I am part of the team that really believes that there is so many interesting molecules and secrets hidden in the fish brain that could actually be part of the puzzle about neuronal regeneration.
If you google for fish brain anatomy you will find lot of figures like this one:
We studied different fish species and all of them, based on their ecological niche have anatomically different brain. For example, if they use scent more their olfactory lobes are more dominant (catfish).
Fish can regenerate their nerve cells 10 times more than mammals and also they produce new neurons constantly, including adult age also. We are not able to do that except in three regions SVZ, olfactory bulb and hippocamus.
This is one figure from our study where you can see BrdU (marker of adult neurogenesis) immunohistochemistry on carp brain
So, based on all of that interesting facts I really belive that secret (or at list part of it) is in the fish brain. :)