High homocysteine levels prevent via H2 S the CoCl2 -induced alteration of lymphocyte viability.

High homocysteine (HCy) levels are associated with lymphocyte-mediated inflammatory responses that are sometimes in turn related to hypoxia. Because adenosine is a potent lymphocyte suppressor produced in hypoxic conditions and shares metabolic pathways with HCy, we addressed the influence of high HCy levels on the hypoxia-induced, adenosine-mediated, alteration of lymphocyte viability. We treated mitogen-stimulated human lymphocytes isolated from healthy individuals and the human lymphoma T-cell line CEM with cobalt chloride (CoCl2 )to reproduce hypoxia. We found that CoCl2 -altered cell viability was dose-dependently reversed using HCy. In turn, the HCy effect was inhibited using DL-propargylglycine, a specific inhibitor of the hydrogen sulphide (H2 S)-synthesizing enzyme cystathionine-γ-lyase involved in HCy catabolism. We then addressed the intracellular metabolic pathway of adenosine and HCy, and the role of the adenosine A2A receptor (A2 A R). We observed that: (i) hypoxic conditions lowered the intracellular concentration of HCy by increasing adenosine production, which resulted in high A2 A R expression and 3', 5'-cyclic adenosine monophosphate production; (ii) increasing intracellular HCy concentration reversed the hypoxia-induced adenosinergic signalling despite high adenosine concentration by promoting both S-adenosylhomocysteine and H2 S production; (iii) DL-propargylglycine that inhibits H2 S production abolished the HCy effect. Together, these data suggest that high HCy levels prevent, via H2 S production and the resulting down-regulation of A2 A R expression, the hypoxia-induced adenosinergic alteration of lymphocyte viability. We point out the relevance of these mechanisms in the pathophysiology of cardiovascular diseases.

Spare Adenosine A2a Receptors Are Associated With Positive Exercise Stress Test In Coronary Artery Disease.

During exercise, cardiac oxygen-consumption increases and the resulting low oxygen level in myocardium triggers coronary vasodilation. This response to hypoxia is controlled notably by the vasodilator adenosine and its A2A receptor (A2AR). According to the "spare receptor" pharmacological model, a strong A2AR-mediated response can occur in the context of a large number of receptors remaining unoccupied, activation of only a weak fraction of A2AR (evaluated using KD) resulting in maximal cAMP production (evaluated using EC50), and hence in maximal coronary vasodilation. In coronary artery disease (CAD), myocardial ischemia limits adaptation to exercise, which is commonly detected using the exercise stress test (EST). We hypothesized that spare A2AR are present in CAD patients to correct ischemia. Seventeen patients with angiographically-documented CAD and 17 control subjects were studied. We addressed adenosine-plasma concentration and A2AR-expression at the mononuclear cell-surface, which reflects cardiovascular expression. The presence of spare A2AR was tested using an innovative pharmacological approach based on a homemade monoclonal antibody with agonist properties. EST was positive in 82% of patients, and in none of the controls. Adenosine plasma-concentration increased by 60% at peak exercise in patients only (p<0.01). Most patients (65%), and none of the controls, had spare A2AR (identified when EC50/KD≤0.1) and a low A2AR-expression (mean: -37% vs controls; p<0.01). All patients with spare A2AR had a positive EST whereas the subjects without spare A2AR had a negative EST (p<0.05). Spare A2AR are therefore associated with positive EST in CAD patients and their detection may be used as a diagnostic marker.

Endogenous adenosine release is involved in the control of heart rate in rats.

Intravenous (i.v.) injections of adenosine exert marked effects on heart rate (HR) and arterial blood pressure (BP), but the role of an endogenous adenosine release by vagal stimulation has not been evaluated. In anaesthetized rats, we examined HR and BP changes induced by 1 min electrical vagal stimulation in the control condition, and then after i.v. injections of (i) atropine, (ii) propranolol, (iii) caffeine, (iv) 8 cyclopentyl-1,3-dipropylxanthine (DPCPX), or (v) dipyridamole to increase the plasma concentration of adenosine (APC). APC was measured by chromatography in the arterial blood before and at the end of vagal stimulation. The decrease in HR in the controls during vagal stimulation was markedly attenuated, but persisted after i.v. injections of atropine and propranolol. When first administered, DPCPX modestly but significantly reduced the HR response to vagal stimulation, but this disappeared after i.v. caffeine administration. Both the HR and BP responses were significantly accentuated after i.v. injection of dipyridamole. Vagal stimulation induced a significant increase in APC, proportional to the magnitude of HR decrease. Our data suggest that the inhibitory effects of electrical vagal stimulations on HR and BP were partly mediated through the activation of A1 and A2 receptors by an endogenous adenosine release. Our experimental data could help to understand the effects of ischemic preconditioning, which are partially mediated by adenosine.

