Mapping of LH-RH-containing projections to the mediobasal hypothalamus by differential deafferentation experiments

Luteinizing hormone-releasing hormone (LH-RH) was immunoassayed in several hypothalamic structures of male rats after complete, anterior, posterior or lateral deafferentation of that structure performed with a rotating knife, as well as after discrete frontal or sagittal transections placed with a glass knife in various parts of the mediobasal hypothalamus (MBH). Survival time in all cases was two weeks. Taken together, the results indicate that most fibers containing LH-RH and originating in the preoptic area of the hypo-thalamus (APO) take a lateral course upon leaving that structure and travel along the medial forebrain bundle (MFB). Along that tract, they proceed caudally over some distance and enter the median eminence from the side; fibers innervating the posterior median eminence bend back towards the midline at a more posterior level than those terminating in the zona externa. A limited amount of fibers also reaches the median eminence from a midsagittal location. In addition, a few medial arcuate-median eminence connections may account for the small proportion of MBH LH-RH spared by complete, anterolateral or lateral transections. Projections to the organum vasculosum laminae terminalis, whether proceeding from the APO in an independent manner or as collaterals of fibers terminating in the median eminence, are unaffected by any of the transections tested. A partly common organization pattern of fibers supplying the median eminence in several neuropeptides (for instance somatostatin, CRF, cholecystokinin or dynorphin) is suggested.