Abstract
Spatial mapping of small molecules, such as neurotransmitters, alongside lipids, can increase our understanding of biological functions of those molecules within the brain. Desorption Electrospray Ionization (DESI) is an ambient ionization technique that can spatially profile the distribution of molecules in research tissue samples. Here we present the utility of DESI imaging to simultaneously detect lipids and neurotransmitters directly in brain tissue samples. Rat brain was harvested and flash-frozen in liquid nitrogen before cryosectioning. Coronal tissue sections (8 microns thick) were mounted on regular glass microscope slides, vacuum dried, and analyzed without any further sample preparation. The DESI imaging platform coupled with a high definition mass spectrometer (HDMS) with ion mobility separation was employed to obtain ion intensities of small molecules and lipids over the entire tissue. DESI Imaging data were collected and processed on a high definition mass spectrometer with ion mobility separation. DESI acquisitions were performed using methanol and water as a DESI spray solvent. The ambient nature of DESI allowed for MS imaging without any matrix application or extensive sample preparation steps. Molecular maps were processed and overlaid with an optical image of the tissue to co-register the molecular distribution based on the anatomical features of the brain, such as the corpus callosum. Small molecules such as amino acids and neurotransmitters were simultaneously detected along with lipids. Molecular identification was aided using high mass accuracy database searches against LipidMaps and HMDB. In addition to the accurate mass and high-fidelity isotopic distribution, collisional cross sections (CCS) or drift time data obtained during ion mobility separation was used to improve confidence in detected molecules. Ion mobility measurements before the MS measurment increased the coverage and added selectivity which helped identification This preliminary work indicated the utility of DESI imaging for clearly distinguishing localized metabolites and lipids to provide insights for neuromolecular research.