Burger R M, Pollak G D
Department of Zoology, University of Texas at Austin, Austin, Texas 78712, USA.
J Neurophysiol. 1998 Oct;80(4):1686-701. doi: 10.1152/jn.1998.80.4.1686.
Neurons in the central nucleus of the inferior colliculus (ICc) typically respond with phase-locked discharges to low rates of sinusoidal amplitude-modulated (SAM) signals and fail to phase-lock to higher SAM rates. Previous studies have shown that comparable phase-locking to SAM occurs in the dorsal nucleus of the lateral lemniscus (DNLL) and medial superior olive (MSO) of the mustache bat. The studies of MSO and DNLL also showed that the restricted phase-locking to low SAM rates is created by the coincidence of phase-locked excitatory and inhibitory inputs that have slightly different latencies. Here we tested the hypothesis that responses to SAM in the mustache bat IC are shaped by the same mechanism that shapes responses to SAM in the two lower nuclei. We recorded responses from ICc neurons evoked by SAM signals before and during the iontophoretic application of several pharmacological agents: bicuculline, a competitive antagonist for gamma-aminobutyric acid-A (GABAA) receptors; strychnine, a competitive antagonist for glycine receptors; the GABAB receptor blocker, phaclofen, and the N-methyl-D-aspartate (NMDA) receptor blocker, (-)-2-amino-5-phosphonopentanoic acid (AP5). The hypothesis that inhibition shapes responses to SAM signals in the ICc was not confirmed. In >90% of the ICc neurons tested, the range of SAM rates to which they phase-locked was unchanged after blocking inhibition with bicuculline, strychnine or phaclofen, applied either individually or in combination. We also considered the possibility that faster alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors follow high temporal rates of incoming excitation but that the slower NMDA receptors could follow only lower rates. Thus at higher SAM rates, NMDA receptors might generate a sustained excitation that "smears" the phase-locked excitation generated by the AMPA receptors. The NMDA hypothesis, like the inhibition hypothesis, was also not confirmed. In none of the cells that we tested did the application of AP5 by itself, or in combination with bicuculline, cause an increase in the range of SAM rates that evoked phase-locking. These results illustrate that the same response property, phase-locking restricted to low SAM rates, is formed in more than one way in the auditory brain stem. In the MSO and DNLL, the mechanism is coincidence of phase-locked excitation and inhibition, whereas in ICc the same response feature is formed by a different but unknown mechanism.
下丘中央核（ICc）中的神经元通常会以锁相放电的方式对低频正弦调幅（SAM）信号做出反应，而对较高的SAM频率则无法实现锁相。先前的研究表明，在髯蝠的外侧丘系背核（DNLL）和内侧上橄榄核（MSO）中也会出现类似的对SAM的锁相现象。对MSO和DNLL的研究还表明，对低SAM频率的有限锁相是由锁相兴奋性和抑制性输入的巧合造成的，这些输入的潜伏期略有不同。在这里，我们测试了一个假设，即髯蝠IC中对SAM的反应是由与两个较低核团中对SAM的反应相同的机制塑造的。我们记录了在离子电泳施加几种药物之前和期间，SAM信号诱发的ICc神经元的反应：荷包牡丹碱，一种γ-氨基丁酸-A（GABAA）受体的竞争性拮抗剂；士的宁，一种甘氨酸受体的竞争性拮抗剂；GABAB受体阻滞剂，巴氯芬，以及N-甲基-D-天冬氨酸（NMDA）受体阻滞剂，（-）-2-氨基-5-磷酸戊酸（AP5）。ICc中抑制作用塑造对SAM信号反应的假设未得到证实。在超过90%的测试ICc神经元中，在用荷包牡丹碱、士的宁或巴氯芬单独或联合阻断抑制作用后，它们实现锁相的SAM频率范围没有变化。我们还考虑了更快的α-氨基-3-羟基-5-甲基异恶唑-4-丙酸（AMPA）受体能够跟随传入兴奋的高时间频率，但较慢的NMDA受体只能跟随较低频率的可能性。因此，在较高的SAM频率下，NMDA受体可能会产生持续的兴奋，从而“模糊”AMPA受体产生的锁相兴奋。与抑制假设一样，NMDA假设也未得到证实。在我们测试的所有细胞中，单独应用AP5或与荷包牡丹碱联合应用，都没有导致诱发锁相的SAM频率范围增加。这些结果表明，在听觉脑干中，同一种反应特性，即仅限于低SAM频率的锁相，是通过多种方式形成的。在MSO和DNLL中，机制是锁相兴奋和抑制的巧合，而在ICc中，相同的反应特征是由一种不同但未知的机制形成的。
