Serotonin pain

Pain researchers devised a method for assessing human pain. A blister was induced to the same part of the hand of each subject, and the blister base used as the testing ground for certain drugs. When serotonin was applied to a blister base, there were 19 sec between the time of 5-HT administration and the perception of pain as reported by human volunteers. The figure below is a graphic representation of pain (y-axis). Higher concentrations of 5-HT caused more intense pain. This experiment verified the hypothesis that the pain of a wasp sting is related to the high 5-HT and histamine content of wasp venom.



In human volunteers, 5-HT was more potent at causing pain than DMT, bufotenin, or tryptophan (Table 1).

From 5-HT pain

An animal model of pain was developed with rats. Each test chemical was injected into the paw of a rat, and the amount of swelling (edema) in the paw was measured, in terms of the thickness of the paw in millimeters. A photo of this procedure is shown below. Chemicals such as 5-HT and histamine injected subcutaneously caused local inflammation of the paw.

From 5-HT pain

Injected 5-HT to the rat paw was 200X more potent than histamine at causing edema. The graph below shows that 5-HT is more painful than histamine, causing more edema at lower doses. A mere dose of 1 ug 5-HT per paw caused a swelling reaction.




The swelling caused by 5-HT injection was prevented by pre-treatment of rats with LSD and certain phenothiazine drugs.


References

Lewis, G. P., Ed. 5-Hydroxytryptamine. Pergamon Press: New York, 1958.