ErbB4+ spinal cord dorsal horn neurons process heat pain

How the spinal cord transmits heat signals from the periphery to the brain remains unclear. In this issue of Neuron, Wang et al., 2022Wang H. Chen W. Dong Z. Xing G. Cui W. Yao L. Zou W.J. Robinson H.L. Bian Y. Liu Z. et al.A novel spinal neuron connection for heat sensation.Neuron. 2022; 110: 2315-2333https://doi.org/10.1016/j.neuron.2022.04.021Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar identify a population of spinal cord neurons functioning in this pathway. How the spinal cord transmits heat signals from the periphery to the brain remains unclear. In this issue of Neuron, Wang et al., 2022Wang H. Chen W. Dong Z. Xing G. Cui W. Yao L. Zou W.J. Robinson H.L. Bian Y. Liu Z. et al.A novel spinal neuron connection for heat sensation.Neuron. 2022; 110: 2315-2333https://doi.org/10.1016/j.neuron.2022.04.021Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar identify a population of spinal cord neurons functioning in this pathway. Thermosensation allows living organisms to sense the surrounding environment and seasonal changes and adjust their physiology and behaviors accordingly. Plants, for example, have optimal temperatures for seed germination and leaf growth, while animals use environmental temperatures to guide hibernation and migratory behavior. Animals will also avoid intense heat or cold that causes bodily harm. This exquisite ability to sense a broad range of temperatures, while essential for survival, can become pathological in conditions like tissue inflammation or nerve injury. In these conditions, innocuous temperatures may trigger pain, a phenomenon called “heat/cold hyperalgesia.” How does the nervous system normally sense temperature, and how are these signals altered during chronic pain to cause hyperalgesia? The molecular nature for thermosensation in mammals was first discovered by the functional cloning of transient receptor potential vanilloid 1 (TRPV1), a cation channel and receptor for capsaicin (Caterina et al., 1997Caterina M.J. Schumacher M.A. Tominaga M. Rosen T.A. Levine J.D. Julius D. The capsaicin receptor: a heat-activated ion channel in the pain pathway.Nature. 1997; 389: 816-824https://doi.org/10.1038/39807Crossref PubMed Scopus (7261) Google Scholar). Heat (>43°C) also activates heterologously expressed TRPV1, while TRPV1 knockout mice show impaired noxious heat detection. Several other receptors and TRP channels have also been identified for their functions in thermosensation, including TRPM8 for sensing cold-range temperatures (Reeh and Fischer, 2022Reeh P.W. Fischer M.J.M. Nobel somatosensations and pain.Pflugers Arch. 2022; 474: 405-420https://doi.org/10.1007/s00424-022-02667-xCrossref Scopus (10) Google Scholar). Accumulating evidence from physiological recordings, calcium imaging, and molecular characterizations, including more recent single-cell RNA sequencing data, suggests that hot and cold information is encoded and transmitted by distinct populations of primary dorsal root ganglia (DRG) and trigeminal ganglia sensory neurons (Yarmolinsky et al., 2016Yarmolinsky D.A. Peng Y. Pogorzala L.A. Rutlin M. Hoon M.A. Zuker C.S. Coding and plasticity in the mammalian thermosensory system.Neuron. 2016; 92: 1079-1092https://doi.org/10.1016/j.neuron.2016.10.021Abstract Full Text Full Text PDF PubMed Scopus (103) Google Scholar). Disrupting selective populations of primary afferents or thermoreceptor molecules further supports the existence of “thermal sensory lines” in the periphery (Reeh and Fischer, 2022Reeh P.W. Fischer M.J.M. Nobel somatosensations and pain.Pflugers Arch. 2022; 474: 405-420https://doi.org/10.1007/s00424-022-02667-xCrossref Scopus (10) Google Scholar). The spinal cord receives thermal information from peripheral sensory afferents and relays it to the brain after integration and processing. Anatomical, calcium imaging, and physiological recording studies in rodents and other mammals have revealed that spinal cord neurons processing thermal information primarily reside in the superficial layer of the dorsal horn (DH) (Basbaum et al., 2009Basbaum A.I. Bautista D.M. Scherrer G. Julius D. Cellular and molecular mechanisms of pain.Cell. 2009; 139: 267-284https://doi.org/10.1016/j.cell.2009.09.028Abstract Full Text Full Text PDF PubMed Scopus (2639) Google Scholar; Harding et al., 2020Harding E.K. Fung S.W. Bonin R.P. Insights into spinal dorsal horn circuit function and dysfunction using optical approaches.Front. Neural Circuits. 