Localization of mRNAs for four membrane-bound guanylyl cyclases (membrane GCs; OIGC3, OIGC4, OIGC5, and OIGC-R2), three soluble guanylyl cyclase subunits (soluble GC; OIGCS-α1, OIGCS-α2, and OIGCS-β1), neuronal nitric oxide synthase (nNOS), and cGMP-dependent protein kinase I (cGK I) was examined in the embryonic and adult retinas of the medaka fish Oryzias latipes by in situ hybridization. All of the membrane GC mRNAs were detected in the photoreceptor cells of the adult and embryonic retinas, but in different parts; the OIGC3 and OIGC5 mRNAs were expressed in the proximal part and the OIGC4 and OIGC-R2 mRNAs were expressed in the outer nuclear layer. The mRNA for nNOS was expressed in a scattered fashion on the inner side of the inner nuclear layer in the adult and embryonic retinas. The mRNAs (OIGCS-α2 and OIGCS-β1) of two soluble GC subunits (α2 and β1) were expressed mainly in the inner nuclear layer and the ganglion cell layer of the embryonic retina while the mRNAs of the soluble GC α1 subunit and cGK I were not detected in either the adult or embryonic retina. These results suggest that NO itself and/or the cGMP generated by soluble GC (α2/β1 heterodimer) play a novel role in the neuronal signaling and neuronal development in the medaka fish embryonic retina in addition to the role played by phototransduction through membrane GCs in the adult and embryonic retinas.
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