Endogenous concentrations of biologically relevant metals in rat brain and cochlea determined by inductively coupled plasma mass spectrometry

Manganese (Mn), iron (Fe), copper (Cu), and zinc (Zn) are essential nutrients which aid in the proper functioning of cells, but high concentrations of these metals can be toxic to various organs. Little is known about the endogenous concentrations of these metals in the cochlea, the auditory portion of the inner ear which is extremely small and difficult to access. To fill this gap, a trace quantitative digestion and inductively coupled plasma mass spectrometry method was developed to determine the concentrations of these metals in the stria vascularis, organ of Corti, and spiral ganglion, three critically important parts of the cochlea (≤1.5 mg); these values were compared to those in specific brain regions (≤20 mg) of rats. Rats were sacrificed and the cochlea and brain regions were carefully isolated, digested, and analyzed to determine baseline concentrations of Mn, Fe, Cu, and Zn. In the cochlea, Mn, Fe, Cu, and Zn concentrations ranged from 3.2-6, 73-300, non-detect, and 13-200 μg/g respectively. In the brain, Mn, Fe, Cu, and Zn concentrations ranged from 1.3-2.72, 21-120, 5.0-10.6, and 33-47 μg/g respectively. Significant differences (p < 0.05) were observed between the tissue types within the cochlea, and between the cochlea and brain. This validated method provides the first quantitative assessment of these metals in the three key subdivisions of the cochlea compared to the levels in the brain; Mn, Fe, and Zn levels were considerably higher in the cochlea than brain.