Institute for Neuroscience of Castilla y León (INCYL), University of Salamanca, C/ Pintor Fernando Gallego, 1, 37007, Salamanca, Spain.
Institute of Biomedical Research of Salamanca (IBSAL), University of Salamanca, Salamanca, Spain.
Brain Struct Funct. 2018 Jul;223(6):2733-2751. doi: 10.1007/s00429-018-1654-9. Epub 2018 Mar 24.
The acoustic startle reflex (ASR) is a short and intense defensive reaction in response to a loud and unexpected acoustic stimulus. In the rat, a primary startle pathway encompasses three serially connected central structures: the cochlear root neurons, the giant neurons of the nucleus reticularis pontis caudalis (PnC), and the spinal motoneurons. As a sensorimotor interface, the PnC has a central role in the ASR circuitry, especially the integration of different sensory stimuli and brain states into initiation of motor responses. Since the basal ganglia circuits control movement and action selection, we hypothesize that their output via the substantia nigra (SN) may interplay with the ASR primary circuit by providing inputs to PnC. Moreover, the pedunculopontine tegmental nucleus (PPTg) has been proposed as a functional and neural extension of the SN, so it is another goal of this study to describe possible anatomical connections from the PPTg to PnC. Here, we made 6-OHDA neurotoxic lesions of the SN pars compacta (SNc) and submitted the rats to a custom-built ASR measurement session to assess amplitude and latency of motor responses. We found that following lesion of the SNc, ASR amplitude decreased and latency increased compared to those values from the sham-surgery and control groups. The number of dopamine neurons remaining in the SNc after lesion was also estimated using a stereological approach, and it correlated with our behavioral results. Moreover, we employed neural tract-tracing techniques to highlight direct projections from the SN to PnC, and indirect projections through the PPTg. Finally, we also measured levels of excitatory amino acid neurotransmitters in the PnC following lesion of the SN, and found that they change following an ipsi/contralateral pattern. Taken together, our results identify nigrofugal efferents onto the primary ASR circuit that may modulate motor responses.
声惊反射（ASR）是一种对大声和意外的听觉刺激产生的短暂而强烈的防御反应。在大鼠中，初级声惊反射通路包括三个依次连接的中枢结构：耳蜗根神经元、桥脑尾侧网状核（PnC）的巨细胞和脊髓运动神经元。作为一个感觉运动接口，PnC 在 ASR 电路中起着核心作用，特别是将不同的感觉刺激和大脑状态整合到运动反应的启动中。由于基底神经节回路控制运动和动作选择，我们假设它们通过向 PnC 提供输入，经由黑质（SN）的输出可能与 ASR 初级回路相互作用。此外，被盖脚桥核（PPTg）已被提议为 SN 的功能和神经延伸，因此本研究的另一个目标是描述 PPTg 到 PnC 的可能解剖连接。在这里，我们对 SN 壳核（SNc）进行了 6-OHDA 神经毒性损伤，并使大鼠接受了定制的 ASR 测量会议，以评估运动反应的幅度和潜伏期。我们发现，与假手术和对照组相比，SNc 损伤后 ASR 幅度降低，潜伏期延长。使用立体学方法估计 SNc 中剩余的多巴胺神经元数量，并且与我们的行为结果相关。此外，我们采用神经束追踪技术来突出 SN 到 PnC 的直接投射，以及通过 PPTg 的间接投射。最后，我们还测量了 SN 损伤后 PnC 中兴奋性氨基酸神经递质的水平，发现它们以同侧/对侧模式发生变化。总之，我们的结果确定了向初级 ASR 回路发出的黑质传出纤维，它们可能调节运动反应。
