Straka H, Debler K, Dieringer N
Physiologisches Institut, München, Germany.
Neuroscience. 1996 Feb;70(3):697-707. doi: 10.1016/s0306-4522(96)83008-9.
Vestibular afferent fibers exhibit a specific, cell size-related uptake of aspartate and glycine [Straka H. et al. (1995) Neuroscience 70, 685-696]. A similar, size-related coexistence of glycine and glutamate had been reported earlier for these fibers [Reichenberger I. and Dieringer N. (1994) J. comp. Neurol. 349, 603-614]. Taken together, these results suggest a size-related co-release of both amino acids and the activation of different glutamate receptors in second order vestibular neurons. To test this hypothesis we stimulated the VIIIth nerve and recorded the responses of central vestibular neurons in the isolated brainstem of frogs before and during the application of the N-methyl-D-aspartate antagonists (7-chlorokynurenic acid and D-(-)-2-amino-5-phosphonovaleric acid). The presence of either one of these antagonists provoked a dose-dependent and Mg(2+)-sensitive partial block of the monosynaptic responses recorded extra- or intracellularly. This implies that afferent-evoked responses in central vestibular neurons are composed of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated components. In most of the intracellularly recorded neurons (21 out of 24) the relative amplitude of the N-methyl-D-aspartate receptor-mediated component decreased with an increase in stimulus intensity. Since electric stimulation recruits thick afferents at a lower current intensity than thin afferent fibers, our results imply a co-activation of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors by thick vestibular afferents. At a given stimulus intensity the amplitude of the N-methyl-D-aspartate receptor-mediated component differed between neurons. The results of this study extend the list of known anatomical, histochemical and physiological properties that distinguish thick from thinner vestibular afferent fibers. In spite of this detailed knowledge, however, the physiological role of thick vestibular afferents is so far unclear. The novel concept of a size-related co-activation of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors by vestibular afferent fibers establishes the basis for more specific physiological hypotheses.
前庭传入纤维对天冬氨酸和甘氨酸表现出一种与细胞大小相关的特异性摄取[施特拉卡H等人（1995年），《神经科学》70卷，685 - 696页]。此前已有报道称，这些纤维中甘氨酸和谷氨酸存在类似的与大小相关的共存现象[赖兴贝格尔I和迪林格N（1994年），《比较神经学杂志》349卷，603 - 614页]。综合这些结果表明，这两种氨基酸存在与大小相关的共同释放，并且在前庭二级神经元中激活了不同的谷氨酸受体。为了验证这一假设，我们在应用N - 甲基 - D - 天冬氨酸拮抗剂（7 - 氯犬尿氨酸和D -（-）- 2 - 氨基 - 5 - 磷酸戊酸）之前和期间，刺激青蛙离体脑干中的第八对脑神经，并记录中枢前庭神经元的反应。这两种拮抗剂中的任何一种的存在都会引起剂量依赖性的、对镁离子敏感的单突触反应的部分阻断，该反应可在细胞外或细胞内记录到。这意味着中枢前庭神经元中传入诱发的反应由N - 甲基 - D - 天冬氨酸受体介导的成分和非N - 甲基 - D - 天冬氨酸受体介导的成分组成。在大多数细胞内记录的神经元（24个中的21个）中，N - 甲基 - D - 天冬氨酸受体介导成分的相对幅度随着刺激强度的增加而降低。由于电刺激在比细传入纤维更低的电流强度下募集粗传入纤维，我们的结果表明粗前庭传入纤维共同激活了N - 甲基 - D - 天冬氨酸受体和非N - 甲基 - D - 天冬氨酸受体。在给定的刺激强度下，N - 甲基 - D - 天冬氨酸受体介导成分的幅度在神经元之间存在差异。本研究结果扩展了区分粗前庭传入纤维和细前庭传入纤维的已知解剖学、组织化学和生理学特性的列表。然而，尽管有这些详细的知识，粗前庭传入纤维的生理作用目前仍不清楚。前庭传入纤维与大小相关的共同激活N - 甲基 - D - 天冬氨酸受体和非N - 甲基 - D - 天冬氨酸受体这一新概念为更具体的生理学假设奠定了基础。
