Date Published: May 24, 2019
Publisher: Public Library of Science
Author(s): Saki Miyake, Hitoshi Higuchi, Yuka Honda-Wakasugi, Maki Fujimoto, Hotaka Kawai, Hitoshi Nagatsuka, Shigeru Maeda, Takuya Miyawaki, Michael Costigan.
Recently, attention has been focused on the role of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in the mechanism of and as a treatment target for neuropathic and inflammatory pain. Ivabradine, a blocker of HCN channels, was demonstrated to have an effect on neuropathic pain in an animal model. Therefore, in the present study, we evaluated the effect of ivabradine on inflammatory pain, and under the hypothesis that ivabradine can directly influence inflammatory responses, we investigated its effect in in vivo and in vitro studies.
After approval from our institution, we studied male Sprague–Dawley rats aged 8 weeks. Peripheral inflammation was induced by the subcutaneous injection of carrageenan into the hindpaw of rats. The paw-withdrawal threshold (pain threshold) was evaluated by applying mechanical stimulation to the injected site with von Frey filaments. Ivabradine was subcutaneously injected, combined with carrageenan, and its effect on the pain threshold was evaluated. In addition, we evaluated the effects of ivabradine on the accumulation of leukocytes and TNF-alpha expression in the injected area of rats. Furthermore, we investigated the effects of ivabradine on LPS-stimulated production of TNF-alpha in incubated mouse macrophage-like cells.
The addition of ivabradine to carrageenan increased the pain threshold lowered by carrageenan injection. Both lamotrigine and forskolin, activators of HCN channels, significantly reversed the inhibitory effect of ivabradine on the pain threshold. Ivabradine inhibited the carrageenan-induced accumulation of leukocytes and TNF-alpha expression in the injected area. Furthermore, ivabradine significantly inhibited LPS-stimulated production of TNF-alpha in the incubated cells.
The results of the present study demonstrated that locally injected ivabradine is effective against carrageenan-induced inflammatory pain via HCN channels. Its effect was considered to involve not only an action on peripheral nerves but also an anti-inflammatory effect.
Neuropathic pain is a chronic pain state, and it frequently impairs patients’ quality of life [1–4]. Many investigations have been conducted on its mechanism and treatment, but the mechanism is complex and remains to be fully clarified [5–8]. Furthermore, not only direct nerve injury but also other conditions, such as inflammation and viral infection, can cause neuropathic pain and increase the complexity [6–8]. Various kinds of drugs, including antiepileptic drugs, antidepressants, pregabalin, N-methyl-D-aspartate (NMDA) receptors blockers, NSAIDs, and opioids are currently used as treatments targeting neuropathic pain, but these drugs may not be sufficient for relief from neuropathic pain [9, 10].
The present study demonstrated that locally injected ivabradine is effective against carrageenan-induced inflammatory pain. This indicates that ivabradine may be a candidate for a new medicine to relieve inflammatory and neuropathic pain. In the present study, the local injection of ivabradine at a concentration of more than 20 μM significantly increased the pain threshold at the carrageenan-injected site of the hindpaw, and this effect was reversed by lamotrigine and forskolin. Lamotrigine directly activates HCN channels without elevating cAMP . Forskolin is an adenylate cyclase activator that increases intracellular cAMP, which binds to a domain in the C-terminal tail on HCN channels , and increased cAMP results in the activation of HCN channels. Therefore, the results suggest that ivabradine acts on peripheral nerves and tissues via HCN channels. Forskolin was reported to have a greater effect on HCN activation than lamotrigine . Forskolin also reversed the action of ivabradine as well as lamotrigine in the present study.
The results of the present study demonstrated that locally injected ivabradine is effective against carrageenan-induced inflammatory pain via HCN channels. Its effect was considered to involve not only an action on peripheral nerves but also an anti-inflammatory effect. The findings suggest that the local injection of ivabradine may be useful for preventing the development of inflammatory and neuropathic pain.