难治性疼痛的高发生率要求新镇痛药的紧急临床前转化。在开发新型镇痛药时,了解动物疼痛的行为读数对于疗效评估至关重要。Mas相关的G蛋白偶联受体D阳性(Mrgprd)和瞬时受体电位香草素1阳性(TRPV1)感觉神经元是C纤维伤害感受器的两个主要非重叠亚群。据报道,它们的激活会引起各种不良行为。然而,什么样的行为可靠地代表主观上有意识的疼痛感知需要重新审视。这里,我们产生了转基因小鼠,其中Mrgprd或TRPV1感觉神经元特异性表达通道视紫红质2(ChR2)。在生理条件下,后爪Mrgprd+传入神经的光遗传学激活诱发的反射行为(提升,等。),但未能产生厌恶。相比之下,TRPV1+传入激活诱发明显的反射行为和情感反应(舔,等。),以及强烈的厌恶。在由备用神经损伤(SNI)引起的神经性疼痛条件下,Mrgprd+传入激励可以引起情感行为和回避。机械上,在幼稚条件下,TRPV1+伤害感受器或SNI后由Mrgprd+伤害感受器激活浅层(层I-IIo)中的脊髓-臂旁核(lPBN)投射神经元,而在幼稚条件下,只有深脊髓神经元被Mrgprd+伤害感受器激活。此外,在正常条件下,从Mrgprd传入到内层II(IIi)内神经元的兴奋性输入是部分门控的。总之,我们得出的结论是,成年Mrgprd伤害性感受器的光遗传学激活在生理条件下通过深脊髓途径驱动非疼痛样反射行为,在病理条件下通过浅层脊髓途径驱动疼痛样情感行为。传递不同形式的伤害行为的不同脊柱途径提供了不同的治疗靶标。此外,这项研究呼吁通过使用全面和合适的行为测定来合理评估临床前镇痛疗效,以及通过评估两种不同途径中的神经活动。
The high incidence of treatment-resistant pain calls for the urgent preclinical translation of new analgesics. Understanding the behavioral readout of pain in animals is crucial for efficacy evaluation when developing novel analgesics. Mas-related G protein-coupled receptor D-positive (
Mrgprd+) and transient receptor potential vanilloid 1-positive (TRPV1+) sensory neurons are two major non-overlapping subpopulations of C-fiber nociceptors. Their activation has been reported to provoke diverse nocifensive behaviors. However, what kind of behavior reliably represents subjectively conscious pain perception needs to be revisited. Here, we generated transgenic mice in which Mrgprd+ or TRPV1+ sensory neurons specifically express channelrhodopsin-2 (ChR2). Under physiological conditions, optogenetic activation of hindpaw
Mrgprd+ afferents evoked reflexive behaviors (lifting, etc.), but failed to produce aversion. In contrast, TRPV1+ afferents activation evoked marked reflexive behaviors and affective responses (licking, etc.), as well as robust aversion. Under neuropathic pain conditions induced by spared nerve injury (SNI), affective behaviors and avoidance can be elicited by Mrgprd+ afferents excitation. Mechanistically, spinal cord-lateral parabrachial nucleus (lPBN) projecting neurons in superficial layers (lamina I-II o ) were activated by TRPV1+ nociceptors in naïve conditions or by Mrgprd+ nociceptors after SNI, whereas only deep spinal cord neurons were activated by
Mrgprd+ nociceptors in naïve conditions. Moreover, the excitatory inputs from
Mrgprd+ afferents to neurons within inner lamina II (II i ) are partially gated under normal conditions. Altogether, we conclude that optogenetic activation of the adult
Mrgprd+ nociceptors drives non-pain-like reflexive behaviors via the deep spinal cord pathway under physiological conditions and drives pain-like affective behaviors via superficial spinal cord pathway under pathological conditions. The distinct spinal pathway transmitting different forms of nocifensive behaviors provides different therapeutic targets. Moreover, this study appeals to the rational evaluation of preclinical analgesic efficacy by using comprehensive and suitable behavioral assays, as well as by assessing neural activity in the two distinct pathways.