Neurobiology and Pain Therapeutics Section, Laboratory Of Sensory Biology, NIDCR, NIH, Bldg 49 Rm 1C2049 Convent Dr, Bethesda, MD 20892, USA.
Mol Pain. 2012 Sep 21;8:70. doi: 10.1186/1744-8069-8-70.
The prevalence of long-term opiate use in treating chronic non-cancer pain is increasing, and prescription opioid abuse and dependence are a major public health concern. To explore alternatives to opioid-based analgesia, the present study investigates a novel allosteric pharmacological approach operating through the cation channel TRPV1. This channel is highly expressed in subpopulations of primary afferent unmyelinated C- and lightly-myelinated Aδ-fibers that detect low and high rates of noxious heating, respectively, and it is also activated by vanilloid agonists and low pH. Sufficient doses of exogenous vanilloid agonists, such as capsaicin or resiniferatoxin, can inactivate/deactivate primary afferent endings due to calcium overload, and we hypothesized that positive allosteric modulation of agonist-activated TRPV1 could produce a selective, temporary inactivation of nociceptive nerve terminals in vivo. We previously identified MRS1477, a 1,4-dihydropyridine that potentiates vanilloid and pH activation of TRPV1 in vitro, but displays no detectable intrinsic agonist activity of its own. To study the in vivo effects of MRS1477, we injected the hind paws of rats with a non-deactivating dose of capsaicin, MRS1477, or the combination. An infrared diode laser was used to stimulate TRPV1-expressing nerve terminals and the latency and intensity of paw withdrawal responses were recorded. qRT-PCR and immunohistochemistry were performed on dorsal root ganglia to examine changes in gene expression and the cellular specificity of such changes following treatment.
Withdrawal responses of the capsaicin-only or MRS1477-only treated paws were not significantly different from the untreated, contralateral paws. However, rats treated with the combination of capsaicin and MRS1477 exhibited increased withdrawal latency and decreased response intensity consistent with agonist potentiation and inactivation or lesion of TRPV1-containing nerve terminals. The loss of nerve endings was manifested by an increase in levels of axotomy markers assessed by qRT-PCR and colocalization of ATF3 in TRPV1+ cells visualized via immunohistochemistry.
The present observations suggest a novel, non-narcotic, selective, long-lasting TRPV1-based approach for analgesia that may be effective in acute, persistent, or chronic pain disorders.
长期使用阿片类药物治疗慢性非癌性疼痛的现象越来越普遍，处方类阿片类药物滥用和依赖已成为主要的公共卫生问题。为了探索阿片类药物镇痛的替代方法，本研究探索了一种通过阳离子通道 TRPV1 发挥作用的新型变构药理学方法。该通道在未髓鞘化 C 纤维和轻度髓鞘化 Aδ 纤维的亚群中高度表达，分别检测到低和高速率的有害加热，并且也被香草素激动剂和低 pH 值激活。足够剂量的外源性香草素激动剂，如辣椒素或树脂毒素，由于钙超载，可以使初级传入末梢失活/失活，我们假设激动剂激活的 TRPV1 的正变构调节可以在体内产生对伤害性神经末梢的选择性、暂时失活。我们之前鉴定出 MRS1477，一种 1,4-二氢吡啶，可在体外增强香草素和 pH 值对 TRPV1 的激活，但本身没有检测到可察觉的内在激动剂活性。为了研究 MRS1477 的体内作用，我们向大鼠的后爪注射了非失活剂量的辣椒素、MRS1477 或两者的组合。使用红外二极管激光刺激 TRPV1 表达的神经末梢，记录爪回缩反应的潜伏期和强度。对背根神经节进行 qRT-PCR 和免疫组织化学检测，以检查治疗后基因表达的变化和这种变化的细胞特异性。
仅用辣椒素或 MRS1477 处理的爪子的退缩反应与未处理的、对侧爪子没有显著差异。然而，用辣椒素和 MRS1477 组合处理的大鼠表现出潜伏期延长和反应强度降低，这与 TRPV1 包含的神经末梢的激动剂增强和失活或损伤一致。通过 qRT-PCR 评估轴突切断标记物的水平增加以及通过免疫组织化学观察到 ATF3 在 TRPV1+细胞中的共定位，证明了神经末梢的丧失。
目前的观察结果表明，一种新型的、非阿片类的、选择性的、长效 TRPV1 为基础的镇痛方法可能对急性、持续性或慢性疼痛障碍有效。
