Noncanonical Ion Channel Behaviour in Pain

The impact of HCN4 channels on CNS brain networks as a new target in pain development

While it is well established that the isoform 2 of the hyperpolarization-activated cyclic nucleotide-gated cation channel (HCN2) plays an important role in the development and maintenance of pain, the role of the closely related HCN4 isoform in the processing of nociceptive signals is not known. HCN4 channels are highly expressed in the thalamus, a region important for stimulus transmission and information processing. We used a brain-specific HCN4-knockout mouse line (HCN4-KO) to explore the role of HCN4 channels in acute nociceptive processing using several behavioral tests as well as a multimodal magnetic resonance imaging (MRI) approach. Functional MRI (fMRI) brain responses were measured during acute peripheral thermal stimulation complemented by resting state (RS) before and after stimulation. The data were analyzed by conventional and graph-theoretical approaches. Finally, high-resolution anatomical brain data were acquired. HCN4-KO animals showed a central thermal, but not a mechanical hypersensitivity in behavioral experiments. The open field analysis showed no significant differences in motor readouts between HCN4-KO and controls but uncovered increased anxiety in the HCN4-KO mice. Thermal stimulus-driven fMRI (s-fMRI) data revealed increased response volumes and response amplitudes for HCN4-KO, most pronounced at lower stimulation temperatures in the subcortical input, the amygdala as well as in limbic/hippocampal regions, and in the cerebellum. These findings could be cross-validated by graph-theoretical analyses. Assessment of short-term RS before and after thermal stimulation revealed that stimulation-related modulations of the functional connectivity only occurred in control animals. This was consistent with the finding that the hippocampus was found to be smaller in HCN4-KO. In summary, the deletion of HCN4 channels impacts on processing of acute nociception, which is remarkably manifested as a thermal hypersensitive phenotype. This was mediated by the key regions hypothalamus, somatosensory cortex, cerebellum and the amygdala. As consequence, HCN4-KO mice were more anxious, and their brain-wide RS functional connectivity could not be modulated by thermal nociceptive stimulation.

Assessing the impact of pain-linked Nav1.7 variants: An example of two variants with no biophysical effect

Mutations in the voltage-gated sodium channel Nav1.7 are linked to human pain. The Nav1.7/N1245S variant was described before in several patients suffering from primary erythromelalgia and/or olfactory hypersensitivity. We have identified this variant in a pain patient and a patient suffering from severe and life-threatening orthostatic hypotension. In addition, we report a female patient suffering from muscle pain and carrying the Nav1.7/E1139K variant. We tested both Nav1.7 variants by whole-cell voltage-clamp recordings in HEK293 cells, revealing a slightly enhanced current density for the N1245S variant when co-expressed with the β1 subunit. This effect was counteracted by an enhanced slow inactivation. Both variants showed similar voltage dependence of activation and steady-state fast inactivation, as well as kinetics of fast inactivation, deactivation, and use-dependency compared to WT Nav1.7. Finally, homology modeling revealed that the N1245S substitution results in different intramolecular interaction partners. Taken together, these experiments do not point to a clear pathogenic effect of either the N1245S or E1139K variant and suggest they may not be solely responsible for the patients' pain symptoms. As discussed previously for other variants, investigations in heterologous expression systems may not sufficiently mimic the pathophysiological situation in pain patients, and single nucleotide variants in other genes or modulatory proteins are necessary for these specific variants to show their effect. Our findings stress that biophysical investigations of ion channel mutations need to be evaluated with care and should preferably be supplemented with studies investigating the mutations in their context, ideally in human sensory neurons.

K+ Channels in Primary Afferents and Their Role in Nerve Injury-Induced Pain

Sensory abnormalities generated by nerve injury, peripheral neuropathy or disease are often expressed as neuropathic pain. This type of pain is frequently resistant to therapeutic intervention and may be intractable. Numerous studies have revealed the importance of enduring increases in primary afferent excitability and persistent spontaneous activity in the onset and maintenance of peripherally induced neuropathic pain. Some of this activity results from modulation, increased activity and /or expression of voltage-gated Na+ channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. K+ channels expressed in dorsal root ganglia (DRG) include delayed rectifiers (Kv1.1, 1.2), A-channels (Kv1.4, 3.3, 3.4, 4.1, 4.2, and 4.3), KCNQ or M-channels (Kv7.2, 7.3, 7.4, and 7.5), ATP-sensitive channels (KIR6.2), Ca2+-activated K+ channels (KCa1.1, 2.1, 2.2, 2.3, and 3.1), Na+-activated K+ channels (KCa4.1 and 4.2) and two pore domain leak channels (K2p; TWIK related channels). Function of all K+ channel types is reduced via a multiplicity of processes leading to altered expression and/or post-translational modification. This also increases excitability of DRG cell bodies and nociceptive free nerve endings, alters axonal conduction and increases neurotransmitter release from primary afferent terminals in the spinal dorsal horn. Correlation of these cellular changes with behavioral studies provides almost indisputable evidence for K+ channel dysfunction in the onset and maintenance of neuropathic pain. This idea is underlined by the observation that selective impairment of just one subtype of DRG K+ channel can produce signs of pain in vivo. Whilst it is established that various mediators, including cytokines and growth factors bring about injury-induced changes in DRG function and excitability, evidence presently available points to a seminal role for interleukin 1β (IL-1β) in control of K+ channel function. Despite the current state of knowledge, attempts to target K+ channels for therapeutic pain management have met with limited success. This situation may change with the advent of personalized medicine. Identification of specific sensory abnormalities and genetic profiling of individual patients may predict therapeutic benefit of K+ channel activators.

