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The Pain Behind the Pull
Aly Skoczek ’18
Lake Forest College
Lake Forest, Illinois 60045
Pain sensation associated with heat and hair pulling in humans and animals is related to cells that contain high threshold mechanoreceptors and that are are positive for calcitonin gene-related peptide. New imaging techniques regarding calcium signaling have been able to identify a new type of cell responsible for the sensation of hair pulling alone, named Circ-HTMRs.
Pulling hair is a common way sisters fight, yet what many people do not know is that the mechanism underlying the pain caused by hair pulling is relatively unknown. What makes the pain caused by hair pulling different from the pain caused by touching a hot surface? Both hurt, but how is the pain different? Ghitani et al. (1) discovered that the cells responsible for hair pulling pain are, indeed, separate from those associated with other types of pain, specifically pain caused by heat.
The sensation of touch allows us to detect mechanical stimuli and helps us understand our surroundings(2). There are currently two well-known mechanosensation receptors in the skin: those that are high threshold mechanoreceptors (HTMRs) and those that are low threshold mechanoreceptors (LTMRs), both of which are responsible for the detection of different types of touch as well as pain (2). HTMRs are responsible for responding to strong stimuli that usually result in pain or could lead to bodily harm, while LTMRs are responsible for light and touch stimuli (3). Stimuli that could cause HTMRs to be activated could include strong, heavy pressure, pain, and heat (2). While it is known that there are different types of HTMR stimuli, it was previously thought that the cells responsible for sensing pain and heat fall under the same class of cells that are positive for calcitonin gene-related peptide (CGRP) (4),
Ghitani et al. attempted to use GCaMP6f, an indicator of calcium in a cell, to identify which cells are active during pain associated with heat and pain associated with hair pulling (5). A cell that is activated and has GCaMP6f as an indicator will become fluorescent and allow for simple identification and analysis (5). The specific cells that are activated are a subgroup of those that express CGRP. Ghitani et al. were able to determine that cells that express CGRP are responsive to both heat stimuli and hair pulling stimuli. However, when the images were put on top of each other, it was discovered that the cells associated with heat pain never overlapped with the cells that responded to hair pulling. They then measured the size of these cells and discovered that the cells that responded to hair pulling were significantly larger than those that responded to heat.
Once Ghitani et al. found that there may be two different types of CGRP-expressing cells responsible for heat and pain, the researchers wanted to see what would happen if the response to heat was inhibited To inhibit heat response, one must first inhibit transient receptor potential vanilloid 1 (TRPV1). TRPV1 is heavily present in sensory neurons and responsible for thermosensation, which includes heat and cold, that can then lead to pain (6). In order to analyze the difference in CGRP cells that respond to heat and those that respond to hair pulling pain, Ghitani et al. used resiniferatoxin (RTX), an agonist of TRPV1. This means that when RTX is injected into the animal neurons that express TRPV1, TRPV1 will be killed and heat sensation will no longer be present (7). This method of TRPV1 inhibition was continuously used to help identify the cells that express CGRP and are activated by hair pulling but not heat.
The researchers used GCamP6f again to identify active cells, this time with RTX. They found that when HTMRs were stimulated with heat and hair pulling when RTX was added, no cells responded to heat while some did respond to hair pulling. This shows that there are cells that express CGRP that are indeed responsible for hair pulling pain and not heat pain. They then measured cell size again and found that in the control, without RTX, cells of all sizes responded to hair pulling and cells of small sizes responded to heat. When RTX was present, no cells responded to heat and only larger cells responded to hair pulling. These results show that there are specific CGRP cells that respond to hair pulling and that these cells have moderate to large size cell bodies. These cells were then dubbed Circ-HTMRs.
To further test the function of these Circ-HTMRs, Ghitani et al. used optogenetics. Optogenetics is the process of using different proteins that are activated by light to either turn on or turn off a cell(8). Ghitani et al. used channelrhodopsin-2 (ChR2) to activate these Circ-HTMR cells. When ChR2 was introduced, a blue light was used to turn on the cells. When the cells are exposed to blue light, Circ-HTMRs are activated, and when the blue light is off, so are Circ-HTMR cells (8,1). In a behavioral test they allowed mice to run in between two chambers that both had no light on. They then turned on the blue light in one side of the chamber. When the blue light was on, mice showed extreme avoidance behavior. This escape response suggests that these Circ-HTMRs caused a specific type of pain sensation similar to that of hair pulling.
Up until these findings, CGRP cells have been known to be responsible for thermosensation but no other pain sensations or behaviors (4). These findings show that while cells that contain CGRP are responsible for heat sensation, the cells responsible for heat sensation are small in size, but there is a second function of these cells. The second function is pain sensation, specifically high threshold pain caused by hair pulling, which is t caused by a distinct set of larger CGRP cells from those that respond to heat.
Ghitani et al. were able to identify these Circ-HTMRs and show that they are responsible for hair pulling high threshold mechanosensation. Future studies can focus on the exact chemical and mechanical pathways that these cells experience to relay their message to the brain from the skin. Overall, this study provides identification of a new type of mechanosensory cell that will be further studied to analyze how we as mammals feel pain, integrate the sensation, and act because of it.
1.) Ghitani, N., Barik, A., Szczot, M., Thompson, J. H., Li, C., Le Pichon, C. E., … & Chesler, A. T. (2017). Specialized mechanosensory nociceptors mediating rapid responses to hair pull. Neuron, 95(4), 944-954.
2.) Roudaut, Y., Lonigro, A., Coste, B., Hao, J., Delmas, P., & Crest, M. (2012). Touch sense: functional organization and molecular determinants of mechanosensitive receptors. Channels, 6(4), 234-245.
3.) Boada, M. D., Ririe, D. G., & Eisenach, J. C. (2017). Post-discharge hyperpolarization is an endogenous modulatory factor limiting input from fast-conducting nociceptors (AHTMRs). Molecular Pain, 131-11. doi:10.1177/1744806917726255
4.) Gibson, S. J., Polak, J. M., Bloom, S. R., Sabate, I. M., Mulderry, P. M., Ghatei, M. A., … & Evans, R. M. (1984). Calcitonin gene-related peptide immunoreactivity in the spinal cord of man and of eight other species. Journal of Neuroscience, 4(12), 3101-3111.
5.) Ye, L., Haroon, M. A., Salinas, A., & Paukert, M. (2017). Comparison of GCaMP3 and GCaMP6f for studying astrocyte Ca2+ dynamics in the awake mouse brain. Plos ONE, 12(7), 1-17. doi:10.1371/journal.pone.0181113
6.) McKemy, D. D., Neuhausser, W. M., & Julius, D. (2002). Identification of a cold receptor reveals a general role for TRP channels in thermosensation. Nature, 416(6876), 52.
7.) Leo, M., Schulte, M., Schmitt, L., Schäfers, M., Kleinschnitz, C., & Hagenacker, T. (2017). Intrathecal Resiniferatoxin Modulates TRPV1 in DRG Neurons and Reduces TNF-Induced Pain-Related Behavior. Mediators Of Inflammation, 1-8. doi:10.1155/2017/2786427
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