Colm J. McGinnity

Attenuation of proliferation in oligodendrocyte precursor cells by activated microglia

Activated microglia can influence the survival of neural cells through the release of cytotoxic factors. Here, we investigated the interaction between Toll‐like receptor 4 (TLR4)‐activated microglia and oligodendrocytes or their precursor cells (OPC). Primary rat or N9 microglial cells were activated by exposure to TLR4‐specifc lipopolysaccharide (LPS), resulting in mitogen‐activated protein kinase activation, increased CD68 and inducible nitric oxide synthase expression, and release of the proinflammatory cytokines tumor necrosis factor (TNF) and interleukin‐6 (IL‐6). Microglial conditioned medium (MGCM) from LPS‐activated microglia attenuated primary OPC proliferation without inducing cell death. The microglial‐induced inhibition of OPC proliferation was reversed by stimulating group III metabotropic glutamate receptors in microglia with the agonist L‐AP4. In contrast to OPC, LPS‐activated MGCM enhanced the survival of mature oligodendrocytes. Further investigation suggested that TNF and IL‐6 released from TLR4‐activated microglia might contribute to the effect of MGCM on OPC proliferation, insofar as TNF depletion of LPS‐activated MGCM reduced the inhibition of OPC proliferation, and direct addition of TNF or IL‐6 attenuated or increased proliferation, respectively. OPC themselves were also found to express proteins involved in TLR4 signalling, including TLR4, MyD88, and MAL. Although LPS stimulation of OPC did not induce proinflammatory cytokine release or affect their survival, it did trigger JNK phosphorylation, suggesting that TLR4 signalling in these cells is active. These findings suggest that OPC survival may be influenced not only by factors released from endotoxin‐activated microglia but also through a direct response to endotoxins. This may have consequences for myelination under conditions in which microglial activation and cerebral infection are both implicated.

Initial Evaluation of 18F-GE-179, a Putative PET Tracer for Activated N-Methyl d-Aspartate Receptors

N-methyl d-aspartate (NMDA) ion channels play a key role in a wide range of physiologic (e.g., memory and learning tasks) and pathologic processes (e.g., excitotoxicity). To date, suitable PET markers of NMDA ion channel activity have not been available. 18F-GE-179 is a novel radioligand that selectively binds to the open/active state of the NMDA receptor ion channel, displacing the binding of 3H-tenocyclidine from the intrachannel binding site with an affinity of 2.4 nM. No significant binding was observed with 10 nM GE-179 at 60 other neuroreceptors, channels, or transporters. We describe the kinetic behavior of the radioligand in vivo in humans. Methods: Nine healthy participants (6 men, 3 women; median age, 37 y) each underwent a 90-min PET scan after an intravenous injection of 18F-GE-179. Continuous arterial blood sampling over the first 15 min was followed by discrete blood sampling over the duration of the scan. Brain radioactivity (KBq/mL) was measured in summation images created from the attenuation- and motion-corrected dynamic images. Metabolite-corrected parent plasma input functions were generated. We assessed the abilities of 1-, 2-, and 3-compartment models to kinetically describe cerebral time–activity curves using 6 bilateral regions of interest. Parametric volume-of-distribution (VT) images were generated by voxelwise rank-shaping regularization of exponential spectral analysis (RS-ESA). Results: A 2-brain-compartment, 4-rate-constant model best described the radioligand’s kinetics in normal gray matter of subjects at rest. At 30 min after injection, 37% of plasma radioactivity represented unmetabolized 18F-GE-179. The highest mean levels of gray matter radioactivity were seen in the putamina and peaked at 7.5 min. A significant positive correlation was observed between K1 and VT (Spearman ρ = 0.398; P = 0.003). Between-subject coefficients of variation of VT ranged between 12% and 16%. Voxelwise RS-ESA yielded similar VTs and coefficients of variation. Conclusion: 18F-GE-179 exhibits high and rapid brain extraction, with a relatively homogeneous distribution in gray matter and acceptable between-subject variability. Despite its rapid peripheral metabolism, quantification of 18F-GE-179 VT is feasible both within regions of interest and at the voxel level. The specificity of 18F-GE-179 binding, however, requires further characterization with in vivo studies using activation and disease models.