The mechanisms of the widespread production of phosphorylated HSP25 after fatiguing muscle stimulation.

We previously showed that a widespread heat shock protein (HSP) response to fatigue of a single hindlimb muscle was responsible for a global adaptive response to an acute localized stress. We also demonstrated that the HSP response resulted from the activation of nerve afferents from the stimulated muscle. However, we did not examine the role played by the different muscle afferents or the efferent arm of HSP response. In the present study we measured the changes in phosphorylated HSP25 (pHSP25) levels in resting hindlimb muscles and the diaphragm, kidney and brain in response to a fatiguing stimulation of one tibialis anterior muscle that was repeated in five series of experiments: (1) intact muscle innervation, (2) during the selective procaine block of conduction in group IV muscle afferents, (3) after muscle nerve transection to suppress all the sensory messages, and under pharmacological blockade of the (4) alpha-adrenergic or (5) glutamatergic neurotransmission. The data showed that: (1) the pHSP25 response in hindlimb muscles resulted from the stimulation of both group III and IV muscle afferents while the pHSP25 response in the diaphragm, kidney and brain resulted from the sole activation of the group IV fibres, and (2) the blockade of alpha-adrenergic, but not glutamatergic, neurotransmission suppressed the pHSP25 response in all explored tissues except the brain. The present study highlights the role played by the group III and IV muscle afferents in the fatigue-induced pHSP25 response and shows that the sympathetic nerve supply to the muscles and kidney represents the efferent arm of the pHSP25 activation. However, the pHSP25 changes in the brain cannot be explained by the pathways investigated here.

Fatiguing stimulation of one skeletal muscle triggers heat shock protein activation in several rat organs: the role of muscle innervation.

We hypothesised that activation of muscle afferents by fatigue triggers a widespread activation of heat shock proteins (HSPs) in resting muscles and different organs. In anaesthetised rats, HSP25 and HSP70 levels were determined in both tibialis anterior (TA) and extensor digitorum longus (EDL) muscles and in the diaphragm, kidney and brain by ELISA, which mostly identifies phosphorylated HSP, and western blotting. One TA muscle was electrically stimulated and tissues were sampled 10 or 60 min after the stimulation had ended. The nerve supply to the stimulated TA or its counterpart in the contralateral limb was left intact or suppressed. In control rats, no muscle stimulation was performed and tissues were sampled at the same time points (10 or 60 min). After TA stimulation, ELISA showed an increased HSP25 content in the contralateral TA, EDL and diaphragm at 10 min but not at 60 min, and HSP70 increased in all sampled tissues at 60 min. Western blotting did not show any changes in HSP25 and HSP70 at 10 min, while at 60 min HSP25 increased in all sampled tissues except the brain and HSP70 was elevated in all tissues. Denervation of the contralateral non-stimulated limb suppressed HSP changes in TA and after denervation of the stimulated TA the widespread activation of HSPs in other organs was absent. Our data suggest that fatigue-induced activation of skeletal muscle afferents triggers an early increase in phosphorylated HSP25 in muscles and a delayed elevation of non-phosphorylated HSP25 and HSP70 in skeletal and respiratory muscles, kidney and brain.

Fall in oxygen tension of culture medium stimulates the adenosinergic signalling of a human T cell line.

We examined the short-course expression of various parameters involved in the adenosinergic signalling of a human T cell line during in vitro decrease of the medium culture oxygen tension mimicking in vivo hypoxia. Fall of 92 mmHg in oxygen tension of culture medium induced in CEM, a CD4+ human T cell line, a continuous production of hypoxia-inducing factor-1α with a plateau value at 9 h, a rapid increase in adenosine production peaking at 3 h and a decrease in adenosine deaminase peaking at 6 h. The adenosine A(2A) receptor (A(2A)R) protein level of CEM cells was enhanced with a peak at 6 h. Intracellular 3',5'-cyclic adenosine monophosphate accumulated in CEM cells with a maximal level at 9 h. These results show that a human-cultured T cells line can upregulate its own adenosine production and A(2A)R expression during exposure to acute hypoxia. Hypoxia-increased stimulation of the adenosinergic signalling of T cells may have immunosuppressive properties and, consequently, A(2A)R agonists may have therapeutic relevance.

