Neurosteroids and the songbird model system.

The brain is now widely recognized as having the capacity to make steroids, neurosteroidogenesis. Although many functions are known for steroids that might be made in the brain, the evolution of and natural biological functions for these neurosteroids are not fully understood. In songbirds, neurosteroids may function in the development of neural circuits controlling song and may also participate in the activation of some steroid-dependent behaviors during the non-breeding season. In addition to neuroanatomical and behavioral evidence, we have physiological, molecular, and biochemical evidence for the expression and activity of steroidogenic enzymes in the brains of developing and adult songbirds. We review the evidence published so far for songbird neurosteroidogenesis and discuss why we believe songbird species are excellent models for the study of brain steroid synthesis and action. (c) 2006 Wiley-Liss, Inc.