The rewarding or reinforcing effects of amphetamines, cocaine, and heroin have been associated with the effect on which major neurotransmitter? a. serotonin b. dopamine c. gaba d. acetylcholine user: parasympathetic activation can result in all of the following except a. decreased blood pressure b. constriction of the pupils and bronchi c. decreased function of the gastrointestinal tract d. decreased heart rate