Transduction of a non-photic cue: from the auditory system to a neuroendocrine response?

Decades of research have shown that exposure to social signals such as conspecific song enhances reproductive function in birds. How such signals are processed by the brain, and particularly how these signals are transduced into an endocrine response, is not well understood in songbirds. Here, we review evidence that in birds, social cues such as song induce in the brain a response that is in many ways similar to the response to photostimulation. Using the expression of immediate early genes to map neuronal responses to environmental cues, we and others have shown that the mediobasal hypothalamus (MBH) undergoes significant changes in patterns of neuronal activity in response to both photic and social cues. Thus, this brain region may participate in a final common pathway for the transduction of both kinds of cues into an endocrine response. Interestingly, female songbirds require prior photostimulation in order to respond either behaviorally or hormonally to hearing male song. We propose that this requirement involves the need for both the MBH and the auditory system to be primed with estrogen in order for auditory cues to initiate an endocrine response, and we present new evidence consistent with this hypothesis.