What foods or medicinal compounds decrease serotonin (i.e. serotonin-mediated neurotransmission)?

There are literally hundreds of pages explaining how to increase serotonin and improve mood, but regulating mood is only one of serotonin's many functions. Most people don't realize that serotonin also exerts an inhibitory effect on acetylcholine and glutamate transmission, and for some, increasing Serotonin can cause confusion symptoms similiar to "sluggish cognitive tempo". Some researchers are looking for serotonin antagonists as a treatment for dementia patients as a way to improve lost cognitive functions. In my opinion, its possible that many people diagnosed with ADD would also benefit from this kind of compound as well. So I wonder, are there any natural ways to reduce serotonin? I have read that a low carb diet would do this, but I would like to find a psychoactive compound if possible. (Remember, it would have to cross the Blood-Brain Barrier!)