What happens in the brain to ensure that animals are motivated to avoid painful and other harmful stimuli? It’s an important question for the pain field, considering the emotional aspects of chronic pain. Now, researchers led by David Engblom, Linköping University, Sweden, explore the role of the hypothalamic melanocortin system, best known as an appetite-suppressing circuit, in this process.
They demonstrate that deleting or inhibiting melanocortin 4 receptors (MC4Rs) in the striatum of mice produces a surprising flip of motivational behavior: Instead of avoiding painful or other aversive stimuli, the mice show indifference to or even a preference for them. Further, the investigators show that this switch in motivational valence depends on alterations in dopamine signaling between proopiomelanocortin (POMC)-expressing neurons in the hypothalamus and medium spiny neurons in the striatum that express dopamine D1 receptors.
“It’s important to look for these types of circuits because no matter the etiology behind a particular form of pain, they all involve this emotional component,” says Greg Corder, Stanford University, Palo Alto, US, who was not involved in the new work. “This study does a beautiful job of identifying a circuit that facilitates this component, and it’s really interesting that disruption of just the melanocortin system is sufficient to induce this flip from aversion to reward.”
The new research was published July 2 in the Journal of Clinical Investigation, along with an accompanying commentary by Alexandra DiFeliceantonio and Paul Kenny, Icahn School of Medicine at Mount Sinai, New York, US.
The melanocortin system
The brain interprets somatosensory information coming in from the environment, encoding that information with either a positive or negative motivational valence so that an organism can learn to seek out or avoid a particular stimulus. When it’s a nociceptive stimulus, the brain assigns a negative valence to it (Dossantos et al., 2017).
“This emotional feature of pain is a distributed network across many different brain regions,” says Corder. “If you target valence, even while maintaining the sensory component, you may effectively remove pain. This is because pain is a unified perception, and without the valence you are just left with an inert sensory phenomenon.”
Similarly, hunger also taps into these emotion-related circuits. A recent study even found that extreme hunger can suppress persistent inflammatory pain, including its affective component (see PRF related news).
The unique intersection among appetite, motivational valence, and pain led Engblom to explore the melanocortin system, which controls appetite and energy balance.
“The melanocortin system is really well described as a satiety circuit,” explained Engblom. “If you inhibit it, you get obesity and increased food intake.”
In this system, following a meal, POMC-expressing cells in the hypothalamus release melanocortin, which binds MC4Rs, also in the hypothalamus, to decrease appetite. But recent studies have demonstrated that MC4Rs are also present in the striatum, which processes reward. Further, these striatal MC4Rs mediate stress-induced anhedonia, or the inability to feel pleasure, which suggested that these receptors might be involved in more than only appetite regulation (Lim et al., 2012).
“These are different circuits,” explains Engblom. “The hypothalamic circuit controls appetite, and it seemed possible that the striatal circuit might control aversive signaling.”
Indifference to—or even a preference for—pain and other aversive stimuli
So co-first authors Anna Mathia Klawonn and Michael Fritz and colleagues explored a possible role for striatal MC4Rs in determining motivational valence to various stimuli by using genetically engineered mice lacking MC4Rs and a series of two-chamber conditioned place aversion (CPA) assays.
In the first such assay, they paired one chamber with the bacterial cell wall component lipopolysaccharide (LPS), which causes pain and inflammation. As expected, wild-type mice avoided the LPS-paired chamber. But, the MC4R knockout mice did not avoid that chamber—in fact, they actually preferred it. Somehow, the animals now assigned a positive valence to LPS, encoding it as rewarding.
The group then paired one chamber with formalin injections into the paw, another aversive stimulus. Here, while wild-type mice avoided the formalin-paired chamber, as expected, the knockouts showed no preference to either chamber. Similar results were seen when pairing one chamber with non-painful aversive stimuli, including injections of lithium chloride to cause nausea, and a kappa opioid receptor agonist to cause dysphoria. Again, wild-type mice avoided the chamber paired with the aversive stimulus, while the MC4R knockouts preferred the paired chamber.
“The effect was shocking because it was so strong,” says Engblom. “By removing the MC4R receptors, you can block aversion completely and, in some cases, even flip it to reward.”
In contrast, the team saw no differences between the knockouts and wild-type animals in a CPA assay using cocaine, with both groups preferring the cocaine-paired chamber.
“They still know how to associate a positive valence to a rewarding stimulus, so this MC4R system seems specific to aversive signaling processes,” according to Engblom.
Because MC4R knockout mice develop obesity, which could affect behavior, the investigators then treated wild-type mice with an MC4R antagonist prior to LPS injection, to avoid this consequence. Here, too, inhibiting MC4R function in this way made the wild-type animals prefer the LPS-paired chamber, just as the knockout animals did.
Dopamine goes up instead of down
The team then examined brain dopamine signaling, which regulates reward processing. They measured dopamine with positron emission tomography (PET), using a radioactively labeled dopamine receptor antagonist called raclopride. When dopamine is released, it displaces raclopride, decreasing the amount of radioactivity.
In the MC4R knockout mice, the researchers injected LPS, followed by raclopride, and then scanned the entire brain with PET. They saw decreased radioactivity in the ventral and dorsal striatum of the knockouts, indicating dopamine release.
“Aversive stimuli should normally reduce dopamine so that the animal interprets it as a negative experience. But without MC4Rs, we see an increase in dopamine,” explains Engblom.
To test whether this dopamine spike flipped aversion to reward, the group administered a dopamine antagonist to the knockout mice prior to LPS administration. This time, the mice avoided the LPS-paired chamber, just as the wild-type animals did, with similar results observed with formalin and lithium chloride.
“We don’t know the exact mechanism here, but it seems that this rise in dopamine is causing aversive events to be interpreted as positive,” says Engblom.
From the hypothalamus to the striatum
Finally, Engblom and colleagues explored the neural pathway between the hypothalamus and striatum that might be involved during the switch from aversion to reward. They genetically re-expressed MC4Rs only in dopamine D1 receptor-expressing medium spiny neurons in the striatum of the knockouts. Doing so restored avoidance of the LPS-paired chamber in the CPA assay.
Next, they used chemogenetics in wild-type animals to specifically activate POMC neurons in the hypothalamus that project to MC4R-expressing neurons in the striatum. The CPA assay showed that activation of only this single pathway caused robust avoidance behaviors.
“This hypothalamic projection to the striatum is sufficient to induce aversion, and so these striatal MC4Rs seem to function as gatekeepers to prevent harmful signals from being interpreted as rewarding,” according to Engblom.
The study adds a new circuit to the list of those that determine the valence of pain, such as those in the anterior cingulate cortex, habenula, and insula. “The pain field is ripe for pushing further into the brain,” says Corder. “And it’s possible you could target this circuit therapeutically.”
Given the effects of the melanocortin system on appetite, however, Engblom thinks targeting the circuit for pain will make sense only in some cases.
“If you inhibit MC4R function, the patient may develop obesity,” says Engblom. “But targeting this pathway may be beneficial in some patients, like those with terminal cancer pain, where appetite side effects may be less important.”
Nathan Fried is an assistant professor at Rutgers University, Camden, US.
Image credit: Wikimedia Commons.
