Pain Mechanisms and Treatments (3)
Botulinum Toxin and Related Neurological Disorders (3)
Nerve injury and regeneration (1)
Advancements in Transdermal Drug Delivery (1)
Fibromyalgia and Chronic Fatigue Syndrome Research (1)
Eberhardt E, Namer B, Neureiter A, Körner J, Jørum E , et al.
Pain •
Spontaneous activity of peripheral sensory nerve fibers is one of the main drivers of neuropathic pain. It can be assessed in microneurography recordings of patients' C fibers and in patch-clamp recordings of dissociated dorsal root ganglia from humans and rodents. In microneurography of human C fibers, a distinct subgroup of neurons, the so-called mechano-insensitive (CMi) or sleeping nociceptors, shows spontaneous activity during neuropathic pain. It was shown before that sensory neurons from patient-derived induced pluripotent stem cells (iSNs) can be used to model this increased spontaneous activity in vitro, suggesting that a disease relevant cell type is generated with this approach. The origin of the spontaneous activity in human C fibers is not fully understood. Derived sensory neurons offer the unique possibility to study patient-derived, single-cell function, allowing for identification of potential mechanisms underlying spontaneous C-fiber activity. Here, we identify 4 distinct functional subtypes of iSNs from healthy donors and a patient suffering from the neuropathic pain syndrome inherited erythromelalgia using patch-clamp recordings. Similar to microneurography recordings from the same patient, spontaneous activity is restricted to 1 functional subgroup that shows tonic firing behavior and seems to be especially prone to develop neuronal hyperexcitability. We demonstrate that spontaneous activity correlates with a reduced voltage threshold of action potential generation and increased spontaneous depolarizing fluctuations of the membrane potential. Our findings reveal that only the tonically firing functional subclass of iSNs shows spontaneous activity and suggest that these neurons may be related to the pathologically active CMi fibers identified during microneurography recordings in patients with pain.
Meents JE, Bressan E, Sontag S, Foerster A, Hautvast P , et al.
Pain •
The chronic pain syndrome inherited erythromelalgia (IEM) is attributed to mutations in the voltage-gated sodium channel (NaV) 1.7. Still, recent studies targeting NaV1.7 in clinical trials have provided conflicting results. Here, we differentiated induced pluripotent stem cells from IEM patients with the NaV1.7/I848T mutation into sensory nociceptors. Action potentials in these IEM nociceptors displayed a decreased firing threshold, an enhanced upstroke, and afterhyperpolarization, all of which may explain the increased pain experienced by patients. Subsequently, we investigated the voltage dependence of the tetrodotoxin-sensitive NaV activation in these human sensory neurons using a specific prepulse voltage protocol. The IEM mutation induced a hyperpolarizing shift of NaV activation, which leads to activation of NaV1.7 at more negative potentials. Our results indicate that NaV1.7 is not active during subthreshold depolarizations, but that its activity defines the action potential threshold and contributes significantly to the action potential upstroke. Thus, our model system with induced pluripotent stem cell-derived sensory neurons provides a new rationale for NaV1.7 function and promises to be valuable as a translational tool to profile and develop more efficacious clinical analgesics.
Kleggetveit IP, Schmidt R, Namer B, Salter H, Helås T , et al.
Brain and behavior •
The sodium channel Nav 1.9 is expressed in peripheral nociceptors and has recently been linked to human pain conditions, but the exact role of Nav 1.9 for human nociceptor excitability is still unclear. C-nociceptors from two patients with late onset of erythromelalgia-like pain, signs of small fiber neuropathy, and rare genetic variants of Nav 1.9 (N1169S, I1293V) were assessed by microneurography. Compared with patients with comparable pain phenotypes (erythromelalgia-like pain without Nav-mutations and painful polyneuropathy), there was a tendency toward more activity-dependent slowing of conduction velocity in mechanoinsensitive C-nociceptors. Hyperexcitability to heating and electrical stimulation were seen in some nociceptors, and other unspecific signs of increased excitability, including spontaneous activity and mechanical sensitization, were also observed. Although the functional roles of these genetic variants are still unknown, the microneurography findings may be compatible with increased C-nociceptor excitability based on increased Nav 1.9 function.
Kist AM, Sagafos D, Rush AM, Neacsu C, Eberhardt E , et al.
PloS one •
Gain-of-function mutations in the tetrodotoxin (TTX) sensitive voltage-gated sodium channel (Nav) Nav1.7 have been identified as a key mechanism underlying chronic pain in inherited erythromelalgia. Mutations in TTX resistant channels, such as Nav1.8 or Nav1.9, were recently connected with inherited chronic pain syndromes. Here, we investigated the effects of the p.M650K mutation in Nav1.8 in a 53 year old patient with erythromelalgia by microneurography and patch-clamp techniques. Recordings of the patient's peripheral nerve fibers showed increased activity dependent slowing (ADS) in CMi and less spontaneous firing compared to a control group of erythromelalgia patients without Nav mutations. To evaluate the impact of the p.M650K mutation on neuronal firing and channel gating, we performed current and voltage-clamp recordings on transfected sensory neurons (DRGs) and neuroblastoma cells. The p.M650K mutation shifted steady-state fast inactivation of Nav1.8 to more hyperpolarized potentials and did not significantly alter any other tested gating behaviors. The AP half-width was significantly broader and the stimulated action potential firing rate was reduced for M650K transfected DRGs compared to WT. We discuss the potential link between enhanced steady state fast inactivation, broader action potential width and the potential physiological consequences.
Namer B, Ørstavik K, Schmidt R, Kleggetveit IP, Weidner C , et al.
Pain •
Seven patients diagnosed with erythromelalgia (EM) were investigated by microneurography to record from unmyelinated nerve fibers in the peroneal nerve. Two patients had characterized variants of sodium channel Nav1.7 (I848T, I228M), whereas no mutations of coding regions of Navs were found in 5 patients with EM. Irrespective of Nav1.7 mutations, more than 50% of the silent nociceptors in the patients with EM showed spontaneous activity. In the patient with mutation I848T, all nociceptors, but not sympathetic efferents, displayed enhanced early subnormal conduction in the velocity recovery cycles and the expected late subnormality was reversed to supranormal conduction. The larger hyperpolarizing shift of activation might explain the difference to the I228M mutation. Sympathetic fibers that lack Nav1.8 did not show supranormal conduction in the patient carrying the I848T mutation, confirming in human subjects that the presence of Nav1.8 crucially modulates conduction in cells expressing EM mutant channels. The characteristic pattern of changes in conduction velocity observed in the patient with the I848T gain-of function mutation in Nav1.7 could be explained by axonal depolarization and concomitant inactivation of Nav1.7. If this were true, activity-dependent hyperpolarization would reverse inactivation of Nav1.7 and account for the supranormal CV. This mechanism might explain normal pain thresholds under resting conditions.