Abstract
A prominent role of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels has been suggested based on their expression and (dys)function in dorsal root ganglion (DRG) neurons, being likely involved in peripheral nociception. Using HCN blockers as antinociceptive drugs is prevented by the widespread distribution of these channels. However, tissue-specific expression of HCN isoforms varies significantly, HCN1 and HCN2 being considered as major players in DRG excitability. We characterized the pharmacological effect of a novel compound, MEL55A, able to block selectively HCN1/HCN2 isoforms, on DRG neuron excitability in-vitro and for its antiallodynic properties in-vivo. HEK293 cells expressing HCN1, HCN2, or HCN4 isoforms were used to verify drug selectivity. The pharmacological profile of MEL55A was tested on mouse DRG neurons by patch-clamp recordings, and in-vivo in oxaliplatin-induced neuropathy by means of thermal hypersensitivity. Results were compared to the non-isoform-selective drug, ivabradine. MEL55A showed a marked preference toward HCN1 and HCN2 isoforms expressed in HEK293, with respect to HCN4. In cultured DRG, MEL55A reduced Ih amplitude, both in basic conditions and after stimulation by forskolin, and cell excitability, its effect being quantitatively similar to that observed with ivabradine. MEL55A was able to relieve chemotherapy-induced neuropathic pain. In conclusion, selective blockade of HCN1/HCN2 channels, over HCN4 isoform, was able to modulate electrophysiological properties of DRG neurons similarly to that reported for classical Ih blockers, ivabradine, resulting in a pain-relieving activity. The availability of small molecules with selectivity toward HCN channel isoforms involved in nociception might represent a safe and effective strategy against chronic pain.
Highlights
During the last decade, hyperpolarizing activated cyclic nucleotide-gated (HCN) channels emerged as key players controlling and facilitating neuron excitability
The population of dorsal root ganglia (DRG) neurons in culture typically consists of cells with different dimensions for which, according to data in literature (Acosta et al, 2012; Schnorr et al, 2014), the proportion of HCN isoforms may vary depending on size
Based on expression profile and localization of HCN isoforms in DRG neurons and previously reported effect on maximal current in heterologously expressed channels (Melchiorre et al, 2010), we further assessed the properties of MEL55A (R-enantiomer, Supplemental Figure 1) in human embryonic kidney cells (HEK293) cells at physiologically relevant potentials
Summary
During the last decade, hyperpolarizing activated cyclic nucleotide-gated (HCN) channels emerged as key players controlling and facilitating neuron excitability. Among the most interesting ones, because of potential pathophysiological implications, are the nociceptive neurons whose bodies reside in the dorsal root ganglia (DRG). Recent promising findings demonstrate that Ih activation plays a facilitating role in neuropathic pain, an ill-treated disease demanding new pharmacological strategies (Tsantoulas et al, 2016). Mechanisms triggering hyperalgesia or allodynia may involve gene reprogramming (Papp et al, 2010; Descoeur et al, 2011; Schnorr et al, 2014) as well as cAMP-mediated channel gating consequent to receptor stimulation by prostaglandin E2 and substance P (Jafri and Weinreich, 1998; Momin et al, 2008; Resta et al, 2016)
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