2020; 14: 31https://doi.org/10.3389/fncir.2020.00031Crossref Scopus (17) Google Scholar; Ran et al., 2016Ran C. Hoon M.A. Chen X. The coding of cutaneous temperature in the spinal cord.Nat. Neurosci. 2016; 19: 1201-1209https://doi.org/10.1038/nn.4350Crossref PubMed Scopus (84) Google Scholar). Recently, two largely non-overlapping mouse spinal cord DH projection neuron populations, marked by the expression of Tacr1 or Gpr83, were identified. They form parallel ascending circuit modules that preferentially convey thermal and tactile signals, respectively (Choi et al., 2020Choi S. Hachisuka J. Brett M.A. Magee A.R. Omori Y. Iqbal N.u.A. Zhang D. DeLisle M.M. Wolfson R.L. Bai L. et al.Parallel ascending spinal pathways for affective touch and pain.Nature. 2020; 587: 258-263https://doi.org/10.1038/s41586-020-2860-1Crossref PubMed Scopus (83) Google Scholar). Nevertheless, the molecularly defined populations of excitatory DH interneurons that might process thermal information have remained elusive. In this issue of Neuron, Wang et al., 2022Wang H. Chen W. Dong Z. Xing G. Cui W. Yao L. Zou W.J. Robinson H.L. Bian Y. Liu Z. et al.A novel spinal neuron connection for heat sensation.Neuron. 2022; 110: 2315-2333https://doi.org/10.1016/j.neuron.2022.04.021Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar identified a population of spinal cord interneurons that plays an important role in heat sensation in mice (Figure 1). The authors first expressed a short half-life EGFP (shEGFP) under the control of c-Fos promoter in spinal cord DH neurons. Noxious heat stimuli (52°C) were applied to induce shEGFP expression in heat-activated neurons. Most GFP+ neurons were found in the superficial layer of the DH (laminae I/II). Single-cell RT-PCR of GFP+ DH neurons revealed a population of excitatory neurons expressing ErbB4. To demonstrate that ErbB4+ DH neurons are required for heat sensation, the authors ablated these neurons and tested ablated mice with a battery of sensory behavioral assays. Interestingly, the ablated mice displayed impaired responses to noxious heat sensation. This deficit became more severe when SST+ and CCK+ interneurons, some of which are also activated by noxious heat stimulation, were additionally ablated. Since noxious heat is mainly sensed by TRPV1+ sensory afferents (Mishra et al., 2011Mishra S.K. Tisel S.M. Orestes P. Bhangoo S.K. Hoon M.A. TRPV1-lineage neurons are required for thermal sensation.EMBO J. 2011; 30: 582-593https://doi.org/10.1038/emboj.2010.325Crossref PubMed Scopus (197) Google Scholar), the authors hypothesized that ErbB4+ DH neurons are downstream of TRPV1+ afferents. They recorded excitatory postsynaptic currents (EPSCs) from ErbB4+ DH neurons in spinal cord slices. 50%–80% of these neurons responded to stimulation of Aδ and C fibers, and around half of the responsive neurons showed monosynaptic connectivity. When TRPV1+ fibers were pharmacologically blocked, ErbB4+ neurons showed reduced postsynaptic responses to C fiber stimuli. In addition, the authors provided morphological and functional evidence for this synaptic connection using immunostaining and optogenetic stimulation. Together, these results indicate that some ErbB4+ DH neurons are directly downstream of TRPV1+ sensory afferents (Figure 1). Do ErbB4+ spinal neurons preferentially respond to heat, or do they respond to multiple types of somatosensory stimuli? Optogenetic activation of axons of MrgprD+ DRG neurons, which are polymodal and important for mechanosensation, failed to evoke EPSCs in most recorded lamina I/II ErbB4+ neurons. In a more physiologically relevant ex vivo preparation, in which the spinal cord remains attached to lumbar nerve roots, DRGs, the saphenous nerve, and hindlimb skin, more than half of lamina I/II ErbB4+ neurons responded to noxious heat applied to the skin while less than 15% responded to mechanical stimuli. Thus, the authors concluded that a subset of ErbB4+ DH neurons show preferential sensitivity to noxious heat. To demonstrate that activity of ErbB4+ DH neurons affects the behavioral response to heat, the authors used chemogenetic strategies to transiently inhibit or activate this population. Inhibition of