The dorsal root ganglion as a target for neurorestoration in neuropathic pain

Abstract

Neuropathic pain is a severe and chronic condition widely found in the general population. The reason for this is the extensive variety of damage or diseases that can spark this unpleasant constant feeling in patients. During the processing of pain, the dorsal root ganglia constitute an important region where dorsal root ganglion neurons play a crucial role in the transmission and propagation of sensory electrical stimulation. Furthermore, the dorsal root ganglia have recently exhibited a regenerative capacity that should not be neglected in the understanding of the development and resolution of neuropathic pain and in the elucidation of innovative therapies. Here, we will review the complex interplay between cells (satellite glial cells and inflammatory cells) and factors (cytokines, neurotrophic factors and genetic factors) that takes place within the dorsal root ganglia and accounts for the generation of the aberrant excitation of primary sensory neurons occurring in neuropathic pain. More importantly, we will summarize an updated view of the current pharmacologic and nonpharmacologic therapies targeting the dorsal root ganglia for the treatment of neuropathic pain.

Introduction

Chronic neuropathic pain is among one of the most widespread patient complaints and is one of the most difficult conditions to treat. Neuropathic pain was defined as “pain arising as a direct consequence of a lesion or disease affecting the somatosensory system,” a definition that was endorsed by the International Association for the Study of Pain in 2011 (Jensen et al., 2011). Epidemiological studies reveal a 7–8% prevalence of chronic neuropathic pain in the general population, which constitutes approximately 20–25% of chronic pain patients (Bouhassira, 2019).

Neuropathic pain is characterized by sensitization and hyperexcitation of primary sensory neurons. Primary somatosensory neurons transmit information about our external environment and internal state to the central nervous system (CNS). They enable us to detect, perceive, and react to a broad range of stimuli, either innocuous or noxious. Their cell bodies reside in sensory ganglia, such as the dorsal root ganglia (DRGs), the trigeminal ganglia and other cranial nerve ganglia.

In the CNS, nociceptive inputs are processed by local spinal circuits, and nociceptive information is projected to the corresponding telencephalon areas via two main di-synaptic routes. Projecting neurons in the spino-thalamic-cortical pathway terminate in different nuclei of the thalamus, from which they target several cortical areas (e.g., the posterior insular cortex, medial parietal operculum or motor sections of the midcingulate cortex as well as primary and secondary somatosensory cortices). In general, processing in these areas is responsible for the discriminative aspects of the nociceptive signals (e.g., localization, timing, or intensity). Projections through the spino-parabrachial-amygdala pathway reach the limbic system via the amygdala complex. The central nucleus of the amygdala, or the “nociceptive amygdala,” receives purely nociceptive inputs from the lateral parabrachial area to process affective emotional responses. This “emotional network” activated by stimuli that are different from pain renders the “salience” attributes of noxious stimuli. Thus, this pathway is responsible for processing the affective-motivational aspects of pain (reviewed in Roza and Martínez-Padilla, 2021).

To date, many methods and therapeutic targets have been attempted to alleviate neuropathic pain, with limited benefits and similar outcomes at most (Varshney et al., 2021). The DRGs are structures located at the communication point moving from the periphery to the CNS. Due to their strategic location, critical reexamination is required to understand their functional role in the initiation and maintenance of chronic pain and to conceive novel therapeutic approaches targeting the DRGs.

Search Strategy and Selection Criteria

Studies cited in this review were obtained from searching the PubMed database (https://pubmed.ncbi.nlm.nih.gov) using the following keywords: dorsal root ganglia, neuropathic pain, chronic pain, satellite glial cells, nerve injury, pharmacologic and non-pharmacologic treatments and sensory neurons. Any additional articles were included from suggestions from citation tracking. Studies cited in this review were published between 1977 and 2023. Most of the studies included in this review (> 50%) were published between 2019 and 2023, and any studies published before 2019 are included to acknowledge the first article describing the issue. The literature search was completed on April 20, 2023.

Anatomy of the Dorsal Root Ganglia

The DRGs are highly complex structures located on either side of the spinal cord that span the length of the spinal column. In humans, 31 right and left paired spinal nerves carry/transmit autonomic, motor, and sensory information between the spinal cord and the periphery. These spinal nerves are composed of afferent sensory dorsal axons (the dorsal root) and motor ventral efferent axons (the ventral root) that emerge from the intervertebral neural foramina between adjacent vertebral segments (Esposito et al., 2019). On its way out from the neural foramina, the dorsal sensory root forms the DRG.

