Neuropeptides in the median eminence: their sources and destinations

By radioimmunoassay and immunocytochemical techniques, 14 neuropeptides have been measured and localized in the rat median eminence. Neuropeptides with inhibitory or stimulatory effects on the anterior pituitary hormones as well as posterior pituitary hormones are present in the median eminence in the highest concentrations of the central nervous system. All these peptides (LH-RH, TRH, somatostatin, CRF, vasopressin, oxytocin) are of preoptic or hypothalamic origin and they are transported to the median eminence by loop-like fiber systems through the lateral retrochiasmatic area. Within the median eminence, the pericapillary space constitutes the main common pathway. Three major transport routes--axons, vessels, liquor spaces--are separated from each others by only basement membranes, which allow free communications downwards to the pituitary but also backwards to the central nervous system.