Anatomical distribution and cellular basis for high levels of aromatase activity in the brain of teleost fish: Aromatase enzyme and mRNA expression identify glia as source

Although teleost fish have higher levels of brain aromatase activity than a ny other vertebrate group, its function remains speculative, and no study h as identified its cellular basis. A previous study determined aromatase act ivity in a vocal fish, the plainfin midshipman (Porichthys notatus), and fo und highest levels in the telencephalon and lower levels in the sonic hindb rain, which was dimorphic between and within (males) sexes. We have now loc alized aromatase-containing cells in the midshipman brain both by immunocyt ochemistry using teleost-specific aromatase antibodies and by in situ hybri dization using midshipman-specific aromatase probes. Aromatase-immunoreacti vity and mRNA hybridization signal are consistent with relative levels of a romatase activity in different brain regions: concentrated in the dimorphic sonic motor nucleus, in a band just beneath the periaqueductal gray in the midbrain, in ventricular regions in the hypothalamus, and highest levels i n the telencephalon especially in preoptic and ventricular areas. Surprisin gly, double-label immunofluorescence does not show aromatase-immunoreactive colocalization in neurons, but instead in radial glia throughout the brain . This is the first study to identify aromatase expression mostly, if not e ntirely, in glial cells under normal rather than brain injury-dependent con ditions. The abundance of aromatase in teleosts may represent an adaptation linked to continual neurogenesis that is known to occur throughout an indi vidual's lifetime among fishes. The localization of aromatase within the in tersexually and intrasexually dimorphic vocal-motor circuit further implies a function in the expression of alternative male reproductive phenotypes a nd, more generally, the development of natural, individual variation of spe cific brain nuclei.