Research reportMagnetic resonance spectroscopic analysis of neurometabolite changes in the developing rat brain at 7 T
Introduction
Currently, developmental neurobiological research is often conducted using traditional ex vivo methods such as histopathological or biochemical evaluations (Corbett, 1990) often in combination with functional assessments. However, with the rapid development of non-invasive in vivo imaging/spectroscopy techniques (Tkac et al., 2003, Huppi, 2001), there is an opportunity to optimize the efficiency and use of animals in developmental research including developmental toxicology using such approaches longitudinally (Pogge and Slikker, 2004). Magnetic resonance spectroscopy (MRS), in particular, offers a unique chance to monitor a number of important neurometabolites in predetermined areas of the brain non-invasively and with sufficient accuracy (Tkac et al., 2003, Larvaron et al., 2006, Mlynarik et al., 2008a, Tkac et al., 2004, Morgan et al., 2013, Kunz et al., 2011). Major advancements in magnetic resonance science have facilitated the acquisition of highly resolvable MRS signals (Mlynarik et al., 2008a, Tkac et al., 2004, Burri et al., 1990, Mlynarik et al., 2008b, Tkac et al., 1999). However, it remains to be seen if there are any differences in the metabolite concentrations and their changing trend in the developing animal brain due to the variations across species from different colonies, or differences in experimental approaches. Our study, apart from longitudinally evaluating the metabolite concentrations during development, also aims to examine the differences between two rat colonies: one in-house and the other from a commercially available source. Here, naïve rats from NCTR’s colony were assessed using 1H-MRS during early development beginning at the age of 2 weeks (PND 14) and continuing up to the age of 10 weeks (PND 70) to provide reference data for future neuro-developmental research.
Previous studies (Tkac et al., 2003, Hida et al., 1992) focused on the postnatal rat brain between the ages of 7 and 28 days, which roughly corresponds to early human brain development from 34 weeks of gestation to 2–3 years after birth, respectively (Dobbing, 1990, Clancy et al., 2007). Since a rat brain at birth is highly immature compared to the brain of the human newborn, neuronal differentiation and glial proliferation rapidly occur postnatally (Hida et al., 1992), which is also accompanied by extensive changes in neurometabolite profiles (Tkac et al., 2003, Larvaron et al., 2006, Terpstra et al., 2010, Ikonomidou et al., 2001, Hashimoto et al., 1995).
The current study reports developmental profiles for metabolite concentrations of two specific brain regions, the hippocampus (HC) and anterior cingulate cortex (ACC). In rodents, the hippocampus has been studied extensively as a part of a brain system responsible for learning, memory and other central nervous system processes (Jarrard, 1993). The ACC plays a role in a wide variety of autonomic functions, such as regulating blood pressure and heart rate, as well as in rational cognitive functions such as reward anticipation and decision-making (Holec et al., 2014, Burns and Wyss, 1985). Since these areas are selectively vulnerable to various neonatal insults (Nelson and Silverstein, 1994, de Deungria et al., 2000, Pfeuffer et al., 1999a, Olney, 2002), we believe that this study may also provide important data that will be useful for future research involving toxicant exposures during development. Our study also mathematically modeled the trends observed in changes of metabolite concentrations during development allowing for an estimate of the age at which the changes in each metabolite reach the plateau, suggesting the maturity for that particular system.
Section snippets
Results
The voxel selection and representative spectrum are shown in Fig. 1, Fig. 2. LCModel output showed signal to noise ratio (SNR) of 6─13 for ACC and 11─22 for HC, which is on the same order or better than cited by other researchers (Morgan et al., 2013). The Cramer-Rao lower bounds (CRLB) for Glu in the HC and ACC were approximately 3% and 10%, respectively. Ten metabolites were positively identified (with the CRLB <20%) in most of the spectra: creatine (Cr), phosphocreatine (PCr),
Discussion
One of the critical processes of neurodevelopment involves significant changes in glutamat-ergic neurotransmitter systems during synaptogenesis (Ikonomidou et al., 2001, Olney, 2002, Szczurowska and NMDA, 2013, Ben-Ari, 2006). This process is vulnerable to a variety of pharmacological and environmental interventions and its disruption may lead to immediate increases in apoptosis, excitotoxicity and long-term CNS impairment (Olney, 2002). Non-invasive MRS at high magnetic field strength presents
Animal handling
Animal handling and MRI/MRS procedures were approved by the National Center for Toxicological Research (NCTR) Institutional Animal Care and Use Committee. Ten male Sprague-Dawley rats from the NCTR breeding colony at postnatal day (PND) 14 were housed with their dams in plastic isolators with hardwood chip bedding until weaning on PND 21. Animals received food and water ad lib and were maintained under a 12 h/12 h light/dark cycle, with testing during the light phase. Each rat was scanned in the
Disclosure
This document has been reviewed in accordance with United States Food and Drug Administration (FDA) policy and approved for publication. Approval does not signify that the contents necessarily reflect the position or opinions of the FDA nor does mention of trade names or commercial products constitute endorsement or recommendation for use. The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the FDA.
Acknowledgements
This work was supported by the National Center for Toxicological Research (NCTR)/U.S. Food and Drug Administration (FDA), protocol P0731. We thank Merle Paule for substantial help in the project execution and preparation of this manuscript and Elena Liachenko for editorial help.
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