Immunolocalization of aromatase- and androgen receptor-positive neurons in the goldfish brain.

Full expression of testosterone (T) actions in the brain requires both direct binding to androgen receptors (AR) and in situ aromatization to estradiol (E2). To determine the cellular basis of constitutively high aromatase and AR binding activities in teleost fish brain, and the neuroanatomic location and spatial relations of cells of each type, an immunocytochemical mapping study of goldfish (Carassius auratus) brain was carried out using antibodies to human placental aromatase and human/rat AR peptide and the avidin-biotin-peroxidase technique. Both antibodies specifically labeled cells that were neuronal in appearance and were most numerous in reproductive control centers: medial and ventral telencephalon (TEL) and preoptic and hypothalamic periventricular nuclei. Additional populations of aromatase- and AR-labeled cells were present in the olfactory bulbs, central telencephalon, and stratum periventriculare of the optic tectum. Anti-aromatase, but not anti-AR, labeled fiber tracts and fibrous layers in visual and auditory pathways, and perikarya and processes of premotor neurons known to integrate sensory input (reticulospinal neurons, Mauthner cells). Anti-AR selectively labeled lateral TEL regions, the nucleus ventromedialis thalami, and discrete cell clusters in the medial tegmental nucleus. Aromatase-immunoreactivity (-ir) was primarily cytoplasmic, whereas AR-ir was primarily nuclear, but relative intensity of nuclear vs cytoplasmic labeling with each antibody differed by brain region. Aromatase- and AR-ir cells were not obviously more numerous in goldfish brain than previously seen in birds and mammals, suggesting that enhanced expression occurs on a per cell basis. We conclude that T exerts its actions coordinately via direct and indirect pathways in most brain regions but independently via AR- or aromatase-mediated mechanisms in selected areas. These studies point to a wide role for androgen in modulating primary sensory signals as well as in classical reproductive processes.