Identification of gonadotropin releasing hormone (GnRH) neurons projecting to the median eminence from third ventricular preoptic area grafts in hypogonadal mice.

Functional gonadotropin releasing hormone (GnRH) neurosecretory activity can be restored in genetically hypogonadal (hpg) adult mice with grafts of GnRH-containing fetal or neonatal septal-preoptic area (S/POA) tissue. Neurons implanted into the third ventricle of the host brain survive and send out axons which innervate one of the normal targets of these neurons, the median eminence (ME). Fibers terminate near primary portal vessels where GnRH is available for release into the vasculature, and this axonal outgrowth is essential for the stimulation of gonadotropin secretion, gonadal and accessory sex structure growth, gametogenesis, and fertility. Although it is known that GnRH axons reach their target, it is not known if all such neurons in a graft contribute to the projection. Taking advantage of the fact that axons in the ME, the sole host target, are outside the blood-brain barrier (BBB), long-term grafted animals were injected intraperitoneally with a retrograde tracer, Fluorogold (FG). Normal male mice were injected for comparison. Animals were sacrificed 5 days after injection and brain sections in the area of the graft were stained immunocytochemically for GnRH. In the normal male mice, two-thirds of the GnRH neurons were double-labeled with FG. In grafted individuals which showed increased gonadal growth, the percentage of labeled cells ranged from 17 to 75%. The results indicate that despite tissue injury, ectopic location, and a vastly reduced population, transplanted fetal GnRH neurons recapitulate a pattern seen in normal intact mice where some but not all neurons were capable of capturing a peripherally delivered tracer.