Daisuke Uta

Nucleic acid aptamers are promising next-generation experimental and therapeutic drugs for uncharacterized biomolecules and intractable diseases. However, efficient aptamer selection remains challenging. We have developed a functional selection method termed extracellular vesicle (EV)-SELEX that efficiently selects DNA aptamers for G protein-coupled receptors (GPCRs), a major class of drug targets via ligand-dependent GPCR endocytosis and subsequent release of GPCR-containing EVs. Using this method, we obtained Dapt-μR, a DNA aptamer that had a high affinity to μ-opioid receptor (MOR) (K_d ≈ 28 nM) but no other types of opioid receptors. In unmodified form, Dapt-μR acted as a morphine-like, naloxone-sensitive MOR agonist, inhibiting cAMP accumulation in cultured cells and reducing Ca²⁺ influx in primary striatal neurons. Furthermore, Dapt-μR selectively bound to the MOR-rich spinal cord dorsal horn and produced an analgesic effect following intrathecal administration in mice. These findings demonstrate that EV-SELEX is a powerful method for selecting DNA aptamers for target GPCRs.

Background: Neuropathic pain remains difficult to treat because current analgesics often provide insufficient efficacy or dose-limiting adverse effects. Nav1.7 is genetically validated as a key regulator of human pain sensation, but the development of selective small-molecule Nav1.7 inhibitors has been limited by the high similarity among voltage-gated sodium channel subtypes. Methods: We generated monoclonal antibodies selectively targeting Nav1.7, humanized them for therapeutic development, and evaluated their binding, selectivity, functional channel inhibition, systemic analgesic efficacy, and effects on neuronal activity in a rat model of partial sciatic nerve ligation-induced neuropathic pain. Results: The humanized antibodies showed high-affinity and selective binding to Nav1.7 and functionally inhibited the channel in cellular assays. After systemic administration to neuropathic pain model rats, the lead antibody produced robust analgesia lasting at least 96 h. Electrophysiological analyses demonstrated reduced mechanically evoked and spontaneous neuronal activity, and immunohistochemistry showed decreased mechanical stimulus-induced phosphorylation of extracellular signal-regulated kinase in dorsal root ganglion neurons. The antibodies did not impair physiological nociception or motor function under the tested conditions. Conclusions: These findings provide preclinical proof of concept that humanized anti-Nav1.7 antibodies can act as systemically administered, long-acting biologic analgesics for neuropathic pain while preserving normal nociceptive and motor functions. The clinical advancement of S-151128 further supports the translational potential of this modality.

Peripheral nerve ischemia–reperfusion injury is considered to contribute to sensory disturbances that impair quality of life in patients with diabetic neuropathy and chemotherapy-induced neuropathy. However, the spinal mechanisms underlying these disturbances remain unclear, partly due to the lack of established animal models and evaluation systems. In the present study, we used a rat hindlimb ischemia–reperfusion model and in vivo extracellular recording to examine bidirectional changes in neuronal activity in the spinal dorsal horn. Ischemia was induced by tightly binding the rat ankle with a rubber band, followed by reperfusion. Behavioral analysis showed a significant increase in hindlimb licking behavior after reperfusion, indicating the development of sensory disturbance-like responses. Extracellular recordings from superficial dorsal horn neurons showed diverse patterns of spontaneous firing and responses to mechanical stimulation, with both hypersensitive and desensitized responses. Furthermore, mRNA expression levels of immediate early genes (Egr1, Egr3, and Fos) were upregulated in the spinal cord after reperfusion. These results suggest that this ischemia–reperfusion model reproduces complex neuronal responses relevant to peripheral neuropathy and provides a useful evaluation system for evaluating both increased and decreased neural activity. This approach may contribute to elucidating the mechanisms of sensory disturbances and to the development of new treatments for neuropathic conditions.

Neuropathic pain represents a critical challenge in medical research and clinical practice. Enhanced peripheral nerve activity and spinal dorsal horn neuronal firing are thought to contribute to the nociceptive hypersensitivities that are observed in chronic pain conditions, including those modeled by partial sciatic nerve ligation (PSNL). However, the detailed in vivo neuronal response dynamics and underlying mechanisms in the PSNL model remain to be fully clarified. To better understand these mechanisms, we evaluated dorsal root ganglion (DRG) and spinal dorsal horn neuronal activity in the PSNL model using in vivo approaches. Von Frey testing revealed sustained mechanical allodynia in PSNL animals; withdrawal thresholds were significantly reduced up to day 14 post-surgery. Immunohistochemistry revealed a stimulation-dependent increase in phosphorylated extracellular signal-regulated kinase (pERK)-positive neurons in the DRG, thereby indicating heightened peripheral nerve activity. Additionally, electrophysiological recordings demonstrated the enhanced firing of spinal dorsal horn neurons in response to the same stimuli. Notably, DRG pERK expression changes correlated with spinal neuronal firing frequency. Together, these findings suggest that peripheral nerve activity drives spinal neuronal sensitization, thus elucidating both pain mechanisms in the PSNL model and activity-dependent signaling in neuropathic pain.

