B. Silverin et al., Distribution of aromatase activity in the brain and peripheral tissues of passerine and nonpasserine avian species, GEN C ENDOC, 117(1), 2000, pp. 34-53
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.