The time course of [F-18]fallypride in the region of the cerebellum is shown in for the decay corrected data. These data reveal high initial uptake of radiotracer followed by relatively rapid clearance with only approximately 0.003% injected dose (I.D.) per cc of tissue at 2 hours post-injection. These data were fit to a bi-exponential curve to provide an estimate of the clearance of radiotracer from the cerebellum, with the results given in .
Cerebellum time-activity curves for [F-18]fallypride normalized to percent injected dose (% I.D.) per cc of tissue.
[F-18]Fallypride kinetics in the cerebellum
The figure reveals relatively close agreement in kinetics across the cohort with the mean cerebellar concentration during the final 30 minutes revealing a coefficient of variation of 27% (0.0027 ± 0.0007 % I.D./cc) and all values within 2 standard deviations of the mean. There was also no significant correlation between cerebellar AUCr and animal weight (r = −0.18, p<0.31).
Regional D2/D3 Dopamine Binding
highlights the extrastriatal regions of the brain demonstrating high focal binding of [F-18]fallypride. Shown in are PET time-activity curves of [F-18]fallypride in the various regions of the brain. The DVR values for the regions of interest across the entire cohort are shown in , with the exclusion of 2 animals having DVR values that were more than 2 standard deviations from the group mean. These data illustrate the range in specific D2/D3 binding for regions of high, medium and low D2/D3 receptor density. The coefficient of variation (s.d./mean*100%) for these regions is shown for both DVR and BPND (= DVR – 1). The thalamus has a highly heterogeneous distribution of D2/D3 receptors throughout the subregions. The thalamic ROI was centered over the midline region of the inferior thalamus, revealing the highest uptake of the thalamic subregions. The rank order of D2/D3 binding for each animal was approximately consistent across all regions, i.e. the monkeys with the highest putamen binding also had the highest binding in the amygdala, substantia nigra, anterior cingulate, etc. The Spearman correlation coefficient for DVR between the regions was highest among the striatal regions with 0.90 > ρ > 0.65 between the putamen, caudate and ventral striatum. For the other regions, the correlations between regions ranged from 0.20 – 0.60 and were positive in all cases. The pituitary revealed no correlations outside the range of −0.10 < r < 0.10, though it must be noted that some of the assumptions of the reference region model fail in this region outside the blood brain barrier.
Figure 2 Extrastriatal regions with high focal uptake of [F-18]fallypride. Coregistered MRIs (top row) reveal two corresponding coronal slices (indicated by c1 and c2 on sagital view) and regions of elevated [F-18]fallypride binding (bottom row) in the thalamus (more ...)
Cohort distribution volume ratio (DVR) of [F-18]fallypride
DVR measurements were also made on the native PET images, prior to spatial processing, to minimize potential intersubject variance due to spatial coregistration and normalization. ROIs were drawn only for the putamen, substantia nigra and frontal cortex based on the PET images. The DVRs for these ROIs were significantly greater compared to the template based values (shown in the ) due to the central ROI placement on the PET data, however, there was a slightly reduced coefficient of variation for each region: putamen (mean = 28.6, COV = 19%, COV BPND = 20%), substantia nigra (3.40, 13%, 18%) and frontal cortex (1.52, 12%, 35%), suggesting that a small but measureable amount of variance is introduced due to the process of spatial normalization with MRI-based ROIs, or that these structures are not homogenous with respect to dopamine response, and/or that a partial volume correction might be needed. There was also a very strong correlation between the native PET and template based DVR values, r = 0.98, 0.92, 0.92 for the putamen, substantia nigra and frontal cortex, respectively.
The effects of the experimental variables, including sex, age, weight, and timing of anesthesia (ketamine and isoflurane) administration on D2/D3 binding using hierarchical multiple regression are shown in . Based on the Welch two sample t-test, there was no significant sex-based difference in age (p=0.31), weight (p=0.11), ketamine timing (p=0.15), isoflurane timing (0.23), fallypride mass (=0.89) or AUCc (p=0.43). Also, there was no significant correlation between age, weight, ketamine timing, isoflurane timing and fallypride mass (μg/kg) explanatory parameters. Though all of the brain regions displayed higher average [F-18]fallypride binding in the females, the only region revealing a significant difference based on sex was the pituitary, with the females displaying 38% greater uptake. Further analysis considerations in the pituitary will be addressed in the Discussion. Within the brain, there were no significant differences (when corrected for multiple comparisons) in [F-18]fallypride binding due to age or weight when controlled for sex. As seen in the table the range in age (3.20 – 5.26 yrs) and weight (4.6 – 8.7 kg) of the subjects was relatively small. Shown in are the DVR values in the putamen and amygdala plotted as a function of age.
Effects of DVR with Experimental Variables
DVR as a function of age in the regions of the putamen and amygdala for both females and males.
Fallypride Mass Effects
also gives the results for the correlation of [F-18]fallypride binding with the body mass scaled fallypride and total integrated nondisplaceable fallypride (AUCc). It can be seen that a statistically significant relationship with AUCc was measured in the substantia nigra, thalamus, and frontal cortex, with only the thalamus showing significance when corrected for multiple comparisons. The individual data points for four of the regions are shown in , using the native PET ROIs. Also plotted is the theoretical relationship between the measured binding (DVR-1) and the total integrated nondisplaceable fallypride (AUCc). This curve is included only to serve as an indicator of the theoretical mass dependent effects on DVR for constant values of Bmax and KD, which cannot be assumed for these data due to intersubject variability.