Hypothesis: a possible role for mast cells and their inflammatory mediators in the pathogenesis of autoimmune encephalomyelitis.

Mast cells and their potent chemical mediators are known to initiate and modulate a number of important inflammatory cascades. With respect to the central nervous system, the role of mast cells as participants in the promotion and resolution of inflammation has been widely underestimated. Mast cell-derived histamine, serotonin, kallikreins, and tumor necrosis factor-alpha (TNF-alpha) can enhance microvascular permeability, leukocyte rolling, adhesion, and extravasation of inflammatory cells into the brain and spinal cord. Mast cell mediators may play an important role in autoimmune encephalomyelitis and multiple sclerosis by promoting the entry of autoreactive T cells and the recruitment of nonspecific monocytes across the blood:brain barrier.

Mast cell-derived histamine and tumour necrosis factor: differences between SJL/J and BALB/c inbred strains of mice.

Mast cells cultured from bone marrow of BALB/c and SJL/J inbred strains of mice using IL-3 showed distinct patterns of growth and marked differences in their content of TNF-alpha and histamine. Mast cells derived from SJL/J mice grew and matured at a faster rate than those from BALB/c bone marrow. SJL/J mast cells were found to contain more than twice the amount of histamine and TNF-alpha in their granules than BALB/c-derived cells. In addition, when triggered by anti-DNP IgE antibody and specific antigen (DNP-albumin), mast cells derived from SJL/J mice released more histamine and TNF-alpha than mast cells derived from BALB/c mice. These results confirm previous observations regarding a genetic basis for mouse strain differences in mast cell growth rates, and extend previous observations to document differences in mast cell mediator contents. These results are consistent with the concept that genetically controlled differences in the numbers of central nervous system (CNS)-associated mast cells and their vasogenic mediators may play an important role in modulating oedema and inflammation in CNS trauma and diseases in mice.

Changes in brain and spinal cord water content during recurrent experimental autoimmune encephalomyelitis in female Lewis rats.

Regional changes in percent water content, a measure of regional levels of edema, were determined in female Lewis rats during key stages of recurrent experimental autoimmune encephalomyelitis (rEAE). The changes in percent water content of the spinal cord and brainstem closely paralleled the clinical and, to a lesser extent, histological course of rEAE (increasing during exacerbations and decreasing during remissions), whereas the percent water content of the forebrain, thalamus/midbrain, hypothalamus, and cerebellum remained constant and equal to control levels at all stages of the disease process. These results suggest that edema formation and resolution in the brainstem and spinal cord may be significant determinants of the transient and recurrent course of neurological dysfunction exhibited by rats with rEAE.

Histamine-induced microvascular leakage in pial venules: differences between the SJL/J and BALB/c inbred strains of mice.

The actions of histamine on pial venule leaky site formation were measured intravitally in two inbred strains of mice (BALB/c and SJL/J). Pial venules were visualized using a cranial window microscopy technique, and microvascular leaky site formation was assessed visually using a fluorescein-dextran indicator. SJL/J mice were found to be sensitive to histamine-induced leakage, whereas the BALB/c strain was refractory. Exposure to pertussis toxin enhanced the sensitivity to histamine in the SJL/J strain, but little effect was observed for BALB/c mice. However, the employment of a polymerase chain reaction (PCR) technique for the detection of mRNA for histamine H1 receptor identified receptor-specific message in isolated cerebrovascular endothelium from both strains of mice. The lack of pial responsiveness in the BALB/c mice remains unexplained. Mast cells in the dura mater were found to be more numerous in SJL/J mice than in BALB/c mice. This observation supports previous observations of strain-specific differences in CNS inflammation. The results support the concept that genetically controlled differences in vascular sensitivity and localization of CNS-associated mast cells may play important roles in the generation of vasogenic edema and inflammation in CNS trauma and disease.

Changes in ocular mast cell numbers and histamine distribution during experimental autoimmune uveitis.

