Loss of inner hair cell-auditory nerve fiber synapses is considered to be an important early stage of neural presbyacusis. Mass potentials, recorded at the cochlear round window, can be used to derive the neural index (NI), a sensitive measure for pharmacologically-induced synapse loss. Here, we investigate the applicability of the NI for measuring age-related auditory synapse loss in young-adult, middle-aged, and old Mongolian gerbils.

Synapse loss, which was progressively evident in the 2 aged groups, correlated weakly with NI when measured at a fixed sound level of 60 dB SPL. However, the NI was confounded by decreases in single-unit firing rates at 60 dB SPL. NI at 30 dB above threshold, when firing rates were similar between age groups, did not correlate with synapse loss.

Our results show that synapse loss is poorly reflected in the NI of aged gerbils, particularly if further peripheral pathologies are present. The NI may therefore not be a reliable clinical tool to assess synapse loss in aged humans with peripheral hearing loss.

内毛细胞-听觉神经纤维突触的丧失被认为是神经性老年性聋的重要早期阶段。在耳蜗圆窗记录的质量电位可用于推导神经指数 (NI)，这是一种药理学诱导的突触损失的敏感测量。在这里，我们研究了 NI 在测量年轻、中年和老年蒙古沙鼠与年龄相关的听觉突触损失方面的适用性。

在 60 dB SPL 的固定声级下测量时，突触损失在 2 个年龄组中逐渐明显，与 NI 相关性较弱。然而，NI 被 60 dB SPL 下单单元发射率的降低所迷惑。NI 在高于阈值 30 dB 时，当年龄组之间的放电率相似时，与突触丢失无关。

我们的研究结果表明，突触损失在老年沙鼠的 NI 中反映不佳，特别是如果存在进一步的外周病变。因此，NI 可能不是评估患有外周性听力损失的老年人突触损失的可靠临床工具。