Fibromyalgia (FM) is a chronic disorder characterized by widespread pain and various accompanying symptoms, with complex mechanisms involving both peripheral and central nervous systems that remain unclear. Herein, we introduce a study employing a reserpine-induced rat model of FM, investigating the spinal dorsal horn in particular. In reserpine-induced FM model rats, we observed pronounced mechanical hypersensitivity of cutaneous and muscular C-fiber nociceptors. Moreover, increased expression of the ASIC3 (one of acid-sensing ion channel) in dorsal root ganglia implicated its role in peripheral sensitization. Within the spinal dorsal horn, enhanced microglial activation was evident; pharmacological inhibition with minocycline significantly attenuated mechanical hyperalgesia, indicating the critical involvement of microglial activity in central pain processing. Furthermore, patch-clamp recordings revealed altered synaptic transmission characterized by enhanced spontaneous excitatory postsynaptic currents (EPSCs) and reduced inhibitory postsynaptic currents (IPSCs), contributing to heightened pain signaling in the FM model. These findings suggest that an imbalance of intensified excitatory and diminished inhibitory neurotransmission in the spinal dorsal horn leads to impaired sensory gating and augmented nociceptive transmission to higher centers, a pivotal mechanism in the pathophysiology of FM.

The expression of hormonal receptors in pain-processing regions complicates understanding the hormonal effects on pain mechanisms. This study investigates androgen receptor (AR) involvement in pain sensitivity and sex differences in pain perception. Mechanical pain thresholds were higher in normal male mice compared to gonadectomized (GDX) male and normal female mice, correlating with serum testosterone levels. In the dorsal root ganglia (DRG), AR was expressed in normal males but undetectable in GDX males and normal females. Androgen receptor overlapped with NeuN, a neuronal nuclei marker, indicating androgen signaling activation in sensory neurons. In male sensory neuron–selective AR conditional knockout (AR-cKO) mice, mechanical pain thresholds were significantly lower than in wild-type males, with the greatest AR depletion in calcitonin gene–related peptide (CGRP) ⁺ neurons. Electrophysiological analyses revealed increased excitability of spinal dorsal horn neurons in both GDX males and AR-cKO males. In female mice, administration of testosterone propionate or dihydrotestosterone significantly raised mechanical pain thresholds, accompanied by increased AR expression in the DRG. This effect was abolished in AR-cKO females, where AR depletion was most prominent in CGRP ⁺ neurons, consistent with male findings. These results indicate that primary sensory neurons, particularly CGRP ⁺ neurons, are critical targets of androgen in regulating mechanical pain sensitivity. Therefore, manipulating androgen signaling in sensory neurons may offer a promising approach to managing mechanical pain.

Background/Objectives: Chronic pain is a significant global health challenge and is associated with diverse conditions, such as diabetic neuropathic pain and spinal stenosis. Understanding the mechanisms of pain transmission is crucial, for both the peripheral and central pathways. However, there are limitations in spinal electrophysiological techniques in terms of the injection method. Traditional methods such as spinal injections may differ in the distributions and concentrations of drugs compared with intrathecal administration during the behavior test. So, we developed a new intrathecal administration method for electrophysiological recordings. Methods: Sprague–Dawley rats were injected with lidocaine intrathecally, and the analgesic effect was evaluated by the von Frey test. In vivo extracellular single-unit recordings of the superficial dorsal horn neurons were performed following a newly developed technique. Lidocaine was intrathecally injected into the arachnoid membrane after laminectomy. After that, the neural responses in the superficial dorsal horn were measured. Results: Newly developed intrathecally administered dye reached the spinal cord and the cauda equina. Intrathecally administrated lidocaine increased the paw withdrawal threshold and suppressed spinal neuronal firing. This suppression correlated with increases in paw withdrawal thresholds. Conclusions: This innovative method provides insights into the central effects of analgesics, which will help the development of therapies for chronic pain.

Chemotherapy-induced peripheral neuropathy (CIPN) is a significant adverse event with unclear mechanisms and limited treatment alternatives. This study aimed to investigate the efficacy of two alkalizing agents, a mixture of potassium citrate and sodium citrate (K/Na citrate) or sodium bicarbonate (NaHCO₃), in preventing and treating paclitaxel (PTX)-induced mechanical allodynia in rodents. The results from rodent models demonstrated that repeated prophylactic administration of K/Na citrate or NaHCO₃ could inhibit the development of PTX-induced mechanical allodynia. Moreover, K/Na citrate was effective in preventing the PTX-induced exacerbation of mechanical allodynia, even when treatment was initiated immediately after the onset of allodynia. K/Na citrate also reduced the levels of the plasma complement component anaphylatoxin C3a in a PTX-induced CIPN rat model. Complement activation, resulting in the production of C3a, has been implicated in the pathogenesis of this model. Additionally, pretreatment with Na citrate significantly prevented the reduction in neurite outgrowth caused by PTX. Furthermore, K/Na citrate inhibited spontaneous and mechanical stimuli-induced firing in spinal dorsal horn neurons. These findings indicate that K/Na citrate may regulate the development of PTX-induced mechanical allodynia by modulating complement activation and providing neuroprotection against PTX-induced peripheral nerve injury. This study implies that alkalization could help prevent PTX-induced peripheral neuropathy and mitigate its exacerbation.