Why sickness hurts: A central mechanism for pain induced by peripheral inflammation

- Experimental inflammation down-regulates the descending inhibitory pain pathway.
- Inflammation induces stronger affective and interoceptive pain related brain activity.
- Women had an overall weaker descending pain inhibition, rendering them more sensitive to the inflammatory provocation.

Low-grade systemic inflammation has been implicated in chronic pain, as well as in comorbid diseases like depression and fatigue. We have previously shown that women's pain perception and regulation is more affected by systemic inflammation than that of men. Here we investigated the neural substrates underlying these effects using an fMRI paradigm previously employed in a clinical population. Fifty-one participants (29 women) were injected with 0.6 ng/kg lipopolysaccharide (LPS) or saline to induce a peripheral inflammatory response. The subjects were then tested with a pressure pain fMRI paradigm designed to capture descending pain inhibitory activity 2 h after injection, and blood was sampled for cytokine analysis.The subjects injected with LPS became more pain sensitive compared to the placebo group, and the heightened pain sensitivity was paralleled by decreased activity in the ventrolateral prefrontal cortex and the rostral anterior cingulate cortex (rACC) compared to placebo; areas involved in descending pain regulation. The LPS group also had higher activity in the anterior insular cortex, an area underpinning affective and interoceptive pain processing. Women displayed overall less pain-evoked rACC activity compared to men, which may have rendered women less resilient to immune provocation, possibly explaining sex differences in LPS-induced pain sensitivity. Our findings elucidate the pain-related brain circuits affected by experimental peripheral inflammation, strengthening the theoretical link between systemic inflammation and weakened pain regulation in chronic pain disorders. The results further suggest a possible mechanism underlying the female predominance in many chronic pain disorders.