Kwan C L, Demaro J A, Hu J W, Jacquin M F, Sessle B J
Faculty of Dentistry, University of Toronto, Toronto, Ontario M5G 1G6, Canada.
J Neurophysiol. 1999 Feb;81(2):435-46. doi: 10.1152/jn.1999.81.2.435.
The effects of C-fiber depletion induced by neonatal capsaicin treatment on the functional properties of vibrissa-sensitive low-threshold mechanoreceptive (LTM) neurons in the rat trigeminal nucleus principalis were examined in adult rats. Neonatal rats were injected either with capsaicin or its vehicle within 48 h of birth. The depletion of unmyelinated afferents was confirmed by the significant decrease in plasma extravasation of Evan's blue dye induced in the hindlimb skin of capsaicin-treated rats by cutaneous application of mustard oil and by the significant decrease of unmyelinated fibers in both the sciatic and infraorbital nerves. The mechanoreceptive field (RF) and response properties of 31 vibrissa-sensitive neurons in capsaicin-treated rats were compared with those of 32 vibrissa-sensitive neurons in control (untreated or vehicle-treated) rats. The use of electronically controlled mechanical stimuli allowed quantitative analysis of response properties of vibrissa-sensitive neurons; these included the number of center- and surround-RF vibrissae within the RF (i.e., those vibrissae which when stimulated elicited >/=1 and <1 action potential per stimulus, respectively), the response magnitude and latency, and the selectivity of responses to stimulation of vibrissae in different directions with emphasis on combining both the response magnitude and direction of vibrissal deflection in a vector analysis. Neonatal capsaicin treatment was associated with significant increases in the total number of vibrissae, in the number of center-RF vibrissae per neuronal RF, and in the percentage of vibrissa-sensitive neurons that also responded to stimulation of other types of orofacial tissues. Compared with control rats, capsaicin-treated rats showed significant increases in the response magnitude to stimulation of surround-RF vibrissae as well as in response latency variability to stimulation of both center- and surround-RF vibrissae. C-fiber depletion also significantly altered the directional selectivity of responses to stimulation of vibrissae. For neurons with multiple center-RF vibrissae, the proportion of center-RF vibrissae with net vector responses oriented toward the same quadrant was significantly less in capsaicin-treated compared with control rats. These changes in the functional properties of principalis vibrissa-sensitive neurons associated with marked depletion of C-fiber afferents are consistent with similarly induced alterations in LTM neurons studied at other levels of the rodent somatosensory system, and indeed may contribute to alterations previously described in the somatosensory cortex of adult rodents. Furthermore, these results provide additional support to the view that C fibers may have an important role in shaping the functional properties of LTM neurons in central somatosensory pathways.
在成年大鼠中，研究了新生期辣椒素处理诱导的C纤维耗竭对大鼠三叉神经主核中触须敏感的低阈值机械感受性（LTM）神经元功能特性的影响。新生大鼠在出生后48小时内注射辣椒素或其赋形剂。通过在辣椒素处理的大鼠后肢皮肤中经皮应用芥子油诱导伊文思蓝染料血浆外渗的显著减少，以及坐骨神经和眶下神经中无髓纤维的显著减少，证实了无髓传入纤维的耗竭。将31只辣椒素处理大鼠中触须敏感神经元的机械感受野（RF）和反应特性与32只对照（未处理或赋形剂处理）大鼠中触须敏感神经元的进行了比较。使用电子控制的机械刺激可以对触须敏感神经元的反应特性进行定量分析；这些特性包括RF内中心RF触须和周围RF触须的数量（即分别在每次刺激时刺激引发≥1和<1个动作电位的那些触须）、反应幅度和潜伏期，以及对不同方向触须刺激反应的选择性，重点是在矢量分析中结合触须偏转的反应幅度和方向。新生期辣椒素处理与触须总数、每个神经元RF中中心RF触须的数量以及对其他类型口面部组织刺激也有反应的触须敏感神经元的百分比显著增加有关。与对照大鼠相比，辣椒素处理的大鼠对周围RF触须刺激的反应幅度以及对中心和周围RF触须刺激的反应潜伏期变异性显著增加。C纤维耗竭还显著改变了对触须刺激反应的方向选择性。对于具有多个中心RF触须的神经元，与对照大鼠相比，辣椒素处理的大鼠中净矢量反应朝向同一象限的中心RF触须的比例显著降低。与C纤维传入显著耗竭相关的主核触须敏感神经元功能特性的这些变化与在啮齿动物体感系统其他水平研究的LTM神经元中类似诱导的改变一致，并且确实可能导致成年啮齿动物体感皮层中先前描述的改变。此外，这些结果为C纤维可能在塑造中枢体感通路中LTM神经元的功能特性方面发挥重要作用的观点提供了额外支持。
