These neurotransmitters include noradrenaline, serotonin, dopamine, histamine, and acetylcholine. The neurotransmitters that these neurons release include dopamine, norepinephrine, serotonin, histamine, acetylcholine, and glutamate. Many neurotransmitters are amines, including epinephrine, norepinephrine, dopamine, serotonin, and histamine. Leukotrienes are released from mast cells along with histamine. It acts as an inverse agonist that decreases activity at histamine H1 receptors. In contrast, histamine H receptor antagonists increase wakefulness. The underlying mechanism typically involves histamine or bradykinin. Gastric secretion is stimulated chiefly by three chemicals&colon; acetylcholine (ACh), histamine, and gastrin. Antihistamine drugs act by blocking histamine action on nerve endings. Antihistamines, originally classified as antagonists of histamine H receptors have been reclassified as inverse agonists. Doxylamine acts primarily as an antagonist or inverse agonist of the histamine H receptor. Cetirizine acts as a highly selective antagonist of the histamine H receptor. This receptor is activated by the biogenic amine histamine. Antihistamines suppress the activity of the histamine. It is specifically a potent inverse agonist of the histamine H receptor.