Drinking alcohol can make a person more vulnerable to developing a cocaine addiction, a study has found.
The research showed that an early exposure to booze can lead to a later dependence on harder substances.
Alcohol consumed over a longer period of time promotes the breakdown of two key proteins that usually act as a block in the brain’s ‘reward circuit’.
The reward circuit refers to a group of structures that release dopamine – linked to pleasure – when they are activated by stimuli, such as addictive drugs.
Scientists already knew that alcohol and nicotine tend to be gateway drugs, or substances which supposedly lead users on to more addictive or dangerous drugs, to cocaine use.
Now researchers at Columbia University Medical Centre say they wanted to understand the biology of how gateway drugs contribute to cocaine addiction.
Research on rats showed alcohol encourages the degradation of two proteins – histone deacetylases 4 and 5 – in the nucleus accumbens, a region in the brain that makes up part of the reward circuit.
Histone deacetylases 4 and 5 play a critical role in regulating how cells work and act as mediators in the reward circuit.
Dr Denise Kandel, professor of sociomedical sciences at Columbia University, said: “The gateway drug hypothesis is based on the observation that when kids get involved in drugs, they follow certain developmental sequences in which they use certain drugs prior to the use of other drugs.
“The issue is – what mechanism accounts for the fact that the use of one drug increases the risk of use of another?”
The researchers evaluated cocaine-seeking behaviours of rats by dividing them into two groups.
The first group was given alcohol 10 days prior to cocaine exposure, while the second group was offered water instead of alcohol.
Study author Dr Edmund Griffin, an assistant professor of Clinical Psychiatry at Columbia University, said: “We found that the animals in the alcohol priming group had enhanced behavioural responses.
“We looked not only at how much cocaine they used but also will they continue to use a drug, even if they have a negative consequence – like a foot shock.”
The rats with longer-term prior alcohol exposure were found to be more persistent in seeking cocaine, pressing a lever to release the drug an average of 58 times during the experiment, compared to rodents without alcohol exposure who used the lever only 18 times.
Dr Griffin said: “We found that alcohol is causing degradation of histone deacetylases which acts as a molecular brake pad inside the reward circuitry of the brain.”
The researchers found the breakdown of the proteins increased compulsive use of the drug.
Dr Griffin added: “Our study helps us to understand how an early exposure to something like alcohol can actually tip the balance and increase a person’s ability to develop addiction.”
The research is published in the journal Science Advances.