Here is an excerpt from an interesting article (by Elizabeth Finkel),please follow link to read the whole article.
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http://www.cosmosmagazine.com/node/1441
AS TIME HAS passed, cannabis dependence syndrome has been described not only in humans, but in rats too. In the past, rats were reassuring because they didn't get addicted to THC when it was added to their food. But according to the traditional rat test, nicotine isn't addictive either – and anyone who has tried to give up smoking tobacco will certainly disagree with that. So could it be that there's something wrong with the test?
Just as people are now given a different test to measure dependence, so are rats. If rats are put in a cage with a red and a green room, and they are injected with THC only in the green room, they will end up spending more time in the green room. Arguably, it mimics the behaviour of a dependent person: they change their life activities to acquire the substance. On this test, rats are revealed as forming dependence on both nicotine and cannabis.
A couple of rat studies undertaken in 1997 also led scientists to view cannabis in a more sinister light. Opiates act on the brain's reward circuitry, which exists to keep us having sex, eating and doing well at tasks. When we do one of those pleasurable things, the brain rewards each of these functions with a hit of dopamine, a naturally produced neurotransmitter. With food, sex and work, our bodies give us feedback to say when we've had enough. But opiates go straight to the brain, to keep activating the pleasure centre. With every hit of heroin, the user gets a hit of dopamine.
If rats are injected with heroin, researchers see surges of dopamine released. In 1997, when researchers at the University of Cagliari in Italy injected THC into rats, they saw a comparable dopamine surge.
For some scientists, studies like this changed their perception of cannabis forever. In a commentary in the U.S. journal, Science, neurobiologist David Friedman at Bowman Gray School of Medicine in North Carolina said, "These studies supply important evidence that cannabis acts on the same neural substrates [brain circuits] and has the same effects as drugs already known to be highly addictive. They send a powerful message that should raise everyone's awareness about the dangers of cannabis use."
Henry Lester, a biologist who studies the chemistry of nicotine addiction at the California Institute of Technology, put it this way, "If I knew as much about the mechanism of action of cannabis as I now know, I would never have touched it 35 years ago. I consider it a highly dangerous drug, akin to the opiates."
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from the same article-
Murat Yucel, a neuropsychologist and brain researcher at the Orygen Research Centre at the University of Melbourne, reads even more into this finding. "We know that 75 per cent of all mental illness has its origins during the teenage years. We've been aware of that for a long time, but now we may understand why. This is a critical time when parts of the brain involved in weighing up one's actions are still maturing and connecting up with the emotional centre. Maybe disrupting that link has something to do with the emergence of mental disorders."
Just as a finished building is better able to withstand damage than the bare foundations, the brains of teenagers are likely to be more vulnerable to the effects of damaging drugs. So far, the evidence incriminating marijuana is sparse, but Yücel sees an intriguing trend: in those who have used both alcohol and marijuana from an early age, the size of the amygdala (where emotions are processed) is larger; while
the hippocampus (where short-term memories are stored) is smaller than in people who used the cocktail later in life. Interestingly,
that is precisely the kind of problem reported by many drug users: mood disorders and memory problems.
Most of the researchers I interviewed, including Yücel, agreed that it remains to be proven whether marijuana permanently affects the function of the teenage brain. Still, none hesitated to proffer that they thought teenagers were at the highest risk from using marijuana, and should avoid it. David Castle's concerns relate not so much to long-term brain damage but the clear-cut impairment to learning and memory. "My concerns are more at a pragmatic level; if youngsters are stoned – and the higher doses today are a major concern – then they are not going to lay down the bedrock of education they need."
There is also agreement that the younger users were more likely to spiral down into dependence followed by the use of harder drugs, dropping out of school, perhaps falling into crime … indeed, the whole nightmare that spooks parents. It seems no-one is going to be able to prove this one, but the warnings seem clear: delay marijuana use as long as possible. Use it moderately. Don't use it at all if you have a personal or family history of mental illness. And stop using it if it makes you feel anxious, distressed or paranoid.
Other harms of regular marijuana use include an increased risk of emphysema (destruction of the elastic wall of the air sacs in the lung) above that of smoking alone. Some studies have shown that it increases the risk of head and neck cancers. And while researchers such as Hall have long been unconvinced about a link between marijuana and lung cancer, in May 2006 researchers at the University of California, Los Angeles reported they found no increased risk of lung cancer in marijuana smokers. Tobacco smoking, however, increased the risk by 20-fold. Again, to quote visiting Oxford pharmacologist Les Iversen: "On balance, the public health risks are small. Bear in mind that millions of people smoke marijuana occasionally and there is no evidence of harm. You have to judge the benefits against the risks. I would say a lot of people benefit from moderate use. But that's not to say it's not a dangerous drug."
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