Wavelet-based resolution recovery using an anatomical prior provides quantitative recovery for human population phantom PET 11C]raclopride data

The purpose of this study was the evaluation of a wavelet-based resolution recovery (RR) method, named structural and functional synergy for RR (SFS-RR), for a variety of simulated human brain [11C]raclopride PET images. Simulated datasets of 15 human phantoms were processed by SFS-RR using an anatomical prior. This anatomical information was in the form of a hybrid segmented-atlas, which combines an MRI for anatomical labelling and a PET image for functional labelling of each anatomical structure. First, the relationship between the FWHM of the original the PET image and its resolution recovered version was investigated. Then quantitative evaluation was performed by comparing caudate, putamen and cerebellum regions of the true image; simulated PET image; and RR image. The spatial resolution of the original PET image effected on how accurately SFS-RR recovers the image counts in striatum regions. The resolution in striatum, and cerebellum regions was successfully recovered for all the 15 human phantoms. The proposed methodology proved effective in the resolution recovery of small structures of brain [11C]raclopride PET images. The improvement was consistent across the anatomical variability of a simulated population, provided accurate anatomical segmentations are available.

NMDA receptor binding in focal epilepsies.

Objective To demonstrate altered N-methyl-d-aspartate (NMDA) receptor availability in patients with focal epilepsies using positron emission tomography (PET) and [18F]GE-179, a ligand that selectively binds to the open NMDA receptor ion channel, which is thought to be overactive in epilepsy. Methods Eleven patients (median age 33 years, 6 males) with known frequent interictal epileptiform discharges had an [18F]GE-179 PET scan, in a cross-sectional study. MRI showed a focal lesion but discordant EEG changes in two, was non-localising with multifocal EEG abnormalities in two, and was normal in the remaining seven patients who all had multifocal EEG changes. Individual patient [18F]GE-179 volume-of-distribution (VT) images were compared between individual patients and a group of 10 healthy controls (47 years, 7 males) using Statistical Parametric Mapping. Results Individual analyses revealed a single cluster of focal VT increase in four patients; one with a single and one with multifocal MRI lesions, and two with normal MRIs. Post hoc analysis revealed that, relative to controls, patients not taking antidepressants had globally increased [18F]GE-179 VT (+28%; p<0.002), and the three patients taking an antidepressant drug had globally reduced [18F]GE-179 VT (−29%; p<0.002). There were no focal abnormalities common to the epilepsy group. Conclusions In patients with focal epilepsies, we detected primarily global increases of [18F]GE-179 VT consistent with increased NMDA channel activation, but reduced availability in those taking antidepressant drugs, consistent with a possible mode of action of this class of drugs. [18F]GE-179 PET showed focal accentuations of NMDA binding in 4 out of 11 patients, with difficult to localise and treat focal epilepsy.

Partial volume correction using structural–functional synergistic resolution recovery: comparison with geometric transfer matrix method

We validated the use of a novel image-based method for partial volume correction (PVC), structural–functional synergistic resolution recovery (SFS-RR) for the accurate quantification of dopamine synthesis capacity measured using [18F]DOPA positron emission tomography. The bias and reliability of SFS-RR were compared with the geometric transfer matrix (GTM) method. Both methodologies were applied to the parametric maps of [18F]DOPA utilization rates (kicer). Validation was first performed by measuring repeatability on test–retest scans. The precision of the methodologies instead was quantified using simulated [18F]DOPA images. The sensitivity to the misspecification of the full-width-half-maximum (FWHM) of the scanner point-spread-function on both approaches was also assessed. In the in-vivo data, the kicer was significantly increased by application of both PVC procedures while the reliability remained high (intraclass correlation coefficients >0.85). The variability was not significantly affected by either PVC approach (<10% variability in both cases). The corrected kicer was significantly influenced by the FWHM applied in both the acquired and simulated data. This study shows that SFS-RR can effectively correct for partial volume effects to a comparable degree to GTM but with the added advantage that it enables voxelwise analyses, and that the FWHM used can affect the PVC result indicating the importance of accurately calibrating the FWHM used in the recovery model.

