Figure 2D. Luteinizing hormone-releasing hormone (LHRH) neuronal system in the rat. D) Neuroendocrine LHRH neurons; neurons projecting to the median eminence. Visualization of neurons containing the retrograde tracer Fast-blue 4 days after its administration in the femoral vein of a cycling female rat. Fast-blue (0.2%) was diluted in sterile saline solution and administered in the morning of proestrus. Four days later, animals were deeply anesthetized and rapidly perfused with 4% paraformaldehyde between 1730 and 1800 h of proestrus afternoon. Frozen brains were mounted on a microtome and were cut into 30-mm coronal slices. It is possible to see a clear example of two LHRH-immunoreactive neurons (panel D) that contain the retrograde tracer Fast-blue indicating that these neurons project to the infundibular system.