Department of Anatomy and Neurobiology, Northeast Ohio Medical University, College of Medicine, Rootstown, Ohio, United States of America.
PLoS One. 2012;7(4):e35831. doi: 10.1371/journal.pone.0035831. Epub 2012 Apr 24.
Neurons in the nucleus laminaris (NL) of birds act as coincidence detectors and encode interaural time difference to localize the sound source in the azimuth plane. GABAergic transmission in a number of CNS nuclei including the NL is subject to a dual modulation by presynaptic GABA(B) receptors (GABA(B)Rs) and metabotropic glutamate receptors (mGluRs). Here, using in vitro whole-cell patch clamp recordings from acute brain slices of the chick, we characterized the following important but unknown properties pertaining to such a dual modulation: (1) emergence of functional GABA synapses in NL neurons; (2) the temporal onset of neuromodulation mediated by GABA(B)Rs and mGluRs; and (3) the physiological conditions under which GABA(B)Rs and mGluRs are activated by endogenous transmitters. We found that (1) GABA(A)R-mediated synaptic responses were observed in about half of the neurons at embryonic day 11 (E11); (2) GABA(B)R-mediated modulation of the GABAergic transmission was detectable at E11, whereas the modulation by mGluRs did not emerge until E15; and (3) endogenous activity of GABA(B)Rs was induced by both low- (5 or 10 Hz) and high-frequency (200 Hz) stimulation of the GABAergic pathway, whereas endogenous activity of mGluRs was induced by high- (200 Hz) but not low-frequency (5 or 10 Hz) stimulation of the glutamatergic pathway. Furthermore, the endogenous activity of mGluRs was mediated by group II but not group III members. Therefore, autoreceptor-mediated modulation of GABAergic transmission emerges at the same time when the GABA synapses become functional. Heteroreceptor-mediated modulation appears at a later time and is receptor type dependent in vitro.
鸟类的核层（NL）神经元充当着巧合检测器，将两耳时间差编码为声源在方位平面上的定位信息。包括 NL 在内的许多中枢神经系统核中的 GABA 能传递受到突触前 GABA(B) 受体（GABA(B)Rs）和代谢型谷氨酸受体（mGluRs）的双重调制。在这里，我们使用来自小鸡急性脑切片的体外全细胞膜片钳记录，对以下这种双重调制的一些重要但未知的特性进行了描述：（1）NL 神经元中功能性 GABA 突触的出现；（2）GABA(B)Rs 和 mGluRs 介导的神经调制的时间起始；（3）内源性递质激活 GABA(B)Rs 和 mGluRs 的生理条件。我们发现：（1）GABA(A)R 介导的突触反应在大约一半的 E11 神经元中被观察到；（2）E11 时可检测到 GABA(B)R 对 GABA 能传递的调制，而 mGluRs 的调制直到 E15 才出现；（3）GABA 能通路的低（5 或 10 Hz）和高频（200 Hz）刺激均可诱导 GABA(B)Rs 的内源性活性，而谷氨酸能通路的高频（200 Hz）刺激而非低频（5 或 10 Hz）刺激可诱导 mGluRs 的内源性活性。此外，mGluRs 的内源性活性是由 II 组而不是 III 组成员介导的。因此，当 GABA 突触变得功能化时，自身受体介导的 GABA 能传递调制就会出现。异源受体介导的调制出现的时间较晚，并且在体外依赖于受体类型。
