Rhodopsins are the members of the family of G protein-coupled receptors (GPCRs). It is considered that rhodopsins have diverged from ligand-binding GPCRs. Vertebrate rhodopsins can bind the inverse-agonist (or antagonist) 11-cis-retinal but cannot bind the agonist all-trans-retinal. In this study, we found that unlike vertebrate rhodopsin, amphioxus rhodopsin can bind the agonist all-trans-retinal, showing that the amphioxus rhodopsin has characteristics of both photoreceptive rhodopsins and ligand-binding GPCRs. Mutational analyses revealed the molecular mechanism that reduces dark noise of visual cells during the molecular evolution of rhodopsins. A possibility of the amphioxus rhodopsin as a GPCR model was also discussed.

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The nematode Caenorhabditis elegans is one of ideal model organisms for neurogenetic and behavioral genetic researches, because the structure of its simple nervous system with 302 neurons is well-described and so much genetic information is available. The functions of sensory neurons and the molecular mechanisms of sensory transduction have been elucidated by genetic and behavioral studies. In this review, I will summarize the physiological and molecular basis for chemosensation and chemotactic response of the nematode. The nematode senses many environmental chemicals and integrates the sensory signals within the nervous system to generate chemotactic behavior. Two bilaterally symmetric amphids and phasmids, located on the head and tail of a nematode respectively, are known for chemosensory organs. I will also describe the behavioral plasticity such as associative learning and the regulation mechanism of population density on the chemotaxis.

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Among avian species that communicate by means of vocalization, vocal learning has been found to occur in three orders of birds: oscine passeriformes (songbirds), psittaciformes (parrots), and trochiliformes (hummingbirds). Male songbirds learn species-specific songs by matching their own vocalization to a memorized tutor's song using auditory feedback during a sensitive period in juveniles. Recent reports including our studies indicate that auditory feedback is also necessary for maintaining the stability of adult song. However, much less is known about the role of auditory feedback in adult since effects of auditory deprivation are different among species. In this review article, we first introduce ethological studies in which we compared altered song pattern induced by deafening among different species. We then present our data that show changes appearing in song control system in adult brain during deafening-induced song alteration, and what may induce these changes. In addition, we introduce recent topics on adult neurogenesis, which are also considered to relate with adult song plasticity. Based on these studies, we discuss the possible function of auditory feedback that regulates adult song maintenance and plasticity.

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Part 4. Rate of information transmission through a noisy channel and other related concepts are formulated as an extension of Shannonian information theory introduced in Part 3. Ergodicity of stochastic process and Gaussian probability distribution are also explained. Auto- and Cross-correlation functions and convolution integral are introduced with their Fourier transforms for elimination of effect of noise in system identification.

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