Sillar K T, Simmers A J
Gatty Marine Laboratory, School of Biological and Medical Sciences, University of St. Andrews, Fife, Scotland.
J Neurosci. 1994 May;14(5 Pt 1):2636-47. doi: 10.1523/JNEUROSCI.14-05-02636.1994.
The effects of the neuromodulatory monoamine 5-HT (serotonin) on a cutaneous mechanosensory (Rohon-Beard, R-B neuron) pathway in the spinal cord of postembryonic Xenopus laevis tadpoles have been examined. In paralyzed animals, exogenous 5-HT at 1-10 microM reversibly inhibits (within 1-2 min) the activation of fictive swimming in response to electrical stimulation of R-B free nerve endings in the skin. At threshold stimulus intensities for swimming under control conditions, intracellularly recorded EPSPs in contralateral motoneurons are completely abolished by 5-HT without any obvious change in neuronal conductance or membrane potential. However, increasing the stimulus voltage can activate swimming with enhanced motor burst discharge on each cycle (Sillar et al., 1992). This suggested that 5-HT inhibits the swim-initiating pathway rather than the motor rhythm-generating circuitry itself. Extracellular recordings from the central projections of R-B neurons indicated that the amine does not impair the generation of mechanoafferent impulses or their propagation into the spinal cord. However, 5-HT application blocks impulse activity in dorsolaterally positioned sensory interneurons (DLis) that are contacted by R-B neurons, suggesting that 5-HT acts at R-B to DLi synapses in the dorsal cord. By recording with microelectrodes from DLis, we find that skin stimulus-evoked EPSPs at this first-order synapse in the swim-initiating pathway are reversibly suppressed by 5-HT. No obvious change in DLi membrane potential or conductance could be detected during the inhibition, suggesting a presynaptic site of action for 5-HT. To investigate this suggestion further, the effects of 5-HT on the spontaneous release of R-B sensory transmitter (excitatory amino acid, EAA) were examined, again by recording postsynaptically from DLis. In quiescent preparations, DLis receive spontaneous glycinergic, GABAergic, and EAA receptor-mediated PSPs. The inhibitory potentials are abolished by strychnine and curare, respectively. The excitatory potentials that remain are not blocked by application of the calcium channel blocker cadmium chloride at 1 mM, but are suppressed by the EAA receptor antagonist kynurenic acid. They therefore resemble the TTX-resistant EPSPs described previously in Xenopus DLis (Sillar and Roberts, 1991), which are presumed to arise from the spontaneous liberation of EAA transmitter from R-B terminals. Bath application of 5-HT dramatically reduces the rate of occurrence of these spontaneous EPSPs consistent with a presynaptic locus for the inhibitory effects of 5-HT.(ABSTRACT TRUNCATED AT 400 WORDS)
研究了神经调节单胺5-羟色胺（5-HT，血清素）对非洲爪蟾蝌蚪胚胎后期脊髓中皮肤机械感觉（罗霍恩-比尔兹，R-B神经元）通路的影响。在瘫痪动物中，1-10微摩尔的外源性5-HT在1-2分钟内可逆性抑制（响应皮肤中R-B游离神经末梢的电刺激）虚构游泳的激活。在对照条件下游泳的阈值刺激强度时，5-HT可完全消除对侧运动神经元细胞内记录的兴奋性突触后电位（EPSP），而神经元电导或膜电位无明显变化。然而，增加刺激电压可激活游泳，且每个周期的运动爆发放电增强（西拉尔等人，1992年）。这表明5-HT抑制游泳启动通路而非运动节律产生电路本身。从R-B神经元的中枢投射进行的细胞外记录表明，该胺类物质不损害机械传入冲动的产生或其向脊髓的传播。然而，应用5-HT可阻断位于背外侧的感觉中间神经元（DLi）中的冲动活动，这些中间神经元与R-B神经元相接触，这表明5-HT作用于脊髓背侧的R-B至DLi突触。通过用微电极记录DLi，我们发现游泳启动通路中该一级突触处皮肤刺激诱发的EPSP被5-HT可逆性抑制。在抑制过程中未检测到DLi膜电位或电导有明显变化，这表明5-HT的作用位点在突触前。为进一步研究这一推测，再次通过从DLi进行突触后记录，研究了5-HT对R-B感觉递质（兴奋性氨基酸，EAA）自发释放的影响。在静止标本中，DLi接受自发的甘氨酸能、GABA能和EAA受体介导的PSP。抑制性电位分别被士的宁和箭毒消除。剩余的兴奋性电位不受1毫摩尔氯化镉（钙通道阻滞剂）的应用所阻断，但被EAA受体拮抗剂犬尿喹啉酸抑制。因此，它们类似于先前在非洲爪蟾DLi中描述的对河豚毒素有抗性的EPSP（西拉尔和罗伯茨，1991年），推测这些EPSP源于EAA递质从R-B终末的自发释放。浴槽应用5-HT显著降低这些自发EPSP的发生率，这与5-HT抑制作用的突触前位点一致。（摘要截断于400字）
