I always understood Yohimbe to work primarily on Norepinephrine and not directly on Serotonin. Maybe I'm just flat wrong here or I'm simplifying this too much, but if I'm correct how could that cause Serotonin Syndrome? If it doesn't act directly on Serotonin how could it be Serotonin Syndrome since Serotonin is downstream from Dopamine and Norepinephrine (DA-> NE -> 5-HT)? If I'm way off on this feel free to educate me. My education is in Engineering, not Chemistry or Biology, but I'm always eager to learn.
I do not dare to educate anyone within chemistry. I myself am educated within electrical engineering, currently trying to finish my Ph.D. studies within applied signal processing. However I do not mind to debate matters which I am interested in since also I am eager to learn. And what better way of learning then describing own understanding of the specific subject to others, to be either agreed on or corrected. Hopefully in the end we all learn something new. I do apology if my previous statements here and in the DMAA thread seemed as a pure fact, which I clearly do not claim.
I know Wikipedia is not the best source of the information, but it can point one in the right direction. If you read the pharmacology part under yohimbine it does state which serotonin 5-HT receptor it affects. If compared with the info in article on 5-HT receptors, the yohimbine is mentioned either as agonist or antagonist under many of the specific types of 5-HT receptors. Then if you read the symptoms of Serotonin syndrome you will find many of the side effects you and many others experience mentioned. I am not saying this may be 100% correct, rather that it could be a possible explanation. It is 1am in Sweden, but I can try to provide specific articles about this matter within near future.
On a side note, I do not think it matters if the serotonin receptors are directly or indirectly affected to cause the serotonin syndrome. From what I can read, the serotonin syndrome can equally be caused by amphetamine or methylphenidate which only affects dopamine and norepinephrine receptors. So the serotonin receptors are probably affected though the chain of events you are describing. But in the end, the serotonin receptors are affected, which gives the possibility to cause serotonin syndrome.
Regarding the varying reactions to yohimbine, I suppose, some people are more sensitive than others. I tried yohimbine two years back in the ALRIs Poison. The first three days it gave me loose or liquid bowel movements, after that no negatives. I got slightly stimulated in contrast to the global version of the Poison that had the yohimbine removed.
