Activating the reward system in the brains of mice directly boosts their immune systems, offering a physiological explanation for the placebo effect.
Shai Shen-Orr, Asya Rolls and their colleagues at the Technion–Israel Institute of Technology in Haifa activated neurons in a part of the mouse brain that processes rewarding activities such as eating and sex. The next day, they injected the mice with the bacterium Escherichia coli. The animals showed increases in both short-term and long-term immune responses to the pathogen, compared with mice in a control group. But these effects were lost when the researchers also inactivated the animals' sympathetic nervous systems, suggesting that this system helps to mediate interactions between the brain and the immune system.
Nature Med. http://dx.doi.org/10.1038/nm.4133 (2016)
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Reward system boosts immunity. Nature 535, 10 (2016). https://doi.org/10.1038/535010b