The DRGs are a collection of cell bodies of neurons that are primarily responsible for the transduction of sensory information from the periphery and the transmission of the information to the CNS. In the DRGs, neurons present a pseudounipolar morphology (Figure 1). Their axons leave the cell bodies (out of the DRG) into the dorsal root and split into two branches (Esposito et al., 2019). The central branch goes to the dorsal horn of the spinal cord, where it forms synapses with neurons of the CNS. Conversely, the peripheral branch travels through the distal dorsal root into the spinal nerve to receptor endings in the periphery (e.g., the skin, joint, and muscle) and is responsible for afferent signaling (Leijnse and D’Herde, 2016).

Figure 1.

Scheme of the dorsal root ganglia.

Organization of primary afferent fibers (Aβ, Aδ and C fibers) of dorsal root ganglion neurons projecting to the spinal cord dorsal horn (laminae I–V). Created with Adobe Photoshop v.8.01 and Microsoft Powerpoint 2010. DREZ: Dorsal root entry zone; DRTZ: dorsal root transitional zone.

The central branch shares the typical features of peripheral nerve tissue, with Schwann cells and a basement lamina. When arriving at the spinal cord, dorsal root axons encounter central glia and course through channels between astrocytes at the dorsal root transitional/entry zone. In the transitional node at the dorsal root entry zone, dorsal axons are myelinated by Schwann cells peripherally before entering the spinal cord and by oligodendrocytes once in the CNS (Berthold and Carlstedt, 1977).

This atypical morphology of the T-junction gives the DRG neuron an important role in the transmission and propagation of the electrical impulse. These neurons act to impede the electrical impulse from a nociceptor to the dorsal root entry zone, participate in the propagation of the electrical pulse, or act as a low-pass filter to electrical information from the periphery (Hao et al., 2023). Therefore, their activity and welfare play an essential role in the conduction of incoming external or internal stimuli but also in the pathogenesis of uncontrolled and aberrant stimulations, as occurs in neuropathic pain.

DRG primary sensory neurons are a pool of diverse categories. Small-diameter DRG neurons extend unmyelinated axons (C-fibers) and thinly myelinated axons (Aδ-fibers) that group nociceptive, thermal and mechanoreceptive signals. These signals generated at peripheral nerve terminals are sent to neurons in laminae I–II of the spinal cord. In contrast, large DRG neurons transmit mechanoreceptive and proprioceptive signals via thickly myelinated afferents (Aβ-fibers) to spinal laminae III–V. Compared to large, myelinated, and high-velocity Aβ-fibers, unmyelinated C-fibers are smaller in diameter and have a much slower conduction velocity. Researchers have demonstrated that these C-fibers play an active role in chronic pain, propagating aberrant pain signaling within the DRG cell bodies (Figure 1).

Despite their relative proximity, in most cases, the cell bodies of DRG neurons do not interact with one another due to the surrounding layers of satellite glial cells (SGCs). However, there is a small proportion (4–9% depending on the species) of DRG neurons that share a common glial envelope, forming a “cluster” with one or two other neurons (Hanani and Spray, 2020), opening the possibility of cross-excitation of several DRG neurons by one stimulus. Moreover, the gap between SGCs and the neuronal surface is approximately 20 nm, which is a similar distance to that occurring in the synaptic space. This enables SGCs to take an active role in cellular communication within the DRG. SGCs, once thought to be mere support of DRG neuron cell bodies to maintain their homeostasis, have recently been shown to play active and important roles in the normal physiology of the ganglia. Hence, as a consequence of this active interaction, SGC dysfunction affects DRG neuron activity and accounts for the generation/maintenance of neuropathic pain (Hanani and Spray, 2020).

The Dorsal Root Ganglia in Neuropathic Chronic Pain

Chronic neuropathic pain usually begins with damage to peripheral nerves, unleashing a cascade of responses within the DRG, with inflammation being the ultimate trigger for pain. The DRG can become inflamed by different causes (degeneration, compression or action of inflammatory mediators), inducing severe pain and other symptoms.