Expeditious synthesis and biological evaluation of new C-6 1,2,3-triazole adenosine derivatives A1 receptor antagonists or agonists.

The synthesis of new C-6 1,2,3-triazole adenosine derivatives via microwave assisted 1,3-dipolar cycloaddition as key step is described. The binding on membranes of cells that over express A(1) adenosine receptors (A(1)AR) was also evaluated. Among them, four compounds increased cAMP production, in a dose-dependent manner acting as antagonists of the A(1)AR, while two compounds act as agonists.

Production of an agonist-like monoclonal antibody to the human A2A receptor of adenosine for clinical use.

The second extracellular loop of the A(2A) receptor (A(2A)R) of adenosine was used to immunize mice for production of Adonis, an IgM monoclonal antibody. Adonis bound to the immunogen peptide and the native receptor in ELISA with K(D) values in 6.51-12.35 nM range. It recognized a linear epitope of 7 amino acids (LFEDVVP) at the C-terminal part of the external loop. Adonis revealed a 45-kDa band in lysate of human peripheral blood mononuclear cells in Western blotting in denaturing conditions. This served to monitor the up-regulation of the A(2A)R expression by caffeine. Adonis stimulated the cAMP production and inhibited the cell proliferation of an A(2A)R transfected stable cell line. These results confirm the immunogenicity and the functional relevance of the second extracellular loop of the A(2A)R. They suggest that Adonis may be of clinical use in various pathological situations to measure the regulation of the A(2A)R expression and to act as A(2A)R agonist drug.

Design, synthesis and biological evaluation of a bivalent micro opiate and adenosine A1 receptor antagonist.

The cross talk between different membrane receptors is the source of increasing research. We designed and synthesized a new hetero-bivalent ligand that has antagonist properties on both A(1) adenosine and mu opiate receptors with a K(i) of 0.8+/-0.05 and 0.7+/-0.03 microM, respectively. This hybrid molecule increases cAMP production in cells that over express the mu receptor as well as those over expressing the A(1) adenosine receptor and reverses the antalgic effects of mu and A(1) adenosine receptor agonists in animals.

Expressions of adenosine A2A receptors in coronary arteries and peripheral blood mononuclear cells are correlated in coronary artery disease patients.

BACKGROUND: Altered coronary blood flow occurs in patients with coronary artery disease (CAD). Adenosine strongly impacts blood flow mostly via adenosine A2A receptor (A2AR) expressed in coronary tissues. As part of a systemic regulation of the adenosinergic system, we compared A2AR expression in situ, and on peripheral blood mononuclear cells (PBMC) in CAD patients. METHODS AND RESULTS: Aortic and coronary tissues, and PBMC were sampled in 20 CAD patients undergoing coronary artery bypass surgery and consecutively included. Controls were PBMC obtained from 15 healthy subjects. Expression and activity of A2AR were studied by Western blotting and cAMP measurement, respectively. A2AR expression on PBMC was lower in patients than in controls (0.83±0.31 vs 1.2±0.35 arbitrary units; p<0.01), and correlated with A2AR expression in coronary and aortic tissues (Pearson's r: 0.77 and 0.59, p<0.01, respectively). Basal and maximal cAMP productions following agonist stimulation of PBMC were significantly lower in patients than in controls (120±42 vs 191±65 and 360±113 vs 560±215pg/106 cells, p<0.05, respectively). In CAD patients, the increase from basal to maximal cAMP production in PBMC and aortic tissues was similar (+300% and +246%, respectively). CONCLUSION: Expression of A2AR on PBMC correlated with those measured in coronary artery and aortic tissues in CAD patients, A2AR activity of PBMC matched that observed in aorta, and A2AR expression and activity in PBMC were found reduced as compared to controls. Measuring the expression level of A2AR on PBMC represents a good tool to address in situ expression in coronary tissues of CAD patients.

Effect of hyperoxic and hyperbaric conditions on the adenosinergic pathway and CD26 expression in rat.

