Normal levels of histamine and Tele-Methylhistamine in mast cell-deficient mouse brain

To determine the contribution by mast cells to the brain content of histamine (HA) and its metabolite tele-methylhistamine (t-MH), the number of mast cells, as well as the levels of HA and t-MH were measured in brain regions of mast cell-deficient (W/Wv) and control (+/+) mice. In agreement with earlier studies, mast cells were identified in control mouse brains, whereas W/Wv brains were devoid of mast cells. Contrary to earlier studies, no differences between these strains were found in the HA levels of any brain region, implying that mouse brain mast cells do not contribute significantly to brain HA levels. Brain t-MH levels were also not different between strains, except in hypothalamus, where W/Wv levels were higher; a significantly smaller W/Wv hypothalamus accounted for this difference. It is not certain that such differences are due to the absence of mast cells, since the W/Wv mutant is pleiomorphic, and the biochemical nature of this mutation remains uncertain. However, the absence of mast cells and presence of HA in the W/Wv mouse brain is direct evidence for the existence of non-mast cell HA in the brain. These results also show that mouse brain t-MH levels are predictive of non-mast cell HA in brain.