Extra-retinal Photoreceptors in Birds —A Morphological and Functional Consideration —

摘要

Brain photoreceptors and the pineal organ are considered as extra-retinal photoreceptors in birds. Deep-brain photoreceptors have not yet been identified morphologically, although physiological studies have accumulated evidence for the existence of aphotoreceptor in the avian brain for several decades. Several pioneering works concerning an extra-retinal photoreceptor in birds have been published in the last decade. First, in 1988, Silver et al. demonstrated that opsin-like immunoreac-tivity in theavian brain was present in cere-brospinal fluid (CSF)-contacting neurons of the lateral septum and infundibulum. Since then, a CSF-contacting neuron has emerged as a viable candidate for a deep-brain photo-receptor in birds. Then, in 1994, Okano et al. isolated a cDNA clone encoding an opsin-like molecule from a chicken pineal cDNA library, and showed that the coded protein with bound 11-cis-retinal exhibits blue-light sensitivity with maximal absorbance at -470nm. They named this protein "pinopsin" inreference to pineal opsin. Most recently, Wada et al. (1998) detected rhodopsin gene expression in the lateral septum of the pigeon. In this paper, I review especially the immunohisto- and cytochemical issues revelant to the localization of the brain photoreceptors in the avian brain and give an overview of the morphological bases for the avian extra-retinal photo-receptor. Before presenting morphological considerations, I describe briefly the physiological background of brain photoreceptors in birds.