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Rat Park was a study into drug addiction conducted in the late 1970s (and published in 1980) by Canadian psychologist Bruce K. Alexander and his colleagues at Simon Fraser University in British Columbia, Canada. Alexander's hypothesis was that drugs do not cause addiction, and that the apparent addiction to opiate drugs commonly observed in laboratory rats exposed to it is attributable to their living conditions, and not to any addictive property of the drug itself.〔Alexander, Bruce K., (2001) ("The Myth of Drug-Induced Addiction" ), a paper delivered to the Canadian Senate, January 2001, retrieved December 12, 2004.〕 He told the Canadian Senate in 2001 that prior experiments in which laboratory rats were kept isolated in cramped metal cages, tethered to a self-injection apparatus, show only that "severely distressed animals, like severely distressed people, will relieve their distress pharmacologically if they can."〔Weissman, D. E. & Haddox, J. D. (1989). "Opioid pseudoaddiction: an iatrogenic syndrome," ''Pain'', 36, 363–366, cited in Alexander 2001, ''op cit''.〕 To test his hypothesis, Alexander built Rat Park, an housing colony, 200 times the floor area of a standard laboratory cage. There were 16–20 rats of both sexes in residence, an abundance of food, balls and wheels for play, and enough space for mating and raising litters.〔Slater, Lauren. (2004) ''Opening Skinner's Box: Great Psychological Experiments of the Twentieth Century'', W.W. Norton & Company.〕 The results of the experiment appeared to support his hypothesis. Rats who had been forced to consume morphine hydrochloride for 57 consecutive days were brought to Rat Park and given a choice between plain tap water and water laced with morphine. For the most part, they chose the plain water. "Nothing that we tried," Alexander wrote, "... produced anything that looked like addiction in rats that were housed in a reasonably normal environment."〔 Control groups of rats isolated in small cages consumed much more morphine in this and several subsequent experiments. The two major science journals, ''Science'' and ''Nature'', rejected Alexander, Coambs, and Hadaway's first paper, which appeared instead in ''Psychopharmacology'', a respectable but much smaller journal in 1978. The paper's publication initially attracted no response.〔Alexander, B.K., Coambs, R.B., and Hadaway, P.F. (1978). "The effect of housing and gender on morphine self-administration in rats," ''Psychopharmacology'', Vol 58, 175–179. PMID 98787〕 Within a few years, Simon Fraser University withdrew Rat Park's funding. ==Disease model of drug addiction== It is not disputed that some substances cause withdrawal symptoms after repeated use, leaving the user in distress if they stop using. Where scientists differ is over the extent to which certain substances can be said to rob the user of self control, causing not only withdrawal〔Jaffe, J.H. Drug addiction and drug abuse. In: Gilman, A.G.; Goodman, L.S.; Rall, T.W.; Murad, F. (eds), The Pharmacological Basis of Therapeutics (7th edition), p 532–581. New York: Macmillan Publishing Co., 1985.〕— but a drug addiction, defined as "a behavioral pattern of drug use, characterized by overwhelming involvement with the use of a drug (compulsive use), the securing of its supply, and a high tendency to relapse after withdrawal."〔 In the 19th century, drug addiction was regarded as a sign of akrasia, immorality, or weakness of the will. However 20th century brain research replaced this moral model with a disease model of addiction, according to which addiction to a drug is a by-product of the chemical structure of the drug itself. According to social psychologist Stanton Peele, the disease model states that "()olerance, withdrawal, and craving are thought to be properties of particular drugs, and sufficient use of these substances is believed to give the organism no choice but to behave in these stereotypical ways."〔Peele, Stanton. ''The Meaning of Addiction. Compulsive Experience and Its Interpretation''. Lexington: Lexington Books, 1985, pp. 1–26. (excerpt )〕 This view of drug addiction is reflected in the policies of the War on Drugs and in slogans such as "Heroin is so good. Don't even try it once," or "Crack cocaine is instantly addictive."〔 Scientists adhering to the disease model believe that behavior is "the business of the brain," according to Avram Goldstein, Professor Emeritus of Pharmacology at Stanford University, and a leading researcher into drug addiction.〔 Goldstein writes that the site of action of heroin and all other addictive drugs is a bundle of neurons deep in the brain called the mesolimbic dopaminergic pathway, a reward pathway that mediates feelings of wanting and motivation. Within this pathway, heroin causes dopamine neurons to release dopamine, a neurotransmitter that determines incentive salience and causes the user to want more. Dopamine neurons are normally held in check by inhibitory neurons, but heroin shuts these down, allowing the dopamine neurons to become overstimulated. The brain responds with feelings of euphoria, but the stimulation is excessive, and in order to protect itself against this, the brain adapts by becoming less sensitive to the heroin.〔Goldstein, Avram. ("Neurobiology of Heroin Addiction and of Methadone Treatment" ), American Association for the Treatment of Opioid Dependence, retrieved July 16, 2006.〕 This has two consequences, according to the disease model. First, more heroin is required to produce the high, and at the same time, the reward pathway becomes less sensitive to the effects of endorphins, which regulate the release of dopamine, so that without heroin, there is a persistent feeling of sickness. After repeated intake, the user becomes tolerant and dependent, and undergoes withdrawal symptoms if the heroin supply is terminated. As the feelings of withdrawal worsen, the user loses control, writes Goldstein, and becomes an addict.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Rat Park」の詳細全文を読む スポンサード リンク
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