Risk factors for reading disability in families with rolandic epilepsy

OBJECTIVE The high prevalence and impact of neurodevelopmental comorbidities in childhood epilepsy are now well known, as are the increased risks and familial aggregation of reading disability (RD) and speech sound disorder (SSD) in rolandic epilepsy (RE). The risk factors for RD in the general population include male sex, SSD, and ADHD, but it is not known if these are the same in RE or whether there is a contributory role of seizure and treatment-related variables. METHODS An observational study of 108 probands with RE (age range: 3.6-22 years) and their 159 siblings (age range: 1-29 years; 83 with EEG data) were singly ascertained in the US or UK through a proband affected by RE. We used a nested case-control design, multiple logistic regression, and generalized estimating equations to test the hypothesis of an association between RD and seizure variables or antiepileptic drug treatment in RE; we also assessed an association between EEG focal sharp waves and RD in siblings. RESULTS Reading disability was reported in 42% of probands and 22% of siblings. Among probands, RD was strongly associated with a history of SSD (OR: 9.64, 95% CI: 2.45-37.21), ADHD symptoms (OR: 10.31, 95% CI: 2.15-49.44), and male sex (OR: 3.62, 95% CI: 1.11-11.75) but not with seizure or treatment variables. Among siblings, RD was independently associated only with SSD (OR: 4.30, 95% CI: 1.42-13.0) and not with the presence of interictal EEG focal sharp waves. SIGNIFICANCE The principal risk factors for RD in RE are SSD, ADHD, and male sex, the same risk factors as for RD without epilepsy. Seizure or treatment variables do not appear to be important risk factors for RD in probands with RE, and there was no evidence to support interictal EEG focal sharp waves as a risk factor for RD in siblings. Future studies should focus on the precise neuropsychological characterization of RD in families with RE and on the effectiveness of standard oral-language and reading interventions.

Test–retest reproducibility of cannabinoid-receptor type 1 availability quantified with the PET ligand [11C]MePPEP

Background Endocannabinoids are involved in normal cognition, and dysfunction in cannabinoid-receptor-mediated neurotransmission has been suggested in a variety of neurological and psychiatric pathologies. The type 1 cannabinoid receptor (CB1) is widely expressed in the human central nervous system. The objective of this study was to quantify the test–retest reproducibility of measures of the PET ligand [11C]MePPEP in order to assess the stability of CB1-receptor quantification in humans in vivo. Methods Fifteen healthy subjects (eight females; median age 32 years, range 25 to 65 years) had a 90-minute PET scan on two occasions after injection of a median dose of [11C]MePPEP of 364 MBq. Metabolite-corrected arterial plasma input functions were obtained for all scans. Eight ROIs, reflecting different levels of receptor densities/concentrations, were defined automatically: hippocampus, anterior cingulate gyrus, inferior frontal gyrus, caudate nucleus, globus pallidus, nucleus accumbens, thalamus, and pons. We used seven quantification methods: reversible compartmental models with one and two tissue classes, two and four rate constants, and a variable blood volume term (2kbv; 4kbv); model-free (spectral) analyses with and without regularisation, including one with voxel-wise quantification; the simplified reference tissue model (SRTM) with pons as a pseudo-reference region; and modified standard uptake values (mSUVs) calculated for the period of ~ 30–60 min after injection. Percentage test–retest change and between-subject variability were both assessed, and test–retest reliability was quantified by the intraclass correlation coefficient (ICC). The ratio of binding estimates pallidum:pons served as an indicator of a methods ability to reflect binding heterogeneity. Results Neither the SRTM nor the 4kbv model produced reliable measures, with ICCs around zero. Very good (> 0.75) or excellent (> 0.80) ICCs were obtained with the other methods. The most reliable were spectral analysis parametric maps (average across regions ± standard deviation 0.83 ± 0.03), rank shaping regularised spectral analysis (0.82 ± 0.05), and the 2kbv model (0.82 ± 0.09), but mSUVs were also reliable for most regions (0.79 ± 0.13). Mean test–retest changes among the five well-performing methods ranged from 12 ± 10% for mSUVs to 16% for 2kbv. Intersubject variability was high, with mean between-subject coefficients of variation ranging from 32 ± 13% for mSUVs to 45% for 2kbv. The highest pallidum:pons ratios of binding estimates were achieved by mSUV (4.2), spectral analysis-derived parametric maps (3.6), and 2kbv (3.6). Conclusion Quantification of CB1 receptor availability using [11C]MePPEP shows good to excellent reproducibility with several kinetic models and model-free analyses, whether applied on a region-of-interest or voxelwise basis. Simple mSUV measures were also reliable for most regions, but do not allow fully quantitative interpretation. [11C]MePPEP PET is well placed as a tool to investigate CB1-receptor mediated neurotransmission in health and disease.

