Le Bon-Jego Morgane, Cattaert Daniel, Pearlstein Edouard
Laboratoire de Neurobiologie des Réseaux, Centre National de la Recherche Scientifique Unité Mixte de Recherche 5816, Biologie Animale, Bâtiment B2, 33405 Talence cedex, France.
J Neurosci. 2004 Jan 14;24(2):398-411. doi: 10.1523/JNEUROSCI.4032-03.2004.
Serotonin (5HT) is an endogenous amine that modifies posture in crustacea. Here, we examined the mechanisms of action of 5HT on the resistance reflex in crayfish legs. This reflex, which counteracts movements imposed on a limb, is based on a negative feedback system formed by proprioceptors that sense joint angle movements and activate opposing motoneurons. We performed intracellular recordings from depressor motoneurons while repetitively stretching and releasing a leg joint proprioceptor in a resting in vitro preparation (i.e., a preparation that lacks spontaneous rhythmic activity). 5HT increased the amplitude of the depolarization during the release phase of the proprioceptor (corresponding to an upward movement of the leg) and the discharge frequency of the motoneurons. The 5HT-induced increase in the resistance reflex is caused, to a large extent, by polysynaptic pathways because it was very attenuated in the presence of high divalent cation solution. In addition to this activation of the polysynaptic pathways, 5HT also has postsynaptic effects that enhance the resistance reflex. 5HT causes a tonic depolarization, as well as an increase in the time constant and input resistance of motoneurons. We developed a simple mathematical model to describe the integrative properties of the motoneurons. The conclusion of this study is that the input frequency and the decay time constant of the EPSPs interact in such a way that small simultaneous changes in these parameters can cause a large effect on summation. Therefore, the conjunction of presynaptic and postsynaptic changes produces a strong cooperative effect on the resistance reflex response.
血清素（5-羟色胺，5HT）是一种能改变甲壳纲动物姿势的内源性胺类物质。在此，我们研究了5HT对小龙虾腿部抵抗反射的作用机制。这种反射可抵消施加于肢体的运动，它基于一个负反馈系统，该系统由感知关节角度运动并激活对抗运动神经元的本体感受器构成。我们在静息的体外标本（即缺乏自发节律活动的标本）中，对抑制性运动神经元进行细胞内记录，同时反复拉伸和释放腿部关节本体感受器。5HT增加了本体感受器释放阶段（对应腿部向上运动）去极化的幅度以及运动神经元的放电频率。5HT诱导的抵抗反射增强在很大程度上是由多突触通路引起的，因为在高浓度二价阳离子溶液存在时这种增强作用会大大减弱。除了激活多突触通路外，5HT还具有增强抵抗反射的突触后效应。5HT会引起持续性去极化，以及运动神经元时间常数和输入电阻的增加。我们建立了一个简单的数学模型来描述运动神经元的整合特性。本研究的结论是，兴奋性突触后电位（EPSP）的输入频率和衰减时间常数相互作用，使得这些参数同时发生的微小变化会对总和产生很大影响。因此，突触前和突触后变化的结合对抵抗反射反应产生了强烈的协同作用。