Effective block by pirfenidone, an antifibrotic pyridone compound (5-methyl-1-phenylpyridin-2[H-1]-one), on hyperpolarization-activated cation current: An additional but distinctive target

Pirfenidone (PFD), a pyridone compound, is well recognized as an antifibrotic agent tailored for the treatment of idiopathic pulmonary fibrosis. Recently, through its anti-inflammatory and anti-oxidant effects, PFD based clinical trial has also been launched for the treatment of coronavirus disease (COVID-19). To what extent this drug can perturb membrane ion currents remains largely unknown. Herein, the exposure to PFD was observed to depress the amplitude of hyperpolarization-activated cation current (I_h) in combination with a considerable slowing in the activation time of the current in pituitary GH₃ cells. In the continued presence of ivabradine or zatebradine, subsequent application of PFD decreased I_h amplitude further. The presence of PFD resulted in a leftward shift in I_h activation curve without changes in the gating charge. The addition of this compound also led to a reduction in area of voltage-dependent hysteresis evoked by long-lasting inverted triangular (downsloping and upsloping) ramp pulse. Neither the amplitude of M-type nor erg-mediated K⁺ current was altered by its presence. In whole-cell potential recordings, addition of PFD reduced the firing frequency, and this effect was accompanied by the depression in the amplitude of sag voltage elicited by hyperpolarizing current stimulus. Overall, this study highlights evidence that PFD is capable of perturbing specific ionic currents, revealing a potential additional impact on functional activities of different excitable cells.

Circadian regulation of chemotherapy-induced peripheral neuropathic pain and the underlying transcriptomic landscape

Growing evidence demonstrates circadian rhythms of pain hypersensitivity in various chronic disorders. In chemotherapy-induced peripheral neuropathy (CIPN), agents such as paclitaxel are known to elicit chronic neuropathic pain in cancer patients and seriously compromise their quality of life. Here, we report that the mechanical threshold for allodynia in paclitaxel-treated rats exhibited a robust circadian oscillation, reaching the nadir during the daytime (inactive phase). Using Per2::LucSV circadian reporter mice expressing a PER2::LUC fusion protein, we isolated dorsal root ganglia (DRG), the primary sensory cell body for peripheral nerve injury generated hypersensitivity, and monitored ex vivo reporter bioluminescence. We observed strong circadian reporter rhythms in DRG neurons which are highly entrainable by external cues. Paclitaxel treatment significantly lengthened DRG circadian periods, with little effects on the amplitude of oscillation. We further observed the core protein BMAL1 and PER2 in DRG neurons and satellite cells. Using DRG and dorsal horn (DH; another key structure for CIPN pain response) tissues from vehicle and paclitaxel treated rats, we performed RNA-sequencing and identified diurnal expression of core clock genes as well as clock-controlled genes in both sites. Interestingly, 20.1% and 30.4% of diurnal differentially expressed genes (DEGs) overlapped with paclitaxel-induced DEGs in the DRG and the DH respectively. In contrast, paclitaxel-induced DEGs displayed only a modest overlap between daytime and nighttime (Zeitgeber Time 8 and 20). Furthermore, paclitaxel treatment induced de novo diurnal DEGs, suggesting reciprocal interaction of circadian rhythms and chemotherapy. Our study therefore demonstrates a circadian oscillation of CIPN and its underlying transcriptomic landscape.

Novel Analgesics with Peripheral Targets

A limited number of peripheral targets generate pain. Inflammatory mediators can sensitize these. The review addresses targets acting exclusively or predominantly on sensory neurons, mediators involved in inflammation targeting sensory neurons, and mediators involved in a more general inflammatory process, of which an analgesic effect secondary to an anti-inflammatory effect can be expected. Different approaches to address these systems are discussed, including scavenging proinflammatory mediators, applying anti-inflammatory mediators, and inhibiting proinflammatory or facilitating anti-inflammatory receptors. New approaches are contrasted to established ones; the current stage of progress is mentioned, in particular considering whether there is data from a molecular and cellular level, from animals, or from human trials, including an early stage after a market release. An overview of publication activity is presented, considering a IuPhar/BPS-curated list of targets with restriction to pain-related publications, which was also used to identify topics.

Ion Channels of Nociception

The special issue “Ion Channels of Nociception” contains 13 articles published by 73 authors from different countries united by the main focusing on the peripheral mechanisms of pain. The content covers the mechanisms of neuropathic, inflammatory, and dental pain as well as pain in migraine and diabetes, nociceptive roles of P2X3, ASIC, Piezo and TRP channels, pain control through GPCRs and pharmacological agents and non-pharmacological treatment with electroacupuncture.