Morphological and functional demonstration of rat dura mater mast cell-neuron interactions in vitro and in vivo

Mast cells derive from a distinct bone marrow precursor and mature in tissues under the influence of stem cell factor, nerve growth factor (NGF) and certain interleukins. Intracranial mast cells first appear in the meninges and are located perivascularly close to neurons. They can be activated by antidromic stimulation of the trigeminal nerve, as well as by acute immobilization stress. Substance P (SP) and corticotropin-releasing hormone (CRH) are particularly potent in stimulating mast cell release of vasoactive, inflammatory and nociceptive molecules. These findings have suggested that mast cells may be involved in neuroinflammatory conditions, such as migraines. In this study, dura mast cells were shown to have characteristics of connective tissue mast cells (CTMC) as they contained histamine, heparin and rat mast cell protease I (RMCP-I). Mast cells were localized close to SP-positive neurons immunocytochemically and mast cell-neuron contacts were also documented using scanning electron microscopy. Dura stimulated by SP and carbachol in situ released histamine. Preincubation of dura with estradiol slightly augmented histamine release by SP, an effect possibly mediated through estrogen receptors identified on dura mast cells. Acute stress by immobilization led to dura mast cell degranulation which was prevented by pretreatment with a neutralizing antibody to CRH or a CRH receptor antagonist. The present results further clarify the biology of intracranial mast cells and support their involvement in the pathophysiology of migraines which are precipitated or worsened by stress.

Migration of mast cells in the developing rat brain

Mast cells are known to derive from the bone marrow and enter the tissues as immature cells which differentiate under local microenvironmental factors. However, it has not been known how and when these cells enter the brain; moreover, the localization of mast cells in the developing rat brain differs from that of the adult animal. Our anatomical and morphological observations showed that during late embryonic stages and the first 11 days after birth, rat brain mast cells were exclusively concentrated within the pia mater surrounding the diencephalon, the choroid fissure and within the choroid plexus. Histochemically these cells contained only a few toluidine blue metachromatic granules, suggesting a 'mucosal' phenotype and the absence of heparin. Later, during a transitional phase from postnatal day 11 to 13, these cells migrated along blood vessels of the fimbria, the hippocampus and the penetrating vessels of the thalamus into the dorsolateral and posterolateral thalamic nuclei. These cells contained more metachromatic granules, and from day 13 on, they assumed their adult perivascular localization within the thalamus with numerous metachromatic granules similar to those described for mature thalamic and serosal mast cells.

Histochemical and ultrastructural characteristics of rat brain perivascular mast cells stimulated with compound 48/80 and carbachol

Mast cells, known for their involvement in allergic reactions where they secrete numerous chemicals in response to immunoglobulin E and specific antigens, have recently been localized in the central nervous system. The function of these brain mast cells has remained speculative as they have not been the subject of any combined functional or detailed morphological studies. Here it is shown that these cells are primarily perivascular and stain metachromatically with Toluidine Blue, but red with Alcian Blue counterstained with Safranin, indicating that they contain proteoglycans quite similar to those of peritoneal, but not mucosal mast cells. Intracardiac administration of the classic mast cell secretagogue, compound 48/80, or the acetylcholine analog, carbachol, caused ultrastructural changes in brain mast cells consistent with secretion, but without exocytosis. However, it is known that the same concentration of carbachol has no effect on homogeneic peritoneal mast cells. These results indicate that brain mast cells share histochemical characteristics with serosal mast cells, but differ in their reactivity to secretagogues, and apparently in the mechanism of secretion. Their ability to respond to neurotransmitters and their distinct mechanism of secretion, which may be selective, expands their possible role in brain pathophysiology.

Serotonin release from rat brain mast cells in vitro

Mast cells are primarily localized in connective tissues, where they secrete numerous mediators. They have also been identified in the mammalian central nervous system on the basis of their histochemical and morphological properties, but their role there remains unknown. A perfusion system was used to investigate in vitro mediator release from rat brain mast cells. Compound 48/80, the classic mast cell secretagogue of connective tissue mast cells, induced dose-dependent and non-cytotoxic release of serotonin, histamine and beta-hexosaminidase from mast cells in the rat thalamus and hypothalamus, but not in the cerebellum which was used as a negative control. Detailed studies were performed on thalamic mast cells, which were identified on the basis of metachromasia with Toluidine Blue and Safranin-positive staining with the Alcian Blue/Safranin technique. Their secretion was characterized by: (a) parallel release of serotonin, histamine and beta-hexosaminidase; (b) lack of dependence on extracellular calcium; (c) susceptibility to inhibition by disodium cromoglycate; and (d) lack of lactate dehydrogenase release. These results indicate that the morphology and secretory characteristics of thalamic mast cells resemble those of connective tissue mast cells. The ability of brain mast cells to secrete their mediators is discussed in the context of their possible involvement in brain pathophysiology.

Endogenous regulation of rat brain mast cell serotonin release

Mast cells are involved in allergic reactions where they release numerous vasoactive and other mediators in response to IgE and antigen. They are also activated by neuropeptides and are found in close contact with neurons. Mast cell heterogeneity has now been documented for mucosal mast cells and connective tissue mast cells. Rat brain mast cells were studied in a perfusion system and were shown to release serotonin in response to the mast cell secretagogue compound 48/80 (C48/80). High-potassium neuronal depolarization also released serotonin, but this was calcium dependent, not associated with beta-hexosaminidase, and was unaffected by prior treatment with C48/80. Neuronal depolarization, however, was associated with somatostatin secretion and substantially reduced subsequent C48/80 stimulation, an effect abolished by neonatal treatment of the animals with capsaicin. Perfusion with somatostatin and substance P also induced brain mast cell serotonin release. C48/80 stimulation of combined thalamic and hypothalamic slices after neuronal depolarization substantially reduced the C48/80 effect, suggesting the possible presence of endogenous inhibitors released from the hypothalamus. Finally, the alpha 2-receptor agonist clonidine had a slight stimulatory effect. These results indicate that brain mast cell serotonin release may be regulated by endogenous neurotransmitters and/or neuromodulators.