Investigating the reproducibility of the novel Alpha-5 GABAA receptor PET ligand [11C]Ro15 4513

Method: Four male healthy controls had two 90-minute dynamic PET scans on a Siemens/ECAT 962 scanner, each after intravenous injection of approximately 444 MBq (mean 432.9 MBq, range 359.3–452 MBq) of [C]Ro15 4513. Data was movement-corrected with a frame-to-frame coregistration method (Hammers et al. 2007) and co-registered with T1-wieghted structural 3D-images obtained on a 3 T magnetic resonance images (MRI). Arterial plasma input function (IFs) were obtained by continuous blood withdrawal for 15 min and combination with discrete plasma samples for correction of radio-labelled metabolites and incorporation of plasma-over-blood ratio within Clickfit Software (MRC Cyclotron Unit, Hammersmith, London, UK). The PET volumes-of-distribution (VD) were quantified using spectral analysis (SA) within RPM (Gunn et al, 1997). The default slow frequency boundary of 0.0008 s−1 in RPM (log10=−3.10) was changed to 0.00063 s−1 (log10=−3.2) in order to match the C decay constant of log10=−3.25 (Hammers et al, 2007). VDs from representative high and low binding regions were extracted via an automated region-of-interest (ROI) definition method (MAPER, Heckemann et al. 2010). Outcome measures were intraclass correlation coefficient (ICCs), the coefficient of variation (CV), and reliability in ROIs.

Test-retest reproducibility of quantitative binding measures of [11C]Ro15-4513, a PET ligand for GABAA receptors containing alpha5 subunits

