Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Ken Yeung

Abstract

Mass spectrometry imaging (MSI) has emerged as a promising technique in providing small molecule compositions and two-dimensional or three-dimensional ion images showing the spatial distributionsand relative abundances of small molecules in biological tissues. Among the ionization techniques compatible with MSI, matrix-assisted laser desorption ionization (MALDI) has been the most widely used ionization method. The careful selection of matrices in MALDI MSI allows the detection and imaging of specific compounds from complex biological samples. A major limitation to the detection of small molecules by MALDI-time-of-flight (TOF) MSI is the interference of matrix-derived peaks from commonly used organic matrices. Inorganic nanostructured materials such as silicon, carbon, metals, and metal oxides, have been explored as matrices to assist the laser desorption/ionization (LDI) mass spectrometry (MS) analysis of small moleculesbecause of their low background signals, but their applications for imaging of small molecules in biological tissues remain limited.

In this thesis, the application of zinc oxide (ZnO) nanoparticles(NP) for LDI MS imaging of small molecules was investigated as a superior alternative to TiO2NP,one of the known NP that can effectively assist LDI MS imaging of small molecules. Compared to TiO2NP, ZnO NP have been found to deliver similar or better results in the imaging of small molecules in the rodent brain tissues. In addition, due to its dissolution in acidic pH, the regular acid washes in the automatic sprayer, which is used to deposit ZnO NP dispersion, were effective in minimizing clogging and maintaining fairly high reproducibility. Moreover, imaging of rodent brain tissues with superior spatial resolution was achieved.

MALDI MSI was next applied to visualize the distributions and analyze the relative abundances of γ-aminobutyric acid (GABA) and glutamate, the primary inhibitory and excitatory neurotransmitters in the central nervous system (CNS), in various well-defined brain regions between the drug-treated and vehicle-treated rats. The effects of two drugs, nicotine and Δ-9-tetrahydrocannabinol (Δ-9-THC), were studied separately on the neurotransmitter levels in the rat brains. Chronic exposure to nicotine and Δ-9-THC during adolescence respectively, has been found to modulate the total GABA and glutamate levels in multiple brain regions in the adolescent rat brains in the short term and in the adult rat brains in the long term differently. The results acquired by MALDI MSI provide us with a more direct comparison between the preclinical studies from animal models and clinical studies using magnetic resonance spectroscopy(MRS).

Summary for Lay Audience

Molecular imaging techniques such as positron emission tomography (PET) and fluorescence imaging have been used to aid the clinical diagnosis of brain cancers for many years by reflecting the activities of patients’ brains at molecular levels. However, these techniques require radioactive and fluorescent agents to target specific compounds in the brain. The use of mass spectrometry imaging (MSI) has emerged as a promising technique in providing the molecular compositions and localization information of multiple compounds in post-mortem tissues without needing to target specific compounds.

Zinc oxide (ZnO) nanoparticles (NP) assisted the detection and imaging of metabolites and small neurotransmitters in rodent brain tissues using laser desorption ionization (LDI) MSI. This method was then used to assess the drug effects on neurotransmitter abundances in rat brains following drug treatments. Two drugs, nicotine, the cause of compulsive tobacco smoking, and Δ-9-tetrahydrocannabinol (Δ-9-THC, THC), the primary psychoactive component of marijuana, were studied separately.

LDI MSI assisted with ZnO NP was applied to examine the localizations and levels of γ-aminobutyric acid (GABA) and glutamate, the main inhibitory and excitatory neurotransmitters in the central nervous system (CNS). Sub-regions of the prefrontal cortex (PFC), nucleus accumbens (NAc), and hippocampus (HPC), which are related to executive functioning, reward processing, and memory, respectively, were studied to answer the functions of the specific sub-regions. Chronic exposure to nicotine and Δ-9-THC during adolescence respectively, has been shown to alter the total GABA and glutamate levels in multiple brain regions in both adolescent and adult rat brains,indicating chronic adolescent drug exposure affects both adolescent and adult rat brains differently.In addition, we are able to correlate some of the results from animal models to what has been found in clinical populations.

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