How do chili peppers relate to inflammatory knee pain?
During the day 3 AM poster session, I managed to snag Sampurna Chakrabarti (Follow her on Twitter), a winner of the SFN Trainee Professional Development Award, to talk about her recent research on mechanisms of inflammatory pain.
To study this, she and her colleagues injected Complete Freund’s Adjuvant (CFA) into one of the knees of a mouse, leaving the other knee as a ‘control’. ‘Adjuvants’ like CFA elicit a strong inflammatory response, and can boost adaptive (primarily lymphocytes like T cells and B cells) immunity. An easy way to remember which cells are which is that T-cells mature in the Thymus gland, while B cells mature in the Bone marrow. Injections of CFA into joints is a widely used model with which to elicit an inflammatory response and study diseases like arthritis.
The dorsal root ganglia (there’s two at almost all vertebrae) relay sensory information arriving from everywhere in the body. They serve a key role in reflex responses (e.g., to a hot grill) that occur before the brain becomes “aware” that something happened, and they also act as a highway to transmit information to the spinal cord and up to the brain (Credit: Quora.com)
Causing this inflammatory reaction in the joint causes mice (and people) to experience pain, severely impairing one’s quality of life and in some cases, mobility. The open question is, “how does this inflammatory reaction cause this pain response?” and “can we prevent this to provide relief for patients with joint pain?”
To measure pain responses in mice, they used a quick behavioral assay that determines how much a mouse digs down into the bedding in it’s cage. Mice naturally dig to form nests and burrows, while mice in pain can’t muster up enough energy to complete this task. As a secondary measure, Sampurna and her colleagues also measured the swelling of the knee as an index of inflammation. Their hypothesis was that inflammation sensitizes sensory neurons (located in the dorsal root ganglia ; DRG) relaying information from the knee to the spinal cord, leading to joint pain.
But how could inflammation ‘sensitize’ a neuron to joint pain? The family of proteins called TRPV (‘trip-Vee’) receptors is widely known to be important in the recognition of painful stimuli. TRPV1, specifically, is most famous for its alternative name, the ‘capsaicin receptor’. Capsaicin is the molecule in chili peppers that causes the painful burning sensation, making it a useful ingredient in things that cause pain, like pepper spray.
Neurons projecting to the inflamed knee (‘knee neurons’ labeled with FB) from the dorsal root ganglion expressed a much higher amount of the capsaicin receptor (TRPV1), without changes in the receptor for Nerve Growth Factor (a molecule associated with increased neural sensitivity; TrkA) (Credit: Chakrabarti et al., 2018; Neuropharmacology)
The researchers assessed whether dorsal root ganglion neurons projecting to the knee were hypersensitive by recording from- and stimulating them using electrophysiology. To identify only neurons that project to the knee of interest, they injected a retrograde label into the joint (called Fast Blue ; FB) to label upstream neurons projecting to the knee. Because neurons labeled with FB ‘glow’ under a microscope, it is easy to see and manipulate only the neurons of interest (so called ‘knee-neurons’).
They observed that following CFA administration, these neurons had a lower threshold for firing action potentials in response to a number of stimuli, including the chili pepper compound, capsaicin. This indicated that they were more sensitive to noxious stimuli, which could explain the sensation of pain elicited by the inflamed joint. But what is causing this ‘sensitization’? They used immunohistochemistry to show that knee-neurons express much higher levels of the capsaicin receptor and the combination of the capsaicin receptor and TrkA (the receptor for nerve growth factor; NGF). This supported the idea that inflammation up-regulates NGF and TRPV1 signaling to sensitize neurons, resulting in pain.
Blocking TRPV1 signaling using a receptor antagonist prevents inflammatory joint pain elicited by injections of CFA. In panels B and C you can see that without the antagonist, the mice fail to show their normal happy digging behaviors. However, with the antagonist, their behavior returns to normal, indicating that they are no longer in pain (Credit: Chakrabarti et al., 2018; Neuropharmacology)
As a final test to see if TRPV1 is really the culprit, they repeated their digging behavior assay after CFA administration with or without the TRPV1 receptor blocker (antagonist) “A-425619”. When the actions of TRPV1 were blocked, mice with inflamed knees no longer showed signs of pain, suggesting that manipulating this pathway may be a good strategy to reduce joint pain. .
Indeed, the researchers are now moving their findings in mice onto humans to see if this effect can be repeated to improve quality of life in patients with arthritis and other joint diseases!
That’s it for today, I’ve been a bit sick so I had to slow down a bit. See you guys tomorrow!