Bursting Enables GRP Neurons to Engage Spinal Itch Circuits
Authors: Hugues Petitjean, Philippe Séguéla, Reza Sharif-Naeini
Journal: Neuron (2019)

In this issue of Neuron, Pagani et al., 2019Pagani M. Albisetti G.W. Sivakumar N. Wildner H. Santello M. Johannssen H.C. Zeilhofer H.U. How gastrin-releasing peptide opens the spinal gate for itch.Neuron. 2019; 103 (this issue): 102-117Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar find that itch signaling occurs only when GRP neurons fire action potentials in bursts. This enables GRP release and the activation of GRPR neurons, which help carry the itch signal to the brain. In this issue of Neuron, Pagani et al., 2019Pagani M. Albisetti G.W. Sivakumar N. Wildner H. Santello M. Johannssen H.C. Zeilhofer H.U. How gastrin-releasing peptide opens the spinal gate for itch.Neuron. 2019; 103 (this issue): 102-117Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar find that itch signaling occurs only when GRP neurons fire action potentials in bursts. This enables GRP release and the activation of GRPR neurons, which help carry the itch signal to the brain. Itch is an evolutionary-conserved sense that protects us from potentially harmful stimuli such as mosquito bites and parasites. Based on the conservation of itch receptors in various species, this somatosensation appeared at least some 400 million years ago, when tetrapods first emerged (Mack and Kim, 2018Mack M.R. Kim B.S. The itch-scratch cycle: a neuroimmune perspective.Trends Immunol. 2018; 39: 980-991Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). However, when itch becomes chronic, defined as lasting more than 6 weeks, it becomes highly debilitating and is associated with a decrease in the quality of life (Mack and Kim, 2018Mack M.R. Kim B.S. The itch-scratch cycle: a neuroimmune perspective.Trends Immunol. 2018; 39: 980-991Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar). This pathology underlies many skin disorders such as atopic dermatitis and psoriasis as well as chronic kidney and liver diseases. The unsuccessful management of chronic itch in these conditions stems from our limited insight into its underlying mechanisms. Therefore, despite recent advances in our understanding of molecules and neuronal pathways involved in the transmission of itch (Dong et al., 2001Dong X. Han S. Zylka M.J. Simon M.I. Anderson D.J. A diverse family of GPCRs expressed in specific subsets of nociceptive sensory neurons.Cell. 2001; 106: 619-632Abstract Full Text Full Text PDF PubMed Scopus (507) Google Scholar, Huang et al., 2018Huang J. Polgár E. Solinski H.J. Mishra S.K. Tseng P.Y. Iwagaki N. Boyle K.A. Dickie A.C. Kriegbaum M.C. Wildner H. et al.Circuit dissection of the role of somatostatin in itch and pain.Nat. Neurosci. 2018; 21: 707-716Crossref PubMed Scopus (131) Google Scholar, Kardon et al., 2014Kardon A.P. Polgár E. Hachisuka J. Snyder L.M. Cameron D. Savage S. Cai X. Karnup S. Fan C.R. Hemenway G.M. et al.Dynorphin acts as a neuromodulator to inhibit itch in the dorsal horn of the spinal cord.Neuron. 2014; 82: 573-586Abstract Full Text Full Text PDF PubMed Scopus (229) Google Scholar), developments in this field remain desperately needed. Itch-producing compounds (pruritogens) activate their receptors on peripheral sensory neurons (pruriceptors), which send their central axons in the superficial layers of the dorsal horn of the spinal cord. A subset of these pruriceptors, expressing the receptor for the anti-malarial drug chloroquine MrgprA3 (Han et al., 2013Han L. Ma C. Liu Q. Weng H.J. Cui Y. Tang Z. Kim Y. Nie H. Qu L. Patel K.N. et al.A subpopulation of nociceptors specifically linked to itch.Nat. Neurosci. 2013; 16: 174-182Crossref PubMed Scopus (380) Google Scholar), forms synaptic connections with excitatory interneurons that contain the neuropeptide gastrin-releasing peptide (GRP) (Albisetti et al., 2019Albisetti G.W. Pagani M. Platonova E. Hösli L. Johannssen H.C. Fritschy J.M. Wildner H. Zeilhofer H.U. Dorsal horn gastrin-releasing peptide expressing neurons transmit spinal itch but not pain signals.J. Neurosci. 