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

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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.