Nature versus nurture is an age-old question about how human traits and personalities are formed. But is drug addiction a result of genetics, life events, or both?

Researchers at the University of British Columbia have genetically engineered mice who don't have the capacity to become addicted to cocaine. These findings were published in Nature and supports the theory that a habit of relying on drug use could be written into someone’s DNA.

Read: Painkiller Dangers: More Children Are Being Poisoned By Prescription Opioids

The team of Canadian researchers administered injections of cocaine to both normal and genetically modified mice before manipulating the animals’ brains to associate the drug with a certain “wallpaper” within a large cage, The Independent reported.

Both groups of mice were then given free range to move around the cage as they wished. The normal animals went to the area associated with cocaine almost twice as frequently as the genetically modified mice. The latter group was changed genetically to produce higher levels of a protein — called cadherin — that normally strengthens the connections between brain cells.

According to the Independent, this reaction to cadherin proved that the modified mice had not developed such strong memories of the drug, which prevented them from being addicted to cocaine.

“Through genetic engineering, we hard-wired in place the synapses in the reward circuits of these mice,” researcher Andrea Globa told the Independent. “By preventing the synapses from strengthening, we prevented the mutant mice from ‘learning’ the memory of cocaine, and thus prevented them from becoming addicted.”

In the future, these findings could help develop new drugs to treat human addiction, lead study author Shernaz Bamji said in a press release from UBC.

Read: Addiction The Biggest Health Problem Facing The Country, Says US Surgeon General

Back in November 2016, United States Surgeon General Dr. Vivek Murthy published a report declaring that addiction is the biggest health problem currently facing the country, Medical Daily reported at the time. Statistics show that substance abuse disorders involving alcohol or other drugs take the lives of more people than all types of cancer combined.

Source: Mills F, Globa AK, Liu S, Cowan CM, Mobasser M, Phillips AG, Borgland SL, Bamji SX. Cadherins mediate cocaine-induced synaptic plasticity and behavioral conditioning. Nature. 2017.

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