2019; 39: 2238-2250Crossref PubMed Scopus (46) Google Scholar). Activation of GRP neurons by primary afferents causes the release of the neuropeptide, along with glutamate, on a second subset of excitatory interneurons, which express the GRP receptor (GRPR). Ablation of GRP neurons (Sun et al., 2017Sun S. Xu Q. Guo C. Guan Y. Liu Q. Dong X. Leaky gate model: intensity-dependent coding of pain and itch in the spinal cord.Neuron. 2017; 93: 840-853.e5Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar) or GRPR neurons (Sun et al., 2009Sun Y.G. Zhao Z.Q. Meng X.L. Yin J. Liu X.Y. Chen Z.F. Cellular basis of itch sensation.Science. 2009; 325: 1531-1534Crossref PubMed Scopus (449) Google Scholar) gives rise to mice with reduced responses to the injection of pruritogens, highlighting the essential role of this peptide/receptor pathway. However, the exact mode of communication between GRP and GRPR neurons in the context of itch signaling remained poorly understood. Particularly, given that GRP neurons are glutamatergic (Sun et al., 2017Sun S. Xu Q. Guo C. Guan Y. Liu Q. Dong X. Leaky gate model: intensity-dependent coding of pain and itch in the spinal cord.Neuron. 2017; 93: 840-853.e5Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar), it is unclear why deleting the GRPR can block itch behavior. Presumably, the release of GRP and its binding to GRPR acts in concert to the activation of postsynaptic glutamate receptors. In this issue of Neuron, the Zeilhofer group uses an elegant series of experimental approaches to reveal the properties of the GRP-GRPR neuron synapse (Pagani et al., 2019Pagani M. Albisetti G.W. Sivakumar N. Wildner H. Santello M. Johannssen H.C. Zeilhofer H.U. How gastrin-releasing peptide opens the spinal gate for itch.Neuron. 2019; 103 (this issue): 102-117Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar). To examine the synaptic communication between these two subsets of neurons, they generated a triple transgenic mouse that expressed channelrhodopsin2-eYFP (ChR2-eYFP) fusion protein in GRP neurons and eGFP in GRPR neurons, allowing them to record from GRPR neurons while optogenetically stimulating GRP neurons and to confirm the monosynaptic nature of this communication. Interestingly, when postsynaptic responses of GRPR neuron were examined in response to single light pulse stimulation of GRP neurons, none of the excitatory GRPR neurons were able to generate action potentials. The excitation produced by such stimulation was restricted to subthreshold depolarizations, leading Pagani et al., 2019Pagani M. Albisetti G.W. Sivakumar N. Wildner H. Santello M. Johannssen H.C. Zeilhofer H.U. How gastrin-releasing peptide opens the spinal gate for itch.Neuron. 2019; 103 (this issue): 102-117Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar to question whether such activation modality truly reflects the activity of GRP neurons activated by peripheral pruriceptors. They thus generated another triple transgenic mouse line in which MrgprA3-expressing pruriceptors expressed ChR2 and GRP neurons expressed eGFP. This enabled them to record, in spinal cord slice preparations, from GRP neurons while stimulating the central terminals of the primary pruriceptors. Surprisingly, their current-clamp recordings indicated that a single (4 ms) light pulse stimulation of MrgprA3 fibers triggered bursts of 2–5 action potentials in GRP neurons. This observation prompted them to determine whether burst-like firing in GRP neurons would be more appropriate when communicating with downstream GRPR neurons. They therefore mimicked this firing pattern in their subsequent experiments on synaptic communication between GRP and GRPR neurons. Through the use of temporally patterned optogenetic light stimulation, they tested whether burst-like stimulation of GRP neurons would produce action potentials in GRPR neurons. Surprisingly, this led to the gradual depolarization of GRPR neurons followed by the generation of action potentials. Furthermore, the depolarization and