Kwan Alex C, Dietz Shelby B, Webb Watt W, Harris-Warrick Ronald M
School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA.
J Neurosci. 2009 Sep 16;29(37):11601-13. doi: 10.1523/JNEUROSCI.1612-09.2009.
Hb9 interneurons (Hb9 INs) are putative components of the mouse spinal locomotor central pattern generator (CPG) and candidates for the rhythm-generating kernel. Studies in slices and hemisected spinal cords showed that Hb9 INs display TTX-resistant membrane potential oscillations, suggesting a role in rhythm generation. To further investigate the roles of Hb9 INs in the locomotor CPG, we used two-photon calcium imaging in the in vitro isolated whole neonatal mouse spinal cord preparation to record the activity of Hb9 INs, which were subsequently stained for unambiguous genetic identification. We elicited fictive locomotion by transmitter application or by electrically stimulating the caudal tip of the spinal cord. Although most Hb9 INs were rhythmically active during fictive locomotion, their activity was sparse and they failed to fire with each cycle of the episode. If Hb9 INs are the principal pacemakers of the CPG in the hemisegment in which they are located, they should direct the firing of motor neurons, with their activity preceding that of their ipsilateral segmental ventral roots. Instead, during each locomotor cycle, onset of Hb9 IN activity lagged behind the onset of the ipsilateral ventral root burst by a mean phase of 0.21 during electrical stimulation and 0.28 during transmitter application. Whole-cell recordings in intact and hemisected spinal cords confirmed the imaging results. Our data suggest that Hb9 INs participate in fictive locomotion, but the delayed onset of activity relative to ipsilateral motoneurons suggests that Hb9 INs are unlikely to be the sole intrasegmental rhythm-generating kernel of the CPG.
Hb9中间神经元（Hb9 INs）被认为是小鼠脊髓运动中枢模式发生器（CPG）的组成部分，也是节律产生核心的候选者。在切片和半切脊髓中的研究表明，Hb9 INs表现出对河豚毒素（TTX）耐受的膜电位振荡，这表明其在节律产生中发挥作用。为了进一步研究Hb9 INs在运动CPG中的作用，我们在体外分离的新生小鼠全脊髓标本中使用双光子钙成像来记录Hb9 INs的活动，随后对其进行染色以进行明确的基因鉴定。我们通过应用递质或电刺激脊髓尾端来诱发虚拟运动。尽管大多数Hb9 INs在虚拟运动期间有节律地活动，但其活动稀疏，并且在每个运动周期中未能同步发放。如果Hb9 INs是其所在半节段CPG的主要起搏器，那么它们应该指导运动神经元的发放，其活动应先于同侧节段腹根的活动。然而，在每个运动周期中，在电刺激期间Hb9 INs活动的起始落后于同侧腹根爆发起始的平均相位为0.21，在应用递质期间为0.28。在完整和半切脊髓中的全细胞记录证实了成像结果。我们的数据表明，Hb9 INs参与虚拟运动，但相对于同侧运动神经元活动起始的延迟表明，Hb9 INs不太可能是CPG唯一的节段内节律产生核心。