The nucleoside adenosine acts on the nervous and cardiovascular systems via the A2A receptor (A2AR). In response to oxygen level in tissues, adenosine plasma concentration is regulated in particular via its synthesis by CD73 and via its degradation by adenosine deaminase (ADA). The cell-surface endopeptidase CD26 controls the concentration of vasoactive and antioxidant peptides and hence regulates the oxygen supply to tissues and oxidative stress response. Although overexpression of adenosine, CD73, ADA, A2AR, and CD26 in response to hypoxia is well documented, the effects of hyperoxic and hyperbaric conditions on these elements deserve further consideration. Rats and a murine Chem-3 cell line that expresses A2AR were exposed to 0.21 bar O2, 0.79 bar N2 (terrestrial conditions; normoxia); 1 bar O2 (hyperoxia); 2 bar O2 (hyperbaric hyperoxia); 0.21 bar O2, 1.79 bar N2 (hyperbaria). Adenosine plasma concentration, CD73, ADA, A2AR expression, and CD26 activity were addressed in vivo, and cAMP production was addressed in cellulo. For in vivo conditions, 1) hyperoxia decreased adenosine plasma level and T cell surface CD26 activity, whereas it increased CD73 expression and ADA level; 2) hyperbaric hyperoxia tended to amplify the trend; and 3) hyperbaria alone lacked significant influence on these parameters. In the brain and in cellulo, 1) hyperoxia decreased A2AR expression; 2) hyperbaric hyperoxia amplified the trend; and 3) hyperbaria alone exhibited the strongest effect. We found a similar pattern regarding both A2AR mRNA synthesis in the brain and cAMP production in Chem-3 cells. Thus a high oxygen level tended to downregulate the adenosinergic pathway and CD26 activity. Hyperbaria alone affected only A2AR expression and cAMP production. We discuss how such mechanisms triggered by hyperoxygenation can limit, through vasoconstriction, the oxygen supply to tissues and the production of reactive oxygen species.

NF-κB enhances hypoxia-driven T-cell immunosuppression via upregulation of adenosine A(2A) receptors.

Hypoxia affects inflammation by modulating T-cell activation via the adenosinergic system. We supposed that, in turn, inflammation influences cell hypoxic behavior and that stimulation of T-cells in inflammatory conditions involves the concerted action of the nuclear factor κB (NF-κB) and the related hypoxia-inducible factor 1α (HIF-1α) on the adenosinergic system. We addressed this hypothesis by monitoring both transcription factors and four adenosinergic signaling parameters - namely adenosine, adenosine deaminase (ADA), adenosine A2A receptor (A2AR) and cAMP - in T-cells stimulated using phorbol myristate acetate and phytohemagglutinin and submitted to hypoxic conditions which were mimicked using CoCl2 treatment. We found that cell viability was more altered in stimulated than in resting cells under hypoxia. Detailed analysis showed that: i) NF-κB activation remained at basal level in resting hypoxic cells but greatly increased following stimulation, stimulated hypoxic cells exhibiting the higher level; ii) HIF-1α production induced by hypoxia was boosted via NF-κB activation in stimulated cells whereas hypoxia increased HIF-1α production in resting cells without further activating NF-κB; iii) A2AR expression and cAMP production increased in stimulated hypoxic cells whereas adenosine level remained unchanged due to ADA regulation; and iv) the presence of H2S, an endogenous signaling molecule in inflammation, reversed the effect of stimulation on cell viability by down-regulating the activity of transcription factors and adenosinergic immunosuppression. We also found that: i) the specific A2AR agonist CGS-21680 increased the suppressive effect of hypoxia on stimulated T-cells, the antagonist ZM-241385 exhibiting the opposite effect; and ii) Rolipram, a selective inhibitor of cAMP-specific phosphodiesterase 4, and 8-Br-cAMP, a cAMP analog which preferentially activates cAMP-dependent protein kinase A (PKA), increased T-cell immunosuppression whereas H-89, a potent and selective inhibitor of cAMP-dependent PKA, restored cell viability. Together, these data indicate that inflammation enhances T-cell sensitivity to hypoxia via NF-κB activation. This process upregulates A2AR expression and enhances cAMP production and PKA activation, resulting in adenosinergic T-cell immunosuppression that can be modulated via H2S.

Ticagrelor increases adenosine plasma concentration in patients with an acute coronary syndrome.

