Why do amphetamines suppress appetite?

Amphetamines are like having a long lasting rush of adrenaline (epinephrine). The sympathetic nervous system is activated, so increased fight or flight response to muscles and breathing and decreased rest and digestion. So the hormones and paracrines factors that stimulate digestion and hunger are suppressed in their secretion. I am not sure if they 'block' the brain receptor directly but you have the right idea. The stomach expands and shrinks to the food in it, so if it does not have alot of food it will be contracted.