoplasmic reticulum (ER) Ca²⁺ stores and stimulates store-operated Ca²⁺ entry (SOCE). The extracellular matrix glycoprotein thrombospondin-4 (TSP4), which is increased after painful nerve injury, decreases Ca²⁺ current (I_Ca) through high-voltage-activated Ca²⁺ channels and increases I_Ca through low-voltage-activated Ca²⁺ channels in dorsal root ganglion neurons, which are events similar to the effect of nerve injury. We therefore examined whether TSP4 plays a critical role in injury-induced disruption of intracellular Ca²⁺ signaling. We found that TSP4 increases PMCA activity, inhibits SERCA, depletes ER Ca²⁺ stores, and enhances store-operated Ca²⁺ influx. Injury-induced changes of SERCA and PMCA function are attenuated in TSP4 knock-out mice. Effects of TSP4 on intracellular Ca²⁺ signaling are attenuated in voltage-gated Ca²⁺ channel α₂δ₁ subunit (Ca_vα₂δ₁) conditional knock-out mice and are also Protein Kinase C (PKC) signaling dependent. These findings suggest that TSP4 elevation may contribute to the pathogenesis of chronic pain following nerve injury by disrupting intracellular Ca²⁺ signaling via interacting with the Ca_vα₂δ₁ and the subsequent PKC signaling pathway. Controlling TSP4 mediated intracellular Ca²⁺ signaling in peripheral sensory neurons may be a target for analgesic drug development for neuropathic pain.

Cardiomyocyte GTP Cyclohydrolase 1 Protects the Heart Against Diabetic Cardiomyopathy

Diabetic cardiomyopathy increases the risk of heart failure and death. At present, there are no effective approaches to preventing its development in the clinic. Here we report that reduction of cardiac GTP cyclohydrolase 1 (GCH1) degradation by genetic and pharmacological approaches protects the heart against diabetic cardiomyopathy. Diabetic cardiomyopathy was induced in C57BL/6 wild-type mice and transgenic mice with cardiomyocyte-specific overexpression of GCH1 with streptozotocin, and control animals were given citrate buffer. We found that diabetes-induced degradation of cardiac GCH1 proteins contributed to adverse cardiac remodeling and dysfunction in C57BL/6 mice, concomitant with decreases in tetrahydrobiopterin, dimeric and phosphorylated neuronal nitric oxide synthase, sarcoplasmic reticulum Ca(2+) handling proteins, intracellular [Ca(2+)]i, and sarcoplasmic reticulum Ca(2+) content and increases in phosphorylated p-38 mitogen-activated protein kinase and superoxide production. Interestingly, GCH-1 overexpression abrogated these detrimental effects of diabetes. Furthermore, we found that MG 132, an inhibitor for 26S proteasome, preserved cardiac GCH1 proteins and ameliorated cardiac remodeling and dysfunction during diabetes. This study deepens our understanding of impaired cardiac function in diabetes, identifies GCH1 as a modulator of cardiac remodeling and function, and reveals a new therapeutic target for diabetic cardiomyopathy.

Combining patient proteomics and in vitro cardiomyocyte phenotype testing to identify potential mediators of heart failure with preserved ejection fraction

BACKGROUND

Heart failure with ejection fraction (HFpEF) is a syndrome resulting from several co-morbidities in which specific mediators are unknown. The platelet proteome responds to disease processes. We hypothesize that the platelet proteome will change composition in patients with HFpEF and may uncover mediators of the syndrome.

METHODS AND RESULTS

Proteomic changes were assessed in platelets from hospitalized subjects with symptoms of HFpEF (n = 9), the same subjects several weeks later without symptoms (n = 7) and control subjects (n = 8). Mass spectrometry identified 6102 proteins with five scans with peptide probabilities of ≥0.85. Of the 6102 proteins, 165 were present only in symptomatic subjects, 78 were only found in outpatient subjects and 157 proteins were unique to the control group. The S100A8 protein was identified consistently in HFpEF samples when compared with controls. We validated the fining that plasma S100A8 levels are increased in subjects with HFpEF (654 ± 391) compared to controls (352 ± 204) in an external cohort (p = 0.002). Recombinant S100A8 had direct effects on the electrophysiological and calcium handling profile in human induced pluripotent stem cell-derived cardiomyocytes.

CONCLUSIONS

Platelets may harbor proteins associated with HFpEF. S100A8 is present in the platelets of subjects with HFpEF and increased in the plasma of the same subjects. We further established a bedside-to-bench translational system that can be utilized as a secondary screen to ascertain whether the biomarkers may be an associated finding or causal to the disease process. S100A8 has been linked with other cardiovascular disease such as atherosclerosis and risk for myocardial infarction, stroke, or death. This is the first report on association of S100A8 with HFpEF.

Analgesia for neuropathic pain by dorsal root ganglion transplantation of genetically engineered mesenchymal stem cells: initial results

Background

Cell-based therapy may hold promise for treatment of chronic pain. Mesenchymal stem cells (MSCs) are readily available and robust, and their secretion of therapeutic peptides can be enhanced by genetically engineering. We explored the analgesic potential of transplanting bone marrow-derived MSCs that have been transduced with lentivectors. To optimize efficacy and safety, primary sensory neurons were targeted by MSC injection into the dorsal root ganglia (DRGs).

Results

MSCs were transduced using lentivectors to express enhanced green fluorescent protein (EGFP) or to co-express the analgesic peptide glial cell line-derived neurotrophic factor (GDNF) and EGFP by a viral 2A bicistronic transgene cassette. Engineered MSCs were injected into the 4^th lumbar (L4) and L5 DRGs of adult allogeneic rats to evaluate survival in the DRGs. MSCs were detected by immunofluorescence staining up to 2–3 weeks after injection, distributed in the extracellular matrix space without disrupting satellite glial cell apposition to sensory neurons, suggesting well-tolerated integration of engrafted MSCs into DRG tissue. To examine their potential for inhibiting development of neuropathic pain, MSCs were injected into the L4 and L5 DRGs ipsilateral to a spinal nerve ligation injury. Animals injected with GDNF-engineered MSCs showed moderate but significant reduction in mechanical allodynia and hyperalgesia compared to controls implanted with MSCs expressing EGFP alone. We also observed diminished long-term survival of allografted MSCs at 3 weeks, and the development of a highly-proliferating population of MSCs in 12% of DRGs after transplantation.

Conclusions

These data indicate that genetically modified MSCs secreting analgesic peptides could potentially be developed as a novel DRG-targeted cell therapy for treating neuropathic pain. However, further work is needed to address the challenges of MSC survival and excess proliferation, possibly with trials of autologous MSCs, evaluation of clonally selected populations of MSCs, and investigation of regulation of MSC proliferation.