The situations that spark inflammation of the dorsal root cover a broad range of conditions, such as peripheral nerve trauma, sciatica, compressive neuropathy, herniated disc osteophytes (Vancamp et al., 2017), spinal stenosis, peripheral neuropathy, meningitis, side effects of several drug treatments, such as some anticancer drugs (Quintao et al., 2019) and antiretroviral drug therapy (Madden et al., 2020), and spinal infections (Ma et al., 2006), including HIV (Lu et al., 2021). This wide variety of common events that cause the noxious feeling of neuropathic pain explains its high prevalence in the population.

In response to inflammation of a peripheral afferent fiber, the DRG undergoes important changes related to glial cells, cytokines, neurotrophic factors (NTs), ion channels, and neurotransmitters, inducing DRG neuron hyperexcitability and genetic changes.

Dorsal Root Ganglion Cells and Proinflammatory Cytokines

The glial cells of the peripheral nervous system (PNS) mainly include Schwann cells and SGCs. Studies of PNS injury have shown that PNS resident macrophages and infiltrated inflammatory cells also play important roles in pain development and maintenance (Bethea and Fischer, 2021).

As mentioned, SGCs play an important role in the normal physiology of the ganglia. As an example of the bidirectional communication between neurons and satellite cells, electric stimulation incites vesicular release of ATP from DRG neurons that activate their SCG P2X7 receptors. P2X7 activation leads to the release of tumor necrosis factor-α from satellite cells. Tumor necrosis factor-α in turn potentiates P2X3 receptor-mediated responses and increases the excitability of DRG neurons (Zhang et al., 2007).

Nerve damage or inflammation activates SGCs in sensory ganglia. As a result of an injury, SGCs increase the expression of the astrocyte marker glial fibrillary acidic protein (GFAP), indicating glial reactivity and gap junction-mediated SGC coupling (Mohr et al., 2021). This increased coupling strength among SGCs contributes to the lowering of the pain threshold (Huang et al., 2010). Other facets that seem to contribute to neuropathic pain are the downregulation of Kir4.1 channels in SGCs (Vit et al., 2008) and increased sensitivity of SGCs to the pain mediator ATP mediated by P2X (ionotropic) receptors (Kushnir et al., 2011). Importantly, activated SGCs release the proinflammatory cytokines IL-1β, IL-6, TNF and fractalkine (Dubovy et al., 2010; Souza et al., 2013; Mitterreiter et al., 2017), which increase neuron excitability and firing and mediate the recruitment of circulating immune cells, including macrophages, T cells and neutrophils.

Upon peripheral nerve injury, inflammation takes place and stretches out to the DRG. In this context, monocyte-derived macrophages (Ydens et al., 2020) arrive at the ganglion of the wounded nerve. In the inflamed DRG, macrophages accumulate in the DRG and are skewed into an M1-like phenotype (Simeoli et al., 2017), initiating and maintaining neuropathic pain (Yu et al., 2020). Depletion of macrophages in the DRG, but not at the peripheral nerve injury site, can prevent the development of nerve injury-induced mechanical hypersensitivity. This has been suggested to be due to cellular interactions between DRG macrophages and sensory neurons as a relevant contribution to the neuropathic pain phenotype (Yu et al., 2020). Exaggerated inflammatory cell responses within the DRG lead to chronic stimulation of sensory neurons and ultimately prolonged sensation of pain.

However, macrophage polarization can shift during the inflammatory process, with macrophages turning into an M2-like phenotype. The M2-like phenotype contributes to tissue healing and the resolution of inflammation as well as the alleviation of neuropathic pain (Niehaus et al., 2021). Although the mechanism by which inflammatory pain resolves is unknown, impressive recent data show that macrophages can actively resolve inflammatory pain by transferring mitochondria to sensory neurons (van der Vlist et al., 2022).

DRG-infiltrating T lymphocytes also play a significant role in inducing neuropathic pain. Notably, blocking functional CD8⁺ T cells at the level of the spinal cord and the DRG is sufficient to reverse chemotherapy-induced mechanical hypersensitivity. Similarly, adoptive transfer of CD8⁺ T cells exacerbated neuropathic pain in this model (Galvin and C, 2021), suggesting that cytotoxic T cells contribute to pain progression. T cells also secrete IFN-γ, which activates glial cells and initiates chronic pain (Ferrara et al., 2022). Moreover, the leukocyte elastase released by T cells and neutrophils after nerve injury induces neuropathic mechanical allodynia (Vicuna et al., 2015; Bali and Kuner, 2017).

