Pharmacological evidence for 5-HT-evoked changes in bold fmri signals in rat brain in vivo

M.A. Preece1, N.R. Sibson2, A.M. Blamire4, P. Styles2 & T. Sharp3 1School of Life Sciences, Kingston University, Kingston upon Thames; 2Experimental Neuroimaging Group, Department of Physiology, Anatomy & Genetics and 3Department of Pharmacology, University of Oxford, Oxford; 4School of Clinical and Laboratory Sciences, University of Newcastle upon Tyne, Newcastle upon Tyne.

The 5-HT transmitter system is implicated in the cause and drug treatment of a range of brain disorders but clinical methods for monitoring 5-HT function are few. In this respect the ability to monitor 5-HT release using non-invasive imaging techniques would be greatly advantageous. To date, attempts to monitor 5-HT release by radioligand displacement using positron emission tomography have met limited success in animals and humans (eg Hirani et al., 2004). However, recent human data suggest that drug-evoked changes in 5-HT may be detectable by blood oxygenation level dependent functional magnetic resonance imaging (BOLD fMRI) ( McKie et al., 2005). Here we used this approach in rats to assess the effect of the 5-HT releasing agent, fenfluramine, administered alone and after 5-HT depletion.

Male Sprague-Dawley rats (n=5/6 per group, 270-320g) were anaesthetised (1% halothane in 60:40 N20:02) and placed in a horizontal bore 7T magnet. Five gradient echo images spanning the rostral-caudal extents of the forebrain (TR=500msec, TE=25msec, 1.5 mm slice thickness) were acquired every 2 min for 100 min. Fenfluramine (10 mg/kg) or saline vehicle were injected i.v. 15 min after scanning commenced. For 5-HT depletion experiments, rats were pre-dosed with p-chlorophenylalanine ( p CPA) (300 mg/kg i.p. administered twice, 24 h apart). Data were analysed using FEAT software (http://www.fmrib.ox.ac.uk) to generate probability maps, which were then used to perform a region of interest analysis.

Fenfluramine produced significant changes in BOLD signal intensity in a number of regions. Effects were greatest in the nucleus accumbens (maximum change -12.1±2.8 % versus predrug values), prefrontal cortex (-10.1±3.2 %), motor cortex (+2.3±1.0 %), septum (-5.8±1.5 %) and caudate nucleus (-4.9±0.8 %) (p<0.05, repeated measures ANOVA versus saline controls). Pre-treatment with pCPA resulted in a complete (prefrontal cortex, septum and caudate) or partial (nucleus accumbens) attenuation of the effects of fenfluramine in all regions except one (motor cortex).
These data demonstrate that the 5-HT releasing agent fenfluramine evokes changes in BOLD signal intensity in various brain regions of the anaesthetized rat. The finding that some (but not all) of these changes are blocked by pCPA indicates the involvement of increased 5-HT release. These findings support the utility of pharmacological fMRI as an approach to investigate the functionality of the brain 5-HT system in vivo.

McKie, S et al., (2005). Psychopharmacology, 180: 680-686.
Hirani, E et al., (2003). Synapse, 50: 251-260.

Supported by EC FP6 Integrated network (NEWMOOD, LMSH-CT-20046503474).