Don't most, if not all, spices increase serotonin in the gut? And at the opposite, bitterness is good (ultra-bitterness, in the case of cascara...). They are often recommended as "brain boosters" as increasing serotonin. They also say that the gut can "tell" when there is spice, just like the mouth, and I would find it intuitive that "hot" in the gut means serotonin or inflammation.