The Use of Pharmacological-challenge fMRI in Pre-clinical Research: Application to the 5-HT System

Abstract

Pharmacological MRI (phMRI) is a new and promising method to study the effects of substances on brain function that can ultimately be used to unravel underlying neurobiological mechanisms behind drug action and neurotransmitter-related disorders, such as depression and ADHD. Like most of the imaging methods (PET, SPECT, CT) it represents a progress in the investigation of brain disorders and the related function of neurotransmitter pathways in a non-invasive way with respect of the overall neuronal connectivity. Moreover it also provides the ideal tool for translation to clinical investigations. MRI, while still behind in molecular imaging strategies compared to PET and SPECT, has the great advantage to have a high spatial resolution and no need for the injection of a contrast-agent or radio-labeled molecules, thereby avoiding the repetitive exposure to ionizing radiations. Functional MRI (fMRI) is extensively used in research and clinical setting, where it is generally combined with a psycho-motor task. phMRI is an adaptation of fMRI enabling the investigation of a specific neurotransmitter system, such as serotonin (5-HT), under physiological or pathological conditions following activation via administration of a specific challenging drug.The aim of the method described here is to assess brain 5-HT function in free-breathing animals. By challenging the 5-HT system while simultaneously acquiring functional MR images over time, the response of the brain to this challenge can be visualized. Several studies in animals have already demonstrated that drug-induced increases in extracellular levels of e.g. 5-HT (releasing agents, selective re-uptake blockers, etc) evoke region-specific changes in blood oxygenation level dependent (BOLD) MRI signals (signal due to a change of the oxygenated/deoxygenated hemoglobin levels occurring during brain activation through an increase of the blood supply to supply the oxygen and glucose to the demanding neurons) providing an index of neurotransmitter function. It has also been shown that these effects can be reversed by treatments that decrease 5-HT availability 16,13,18,7 . In adult rats, BOLD signal changes following acute SSRI administration have been described in several 5-HT related brain regions, i.e. cortical areas, hippocampus, hypothalamus and thalamus 9,16,15 . Stimulation of the 5-HT system and its response to this challenge can be thus used as a measure of its function in both animals and humans 2,11 . Video LinkThe video component of this article can be found at https://www.jove.com/video/3956/ Protocol Preparing animal for in vivo MRI imaging 1. Surgical Cannulation 1. Anesthetize the rat (male Wistar rat, 200-300 g) with isoflurane (5% induction and then reduced to 1.5-2% for maintenance of anesthesia during animal preparation and scanning) given in medical air (21% O 2 , BOC UK). Ensure that the animal is well anesthetized and exhibits no response to a toe pinch. The femoral artery and vein are cannulated for blood gas and blood pressure measurements and a...

Animal Functional Magnetic Resonance Imaging: Trends and Path Toward Standardization

“…Modulating the brain with pharmacological agents during animal fMRI has a wide variety of traditional applications such as studying the global effects of compounds and their target neurotransmitter systems (Mueggler et al, 2001;Shah et al, 2004;Ferrari et al, 2012;Razoux et al, 2013;van der Marel et al, 2013;Jonckers et al, 2015). This approach does not require surgical methods, and is apt for identifying global or regional changes in function associated with new or existing drug therapies for neurotransmitter-related brain disorders (Leslie and James, 2000;Martin and Sibson, 2008;Canese et al, 2011;Bifone and Gozzi, 2012;Klomp et al, 2012;Minzenberg, 2012;Medhi et al, 2014), or to map the effect of exogenously administered neuromodulators. In addition, pharmacological challenges can be used to probe how targets and neurotransmitter systems modulate BOLD responses evoked by other stimuli or pharmacological agents (Marota et al, 2000;Hess et al, 2007;Schwarz et al, 2007;Knabl et al, 2008;Rauch et al, 2008;Shih et al, 2012a;Squillace et al, 2014;Shah et al, 2016;Decot et al, 2017;Bruinsma et al, 2018;Griessner et al, 2018).…”

“…Ideally several doses of drug, and sufficiently long time series should be included in order to interpret the results according to dose-response and absorption/elimination functions (Leslie and James, 2000;Marota et al, 2000;Mueggler et al, 2001;Steward et al, 2005;Ferris et al, 2006;Rauch et al, 2008;Jenkins, 2012;Minzenberg, 2012;Jonckers et al, 2015;Shah et al, 2015;Bruinsma et al, 2018). Indeed, many pharmacological agents have known systemic effects which can influence animal physiology and the BOLD signal (Shah et al, 2004;Wang et al, 2006;Martin and Sibson, 2008;Ferrari et al, 2012;Klomp et al, 2012), and some drugs have direct effects on the vascular endothelium in the brain, which could alter properties of the hemodynamic response (Luo et al, 2003;Gozzi et al, 2007;Shih et al, 2012b). It is imperative to closely control and monitor animal physiology, and use appropriate doses in order to control for unwanted side effects.…”

The power of using functional fMRI on small rodents to study brain pharmacology and disease

“…Treatment of depression disorders consists of increasing the level of 5HT, for example with SSRI. The resulting acute activation of the 5-HT system can be picked up with phMRI ( Sekar et al, 2011a , b ; Klomp et al, 2012a ). Interestingly, phMRI studies show that the response to acute SSRI challenge changes in chronically SSRI treated adult animals ( Klomp et al, 2012b ).…”

Brainwide mapping of endogenous serotonergic transmission via chemogenetic-fMRI

“…The vast majority of these studies have primarily focused on the use of functional magnetic resonance imaging (fMRI) as a proxy for the neuronal activity directly elicited by pharmacological agents, or by mapping second-order modulatory effects on task-elicited fMRI responses (Anderson et al, 2008). This approach, termed pharmacological fMRI, has also been backtranslated to animal studies (Gozzi et al, 2012;Klomp et al, 2012). However, while useful in identifying possible brainwide substrates of neuromodulatory action, drug-based approaches may be contaminated by off-target receptor contributions (Ofek et al, 2012), and typically engage subtypespecific receptors systems that may not be representative of the overall neuromodulatory action of a transmitter systems.…”