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J Physiol. 2019 Jul 9. doi: 10.1113/JP278211. [Epub ahead of print]

HCN3 ion channels: roles in sensory neuronal excitability and pain.

Author information

1
Wolfson Centre for Age-Related Research, King´s College London, Guy´s Campus, London, SE1 1UL, UK.
2
Center for Integrated Protein Science (CIPS-M) and Center for Drug Research, Department of Pharmacy, Ludwig-Maximilians-Universität Munchen, Munich, Germany.

Abstract

KEY POINTS:

HCN ion channels conducting the Ih current control the frequency of firing in peripheral sensory neurons signalling pain Previous studies have demonstrated a major role for the HCN2 subunit in chronic pain but a potential involvement of HCN3 in pain has not been investigated HCN3 was found to be widely expressed in all classes of sensory neurons (small, medium, large) where it contributes to Ih HCN3 deletion increased the firing rate of medium, but not small, sensory neurons Pain sensitivity both acutely and following neuropathic injury was largely unaffected by HCN3 deletion, with the exception of a small decrease of mechanical hyperalgesia in response to a pinprick We conclude that HCN3 plays little role in either acute or chronic pain sensation HCN ion channels generate an inward current that can regulate action potential firing in somatosensory nerve fibres and can play an important role in pain sensation. The HCN1 isoform plays a limited role only in cold sensation following nerve injury. HCN2, on the other hand, is a key regulator of excitability in nociceptive nerve fibres, and controls the perception of inflammatory and neuropathic pain, but has no influence on acute pain sensation. Here we examine a potential role for the HCN3 isoform in neuronal excitability and pain. HCN3 is widely expressed in somatosensory neurons, and contributes to the regulation of firing of action potentials in medium-sized neurons, amongst which many have a nociceptive function. Genetic deletion of HCN3, however, had little impact on acute pain sensation, on inflammatory pain, nor on pain following nerve injury (neuropathic pain). We conclude that HCN3 does not play an important role in pain sensation.

ABSTRACT:

HCN ion channels govern the firing rate of action potentials in the pacemaker region of the heart and in pain-sensitive (nociceptive) nerve fibres. Intracellular cAMP promotes activation of the HCN4 and HCN2 isoforms, while HCN1 and HCN3 are relatively insensitive to cAMP. HCN2 modulates action potential firing rate in nociceptive neurons and plays a critical role in all modes of inflammatory and neuropathic pain, but the role of HCN3 in nociceptive excitability and pain is less studied. Using antibody staining, we found that HCN3 is expressed in all classes of somatosensory neurons. In small nociceptive neurons, genetic deletion of HCN2 abolished the voltage shift of the Ih current carried by HCN isoforms following cAMP elevation, while the voltage shift was retained following deletion of HCN3, consistent with the sensitivity of HCN2 but not HCN3 to cAMP. Deletion of HCN3 had little effect on the evoked firing frequency in small neurons, but enhanced the firing of medium-sized neurons, showing that HCN3 makes a significant contribution to the input resistance only in medium-sized neurons. Genetic deletion of HCN3 had no effect on acute thresholds to heat or mechanical stimuli in vivo, and did not affect inflammatory pain measured with the formalin test. Nerve-injured HCN3 KO mice exhibited similar levels of mechanical allodynia and thermal hyperalgesia to WT mice, but reduced mechanical hyperalgesia in response to a pinprick. These results show that HCN3 makes some contribution to excitability, particularly in medium-sized neurons, but has no major influence on acute or neuropathic pain processing. This article is protected by copyright. All rights reserved.

PMID:
31290157
DOI:
10.1113/JP278211

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