Divergent effects of painful nerve injury on mitochondrial Ca(2+) buffering in axotomized and adjacent sensory neurons

Mitochondria critically regulate cytoplasmic Ca(2+) concentration ([Ca(2+)]c), but the effects of sensory neuron injury have not been examined. Using FCCP (1µM) to eliminate mitochondrial Ca(2+) uptake combined with oligomycin (10µM) to prevent ATP depletion, we first identified features of depolarization-induced neuronal [Ca(2+)]c transients that are sensitive to blockade of mitochondrial Ca(2+) buffering in order to assess mitochondrial contributions to [Ca(2+)]c regulation. This established the loss of a shoulder during the recovery of the depolarization (K(+))-induced transient, increased transient peak and area, and elevated shoulder level as evidence of diminished mitochondrial Ca(2+) buffering. We then examined transients in Control neurons and neurons from the 4th lumbar (L4) and 5th lumbar (L5) dorsal root ganglia after L5 spinal nerve ligation (SNL). The SNL L4 neurons showed decreased transient peak and area compared to control neurons, while the SNL L5 neurons showed increased shoulder level. Additionally, SNL L4 neurons developed shoulders following transients with lower peaks than Control neurons. Application of FCCP plus oligomycin elevated resting [Ca(2+)]c in SNL L4 neurons more than in Control neurons. Whereas application of FCCP plus oligomycin 2s after neuronal depolarization initiated mitochondrial Ca(2+) release in most Control and SNL L4 neurons, this usually failed to release mitochondrial Ca(2+) from SNL L5 neurons. For comparable cytoplasmic Ca(2+) loads, the releasable mitochondrial Ca(2+) in SNL L5 neurons was less than Control while it was increased in SNL L4 neurons. These findings show diminished mitochondrial Ca(2+) buffering in axotomized SNL L5 neurons but enhanced Ca(2+) buffering by neurons in adjacent SNL L4 neurons.

Regulation of voltage-gated Ca(2+) currents by Ca(2+)/calmodulin-dependent protein kinase II in resting sensory neurons

Calcium/calmodulin-dependent protein kinase II (CaMKII) is recognized as a key element in encoding depolarization activity of excitable cells into facilitated voltage-gated Ca(2+) channel (VGCC) function. Less is known about the participation of CaMKII in regulating VGCCs in resting cells. We examined constitutive CaMKII control of Ca(2+) currents in peripheral sensory neurons acutely isolated from dorsal root ganglia (DRGs) of adult rats. The small molecule CaMKII inhibitor KN-93 (1.0μM) reduced depolarization-induced ICa by 16-30% in excess of the effects produced by the inactive homolog KN-92. The specificity of CaMKII inhibition on VGCC function was shown by the efficacy of the selective CaMKII blocking peptide autocamtide-2-related inhibitory peptide in a membrane-permeable myristoylated form, which also reduced VGCC current in resting neurons. Loss of VGCC currents is primarily due to reduced N-type current, as application of mAIP selectively reduced N-type current by approximately 30%, and prior N-type current inhibition eliminated the effect of mAIP on VGCCs, while prior block of L-type channels did not reduce the effect of mAIP on total ICa. T-type currents were not affected by mAIP in resting DRG neurons. Transduction of sensory neurons in vivo by DRG injection of an adeno-associated virus expressing AIP also resulted in a loss of N-type currents. Together, these findings reveal a novel molecular adaptation whereby sensory neurons retain CaMKII support of VGCCs despite remaining quiescent.

Painful nerve injury increases plasma membrane Ca2+-ATPase activity in axotomized sensory neurons

BACKGROUND

The plasma membrane Ca2+-ATPase (PMCA) is the principal means by which sensory neurons expel Ca2+ and thereby regulate the concentration of cytoplasmic Ca2+ and the processes controlled by this critical second messenger. We have previously found that painful nerve injury decreases resting cytoplasmic Ca2+ levels and activity-induced cytoplasmic Ca2+ accumulation in axotomized sensory neurons. Here we examine the contribution of PMCA after nerve injury in a rat model of neuropathic pain.

RESULTS

PMCA function was isolated in dissociated sensory neurons by blocking intracellular Ca2+ sequestration with thapsigargin, and cytoplasmic Ca2+ concentration was recorded with Fura-2 fluorometry. Compared to control neurons, the rate at which depolarization-induced Ca2+ transients resolved was increased in axotomized neurons after spinal nerve ligation, indicating accelerated PMCA function. Electrophysiological recordings showed that blockade of PMCA by vanadate prolonged the action potential afterhyperpolarization, and also decreased the rate at which neurons could fire repetitively.

CONCLUSION

We found that PMCA function is elevated in axotomized sensory neurons, which contributes to neuronal hyperexcitability. Accelerated PMCA function in the primary sensory neuron may contribute to the generation of neuropathic pain, and thus its modulation could provide a new pathway for peripheral treatment of post-traumatic neuropathic pain.

Low-dose memantine attenuated morphine addictive behavior through its anti-inflammation and neurotrophic effects in rats

Opioid abuse and dependency are international problems. Studies have shown that neuronal inflammation and degeneration might be related to the development of opioid addiction. Thus, using neuroprotective agents might be beneficial for treating opioid addiction. Memantine, an Alzheimer's disease medication, has neuroprotective effects in vitro and in vivo. In this study, we evaluated whether a low dose of memantine prevents opioid-induced drug-seeking behavior in rats and analyzed its mechanism. A conditioned-place-preference test was used to investigate the morphine-induced drug-seeking behaviors in rats. We found that a low-dose (0.2-1 mg/kg) of subcutaneous memantine significantly attenuated the chronic morphine-induced place-preference in rats. To clarify the effects of chronic morphine and low-dose memantine, serum and brain levels of cytokines and brain-derived neurotrophic factor (BDNF) were measured. After 6 days of morphine treatment, cytokine (IL-1β, IL-6) levels had significantly increased in serum; IL-1β and IL-6 mRNA levels had significantly increased in the nucleus accumbens and medial prefrontal cortex, both addiction-related brain areas; and BDNF levels had significantly decreased, both in serum and in addiction-related brain areas. Pretreatment with low-dose memantine significantly attenuated chronic morphine-induced increases in serum and brain cytokines. Low-dose memantine also significantly potentiated serum and brain BDNF levels. We hypothesize that neuronal inflammation and BDNF downregulation are related to the progression of opioid addiction. We hypothesize that the mechanism low-dose memantine uses to attenuate morphine-induced addiction behavior is its anti-inflammatory and neurotrophic effects.

