Question: Vivitrol is a once-a-month shot given to opiate-addicted individuals after they have been through a detoxification program. Vivitrol is a molecule that binds to dopamine receptors, which prevents heroin and other opiates from having most (but not all) of their positive-reinforcing effects. The makers of Vivitrol decreased the reinforcing efficacy of heroin by manipulating what dimension of effective reinforcement?