Edited bits from
Research into addiction - Health Report - 17 August 2009
Barry Everitt is Professor of Behavioural Neuroscience in the Department of Experimental Psychology at the University of Cambridge.
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a huge proportion of the population take drugs and by that I would include alcohol and tobacco, very few proportionally go on to become what we think is truly addicted.
So why is it that some individuals who are exposed to drugs have a brain that reacts in a way that leads them down this path to compulsion and others don't?
Barry Everitt: The definitions of addiction have changed over the years and 20 or so years ago the notion was that unless you had or displayed a physical withdrawal syndrome like that that occurs on withdrawal from opiates or alcohol you couldn't really be addicted and indeed
stimulant drugs like cocaine weren't even classed as addictive drugs until relatively recently.
I think the view of addiction as a dependence syndrome has changed to one in which life's activities are given over to procuring and taking drugs and we talk about addicts compulsively seeking and taking drugs at the expense of other activities in their lives and it's very disruptive.
They may or may not have the withdrawal or dependent syndrome typically though we think of them as compulsive drug takers and it's one of the newer views that's brought other pathological states like pathological gambling for example into the same remit of
compulsive behaviour.
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But many people do that, get that hit, and do it again, and again, and again but will
never go on to fulfil the criteria of being addicted.
It's controlled use, it might be social use, it might border on abuse but it's controlled. It doesn't explain why this smaller group, and it's often in human studies,
it's often estimated to be about 20% of those that initially take drugs that go on to lose control over their intake.
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Addicts, human addicts are often described as being
impulsive, sensation seekers, risk takers as well as being compulsive and one has never known with the many studies that have been conducted now on alcoholics, cocaine addicts, heroin addicts is whether that
impulsivity and risk taking...
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So he and others have made a link between propensity to drug addiction and impulsivity.
The question is whether that stretches to the problem which has
impulsivity as its cornerstone, Attention Deficit Hyperactivity Disorder (ADHD). Could the argument that untreated ADHD leads to drug use be right?
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And it turns out intriguingly that impulsive animals, when they take cocaine, cocaine we think of as a stimulant, would make you impulsive it turns out that cocaine medicates the impulsivity. So it makes the impulsivity reduce and you raised ADHD just now, a stimulant amphetamine...
Norman Swan: Has this paradoxical effect.
Barry Everitt: Methal phenidate is the treatment which is a stimulant but it reduces that kind of hyper-activity in ADHD
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The role of memory
Barry Everitt: Absolutely, so the question then arises could you modify them? I mean presumably we have this mechanism so we can update memories, you know strengthen them, add new bits of information but there's a risk.
And you know people who are banged on the head whilst retrieving memories suffer amnesia, animal experimental studies show that if you inhibit protein synthesis or knock down specific plasticity related genes that make those proteins you can actually erase the memory and we've been trying to understand what the neurochemical mechanisms might be.
And one particular transmitter seems very important in this which is
glutamate. Glutamate in certain bits of the brain for emotional memories like the amigdala seems to be a key component of that restabilisation process.
And Amy Milton in my lab and Jonathan Lee before her have shown that
if you inhibit glutamate transmission at memory retrieval and then come back a day or a week later and look and see whether the memory is intact it's gone.
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one that has long been implicated in mediating the positive effects of addictive drugs and it turned out that these
impulsive animals had very low levels of dopamine D2 and D3. Or you can't really differentiate them so a sub type of dopamine receptor D2/D3 specifically in the ventral parts of the striatum, now the striatum is that area that receives the biggest dopaminergic innervation in the brain.
Norman Swan: Where about is it?
Barry Everitt: It's sub-cortical.
Norman Swan: Under the surface.
Barry Everitt: Under the surface and above the brain stem and we know about the striatum and dopamine because that's the locus of the pathology in Parkinson's Disease for example but that's the dorsal part of the striatum. This was the ventral striatum, an area known as the nucleus accumbens,
which for the last 30 years has been regarded as the hot spot where addictive drugs act to bring about their positive reinforcing or rewarding effects.
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Norman Swan: So in summary then you are really describing somebody who's addicted as, to some extent, significantly entrained in their makeup and therefore the ability of just to get a hold of yourself and stop this stuff is limited?
Although 10% of people who are addicted to heroin do that every year.
Barry Everitt: That's right, it's complex. I think the notion is that there are vulnerable individuals who when they take drugs will be led down a path of behavioural and cognitive change that they have no control over, there's an interaction between their genetic therefore brain makeup and the effects that the drug has but at some point individuals, most individuals do recognise that the drug taking is damaging them and they want to stop so you have this volition to stop.
It's whether you can actually bring that to play and gain control over your own behaviour.
And I think this does hark back to why I think the drug addiction field is being so ignored by the pharmaceutical industry and not really treated that seriously by the medical profession until recently which is addicts are just people who show a weak will and all they need to do is say no.
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What causes drug addiction? How should it be treated?
