Research reportDetection of pharmacologically mediated changes in cerebral activity by functional magnetic resonance imaging: the effects of sulpiride in the brain of the anaesthetised rat
Introduction
Regional changes in brain functional activity following administration of psychotropic agents have previously been assessed in animals by use of a number of ex vivo techniques. Chief among these is the quantitative uptake of [14C]2-deoxy-d-glucose (2-DG) into brain tissue as determined by autoradiography [17], [24]. Although it provides good anatomical resolution, and its widespread use has indicated its value for this purpose, 2-DG autoradiography can only map the changes in metabolic activity at one time point in each experimental animal. An in vivo method to study the entire time course of drug action in a single animal would be preferable, enabling a within-subject experimental design, thus circumventing problems associated with between animal variability.
Such limitations of ex vivo techniques have spurred interest in the application of functional magnetic resonance imaging (fMRI) to map changes in brain activity. This can be achieved through the use of T2*-weighted pulse sequences that are capable of detecting changes in blood oxygenation level dependent (BOLD) contrast. BOLD signals in the brain increase locally when paramagnetic deoxyhaemoglobin levels decrease relative to those of diamagnetic oxyhaemoglobin in blood vessels. Fox and Raichle [7] suggested that neuronal activation uncouples cerebral blood flow (CBF) from cerebral metabolic oxygen consumption (CMRO2); thus local CBF changes resulting from neuronal stimulation increase disproportionately in relation to CMRO2. The much increased local CBF following neuronal activation results in an increased proportion of oxyhaemoglobin relative to deoxyhaemoglobin and a consequent increase in BOLD contrast.
The BOLD phenomenon, first described by Ogawa et al. [21], has been employed to detect neuronal activation resulting from somatosensory stimulation [29], cognitive task performance [1], and more recently, pharmacological manipulation [4], [5], [33]. To date, the central transmitter system that has been most extensively studied using fMRI is the dopaminergic system, of particular interest in psychiatric research because dysfunctions in this system have long been associated with schizophrenia [3], [18].
Experimental studies that are able to follow the effects of psychoactive compounds on cerebral function over time will provide greater insight into their mode of action. Such studies should be able to identify the anatomical loci of drug action as well as the relative timing of the onset of drug effects. Different loci may be associated with therapeutically desirable actions as well as unwanted side-effects, thus providing greater insight into the properties of psychotropic agents.
The present study used fMRI to characterise the effects of acute administration of the dopamine D2-like receptor antagonist, sulpiride, upon BOLD contrast in the brains of the α-chloralose anaesthetised rats. Sulpiride was chosen as a well characterised test drug that shows selectivity for dopamine D2 and D3 receptors over other receptor types [13], and also because of its reported efficacy in the amelioration of both positive and negative symptoms of schizophrenia [31], and thus large potential spectrum of activity.
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Animals
All of the experimental procedures described here were performed in accordance with the UK Home Office Animals (Scientific Procedures) Act 1986, and adequate measures taken to minimize pain or discomfort. Male Sprague–Dawley rats (n=15, 270–320 g, Charles River, UK) were group housed with ad libitum access to food and water under a 12:12-h light/dark cycle (lights on 08:00 h) until required for experimental purposes.
Materials
(−)-Sulpiride, α-chloralose and Borax buffer were obtained from the Sigma
Results
To determine that hardware stability was appropriate for each experiment, quality assurance tests were performed prior to each experiment, using a T1-weighted gradient echo pulse sequence, and gave excellent signal-to-noise ratios (defined as mean signal intensity of phantom/standard deviation of background signal intensity) in the range 193–216. Signal instability, defined as (maximum signal intensity−minimum signal intensity)/mean signal intensity, did not exceed 1.5% (using an ASE-EPI pulse
Discussion
We have evaluated the acute effects of the dopamine D2-like receptor antagonist sulpiride in the anaesthetised rat brain using fMRI. We report that sulpiride produces highly consistent yet complex effects that nevertheless support current views of its influence on cerebral activation. The present data indicated that acute intravenous administration of sulpiride, at a dose of 10 mg/kg, resulted in a robust bilateral increase in BOLD signal intensity in the frontal cortex of the α-chloralose
Acknowledgements
It is a pleasure to thank Dr Herchel Smith for his endowment, which funded the Herchel Smith Laboratory for Medicinal Chemistry, and Charlie Reavil and Christian Heidbreder for useful discussions. M.P. held a Postdoctoral Research Fellowship supported by GlaxoSmithKline. C.L.-H.H. thanks the Royal Society and the BBSRC for grant funding support.
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