OBJECTIVES: This study aimed to investigate the impact of ticagrelor on adenosine plasma concentration (APC) in acute coronary syndrome (ACS) patients. BACKGROUND: Ticagrelor is a direct-acting P2Y12-adenosine diphosphate receptor blocker. The clinical benefit of ticagrelor compared with clopidogrel in ACS patients suggests that the drug has non-platelet-directed properties. Animal and in vitro models suggested that the "pleiotropic" properties of ticagrelor may be related to an interaction with adenosine metabolism. METHODS: We prospectively randomized 60 ACS patients to receive ticagrelor or clopidogrel. The APC was measured by liquid chromatography. To assess the mechanism of APC variation, we measured adenosine deaminase concentration, adenosine uptake by red blood cells, and cyclic adenosine monophosphate production by cells overexpressing adenosine receptors. The P2Y12-adenosine diphosphate receptor blockade was assessed by the vasodilator-stimulated phosphoprotein index. RESULTS: Patients receiving ticagrelor had significantly higher APC than patients receiving clopidogrel (1.5 µM [interquartile range: 0.98 to 1.7 µM] vs. 0.68 µM [interquartile range: 0.49 to 0.78 µM]; p < 0.01). The APC was not correlated with vasodilator-stimulated phosphoprotein (p = 0.16). Serum-containing ticagrelor inhibited adenosine uptake by red blood cells compared with clopidogrel or controls (p < 0.01 for both comparisons). Adenosine deaminase activity was similar in serum of patients receiving clopidogrel or ticagrelor (p = 0.1). Ticagrelor and clopidogrel had no direct impact on adenosine receptors (p = not significant). CONCLUSIONS: Ticagrelor increases APC in ACS patients compared with clopidogrel by inhibiting adenosine uptake by red blood cells.

Search for adenosine A2A spare receptors on peripheral human lymphocytes.

Some ligand-receptor couples involve spare receptors, which are apparent when a maximal response is achieved with only a small fraction of the receptor population occupied. This situation favours cross-reactions with low-affinity ligands, which may be detrimental for cell signaling. In the case of the adenosine A2A receptors (A2AR), which have an immunosuppressive effect on lymphocytes through cAMP production, the presence of spare A2AR remains to be established. We examined the situation using patients over-expressing lymphocyte A2AR and an agonist-like mAb to A2AR. We found that maximal mAb binding and functional response varied among the patients whereas the dissociation constant and half-maximal effective concentration had similar mean values (0.19 and 0.18 µM, respectively). Lymphocyte A2AR expression was correlated to plasma adenosine level and A2AR occupation but not to A2AR response. These results are consistent with a lack of a reserve of functional A2AR on human lymphocytes as a general rule and suggest that the amount and functional state of the expressed A2AR determine the maximal level of the lymphocyte response to adenosine.

SKCa Channels Blockage Increases the Expression of Adenosine A2A Receptor in Jurkat Human T Cells.

Adenosine is a nucleoside displaying various biological effects via stimulation of four G-protein-coupled receptors, A1, A2A, A2B, and A3. Adenosine also modulates voltage-gated (Kv) and small conductance calcium-activated (SKCa) potassium channels. The effect of these potassium channels on the expression of adenosine receptors is poorly understood. We evaluated the action of BgK (a natural Kv channel blocker) and Lei-Dab7 (a synthetic SKCa channel blocker) on the expression of adenosine A2A receptors (A2AR) in Jurkat human T cells. We found that Lei-Dab7, but not BgK, increased the maximal binding value of the tritiated ligand ZM241385 to A2AR in a dose-dependent manner (+45% at 5 nM; +70% at 50 nM as compared to control). These results were further confirmed by Western blotting using a specific monoclonal antibody to human A2AR. The ligand affinity-related dissociation constant and A2AR mRNA amount were not significantly modified by either drug. We suggest that modulation of SKCa channels can influence membrane expression of A2AR and thus has a therapeutic potential.

Adenosine plasma level and A2A adenosine receptor expression: correlation with laboratory tests in patients with neurally mediated syncope.

