<prompt> Tone of LES is maintained byAcetyl cholineDopamineSerotonineVIP  <response>: Acetylcholine and tachykinins are involved in the LOS contraction acting through acetylcholine M3 and tachykinin NK2 receptors. Nitric oxide can also be involved in the regulation of LOS contraction. Ref: guyton and hall textbook of medical physiology 12 edition page number:443,444,445 