Contribution of dorsal horn CGRP-expressing interneurons to mechanical sensitivity.

Line S Löken,Mollie Bernstein,Julia Kuhn,Joao M Braz,Madison Jewell,Katherine Hamel,Alexander Etlin,Mahsa Sadeghi,Allan Basbaum,Marilyn Steyert,Ida J Llewellyn-Smith

doi:10.7554/elife.59751

Line S Löken, Mollie Bernstein + Show 9 more

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https://doi.org/10.7554/elife.59751

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Abstract

Primary sensory neurons are generally considered the only source of dorsal horn calcitonin gene-related peptide (CGRP), a neuropeptide critical to the transmission of pain messages. Using a tamoxifen-inducible CalcaCreER transgenic mouse, here we identified a distinct population of CGRP-expressing excitatory interneurons in lamina III of the spinal cord dorsal horn and trigeminal nucleus caudalis. These interneurons have spine-laden, dorsally directed, dendrites, and ventrally directed axons. As under resting conditions, CGRP interneurons are under tonic inhibitory control, neither innocuous nor noxious stimulation provoked significant Fos expression in these neurons. However, synchronous, electrical non-nociceptive Aβ primary afferent stimulation of dorsal roots depolarized the CGRP interneurons, consistent with their receipt of a VGLUT1 innervation. On the other hand, chemogenetic activation of the neurons produced a mechanical hypersensitivity in response to von Frey stimulation, whereas their caspase-mediated ablation led to mechanical hyposensitivity. Finally, after partial peripheral nerve injury, innocuous stimulation (brush) induced significant Fos expression in the CGRP interneurons. These findings suggest that CGRP interneurons become hyperexcitable and contribute either to ascending circuits originating in deep dorsal horn or to the reflex circuits in baseline conditions, but not in the setting of nerve injury.

Highlights

We conclude that the pattern of calcitonin gene-related peptide (CGRP)-expression observed in the CalcaCreER mouse provides a reliable marker of CGRP-expressing neurons in the adult. 116 Unexpectedly, we found large numbers of small tdTomato-positive neurons in the superficial dorsal horn and nucleus caudalis and occasionally in more superficial layers (Figures 1e-f: Figure 8 – figure supplement 1c)
Which included injections into the ventrobasal and nucleus submedius (Yoshida, Dostrovsky, Sessle, & Chiang, 1991) of the thalamus, lateral parabrachial nucleus, and dorsal column nuclei, which are targeted by postsynaptic dorsal column neurons located in the region of lamina IV of the dorsal horn, we found no evidence of CGRP-expressing projection neurons
Whether the CGRP interneurons are engaged in conditions in which mechanical stimulation can trigger itch remains to be determined. 350 DISCUSSION Despite overwhelming evidence that primary sensory neurons are the predominant source of dorsal horn CGRP, here we describe a morphologically uniform population of dorsal horn CGRP-expressing interneurons

Summary

31 Abstract

Primary sensory neurons are generally considered the only source of dorsal horn calcitonin gene-related peptide (CGRP), a neuropeptide critical to the transmission of pain messages. When released into the superficial dorsal horn from the central branches of sensory neurons, CGRP, along with its co-occurring neuropeptide, substance P, potentiates the glutamatergic excitation of postsynaptic neurons, contributing to injury-provoked central sensitization (Ryu, Gerber, Murase, & Randic, 1988; Woolf & Wiesenfeld-Hallin, 1986) The latter process, in turn, contributes to the ongoing pain and profound hypersensitivity characteristic of both inflammatory and neuropathic pains. Caspase-mediated ablation of the neurons increased mechanical thresholds We conclude that these CGRP-expressing interneurons engage deep dorsal horn nociresponsive circuits that contribute either to ascending circuits originating in deep dorsal horn or to the reflex circuits in baseline conditions, but not in the setting of nerve injury

RESULTS

350 DISCUSSION

MATERIALS AND METHODS