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

The vertebrate hippocampus (HP) plays a critical role in the organization of memory. Estrogens alter synaptic morphology and function in the mammalian HP and may potentiate memory performance. Previous studies suggest that the songbird HP itself is a site of significant aromatase expression, intimating that local estrogen synthesis may provide a source of this steroid to estrogen-sensitive neural circuits. To explore the potential role of local estrogen synthesis on HP structure and function, we have characterized aromatase message and activity in the zebra finch HP. Toward this end we have compared (a) HP aromatase mRNA with that at other neural loci, (b) HP aromatase activity between adults of both sexes, and (c) HP and hypothalamic preoptic area (HPOA) aromatase activity among songbirds and nonsongbirds. Finally we asked whether aromatase activity was intrinsic to the HP by maintaining it in culture, isolated from the rest of the telencephalon. The HP of every songbird studied expresses aromatase, with comparable levels across sexes. Notably, aromatase activity was found at higher levels in the songbird HP than in the HPOA. In both nonsongbird species investigated, however, HP aromatase activity was undetectable under identical assay conditions. Additionally, the developing songbird HP continues to express aromatase when cultured in isolation from the rest of the telencephalon. The data suggest that HP aromatase is characteristic of passeriformes and, as in the HPOA, may represent a mechanism whereby estrogen is made available to neural circuits. Passerines may prove invaluable animal models for investigations of the estrogenic modulation of HP structure and function.