|MadSci Network: Biochemistry|
I have restated your questions and tried to answer them individually. How do antiemetics work? (Reference 1) Several classes of antiemetic drugs are now available. These agents block the actions of different neurotransmitters (histamine, acetylcholine, serotonin and dopamine) in the body. These agents act at several sites in the complex processes involved in the vomiting reflex. The vomiting reflex is coordinated in the vomiting center in what is called the "lateral reticluar formation", which is found in the medulla oblongata (a part of the central nervous system present at the base of your brain). This region receives information from various parts of the body, including the "chemoreceptor trigger zone" (which is present in the brain), the vestibular system (part of which is formed by the parts of the middle ear) and from various sites in the stomach and intestines. Do dramamine and meclizine act in different ways on the body? (reference 1) Dimenhydrinate (DRAMAMINE) and meclizine (ANTIVERT) both block certain actions of histamine , as well as certain actions of acetylcholine, in the body. How do they prevent nausea, vomiting and dizziness? (reference 1) Antihistamines and anticholinergics (agents that block the action of acetylcholine) may be effective in the treatment of motion sickness as the result of a similar action: the ability to block the transmission of information from the vestibular apparatus (the part of the middle ear that is involved in balance) to the emetic center in the medulla oblongata, which is a part of the brain involved in coordinating various reflexes (e.g., swallowing, vomiting). Why is drowsiness a usual side effect? (reference 1) Histamine, acting via H1 receptors in the central nervous system, increases wakefulness. Therefore, antihistamines that block the binding of histamine to H1 receptors and which enter the central nervous system cause drowsiness. In addition, anticholinergic agents cause drowsiness and, thus, antihistamines which possess anticholinergic activity also produce drowsiness via this action. Thus, older agents, such as diphenhydramine, which enter the central nervous system, cause sedation, while newer antihistamines, such as astemizole (HISMANAL) and loratadine (CLARITIN), which poorly penetrate into the CNS, are nonsedating. Patients vary in their susceptibility to the sedative effects of antihistamines. Do all antihistamines have the same effect on motion sickness? (reference 1) Not all "antihistamines" are effective in the treatment of motion sickness. The antihistamines that are effective for the treatment of motion sickness all have significant anticholinergic activity and, thus, it may be the anticholinergic (rather than antihistaminergic) action of these drugs that make them effective for treating motion sickness. The most effective of the antihistamines for the treatment of motion sickness are: dimenhydrinate (DRAMAMINE), diphenhydramine (BENADRYL), promethazine (PHENERGAN) cyclizine (MEREZINE) and meclizine (ANTIVERT). It should be pointed out that, although the term "antihistamine" is commonly thought to refer to medications that are useful for treating allergies and motion sickness, it actually refers to any drug that blocks the action of histamine. Histamine, like many substances in the body, exerts a variety of effects, depending on the tissue in which it is acting and the type of "receptor" with which it interacts. There are three types of histamine receptors: so called, H1, H2 and H3 receptors. The agents that are effective for treating motion sickness are H1 receptor antagonists (with significant anticholinergic actions). I have heard ginger (which acts on the stomach rather than the nervous system) is also effective... what's the most effective? There are varying reports on the effectiveness of ginger as an anti-nausea and antiemetic agent. Gary P. Barnas, MD, Medical Director of the Medical College of Wisconsin International Travelers Clinic states, "Recent studies have shown that Ginger root may be as effective as the other drug treatments [for motion sickness] but is associated with fewer side effects." (reference 2). In contrast, a study by Stewart and colleagues concludes, "…ginger does not possess antimotion sickness activity" (reference 3). I am not certain which is the most effective medication for the treatment of motion sickness. It seems that there is significant variability in patient responses to the available drugs and in the severity of the side effects that these drugs produce. The U.S. Food and Drug Administration states, "The Food and Drug Administration considers four active ingredients to be safe and effective for use in OTC drugs for motion sickness, says Gerald Rachanow, deputy director of the monograph review staff in FDA's Office of OTC Drug Evaluation. The ingredients are cyclizine (Marezine and others), dimenhydrinate (Dramamine and others), diphenhydramine (Benadryl and others), and meclizine (Bonine and others). (reference 5) Finally, I suggest that you take a look at the following MadSci page for a description of motion sickness: http://www.madsci.org/posts/archives/may99/926654723.Ns.r.html REFERENCES 1 Goodman & Gilman's The Pharmacological Basis of Therapeutics, Ninth Edition (1996), edited by J.G. Hardman et al., pp. 928-932 2 http://healthlink.mcw.edu/article/907367055.html 3 Stewart J.J., Wood, M.J., Wood, C.D. and Mims, M.E. (1991) Effects of ginger on motion sickness susceptibility and gastric function. Pharmacology 42 (2), pg. 111-120. 4 http://www.hc-sc.gc.ca/hpb/lcdc/publicat/ccdr/96vol22/dr2213ea.html 5 http://www.fda.gov/opacom/catalog/tummy.html
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