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Wednesday January 7 6:27 PM EST Nerve Receptors Control Nicotine Urge NEW YORK (Reuters) -- Chemical receptors lying on the surface of certain brain cells may play a key role in nicotine addiction, researchers say. A study led by Dr. Jean-Pierre Changeux of the Institut Pasteur in Paris, France, labels the 'nicotinic acetylcholine receptor' "one of the primary (chemical pathways) for the addictive properties of nicotine." The study, published in the current issue of the journal Nature, focused on nerve cells in the mesolimbic system, a primitive area of the brain long associated with various types of addiction. The researchers had already theorized that nicotine ingestion works by stimulating production of the neurotransmitter dopamine within the mesolimbic area. Dopamine seems to play a crucial role in stimulating both physical activity and what scientists label the 'reinforcing' impulse: the pursuit of behaviors (such as hunting or eating), which supply some type of survival 'benefit'. Many experts now believe that addictive substances can short-circuit this otherwise positive impulse, driving individuals to ingest or inhale substances which, in fact, do them more harm than good. The French study focused on the nerve processes of mice, whose cellular brain chemistry shares remarkable similarities with our own. The investigators knew that laboratory mice, when offered nicotine in special mouse-activated dispensers, will repeatedly seek out the drug as their addiction increases. However, the French experts bred a strain of mice which lacked a portion of the 'nicotinic' receptor thought to be especially reactive to nicotine. Removal of this 'subunit' effectively disabled the receptor. The researchers say that, unlike their 'normal' cousins, the receptor-impaired mice displayed no special interest in triggering the nicotine-dispensers. Changeux and his team theorized that because the mice lacked working receptors, the ingestion of nicotine failed to stimulate the production of dopamine -- and these mice were less likely to become addicted. Microscopic examination of mouse brain cells showed similar responses on the cellular level -- neurons without fully-functioning receptors displayed little or no dopaminergic activity when exposed to nicotine. Unfortunately, the French team claim there is a down-side to this disconnection of the nicotinic receptors. Locomotion decreased along with addiction -- the study authors say the overall physical activity of the 'switched-off' mice "decreased by 50%," compared with their healthy cousins. SOURCE: Nature (1998;391:173-177) Copyright © 1997 Reuters Limited. Judith Richards [log in to unmask]