Is there a difference between histamine and methacholine?

Yes different: Histamine a compound that is released by cells in response to injury and in allergic and inflammatory reactions, causing contraction of smooth muscle and dilation of capillaries. Methacholine is a synthetic choline ester that acts as a non-selective muscarinic receptor agonist in the parasympathetic nervous system. both are use to test for airway reactivity in asthma.