A mechanism for ADHD stimulant medication tolerance has now been found. See http://neurosciencenews.com/adhd-medication-patient-brains-adapt-dat/.
Here's the thing though: what exactly is the "signal" that the body uses to interpret whether or not it needs to upregulate the amount of DAT (dopamine transporter) that exist? Is it a dopamine-based signal that somehow then gets carried to a transcription factor that gets carried to the nucleus for additional transcription?