In addition, in a model of chronic neuropathic pain, i.e., constriction injury, DRG-infiltrated neutrophils express the chemokine MCP-1/CCL2, which sensitizes peripheral nociceptors (Dansereau et al., 2021). Prostaglandin E2 (PGE2), also released by these neutrophils, induces chronic pain by stimulating DRG neurons to produce pain-related neuropeptides (such as substance P (SP) and calcitonin gene-related peptide (CGRP)) and interleukin (IL)-18, highlighting the effect that these immune cells have in chronic pain as well.

Neurotrophic factors

Injuries and compression of the peripheral nerves can release NTs. Unlike their potentiating role in neural growth and regeneration, NTs contribute to the pathogenesis of neuropathic pain, as they have key roles in the mechanisms of peripheral and central sensitization (Khan and Smith, 2015).

DRG glia release nerve growth factor (NGF), a prototypical member of a family of target-derived neurotrophic molecules, following constriction injury in mice. This factor increases adrenergic sprouting into the DRG, causing neuropathic pain (Dai et al., 2020) . NGF expression is also increased in the DRG of rat pups during postnatal life after peripheral inflammation induced by complete Freund’s adjuvant (Yuan et al., 2020). One of the proposed mechanisms of action of NGF might be through the upregulation of several pain-related genes in the primary sensory neurons of the DRG, such as SP, CGRP, transient receptor potential vanilloid subtype 1 (TRPV1), Na(v)1.8 and Na(v)1.9 sodium channels and mu opioid receptor (MOR) (Siniscalco et al., 2011).

BDNF, another important NT, shows hyperalgesic effects similar to those of NGF. BDNF is strongly involved in the axonal sprouting of intraspinal serotonergic fibers following dorsal root injuries (DRIs), a model of neuropathic pain that leads to sensory impairments, such as loss of sensation and axonal sprouting of intraspinal serotonergic fibers (Cragg et al., 2010). In another model of neuropathic pain, that is, lumbar 5 ventral root transection, BDNF expression is also significantly upregulated in DRG sensory neurons, inducing neuropathic pain (Li et al., 2006).

DRG neuron hyperexcitability

Major channels that participate in/contribute to the propagation of action potentials in DRG neurons include sodium potassium and calcium channels. The upregulation of ion channels after nerve injury leads to neuronal hyperexcitability of the cells and constant stimulation of the neurons within the DRG. This further sensitization incites chronic pain perception (Ma et al., 2019; Smith, 2020).

Upregulation of a type of sodium channel (Nav1.8 Na⁺), a tetrodotoxin (TTX)-insensitive sodium channel that is preferentially expressed in DRG neurons, is responsible for spontaneous action potential activity and hyperexcitability within the DRG of damaged sensory axons. This upregulation contributes to the development of ectopic mechanosensitivity and neuropathic pain (Theile and Cummins, 2011).

In inflamed DRGs, TNF-α increases membrane outward potassium channel currents and calcium channel currents, leading to DRG neuronal hyperpolarization. This hyperpolarization results in a hyperexcitability of the DRG neurons that causes neuropathic pain. Axotomy leads to a reduction in voltage-gated potassium channels, causing increased depolarization in DRG neurons, which is also suggested as a potential molecular mechanism for the hyperexcitability of injured nerves (Wei et al., 2021).

Furthermore, after constriction injury of a peripheral axon, calcium currents are gated by significantly reduced low-threshold voltage. Persistent inflammation alters the density and distribution of voltage-activated calcium channel populations of rat DRG neurons(Harding and Zamponi, 2022), thereby participating in the development of neuropathic pain.

In addition, modulation of synaptic transmission through regulation of the release of neurotransmitters is performed by voltage-gated calcium channels and glutamate receptors, which are expressed on presynaptic membranes at the terminals of the primary afferents of the dorsal horn of the spinal cord. After peripheral afferent fiber injury, DRG neurons in vitro become hyperexcitable, and they exhibit ectopic firing (Liu et al., 1999). Neuropathic pain is associated with hyperactivity of excitatory (glutamatergic) transmission. Glutamate released by C-fibers leads to an increasingly enhanced response of the dorsal horn neurons: this phenomenon of central sensitization is called “wind-up”(Mendell, 2022).