(+)-Morphine attenuates the (-)-morphine-produced tail-flick inhibition via the sigma-1 receptor in the mouse spinal cord

AIMS

We have previously demonstrated that pretreatment with (+)-morphine given intrathecally attenuates the intrathecal (-)-morphine-produced tail-flick inhibition. The phenomenon has been defined as antianalgesia against (-)-morphine-produced analgesia. Present experiments were then undertaken to determine if the antianalgesic effect induced by (+)-morphine given spinally is mediated by the stimulation of the sigma-1 receptor in the mouse spinal cord.

MAIN METHODS

Sigma-1 receptor ligands, N-[2-(3,4-Dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine dihydrobromide (BD1047) and (+)-pentazocine were used to determine if (+)-morphine-induced antianalgesia is mediated by the stimulation of sigma-1 receptors in the mouse spinal cord. Tail-flick test was employed to measure the nociceptive response. All compounds were given intrathecally.

KEY FINDINGS

Pretreatment with BD1047 (1-10 μg) or (+)-pentazocine (0.1-10 μg) dose-dependently reversed the attenuation of the (-)-morphine-produced tail-flick inhibition induced by (+)-morphine (10 pg). BD1047 and (+)-pentazocine injected alone did not affect (-)-morphine-produced tail-flick inhibition.

SIGNIFICANCE

The finding indicates that (+)-morphine attenuates the (-)-morphine-produced tail-flick inhibition via the activation of the sigma-1 receptors in the mouse spinal cord. Sigma-1 receptors may play an important role in opioid analgesia in the mouse spinal cord.

Learned avoidance from noxious mechanical simulation but not threshold semmes weinstein filament stimulation after nerve injury in rats

Noxious mechanical stimulation evokes a complex and sustained hyperalgesic motor response after peripheral nerve injury that contrasts with a brief and simple withdrawal seen after noxious stimulation in control animals or after threshold punctate mechanical stimulation by the von Frey technique. To test which of these behaviors indicate pain, the aversiveness of the experience associated with each was determined using a passive avoidance test in rats after sciatic nerve ligation (SNL) or skin incision alone. After 18 days, step-down latency was measured during 9 sequential trials at 10-minute intervals. At each trial, rats received either no stimulus, needle stimuli, or threshold Semmes Weinstein (SW) filament stimuli after stepping down. Reactions were either a hyperalgesic response or a brief reflexive withdrawal. In SNL animals, needle stimulation produced substantial learned avoidance when animals showed hyperalgesic responses but produced minimal prolonged latency in SNL animals that showed only simple withdrawal responses. No learned avoidance developed using threshold SW testing in SNL animals. These findings show that needle stimulation is aversive in rats responding with hyperalgesic behavior. In contrast, SW stimulation, as well as needle stimulation that produced mere withdrawal, is minimally aversive.

PERSPECTIVE

The validity of measures of pain in animals is open to question. We demonstrated that needle stimulation is aversive in rats that respond with hyperalgesic-type behavior and is therefore a valid indicator of pain. Stimulation by SW is minimally aversive and is a problematic indicator of pain.

Axotomy depletes intracellular calcium stores in primary sensory neurons

BACKGROUND

The cellular mechanisms of neuropathic pain are inadequately understood. Previous investigations have revealed disrupted Ca signaling in primary sensory neurons after injury. The authors examined the effect of injury on intracellular Ca stores of the endoplasmic reticulum, which critically regulate the Ca signal and neuronal function.

METHODS

Intracellular Ca levels were measured with Fura-2 or mag-Fura-2 microfluorometry in axotomized fifth lumbar (L5) dorsal root ganglion neurons and adjacent L4 neurons isolated from hyperalgesic rats after L5 spinal nerve ligation, compared to neurons from control animals.

RESULTS

Endoplasmic reticulum Ca stores released by the ryanodine-receptor agonist caffeine decreased by 46% in axotomized small neurons. This effect persisted in Ca-free bath solution, which removes the contribution of store-operated membrane Ca channels, and after blockade of the mitochondrial, sarco-endoplasmic Ca-ATPase and the plasma membrane Ca ATPase pathways. Ca released by the sarco-endoplasmic Ca-ATPase blocker thapsigargin and by the Ca-ionophore ionomycin was also diminished by 25% and 41%, respectively. In contrast to control neurons, Ca stores in axotomized neurons were not expanded by neuronal activation by K depolarization, and the proportionate rate of refilling by sarco-endoplasmic Ca-ATPase was normal. Luminal Ca concentration was also reduced by 38% in axotomized neurons in permeabilized neurons. The adjacent neurons of the L4 dorsal root ganglia showed modest and inconsistent changes after L5 spinal nerve ligation.

CONCLUSIONS

Painful nerve injury leads to diminished releasable endoplasmic reticulum Ca stores and a reduced luminal Ca concentration. Depletion of Ca stores may contribute to the pathogenesis of neuropathic pain.

Nitric oxide activates ATP-sensitive potassium channels in mammalian sensory neurons: action by direct S-nitrosylation

Background

ATP-sensitive potassium (K_ATP) channels in neurons regulate excitability, neurotransmitter release and mediate protection from cell-death. Furthermore, activation of K_ATP channels is suppressed in DRG neurons after painful-like nerve injury. NO-dependent mechanisms modulate both K_ATP channels and participate in the pathophysiology and pharmacology of neuropathic pain. Therefore, we investigated NO modulation of K_ATP channels in control and axotomized DRG neurons.

Results

Cell-attached and cell-free recordings of K_ATP currents in large DRG neurons from control rats (sham surgery, SS) revealed activation of K_ATP channels by NO exogenously released by the NO donor SNAP, through decreased sensitivity to [ATP]i.

This NO-induced K_ATP channel activation was not altered in ganglia from animals that demonstrated sustained hyperalgesia-type response to nociceptive stimulation following spinal nerve ligation. However, baseline opening of K_ATP channels and their activation induced by metabolic inhibition was suppressed by axotomy. Failure to block the NO-mediated amplification of K_ATP currents with specific inhibitors of sGC and PKG indicated that the classical sGC/cGMP/PKG signaling pathway was not involved in the activation by SNAP. NO-induced activation of K_ATP channels remained intact in cell-free patches, was reversed by DTT, a thiol-reducing agent, and prevented by NEM, a thiol-alkylating agent. Other findings indicated that the mechanisms by which NO activates K_ATP channels involve direct S-nitrosylation of cysteine residues in the SUR1 subunit. Specifically, current through recombinant wild-type SUR1/Kir6.2 channels expressed in COS7 cells was activated by NO, but channels formed only from truncated isoform Kir6.2 subunits without SUR1 subunits were insensitive to NO. Further, mutagenesis of SUR1 indicated that NO-induced K_ATP channel activation involves interaction of NO with residues in the NBD1 of the SUR1 subunit.

