Animal Studies

The adult K/BxN transgenic mouse develops spontaneous autoimmune arthritis with joint remodeling and profound bone loss. We report that both males and females display a severe sustained tactile allodynia which is reduced by gabapentin but not the potent cyclooxygenase inhibitor ketorolac. In dorsal horn, males and females show increased GFAP astrocytic cells; however, only males demonstrate an increase in Iba1 microglia. In dorsal root ganglia (DRG), there is an increase in CGRP, TH, and Iba1 (macrophage) labeling, but no increase in ATF3 cells. At the ankle there is increased CGRP, TH, and GAP-43 fiber synovial innervation. Thus, based on the changes in dorsal horn, DRG and peripheral innervation, we suggest that the adult K/BxN transgenic arthritic mice display a neuropathic phenotype, an assertion consistent with the analgesic pharmacology seen in this animal. These results indicate the relevance of this model to our understanding of the nociceptive processing which underlies the chronic pain state that evolves secondary to persistent joint inflammation.

The dissemination of cancer to bone can cause significant cancer-induced bone pain (CIBP), severely impairing the patient's quality of life. Several rodent models have been developed to explore the nociceptive mechanisms of CIBP, including intratibial inoculation of breast carcinoma cells in syngeneic Sprague Dawley rats. Using this model, we investigated whether resident spinal microglial cells are involved in the transmission and modulation of CIBP, a long-debated disease feature. Immunohistochemical staining of ionizing calcium-binding adaptor molecule 1 (Iba-1) and phosphorylated p38-mitogen-activated protein kinase (P-p38 MAPK) showed no spinal microglial reaction in cancer-bearing rats, independently of disease stage, sex, or carcinoma cell line. As a positive control, significant upregulation of both Iba-1 and P-p38 was observed in a rat model of neuropathic pain. Additionally, intrathecal administration of the microglial inhibitor minocycline did not ameliorate pain-like behaviors in cancer-bearing rats, in contrast to spinal morphine administration. Our results indicate that microglial reaction is not a main player in CIBP, adding to the debate that even within the same models of CIBP, significant variations are seen in disease features considered potential drug targets. We suggest that this heterogeneity may reflect the clinical landscape, underscoring the need for understanding the translational value of CIBP models.

Mutations within the SCN11A gene which encodes the voltage-gated sodium channel Na1.9 mainly expressed in small fiber sensory neurons have been associated with neuropathic disorders; however, suitable medications have not been fully investigated. To develop drug therapies against Na1.9-related neuropathic pain, we aimed to establish a novel model using mice carrying the Scn11a p.R222S mutation initially identified in patients with familial episodic limb pain that is characterized by paroxysmal pain induced by fatigue or bad weather conditions. We investigated the influence of cold exposure (4 °C, overnight) on the behavioral and biochemical phenotypes of Scn11a p.R222S mutant (R222S) and wild type C57BL/6N (WT) mice. We also tested the effects of acetaminophen (125, 250 mg/kg, perorally, p.o.) and traditional Japanese medicine, goshajinkigan (0.5 or 1.0 g/kg, p.o.), which are analgesic drugs prescribed to patients with neuropathic pain, in this model of cold-induced mechanical allodynia in R222S mice.Cold-exposed R222S mice exhibited enhanced mechanical allodynia and thermal hypersensitivity compared with WT mice. The decrease of the mechanical withdrawal threshold in R222S mice was reversible 24 h after housing at room temperature. There was no significant change in the levels of interleukin-1β, interleukin-6, tumor necrosis factor-α, or interferon-γ in the plasma or spinal cords of WT and R222S mice after cold exposure. Both acetaminophen (250 mg/kg) and goshajinkigan (1.0 g/kg) significantly attenuated mechanical allodynia in R222S mice. The model of cold-induced mechanical allodynia in mice with the Scn11a p.R222S mutation is novel and useful for evaluating analgesic drugs for intractable neuropathies related to Na1.9.

Diabetic peripheral neuropathy (DPN) is a chronic microvascular complication of diabetes. The purpose of this study is to find the underlying mechanism for the effects of acupuncture in DPN rats. Rats were rendered diabetic with a single injection of 35 mg/kg streptozotocin (STZ). These STZ-diabetic rats were treated with acupuncture for 20 min once daily. The therapeutic efficacy of acupuncture was assessed using mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) evaluations. After 14 days treatment, acupuncture markedly reduced the pathological injury in STZ-diabetic rats. Moreover, it significantly down-regulated P2X4 and OX42 expression along with the reduced levels of inflammatory factors (CXCR3, TNF-α, IL-1β, IL-6), GSP and lipid metabolisms in the spinal cord of the DPN rats. Acupuncture could relieve DPN in rats by regulating P2X4 expression and inflammation in spinal microglia.

Recently programmed death-ligand 1 (PD-L1) receptor PD-1 was found in dorsal root ganglion (DRG) neurons, and PD-L1 activates PD-1 to inhibit inflammatory and neuropathic pain by modulating neuronal excitability. However, the downstream signaling of PD-1 in sensory neurons remains unclear. Here, we show that PD-L1 activates Src homology 2 domain-containing tyrosine phosphatase-1 (SHP-1) to downregulate transient receptor potential vanilloid 1 (TRPV1) in DRG neurons and inhibit bone cancer pain in mice. Local injection of PD-L1 produced analgesia. PD-1 in DRG neurons colocalized with TRPV1 and SHP-1. PD-L1 induced the phosphorylation of SHP-1 in DRG TRPV1 neurons and inhibited TRPV1 currents. Loss of TRPV1 in mice abolished bone cancer-induced thermal hyperalgesia and PD-L1 analgesia. Conditioned deletion of SHP-1 in NaV1.8+ neurons aggravated bone cancer pain and diminished the inhibition of PD-L1 on TRPV1 currents and pain. Together, our findings suggest that PD-L1/PD1 signaling suppress bone cancer pain via inhibition of TRPV1 activity. Our results also suggest that SHP-1 in sensory neurons is an endogenous pain inhibitor and delayed the development of bone cancer pain via suppressing TRPV1 function.