Distribution and regulation of telencephalic aromatase expression in the zebra finch revealed with a specific antibody

In songbirds, aromatase (estrogen synthase) activity and mRNA are readily detectable in the brain. This neural aromatization presumably provides estrogen to steroid‐sensitive targets via autocrine, paracrine, and synaptic mechanisms. The location of immunoreactive protein, however, has been difficult to describe completely, particularly in distal dendrites, axons, and terminals of the forebrain. Here we describe the neuroanatomical distribution of aromatase in the zebra finch by using a novel antibody raised specifically against zebra finch aromatase. The distribution of aromatase‐positive somata in the zebra finch brain is in excellent agreement with previous reports. Additionally, this antibody reveals elaborate, spinous dendritic arbors, fine‐beaded axons, and punctate terminals of telencephalic neurons that may synthesize estrogen. Some of these axon‐like fibers extend into the high vocal center (HVC) and the robust nucleus of the archistriatum (RA) in males and females, suggesting a role for presynaptic aromatization in cellular processes within these loci. Adult males have more aromatase‐positive fibers in the caudomedial neostriatum (NCM) and the preoptic area (POA) compared to females, despite the lack of detectable sex differences in the number of immunoreactive somata at these loci. Thus, the compartmentalization of aromatase in dendrites and axons may serve a sexually dimorphic function in the songbird. Finally, in adult males, aromatase expression is down‐regulated by circulating estradiol in the hippocampus, but not in the NCM or POA. The distribution of aromatase suggests a role for aromatization in the regulation of pre‐ and postsynaptic function in steroid sensitive areas of the songbird forebrain. J. Comp. Neurol. 423:619–630, 2000. © 2000 Wiley‐Liss, Inc.