Imaging Nicotine in Rat Brain Tissue by Use of Nanospray Desorption Electrospray Ionization Mass Spectrometry

Imaging mass spectrometry offers simultaneous spatially resolved detection of drugs, drug metabolites, and endogenous substances in a single experiment. This is important when evaluating effects of a drug on a complex organ system such as the brain, where there is a need to understand how regional drug distribution impacts function. Nanospray desorption electrospray ionization, nano-DESI, is a new ambient technique that enables spatially resolved analysis of a variety of samples without special sample pretreatment. This study introduces an experimental approach for accurate spatial mapping of drugs and metabolites in tissue sections by nano-DESI imaging. In this approach, an isotopically labeled standard is added to the nano-DESI solvent to compensate for matrix effects and ion suppression. The analyte image is obtained by normalizing the analyte signal to the signal of the standard in each pixel. We demonstrate that the presence of internal standard enables online quantification of analyte molecules extracted from tissue sections. Ion images are subsequently mapped to the anatomical brain regions in the analyzed section by use of an atlas mesh deformed to match the optical image of the section. Atlas-based registration accounts for the physical variability between animals, which is important for data interpretation. The new approach was used for mapping the distribution of nicotine in rat brain tissue sections following in vivo drug administration. We demonstrate the utility of nano-DESI imaging for sensitive detection of the drug in tissue sections with subfemtomole sensitivity in each pixel of a 27 μm × 150 μm area. Such sensitivity is necessary for spatially resolved detection of low-abundance molecules in complex matrices.