Bicuculline-Induced Brain Activation in Mice Detected by Functional Magnetic Resonance Imaging Thomas Mueggler, Diana Baumann, Martin Rausch, and Markus Rudin * Dynamic measurements of local changes in relative cerebral blood volume (CBV rel ) during a pharmacological stimulation paradigm were performed in mice. Using magnetite nanopar- ticles as an intravascular contrast agent, high-resolution CBV rel maps were obtained. Intravenous administration of the GABA A antagonist bicuculline prompted increases in local CBV rel as assessed by MRI with a high spatial resolution of 0.2  0.2 mm 2 and a temporal resolution of 21 s. Signal changes occurred 20 –30 s after the onset of drug infusion in the somatosensory and motor cortex, followed by other cortical and subcortical structures. The magnitudes of the CBV rel increases were 18%  4%, 46%  14%, and 67%  7%, as compared to prestimulation values for the cortex, and 9%  3%, 25%  4%, and 36%  7% for the caudate putamen for bicuculline doses of 0.6, 1.25, and 1.5 mg/kg, respectively. On-line monitoring of trans- cutaneous carbon dioxide tension PtcCO 2 reflecting arte- rial PaCO 2 did not show any alteration during the stimulation paradigm. One of five of the mice receiving the highest bicu- culline dose, and three of seven receiving the intermediate dose displayed a different cortical response pattern. After a CBV rel increase of 40% lasting for approximately 1 min, significant CBV rel reductions by 80% have been observed. Subcortical structures did not display this behavior. The present study sug- gests that this noninvasive approach of functional MRI (fMRI) can be applied to study drug-induced brain activation by central nervous system (CNS) drugs in mice under normal and patho- logical situations. Magn Reson Med 46:292–298, 2001. © 2001 Wiley-Liss, Inc. Key words: functional magnetic resonance imaging; relative cerebral blood volume; bicuculline; pharmacological stimula- tion; mouse; iron-oxide nanoparticles; transcutaneous blood gas monitoring Genetic engineering and transgenic technology have led to advances in the generation of animal models that develop aspects of various brain pathologies, and have emphasized the need to develop accurate methods when phenotyping these animals. Such models would benefit from the devel- opment of noninvasive methods to study disease progres- sion while allowing for a correlation with behavioral stud- ies. The purpose of the present study was to assess the feasibility of functional magnetic resonance imaging (fMRI) in mice by monitoring changes in local relative cerebral blood volume (CBV rel ) during pharmacological stimulation under controlled physiological conditions, and hence to assess the potential of pharmacological fMRI as a noninvasive method to characterize transgenic and knock-out mice models. fMRI has proven its utility for the noninvasive mapping of brain function with high temporal and spatial resolu- tion. Depending on the method applied, image contrast in fMRI is determined either by the oxygenation state of hemoglobin (BOLD) (1) or by changes in cerebral hemody- namics, i.e., cerebral blood flow (CBF) (2,3) and CBV (4). Using superparamagnetic iron-oxide nanoparticles as a blood-pool contrast agent, a local CBV rel increase is re- flected by an increase in the amount of magnetite nano- particles in the activated tissue and, correspondingly, by a decrease in the signal intensities in T 2 -weighted images. High-resolution CBV rel maps can be derived from the sig- nal attenuation once the tracer has reached a steady-state blood concentration. Applying this CBV rel -fMRI method, activation in the rat sensory cortex caused by electrical stimulation of the forepaw was demonstrated (5,6). Hemo- dynamic changes associated with a pharmacological stim- ulation paradigm have been demonstrated in rats, and could be used to study drugs at the specific receptor level. Systemic infusion of the GABA A antagonist bicuculline led to an alteration in neuronal activity in cortical and subcortical brain structures. Dose-dependent CBV rel in- creases have been measured in various brain structures (e.g., the cortex, thalamus, and caudate putamen) in the rat (7). It is well known that physiological parameters, such as body temperature and blood gases, will affect CBV values. Mandeville et al. (6) reported CBV rel increases on the order of 50% in rats during hypercarbia by increasing arterial CO 2 tension (PaCO 2 ) from 25–55 mmHg. Although reports on the quantitative relationship between changes in CBV rel and PaCO 2 differ considerably, it is obvious that control and continuous monitoring of blood CO 2 and blood oxy- genation are critical for detecting small alterations of ce- rebrovascular parameters in the mouse. Although larger animals can sustain frequent blood samplings, noninva- sive monitoring of blood gases has been found to be essen- tial when studying mice in order to minimize blood loss, which might affect cerebral hemodynamics. Transcutane- ous blood gas monitoring is a noninvasive method used clinically (8 –10), and it has also been applied to MRI measurement of the rat to monitor alterations in transcu- taneous CO 2 tension (PtcCO 2 ) under different respiration conditions and hypercarbia (11). The purpose of this study was to demonstrate the feasibility of fMRI in anesthetized, ventilated mice. Pharmacological stimulation with the GABA A antago- nist bicuculline, which has been well characterized in fMRI studies in rats, was used as the stimulation para- digm. PtcCO 2 was measured using an on-line transcuta- neous blood gas monitoring system with a Severing- haus-type CO 2 electrode. Core Technologies Area, Novartis Pharma Ltd, Basel, Switzerland. *Correspondence to: Markus Rudin, Ph.D., CTA/Analytical and Imaging Sci- ences Unit, WSJ-386.2.02, Novartis Pharma AG, CH-4002 Basel, Switzer- land. E-mail: markus-1.rudin@pharma.novartis.com Received 20 September 2000; revised 25 January 2001; accepted 12 February 2001. Magnetic Resonance in Medicine 46:292–298 (2001) © 2001 Wiley-Liss, Inc. 292