Volume 28, Issue 2 - Special Issue: The Challenge of Chronic Pain; Guest Editors: Rohini Kuner and Herta Flor

Voltage-gated sodium channels are crucial for pain perception. This is illustrated by several human genetic conditions that lead to either chronic pain or, vice versa, to congenital painlessness. The type of mutation, its impact on neuron excitability as well as the affected sodium channel subtype delineates a complex picture of the disorders. Genetic variants in sodium channels may affect the complex biophysical gating and also their trafficking, association with other proteins and more complex regulations of the channel protein and function, thus allowing us to explore the subtle but impactful effects of their dysregulation for human nociception. A detailed understanding of these pain disorders provides a unique chance to understand the detailed intricacies of nociception and pathological conditions such as neuropathic pain. With increasing awareness of the importance of sodium channel variants in neuropathic pain, more patients are genetically screened, sometimes identifying variants of unclear significance (VUS). Bioinformatic tools help to assess their potential disease causing impact, but functional studies using patch-clamp experiments in cell lines are needed to allow for reliable conclusions. Often cell lines are not sufficient to show a physiologically relevant phenotype and more complex, time intensive models, such as induced pluripotent stem cells (iPS-cells) are employed. A challenge remains to identify the role of each sodium channel VUS in the context of the detailed cellular genetic and functional context. To lay the grounds for such a detailed interpretation, we need a correlation of cellular function and genetic transcription on a single cell basis, as it is possible with the Patch-Seq technique. The more detailed our knowledge becomes on functional and genetic sensory neurons subtypes and their role in the generation of neuropathic pain, the more targeted the personal or population-based treatment can be.

Interactions of the immune system and the nociceptive system play an important role in the generation and maintenance of pain in musculoskeletal diseases and in disease development. In inflamed tissue peripheral nociceptive neurons are rendered hyperexcitable by proinflammatory cytokines, antigen/antibody complexes and other immune mediators. Spinal nociceptive neurons are rendered hyperexcitable with the support of microglial cells, the immune cells of the central nervous system. The so-elicited sensitization of pain pathways has a strong impact on pain processing in the brain. On the other hand, immune processes are regulated by the nervous system. Sensory neurons, by releasing neuropeptides, and efferent neurons of the sympathetic nervous system support immune processes which promote disease development.

Several forms of chronic pain do not respond to the conventional analgesics, such as opioids, but can be treated with antidepressants, such as serotonin and noradrenalin reuptake inhibitors (SNRIs). Recent studies indicate that noradrenalin signalling is a key target for SNRI-induced analgesia in neuropathic pain. SNRIs inhibit chronic pain by blocking reuptake of noradrenalin and subsequent activation of adrenergic receptors on neurons in the dorsal horn of the spinal cord. However, in the nervous system, various subtypes of adrenergic receptors are highly expressed by astrocytes and microglial cells. Activation of these receptors on astrocytes engages complex intracellular signalling pathways and prevents inflammatory changes of microglia, which in turn can affect neuronal activity. Hence, SNRIs-induced modulations of the glial cell physiology can impact neural circuit functions and pain perception. In this review, we summarize our current knowledge on the impact of SNRIs on glial cells and in modulating chronic pain in experimental animal models.

Brain circuits involved in pain chronicity shift from areas involved in nociceptive processing to those associated with emotional and motivational processes. They overlap with circuits relevant for anxiety, fear and depression and are characterized by deficient prefrontal control mechanisms. Noninvasive brain stimulation techniques such as repetitive transcranial magnetic stimulation, transcranial direct and alternating current stimulation directly impact on these circuits and pain. Neurofeedback and brain-computer interfaces as well as various types of cognitive and behavioral interventions also alter these circuits. The analysis of brain changes related to pain chronicity helps to mechanistically tailor interventions to patient characteristics, can increase treatment efficacy and efficiency and can identify new treatment approaches.

According to best current estimates, approximately 10% of those infected with SARS-CoV-2-virus experience long-term clinical and nonspecific neurological symptoms that may last for several weeks or months. This is currently referred to as “Long-COVID” or “Post-COVID-Syndrome”. Based on current knowledge, the most common long-term symptoms of COVID-19 disease include fatigue and poor concentration, but particularly also headache and musculoskeletal pain. However, given the novelty of COVID-19, only a few studies have systematically evaluated the central nervous alterations in the pain processing structures of our brain. Those first insights are yet important in order to offer patients adequate therapeutic options. Based on a systematic review of the literature, we will therefore provide an overview of the central nervous alterations in the brain described in the context of SARS-CoV-2 infection, focusing on findings with brain imaging.