In human retina, three kinds of cone opsins, which are sensitive to red, green, and blue, respectively, are expressed in separate cone cells. The difference of their spectral sensitivities generates color opponency in retinal ganglion cells and enables us to percept colors. Similar to retina, the ganglion cells in pineal-related organs from cyclostomes to reptiles are also known to exhibit color opponency, the neural firing inhibition and promotion by UV and visible light irradiation, respectively. We have found parapinopsin, a UV-sensitive pineal opsin, is a common molecule responsible for UV-sensitivity in pineal color opponency. Parapinopsin has bistability, the property of photo-interconvertibility between the UV-sensitive dark state and visible light sensitive photoproduct. This property is never seen in visual opsins, such as cone opsins. To understand a physiological significance of the bistable nature of parapinopsin, we addressed two-photon imaging using transgenic and parapinopsin-deficient zebrafish and found the bistable nature contributes to generating color opponency in the single photoreceptor cell. We further found the photoreceptor cell exhibits color opponency under white light, whose spectral distribution is similar to natural sunlight. These findings provide the new concept that color opponency can be generated with a single kind of bistable opsin without the calculation of outputs from multiple opsins.

Zebrafish is a good vertebrate model to study motor development. Zebrafish show touch response and auditory startle response at the onset of 1 day and 3 day of development, respectively. At 5 day, zebrafish also show the plasticity of auditory startle response, in which they cease to show sound-induced escape response after they repetitively hear white noise sound at subthreshold level (insufficient in loudness to evoke an escape response). When they hear the white noise, a protein kinase CaMKII is activated in the hindbrain Mauthner cells, which is the command neuron for the auditory startle response. Activated CaMKII induces phosphorylation of the scaffolding protein gephyrin at inhibitory synapse that in turn enhances synaptic accumulation of inhibitory GlyR proteins. Reinforcement of the inhibitory inputs into the Mauthner cells suppresses the Mauthner cell activation, thereby reducing the probability of auditory startle response. In general, fish is intrinsically sensitive to a splash sound, and the white noise that can reduce the escape response is identical to the sound of rain. We therefore assume that fish is endowed with a capacity to change splash (e.g. bird attack diving into water)-induced escape behavior for the adaptation to weather change.

Behavioral analysis plays an important role and is frequently performed in biological sciences. However, in the fields of comparative biology, researchers tend to perform manual video-recording and behavioral annotation, which are laborious and take much time. Unlike the situation of model species, it is often difficult to apply the automated behavioral recording/analysis systems for nonmodel animals, since they are exclusively designed for certain model species and/or behavior. In addition, manual operations can generate unintentional human operational errors, and it is difficult to keep the analysis in good quality. To solve the problem, we established an opensource behavioral recording system using Raspberry Pi, which automatically performs video-recording and systematic file-sorting, and the behavioral recording without unintentional human errors. We also developed an Excel macro for behavioral annotation, which enables us to easily perform behavioral annotation with simple manipulation. This tool enables us to perform the efficient and informative behavioral analysis compared with conventional manual annotation. Thus, we succeeded in developing an analysis suite that may help all researchers performing behavioral analyses using any species. By using this suite, we analyzed the sexual behavior of a laboratory and a wild medaka strain and found a difference in sexual motivation perhaps resulted from domestication.