Topography and ultrastructure of LHRH- and somatostatin-containing axonal terminals in the median eminence of rats.

The topography and ultrastructure of LHRH and somatostatin nerve fibers in the median eminence in male rats were investigated with special reference to the relation between the fiber terminals and the perivascular space of the portal capillaries. Vibratome sections taken from the rostral and preinfundibular portions of the median eminence were immunostained with anti-LHRH or anti-somatostatin serum before plastic embedding for electron microscopy. In the external layer of the median eminence, the LHRH fibers did not directly contact the perivascular space, although they were in close proximity to the external surface of the median eminence. On the contrary, somatostatin fibers divided into branches in the external layer of the median eminence and terminated in direct contact with the perivascular space in the median eminence. LHRH fiber terminals contain immunoreactive granules, whereas somatostatin fibers are characterized by the presence of synaptic vesicle-like structures, vesicle ghosts and immunoreactive granules in their terminals. These topographical and ultrastructural differences might be regarded as a morphological manifestation of different mechanisms in the release of neurohormones.