Choroidal mast cells have been implicated in experimental autoimmune uveitis (EAU), an ocular inflammatory disease induced by S-antigen. Our data confirm that choroidal mast cell numbers decrease with clinical onset of S-antigen-induced EAU in Lewis rats, and establish that the decrease is statistically significant. In addition, we find that the numbers of limbal mast cells also decrease during S-antigen-induced EAU, and that this decrease occurs earlier in the course of the disease than that observed for choroidal mast cells. Activation and degranulation of mast cells, as evidenced by decreases in mast cell number, result in the synthesis and/or release of large quantities of mast cell mediators, such as histamine. Histamine levels in EAU were found to change significantly, decreasing in the anterior portion of the eye and increasing in the choroid and retina, in concert with changes in mast cell number over the course of EAU. Mast cell mediators may actively contribute to the pathogenesis of EAU through direct enhancement of the inflammation, by stimulation of other elements of the immune system, and/or through facilitation of the blood-retinal barrier breakdown that occurs in EAU. Overall, these results add to the evidence for a mast cell role in EAU, and, in addition, show that the mast cell involvement in EAU includes the mast cells of the limbus.

Attenuation of experimental autoimmune encephalomyelitis by Compound 48/80 in Lewis rats.

The appearance of increased levels of histamine in the central nervous system (CNS) concomitant with the development of clinically significant acute experimental autoimmune encephalomyelitis (EAE) in male Lewis rats suggests that CNS-associated mast cells may mediate acute EAE in Lewis rats. We now report that, compared to controls, rats with acute EAE exhibit fewer detectable mast cells in their dura mater and velum interpositum. In addition, intracisternal, but not intraperitoneal administration of Compound 48/80 just prior to the appearance of clinical signs of acute or recurrent EAE in male and female rats, respectively, significantly attenuates the clinical severity of both forms of EAE. These results further support the hypothesis that CNS-associated, but not peripheral mast cells are mediators or modulators of acute and recurrent EAE in Lewis rats.

Brain and spinal cord levels of histamine in Lewis rats with acute experimental autoimmune encephalomyelitis.

Acute experimental autoimmune encephalomyelitis (EAE) was induced in Lewis rats by inoculation with guinea pig spinal cord homogenate emulsified with Mycobacterium tuberculosis-enriched complete Freund's adjuvant (CFA). Control rats were inoculated with CFA alone. Control and EAE rats were killed on days 7, 9, 11, and 13 postinoculation, and regional brain and spinal cord levels of histamine were determined. No regional differences in histamine content between control and EAE rats were seen on day 7 or 9 postinoculation. However, depending on the region, EAE rats exhibited significantly higher levels of histamine in their CNS on day 11 or 13 postinoculation or on both. Thus, regionally and temporally specific increases in brain and spinal cord levels of histamine develop concomitant with or just after the appearance (on day 10 postinoculation) of clinical signs of acute EAE, a finding suggesting that histamine may be involved in the development or expression of acute EAE in Lewis rats.

An assessment of mast-cell deficient mice (W/Wv) as a model system to study the role of histamine in implantation and deciduoma formation.

The ovaries from mast cell-normal (+/+) and mast cell-deficient (W/Wv) mice were examined with light and electron microscopy. In addition the effect of ovariectomy and subsequent steroid treatment on total uterine histamine content, total mast cell numbers and surface and glandular epithelial cell heights was measured. The ovaries of +/+ mice were normal, displaying various stages of follicular growth and atresia and numerous corpora lutea; the ovaries of W/Wv mice lacked follicles and corpora lutea but contained numerous hyperplastic interstitial cells which contained numerous lipid droplets, vesiculated mitochondria and abundant endoplasmic reticulum suggestive of steroid synthesis. Steroid treatment of ovariectomized +/+ and W/Wv mice caused a significant increase in uterine wet weight and endometrial surface and glandular epithelial cell heights. In +/+ mice, steroid treatment caused a concomitant increase in total mast cells per uterine horn while mast cells were totally absent in W/Wv mice. The increase in uterine histamine in +/+ mice is consistent with the increase in mast cell numbers. Measurable amounts of uterine histamine, which increases slightly after steroid treatment, were demonstrated in W/Wv mice. Since the uteri of +/+ and W/Wv mice respond to steroids in a similar manner with the sole exception being histamine content and mast cell numbers, our results demonstrate the potential of using these animals to investigate the role(s) of uterine mast cells and non-mast cell uterine histamine in the process of implantation and the formation of a decidual cell response.

