Why Is Craniofacial Pain Often Worse Than Body Pain? An Explanation Emerges
Patients often describe head and face pain (craniofacial pain) as more severe and emotionally unpleasant than pain from the rest of the body. These regions are also the site of some of the most common chronic pain disorders including temporomandibular disorder, headache, and trigeminal neuralgia. Now, a new preclinical study led by Fan Wang, Duke University Medical Center, Durham, US, shows that a previously uncharacterized neural pathway between the trigeminal ganglion (TG), a key relay site of facial pain, and the lateral parabrachial nucleus (PBL), a brain region critical for encoding the unpleasantness and averseness of pain, may explain the heightened affective response to craniofacial pain.
The authors employed their novel CANE (capturing activated neuronal ensembles) technology for viral-genetic tagging of transiently activated neurons, along with extensive mapping of axonal projections and optogenetic manipulation of neurons. These techniques allowed them to identify a monosynaptic connection between the TG and PBL with important functions in craniofacial pain-related aversive behaviors in mice.
“The implications of this study are very interesting, and potentially explain this long-discussed phenomenon of the differential impact of head pain versus bodily pain,” said Greg Dussor, University of Texas at Dallas, US, who was not involved in the study.
“It’s true that head pain is more troubling than pain from the rest of the body, but it’s also true that deep tissue pain is more troubling than superficial cutaneous pain,” according to Dussor. “From that you could infer that the most troubling pain is deep tissue head pain, which is essentially headache, so I think there are big implications of this study for the headache field, too.”
The research was published online November 13 in Nature Neuroscience.
Arguably, two of the most severe chronic pain conditions in terms of patient suffering are trigeminal neuralgia and cluster headaches. This suffering or emotional burden is known as the affective component of pain and can be distinct from the sensory component of pain. It has been hypothesized that craniofacial pain has greater biological relevance than pain from other parts of the body, as it signals potential danger or injury to vital cognitive functions (Schmidt et al., 2016; Meier et al., 2014).
Wang and colleagues were intrigued by why craniofacial pain is generally associated with more suffering than bodily pain. “It was a really naïve question that started this whole research: Mainly, could we find a neural circuit for the basis of this increased pain severity? And as we were more focused on the affective aspects of pain, we chose to examine the parabrachial nucleus, a critical relay node for those aspects,” said Wang.
Tracing projections
To begin, lead author Erica Rodriguez and colleagues injected the noxious chemical formalin into either the whisker pad or hindpaw of mice. Subsequently, they used immunohistochemistry to stain the PBL for c-Fos, a commonly used surrogate marker of neuronal activity. They found that whisker pad formalin resulted in substantially more c-Fos expression (that is, increased neuronal activation) in the PBL, which is located in the brainstem, compared to hindpaw formalin.
To gain a better idea of why noxious facial stimuli activated more PBL neurons compared to noxious bodily stimuli, the authors sought to identify neurons that provided presynaptic input to the PBL neurons. So they turned to their novel CANE technology; this is a viral-genetic tagging approach in which neurons that are Fos+ transiently express a foreign receptor on their surface, which enables engineered viruses that are infused into the PBL to selectively infect those neurons (Sakurai et al., 2016). To selectively label transiently activated neurons, an initial noxious stimulus (formalin or capsaicin) was delivered to the whisker pad, and three weeks later a second noxious stimulus was administered. The PBL was subsequently isolated and examined for c-Fos labeling. The second noxious stimulus reactivated many (approximately 55 percent) of the same cells activated by the first stimulus, creating a map of nociceptive PBL neurons.
Capturing and mapping the axonal projection targets of PBL-nociceptive neurons. k–p, Representative images of axonal projections from captured formalin-activated PBL neurons (magenta) in several brain nuclei expressing Fos (green) induced by formalin. Insets, schematics of coronal view of location (in red box) in brain. BNST, bed nucleus of the stria terminalis; CeAC, central amygdalar nucleus, capsular part; PVH, paraventricular hypothalamic nucleus; PVT, paraventricular nucleus of the thalamus; SNpc, substantia nigra pars compacta; PAGvl, ventrolateral periaqueductal gray. Image and caption from Rodriguez et al., 2017. Reprinted by permission from Springer Nature.
Furthermore, using a CANE-based circuit tracing method with a fluorescent marker, Wang and colleagues discovered that these PBL nociceptive neurons projected to multiple regions in the brain associated with emotion- and instinct-related behaviors, including the hypothalamus and central nucleus of the amygdala. PBL nociceptive neurons also received presynaptic inputs from emotion-related limbic regions and brainstem nuclei including the ventrolateral periaqueductal grey (PAG) and bed nucleus stria terminalis.
A new pathway
Perhaps most interesting of all was the observation of trans-synaptically labeled presynaptic input neurons in the ipsilateral trigeminal ganglion (TG), indicating that CANE-captured PBL nociceptive neurons receive direct inputs from the TG. Previous studies had only hinted at a direct TG-PBL pathway by examining projection patterns of the ethmoidal nerve, which originates from the TG and innervates the nasal cavity, and had reported labeled afferents from this nerve in regions adjacent to the PBL (Panneton et al., 2006; Panneton and Gan, 2014).
The new finding suggested, for the first time, that TG neurons innervating the head and face provide direct monosynaptic inputs to ipsilateral PBL nociceptive neurons. Moreover, using whole-cell patch-clamp recording of PBL neurons, the authors found these direct inputs to be excitatory in nature. Such input patterns, the authors say, provide a neural circuit basis for the perception of trigeminal nerve pain as being more severe and emotionally draining than body pain.
“The head and face region has all five senses compared to the rest of the body, which only has somatosensation, so biologically speaking, there must be a need for rapid, instinctive responses to anything noxious to this region,” explained Wang. “The existence of this TG-PBL pathway enables direct access to powerful and heightened affective responses to help protect these important vital organs in this area.”
Dussor agrees. “The head is one of the most critical locations in the body, and pain originating from it needs to be extremely emotionally disturbing. It might be the case that it also makes the rest of the body hypersensitive to ensure adequate protection,” he said.
Behavioral correlates
Forty percent of the trans-synaptically labeled TG-PBL neurons were transient receptor potential vanilloid type 1 (TRPV1)-positive, suggesting that TRPV1 fibers may be a major source of noxious inputs from the TG to the PBL. So the team generated mice in which all TRPV1+ primary afferent axons were selectively labeled.
To investigate if activation of the direct TG-PBL pathway produced aversive behavior in these mice, the authors used a combination of optogenetics and a real-time place escape/avoidance paradigm (PEAP), which is used to model the affective/unpleasant component of pain in rodents. Optogenetic activation of TRPV1+ axon terminals in the PBL produced immediate and robust avoidance of the preferred chamber in the PEAP and was also accompanied by audible vocalizations in the mice, two strong indications that activation of the TG-PBL monosynaptic circuit produces affective pain behavior.
Finally, the team hypothesized that optogenetic silencing of the TRPV1+ neurons would reduce facial pain, and, indeed, this was the case. Following capsaicin injection into the whisker pad, silencing of TRPV1+ axon terminals in the PBL reduced pain thresholds to mechanical stimulation.
As the PBL receives inputs from many different regions intricately involved in emotional regulation, future research could aim to investigate how these other brain areas may modulate the pain experience.
“We are really interested in these top-down projections coming into the PBL, many of which are inhibitory, but in different emotional states, could these projections be facilitatory?” said Wang.
Dara Bree is a postdoctoral fellow at Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, US.
Image credit: Rodriguez et al., 2017.
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