[Median eminence in vitro superfusion membrane dynamics in a presynaptic compartment]

Gallardo and Ramirez developed in 1977 an in vitro superfusion procedure for rat hypothalamus; this procedure has been adapted for preserving tissues in order to make ultrastructural analysis. The median eminences were superfused with both, standard media and with media rich in potassium. After superfusion, the specimens were fixed either by immersion in chemical fixative, or quick-frozen utilizing the Cryoblock designed by Escaig, at 6 degrees K. A better preservation of granular vesicles and cytoskeleton is obtained by freezing. Both procedures support the existence of microvesicular depletion after potassium stimulation as well as the existence of two types of vacuoles (smooth and coated vacuoles) taking their origins in the plasmalemma. A double mechanism of retrieval of exocytosed membranes can be postulated in the median eminence axonal endings. Smooth vacuoles could be the way to retrieve microvesicles and complete a short exoendocytic cycle, while coated vacuoles could be intermediate elements to retrieve membranes belonging to secretory granules.