Loss of responses to odorants and pheromones in mPRγ (paqr5b)-knockout zebrafish

In our previous study, we generated a membrane-type progesterone receptor γ (paqr5b) knockout zebrafish line. Knocking out paqr5b by genome editing resulted in the loss of neurons in the olfactory rosette (OR). These findings indicated that Paqr5b plays an essential role in the formation of olfactory neurons. In this study, we investigated the extent to which paqr5b^-/- fish lacking olfactory neurons retain their sense of smell. We used a reported tank for zebrafish olfactory analysis with a dividing plate in the middle. The tank was divided into three zones: the right zone, where test substances were added; the neutral zone; and the left zone. The fish were released into the neutral zone at the beginning of each test, the chemical was added to the corner of the right zone, and a 3-min video was taken to track the movements of the fish. The video was then played back, and the time spent in the three zones was counted manually. Both male and female paqr5b^+/+ and paqr5b^-/- zebrafish were separately analyzed for time spent in the three zones after exposure to ATP, cadaverine, and the pheromone 17α,20β-dihydroxy-4-pregnen-3-one (DHP). Both male and female paqr5b^+/+ zebrafish were strongly attracted to ATP and stayed in the right zone for approximately 2 min (67%). In addition, paqr5b^+/+ zebrafish avoided cadaverine and stayed longer in the left zone than in any other zone. In contrast, paqr5b^-/- zebrafish stayed in all three zones for approximately the same amount of time, even after exposure to ATP and cadaverine. The paqr5b^+/+ fish were attracted to DHP and stayed in the right zone longer. Paqr5b^-/- zebrafish of either sex were not reactive to DHP exposure. These results showed that paqr5b^-/- zebrafish lacking olfactory neurons lost responsiveness to odorants and pheromones.