Activation of one of the brain’s reward centers - composed of neural circuits that are normally engaged by naturally rewarding stimuli and positive expectations - influences the body’s immune defense against a specific bacterial pathogen (Escherichia coli), reports a study in mice published online this week in Nature Medicine.
Positive emotions and expectations activate the reward circuitry in the brain, which affects multiple physiological processes, including the functioning of the immune system. However, the precise influence of specific brain regions and cell types on peripheral immunity has remained unknown.
Asya Rolls, Shai Shen-Orr and colleagues employed genetic methods to achieve the expression of engineered receptors called DREADDs, or designer receptors exclusively activated by designer drugs, exclusively within dopaminergic neurons of the ventral tegmental area (VTA) of the mouse brain. To enhance activity in this reward center, the authors activated these receptors one day before an immune challenge with Escherichia coli bacteria, and observed an increase in the ability of peripheral immune cells to defend against the bacteria in both short- and long-term experiments. These effects are suppressed by the inactivation of the peripheral sympathetic nervous system, which suggests that this system acts as a bridge between the central brain reward circuits and the peripheral immune system.
Although the idea has not yet been experimentally tested, this study may provide a biological basis for the well-known placebo effect, in which a patient’s positive expectations can lead to physiological improvements in a variety of health conditions. Further work will be required to investigate the circumstances under which this brain-immune system might operate, although the authors speculate that naturally rewarding stimuli, such as mating or feeding behaviors, might both activate the VTA and increase the likelihood of potential exposure to pathogens. In addition, understanding whether this brain-immune system connection influences immune defense against other pathogens, such as viruses, will require further investigation.