Conclusion

NO activates K_ATP channels in large DRG neurons via direct S-nitrosylation of cysteine residues in the SUR1 subunit. The capacity of NO to activate K_ATP channels via this mechanism remains intact even after spinal nerve ligation, thus providing opportunities for selective pharmacological enhancement of K_ATP current even after decrease of this current by painful-like nerve injury.

(+)-Morphine attenuates the (-)-morphine-produced conditioned place preference and the mu-opioid receptor-mediated dopamine increase in the posterior nucleus accumbens of the rat

An unbiased conditioned place preference paradigm and the microdialysis technique was used to evaluate the effect of (+)-morphine pretreatment on the conditioned place preference produced by (-)-morphine and the increased release of the dopamine produced by mu-opioid ligand endomorphin-1, respectively, in the posterior nucleus accumbens shell of the male CD rat. (-)-Morphine (2.5-10 microg) microinjected into the posterior nucleus accumbens shell dose-dependently produced the conditioned place preference. Pretreatment with (+)-morphine (0.1-10 pg) given into the posterior accumbens shell for 45 min dose-dependently attenuated the conditioned place preference produced by (-)-morphine (5 microg) given into the same posterior accumbens shell. However, higher doses of (+)-morphine (0.1 and 1 ng) were less effective in attenuating the (-)-morphine-produced conditioned place preference. Thus, like given systemically, (+)-morphine given into the posterior nucleus accumbens shell also induces a U-shaped dose-response curve for attenuating the (-)-morphine-produced conditioned place preference. Microinjection of mu-opioid agonist endomorphin-1 (1-10 microg) given into the ventral tegmental area dose-dependently increased the release of the extracellular dopamine in the posterior nucleus accumbens shell in the urethane-anesthetized rats. The increased dopamine caused by endomorphin-1 (10 microg) was completed blocked by the (+)-morphine (10 pg) pretreatment given into ventral tegmental area. It is concluded that (+)-morphine attenuates the (-)-morphine-produced conditioned place preference and the mu-opioid receptor-mediated increase of extracellular dopamine in the posterior nucleus accumbens shell of the rat.

Antinociception produced by 14,15-epoxyeicosatrienoic acid is mediated by the activation of beta-endorphin and met-enkephalin in the rat ventrolateral periaqueductal gray

Cytochrome P450 genes catalyze formation of epoxyeicosatrienoic acids (EETs) from arachidonic acid. The effects of 5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET microinjected into the ventrolateral periaqueductal gray (vlPAG) on the thermally produced tail-flick response were studied in male Sprague-Dawley rats. 14,15-EET microinjected into vlPAG (3-156 pmol) dose-dependently inhibited the tail-flick response (ED50 = 32.5 pmol). In contrast, 5,6-EET, 8,9-EET, and 11,12-EET at a dose of 156 pmol were not active when injected into the vlPAG. 14,15-EET failed to displace the radiobinding of [3H][D-Ala2,NHPe4, Gly-ol5]enkephalin (mu-opioid receptor ligand) or [3H]naltrindole (delta-opioid receptor ligand) in crude membrane fractions of rat brain. Tail-flick inhibition produced by 14,15-EET from vlPAG was blocked by intra-vlPAG pretreatment with antiserum against beta-endorphin or Met-enkephalin or the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) or the delta-opioid receptor antagonist naltrindole but not with dynorphin A[1-17] antiserum or the kappa-opioid receptor antagonist nor-binaltorphimine. In addition, tail-flick inhibition produced by 14,15-EET treatment was blocked by intrathecal pretreatment with Met-enkephalin antiserum, naltrindole, or CTOP but not with beta-endorphin antiserum. It is concluded that 1) 14,15-EET itself does not have any affinity for mu- or delta-opioid receptors and 2) 14,15-EET activates beta-endorphin and Met-enkephalin, which subsequently act on mu- and delta-opioid receptors to produce antinociception.

dextro-Naloxone or levo-naloxone reverses the attenuation of morphine antinociception induced by lipopolysaccharide in the mouse spinal cord via a non-opioid mechanism

Glial stimulation by intrathecal injection of lipopolysaccharide (LPS) attenuated the tail-flick inhibition produced by morphine given intrathecally in the spinal cord of the male CD-1 mice. The phenomenon has been defined as antianalgesia. The effects of dextro-naloxone or levo-naloxone on the attenuation of morphine-produced tail-flick inhibition induced by LPS were then studied. Pretreatment with dextro-naloxone or levo-naloxone reversed the attenuation of the morphine-produced tail-flick inhibition induced by LPS. Pretreatment with dextro-naloxone or levo-naloxone alone did not affect the morphine-produced tail-flick inhibition. It is concluded that dextro-naloxone and levo-naloxone block the LPS-induced antianalgesia against morphine antinociception via a non-opioid mechanism.

Paradoxical hyperalgesia induced by mu-opioid receptor agonist endomorphin-2, but not endomorphin-1, microinjected into the centromedial amygdala of the rat

The effects of endomorphin-2 or endomorphin-1 microinjected into the centromedial amygdala on the thermally-induced tail-flick response were studied in male CD rats. Microinjection of endomorphin-2 (8.7-35.0 nmol) given into the centromedial amygdala time- and dose-dependently decreased the tail-flick latencies. On the other hand, endomorphin-1 (8-32.6 nmol) given into the same site did not cause any change of the tail-flick latency. However, endomorphin-1 (32.6 nmol) or endomorphin-2 (35.0 nmol) given into the basolateral site of amygdala did not affect the tail-flick latency. Pretreatment with the antiserum against dynorphin A(1-17) (200 microg) significantly reversed the decrease of the tail-flick latency induced by endomorphin-2. The decrease of the tail-flick latency induced by endomorphin-2 was also blocked by the endomorphin-2 selective micro-opioid receptor antagonist 3-methoxynaltrexone (6.4 pmol) and by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (30 nmol), but not by the kappa-opioid receptor antagonist nor-binaltorphimine (6.6 nmol). It is concluded that endomorphin-2, but not endomorphin-1, given into the centromedial amygdala stimulates a 3-methoxynaltrexone-sensitive mu-opioid receptor subtype to induce the release of dynorphin A(1-17), which then acts on the NMDA receptor, but not kappa-opioid receptor for producing hyperalgesia. This conclusion is further supported by the additional findings that dynorphin A(1-17) (2.3 nmol) given into the centromedial amygdala also caused the decrease of the tail-flick latency, which was similarly blocked by the NMDA receptor antagonist MK-801 (30 nmol), but not kappa-opioid receptor antagonist nor-binaltorphimine (6.6 nmol).

