Concurrent pharmacological MRI and in situ microdialysis of cocaine reveal a complex relationship between the central hemodynamic response and local dopamine concentration

Abstract

The mechanisms underlying the signal changes observed with pharmacological magnetic resonance imaging (phMRI) remain to be fully elucidated. In this study, we obtained microdialysis samples in situ at 5-min intervals during phMRI experiments using a blood pool contrast agent to correlate relative cerebral blood volume (rCBV) changes with changes in dopamine and cocaine concentrations following acute cocaine challenge (0.5 mg/kg iv) in the rat over a duration of 30 min. Three brain areas were investigated: the dorsal striatum (n = 8), the medial prefrontal cortex (mPFC; n = 5), and the primary motor cortex (n = 8). In the striatum and mPFC groups, cocaine and dopamine temporal profiles were tightly correlated, peaking during the first 5-min period postinjection, then rapidly decreasing. However, the local rCBV changes were uncorrelated and exhibited broader temporal profiles than those of cocaine and dopamine, attaining maximal response 5–10 min later. This demonstrates that direct vasoactivity of dopamine is not the dominant component of the hemodynamic response in these regions. In the motor cortex group, microdialysis revealed no local change in dopamine in any of the animals, despite large local cocaine increase and strong rCBV response, indicating that the central hemodynamic response following acute iv cocaine challenge is not driven directly by local dopamine changes in the motor cortex. The combination of phMRI and in situ microdialysis promises to be of great value in elucidating the relationship between the phMRI response to psychoactive drugs and underlying neurochemical changes.

Introduction

Magnetic resonance imaging (MRI) methods can be applied to study the effects of pharmacological agents on the spatiotemporal patterns of brain activity in humans Breiter et al., 1997, Stein et al., 1998 and laboratory animals Chen et al., 1997, Jenkins et al., 2003, Marota et al., 2000, Xu et al., 2000. More specifically, the pharmacological MRI (phMRI) approach tracks signal changes reflecting the central hemodynamic response induced by acute pharmacological challenge (Jenkins et al., 2003). Different pharmacological tools have been used to activate neurotransmitter systems and to study the modulatory action of more selective receptor agonists and antagonists Jenkins et al., 2003, Leslie and James, 2000, Morris, 1999. However, the mechanisms underlying the signal changes observed by phMRI and their correlation with modulations in neurotransmitter concentrations remain to be fully elucidated.

Cocaine produces a widespread central hemodynamic response Stein and Fuller, 1992, Stein and Fuller, 1993 and has been widely used in phMRI studies in the rat Luo et al., 2003, Mandeville et al., 2001, Marota et al., 2000, where intravenous (iv) administration results in temporal profiles that vary with anatomical location (Marota et al., 2000). One of the primary actions of cocaine is dopamine transporter blockade, leading to increased intrasynaptic dopamine. These modulations in the mesolimbic dopamine system are thought to be a fundamental mechanism underlying the psychostimulant effects and reinforcing properties of cocaine and other drugs of abuse (Koob and Le Moal, 1997). Other pharmacological stimuli affecting the dopaminergic system [amphetamine and CFT (2β-carbomethoxy-3β-(4-fluorophenyl)tropane)] have been shown to yield rCBV changes that correlate in amplitude (Chen et al., 1997) and temporal profile (Chen et al., 1999) in the rat striatum with changes in dopamine following intravenous challenge. This led to the hypothesis that dopamine release and the hemodynamic response are tightly coupled, with relative cerebral blood volume (rCBV) changes driven by the dopamine concentration Chen et al., 1997, Chen et al., 1999, Jenkins et al., 2003. This could reflect increased metabolic demand following neural activation at dopaminergic synapses. However, dopaminergic neurons also project postsynaptically onto the vasculature Goldman-Rakic et al., 1992, Williams and Goldman-Rakic, 1998, and dopamine itself has a rapid and direct vasoactive action Edvinsson and Krause, 2002, Krimer et al., 1998. The substantial attenuation of the (rCBV) phMRI response to cocaine (Marota et al., 2000) and amphetamine (Choi et al., 2003) by selective dopamine D₁ receptor blockade, and the observation that dopamine D_1/5 receptors are expressed on microvessels and capillaries in the cortex (Choi et al., 2003), suggested that direct action on dopamine D₁ receptors on the cerebral vasculature may be a major component of the hemodynamic response to dopaminergic ligands Choi et al., 2003, Jenkins et al., 2003. Under the hypothesis that dopamine release and the central hemodynamic response to cocaine are tightly coupled, interregional differences in phMRI time courses (Marota et al., 2000) might reflect different temporal profiles of dopamine concentration. Moreover, interanimal variability in the amplitude of the phMRI response could indicate an underlying variability in neurotransmitter changes.

To test these hypotheses and determine more fully the relationship between the central hemodynamic response and modulations in dopamine concentration following cocaine challenge, we established methodology enabling in situ microdialysis sampling during the phMRI experiment. Cocaine has relatively fast pharmacokinetics, with an intravenous plasma half-life of 15–20 min in the rat (Stein and Fuller, 1993), and differences in rCBV profile occur primarily during the first 10–15 min (Marota et al., 2000). The time resolution of the microdialysis sampling was thus increased to 5 min to approximate more closely the time scale of rCBV changes. A specific and highly sensitive liquid chromatography or mass spectrometry assay (LC/MS-MS) was developed to provide the concurrent determination of the cocaine and dopamine levels in the collected microdialysis samples. This enabled analysis of simultaneous dynamic changes in the concentrations of cocaine ([CA]) and dopamine ([DA]) and the rCBV component of the hemodynamic response within the same animals. We focused this study on three structures: the dorsal striatum (dStr), primary motor cortex (M1), and medial prefrontal cortex (mPFC). Each of these regions shows a different rCBV temporal response profile, and each has a different relationship with the major dopaminergic system projections Fallon and Moore, 1978, Groenewegen et al., 1997, Heidbreder and Groenewegen, 2003, Steketee, 2003, Wu et al., 2003. Microdialysis samples were obtained from either the striatum, motor cortex, or mPFC concurrent with rCBV-phMRI acquisition, enabling correlation of cocaine and dopamine modulation with rCBV changes within the same animal.