Antihypersensitivity effect of betanin (red beetroot extract) via modulation of microglial activation in a mouse model of neuropathic pain.

BACKGROUND: Neuropathic pain (NeP) medications have several side effects that affect NeP patients' quality of life. Betanin, the most common betacyanin pigment, has been shown to have potent antioxidant and anti-inflammatory properties in vivo; thus, it has potential as a healthcare treatment. In this study, we focused on betanin (red beetroot extract) as a potential therapy for NeP. METHODS: Mice model of NeP were made by chronic constriction injury (CCI), and the development of mechanical hypersensitivity was confirmed using the von Frey test. Motor coordination and locomotor activity were assessed using open field tests and rotarod tests, respectively. The expression level of glial markers in the spinal cords was analyzed by immunostaining. The direct effects of betanin on microglial cells were investigated using primary cultured microglial cells. RESULTS: In CCI model mice, repeated betanin treatment, both intraperitoneally and orally, attenuated developing mechanical hypersensitivity in a dose-dependent manner without impairing motor coordination. Betanin treatment also attenuated mechanical hypersensitivity that had developed and prevented the onset of mechanical hypersensitivity in CCI mice. Microglial activation in the spinal cord is known to play a key role in the development of NeP; betanin treatment reduced CCI-induced microglial activation in the spinal cord of model mice. Moreover, in primary microglia cultured cells, the activation of microglia by lipopolysaccharide application was suppressed by betanin treatment. CONCLUSION: Betanin treatment appears to ameliorate mechanical hypersensitivity related to CCI-induced NeP in mice by inhibiting microglial activation. SIGNIFICANCE: This article supports findings of the effect of betanin on NeP and provides a potential therapeutic candidate for NeP. Furthermore, elucidating the underlying mechanism of the effect of betanin on microglial activation could assist the development of new treatments for chronic pain.

Changes in P2X3 receptor expression in the trigeminal ganglion following monoarthritis of the temporomandibular joint in rats.

The pathophysiological mechanisms of orofacial deep-tissue pain is still unclear. Previously, P2X receptors (P2XR) in sensory neurons have been shown to play a role in the signal transduction of cutaneous pain. We investigated the functional significance of P2X3R in relation to orofacial deep-tissue pain caused by monoarthritis of the temporomandibular joint (TMJ). Monoarthritis was induced by the injection of complete Freund's adjuvant (CFA) into the unilateral TMJ of the rat. The pain associated with monoarthritis was assessed by the pressure pain threshold (PPT), which was defined as the amount of pressure required to induce vocalization. Fifteen days after CFA-treatment, changes in PPT were examined after injection of P2XR agonists or antagonists into the TMJ. The number of cells expressing P2X3R in trigeminal ganglia (TG) was investigated by immunohistochemistry. Inflamed TMJ showed a continuous decline in PPT during the experimental period (P<0.001). Injection of alpha,beta-meATP, an agonist of P2X1,3,2/3R, dramatically reduced the bilateral PPTs of both inflamed and non-inflamed TMJs (P<0.01) although beta,gamma-me-l-ATP, a selective agonist of P2X1R, did not. The decreased PPTs of inflamed TMJ were reversed either by PPADS, an antagonist of P2X1,2,3,5,1/5,4/5R, or by TNP-ATP, an antagonist of P2X1,3,2/3,1/5R. Immunohistochemically, the number of P2X3R-positive cells increased in the small cell group in TG (P<0.01), whereas there was no change in medium or large cell groups after the CFA-injection. Retrograde tracing confirmed that TMJ neurons in the TG exhibited P2X3R immunoreactivity. Our results suggested that P2X3R plays an important role in orofacial pressure pain caused by monoarthritis of TMJ.

Nerve terminals extend into the temporomandibular joint of adjuvant arthritic rats.

The innervation of the temporomandibular joint (TMJ) has attracted particular interest because of the close association with complex mandibular movement. Although the pathological changes of disk innervation may have a crucial role in the development of TMJ pain, the innervation of the TMJ disk by experimentally induced arthritis has rarely been examined in detail. Arthritic rats were induced by injection with 0.1ml solution of Complete Freund's adjuvant (CFA). We investigated three-dimensional distribution of nerve fibers in the TMJ disk using immunohistochemistry for protein gene product-9.5 (PGP-9.5) and calcitonin gene-related peptide (CGRP) in naive and arthritic rats. To clarify the possible role of nerve growth factor (NGF) and its receptor on changes in peripheral innervation of the TMJ, the expressions of trkA and p75 receptor in trigeminal ganglia were examined. Although PGP-9.5 and CGRP immunoreactive (ir) fibers were seen in the peripheral part of the TMJ disk, they were not seen in its central part. The total length and the length density of PGP-9.5 ir and CGRP ir nerve fibers increased in arthritic rats. The innervation area of fibers proliferating in the rostro-medial part merged with that of fibers in the rostro-lateral part in the arthritic rats. In addition, the ratio of trkA- and p75-positive small- and medium-sized cells increased in trigeminal ganglia. It is assumed that increasing innervation of the TMJ disk may be important for the pathophysiology of TMJ pain. NGF and its receptors are likely involved in pathological changes of the TMJ disk.