p38 mitogen-activated protein kinase inhibitor SB203580 reverses the antianalgesia induced by dextro-morphine or morphine in the mouse spinal cord

We have previously demonstrated that intrathecal pretreatment with dextro-morphine or morphine attenuates the morphine-produced antinociception. The phenomenon has been defined as antianalgesia, which is mediated by a non-opioid receptor [Wu, H., Thompson, J., Sun, H., Terashvili, M., Tseng, L.F., 2005. Antianalgesia: stereo-selective action of dextro-morphine over levo-morphine on glia in the mouse spinal cord. J. Pharmacol. Exp. Ther. 314, 1101-1108]. To determine if p38 mitogen-activated protein kinase (MAPK) is involved in the antianalgesia, the effects of p38 MAPK inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580) on the attenuation of the morphine-produced tail-flick inhibition induced by dextro-morphine or morphine were studied in male CD-1 mice. Intrathecal pretreatment with SB203580 (24.2 nmol) reversed the attenuation of the morphine-produced tail-flick inhibition induced by dextro-morphine (33 fmol) or morphine (0.3 nmol) pretreatment. The finding indicates that the antianalgesia induced by dextro-morphine or morphine is mediated by the activation of p38 MAPK in the mouse spinal cord.

Pretreatment with antiserum against dynorphin, substance P, or cholecystokinin enhances the morphine-produced anti-allodynia in the sciatic nerve ligated mice

It is generally accepted that neuropathic pain is resistant to amelioration by morphine in clinical studies and insensitivity to intrathecal (i.t.) administered morphine in experimental models of neuropathic pain has been demonstrated. This study is to determine if endogenous dynorphin, substance P or cholecystokinin is involved in the lack of anti-allodynia of morphine in a partial sciatic nerve ligation (PSL) model of CD-1 mice. Mice exhibited tactile allodynia in the ipsilateral hind paw 1 day after PSL, and reached its maximal allodynic effect at 2 days and remained allodynic for 7 days. Morphine (3.0 nmol) given i.t. did not alter the tactile allodynic threshold in ipsilateral paw of mice pretreated i.t. with normal rabbit serum 2 days after PSL. However, the same dose of morphine (3.0 nmol) given i.t. reduced markedly allodynia in mice pretreated for 2h with antiserum against dynorphin A(1-17) (200 microg); the morphine-produced anti-allodynia developed slowly, reached its peak effect at 30 min and returned to an allodynic state in 60 min. Similarly, i.t. injection of morphine reduced the allodynia in PSL mice pretreated with antiserum against substance P (10 microg) or cholecystokinin (200 microg) for 2h. Intrathecal pretreatment with antiserum against dynorphin A(1-17), substance P or cholecystokinin for 2h injected alone did not affect the baseline mechanical tactile threshold in ipsilateral paw 2 days after PSL. The results indicate that endogenous dynorphin A(1-17), substance P and cholecystokinin are involved in PSL-induced neuropathic allodynia to attenuate the anti-allodynic effect of morphine.

Antianalgesia: stereoselective action of dextro-morphine over levo-morphine on glia in the mouse spinal cord

We have previously shown that the naturally occurring levo-morphine at a subanalgesic picomolar dose pretreated i.t. induces antianalgesia against levo-morphine-produced antinociception. We now report that the synthetic stereo-enantiomer dextro-morphine, even at an extremely low femtomolar dose, induces antianalgesia against levo-morphine-produced antinociception using the tail-flick (TF) test in male CD-1 mice. Intrathecal pretreatment with dextro-morphine (33 fmol) time-dependently attenuated the i.t. levo-morphine-produced TF inhibition for 4 h and returned to the preinjection control level at 24 h. Intrathecal pretreatment with dextro-morphine (0.3-33 fmol), which injected alone did not affect the baseline TF latency, dose-dependently attenuated the TF inhibition produced by i.t.-administered levo-morphine (3.0 nmol). The ED(50) value for dextro-morphine to induce antianalgesia was estimated to be 1.07 fmol, which is 71,000-fold more potent than the ED(50) value of levo-morphine, indicating the high stereoselective action of dextro-morphine over levo-morphine for the induction of antianalgesia. Like levo-morphine, the dextro-morphine-induced antianalgesia against levo-morphine-produced TF inhibition was dose-dependently blocked by the nonopioid dextro-naloxone and its stereo-enantiomer levo-naloxone, a nonselective mu-opioid receptor antagonist. The antianalgesia induced by levo-morphine and dextro-morphine is reversed by the pretreatment with the glial inhibitor propentofylline (3.3-65 nmol), indicating that the antianalgesia is mediated by glial stimulation. The findings strongly indicate that the antianalgesia induced by levo-morphine and dextro-morphine is mediated by the stimulation of a novel nonopioid receptor on glial cells.

Differential mechanisms of antianalgesia induced by endomorphin-1 and endomorphin-2 in the ventral periaqueductal gray of the rat

The effects of pretreatment with endomorphin-1 (EM-1) and endomorphin-2 (EM-2) given into the ventral periaqueductal gray (vPAG) to induce antianalgesia against the tail-flick (TF) inhibition produced by morphine given into the vPAG were studied in rats. Pretreatment with EM-1 (3.5-28 nmol) given into vPAG for 45 min dose-dependently attenuated the TF inhibition produced by morphine (9 nmol) given into vPAG. Similarly, pretreatment with EM-2 (1.7-7.0 nmol) for 45 min also attenuated the TF inhibition induced by morphine; however, a high dose of EM-2 (14 nmol) did not attenuate the morphine-produced TF inhibition. The attenuation of morphine-produced TF inhibition induced by EM-2 or EM-1 pretreatment was blocked by pretreatment with mu-opioid antagonist (-)-naloxone (55 pmol) but not nonopioid (+)-naloxone (55 pmol). However, pretreatment with a morphine-6beta-glucuronide-sensitive mu-opioid receptor antagonist 3-methoxynaltrexone (6.4 pmol) selectively blocked EM-2- but not EM-1-induced antianalgesia. Pretreatment with dynorphin A(1-17) antiserum reversed only EM-2- but not EM-1-induced antianalgesia. Pretreatment with antiserum against beta-endorphin, [Met(5)]enkephalin, [Leu(5)]enkephalin, substance P or cholecystokinin, or with delta-opioid receptor antagonist naltrindole (2.2 nmol) or kappa-opioid receptor antagonist norbinaltorphimine (6.6 nmol) did not affect EM-2-induced antianalgesia. It is concluded that EM-2 selectively releases dynorphin A(1-17) by stimulation of a novel subtype of mu-opioid receptor, tentatively designated as mu(3) in the vPAG to induce antianalgesia, whereas the antianalgesia induced by EM-1 is mediated by the stimulation of another subtype of mu(1)- or mu(2)-opioid receptor.

