Yes, the dysregulation in the stuttering brains dopaminergic system is the issue. Some of us have hyperactivity in the dopaminergic system, which is why dopamine antagonists (Ecopiman etc) work, and some of us have an underactive dopaminergic system, which is why dopamine agonists (Vyvanse etc) work for those subsets of people. The down or up regulation of dopamine into the optimum zone improves the timing in the speech motor networks, giving us better fluency whilst on the drug. Stutters can be in either camp depending on their neurological makeup, which is why certain drugs work and others don’t. Dr. Gerald Maguire goes into this deeply in one of his YouTube presentations.