Effects of Mlx-8, a phospholipase A ₂ from Brazilian coralsnake Micrurus lemniscatus venom, on muscarinic acetylcholine receptors in rat hippocampus

Abstract

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 [ ³H]QNB and inhibition of the total [ ³H]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 [ ³H]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 [ ³H]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 [ ³H]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 [ ³H]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 [ ³H]QNB in hippocampus of rats. In addition, Mlx-8 also blocked the accumulation of total [ ³H]inositol phosphate induced by muscarinic agonist carbachol. Thus, Mlx-8 may be a new pharmacological tool for examining muscarinic cholinergic function.