Environmental Modulation of Reproductive Function
Most living things use cues from the environment to time their reproductive efforts. For example, many seasonally breeding animals increase the secretion of reproductive hormones in response to lengthening days in the spring. Other species use different kinds of cues, such as social context or food availability. How such signals are processed by the brain, and particularly how these signals are transduced into an endocrine response, is not well understood. Using a marker for new protein synthesis (Egr-1), we can map the brain’s response to cues in white-throated sparrows in the lab. By studying a model organism that responds to light cues with increases in hormone release, we can see that after just one long day, the mediobasal hypothalamus, a region at the base of the brain where reproductive hormones are released into the bloodstream, is actively producing Egr-1 (see figure at right). The Egr-1 indicates that this brain region is actively engaging in new gene transcription; in other words, responding to the environmental signal.
One long day
Since reproductive development can be accelerated in females when they hear male song, we played recordings of song to females in the lab in order to map the brain response. We found that within minutes, hearing song induces Egr-1 expression in the mediobasal hypothalamus and also triggers release of reproductive hormones. Our results suggest that cells in the mediobasal hypothalamus are part of a final common pathway for both kinds of environmental cue (light and sound). The research helps us to understand how the brain uses information from the external environment to bring about adaptive changes in the organism.