Physiological Actions::
Gastrointestinal Tract:
Serotonin stimulates intestinal peristalsis by decreasing the threshold of nerves which evoke peristalsis lysergic acid diethylamide (LSD) is an antiserotonin drug which has the action of decreasing peristalsis by virtue of its power to neutralize the serotonin action ninety percent of the total 5-HT is found in the intestinal wall (Enterochromaffin cells) Tumor of enterochromaffin cells causes carcinoid syndrome which is characterized by diarrhoea bronchoconstriction flushing decrease in blood pressure cardiac fibrosis pellagra etc methysergide cyproheptadiene sumatriptan etc the 5-HT antagonists are used in this condition
Smooth muscles::
Smooth muscles of other organs such as stomach bronchioles etc are also contracted by 5-HT
Platelets::
In the process of coagulation of blood the platelets have to break liberating thrombokinase and 5-hydroxytryptamine the former takes part in the clotting process while the latter exerts a vasoconstrictor action on blood vessels thus indirectly it checks bleeding thus serotonin stops haemorrhage
Cardiovascular system::
5-HT produces positive inotropic and chronotropic action on heart This action is mediated through 5-HT2 receptors that cause release of adrenaline from adrenal medulla and 5-HT3 /HT4 receptors causing direct tachycardia in coronary blood vessels 5-HT causes vasodilatation
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