Leucokinins: Multifunctional Neuropeptides and Hormones in Insects and Other Invertebrates

Abstract

Leucokinins (LKs) constitute a neuropeptide family first discovered in a cockroach and later identified in numerous insects and several other invertebrates. The LK receptors are only distantly related to other known receptors. Among insects, there are many examples of species where genes encoding LKs and their receptors are absent. Furthermore, genomics has revealed that LK signaling is lacking in several of the invertebrate phyla and in vertebrates. In insects, the number and complexity of LK expressing neurons vary, from the simple pattern in the larva of Drosophila where the entire CNS has 20 neurons of three main types, to cockroaches with about 250 of many different types. Common to all studied insects is the presence or 1-3 pairs of LK-expressing neurosecretory cells in each abdominal neuromere of the ventral nerve cord, that, at least in some insects, regulate secretion in Malpighian tubules. This review summarizes the diverse functional roles of LK signaling in insects, as well as other arthropods and mollusks. These functions include regulation of ion and water homeostasis, feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. Other functions are implied by the neuronal distribution of LK, but remain to be investigated.

Functional studies on neuropeptides and receptors in model animal <italic>Aplysia</italic>

“…(crustacean-cardioactive peptide)信号途径相似 [53,56,57] 。 重要的是，神经肽受体的鉴定对于神经肽发挥作用的分子机制研究具有关键作用。 [18] [6,7,64] 。 例如， 随着模式动物线虫基因组测序的完成， 线虫中大量的神经肽被发现， 包括 insulin-like peptides，FMRFamide，non-insulin peptides 等，其中 insulin-like peptides 在哺乳动物中有同源 神经肽 [32,33] 。研究表明它们在摄食、位置移动和生殖等动物行为中发挥调控作用 [65] ，这对脊 椎动物中相关神经肽的研究具有指导或参考作用。 另外， 昆虫类的模式动物也有一定的优势。 比如神经肽 leucokinins [66] 最先在蟑螂中发现。 这归因于蟑螂繁殖能力强，可以短期内获得大量可用神经组织的特点，适合采用生物化学的方 法从组织中提取并鉴定神经肽。此外，果蝇的基因组测序也已经完成，研究者可以利用生物信 息学的方法发现大量的神经肽 [7,67] ，重要的是，还可以采用分子遗传学手段探索神经肽的信号 途径及其功能，这也使得果蝇成为研究神经肽的一种重要的优势模型动物。 此外，甲壳纲动物(比如螃蟹及龙虾)的神经系统，尤其是其口胃神经系统(stomatogastric nervous system, STNS)是研究神经肽的另一个优势模式系统 [6,68] 脊椎动物一样，海兔也具有所有动物的基本行为类型，包括觅食、位置移动和生殖等 [73] 。目 前海兔神经肽的研究进展相当一部分来自于对摄食行为调控机制的研究 [64,74] 。 海兔通过口器里的一个被称为齿舌(类似于舌头)的器官进行摄食，该器官能够牢牢咬住 食物，使食物可以被摄入食道或者从口器中吐出。海兔的摄食行为具有节律性，每个周期都由 齿舌的"伸长" (protraction)和"回缩" (retraction) ，以及齿舌的"打开"和"咬合"两组动 作组成。绝大部分摄食行为都是以齿舌伸长阶段开始，回缩阶段结束。摄食相关的行为主要有 两种类别：即摄入和反刍。在摄入和反刍行为中，齿舌伸长、回缩与齿舌打开、咬合这两组运 动的联合方式是不同的。在摄入行为里，齿舌在回缩时是咬合的，从而摄入食物；在反刍行为 里，齿舌在伸长时是咬合的，从而推出食物或不能吃的物体 [75,76] (图 3 B)。…”