Department of Pharmacology, School of Medicine, Kanazawa Medical University, Uchinada, Ishikawa, Japan.
Department of Neurophysiology, Kagawa School of Pharmaceutical Sciences, Tokushima Bunri University, Sanuki, Kagawa, Japan.
Br J Pharmacol. 2020 Sep;177(18):4223-4241. doi: 10.1111/bph.15176. Epub 2020 Jul 20.
Glutamate and metabotropic glutamate (mGlu) receptors on primary sensory neurons are crucial in modulating pain sensitivity. However, it is unclear how inflammation affects mGlu receptor-mediated nociceptive responses. We therefore investigated the effects of mGlu receptor agonists on pain-related behaviour during persistent inflammation and their underlying mechanisms.
Effects of a mGlu receptor agonist on pain-related behaviour during inflammation was assessed in mice. Intracellular calcium responses, membrane current responses, and protein expression in primary sensory neurons were examined using cultured dorsal root ganglion (DRG) neurons, dissociated from wild-type and gene knockout mice.
Persistent inflammation induced by complete Freund's adjuvant increased the duration of mGlu receptor-mediated pain behaviour, which was antagonized by inhibition of nerve growth factor (NGF)-tropomyosin receptor kinase A (TrkA) signalling. Calcium imaging revealed that NGF treatment increased the number of cultured DRG neurons responding to mGlu receptor activation. Stimulation of mGlu receptors in NGF-treated DRG neurons induced inward currents through TRPV1 channels in association with PLC but not with IP receptors. NGF treatment also increased the number of neurons responding to a DAG analogue via TRPV1 channel activation. Furthermore, NGF up-regulated expression of TRPV1 and A-kinase anchoring protein 5 (AKAP5), resulting in increased AKAP5-dependent TRPV1 phosphorylation. AKAP5 knockout mice did not exhibit mGlu receptor-mediated excitation in NGF-treated DRG neurons or pain response facilitation under inflammatory conditions.
NGF augments glutamate- and mGlu receptor-mediated excitation of nociceptive neurons by AKAP5-dependent phosphorylation of TRPV1 channels, potentiating hypersensitivity to glutamate in inflamed tissues.
初级感觉神经元上的谷氨酸和代谢型谷氨酸（mGlu）受体在调节痛觉敏感性方面至关重要。然而，炎症如何影响 mGlu 受体介导的伤害性反应尚不清楚。因此，我们研究了 mGlu 受体激动剂在持续性炎症期间对疼痛相关行为的影响及其潜在机制。
在小鼠中评估 mGlu 受体激动剂在炎症期间对疼痛相关行为的影响。使用从野生型和基因敲除小鼠分离的培养背根神经节（DRG）神经元，检测 mGlu 受体激动剂对疼痛相关行为的影响。检测了初级感觉神经元中的细胞内钙反应、膜电流反应和蛋白表达。
完全弗氏佐剂诱导的持续性炎症增加了 mGlu 受体介导的疼痛行为的持续时间，而神经生长因子（NGF）-原肌球蛋白受体激酶 A（TrkA）信号的抑制则拮抗了这种作用。钙成像显示，NGF 处理增加了对 mGlu 受体激活有反应的培养 DRG 神经元的数量。在 NGF 处理的 DRG 神经元中刺激 mGlu 受体，通过与 PLC 而不是 IP 受体相关联，诱导内向电流通过 TRPV1 通道。NGF 处理还增加了对 DAG 类似物通过 TRPV1 通道激活的神经元数量。此外，NGF 上调 TRPV1 和 A-激酶锚定蛋白 5（AKAP5）的表达，导致 AKAP5 依赖性 TRPV1 磷酸化增加。在 NGF 处理的 DRG 神经元中，AKAP5 敲除小鼠没有表现出 mGlu 受体介导的兴奋，也没有在炎症条件下促进疼痛反应。
NGF 通过 AKAP5 依赖性 TRPV1 通道磷酸化增强谷氨酸和 mGlu 受体介导的伤害性神经元兴奋，增强炎症组织中对谷氨酸的敏感性。