Genetic changes

Many studies have reported dramatic changes in individual molecules in the DRG after nerve injury that are implicated in the generation and maintenance of pain (Martin et al., 2019; Du et al., 2022; Ikuma et al., 2023).

Although the extensive study of the individual changes in particular genes in the DRG lacks a comprehensive overview of the alterations in global gene expression to understand the underlying mechanisms of neuropathic pain and to develop new therapies, interesting data over the past year have significantly improved our knowledge of neuropathic pain pathogenesis. Broader approaches, such as cDNA array (Xiao et al., 2002), microarrays (LaCroix-Fralish et al., 2011) and RNA sequencing (RNA-Seq) data (Pokhilko et al., 2020), followed by proper meta-analysis have allowed us to gain a global view of DRG gene expression changes operating in different models of neuropathic pain.

An initial attempt at cDNA array analysis of the genes from the cDNA libraries of the lumbar DRGs of normal and axotomized rats has evidenced differential expression of approximately 80% of the 122 analyzed genes, which were not previously identified in DRGs after nerve injury, especially neuropeptides, of which up to 50% were affected (Xiao et al., 2002).

Deep analysis by RNA-seq has shown that 60% of the common rodent gene response after injury occurs in nociceptors of the DRG, mainly at 1 week after nerve injury, with smaller changes at later 3- to 4-week time points (Pokhilko et al., 2020). The analysis of these data with cross analysis of the already developed interactome of connecting pain-specific protein interactions (Jamieson et al., 2014) has revealed a highly connected network centered on opioid signaling and a substantial overlap between rodent pain network genes and human genes involved in chronic pain.

Single-cell RNA sequencing was used to characterize subtype-specific perturbations in the transcriptomes of DRG neurons in another mouse model of neuropathic pain, namely, constriction injury. This study demonstrated that there are subtype-specific transcriptomic changes in injured neurons and highlighted a transcriptomic sexual dimorphism in DRG neurons after nerve injury (Zhang et al., 2022).

All these findings reveal dynamic and complex changes in the molecular diversity among DRG neurons in response to nerve injury, but this information still remains to be completed.

Dorsal Root Ganglion Stemness and Regeneration

The role of DRGs and their malfunction in the generation and maintenance of neuropathic pain raise questions about the capacity of DRGs to regenerate their cell components and regain the correct function, alleviating/reverting the pain feeling.

After peripheral nerve injury, the number of DRG sensory neurons is initially reduced but recovers after several months in mice models (Gallaher et al., 2014), suggesting the presence of neural crest stem/progenitor cells (NPCs) that cope with cell loss. Over the last years, different studies have highlighted the presence of a stem-like population of NPCs in the adult DRG due to their ability to form spheres and differentiate into multiple peripheral lineages in vitro (Nagoshi et al., 2008). DRGs can generate multiple differentiated cell types after transplantation into the CNS, including remyelinating cells, as demonstrated in adult DRG-NPCs in response to spinal cord demyelination (Vidal et al., 2015).

Although the stemness potential (Snippert and Clevers, 2011) of adult DRGs has been hinted at by their capacity to form spheres from explants in vitro (Namaka et al., 2001; Li et al., 2007) and to differentiate into neurons, glial cells, and myofibroblasts in culture, their identity has been identified only recently. Using transgenic tools, Manglier and colleagues (Maniglier et al., 2022) demonstrated that adult mouse DRGs contain stem cells and progenitor cells. The stemness capacity was shown in a distinct subpopulation of SGCs that have the capacity to generate neurons and glia in vitro but also in vivo. In response to sciatic nerve axotomy, long-term lineage tracing of adult DRG SGCs revealed that activated SGCs give rise in the ipsilateral DRGs of injured animals to a great majority of the newly generated SGCs and Schwann cells but also to a minor but existent population of neurons (Maniglier et al., 2022). SGCs originate from NPCs during development (Maro et al., 2004), which leads to the hypothesis that the stemness potential is retained from that stage in adulthood.

Thus, under physiological conditions, DRG stem cells are responsible for DRG gliogenesis, and turnover of the mature SGC population occurs to maintain glial homeostasis. However, when SGCs are activated by nerve injury, environmental cues activate SGCs to generate neurons in addition to SGCs to replace neurons lost to injury (Maniglier et al., 2022). Whether these new neurons functionally reconnect in the proper fashion or erroneously, possibly contributing to pain, remains an open question. Their existence, activation, and involvement in glial/neuronal renewal under pathophysiological conditions open new research perspectives, especially in the neuropathic field.

