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