Increased release of immunoreactive dynorphin A1–17 from the spinal cord after intrathecal treatment with endomorphin-2 in anesthetized rats

We previously demonstrated pretreatment with antiserum against dynorphin A1-17 attenuates endomorphin-2-induced analgesia and antianalgesia, suggesting that these endomorphin-2 effects are mediated by the release of dynorphin A1-17. Lumbar-cisternal spinal perfusion was used to measure the release of immunoreactive dynorphin A1-17 into spinal perfusates from urethane-anesthetized rats following endomorphin-2 or endomorphin-1 treatment within the perfusion solution. Treatment with endomorphin-2 (5-50 nmol) for 3 min caused a dose-dependent increase of immunoreactive dynorphin A1-17 in spinal perfusates, with a maximal increase detected between 24 and 48 min after endomorphin-2 treatment, while levels returned to baseline within 60 min. Endomorphin-2-induced release of immunoreactive dynorphin A1-17 was attenuated by pretreatment with mu-opioid receptor antagonist naloxone or 3-methoxynaltrexone. Endomorphin-1 induced a slight increase in immunoreactive dynorphin1-17 as well, but only at the highest dose used (50 nmol). Our results suggest that endomorphin-2 stimulated a specific subtype of mu-opioid receptor to induce the release of immunoreactive dynorphin A1-17 in spinal cords of rats.

Nonopioidergic mechanism mediating morphine-induced antianalgesia in the mouse spinal cord

Intrathecal (i.t.) pretreatment with a low dose (0.3 nmol) of morphine causes an attenuation of i.t. morphine-produced analgesia; the phenomenon has been defined as morphine-induced antianalgesia. The opioid-produced analgesia was measured with the tail-flick (TF) test in male CD-1 mice. Intrathecal pretreatment with low dose (0.3 nmol) of morphine time dependently attenuated i.t. morphine-produced (3.0 nmol) TF inhibition and reached a maximal effect at 45 min. Intrathecal pretreatment with morphine (0.009-0.3 nmol) for 45 min also dose dependently attenuated morphine-produced TF inhibition. The i.t. morphine-induced antianalgesia was dose dependently blocked by the nonselective mu-opioid receptor antagonist (-)-naloxone and by its nonopioid enantiomer (+)-naloxone, but not by endomorphin-2-sensitive mu-opioid receptor antagonist 3-methoxynaltrexone. Blockade of delta-opioid receptors, kappa-opioid receptors, and N-methyl-D-aspartate (NMDA) receptors by i.t. pretreatment with naltrindole, nor-binaltorphimine, and (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), respectively, did not affect the i.t. morphine-induced antianalgesia. Intrathecal pretreatment with antiserum against dynorphin A(1-17), [Leu]-enkephalin, [Met]-enkephalin, beta-endorphin, cholecystokinin, or substance P also did not affect the i.t. morphine-induced antianalgesia. The i.t. morphine pretreatment also attenuated the TF inhibition produced by opioid muagonist [D-Ala2, N-Me-Phe4,Gly-ol5]-enkephalin, delta-agonist deltorphin II, and kappa-agonist U50,488H. It is concluded that low doses (0.009-0.3 nmol) of morphine given i.t. activate an antianalgesic system to attenuate opioid mu-, delta-, and kappa-agonist-produced analgesia. The morphine-induced antianalgesia is not mediated by the stimulation of opioid mu-, delta-, or kappa-receptors or NMDA receptors. Neuropeptides such as dynorphin A(1-17), [Leu]-enkephalin, [Met]-enkephalin, beta-endorphin, cholecystokinin, and substance P are not involved in this low-dose morphine-induced antianalgesia.

Lack of mu-opioid receptor-mediated G-protein activation in the spinal cord of mice lacking Exon 1 or Exons 2 and 3 of the MOR-1 gene

The G-protein activation induced by mu-opioid receptor agonists was determined in spinal cord membranes from two types of mu-opioid receptor knockout mice: mice with a disruption of exon 1 (MOR (Exon 1)-KO) or exons 2 and 3 (MOR (Exons 2 and 3)-KO) of the mu-opioid receptor gene. The G-protein activation induced by the opioid agonists was measured by monitoring the increases of guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTP gamma S) binding. The mu-opioid receptor agonists (D-Ala(2),N-MePhe (4),Gly-ol(5)]enkephalin, endomorphin-1, endomorphin-2, morphine, morphine-6 beta-glucuronide, and fentanyl produced concentration-dependent increases of [(35)S]GTP gamma S binding to spinal cord membranes in wild-type mice, but not in MOR (Exon 1)-KO mice or MOR (Exons 2 and 3)-KO mice. On the other hand, the delta-opioid receptor agonist [D-Pen (2,5)]enkephalin, the kappa-opioid receptor agonist (-)U50,488H, or the ORL1-receptor agonist nociception increased [(35)S]GTP gamma S binding in the spinal cord membranes from both MOR (Exon 1)-KO mice and MOR (Exons 2 and 3)-KO mice to the same extent as in the corresponding wild-type mice. The results provide further information about the important roles of the sequences encoded within exon 1 and exons 2 and 3 of mu-opioid receptor gene for the activation of G-proteins by mu-opioid receptor agonists in the mouse spinal cord.

