Fénelon Valérie, Le Feuvre Yves, Bem Tiaza, Meyrand Pierre
Laboratoire de Neurobiologie des Réseaux, UMR 5816 Université Bordeaux I CNRS, 1 Avenue des Facultes, 33405 Talence, France.
J Physiol Paris. 2003 Jan;97(1):59-68. doi: 10.1016/j.jphysparis.2003.10.007.
Modulatory systems are well known for their roles in tuning the cellular and synaptic properties in the adult neuronal networks, and play a major role in the control of the flexibility of functional outputs. However far less is known concerning their role in the maturation of neural networks during the development. In this review, using the stomatogastric nervous system of lobster, we will show that the neuromodulatory system exerts a powerful influence on developing neural networks. In the adult the number of both motor target neurons and their modulatory neurons is restricted to tens of identifiable cells. They are therefore well characterized in terms of cellular, synaptic and morphological properties. In the embryo, these target cells and their neuromodulatory population are already present from mid-embryonic life. However, the motor output generated by the system is quite different: while in the embryo all the target neurons are organized into a single network generating unique motor pattern, in the adult this population splits into two distinct networks generating separate patterns. This ontogenetic partitioning does not rely on progressive acquisition of adult properties but rather on a switch between two possible network operations. Indeed, adult networks are present early in the embryonic life but their expression is repressed by central modulatory neurons. Moreover, embryonic networks can be revealed in the adult system again by altering modulatory influences. Therefore, independently of the developmental age, two potential network phenotypes co-exist within the same neuronal architecture: when one is expressed, the other one is hidden and vice versa. These transitions do not necessarily need dramatic changes such as growth/retraction of processes, acquisition of new intra-membrane proteins etc. but rather, as shown by modelling studies, it may simply rely on a subtle tuning of pre-existing intercellular electrical coupling. This in turn suggests that progressive ontogenetic alteration may not take place at the level of the target network but rather at the level of modulatory input neurons.
调节系统因其在调节成年神经元网络中的细胞和突触特性方面所起的作用而广为人知，并在控制功能输出的灵活性方面发挥着重要作用。然而，关于它们在发育过程中神经网络成熟方面的作用，人们了解得要少得多。在这篇综述中，我们将以龙虾的口胃神经系统为例，表明神经调节系统对发育中的神经网络有强大的影响。在成体中，运动目标神经元及其调节神经元的数量都限制在几十种可识别的细胞内。因此，它们在细胞、突触和形态学特性方面都有很好的特征描述。在胚胎中，这些目标细胞及其神经调节群体在胚胎中期就已经存在。然而，该系统产生的运动输出却大不相同：在胚胎中，所有目标神经元都组织成一个单一的网络，产生独特的运动模式，而在成体中，这个群体分成两个不同的网络，产生不同的模式。这种个体发育过程中的划分并不依赖于逐渐获得成体特性，而是依赖于两种可能的网络运作之间的转换。事实上，成体网络在胚胎早期就已存在，但其表达受到中枢调节神经元的抑制。此外，通过改变调节影响，胚胎网络可以在成体系统中再次显现出来。因此，与发育年龄无关，两种潜在的网络表型在同一神经元结构中共存：当一种表型表达时，另一种就被隐藏，反之亦然。这些转变不一定需要诸如神经突生长/回缩、获得新的膜内蛋白等剧烈变化，而是，正如模型研究所示，它可能仅仅依赖于对预先存在的细胞间电耦合的微调。这反过来表明，个体发育的渐进变化可能不是发生在目标网络层面，而是发生在调节输入神经元层面。
