The passerine hippocampus is a site of high aromatase: Inter- and intraspecies comparisons

The vertebrate hippocampus (HP) plays a critical role in the organization o f memory. Estrogens alter synaptic morphology and function in the mammalian HP and may potentiate memory performance. Previous studies suggest that th e songbird HP itself is a site of significant aromatase expression, intimat ing that local estrogen synthesis may provide a source of this steroid to e strogen-sensitive neural circuits. To explore the potential role of local e strogen synthesis on HP structure and function, we have characterized aroma tase message and activity in the zebra finch HP. Toward this end we have co mpared (a) HP aromatase mRNA with that at other neural loci, (b) HP aromata se activity between adults of both sexes, and (c) HP and hypothalamic preop tic area (HPOA) aromatase activity among songbirds and nonsongbirds. Finall y we asked whether aromatase activity was intrinsic to the HP by maintainin g it in culture, isolated from the rest of the telencephalon. The HP of eve ry songbird studied expresses aromatase, with comparable levels across sexe s. Notably, aromatase activity was found at higher levels in the songbird H P than in the HPOA. In both nonsongbird species investigated, however, HP a romatase activity was undetectable under identical assay conditions. Additi onally, the developing songbird HP continues to express aromatase when cult ured in isolation from the rest of the telencephalon. The data suggest that HP aromatase is characteristic of passeriformes and, as in the HPOA, may r epresent a mechanism whereby estrogen is made available to neural circuits. Passerines may prove invaluable animal models for investigations of the es trogenic modulation of HP structure and function. (C) 1998 Academic Press.