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

Abstract

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.

Surface analysis of lipids by mass spectrometry: More than just imaging

“…Furthermore, it is possible that liquid extraction and spray-based approaches will find success for relative quantitation due to the ease in which internal standards may be added to the extraction or spray solvent [276], although to our knowledge this has yet to be reported for lipid analysis. The lack of current quantitative approaches perhaps represents the greatest challenge currently facing direct surface analysis methods if they are to find more widespread incorporation into lipidomic investigations …”

High Resolution Tissue Imaging Using the Single-probe Mass Spectrometry under Ambient Conditions

“…MSI involves the gathering of MS spectra in a spatially defined manner for a particular sample of interest such that the distribution of a large number of biochemicals such as metabolites and proteins can be obtained within a single experiment. The potential of MSI to improve analysis of biological samples has already been demonstrated in many previous studies, such as the ability to image metabolite distributions in mouse brain sections [2], determine tissue type in tumors [3], to image protein localizations in formalin fixed tissues [4], biomarker discovery [5], and many others.…”

Direct Analysis Mass Spectrometry

“…Surface imaging is a major area of application of DESI. The use of differential mobility was applied to image biological tissue in sea algae and rat brain tissue [199]; flat thin layer chromatographic imprints of leaves and petals were imaged using DESI to provide knowledge of varietal differences, toxic metabolite production, and pest/pathogen attack [200]; mouse uterine sections of implantation sites on day 6 of pregnancy were analyzed by imaging without sample pretreatment [201] and nicotine has been imaged in rat brain tissue following in vivo drug administration [202].…”