This is another PET image, but this time comparing the brain of a non-smoker to a smoker and with a different radioligand. This time, it is not measuring glucose metabolism but instead MAO-B (monoamine oxidase type B) levels. MAO-B breaks down dopamine in the brain. There is something about smoking that hinders the breakdown of dopamine, the feel-good neurotransmitter that is the culprit in addiction (pretty much all drugs of addiction skyrocket dopamine levels in the brain- which is not to say that dopamine itself is bad, it’s actually very important, but such high levels due to a foreign compound cause that compound to be addicting). This is probably the way that smoking is addictive, because it will increase the levels of dopamine in the brain by hindering dopaminergic breakdown. You can see that smoking has a huge effect on MAO-B levels.