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Background:   Here, we described the presence of a neurotoxin with phospholipase A 2  activity isolated from  Micrurus lemniscatus  venom (Mlx-8) with affinity for muscarinic acetylcholine receptors (mAChRs).Methods:   The purification, molecular mass determination, partial amino acid sequencing, phospholipase A 2  activity determination, inhibition of the binding of the selective muscarinic ligand [ 3 H]QNB and inhibition of the total [ 3 H]inositol phosphate accumulation in rat hippocampus of the Mlx-8 were determined.Results:   Thirty-one fractions were collected from HPLC chromatography, and the Mlx-8 toxin was used in this work. The molecular mass of Mlx-8 is 13.628 Da. Edman degradation yielded the following sequence: NLYQFKNMIQCTNTRSWL-DFADYG-CYCGRGGSGT. The Mlx-8 had phospholipase A 2  enzymatic activity. The pK i  values were determined for Mlx-8 toxin and the M 1  selective muscarinic antagonist pirenzepine in hippocampus membranes via [ 3 H]QNB competition binding assays. The pK i  values obtained from the analysis of Mlx-8 and pirenzepine displacement curves were 7.32 ± 0.15, n = 4 and 5.84 ± 0.18, n = 4, respectively. These results indicate that Mlx-8 has affinity for mAChRs. There was no effect on the inhibition ability of the [ 3 H]QNB binding in hippocampus membranes when 1 µM Mlx-8 was incubated with 200 µM DEDA, an inhibitor of phospholipase A 2 . This suggests that the inhibition of the phospholipase A 2  activity of the venom did not alter its ability to bind to displace [ 3 H]QNB binding. In addition, the Mlx-8 toxin caused a blockade of 43.31 ± 8.86%, n = 3 and 97.42 ± 2.02%, n = 3 for 0.1 and 1 µM Mlx-8, respectively, on the total [ 3 H]inositol phosphate content induced by 10 µM carbachol. This suggests that Mlx-8 inhibits the intracellular signaling pathway linked to activation of mAChRs in hippocampus.Conclusion:   The results of the present work show, for the first time, that muscarinic receptors are also affected by the Mlx-8 toxin, a muscarinic ligand with phospholipase A 2  characteristics, obtained from the venom of the Elapidae snake  Micrurus lemniscatus , since this toxin was able to compete with muscarinic ligand [ 3 H]QNB in hippocampus of rats. In addition, Mlx-8 also blocked the accumulation of total [ 3 H]inositol phosphate induced by muscarinic agonist carbachol. Thus, Mlx-8 may be a new pharmacological tool for examining muscarinic cholinergic function.

Effects of Mlx-8, a phospholipase A2 from Brazilian coralsnake Micrurus lemniscatus venom, on muscarinic acetylcholine receptors in rat hippocampus