The neuroendocrine system of annelids 1

In vertebrates the neuroendocrine system is based on chemical signaling between neural and endocrine structures. Final outcomes may be realized via chemical messengers traveling through circulatory conduits to their specific target sites. This process may rely, in part, on neurosecretion of the signaling molecules. The complexity of this system can be readily visualized when one considers the way in which interactions among classical neurotransmitters, cytokines, growth factors, and neuroendocrine hormones, in combination with autocrine and paracrine communication, can regulate cells and tissues. Apart from the neuroendocrine system there is also neuroimmune communication, consisting of reciprocal signaling between neuroendocrine and immune cells, which use the same molecules to coordinate their activity. Thus, our concept of the neuroendocrine system is constantly growing, despite its complexity, but it may be simply summarized as allowing bidirectional communication between neural and endocrine structures over distances greater than that achieved by synaptic communication. In the light of this, I demonstrate in this review that annelids, which are considered “simple” animals, also possess a neuroendocrine system. Résumé : Chez les vertébrés, le système endocrinien est basé sur l’échange de signaux chimiques entre les neurones et les structures endocriniennes. Le résultat final peut être obtenu via les messages chimiques envoyés des conduits circulatoires à des sites cibles spécifiques. Ce processus dépend en partie de la neurosécrétion des molécules signalisatrices. La complexité de ce système peut se visualiser facilement si on se rappelle de quelle façon les interactions entre les neurotransmetteurs classiques, le cytokines, les facteurs de croissance et les hormones neuroendocriniennes, avec les communications autocrine et paracrine, peuvent assurer le contrôle des cellules et des tissus. Outre le système endocrinien, il existe un système de communication neuroimmunitaire dans lequel il y a échange de signaux entre les cellules endocriniennes et les cellules immunitaires, qui utilisent les mêmes molécules pour coordonner leur activité. Par conséquent, notre conception du système endocrinien est constamment remise en question, vu sa complexité, mais peut se résumer simplement comme étant la résultante des communications bidirectionnelles entre les systèmes nerveux et endocrinien sur une distance plus grande que celle qui prévaut en communication synaptique. En tenant compte de cela, nous démontrons dans cette synthèse que les annélides, qui sont considérés comme des animaux simples, possèdent un système neuroendocrinien. [Traduit par la Rédaction] 191 Reviews / Synthèses Introduction During the last two decades the advent of immunocytochemical techniques has strongly influenced thinking concerning invertebrate endocrinology (De Loof and Schoofs 1990; Veelaert et al. 1998). With the use of antisera directed against various vertebrate neuropeptides, a large number of types of immunoreactivity have been demonstrated at the level of fibers and neurons. Most of these studies have been performed on Polychaeta (Dhainaut et al. 1985) and Achaeta (see below), and some maps of the complete nervous system have been constructed using such techniques (Al-Yousuf 1990; Verger-Bocquet et al. 1992; Fewou and Dhainaut-Courtois 1995). Improved chromatographic, immunological, and molecular genetic techniques have greatly facilitated the purification of annelid neuropeptides and the determination of their sequences (Table 1). Sequence comparisons between peptides isolated from annelids and vertebrates have shown the existence of neuropeptide families conserved during evolution, e.g., RFamide, opioids, the oxytocin/vasopressin peptides, and the renin–angiotensin system (RAS). However, some peptides specific to the phylum Annelida have also been discovered (Table 1), and in this review I present a proposed exhaustive list of characterized annelid neuropeptides and their biological functions, particularly osmoregulation, reproduction, and neuroimmune defense. Can. J. Zool. 79: 175–191 (2001) © 2001 NRC Canada 175 DOI: 10.1139/cjz-79-2-175 Received January 21, 2000. Accepted October 25, 2000. Published on the NRC Research Press Web site on February 9, 2001. M. Salzet. Laboratoire d’Endocrinologie des Annélides, Unité propre de la recherche et de l’enseignement associée n° 8017, Centre National de la Recherche Scientifique, SN3, Institut Fédératif de recherche INSERM, Université des Sciences et Technologies de Lille, 59650 Villeneuve d’Ascq, France (e-mail: michel.salzet@univ-lille1.fr). 1This review is one of a series dealing with aspects of the biology of the phylum Annelida. This series is one of several virtual symposia on the biology of neglected groups that will be published in the Journal from time to time. Biology of neglected groups: Annelida / Biologie des groupes négligés : Annelida J:\cjz\cjz79\cjz-02\Z00-195.vp Tuesday, February 06, 2001 12:01:21 PM Color profile: Disabled Composite Default screen