ErbB4+ neurons decreased sensitivity to heat stimuli in hot plate and Hargreaves tests, while activation of these neurons had the reverse effect. Manipulating activity of these neurons did not affect behavioral responses to mechanical, cold, cool, warm, or itch stimuli. These results suggest a dedicated role for ErbB4+ DH neurons in noxious heat sensation (Figure 1). Moreover, the authors took advantage of their expertise with genetic tools for studying receptor tyrosine kinase signaling and elegantly demonstrated that NRG1-ErbB4 signaling is important for transmitting heat sensation (Figure 1). They showed that heat stimuli caused increased levels of NRG1, the ligand of ErbB4, and active (phosphorylated) ErbB4 in the DH, suggesting enhanced NRG1-ErbB4 signaling in response to noxious heat. Knockout of ErbB4 in all tissues except for the heart, or deletion of ErbB4 specifically in the spinal cord, decreased mouse sensitivity to heat but not mechanical stimuli. The authors also found that the kinase activity of ErbB4 is required for heat sensation, because inhibition of ErbB4 kinases with a small molecule (1NMPP1) decreased behavioral sensitivity to heat. To determine whether endogenous NRG1 is important for heat sensation, the authors neutralized NRG1 by intrathecally injecting an extracellular portion of ErbB4 (ecto-ErbB4). This resulted in a decreased level of phosphorylated ErbB4 and reduced noxious heat sensitivity. Moreover, conditional knockout of NRG1 in DRG neurons, but not spinal cord neurons, recapitulated this phenotype, suggesting that NRG1 is mainly released from sensory neurons to transmit heat sensation. How does NRG1-ErbB4 signaling facilitate noxious heat sensation? The majority of heat-responsive ErbB4+ DH neurons are excitatory. Activation of these neurons in spinal cord slices triggered glutamatergic EPSCs in surrounding ErbB4− neurons. Application of NRG1 increased the amplitudes of evoked EPSCs, while treatment with 1NMPP1 reduced EPSC amplitudes, suggesting that NRG1-ErbB4 signaling potentiates downstream glutamatergic transmission. Lastly, the authors tested whether targeting the NRG1-ErbB4 signaling pathway could alleviate heat hyperalgesia in chronic pain. They found that decreasing either NRG1 or ErbB4 activation, by intrathecal injection of ecto-ErbB4 or 1NMPP1, reduced thermal hypersensitivity in inflammatory and neuropathic pain mouse models. Though most DH neurons are polymodal in responsiveness, some display a functional preference toward one sensory modality when activated or disrupted. For example, UCN3+ and GRPR+ excitatory interneurons preferentially mediate mechanical itch and chemical itch, respectively (Chen and Sun, 2020Chen X.J. Sun Y.G. Central circuit mechanisms of itch.Nat. Commun. 2020; 11: 3052https://doi.org/10.1038/s41467-020-16859-5Crossref PubMed Scopus (65) Google Scholar). In this study, Wang et al., 2022Wang H. Chen W. Dong Z. Xing G. Cui W. Yao L. Zou W.J. Robinson H.L. Bian Y. Liu Z. et al.A novel spinal neuron connection for heat sensation.Neuron. 2022; 110: 2315-2333https://doi.org/10.1016/j.neuron.2022.04.021Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar added a missing piece to the puzzle by identifying ErbB4+ DH neurons that transmit heat pain. This study also reveals a novel physiological role for NRG1-ErbB4 signaling in noxious heat sensation, providing potential molecular targets for treating heat hypersensitivity associated with inflammatory and neuropathic pain. Future studies are needed to understand how these interneurons connect to the projection neurons that transmit noxious heat to the brain. Last, but not least, DH neurons for sensing other temperature ranges, such as innocuous temperatures and noxious cold, still remain to be identified. This work is supported by NIH R01 grants (NS083702) and R34 grants (NS118411) to W.L. The authors declare no competing interests. A novel spinal neuron connection for heat sensationWang et al.NeuronMay 11, 2022In BriefHow heat signals are processed in the spinal cord remains unclear. Wang et al. found that ErbB4+ excitatory interneurons are activated by noxious heat, and they participate in heat sensation in mice. In addition, the neuregulin-ErbB4 signaling regulates heat sensation and contributes to heat hypersensitivity under pathological conditions. Full-Text PDF Open Archive