We know that schizophrenia is a condition in which there is too much dopamine in the brain? So could this mean that they're less likely to develop tolerance to too much dopamine than others are? That would be interesting...

BTW, does acetylcholine also play a role in ADD? Increasing acetylcholine can help with some ADD symptoms.

Acetylcholine is thought to play a role in memory, and acetylcholine levels in the hippocampus are thought to play a role in Alzheimer's disease, for instance. Also, there are some areas of the brain, from what I understand, where acetylcholine will lead to a downstream cascade of dopamine. In other words, acetylcholine activates neurons, which then release dopamine elsewhere.

As for schizophrenia and dopamine, keep in mind that while schizophrenia involves excess dopamine, it involves excess dopamine in specific areas, and it may be that dopamine tolerance in these regions works differently from the dopamine tolerance that can develop in the reward pathways. I don't know much about schizophrenia, though, so I could be wrong. I also wouldn't be surprised if serotonin played a role in that disorder, given its role in sensory perception in some regions, as well as the fact that many hallucinogenic drugs work on a specific subset of serotonin receptors.