El Manira A, Zhang W, Svensson E, Bussières N
The Nobel Institute for Neurophysiology, Department of Neuroscience, Karolinska Institutet, S-171 77 Stockholm, Sweden.
J Neurosci. 1997 Mar 1;17(5):1786-94. doi: 10.1523/JNEUROSCI.17-05-01786.1997.
In the lamprey spinal cord, 5-hydroxytryptamine (5-HT) immunoreactivity (ir) is present in the ventromedial plexus originating from intraspinal neurons, ventrolateral column arising from the brainstem, and dorsal column. The latter 5-HT system originates from small dorsal root ganglion neurons. Combined Lucifer yellow intracellular labeling of the intraspinal sensory neurons, dorsal cells, and 5-HT immunohistochemistry showed close appositions between 5-HT-ir fibers and dorsal cell axons. Application of 5-HT depressed monosynaptic EPSPs evoked in giant interneurons by stimulation of single dorsal cells, dorsal roots, or dorsal column without any detectable change in the input resistance of postsynaptic neurons. Furthermore, the amplitude of AMPA-evoked depolarizations in giant interneurons was unaffected by 5-HT. The lack of postsynaptic effects of 5-HT indicates that the decrease of the amplitude of sensory monosynaptic EPSPs by 5-HT is mediated by presynaptic mechanisms. The inhibition of monosynaptic EPSPs by 5-HT was not counteracted by an antagonist of 5-HT1A receptors. 5-HT also reduced the amplitude of the calcium current recorded in isolated dorsal cells and slowed down its kinetics. The inhibition of calcium channels could represent the mechanism mediating the depression of synaptic transmission at the axonal level. These results show that activation of 5-HT receptors on dorsal cell axons as well as on other sensory neurons mediates inhibition of sensory synaptic transmission to giant interneurons. In intact animals, 5-HT could be released from small 5-HT neurons in dorsal root ganglia, which thus may underlie direct sensory-sensory interactions.
在七鳃鳗脊髓中，5-羟色胺（5-HT）免疫反应性（ir）存在于源自脊髓内神经元的腹内侧丛、源自脑干的腹外侧柱和背柱中。后一个5-HT系统起源于小的背根神经节神经元。脊髓内感觉神经元、背侧细胞的联合荧光黄细胞内标记以及5-HT免疫组织化学显示，5-HT-ir纤维与背侧细胞轴突之间紧密相邻。应用5-HT可抑制单个背侧细胞、背根或背柱刺激在巨大中间神经元中诱发的单突触兴奋性突触后电位（EPSP），而突触后神经元的输入电阻没有任何可检测到的变化。此外，5-HT对巨大中间神经元中AMPA诱发的去极化幅度没有影响。5-HT缺乏突触后效应表明，5-HT导致感觉单突触EPSP幅度降低是由突触前机制介导的。5-HT对单突触EPSP的抑制作用未被5-HT1A受体拮抗剂抵消。5-HT还降低了在分离的背侧细胞中记录的钙电流幅度，并减慢了其动力学。钙通道的抑制可能代表了在轴突水平介导突触传递抑制的机制。这些结果表明，背侧细胞轴突以及其他感觉神经元上5-HT受体的激活介导了对向巨大中间神经元的感觉突触传递的抑制。在完整动物中，5-HT可能从背根神经节中的小5-HT神经元释放，因此这可能是直接感觉-感觉相互作用的基础。
