Tang Zheng-Quan, Gao Hongxiang, Lu Yong
Department of Anatomy and Neurobiology, Northeastern Ohio University College of Medicine, 4209 State Route 44, PO Box 95, Rootstown, OH 44272, USA.
J Neurophysiol. 2009 Sep;102(3):1672-83. doi: 10.1152/jn.00419.2009. Epub 2009 Jul 1.
Neurons in the chicken nucleus laminaris (NL), the third-order auditory neurons that detect the interaural time differences that enable animals to localize sounds in the horizontal plane, receive glutamatergic excitation from the cochlear nucleus magnocellularis (NM) and GABAergic inhibition from the ipsilateral superior olivary nucleus. Here, we study metabotropic glutamate receptor (mGluR)- and GABAB receptor (GABABR)-mediated modulation of synaptic transmission in NL neurons. Gramicidin-perforated recordings from acute brain stem slice preparations showed that the reversal potential of the GABAergic responses in NL neurons was more depolarized than the spike threshold. Activation of the GABAergic input produced a mix of inhibitory and excitatory actions in NL neurons. The inhibitory action is known to be critical in improving the acuity of temporal processing of sounds. The excitatory action, however, would reduce the phase locking fidelity of NL neurons in response to their excitatory inputs from the NM. We show that activation of presynaptic mGluRs or GABABRs by either exogenous agonists or synaptically released neurotransmitters reduced the GABAergic responses, preventing the excitatory action of GABA while leaving the inhibitory action intact. Unlike most CNS synapses, the glutamatergic transmission in the NL was not modulated by either mGluRs or GABABRs, indicating that fixed (nonmodulatory) excitatory inputs to the NL may be optimal for coincidence detection. This study contributes to our understanding of how selective neuromodulation is achieved to suit a particular function of neuronal circuits in the brain.
鸡层状核（NL）中的神经元是三阶听觉神经元，可检测使动物能够在水平面定位声音的双耳时间差，它们从耳蜗大细胞核（NM）接收谷氨酸能兴奋性输入，并从同侧上橄榄核接收GABA能抑制性输入。在此，我们研究代谢型谷氨酸受体（mGluR）和GABAB受体（GABABR）介导的对NL神经元突触传递的调节。急性脑干切片制备的短杆菌肽穿孔记录显示，NL神经元中GABA能反应的反转电位比动作电位阈值更去极化。GABA能输入的激活在NL神经元中产生了抑制性和兴奋性作用的混合。已知抑制性作用对于提高声音时间处理的敏锐度至关重要。然而，兴奋性作用会降低NL神经元对来自NM的兴奋性输入的锁相保真度。我们表明，外源性激动剂或突触释放的神经递质激活突触前mGluRs或GABABRs会降低GABA能反应，阻止GABA的兴奋性作用，同时保持抑制性作用不变。与大多数中枢神经系统突触不同，NL中的谷氨酸能传递不受mGluRs或GABABRs的调节，这表明对NL的固定（非调节性）兴奋性输入可能最适合用于同时检测。这项研究有助于我们理解如何实现选择性神经调节以适应大脑中神经元回路的特定功能。