Dynorphinergic Mechanism Mediating Endomorphin-2-Induced Antianalgesia in the Mouse Spinal Cord

We have previously demonstrated that both endomorphin-1 (EM-1) and endomorphin-2 (EM-2) at high doses (1.75-35 nmol) given intrathecally (i.t.) or intracerebroventricularly produce antinociception by stimulation of mu-opioid receptors. Now, we report that EM-2 at small doses (0.05-1.75 nmol), which injected alone did not produce antinociception, produces anti-analgesia against opioid agonist-induced antinociception. The tail-flick (TF) response was used to test the antinociception in male CD-1 mice. Intrathecal pretreatment with EM-2 (0.02-1.75 nmol) 45 min before i.t. morphine (3.0 nmol) injection dose dependently attenuated morphine-induced TF inhibition. On the other hand, a similar dose of EM-1 (1.64 nmol) failed to produce any antianalgesic effect. The EM-2 (1.75 nmol)-produced anti-analgesia against morphine-induced TF inhibition was blocked by i.t. pretreatment with the mu-opioid antagonist naloxone or 3-methoxynaltrexone, but not delta-opioid receptor antagonist naltrindole, kappa-opioid receptor antagonist nor-binaltorphimine, or N-methyl-d-aspartate (NMDA) receptor antagonist MK-801. The EM-2-induced antianalgesic effect against morphine-induced TF inhibition was blocked by i.t. pretreatment with antiserum against dynorphin A(1-17), but not beta-endorphin, [Met]-enkephalin, [Leu]-enkephalin, or cholecystokinin antiserum (200 microg each). The i.t. EM-2 pretreatment also attenuated the TF inhibition induced by other mu-opioid agonists, [d-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin, EM-1 and EM-2, delta-opioid agonist deltorphin II, and kappa-opioid agonist (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide methane-sulfonate hydrate (U50,488H). It is concluded that EM-2 at subanalgesic doses presumably stimulates a subtype of mu-opioid receptor and subsequently induces the release of dynorphin A(1-17) to produce antianalgesic effects against mu-, delta-, or kappa-agonists-induced antinociception. The EM-2-induced antianalgesia is not mediated by the release of [Met]-enkephalin, [Leu]-enkephalin, beta-endorphin, or cholecystokinin, nor does it involve kappa- or delta-opioid or NMDA receptors in the spinal cord.

Acute antinociceptive tolerance and partial cross-tolerance to endomorphin-1 and endomorphin-2 given intrathecally in the mouse

The effect of pretreatment with endomorphin-1 (EM-1) or endomorphin-2 (EM-2) given intrathecally (i.t.) on the tail-flick inhibition induced by subsequent i.t. injection of EM-1 or EM-2 was studied in CD-1 mice. Pretreatment with EM-1 (32.7 nM) for 1-3 h, but not 4 h, attenuated the tail-flick inhibition induced by i.t. EM-1 (16.3 nM), while pretreatment with EM-2 (70 nM) for 0.5-1.5 h, but not 2 h, attenuated tail-flick inhibition induced by i.t. EM-2 (35 nM). EM-1 (32.7 nM) pretreatment for 1.5 h produced 5.3- and only 2.4-folds shift to the right of the dose-response curves for EM-1- and EM-2-induced tail-flick inhibition, respectively, while EM-2 (70 nM) pretreatment for 1 h caused 4.3- and 4.5-folds shift to the right for EM-2- and EM-1-induced tail-flick inhibition, respectively. Thus, mice made antinociceptive tolerant to EM-1 were partially cross-tolerant to EM-2 and mice made antinociceptive tolerant to EM-2 were completely cross-tolerant to EM-1. It is proposed that EM-1- and EM-2-induced antinociception are mediated by the stimulation of two different subtypes of mu-opioid receptors in mouse spinal cord; one subtype is stimulated by both EM-1 and EM-2, and another subtype is stimulated by EM-2.

Buprenorphine blocks epsilon- and micro-opioid receptor-mediated antinociception in the mouse

Antagonistic properties of buprenorphine for epsilon- and micro -opioid receptors were characterized in beta-endorphin- and [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO)-induced antinociception, respectively, with the tail-flick test in male ICR mice. epsilon-Opioid receptor agonist beta-endorphin (0.1-1 micro g), micro -opioid receptor agonist DAMGO (0.5-20 ng), or buprenorphine (0.1-20 micro g) administered i.c.v. dose dependently produced antinociception. The antinociception induced by 10 micro g of buprenorphine given i.c.v. was completely blocked by the pretreatment with beta-funaltrexamine (beta-FNA) (0.3 micro g i.c.v.), indicating that the buprenophine-induced antinociception is mediated by the stimulation of the micro -opioid receptor. The antinociceptive effects induced by beta-endorphin (1 micro g i.c.v.) and DAMGO (16 ng i.c.v.) were dose dependently blocked by pretreatment with smaller doses of buprenorphine (0.001-1 micro g i.c.v.), but not by a higher dose of buprenorphine (10 micro g i.c.v.). beta-FNA at a dose (0.3 micro g i.c.v.) that strongly attenuated DAMGO-induced antinociception had no effect on the antinociception produced by beta-endorphin (1 micro g i.c.v.). However, pretreatment with buprenorphine (0.1-10 micro g) in mice pretreated with this same dose of beta-FNA was effective in blocking beta-endorphin-induced antinociception. beta-FNA was 226-fold more effective at antagonizing the antinociception induced by DAMGO (16 ng i.c.v.) than by beta-endorphin (1 micro g i.c.v.). The antinociception induced by delta-opioid receptor agonist [d-Ala2]deltorphin II (10 micro g i.c.v.) or kappa1-opioid receptor agonist trans-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl)benzeneacetamine methanesulfonate salt [(-)-U50,488H] (75 micro g i.c.v.) was not affected by pretreatment with buprenorphine (0.1-1.0 micro g i.c.v.). It is concluded that buprenorphine, at small doses, blocks epsilon-opioid receptor-mediated beta-endorphin-induced antinociception and micro -opioid receptor-mediated DAMGO-induced antinociception, and at high doses produces a micro -opioid receptor-mediated antinociception.

Spinal pretreatment with antisense oligodeoxynucleotides against exon-1, -4, or -8 of mu-opioid receptor clone leads to differential loss of spinal endomorphin-1-and endomorphin-2-induced antinociception in the mouse

Intrathecal (i.t.) pretreatments with antisense oligodeoxynucleotides (AS ODNs) against exon-1, -4, or -8 of mu-opioid receptor clone (MOR-1) to knockdown different variants of MOR-1 on the antinociception induced by endomorphin-1, enomorphin-2, or [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) given i.t. were investigated in male CD-1 mice. The antinociception was measured with the tail-flick test. AS ODNs against exon-1 (5 microg) given i.t. once daily for 3 days attenuated the antinociception induced by endomorphin-1 and endomorphin-2 with the dose-response curves shifted to the right by 4.5- and 5.3-fold, respectively. AS ODNs against exon-4 (5 microg) attenuated the antinociception induced by endomorphin-1 and endomorphin-2 with the dose-response curves shifted to the right by 2.4- and 5.3-fold, respectively. However, AS ODNs against exon-8 (5 microg) attenuated only the antinociception induced by endomorphin-1, but not endomorphin-2 with the dose-response curves shifted to the right by 3.9- and 1.3-fold, respectively. One more day of pretreatment with antisense probes failed to further reduce the antinociception. The antinociception induced by DAMGO was attenuated by i.t. pretreatment with AS ODNs directed against exon-1, and, to a lesser extent, by AS ODNs directed against exon-8. The mismatch AS ODNs against respective exon-1, -4, and -8 failed to exert significant effects. The selective actions of antisense probes directed against different exons of the MOR-1 in attenuating the antinociception induced by endomorphin-1, endomorphin-2, and DAMGO suggest that multiple splice variants of the MOR-1 exist and support the view that different subtypes of mu-opioid receptors are involved in antinociception induced by endomorphin-1, endomorphin-2, and DAMGO.

