The Cingulate Cortex and Pain: Let’s Get Specific
A newly described pathway from the midcingulate cortex (MCC) to the posterior insula (PI) has a critical part in the regulation of nociceptive hypersensitivity, according to a new study in mice. Activation of this pathway is sufficient to induce hypersensitivity in the absence of peripheral nociceptive input, and it modulates pain independent of fear or negative affect.
“This is a great study. It adds significantly to our understanding of the role of cortical circuits in pain modulation,” says Min Zhuo, University of Toronto, Canada, who studies the role of the anterior cingulate cortex (ACC), another region of the cingulate cortex, in pain, and was not involved in the new work.
The research, led by Rohini Kuner, Heidelberg University, Germany, appears in the November 2017 issue of Nature Neuroscience, along with an accompanying News and Views by Thomas Nevian, University of Bern, Switzerland.
“This study places the midcingulate cortex (MCC) in the spotlight, attributing to it a cortical mechanism of pain sensitization,” Nevian writes. “Furthermore, the study presents striking evidence that different aspects of pain processing are associated with distinct parts of the cingulate cortex and that pain-related memory processes can be found in the MCC that might be instrumental for the development of chronic pain.”
The cingulate cortex plays an important role in chronic pain. A number of imaging studies have shown that this region is activated during the perception of pain. In addition, Zhuo has found that changes in synaptic plasticity in the ACC contribute to the generation of pain (see PRF related news stories here and here). More recently, deep brain stimulation of the ACC has been demonstrated to be an effective treatment for chronic, treatment-resistant neuropathic pain (Boccard et al., 2017).
But, “while the cingulate cortex has been very well studied in both humans and rodents, most studies have ignored the fact that it is not one homogeneous entity, but instead is made up of cytoarchitecturally distinct subdomains that may have very different functions,” said Kuner, including the rostral (or pregenual), dorsal (where the MCC is located) and caudal cingulate cortices. The current study, Kuner said, is the first to specifically examine the contributions of the MCC to pain.
Turning the MCC on and off with light
In the new study, first author Linette Tan and colleagues used optogenetics to activate or silence the MCC. The proton pump archaerhodopsin (ArchT) was used to silence neurons, and the cationic channel channelrhodopsin 2 (ChR2) was used to activate neurons.
Capsaicin injection into the hindpaw of mice produced nocifensive behaviors such as licking and lifting of the affected paw. It also activated neurons across all layers of the MCC, as indicated by an increase in expression of the immediate early gene c-Fos, which is induced in response to neuronal activation. But blocking MCC neuronal activity with photoactivation of virally transduced ArchT 15 to 30 minutes after the capsaicin injection did not affect the animals’ nocifensive behaviors.
Similarly, silencing the hind limb representation area of primary somatosensory cortex (S1HL), another cortical area linked to pain, did not alter nocifensive behaviors following capsaicin. In addition, pain-related behaviors persisted in response to mechanical stimulation with von Frey filaments when the MCC or S1 was blocked.
“To our surprise, inactivating either of these regions did not affect basal pain sensitivity,” said Kuner. This suggests that pain is encoded in highly redundant networks, so removing one part of the network does not produce a net effect, she added. “Pain has such an important function for the body that it is not dependent on individual brain regions,” Kuner said.
The researchers then examined the functions of the MCC and S1HL in a model of nociceptive activity-induced central plasticity, again using capsaicin. In both people and rodents, capsaicin not only leads to acute pain and sensitization at the site of injection, but also a longer-lasting sensitization in neighboring areas. Using optogenetics, the researchers found that the MCC plays an important role in the persistence of nociceptive hypersensitivity. Mice whose MCC or S1HL was continuously silenced during mechanical testing 15 to 30 minutes following capsaicin did not display mechanical hypersensitivity. But mice whose S1HL was silenced 45 to 60 minutes after capsaicin did display hypersensitivity, while those whose MCC was turned off during the same time period did not. The results suggest that the persistence of nociceptive hypersensitivity occurs through MCC-mediated ongoing excitation in central circuits, Kuner said.
