Structure and function of brain and nerve cord-endocrine system in amphioxus

从形态学和分子生物学的角度研究了从无脊椎动物到脊椎动物的脑神经元和内分泌器官系统的进化过程。文昌鱼是最接近脊椎动物的无脊椎动物，被认为具有原始的脑和内分泌器官，如垂体、甲状腺、肾等。利用文昌鱼神经索EST分析了文昌鱼表达基因的序列。通过同源性检索，从7800个EST克隆中发现了内分泌基因。分别是RFamide、神经肽Y、降钙素等神经肽和P450scc、P450c17、17α、20β-HSD等甾体原酶。用原位杂交法检测了部分神经肽克隆在神经索中的定位。编码甾体生成酶的基因在神经索和神经元的神经细胞中表达。甾体原酶、3β-HSD和芳香化酶在EST无性系中均未发现，并通过免疫组化方法在卵巢中检测。孕酮和雌二醇-17β浓度在繁殖季节升高。研究了EST序列获得的视蛋白基因在神经索中的表达和吸收谱。光感受器在神经细胞中的存在被怀疑可能与光刺激诱导神经元的激活和信号传导有关。本研究表明文昌鱼的繁殖可能受到性类固醇的调节，并可能通过神经系统受到光的刺激。在进一步的研究中，性激素在文昌鱼神经索和性腺中的功能可能揭示神经类固醇在脊椎动物脑中的功能。

The evolution of the brain-neuron and endocrine organ system from invertebrate to vertebrate has been studied on morphological and molecular aspects in changes of their organs. Amphioxus is the closest invertebrate to vertebrate and considered to possess primitive brain and endocrine organs such as pituitary, thyroid and kidney and so on. Sequences of expressed genes in amphioxus were analyzed by EST conducted from amphioxus nerve cord. Endocrine genes were found from 7,800 EST clones by a homology search. They were neuropeptides including RFamide, neuropeptide Y, and calcitonin and so on, and steroidogenic enzymes including P450scc, P450c17 and 17α, 20β-HSD. Some neuropeptide clones were detected their localizations in nerve cord by in situ hybridization. Genes encoded steroidogenic enzymes were expressed in nerve cells in nerve cord and neurons. Steroidogenic enzymes, 3β-HSD and aromatase were not fond in EST clones, and detected in ovary by using immunohistochemistry. Concentrations of progesterone and estradiol-17β were measured by RIA and increased in breeding season. Opsin gene obtained from EST sequences was studied on the expression in nerve cord and absorbance spectrum. The existence of photoreceptor in nerve cells was suspected the possibility of relations of activation and signaling of neuron induced by photo-stimulation. This study showed the possibility that reproduction in amphioxus was regulated by sex steroids and might stimulate by light through nerve system. In further study, functions of sex steroids in nerve cord and gonad in amphioxus might reveal the neurosteroid function in vertebrate brain.