Urban L, Dray A
Sandoz Institute for Medical Research, London, U.K.
Neuroscience. 1992;47(3):693-702. doi: 10.1016/0306-4522(92)90177-4.
Low doses (0.2-0.8 microM) of capsaicin were used to achieve selective excitation of C-fibres and the consequent synaptic activation of dorsal horn neurons (laminae I-VI) in the spinal cord of the 12-20-day-old mouse, maintained in vitro. Most dorsal horn cells were activated by application of capsaicin to dorsal root ganglia. The response consisted of a long-lasting membrane depolarization with increased regenerative (synaptic) activity in 79% of the cells, and in a further 7% only an increased synaptic activity was evoked. These effects of capsaicin were completely blocked by removing extracellular calcium ions from the superfusate to the spinal cord, or by the addition of 1 microM tetrodotoxin, suggesting a presynaptic origin of the capsaicin action. Only 67% of cells excited by capsaicin were sensitive to exogenous substance P. The excitatory amino acid antagonists, kynurenic acid (50-100 microM) or (-)-2-amino-5-phosphonovaleric acid (10-20 microM) completely blocked the capsaicin-evoked response in deep dorsal horn cells, indicating the involvement of excitatory amino acid receptors in the synaptic pathway. However, in superficial dorsal horn neurons these antagonists attenuated, but never completely abolished, the capsaicin-evoked depolarization. The kynurenic acid-resistant component of the capsaicin-evoked excitation in superficial dorsal horn cells suggests the involvement of non-amino acid excitatory transmitters--possibly neuropeptides--in the synaptic transmission. Activation of primary afferents by high-intensity electrical stimulation of the dorsal roots induced a prolonged (0.5-3 s) postsynaptic excitation in the majority of deep dorsal horn cells. The duration of the synaptic response was significantly reduced by (-)-2-amino-5-phosphonovaleric acid. Following repeated application of capsaicin, desensitization of the capsaicin-evoked synaptic activation of dorsal horn cells occurred. This effect was paralleled with the loss of the prolonged (-)-2-amino-5-phosphonovaleric acid-sensitive phase of the excitatory postsynaptic potential evoked by the high-intensity electrical stimulation of dorsal roots. This observation suggested that activation of the N-methyl-D-aspartate receptors in the dorsal horn can be activated by small-calibre capsaicin-sensitive fibres. In summary, our data suggest that the selective activation of C-fibre afferents with capsaicin produces synaptic activity in the dorsal horn which has a strong excitatory amino acid component as well as a non-excitatory amino acid, possibly peptidergic, component.
采用低剂量（0.2 - 0.8微摩尔）的辣椒素，对体外培养的12 - 20日龄小鼠脊髓中的C纤维进行选择性激发，进而使脊髓背角神经元（I - VI层）发生突触激活。将辣椒素施加于背根神经节时，大多数背角细胞被激活。反应包括79%的细胞出现持久的膜去极化，再生性（突触）活动增强，另有7%的细胞仅出现突触活动增强。从灌流液中去除细胞外钙离子或添加1微摩尔的河豚毒素，可完全阻断辣椒素的这些作用，提示辣椒素作用于突触前。辣椒素激发的细胞中，只有67%对外源性P物质敏感。兴奋性氨基酸拮抗剂犬尿氨酸（50 - 100微摩尔）或（ - ） - 2 - 氨基 - 5 - 磷酸戊酸（10 - 20微摩尔）可完全阻断背角深层细胞中辣椒素诱发的反应，表明兴奋性氨基酸受体参与了突触通路。然而，在背角浅层神经元中，这些拮抗剂可减弱但从未完全消除辣椒素诱发的去极化。背角浅层细胞中辣椒素诱发的兴奋对犬尿氨酸耐药的成分提示，非氨基酸兴奋性递质——可能是神经肽——参与了突触传递。通过高强度电刺激背根激活初级传入纤维，可在大多数背角深层细胞中诱发持续时间较长（0.5 - 3秒）的突触后兴奋。（ - ） - 2 - 氨基 - 5 - 磷酸戊酸可显著缩短突触反应的持续时间。反复应用辣椒素后，背角细胞对辣椒素诱发的突触激活出现脱敏。这种效应与背根高强度电刺激诱发的兴奋性突触后电位中对（ - ） - 2 - 氨基 - 5 - 磷酸戊酸敏感的延长相的丧失同时出现。这一观察结果提示，背角中的N - 甲基 - D - 天冬氨酸受体可被小口径辣椒素敏感纤维激活。总之，我们的数据表明，用辣椒素选择性激活C纤维传入纤维可在背角产生突触活动，该活动具有强烈的兴奋性氨基酸成分以及非兴奋性氨基酸（可能是肽能）成分。
