Illustration of the reward pathway in the human brain
FERNANDO DA CUNHA/SCIENCE PHOTO LIBRARY
Persistent use of drugs such as cocaine and morphine is thought to affect the way the brain prioritises the body’s basic needs — and we are now getting to the bottom of how this comes about.
When people repeatedly misuse drugs, they might see long-term changes in their behaviour that lead them to choose to take drugs instead of doing essential things like eating and drinking.
A brain pathway called the mesolimbic reward system is suspected to be involved in this process, but few studies have directly compared the system’s response to taking drugs with the response of innate needs being met.
Now, Bowen Tan at Rockefeller University in New York and his colleagues have shown that the same neurons are activated in these two circumstances. They uncovered this using a sophisticated microscopy set-up that allowed them to track the activity of individual neurons in the brains of mice going through withdrawal following repeated exposures to these drugs.
“The field has long been debating whether there is a specialised cell type that encodes drug value only and a specialised cell type that encodes natural reward value only,” says Tan. “What we saw is that these drugs of abuse commonly activate the same set of neurons as the natural rewards.”
The researchers also observed that the neural response to satisfying basic needs became disorganised after the mice were given cocaine or morphine, which occurred alongside a decreased consumption of food and water.
“What is really notable about this finding is that strong neural responses to food or water almost become displaced by responses to drugs,” says Jeremy Day at the University of Alabama at Birmingham. “[This suggests] that drug rewards are able to override how the brain translates need states into behaviours that satisfy those needs.”
Tan and his team also identified a gene, called Rheb, that seems to be necessary for drugs to have this effect. Rheb is part of a cell-signalling pathway that is also found in people, so future work could investigate how inhibiting this pathway could be used as a therapy for substance misuse, he says.
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