Abstract
As in osmoregulation, mineralocorticoid signaling is implicated in the control of brain-behavior actions. Nevertheless, the understanding of this role is limited, partly due to the mortality of mineralocorticoid receptor (MR)-knockout (KO) mice due to impaired Na+ reabsorption. In teleost fish, a distinct mineralocorticoid system has only been identified recently. Here, we generated a constitutive MR-KO medaka as the first adult-viable MR-KO animal, since MR expression is modest in osmoregulatory organs but high in the brain of adult medaka as for most teleosts. Hyper- and hypo-osmoregulation were normal in MR-KO medaka. When we studied the behavioral phenotypes based on the central MR localization, however, MR-KO medaka failed to track moving dots despite having an increase in acceleration of swimming. These findings reinforce previous results showing a minor role for mineralocorticoid signaling in fish osmoregulation, and provide the first convincing evidence that MR is required for normal locomotor activity in response to visual motion stimuli, but not for the recognition of these stimuli per se. We suggest that MR potentially integrates brain-behavioral and visual responses, which might be a conserved function of mineralocorticoid signaling through vertebrates. Importantly, this fish model allows for the possible identification of novel aspects of mineralocorticoid signaling.
Highlights
The mineralocorticoid system in fish may have biological actions in the brain and in behavior rather than in osmoregulation[8,12,13,14], in visuomotor performance, based on mineralocorticoid receptor (MR) localization found in medaka (Oryzias latipes) in the present study
Our results indicate that MR is required for behaviors affected by visual stimuli, but is not essential for osmoregulation in teleost fish
Studies using treatment with ligands, analysis of endogenous ligand-MR dynamics and knockdown of MR have led to somewhat contradictory results[8,18,22,23,24,25]
Summary
The mineralocorticoid system in fish may have biological actions in the brain and in behavior rather than in osmoregulation[8,12,13,14], in visuomotor performance, based on MR localization found in medaka (Oryzias latipes) in the present study. This fish is a useful model organism for studying these functions due to its tolerance to a wide range of salinities and several quantifiable behaviors. Our results indicate that MR is required for behaviors affected by visual stimuli, but is not essential for osmoregulation in teleost fish. These results may reveal a phylogenetically conserved link between mineralocorticoid signaling and behavior
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