Addictive drugs hijack a brain reward mechanism to strengthen drug-related memories and perpetuate drug use, according to a paper published in Neuron this week.
It’s already known that dopamine, the brain’s feel-good reward chemical, plays a role in addiction, and also participates in a process called synaptic potentiation – the strengthening of nerve connections that happens during learning.
To find out if dopamine would encourage synaptic potentiation as a result of exposure to drugs, John Dani from the Baylor College of Medicine in Houston, Texas, and colleagues gave physiologically significant doses of nicotine to freely moving mice while recording brain activity.
They noticed increased synaptic potentiation that correlated with the mice learning to prefer a location that was associated with the nicotine dose. They also recorded a local dopamine signal in the hippocampus, a region towards the centre of the brain that is critical for the formation of new memories, reinforcing the view that dopamine enables memories of specific events to be formed.
This starts a vicious cycle, where dopamine strengthens drug associated memories and thus attaches more importance to them, and increases the likelihood of future drug use. In any normal situation, this dopamine signal would mark memories and feelings about the environment as important, and this would mark an important part of the learning process, but in the presence of an addictive substance, the system is subverted.
John Dani summarised the importance of this for understanding addiction: