Miyata Mariko, Kashiwadani Hideki, Fukaya Masahiro, Hayashi Takayuki, Wu Dianqing, Suzuki Tutomu, Watanabe Masahiko, Kawakami Yoriko
Department of Information Physiology, National Institute for Physiological Sciences, Myodaiji, Okazaki 444-8585, Japan.
J Neurosci. 2003 Sep 3;23(22):8098-108. doi: 10.1523/JNEUROSCI.23-22-08098.2003.
Phospholipase C (PLC) beta4, one of the four isoforms of PLCbetas, is the sole isoform expressed in the mouse ventral posterolateral thalamic nucleus (VPL), a key station in pain processing. The mouse thalamus also has been shown to express a high level of metabotropic glutamate receptor type 1 (mGluR1), which stimulates PLCbetas through activation of Galphaq/11 protein. It is therefore expected that the thalamic mGluR1-PLCbeta4 cascade may play a functional role in nociceptive transmission. To test this hypothesis, we first studied behavioral responses to various nociceptive stimuli in PLCbeta4 knock-out mice. We performed the formalin test and found no difference in the pain behavior in the first phase of the formalin test, which is attributed to acute nociception, between PLCbeta4 knock-out and wild-type mice. Consistent with this result, acute pain responses in the hot plate and tail flick tests were also unaffected in the PLCbeta4 knock-out mice. However, the nociceptive behavior in the second phase of the formalin test, resulting from the tissue inflammation, was attenuated in PLCbeta4 knock-out mice. In the dorsal horn of the spinal cord where PLCbeta1 and PLCbeta4 mRNAs are expressed, no difference was found between the wild-type and knock-out mice in the number of Fos-like immunoreactive neurons, which represent neuronal activity in the second phase in the formalin test. Thus, it is unlikely that spinal PLCbeta4 is involved in the formalin-induced inflammatory pain. Next, we found that pretreatment with PLC inhibitors, mGluR1 antagonists, or both, by either intracerebroventricular or intrathalamic injection, attenuated the formalin-induced pain behavior in the second phase in wild-type mice. Furthermore, activation of mGluR1 at the VPL enhanced pain behavior in the second phase in the wild-type mice. In contrast, PLCbeta4 knock-out mice did not show such enhancement, indicating that mGluR1 is connected to PLCbeta4 in the VPL. Finally, in parallel with the behavioral results, we showed in an electrophysiological study that the time course of firing discharges in VPL corresponds well to that of pain behavior in the formalin test in both wild-type and PLCbeta4 knock-out mice. These findings indicate that the thalamic mGluR1-PLCbeta4 cascade is indispensable for the formalin-induced inflammatory pain by regulating the response of VPL neurons.
磷脂酶C（PLC）β4是PLCβ亚型中的四种之一，是在小鼠腹后外侧丘脑核（VPL）中表达的唯一亚型，VPL是疼痛处理中的关键部位。小鼠丘脑也已被证明高水平表达代谢型谷氨酸受体1（mGluR1），其通过激活Gαq/11蛋白来刺激PLCβ。因此，预计丘脑mGluR1 - PLCβ4级联可能在伤害性感受传递中发挥功能作用。为了验证这一假设，我们首先研究了PLCβ4基因敲除小鼠对各种伤害性刺激的行为反应。我们进行了福尔马林试验，发现在福尔马林试验的第一阶段（归因于急性伤害感受），PLCβ4基因敲除小鼠和野生型小鼠的疼痛行为没有差异。与该结果一致，热板试验和甩尾试验中的急性疼痛反应在PLCβ4基因敲除小鼠中也未受影响。然而，福尔马林试验第二阶段由组织炎症引起的伤害性感受行为在PLCβ4基因敲除小鼠中减弱。在脊髓背角，PLCβ1和PLCβ4的mRNA均有表达，在野生型和基因敲除小鼠之间，福尔马林试验第二阶段代表神经元活动的Fos样免疫反应性神经元数量没有差异。因此，脊髓中的PLCβ4不太可能参与福尔马林诱导的炎性疼痛。接下来，我们发现通过脑室内或丘脑内注射用PLC抑制剂、mGluR1拮抗剂或两者进行预处理，可减弱野生型小鼠福尔马林试验第二阶段的疼痛行为。此外，在VPL激活mGluR1可增强野生型小鼠福尔马林试验第二阶段的疼痛行为。相比之下，PLCβ4基因敲除小鼠没有表现出这种增强，表明在VPL中mGluR1与PLCβ4相连接。最后，与行为学结果一致，我们在一项电生理研究中表明，在野生型和PLCβ4基因敲除小鼠中，VPL的放电时间进程与福尔马林试验中的疼痛行为时间进程高度对应。这些发现表明，丘脑mGluR1 - PLCβ4级联通过调节VPL神经元的反应对于福尔马林诱导的炎性疼痛是必不可少的。