Next, the team examined whether an acute blockage of the MCC could reverse chronic pain hypersensitivity. MCC inhibition 24 hours after the onset of long-lasting inflammatory pain resulting from injection of complete Freund’s adjuvant into the hindpaw partially reduced hypersensitivity. But mechanical allodynia in the spared nerve injury model of chronic pain was unaffected. Taken together, these results suggest that the MCC plays a role in some persistent pain states but not others.
Finally, the researchers then found that they were able to bring about hypersensitivity by directly activating the MCC. “This suggested that pain can be generated in the brain in the absence of external triggers, much like what occurs in some patients with chronic pain,” said Kuner. The current study is the first time this has been shown so explicitly, she added.
Uncovering a mechanism
Just activating or inactivating a region in the brain is a highly artificial way to understand the role of that region in pain, Kuner continued. “A study like this really has limited value unless you try to get at a mechanism,” she said.
To identify one, Kuner and colleagues looked at c-Fos expression patterns across several brain areas, searching for regions that showed opposite changes in activity in the two models (switching on the MCC using ChR2 in the absence of capsaicin or off using ArchT and capsaicin injection).
While many brain regions showed a change in activity in one of the models, only three regions showed opposite changes in activity in both models: the nucleus accumbens (NAc), the posterior insula (PI), and an adjoining structure, the claustrum. Interestingly, the NAc has been traditionally associated with reward and addiction, but has more recently been implicated in pain (Lee et al., 2015).
Examples of viral tracing of projections of MCC excitatory neurons expressing ArchT or ChR2 to the basolateral amygdala (BLA; scale bar, 500 μm), claustrum, PI, and NAc (n = 4 mice; scale bars, 200 μm); aca, anterior commissure anterior part. Image and caption from Tan et al., 2017, reprinted by permission from Macmillan Publishers Ltd.
Using viral tracing, the researchers then found direct afferent projections from the MCC to the NAc, but optogenetic activation or inactivation of these afferents didn’t have a large effect on nociceptive hypersensitivity.
In contrast, the authors also found a new pathway between excitatory neurons in the MCC and layer 2/3 of the PI that, when stimulated or inactivated, produced very similar phenotypes to when the MCC alone was stimulated or inactivated. Indeed, stimulation of these MCC-to-PI afferent projections induced mechanical hypersensitivity in the absence of capsaicin, while silencing the projections, as well as the PI itself, inhibited mechanical hypersensitivity following capsaicin.
Interestingly, modulating the MCC-to-PI pathway did not alter fear-related behavior such as freezing in the animals. This suggests that the MCC has a function in pain distinct from the rostral cingulate cortex’s previously reported role in negative affect associated with pain.
“Our study underscores the importance of precision when it comes to studying the cingulate cortex,” said Kuner. “Different areas of the cingulate seem to contribute differently to pain.”
In a final set of experiments, the researchers investigated whether the MCC or the MCC-to-PI pathway sensitizes nociceptive behaviors via supraspinal mechanisms alone, or whether they affect spinal nociceptive processing via descending modulation. Intrathecal injections of the drug granisetron, which blocks descending serotonergic facilitation to the spinal cord, prevented the hypersensitivity induced by activation of the MCC or MCC-to-PI pathway. In addition, viral tracing showed that excitatory projections arising from the PI terminated in the raphe magnus nucleus, where descending serotonergic projections originate, again revealing a role for descending modulation in response to activity in the pathway.
The current work is part of a larger effort to understand the contribution of different cortices to pain processing, Kuner said. In particular, the team is also studying the role of the prelimbic cortex and insula in pain. In the future, they plan to investigate not just individual brain regions but also how these regions function together as networks across time, she added. “One could then use that information to stimulate or inhibit certain patterns in order to interfere with the percept of pathological pain,” Kuner told PRF.
Allison Marin, PhD, is a neuroscientist-turned-science writer who resides in Pittsburgh, US.
Image credit: Tan et al., 2017
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