Abstract
In order to determine the effect of gravity perception on brain activity, we immunohistochemically studied the density of c-Fos protein-positive cells in various brain regions of adult wild-type and ha mutant medaka fish. Some ha fish form utricular otoliths (lapilli) in later growing stages and are designated ha(3-3), whereas others continue to lack lapilli and are designated ha(2-2). In most regions, wild-type fish expressed a higher density of c-Fos-positive cells than ha(3-3) and ha(2-2) mutant fish following exposure to hypergravity (3 G) for 3 h. Prominent differences between wild-type and ha mutants were quantitatively demonstrated in the telencephalon, fasciculus longitudinalis medialis, and posterior octavus nucleus with the caudal nucleus. An exception was the stratum periventriculare region, which showed the highest density in ha(2-2) fish and the lowest in wild-types. The results showed that the lapillus is the otolith mainly responsible for the gravity perception of medaka and that the ha mutant can serve as a model for fish born and raised in microgravity. The use of ha fish can help us to understand what changes occur in the gravity-sensing system of medaka fish born and raised for life cycle experiments aboard the International Space Station (ISS).
