Kettunen Petronella, Hess Dietmar, El Manira Abdeljabbar
Nobel Institute for Neurophysiology, Department of Neuroscience, Karolinska Institutet, SE-171 77 Stockholm, Sweden.
J Neurophysiol. 2003 Oct;90(4):2341-8. doi: 10.1152/jn.01132.2002. Epub 2003 Jun 18.
The modulation of neuronal excitability by group I metabotropic glutamate receptors (mGluRs) was studied in isolated lamprey spinal cord. At resting potential, application of the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) slightly depolarized the cells. However, at depolarized membrane potentials, this agonist induced repetitive firing. When Na+ channels were blocked by TTX, DHPG induced a slight depolarization at rest that increased in amplitude as the neurons were held at more depolarized membrane potentials. In voltage-clamp conditions, DHPG application induced an inward current associated with a decrease in membrane conductance when cells were held at -40 mV. At resting membrane potential, no significant change in the current was induced by DHPG, although a decrease in membrane conductance was seen. The conductance blocked by DHPG corresponded to a leak current, since DHPG had no effect on the voltage-gated current elicited by a voltage step from -60 to -40 mV, when leak currents were subtracted. The leak current blocked by DHPG is mediated by fluxes of both K+ and Na+. The subtype of group I mGluR mediating the block of the leak current was characterized using specific antagonists for mGluR1 and mGluR5. The inhibition of the leak current was blocked by the mGluR1 antagonist LY 367385 but not by the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP). The DHPG-induced blockage of the leak current required phospholipase C (PLC)-activation and release of Ca2+ from internal stores as the effect of DHPG was suppressed by the PLC-blocker U-73122 and after depletion of intracellular Ca2+ pools by thapsigargin. Our results thus show that mGluR1 activation depolarizes spinal neurons by inhibiting a leak current. This will boost membrane depolarization and result in an increase in the excitability of spinal cord neurons, which could contribute to the modulation of the activity of the spinal locomotor network.
在分离的七鳃鳗脊髓中研究了I组代谢型谷氨酸受体（mGluRs）对神经元兴奋性的调节作用。在静息电位时，应用I组mGluR激动剂（R，S）-3,5-二羟基苯甘氨酸（DHPG）会使细胞轻微去极化。然而，在去极化膜电位时，该激动剂会诱发重复放电。当Na⁺通道被河豚毒素（TTX）阻断时，DHPG在静息时会诱发轻微去极化，随着神经元被钳制在更去极化的膜电位，其幅度会增加。在电压钳条件下，当细胞被钳制在-40 mV时，应用DHPG会诱发与膜电导降低相关的内向电流。在静息膜电位时，DHPG不会诱发电流的显著变化，尽管会观察到膜电导降低。被DHPG阻断的电导对应于漏电流，因为当减去漏电流时，DHPG对从-60 mV到-40 mV的电压阶跃所引发的电压门控电流没有影响。被DHPG阻断的漏电流由K⁺和Na⁺的通量介导。使用mGluR1和mGluR5的特异性拮抗剂对介导漏电流阻断的I组mGluR亚型进行了表征。mGluR1拮抗剂LY 367385可阻断漏电流的抑制，而mGluR5拮抗剂2-甲基-6-（苯乙炔基）吡啶（MPEP）则不能。DHPG诱导的漏电流阻断需要磷脂酶C（PLC）激活以及从内部储存库释放Ca²⁺，因为PLC阻断剂U-73122以及毒胡萝卜素耗尽细胞内Ca²⁺池后，DHPG的作用会受到抑制。因此，我们的结果表明，mGluR1激活通过抑制漏电流使脊髓神经元去极化。这将增强膜去极化并导致脊髓神经元兴奋性增加，这可能有助于调节脊髓运动网络的活动。
