The action of serotonin (5-hydroxytryptamine) in vivo upon the microfilariae of Dirofilaria, Loa and five other species.

Abstract

When serotonin is injected intravenously into certain animals containing microfilariae it causes a sharp short rise in the microfilaria count in the blood, i.e. liberation of microfilariae from the lungs. This happens in the case of Dirofilaria immitis (in dog), D. repens (in dog), Loa loa (in drill), Edesonfilaria malayensis and another Dirofilaria-like microfilaria (in monkey), Monnigofilaria setariosa (in mongoose), and Dipetalonema gracile (in squirrel monkey); but not with Dip. witei (in gerbil) and Litomosoides carinii in cotton rats.This action (upon D. immitis) is not inhibited by atropine. Administration of reserpine, harmine or harmaline to dogs did not exert any definite action on the microfilaria count of D. immitis. Serotonin had no obvious effect on the in vitro activity of microfilariae of L. loa or E. malayensis. Melatonin had no action in vivo upon any of the above microfilariae.The action of serotonin upon microfilariae in vivo is similar to that of acetylcholine but it is not identical with it since the sensitivity of Dip. gracile and Dip. witei to the two compounds is different.Apparently this action is not involved in the production of the 24 h periodicity shown by many microfilariae. It may depend upon disturbance of the normal neuromuscular mechanisms of the microfilariae by which they are retained in the small vessels of the lungs.