Introduction: Alteration of &ggr;‐aminobutyric acid “A” (GABAA) receptor‐mediated neurotransmission has been associated with various neurological and psychiatric disorders. [11C]Ro15‐4513 is a PET ligand with high affinity for &agr;5‐subunit‐containing GABAA receptors, which are highly expressed in limbic regions of the human brain (Sur et al., 1998). We quantified the test‐retest reproducibility of measures of [11C]Ro15‐4513 binding derived from six different quantification methods (12 variants). Methods: Five healthy males (median age 40 years, range 38–49 years) had a 90‐min PET scan on two occasions (median interval 12 days, range 11–30 days), after injection of a median dose of 441 MegaBequerels of [11C]Ro15‐4513. Metabolite‐corrected arterial plasma input functions (parent plasma input functions, ppIFs) were generated for all scans. We quantified regional binding using six methods (12 variants), some of which were region‐based (applied to the average time‐activity curve within a region) and others were voxel‐based: 1) Models requiring arterial ppIFs – regional reversible compartmental models with one and two tissue compartments (2kbv and 4kbv); 2) Regional and voxelwise Logan’s graphical analyses (Logan et al., 1990), which required arterial ppIFs; 3) Model‐free regional and voxelwise (exponential) spectral analyses (SA; (Cunningham and Jones, 1993)), which also required arterial ppIFs; 4) methods not requiring arterial ppIFs – voxelwise standardised uptake values (Kenney et al., 1941), and regional and voxelwise simplified reference tissue models (SRTM/SRTM2) using brainstem or alternatively cerebellum as pseudo‐reference regions (Lammertsma and Hume, 1996; Gunn et al., 1997). To compare the variants, we sampled the mean values of the outcome parameters within six bilateral, non‐reference grey matter regions‐of‐interest. Reliability was quantified in terms of median absolute percentage test‐retest differences (MA‐TDs; preferentially low) and between‐subject coefficient of variation (BS‐CV, preferentially high), both compounded by the intraclass correlation coefficient (ICC). These measures were compared between variants, with particular interest in the hippocampus. Results: Two of the six methods (5/12 variants) yielded reproducible data (i.e. MA‐TD <10%): regional SRTMs and voxelwise SRTM2s, both using either the brainstem or the cerebellum; and voxelwise SA. However, the SRTMs using the brainstem yielded a lower median BS‐CV (7% for regional, 7% voxelwise) than the other variants (8–11%), resulting in lower ICCs. The median ICCs across six regions were 0.89 (interquartile range 0.75–0.90) for voxelwise SA, 0.71 (0.64–0.84) for regional SRTM‐cerebellum and 0.83 (0.70–0.86) for voxelwise SRTM‐cerebellum. The ICCs for the hippocampus were 0.89 for voxelwise SA, 0.95 for regional SRTM‐cerebellum and 0.93 for voxelwise SRTM‐cerebellum. Conclusion: Quantification of [11C]Ro15‐4513 binding shows very good to excellent reproducibility with SRTM and with voxelwise SA which, however, requires an arterial ppIF. Quantification in the &agr;5 subunit‐rich hippocampus is particularly reliable. The very low expression of the &agr;5 in the cerebellum (Fritschy and Mohler, 1995; Veronese et al., 2016) and the substantial &agr;1 subunit density in this region may hamper the application of reference tissue methods.

Using [(11)C]Ro15 4513 PET to characterise GABA-benzodiazepine receptors in opiate addiction: Similarities and differences with alcoholism.

The importance of the GABA-benzodiazepine receptor complex and its subtypes are increasingly recognised in addiction. Using the α1/α5 benzodiazepine receptor PET radioligand [11C]Ro15 4513, we previously showed reduced binding in the nucleus accumbens and hippocampus in abstinent alcohol dependence. We proposed that reduced [11C]Ro15 4513 binding in the nucleus accumbens was a marker of addiction whilst the reduction in hippocampus and positive relationship with memory was a consequence of chronic alcohol abuse. To examine this further we assessed [11C]Ro15 4513 binding in another addiction, opiate dependence, and used spectral analysis to estimate contributions of α1 and α5 subtypes to [11C]Ro15 4513 binding in opiate and previously acquired alcohol-dependent groups. Opiate substitute maintained opiate-dependent men (n = 12) underwent an [11C]Ro15 4513 PET scan and compared with matched healthy controls (n = 13). We found a significant reduction in [11C]Ro15 4513 binding in the nucleus accumbens in the opiate-dependent compared with the healthy control group. There was no relationship between [11C]Ro15 4513 binding in the hippocampus with memory. We found that reduced [11C]Ro15 4513 binding was associated with reduced α5 but not α1 subtypes in the opiate-dependent group. This was also seen in an alcohol-dependent group where an association between memory performance and [11C]Ro15 4513 binding was primarily driven by α5 and not α1 subtype. We suggest that reduced α5 levels in the nucleus accumbens are associated with addiction since we have now shown this in dependence to two pharmacologically different substances, alcohol and opiates.