Nevertheless, to restore sensory functions, injured dorsal root axons must grow from a permissive peripheral milieu into the nonpermissive spinal cord environment. To penetrate the CNS, axonal cones need guidance signaling. New evidence in recent years has highlighted the parallels between axonal and vascular growth; both axonal growth cones and endothelial tip cells respond to related-family signals, i.e., Slit/Roundabout receptors, Ephrin/Eph receptors and netrins/UNC5 pathways, or semaphorins and their primary receptors neuropilins and plexins (Adams and Eichmann, 2010). In addition, similar to the vascular migration of neurons within the brain (Fujioka et al., 2017), Schwann cells migrate along blood vessels in regenerating nerves and use the same route to invade the demyelinated CNS (Garcia-Diaz et al., 2019). The vascular system might thus constitute a perfect paved way for Schwann cells and possibly their progenitors to pass through the dorsal root entry zone and conquer the CNS, restoring the loss of motor or sensory functions. Given their important role in migration and axonal growth, angiogenesis and lympho-angiogenesis are novel features to examine in the pursuit of innovative therapies for neuropathic pain.

Pharmacologic and Nonpharmacologic Treatment Therapies Targeting the Dorsal Root Ganglion

Although there are numerous therapeutic alternatives for treating neuropathic pain, both pharmacological and nonpharmacological, the reality is that many of them are ineffective, and there are a certain number of patients for whom no treatment is possible. For this reason, efforts are being made to improve diagnosis, to study the different pathophysiological mechanisms of pain at different levels to find new therapeutic targets and to increase the knowledge of effective drugs. In addition, nonpharmacological treatments, including neurostimulation and psychotherapy, are progressively being recommended to patients with neuropathic pain, most of whom receive a combination of therapeutic approaches. Although most guidelines and recommendations focus on presenting pharmacological or neurostimulation treatments as the main strategy, it is increasingly necessary to address other alternatives, including those that are not evidence-based, as has recently been proposed (Moisset et al., 2020).

The DRG is an excellent clinical target for pain control because it modulates peripheral and sensory processing specifically involved in neuropathic pain as well as its anatomical ease of access (Esposito et al., 2019). Recent research findings on the benefits of targeting the DRG for the treatment of neuropathic pain are extensive. However, much of the research should be considered preliminary and needs to be confirmed in high-quality trials with sufficient numbers of participants.

Neurostimulation therapy has been used with pulsed radiofrequency application to the DRG via a catheter needle. This technique results in the interruption of nociceptive afferent pathways, and it has the advantage, compared to the conventional technique, of avoiding possible injuries due to temperature increase. Remarkably, there are data from high-quality randomized controlled trials on its positive results on cervical radicular pain, lumbosacral radicular pain, and thoracic postherpetic neuralgia (Koh et al., 2015; Makharita et al., 2018). Similarly, although invasive, electrical neurostimulation of DRG neurons may modulate neuropathic pain signals. Moderate-quality evidence from a randomized comparative effectiveness trial in 152 subjects diagnosed with complex regional pain syndrome or causalgia in the lower extremities showed that DRG stimulation provided a higher rate of treatment success (pain relief), with improvements in quality of life and less postural variation in paresthesia intensity compared to the dorsal column (spinal cord stimulation) (Deer et al., 2017). In this sense, the Neuromodulation Appropriateness Consensus Committee of the International Neuromodulation Society published a consensus paper for DRG stimulation, with recommendations for each neuromodulation therapy (Deer et al., 2019).

Although most existing DRG neuromodulations apply electrical stimulation, ultrasonic modulation on dissociated DRG neurons has shown that it evokes action potentials in DRG neurons, which may involve the activation of sodium, calcium, and nonselective ion channels, and that its therapeutic potential should be considered, given its non-thermal noncavitation bioeffect (Feng et al., 2019).

