Gonadotropin-releasing hormone axons target the median eminence: in vitro evidence for diffusible chemoattractive signals from the mediobasal hypothalamus.

The projection of GnRH neurons to the median eminence of the medial basal hypothalamus (MBH) is established early in development and is also seen when preoptic area-derived GnRH cell-containing grafts are placed in the third ventricle of hypogonadal mice. To further study the factors directing GnRH axonal targeting, we cultivated embryonic or postnatal day 1 preoptic area with a coexplant on collagen- and laminin-coated membranes in insert chambers. After 7 days of culture, GnRH-immunoreactive fibers extended significantly farther and in greater number onto the sector of membrane facing a MBH coexplant than in the opposite sector, but not toward coexplants of control tissue. Moreover, such effects were specific, as outgrowth of a general axonal population, immunoreactive for growth-associated protein 43 was not influenced by the presence of the MBH. Preferential GnRH outgrowth toward the MBH was established early and was maintained during 10 days of culture. The importance of substrate-derived guidance was also assessed with confocal microscopy. GnRH axons consistently traveled in the company of growth-associated protein 43-labeled axons, but only erratic associations were seen between GnRH and glial processes extending on the membrane. We suggest that although employing an axonal substrate, GnRH axons follow a diffusible chemoattractive signal(s) secreted by the MBH.