D-pro(2)-endomorphin-1 and D-pro(2)-endomorphin-2, respectively, attenuate the antinociception induced by endomorphin-1 and endomorphin-2 given intrathecally in the mouse

First, the antinociception with the tail-flick test of D-Pro(2)-endomorphin-1 and D-Pro(2)-endomorphin-2 given i.t. was compared with that produced by endomorphin-1 and -2 in male CD-1 mice. High doses of D-Pro(2)-endomorphin-1 (0.2-0.4 pmol) and D-Pro(2)-endomorphin-2 (300-800 pmol) given i.t. produced antinociception with low intrinsic activity [about 25% maximum possible effect (MPE)] compared with that of endomorphin-1 (16.4 nmol) and endomorphin-2 (35 nmol) (>90% MPE). Second, coadministration of a low dose of D-Pro(2)-endomorphin-1 (0.1 pmol), which given alone did not affect the tail-flick latencies, markedly attenuated the antinociception induced by endomorphin-1 (16.4 nmol) but not by endomorphin-2 (35 nmol). Similarly, coadministration of a low dose of D-Pro(2)-endomorphin-2 (200 pmol), which given alone did not affect the tail-flick latencies, significantly attenuated the antinociception induced by endomorphin-2 (35 nmol) and, to a much lesser extent, endomorphin-1 (16.4 nmol). It is concluded that D-Pro(2)-endomorphin-1 and D-Pro(2)-endomorphin-2 at high doses were partial opioid receptor agonists to produce antinociception, and at low doses were opioid receptor antagonists to block selectively the antinociception induced by endomorphin-1 and endomorphin-2, respectively. Furthermore, our results are consistent with the view that the antinociception induced by endomorphin-1 and endomorphin-2 is mediated by the stimulation of different subtypes of mu-opioid receptors.

Roles of endogenous opioid peptides in modulation of nocifensive response to formalin

Roles of endogenous opioid peptides and their receptors in modulation of the nocifensive responses to formalin in mice were studied. Mice were pretreated i.c.v. or intrathecally (i.t.) with selective opioid receptor antagonists or intrathecally with antisera against endogenous opioid peptides and the nocifensive licking responses to intraplantar injection of formalin (0.5%, 25 microl) were then observed. Pretreatment with the epsilon-opioid receptor antagonist beta-endorphin(1-27) or the selective mu-opioid receptor antagonist D-Phe-Cys-Tyr-Orn-Thr-Pen-Thr-NH(2) (CTOP) given i.c.v. dose dependently enhanced the second, but not the first phase of the nocifensive response. However, i.c.v. pretreatment with the selective delta-receptor antagonist naltrindole or kappa-receptor antagonist nor-binaltrophimine did not affect the nocifensive responses. Intrathecal pretreatment with selective delta(1)-opioid antagonist 7-benzylidene naltrexamine significantly enhanced both the first and second phases of nocifension. Intrathecal pretreatment with CTOP also increased the second but not the first phase of the nocifension. However, i.t. pretreatment with the selective delta(2)-receptor antagonist naltriben or nor-binaltrophimine did not affect the second phase of the nocifension. Intrathecal pretreatment with antiserum against Leu-enkephalin, Met-enkephalin, or dynorphin A(1-17), but not beta-endorphin, enhanced only the second phase of nocifensive response to formalin. It is concluded that the blockade of epsilon- and mu-receptors, but not delta- or kappa-receptors, at the supraspinal sites enhanced the second phase of formalin-induced nocifension. In the spinal cord, Leu-enkephalin, and to a lesser extent, Met-enkephalin and dynorphin A(1-17) and mu- and delta(1)-opioid receptors, but not delta(2)- or kappa-opioid receptors, are involved in modulating the feedback inhibition of the second phase of formalin-induced nocifension.

Acute antinociceptive tolerance and asymmetric cross-tolerance between endomorphin-1 and endomorphin-2 given intracerebroventricularly in the mouse

Development of tolerance in mice pretreated intracerebroventricularly with mu-opioid receptor agonist endomorphin-1, endomorphin-2, or [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO) was compared between endomorphin-1- and endomorphin-2-induced antinociception with the tail-flick test. A 2-h pretreatment with endomorphin-1 (30 nmol) produced a 3-fold shift to the right in the dose-response curve for endomorphin-1. Similarly, a 1-h pretreatment with endomorphin-2 (70 nmol) caused a 3.9-fold shift to the right for endomorphin-2. In cross-tolerance experiments, pretreatment with endomorphin-2 (70 nmol) caused a 2.3-fold shift of the dose-response curve for endomorphin-1, whereas pretreatment with endomorphin-1 (30 nmol) caused no change of the endomorphin-2 dose-response curve. Thus, mice acutely tolerant to endomorphin-1 were not cross-tolerant to endomorphin-2, although mice made tolerant to endomorphin-2 were partially cross-tolerant to endomorphin-1; an asymmetric cross-tolerance occurred. Pretreatment with DAMGO 3 h before intracerebroventricular injection of endomorphin-1, endomorphin-2, or DAMGO attenuated markedly the antinociception induced by endomorphin-1 and DAMGO but not endomorphin-2. It is proposed that two separate subtypes of mu-opioid receptors are involved in antinociceptive effects induced by endomorphin-1 and endomorphin-2. One subtype of opioid mu-receptors is stimulated by DAMGO, endomorphin-1, and endomorphin-2, and another subtype of mu-opioid receptors is stimulated solely by endomorphin-2.

Partial agonistic action of endomorphins in the mouse spinal cord

The partial agonistic properties of endogenous mu-opioid peptides endomorphin-1 and endomorphin-2 for G-protein activation were determined in the mouse spinal cord, monitoring the increases in guanosine-5'-o-(3-[35S]thio)triphosphate binding. The G-protein activation induced by endogenous opioid peptide beta-endorphin in the spinal cord was significantly, but partially, attenuated by co-incubation with endomorphin-1 or endomorphin-2. The data indicates that endomorphin-1 and endomorphin-2 are endogenous partial agonists for mu-opioid receptor in the mouse spinal cord.