spontaneous activity that were generated persisted for minutes after the synaptic stimulation had stopped. To examine whether the gradual depolarization and excitation produced by burst-like activity in GRP neurons was due to GRP release or to glutamate receptor-dependent plasticity, they repeated the experiments in the presence of GRPR or AMPA/NMDA receptor antagonists, respectively. The progressive depolarization and excitation persisted in the presence of glutamate receptor antagonists but were almost completely abolished by GRPR antagonists, thus highlighting the critical role of peptidergic signaling at this synapse. The metabotropic signaling pathway engaged by GRP was examined by exogenous application of the peptide and indicated that the depolarization was associated with an increase in input resistance, suggesting that some background potassium channels had closed. Through a series of pharmacological and electrophysiological approaches, these channels were identified as Kir-like and Kv4.2-like potassium channels (Figure 1). From these findings, Pagani et al., 2019Pagani M. Albisetti G.W. Sivakumar N. Wildner H. Santello M. Johannssen H.C. Zeilhofer H.U. How gastrin-releasing peptide opens the spinal gate for itch.Neuron. 2019; 103 (this issue): 102-117Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar predicted that burst-like activity in GRP neurons was necessary to elicit GRP release, which would produce spiking in excitatory GRPR neurons and lead to the activation of spinal relays of pruriceptive information. To test this prediction in vivo, they chronically implanted GRP-ChR2 mice with fiber optics directed toward the dorsal horn and stimulated GRP neurons with brief pulses of light, applied either as a single stimulus or as a burst. Their results indicated that only during burst-like stimulation of GRP neurons was an aversive reaction elicited in the animals. This effect occurred with a certain delay and outlasted the stimulation period by several minutes, in agreement with the different time courses of pain and itch responses. The past decade has seen important advances in our understanding of how itch is perceived at the molecular level and how it is transmitted to the nervous system at the circuit level. A more challenging task has been to understand how these mechanisms differ from those involved in pain transmission in their necessity for peptidergic transmission. This involves a careful examination of the synaptic physiology of these synapses. Through a "tour de force" approach involving the generation of triple transgenic mice, in vivo optogenetic modulation of itch pathways and slice electrophysiology, the Zeilhofer lab identified a mechanism that explains how the synaptic transmission of itch signals differs from pain signals. Future research may explain how this intracellular cascade can interact with others activated in GRPR neurons. For instance, it was recently demonstrated that these neurons also express the kappa-opioid receptor (KOR), whose activation can desensitize GRPR (Munanairi et al., 2018Munanairi A. Liu X.Y. Barry D.M. Yang Q. Yin J.B. Jin H. Li H. Meng Q.T. Peng J.H. Wu Z.Y. et al.Non-canonical opioid signaling inhibits itch transmission in the spinal cord of nice.Cell Rep. 2018; 23: 866-877Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar). Finally, the findings of the Zeilhofer lab open interesting avenues for integrating the unique role of the synapse between spinal GRP and GRPR neurons in the pathophysiology of chronic itch. How Gastrin-Releasing Peptide Opens the Spinal Gate for ItchPagani et al.NeuronMay 15, 2019In BriefPagani et al. investigated the temporal and spatial signature of GRP signaling, a key element of spinal itch relay. They find that GRP signals build up slowly and rather diffusely during stimulation, possibly explaining why itch is difficult to localize. Full-Text PDF Open AccessBursting Enables GRP Neurons to Engage Spinal Itch CircuitsPetitjean et al.NeuronAugust 07, 2019In Brief(Neuron 103, 5–7; July 3, 2019) Full-Text PDF Open Archive