Department of Biological and Biomedical Sciences, Glasgow Caledonian University, Glasgow G4 0BA, UK.
J Exp Biol. 2010 Jun 15;213(Pt 12):2104-15. doi: 10.1242/jeb.039578.
The crinoid echinoderm Antedon mediterranea autotomises its arms at specialised skeletal joints known as syzygies that occur at regular intervals along the length of each arm. Detachment is achieved through the nervously mediated destabilisation of ligament fibres at a particular syzygy. The aim of this investigation was to identify neurotransmitters that are involved in the autotomy response. Physiological experiments were conducted on isolated preparations of syzygial joints, which can be induced to undergo autotomy-like fracture by applying stimulatory agents such as elevated K(+). Initial experiments with elevated K(+) showed that the autotomy threshold (the minimum amount of stimulation required to provoke autotomy) is lowest in syzygies at the arm base and rises distally. Of a range of neurotransmitter agonists tested, only l-glutamate invoked syzygial destabilisation, as did its analogues l-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate, but not l-(+)-2-amino-4-phosphonobutyrate (l-AP4) or N-methyl-d-aspartate (NMDA). The implication that l-glutamate stimulates syzygial fracture through AMPA/kainate-like receptors was supported by the finding that the action of l-glutamate was inhibited by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Acetylcholine depressed the response of syzygial preparations to l-glutamate, suggesting a possible mechanism by which the autotomy threshold could be varied constitutively and facultatively. An immunocytochemical method employing a polyclonal antibody against l-glutamate conjugated to glutaraldehyde revealed l-glutamate-like immunoreactivity in all components of the putative neural pathway controlling the autotomy reflex, including the epidermis, brachial nerve, syzygial nerves and cellular elements close to the syzygial ligaments. We conclude that it is highly probable that l-glutamate acts as an excitatory neurotransmitter in the activation of arm autotomy in A. mediterranea.
海百合腕足动物 Antedon mediterranea 会在其腕足上的特定骨骼关节（称为 syzygies）处自动断裂，这些关节沿着腕足的长度以固定间隔出现。通过在特定的 syzygy 处神经介导的不稳定化来实现分离。这项研究的目的是确定参与自动断裂反应的神经递质。在 syzygial 关节的分离标本上进行了生理实验，通过施加刺激剂（如升高的 K(+)）可以诱导这些标本发生类似于自动断裂的骨折。使用升高的 K(+) 的初始实验表明，自动断裂阈值（引起自动断裂所需的最小刺激量）在腕足基部的 syzygies 处最低，并向远端升高。在所测试的一系列神经递质激动剂中，只有 l-谷氨酸引起 syzygial 不稳定，其类似物 l-天冬氨酸、α-氨基-3-羟基-5-甲基-4-异恶唑丙酸（AMPA）和 kainate 也是如此，但 l-(+)-2-氨基-4-膦酸丁酸（l-AP4）或 N-甲基-d-天冬氨酸（NMDA）则不然。l-谷氨酸通过 AMPA/kainate 样受体刺激 syzygial 骨折的含义得到了支持，因为发现 l-谷氨酸的作用被 AMPA/kainate 受体拮抗剂 6-氰基-7-硝基喹喔啉-2,3-二酮（CNQX）抑制。乙酰胆碱抑制了 syzygial 标本对 l-谷氨酸的反应，这表明自动断裂阈值可以通过组成型和兼性方式发生变化的可能机制。使用针对与戊二醛结合的 l-谷氨酸的多克隆抗体的免疫细胞化学方法显示，控制自动断裂反射的假定神经通路的所有成分都具有 l-谷氨酸样免疫反应性，包括表皮、臂神经、syzygial 神经和靠近 syzygial 韧带的细胞成分。我们得出结论，l-谷氨酸极有可能在 A. mediterranea 的腕足自动断裂激活中充当兴奋性神经递质。
