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1. Biological Model of Addiction<br />Web link HYPERLINK http://learn.genetics.utah.edu/content/addiction/ …
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  • 1. Biological Model of Addiction<br />Web link HYPERLINK " http://learn.genetics.utah.edu/content/addiction/" http://learn.genetics.utah.edu/content/addiction/<br />Biochemistry of the Brain; Neurotransmitters<br />(i) Initiation<br />In the centre of the brain sits the reward pathway, which is responsible for driving our feelings of motivation, reward and behaviour. Your memory tells you that a particular behaviour will make you feel good; the brain tells the body to initiate the behaviour. Special neurons in the reward pathway release the chemical dopamine (VTA), which gives you a sense of pleasure (NA). In addition, the reward pathway is responsible for making sure you repeat the behaviour. It does this by connecting to regions of the brain that control memory and behaviour. This increases the likelihood that you will repeat the behaviour. When the reward pathway signals the brain’s motor centre, it strengthens the wiring for behaviours that help you achieve your reward. <br />The Reward Pathway and associated areas of the Brain<br />(ii) Maintenance<br />Addictive behaviours and addictive substances can induce change in the structure and function of the reward systems neurons that can be shown, using PET scans to last for weeks and months. These changes contribute to tolerance, dependence and craving. The VTA sends neuron projections into the medial forebrain connecting to the NA, amygdala and PFC. Stimulation of these areas collectively, produces pleasure and reinforcement of that behaviour. Most drugs (such as nicotine which is a psychoactive drug) and activities such as gambling release dopamine into the NA area, prompting incentive to continue and increase the behaviour. The PFC function which normally controls decision-making and inhibits risky behaviours is impaired in addicts, allowing them to choose immediate rewards even in the face of long term negative consequences. Continued over-production of dopamine leads to desensitisation in receptors to compensate. This leads to increased desire to engage in the addictive behaviour to return to the same level of ‘dopamine high’ ie the subject is becoming tolerant.<br />(iii) Relapse<br />The VTA –NA pathways also link to other areas of the brain including the memory areas and help make addicts highly sensitive to reminders of past highs, vulnerable to relapse when stressed and unable to control the urge to repeat the addictive behaviour.<br />Genetics<br />When scientists look for " addiction genes," what they are looking for biological differences that may make someone more or less vulnerable to addiction. It may be harder for people with certain genes to stop behaviour once it starts. Or they may experience more severe withdrawal symptoms. Factors that make it harder to become addicted also may be genetic. For example, an individual may feel sick from a drug that makes other people feel good. But someone's genetic make up will never lead them to inevitably become an addict. Environment makes up a large part of the addiction risk.<br />Scientists will never find just one single addiction gene. Susceptibility to addiction is the result of many interacting genes. Social and environmental factors contribute to this risk of addiction. Like other behavioural diseases, addiction vulnerability is a very complex trait. <br />" Just because you are prone to addiction doesn't mean you're going to become addicted. It just means you've got to be careful." <br /> <br />Researchers construct pedigrees of large families with addiction as a first step to understanding the disease. A pedigree can reveal whether or not a trait has a genetic component. That is, whether or not it is passed down from parent to child by way of genes. Various genes and environmental factors can add up or cancel each other out. Not every addict will carry the same gene, and not everyone who carries an addiction gene will exhibit the trait. Using pedigree data, researchers can hunt down genes. They begin by comparing DNA sequences of individuals who have the disease with those who do not. They can then narrow down the possibilities to identify a small number of so-called " candidate genes" for addiction.<br />Below are samples of some of the genes likely to play a role in addiction;<br />The A1 allele of the dopamine receptor gene DRD2 is more common in people addicted to alcohol or cocaine.<br />Increased expression of the Mpdz gene results in mice experiencing less severe withdrawal symptoms from sedative-hypnotic drugs such as barbiturates.<br />Mice bred to lack the serotonin receptor gene Htr1b are more attracted to cocaine and alcohol.<br />Mice with low levels of neuropeptide Y drink more alcohol, whereas those with higher levels tend to abstain.<br />Mice mutated with a defective Per2 gene drink three times more alcohol than normal.<br />Non-smokers are more likely than smokers to carry a protective gene, CYP2A6, which causes them to feel more nausea and dizziness from smoking.<br />Alcoholism is rare in people with two copies of the ALDH*2 gene variation.<br /> <br />Most genes wouldn't have been discovered without the use of animal models, especially mice.<br />The reward pathway functions in much the same way in mice as it does in people. When researchers discover a gene in mice that plays a role in addiction, they can then identify the counterpart gene in humans by comparing DNA sequences.<br />Developing improved treatments for addiction is now becoming due to the discovery of addiction susceptibility genes. Each new addiction gene identified becomes a potential " drug target." Researchers can focus on one gene product and develop a drug that modifies its activity. In so doing, signals or pathways in the brain may be reversed or stabilized to restore proper brain function.<br /> <br />The effectiveness of addiction medications vary from person to person due to differences in genetic make-up. In the future, genetic tests could be used to determine which genetic variation a patient has and prescribe the best addiction medication for that individual. <br />Twin Studies<br /> <br />……………………………………………………<br />Draw a mind map to brainstorm the Synoptic concepts; Approaches, Issues & Debates.<br />Here are some areas you might consider;<br />Adv & Disadv of the Biological approach<br />Ethical issues<br />Socially sensitive research<br />Use of non-human animals<br />The findings suggest that the progression from the use of cocaine or marijuana to abuse or dependence was due largely to genetic factors." A US study by K Kendler found that concordance rates -both twins using, abusing, or being dependent on drugs- were higher for identical than fraternal twins (see chart). For cocaine use, concordance was 54 percent in identical twins and 42 percent in fraternal twins; for abuse, 47 percent in identical twins and 8 percent in fraternal twins; and for dependence, 35 percent in identical twins and zero for fraternal twins.<br />……………………………………………………………………….<br />Nature- Nurture<br />Free will or determinism<br />Reductionism<br />
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