OBJECTIVES: The purpose of this study was to investigate the hypothesis that responses to the ATP test and head-up tilt test (HUT) may be correlated with different purinergic profiles. DESIGN AND SETTING: The ATP and HUT identify distinct subsets of patients with neurally mediated syncope (NMS). Adenosine and its A(2A) receptors (A(2A)R) may be implicated in the pathophysiology of NMS in patients with positive HUT. Nothing is known about the purinergic profile of patients with positive ATP. PATIENTS AND MEASURES: This prospective study includes a consecutive series of patients with suspected NMS. All patients underwent both HUT and ATP. Before testing, samples were collected for measurement of baseline adenosine plasma level (APL) and expression. RESULTS: A total of 46 patients (25 men and 21 women) with a mean age of 57±18 years were enrolled. The HUT test was positive in 27 patients and the ATP test in 20. Both tests were positive in 9 and negative in 8. High APL was associated with high probability of positive HUT while low APL was associated with high probability of positive ATP. Expression of A(2A)R was lower in patients with positive ATP than in those with positive HUT. CONCLUSION: These findings indicate that patients with NMS present different purinergic profiles and that responses to HUT and ATP are correlated with these profiles.

Intracerebroventricular injection of an agonist-like monoclonal antibody to adenosine A(2A) receptor has antinociceptive effects in mice.

Adenosine is a modulator of nociceptive pathways, both at the spinal and supraspinal levels. Adenosine A(1) and A(2A) receptors (A(1)R, A(2A)R) are expressed in the basal ganglia where they are the target of caffeine, the most widely use psychoactive drug which acts as an antagonist to both types of receptors. Given the controversial role of A(2A)R versus A(1)R in modulating pain in brain areas, mice received intracerebroventricular injection of Adonis, an agonist-like monoclonal antibody with high specificity for the A(2A)R and were subjected to behavioral tests investigating nociceptive thresholds. We report that Adonis led to a significant dose-dependent increase in hot-plate and tail-flick latencies in mice and that such increase was prevented by caffeine and ZM 241385, a specific A(2A)R antagonist. The Adonis antinociceptive effects were also inhibited by naloxone, a non selective antagonist for opioid receptors, suggesting that Adonis acts, at least in part, through the stimulation of the endogenous opioid system. These results confirm the A(2A)R as a target for pain control and Adonis as a potential drug with therapeutic interest.

Monoclonal antibody-assisted stimulation of adenosine A2A receptors induces simultaneous downregulation of CXCR4 and CCR5 on CD4+ T-cells.

Immunocompetent cells express various G-protein-coupled receptors that transduce extracellular signals across the plasma membrane. Among them, CXCR4 and CCR5 chemokines receptors and adenosine A(2A) receptors (A(2A)R) are involved in inflammatory processes. Considering that A(2A)R activation may have incidence on CXCR4 and CCR5 protein expression through heterologous desensitization process, we tested Adonis, an agonist-like monoclonal antibody to A(2A)R on CD4+ CEM T-cells. We found that Adonis inhibited the CEM cell growth, upregulated A(2A)R and downregulated CXCR4 and CCR5 without modifying the CD4 expression. By reducing the expression of CXCR4 and CCR5 chemokines receptors utilized as entry co-receptors by HIV-1 during viral infection of CD4 expressing cells, Adonis stimulation of A(2A)R appears as a valuable means to treat infected cells.

Involvement of endogenous opioid system in scorpion toxin-induced antinociception in mice.

The present study analyzes the involvement of the endogenous opioid system in the antinociceptive effects produced in mammals after alpha- or beta- scorpion toxin injections. The analgesic effects on mice of the alpha-anatoxin Amm VIII, a weak modulator of Na(v)1.2 channel, and the depressant insect-selective beta-toxin LqqIT2 were evaluated by intraperitoneal route. The two toxins increased hot plate and tail flick latencies in a dose-dependent manner. We also compared the effects of the toxins with those obtained after acetic acid administration or cold-water tail immersion, which both induce pain relief through the activation of diffuse noxious inhibitory controls (DNIC) and the release of endogenous opioids. The increased latencies obtained with the toxins, acetic acid, or cold-water tail immersion were partly reversed by the co-administration of the opioid receptor antagonist naloxone. Finally, AmmVIII, LqqIT2, or acetic acid, induced increased c-fos mRNA expression in spinal cord. This increase disappeared when the toxins were co-injected with acetic acid. In conclusion, we show for the first time that an alpha-anatoxin exhibits a potent analgesic activity and confirm that depressant beta-toxins are able to reduce nociception. We hypothesize that pain relief induced by these scorpion toxins may implicate the activation of an endogenous opioid system and may be partly the result of a counter irritation phenomenon, which could be due to the activation of DNIC.