ELEVATED CONCENTRATIONS OF HISTAMINE IN THE RAT BRAIN

—Whole brain concentrations of histamine (Hm) in the rat were measured with a sensitive and specific single‐isotope enzymatic microassay and found to be elevated about 2‐fold in rats killed by focused microwave irradiation compared with rats killed by decapitation. However, regardless of the method of killing, whole brain Hm concentrations were elevated about 4‐fold when the brains were homogenized in 12.5 vol vs 2.5 vol of water or phosphate buffer. Homogenization of brain regions in 12.5 vol of water resulted in Hm concentrations that were about 10 times greater than previously reported values (except in the hypothalamus). Since (1) the‘extra’Hm could be extracted from pellets of low volume homogenates, and (2) synthesis of Hm is not likely to occur through an increase in the volume of water or buffer used for homogenization, we suggest that the elevated concentrations of Hm seen after high volume homogenization of brain tissue are due to a more complete extraction of Hm from brain tissue, and not to an artifactual production of this amine from its precursor (histidine) or some other compound(s).