| Research Abstract |
Our research results from 1997 to 1999 are summarized as follows.
(1) Although it has been presumed that the retina of the leptocephalus of the European eel possesses a homogenous pattern of rod-like photoreceptors, we found the cone-like photoreceptor cells in the retina of the eel leptocephalus collected from the Pacific Ocean close to the spawning area and also from the new born eel larvae cultivated in laboratory aquarium. The electron microscopic study also revealed that the retina of larval eel possesses regular cone-like photoreceptor cells at least during the early stage of their development, as same as the other pelagic fish (Omura et al. 1997a).
(2) The development of the inner ear, especially sensory maculae and otoliths was studied in the ayu plecoglossus altivelis and rainbow trout Oncorhychus mykiss, using light and electron microscopy. Consequently, it was suggested that fishes, while the initiation and oscillation of the daily increment formation may be under circadian en
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docrine control in the rainbow trout (Salem & Omura, 1998a, b).
(3) Immunolocalization of taurine was investigated in the pineal organ and retina of an anadromous fish Plecoglossus altivelis, in the developing retina of a lefteye flounder paralichthys olivaceus, and also between the dark- and light- adapted retinas of the glass eel young goldfish. Intense immunostaining was found in the outer plexiform layer, but not in the outer segments of rod cells. Consequently, it was suggested that the lack of immunoreactivity in the rod outer segment may depend on light stimulation (Omura et al. 1997b, 1999, Omura & Yoshida 1999, Omura & Inagaki 2000).
(4) Field potential responses to local electrical stimulation were analyzed in slices of goldfish optic tectum in vitro. Slowly developing potentiation in goldfish retino-tectal synapses is masked by prior stimulation, suggesting an involvement of glutamatergic field excitatory post-synaptic potential. An involvement of NMDA receptors and a possible involvement of adenosine were also examined (Ishikawa & Matsushima 1997, Ishikawa et al. 1997, 1999). Less
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