Imaging cocaine-induced changes in the mesocorticolimbic dopaminergic system of conscious rats

Abstract

Functional magnetic resonance imaging (fMRI) was used to assess the effects of cocaine on brain activation in fully conscious rats. Methods were developed to image cocaine-induced changes in blood-oxygen-level-dependent (BOLD) signal without the peripheral cardiac and respiratory complications associated with psychostimulant administration. Using spin echo planar imaging (EPI), conscious rats were imaged in a 4.7 T spectrometer prior to and following the intracerebroventricular injection of cocaine (20 microg) in artificial cerebrospinal fluid (10 uL). Within 5 min of injection, there was a significant increase in BOLD signal intensity in the substantia nigra, ventral tegmental area, nucleus accumbens, dorsal striatum and prefrontal cortex, as compared to vehicle controls. Minimal negative BOLD signal changes were observed in response to cocaine and no significant perturbations in normal cardiovascular and respiratory function. These findings demonstrate the technical feasibility of studying psychostimulant-induced brain activity using functional MRI in conscious rats.

Fig. 1

Comparison between spin echo and gradient echo planar imaging (EPI) pulse sequences. Functional images shown to the left are at the level of the ventral tegmental area and images to the right are at the level of the prefrontal cortex. VTA, ventral tegmental area; SN, substantia nigra; SNR, signal-to-noise.

Fig. 2

Intracerebroventricular cannula placement and functional imaging paradigm used in the present study. (A) Prior to imaging sessions a non-magnetic polyethylene cannula was placed into the lateral cerebral ventricle to inject 20 ug of cocaine in 10 ul of artificial cerebrospinal fluid vehicle. (B) A functional imaging session conducted during a 20 min period and cocaine injected 5 min into the scan.

Fig. 3

Effect of intracerebroventricular cocaine administration on behavioral activity and dopamine metabolism in the nucleus accumbens. (A) Automated horizontal and vertical activity counts in response to three ICV cocaine doses. Only rats receiving 20 ug of cocaine showed behavioral activation above vehicle control levels (*P < 0.05). (B) The levels of dopamine metabolites, DOPAC and HVA, were increased in the nucleus accumbens following ICV administration of 20 ug cocaine (^†P < 0.01). (C) Representative coronal sections of the rat brain showing the sites of microdialysis cannula placement into the nucleus accumbens (adapted from Paxinos and Watson rat brain atlas, 1997).

Fig. 4

Activation maps of blood-oxygenation-level-dependent (BOLD) signal change in response to intravenous cocaine administration (1 mg/kg). The colored pixels represent brain tissue volume elements (voxels) that showed signal intensity values significantly different from baseline. Statistical significance was determined with the Stimulate software (Strupp, 1996) using a pixel-by-pixel t-test analysis (P < 0.05) comparing baseline to a post-cocaine injection period. Pixels from each rat were overlaid on their corresponding high resolution anatomy.

Fig. 5

Activation maps of positive blood-oxygenation-level-dependent (BOLD) signal in response to intracerebroventricular cocaine administration (20 ug/10 uL). The colored pixels represent brain areas that showed signal intensity values significantly different from baseline. Statistical significance was determined with the Stimulate software (Strupp, 1996) using a pixel-by-pixel t-test analysis (P < 0.05) comparing baseline to a post-cocaine injection period. Activated pixels from each rat were overlaid on their corresponding anatomy. PFC, prefrontal cortex; NAcc, nucleus accumbens; STR, dorsal striatum; VTA, ventral tegmental area; SN, substantia nigra.

Fig. 6

Percent change in BOLD signal intensity over time in mesocorticolimbic and nigrostriatal brain areas. Data represent average signal intensity from voxels within the regions of interest of each rat. Percent change in signal intensity from baseline was calculated for each subject. Red lines represent cocaine injected rats and the black lines show data for vehicle injected animals. Arrows indicate the time of injection during the 20 min imaging session.