Distribution of aromatase activity in the brain and peripheral tissues of passerine and nonpasserine avian species

Many behavioral effects of testosterone on hypothalamic and limbic brain ar eas are mediated by the action, at the cellular level, of estrogens derived from local testosterone aromatization. Aromatase activity and cells contai ning the aromatase protein and mRNA have accordingly been identified in the brain areas involved in the control of behavior. The presence of an unusua lly high level of aromatase activity has been detected in the telencephalon of one songbird species, the zebra finch (Taeniopygia guttata), and it is suspected that this high telencephalic aromatase may be a specific feature of songbirds but this idea is supported only by few experimental data. The distribution of aromatase activity in the brain of zebra finches and of one nonsongbird species, the Japanese quail (Coturnix japonica), was compared with the distribution of aromatase activity in the brain of four species of free-living European songbirds, the chaffinch (Fringilla coelebs, Fringill idae), willow warbler (Phylloscopus trochilus, Sylviidae), great tit (Pa;ru s major, Paridae), and pied flycatcher (Ficedula hypoleuca, Muscicapidae), High levels of enzyme activity were observed in the diencephalon of all spe cies. The high levels of aromatase activity that had been observed in the z ebra finch telencephalon and were thought to be typical of songbirds were a lso present in the four wild oscine species but not in quail. None of these songbird species had, however, a telencephalic aromatase activity as high as that in the zebra finch, which may represent an extreme as far as the ac tivity of this enzyme in the telencephalon is concerned. Measurable levels of aromatase activity were also detected in all songbird species in the liv er and in the three other brain areas that were assayed, the optic lobes, c erebellum, and brain stem, with the exception of the cerebellum in willow w arblers and quail, but no detectable activity was observed in the testes, m uscle, and adrenals of all species, Additional studies will be needed to id entify the Functional significance of estrogen synthesis in areas that are not classically known to be implicated in the control of reproduction. With in a given species, the birds that had the highest plasma testosterone leve ls also displayed the highest levels of diencephalic aromatase activity and the interspecies differences in the two variables were positively related. This raises the possibility that the absolute level of diencephalic aromat ase represents a species-specific characteristic under the control of plasm a testosterone levels. There was, in contrast, no correlation between the a romatase activity in the telencephalon and the plasma testosterone levels b ut the enzyme activity was-correlated with the plasma levels of luteinizing hormone. These data bring additional support to the idea that the dienceph alic and telencephalic aromatases are controlled by independent mechanisms. (C) 2000 Academic Press.