To map the neural circuits involving GnRH neurons, the researchers used a genetic tracing method that they previously developed for charting neural pathways. They first engineered mice in which GnRH neurons produce barley lectin (BL), a tracer molecule that travels upstream and downstream to connected neurons, and green fluorescent protein (GFP), to mark the producing cells. By visualizing the locations of BL, GFP, and GnRH neurons and their axons, the researchers were able to identify neurons directly connected to GnRH neurons. They also determined which neurons sent signals to GnRH neurons and which neurons received signals from GnRH neurons.
These studies revealed that connections go in both directions between GnRH neurons and relay stations in the brain that process signals from both the olfactory and vomeronasal systems, said Buck.
In mice and other mammals, the olfactory and vomeronasal systems are distinct pathways for sensing chemicals in the environment. While the main olfactory system that begins in the nose processes odors, the vomeronasal (accessory) system receives signals -- triggered by pheromones -- from the vomeronasal organ (VNO) in the nasal septum. However, the systems are not entirely parallel. Buck and her colleagues have shown that the VNO can detect some odorants. And conversely, there is evidence that some pheromone signals require input from the nose in addition to the VNO.
"Our findings suggest that both odor and pheromone relay areas in the brain are sending pheromone signals to GnRH neurons. Moreover, GnRH neurons, in turn, are sending information back to those relay areas," said Buck. "This surprising finding suggests that the GnRH neurons are influencing the processing of odor and ph
Source:Howard Hughes Medical Institute