Dural mast cells: source of contaminating histamine in analyses of mouse brain histamine levels.

Histamine levels were determined in mouse brains from WBB6F1-+/+ (mast cell normal) and WBB6F1-W/Wv (mast cell-deficient) mice whose brains were dissected immediately after decapitation or after freezing the severed heads in liquid nitrogen for 10 s. In WBB6F1-+/+ mice, brains obtained from frozen heads contained significantly higher levels of histamine than those obtained from unfrozen heads. The converse was found in brains obtained from the WBB6F1-W/Wv mice. When CF-1 mice (which also contain brain-associated mast cells) were treated as described above, results very similar to those found with the WBB6F1-+/+ mice were obtained. Further, the high levels of histamine found in CF-1 mice whose brains had been frozen in situ were accompanied by an extensive degranulation of mast cells in the dura mater of these mice. Because of this degranulation of mast cells, and the fact that increased levels of brain histamine were not found in mast cell-deficient mice, it is concluded that dural mast cells are the likely source of the artifactually higher levels of histamine seen in brains frozen in situ.

The significance of mast cells as a source of histamine in the mouse brain.

Knowledge of the relative contributions of mast cells and neurons to the overall pool of histamine in the brain is a prerequisite to determining the significance and role of this amine in brain function. Consequently, we analyzed the levels of brain histamine in four genotypes (+/+, W/+, Wv/+, and W/Wv) of WBB6F1 mice, whose numbers of brain-associated mast cells vary in a genotypically specific manner. Although mast cell numbers ranged from a total absence of mast cells (W/Wv) to an average of about 500 mast cells/brain (W/+), no significant differences between genotypes were found in the quantities of histamine in whole brains, brain regions, or crude subcellular fractions. Thus, in this strain of mice, mast cells are not a significant source of histamine in the brain. This suggests that most of the histamine is of neuronal origin. Since neuronal histamine levels are maintained only by continued histidine decarboxylase activity, complete inhibition of this enzyme by alpha-fluoromethylhistidine, a "suicide" inhibitor of histidine decarboxylase, would totally deplete W/Wv mice of brain histamine. This was not found to occur in the W/Wv mice, suggesting that neuronal stores of histamine can be maintained in the absence of histidine decarboxylase, or that an additional nonneuronal, non-mast cell source of histamine exists in the W/Wv mouse brain.

The effects of castration on histamine levels and 24-hour rhythm in the male rat hypothalamus.

Holtzman male rats were castrated or sham-operated at 22 days of age and raised in a 12 h light: 12 h dark illumination cycle. At age 63 days they were sacrificed by decapitation at six different times during the light:dark (L:D) cycle, and hypothalamic histamine (H) concentrations were measured using a modified single isotope-enzyme microassay. Hypothalamic H was significantly elevated in the castrated rats at all but two of the six times sampled during the L:D cycle, when compared with the sham-operated controls. Both surgical groups had similar 24-h rhythms of hypothalamic H concentrations, with the peak concentration occurring during the light phase followed by a rapid drop to the minimum 2-3 h later. However, the castrated rats appeared to attain both maximal and minimal concentrations somewhat earlier in the day. These results provide circumstantial evidence that hypothalamic H may have a role in the hypothalamo-hypophyseal-gonadal axis in the male rat.