In patients who have failed physical therapy and other conservative pain management modalities, palliative neuroablative techniques are the next options. Given the special characteristics of radiofrequency application, it has been combined with other strategies aimed at minimizing its effects and increasing the permanency and stability of pain relief. In the case of coblation technology, low-temperature pulsed radiofrequency ablation is used to generate energized plasma (ionized gas) so that the charged particles in the plasma disintegrate the underlying tissue, providing an alternative for neuropathic pain treatment (Varshney et al., 2021). DRG coblation has been used in phantom limb pain wherein by preventing nociceptive conduction, it could suppress DRG activities, thus reducing the erroneous ectopic input to the CNS and relieving the pain (Li et al., 2018).

As an alternative neuroablative technique, cryoneuroablation, which is minimally invasive, not permanent, and well tolerated by the patient with only local anesthesia, as well as cryoneurolysis, that is, freezing the nerves and preventing sensory nerve conduction, have been properly used and proposed (Shinu et al., 2022) as novel therapies for the management of pain in nonsurgical anterior knee pain and refractory chronic peripheral neuropathic pain, respectively (McLean et al., 2020; Varshney et al., 2021).

Pharmacological approaches have shown that tricyclic antidepressants, gabapentinoids (gabapentin and pregabalin), selective serotonin-norepinephrine reuptake inhibitors, lidocaine, and capsaicin are the most efficacious pharmacologic agents for neuropathic pain alleviation (Bates et al., 2019; Varshney et al., 2021). As previously cited, chronic pain involves the upregulation of ion channels and the subsequent hyperexcitability of DRG neurons, which richly express voltage-dependent calcium channels (Liem et al., 2016). However, oral (gabapentin) or intrathecal (ziconotide) off-target blockade of N-type (Cav2) and T-type (Cav3) calcium channels has adverse effects and limitations of effectiveness that have led to the proposal of direct administration at the DRG as a better therapeutic option (Liem et al., 2016). In addition to the regulation of voltage-gated calcium channels, DRG neurons contain large amounts of transient TRPV1 chemoreceptors, allowing the topical application of agents such as capsaicin to exert an analgesic effect by inhibiting these TRPV1 receptors on Aδ and C-nerve fibers; thus, this drug is also recommended as a first-line drug in patients with peripheral neuropathic pain (Bates et al., 2019).

Moreover, natural compounds have been investigated for the management of many diseases but also as candidates for the development of new drugs to treat neuropathic pain. Puerarin, isolated from Radix puerariae, is a potent antioxidant and anti-inflammatory agent used in traditional Chinese medicines that, when intraperitoneally administered, ameliorates mechanical allodynia in rats with peripheral nerve injury and decreases TRPV1 and TRPA1 expression levels in the DRG (Wu et al., 2019b). Most drugs are marketed as monotherapies. However, because of their incomplete efficacy and dose-limiting adverse effects, combination pharmacotherapy has been proposed for the treatment of neuropathic pain in adults. A recent extensive review of this matter on more than 40 published trials, representing an approximate doubling of the research in this area over the past decade, allowed meta-analyses of 3 drug class combinations, i.e., opioid-gabapentinoid, opioid-antidepressant, and gabapentinoid-antidepressant, but it failed to demonstrate superiority over monotherapies (Balanaser et al., 2023).

As an alternative to some pharmacological approaches, the use of viral vector-mediated gene therapy targeting the DRG has been investigated in preclinical models (Skorput et al., 2022). Such is the case for the use of recombinant adeno-associated virus as a vector to transfer vectors encoding for GAD65 (glutamic acid decarboxylase 65-kDa isoform) and vesicular GABA transporter genes, affecting the GABA (gamma-aminobutyric acid) inhibitory system in mice with neuropathic pain (Tadokoro et al., 2022). More recently, it has been demonstrated that the overexpression of ten-eleven translocation methylcytosine dioxygenase 1 in the DRG through microinjection of herpes simplex virus expressing full-length ten-eleven translocation methylcytosine dioxygenase 1 mRNA into injured rat DRGs significantly alleviated the induced pain hypersensitivities, likely through DNA demethylation rescue of mu-opioid receptor and voltage-gated potassium channel subunit Kv1.2 expression in the DRG (Wu et al., 2019a). In vitro analysis of isolated DRG neurons also provides preclinical therapeutic information. Thus, the activation of adenosine A3 receptors, involved in inflammation, metabolism or cell-to-cell communication, has been shown to inhibit pronociceptive voltage-dependent calcium channels, which are associated with neuropathic pain (Coppi et al., 2019), in accordance with similar studies in mice (reviewed in Coppi et al., 2022). All these techniques are under preclinical study and have not yet been used in humans.

