DEFINITIVE CHARACTERIZATION OF RAT HYPOTHALAMIC MAST-CELLS

Mast cells have previously been identified in mammalian brain by histo chemistry and histamine fluorescence, particularly in the rat thalamus and hypothalamus. However, the nature of brain mast cells has continu ed to be questioned, especially because the electron microscopic appea rance often shows secretory granule morphology distinct from that of t ypical connective tissue mast cells. Here we report that mast cells in the rat hypothalamus, identified based on metachromatic staining with Toluidine Blue, fluoresced after staining with berberine sulfate, ind icating the presence of heparin. These cells were also positive immuno histochemically for histamine, as well as for rat mast cell protease I , an enzyme characteristically present in rat connective tissue mast c ells. In addition, these same cells showed a very strong signal with i n situ hybridization for immunoglobulin E binding protein messenger RN A. However, use of antibodies directed towards immunoglobulin E or its binding protein did not label any cells, which may mean either the bi nding protein is below the level of detection of the techniques used o r that it is not expressed except in pathological conditions when the blood-brain barrier becomes permeable. Al the ultrastructural level, p erivascular mast cells contained numerous, intact, electron-dense gran ules which were labeled by gold-labeled anti-rat mast cell protease I. These results clearly demonstrate the presence of perivascular mast c ells in the rat hypothalamus, where they may participate in homeostati c processes. Copyright (C) 1996 IBRO. Published by Elsevier Science Lt d.