Androgen metabolism in the juvenile oscine forebrain: A cross-species analysis at neural sites implicated in memory function

Juvenile songbirds are useful models for studying the neural bases of memor y. Memory-reliant behaviors demonstrated at this stage include song learnin g (most songbirds) and food caching (food-storing songbirds). Sex steroids are implicated in the modulation of memory processes in several vertebrates , The songbird forebrain expresses aromatase, 5 alpha-reductase and 5 beta- reductase, enzymes which convert testosterone to estradiol, 5 alpha-, and 5 beta-dihydrotestosterone, respectively. To explore the role of local andro gen metabolism on memory processes, we documented the activities of these e nzymes in the anterior neostriatum (NAN), caudomedial neostriatum (NCM), an d hippocampus (HP) of four species of juvenile songbird, two of which are f ood storers, Areas were dissected, homogenized, and provided with radiolabe led substrate; and formed estrogens, and 5 alpha- and 5 beta-reduced androg ens were measured, In the NAN, 5 beta-reductase was the predominant enzyme, suggesting that local inactivation of testosterone may preserve the sensit ive period of song acquisition. In the NCM, estrogens were formed in abunda nce despite high 5 beta-reductase, suggesting that locally high estrogen sy nthesis may play a role in processes of song perception, In the HP, both es trogens and 5 alpha reduced androgens were formed, suggesting that HP funct ion may be modulated by both estrogens and androgens, Finally, a derived me asure of steroid-differential reveals that food-storing songbirds differ fr om nonstorers in the steroidal milleiu within the HP, but not in the NAN or NCM. Thus, distinct loci within the juvenile songbird forebrain are expose d to different patterns of androgen metabolites. This local conversion may play a role in the neuroendocrine modulation of memory in these birds. (C) 1999 John Wiley gr Sons, Inc. J Neurobiol 40: 397-406, 1999.