12 Feb 2015

Drug Addiction in the Genes

From Our Changing World, 9:20 pm on 12 February 2015

By Sonia Sly

The abuse of amphetamine-type stimulants is reported to be higher in New Zealand than anywhere else in the world, and costs associated with drug and alcohol abuse are soaring at an estimated $7 billion per year. With these figures in mind, research is being undertaken at Victoria University to look into ways of preventing drug and alcohol addiction.

Bart Ellenbroek, has been conducting this research and believes that those suffering from drug and alcohol dependency cannot be held responsible for what is essentially an illness and disorder of the brain. The research points to links between a person's levels of serotonin, a mood-altering brain chemical, and susceptibility to drug dependence. With more than 30-years of experience in the field of neuroscience, Bart Ellenbroek says that one identifiable gene is a contributing factor to addiction.

The Serotonin Transporter is a protein that recycles serotonin, and a change in the gene that codes for this protein has been shown to affect the rate of serotonin uptake. It is known to predispose people to anxiety and depression and it may play a role in addiction. Bart Ellenbroek says this gene is a significant contributing factor to drug and alcohol addiction, but he is quick to reinforce that it is not the only one.

His studies have been undertaken in animal models, and some surprising discoveries have emerged that allude to the notion that not all drug addictions are alike.

'We found indeed that these animals are much more sensitive to cocaine and especially to ecstasy … interestingly, they are not more sensitive to heroin …' _Bart Ellenbroek

The animals also acquired addiction at a much faster rate than those without it, self-administering more frequently and much earlier, with reinstatement of the addiction adopted very quickly after a period of withdrawal.

Sarah Bradbury, who graduated with a PhD in psychology last year, has shown in her research that the development of drug addiction is related to brain levels of serotonin.

What did you set out to find?

I set out to find why some drug users become drug-dependent, when the majority of drug users do not become dependent. I specifically researched the role of two neurotransmitters - serotonin and dopamine - in the progression of MDMA and cocaine use to abuse.

What did you discover?

The results suggest that during initial exposures to a drug, the susceptibility to drug abuse is related to the drug-produced serotonin response: a greater drug-produced serotonin response was associated with a decreased likelihood of progression to drug abuse. Although a wealth of research has implicated dopamine in drug use and abuse, my research failed to find an association between dopamine and susceptibility to drug abuse. Therefore the drug-produced serotonin response appears to be a critical factor in the progression from drug use to drug abuse. This finding was supported by the results of a subsequent study which showed that a depleted serotonin system increased drug-taking during initial exposures to a drug.

How low do serotonin levels have to be in order for people to be susceptible to addiction?

This study is the first to suggest that susceptibility to drug dependence/addiction is associated with inherent individual variability in the serotonin response to drugs. Further research will be needed to gain a better understanding of this relationship, and to get an idea of the relevant serotonin levels.

People talk about addictive personalities. Is there anything from your findings suggesting this to be the case and, if so, what would a low serotonin personality look like?

The research does not directly suggest anything about 'addictive personalities'. However, because a depleted serotonin system resulted in an increase in either MDMA or cocaine intake, the research supports a number of other studies which have shown that the biological basis for all drug addictions is similar. There are also a number of studies which have shown a similar biological basis for addictions to things other than drugs.
In terms of what a low-serotonin personality may look like ... Abnormalities in brain levels of serotonin have been implicated in a number of mood disorders, and people who suffer from mood disorders are more likely than the general population to be drug abusers.

In your study you discovered that levels of serotonin on initial  intake of drugs makes a difference in terms of subsequent abuse, why is this?

The mechanism by which the serotonin response influences drug use is not yet clear. The mechanism might be due to alterations of serotonin receptor function within the brain, and it is likely to involve complex interactions with other neurotransmitter systems.

Do our serotonin levels remain roughly the same or do they change frequently depending on activities, food intake etc.?

Serotonin levels fluctuate but there is an effective regulatory system that neutralises these fluctuations. Any daily fluctuations in serotonin levels are unlikely to be significant enough to make a person more or less susceptible to drug abuse. Abnormal alterations of serotonin levels are associated with changes in normal functioning. For example, depression is associated with lower levels of serotonin, and is therefore treated with drugs that increase serotonin levels.