Abstract Patients with bi-allelic loss of function mutations in the voltage-gated sodium channel Nav1.7 present with congenital insensitivity to pain (CIP), whilst low threshold mechanosensation is reportedly normal. Using psychophysics (n = 6 CIP participants and n = 86 healthy controls) and facial electromyography (n = 3 CIP participants and n = 8 healthy controls), we found that these patients also have abnormalities in the encoding of affective touch, which is mediated by the specialized afferents C-low threshold mechanoreceptors (C-LTMRs). In the mouse, we found that C-LTMRs express high levels of Nav1.7. Genetic loss or selective pharmacological inhibition of Nav1.7 in C-LTMRs resulted in a significant reduction in the total sodium current density, an increased mechanical threshold and reduced sensitivity to non-noxious cooling. The behavioural consequence of loss of Nav1.7 in C-LTMRs in mice was an elevation in the von Frey mechanical threshold and less sensitivity to cooling on a thermal gradient. Nav1.7 is therefore not only essential for normal pain perception but also for normal C-LTMR function, cool sensitivity and affective touch. 2022 https://creativecommons.org/licenses/by/4.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( https://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Patients with bi-allelic loss of function mutations in the voltage-gated sodium channel Na v 1.7 present with congenital insensitivity to pain (CIP), whilst low threshold mechanosensation is reportedly normal. Noxious thermal sensitivity was assessed using a 53°C hotplate, and thermal preference was assessed using a thermal gradient apparatus (6–54°C). For further details see Supplementary material . Whole-cell patch clamp recordings Voltage-clamp recordings using an Axopatch 200B amplifier and Digidata 1550 acquisition system (Molecular Devices) were performed at room temperature (21°C). Data were sampled at 20 kHz and low-pass filtered at 5 kHz. Series resistance was compensated 80–90% to reduce voltage errors. All data were analyzed by Clampfit 10 software (Molecular Devices). As expected, the loss of Na v 1.7 in C-LTMRs did not affect the latency to withdraw from a nocioceptive hotplate ( Fig. 4D ). Mouse and human C-LTMRs are known to respond to cooling stimuli, and we have previously found that CIP participants show hyposensitivity to cold and cool stimuli ( Supplementary Fig. 4C ). We therefore sought to assess the loss of Na v 1.7 in cold stimuli coding. We allowed mice to explore a temperature gradient apparatus freely, which ranged from 6–54°C for 30 min. Na v 1.7-WT mice had a bell shaped (inverted U-shaped) response to the thermal gradient apparatus while TH CreERT2 :Na v 1.7-KO mice spent more time in cooler zones as seen by a significant leftward shift in the non-linear regression Gaussian curve ( Fig. 4E ), suggestive of a deficit in cool detection. Additionally, there was a ∼5°C reduction in the average preferred temperature (the temperature at which mice spent most of their time) in TH CreERT2 :Na v 1.7-KO mice (23.16 ± 1.59°C) compared with the WT mice (28.45 ± 1.21°C) ( Fig. 4F ). We also investigated the stimulus response functions in response to repeated punctate mechanical stimuli, where each stimulus (which is a downward indentation of the skin) increases its velocity. Rodent C-LTMRs from TH CreERT2 :Na v 1.7-KO mice exhibited a reduced firing frequency and are hypo-excitable to moving punctate stimuli compared with Na v 1.7-WT mice ( Fig. 5K ). Due to the mouse behavioural and human psychophysical data demonstrating cool detection abnormalities, we directly analysed the temperature sensibility of mouse C-LTMRs ( Fig. 5L–N ). Using three temperature ramps restricted to identified C-LTMR receptive fields, their response to cooling and warming stimuli was assessed in detail. The first temperature ramp started at 31°C (skin temperature) and cooled the receptive field to 14°C ( Fig. 5L ), the second ramp warmed the receptive field from 14°C to 42°C ( Fig. 5M ) and finally the third ramp cooled the receptive field from 42°C to A global vGLUT3 KO, initially thought to be C-LTMR-specific, resulted in altered noxious mechanical thresholds, 10 a phenotype which was later shown to be driven by loss of spinal vGLUT3. 54 Other studies suggest deficits in acute light touch, cold detection and chemical pain responses when Na v 1.8 positive sensory neurons (which include C-LTMRs) lack Cav3.2, a voltage-gated calcium channel enriched in C-LTMRs. 55 In contrast, the global KO of the chemokine-like protein Tafa4, which is thought to only be expressed and released by C-LTMRs, resulted in a pro-nocioceptive phenotype. There is closer alliance and more physiological evidence that the TH/vGLUT3/Tafa4 population is the likely species equivalent of human C-LTMRs. 10 , 11 , 13 However, we cannot exclude the possibility that both populations co-exist and that perhaps relates to modality-specific pleasure perception. Hitherto, there has been a lack of transgenic tools available to selectively target the C-LTMR population; the discovery that TH is a marker of C-LTMRs and development of the TH CreERT2 line has helped delineate the physiology and connectivity of these neurons in the rodent. 65 replicated the typical speed-dependent inverted U-shaped response in C-LTMRs, together with increased C-LTMR-firing to neutral (32°C) compared with warm (42°C) and cool (18°C) stroking temperatures. Importantly, the speed-dependent vigorous response to stroking stimuli at neutral temperatures was positively correlated to self-reported pleasantness, indicating a link between C-LTMR firing properties and subjective preference. Our findings in the mouse also support previous studies that implicate C-LMTRs in thermal preference.

Clinical Medical School University of Cambridge Cambridge, United Kingdom Email: ku.ca.mac@743wc Initial Posting: February 8, 2018 ; Last Revision: June 11, 2020 . Estimated reading time: 24 minutes Summary The purpose of this overview is to: 1. Describe the clinical characteristics of congenital insensitivity to pain; 2. Review the causes of congenital insensitivity to pain; 3. Provide an evaluation strategy to identify the genetic cause of congenital insensitivity to pain in a proband ; 4. Provide a brief summary of management of congenital insensitivity to pain; 5. Inform genetic risk assessment of family members of a proband with congenital insensitivity to pain. 1. Clinical Characteristics of Congenital Insensitivity to Pain Congenital insensitivity to pain (CIP) is an extremely rare phenotype characterized by the inability to perceive pain (absence of nociception) from birth. Individuals with CIP do not feel pain from any noxious stimuli, including inflammation and heat [ Goldberg et al 2007 ]. This review does not cover conditions that cause a generalized sensory neuropathy. Inability to feel pain leads to repeated injuries and prevents normal healing. Characteristic Findings Age-Related Infants and young children Self-mutilating injuries of the fingers (biting off fingertips) and oral cavity such as loss of the tongue tip, injuries to the inside of the teeth/gums, and avulsion of teeth are common ( Figure 1A&B ). Cuts and bruises may be present. Burns due to impaired temperature sensation [ Cox et al 2006 ] can occur. Recurrent otitis media may be due to selectively reduced immunity to Staphylococcus aureus (see Infections ) [ Shatzky et al 2000 ]. Figure 1. Examples of clinical findings in individuals with congenital insensitivity to pain (CIP) A. Typical loss of fingertips secondary to trauma, poor wound healing, and chronic Staphylococcal aureus infections in a child age nine years with NTRK1- CIP Note: (1) Affected individuals may be able to diff