Introduction
The cholinergic antagonist scopolamine mimics aspects of cognitive dysfunction that occur in dementias like Alzheimer's disease. In particular, scopolamine-induced amnesia in rodents is frequently used to test putative cognition-enhancing therapies. Here, we have further characterised this model by monitoring the cerebral effects of scopolamine (and its analogue, butyl-scopolamine, which does not cross the blood brain barrier) in rats using functional MRI.
Methods
3 groups of 8 SD male rats (286±25 g) received either scopolamine hydrobromide (0.8 mg/kg), scopolamine-N-butyl bromide (0.8 mg/kg) or vehicle saline i.p. one hour after commencement of fMRI. All rats were anaesthetised with iv α-chloralose (65 mg/kg bolus followed by 30 mg/kg/h infusion). Mean arterial blood pressure, pulse and respiration were measured continuously in all animals, whilst blood glucose, pH and paCO2 were measured at the start and end of scanning in over half the animals. MRI was performed using a 4.7 T magnet and a conventional GE T2*-sensitive sequence (effective TE 10 ms, TR 0.94 s, isotropic 0.125 mm3 voxels) producing a time-series of 130 whole brain scans over 2.5 h. For group mapping, all images were movement corrected, normalised to standard space and smoothed using SPM'99 software, followed by noise filtering with MELODIC software (www.fmrib.ox.ac.uk). Data were scaled to the global brain signal and activation maps (SPM'99) were derived representing significant correlations with a pharmacokinetic input function.
Results
There were no significant variations in any physiological parameters except for blood glucose, which was lowered by scopolamine (Table 1). Brain activation maps revealed significant increases in BOLD contrast after scopolamine injection in rostral regions including frontal (orbital) cortex and olfactory nuclei (p<0.05 corrected for multiple comparisons); in caudal regions BOLD increases were mostly localised to the retrosplenial cortex but extended to the CA1 hippocampal layer at a lower statistical threshold (p<0.001, uncorrected). Such changes were absent from both saline and butyl-scopolamine treated rats.
Mean±sd, n=4.8 per group.
p<0.05 from pre, 2-way Anova. Mean±sd, n=4.8 per group.
Conclusions
It is possible that the effects of scopolamine result from increased release of acetylcholine as their distribution encompasses areas innervated by cholinergic fibres from the basal forebrain. However, the increased regional BOLD contrast may also reflect a mismatch between CBF and glucose utilization, both of which are reportedly affected by scopolamine1, 2. The nature of these changes therefore requires further investigation using more direct techniques. In summary, this study demonstrates the utility of using fMRI with scopolamine challenge for in vivo investigation of the cholinergic basis of dementias (See Figure 1).
Group statistical parametric maps showing statistically significant (p<0.05 corrected) signal increases (red) and decreases (blue) at app. −4.3 mm, 4.2 mm and 5.2 mm from Bregma, left to right.