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Imaging Nicotine in Rat Brain Tissue Using Nanospray Desorption Electrospray Ionization Mass Spectrometry Ingela Lanekoff, Mathew Thomas, James P Carson, Jordan N Smith, Charles Timchalk, and Julia Laskin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac302308p • Publication Date (Web): 20 Dec 2012 Downloaded from http://pubs.acs.org on January 2, 2013
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Analytical Chemistry
Imaging Nicotine in Rat Brain Tissue Using Nanospray Desorption Electrospray Ionization Mass Spectrometry Ingela Lanekoff a)*, Mathew Thomasb), James P. Carson Timchalkb), Julia Laskina)* a)
b)
b)
, Jordan N. Smithb), Charles
Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, Richland, WA, 99352 Biological Sciences Division, Pacific Northwest National Laboratory, Richland, WA, 99352
Authors for correspondence: Ingela Lanekoff (
[email protected]) Julia Laskin (
[email protected]) Chemical and Materials Sciences Division Pacific Northwest National Laboratory PO Box 999, K8-88 Richland, WA 99352 USA
Keywords: imaging mass spectrometry, nanospray desorption electrospray ionization (nanoDESI), rat brain, nicotine, ion suppression, matrix effects, dentate gyrus
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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 using nanoDESI 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 using an atlas mesh deformed to match the optical image of the section. Atlasbased 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 nanoDESI imaging for sensitive detection of the drug in tissue sections with sub-femtomole sensitivity in each pixel of 27 µm x 150 µm area. Such sensitivity is necessary for spatiallyresolved detection of low abundant molecules in complex matrices.
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Analytical Chemistry
Introduction Nicotine, a highly addictive drug found in tobacco, affects the reward system by activating nicotinic acetylcholine receptors in the brain. This activation of the cholinergic system leads to a cascade of events within the nerve cell that results in the release of the neurotransmitter, dopamine. Increased amounts of dopamine in the brain convey feelings of well-being and pleasure, which may encourage further nicotine intake and lead to nicotine addiction.1-3 Even without toxic substances found in cigarette smoke, long-term nicotine exposure induces biochemical changes in the brain.4-6 Localization of nicotinic acetylcholine receptors previously has been studied using several techniques, such as micro-positron emission tomography (microPET),7 autoradiography,8 and immunolabeling.9 Although these techniques provide high-quality images, they rely on labeling the molecule of interest, and therefore enable visualization of only a single species at a time. In contrast, mass spectrometry imaging (MSI) does not require any labeling. As a result, spatial distribution of all ionizable compounds from a sample can be obtained in one experiment. In MSI, a mass spectrum is created from a specific location, a pixel, on the sample. Many pixels form an image, displaying the distribution of any chosen peak from the mass spectrum. Traditionally, several ionization techniques are used in MSI experiments for imaging lipids, drugs and metabolites in tissue samples.10-14 In matrix-assisted laser desorption ionization (MALDI),15 laser energy is used to ionize a matrix deposited on the sample. Then, the matrix ionizes sample molecules. MALDI is the MSI technique most widely used to image drugs in various animal tissue samples, such as whole body,16-19 lung,20-21 splenic tissue,22 kidney,23-24 liver,25-26 tumor,27 and brain.28-30 Secondary ion mass spectrometry (SIMS) uses a focused primary ion beam for sputtering molecules from the sample surface to generate secondary ions. SIMS produces images of endogenous substances in tissue with high spatial resolution but suffers from extensive fragmentation of the sputtered molecules.31-35 While both MALDI and SIMS are operated in vacuum, desorption electrospray ionization (DESI) is an ambient technique that has been extensively used for tissue imaging.36-37 DESI uses a mist of charged solvent droplets for desorption and ionization of molecules from a solid or liquid sample. DESI has been used to visualize distribution of lipids,38-41 metabolites,42 and drugs37,43-44 in tissue samples. Nanostructure initiator mass spectrometry (NIMS)45 uses nanostructured surface under the 3 ACS Paragon Plus Environment
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sample, which is then irradiated with a laser. NIMS has been used for imaging biomolecules and drugs in tissue sections.45-48 Laser ablation electrospray ionization (LAESI)49 is an ambient ionization technique that rasters a laser over the tissue sample for ablation of the analyte molecules, which are subsequently picked up by electrospray droplets and generate ions.50 Because no matrix is applied to the sample, LAESI enables simultaneous detection of metabolites and lipids in tissue sections.50-51 Other atmospheric pressure surface ionization techniques that enable localized analysis of tissue sections include liquid microjunction (LMJ) surface sampling44 and liquid extraction surface analysis (LESA).52-53 Nanospray desorption electrospray ionization, or nano-DESI,54 is an ambient surface ionization technique that has been recently used for tissue imaging.55-56 In nano-DESI, a primary capillary delivers a solvent to the solid sample, and the sample molecules are desorbed into the solvent. A secondary capillary, the nanospray capillary, then transfers dissolved analyte molecules to the mass spectrometer inlet, where they are ionized by nanoelectrospray. An image of the sample is created by moving the sample under the nano-DESI probe (i.e., the liquid bridge between the two capillaries) in several lines while continuously desorbing analyte molecules and acquiring mass spectra.55-56 We have recently described a new experimental platform that enables precise control of the distance between the nano-DESI probe and the sample for robust data acquisition over the course of several hours required for tissue imaging experiments.56 Furthermore, by coupling nano-DESI to an LTQ-Orbitrap mass spectrometer, spectra with high mass resolution (m/∆m = 60 000 at m/z 412) and high mass accuracy (