Future pharmacological strategies will rely on molecular and genetic studies. Preclinical models and human studies have allowed us to perform large-scale transcriptomic screening to characterize pathological pathways and identify potential therapeutic targets. In an extensive work, Ray and collaborators (Ray et al., 2018) performed RNA-seq on human DRGs obtained from organ donors to generate a transcriptional landscape, characterize tissue-restricted gene coexpression patterns and identify putative transcriptional regulators. This study revealed novel human DRG-enriched protein-coding gene sets not previously described in the context of the DRG or pain signaling, pointing to the development of novel therapeutics. Numerous studies have investigated the gene expression profiles of tissues involved in neuroimmune interactions in the DRG and neuropathic pain (Ghazisaeidi et al., 2023). As an example, transcriptome analysis of patch-clamp electrophysiology and RNA sequencing of DRGs from patients with varying levels of radicular/neuropathic pain has suggested sex-specific differences and revealed the involvement of the oncostatin M/gp-130 (a common component of the IL-6 cytokine family) signaling pathway in human neuropathic pain, signifying implications for the development of strategies targeting this signaling system (North et al., 2019). The application of integrated bioinformatics analysis to search for differentially expressed genes in the DRG of neuropathic pain has been similarly used to identify specific and significant genetic targets for treatments (Tang et al., 2020). Eighty genes were identified in a study using a spared nerve injury model. Subsequent functional analysis to examine the biological processes and signaling pathways demonstrated that neuropeptide Y (NPY) and activating transcription factor (Atf3) may serve as prognostic and therapeutic targets of neuropathic pain (Tang et al., 2020).

Although the mechanisms underlying the alleviation of pain behavior are not yet fully understood, cell therapy has been shown to positively regulate it. Thus, intrathecal injection of bone marrow mesenchymal stem cells has been shown to be effective in pain relief (Teng et al., 2019). The same authors carried out scaffold-based neural crest stem cell transplantation to the sciatic nerve after sciatic nerve transection on the spinal cord and demonstrated that the transplants ameliorated neuropathic pain and enhanced locomotion by inhibiting microglial activation as well as ERK and NF-κB signals (Zhang et al., 2021). In vitro experiments have shown that multipotent neural crest cells secrete biologically active trophic factors that stimulate rat primary DRG neuron outgrowth (Jones et al., 2021). This finding agrees with previous studies demonstrating the restoration of sensory functions after dorsal root avulsion in mice if avulsed sensory fibers are bridged with the spinal cord by human neural progenitor transplants (Hoeber et al., 2015). The responses to peripheral mechanical sensory stimulation and nociceptive somatosensory function were significantly improved in transplanted animals, supporting the use of a stem cell-based treatment to assist in the interactions and plasticity between dorsal root axons and dorsal horn neurons and to replace lost dorsal horn neurons in the host spinal cord (Hoeber et al., 2015). Many studies have focused on the ability of neural progenitor cells to restore connectivity after spinal cord injury (Fischer et al., 2020), although selective transplantation of specific neuronal subpopulations of cells may also be desirable to facilitate treatment. GABAergic progenitor neurons, transplanted to alleviate neuropathic pain in spinal cord injury models, moderate the loss of presynaptic inhibition onto the dorsal horn neurons (Dugan et al., 2020).

Conclusion

In brief, the DRG constitutes an excellent target to treat neuropathic pain, which is mainly due to its strategic role and involvement in the neuropathomechanism of pain but is also attributed to its regenerative capacity to restore cell loss after an injury. As reviewed here, a long list of factors within the DRG (cells, cytokines or NT) account for the triggering and maintenance of neuropathic pain and are susceptible to being targeted by potential therapies. However, the main obstacle or limitation to the progression in the search for consensus and therapeutic approaches in the treatment of neuropathic pain and, by extension, in the majority of pathologies, is the availability of high-quality clinical trials with a sufficient number of patients to be able to translate preclinical findings to routine clinical practice. Nevertheless, the current variety of preclinical studies, or the application of extensive database and bioinformatics studies, allows for optimism about the therapeutic management of neuropathic pain.

Additional file: Open peer review report 1 ^{(88.6KB, pdf)} .