Elizabeth O. Ahlers

Tai chi training reduces self-report of inattention in healthy young adults

It is important to identify effective non-pharmacological alternatives to stimulant medications that reduce symptoms of attention deficit hyperactivity disorder (ADHD). In this study of healthy young adults, we measured the effects of training in tai chi, which involves mindful attention to the body during movement. Using a non-randomized, controlled, parallel design, students in a 15-week introductory tai chi course (n = 28) and control participants (n = 44) were tested for ADHD indicators and cognitive function at three points over the course of the 15-weeks. The tai chi students’ self-report of attention, but not hyperactivity–impulsivity, improved compared to controls. At baseline, inattention correlated positively with reaction time variability in an affective go/no-go task across all participants, and improvements in attention correlated with reductions in reaction time variability across the tai chi students. Affective bias changed in the tai chi students, as reaction times to positive- and negative-valenced words equalized over time. These results converge to suggest that tai chi training may help improve attention in healthy young adults. Further studies are needed to confirm these results and to evaluate tai chi as therapy for individuals with ADHD.

PET Imaging of α4β2* Nicotinic Acetylcholine Receptors: Quantitative Analysis of 18F-Nifene Kinetics in the Nonhuman Primate

The PET radioligand 2-fluoro-3-[2-((S)-3-pyrrolinyl)methoxy]pyridine (18F-nifene) is an α4β2* nicotinic acetylcholine receptor (nAChR) agonist developed to provide accelerated in vivo equilibrium compared with existing α4β2* radioligands. The goal of this work was to analyze the in vivo kinetic properties of 18F-nifene with both kinetic modeling and graphical analysis techniques. Methods: Dynamic PET experiments were performed on 4 rhesus monkeys (female; age range, 9–13 y) using a small-animal PET scanner. Studies began with a high-specific-activity 18F-nifene injection, followed by a coinjection of 18F-nifene and unlabeled nifene at 60 min. Sampling of arterial blood with metabolite analysis was performed throughout the experiment to provide a parent radioligand input function. In vivo kinetics were characterized with both a 1-tissue-compartment model (1TCM) and a 2-tissue-compartment model, Logan graphical methods (both with and without blood sampling), and the multilinear reference tissue model. Total distribution volumes and nondisplaceable binding potentials (BPND) were used to compare regional binding of 18F-nifene. Regions examined include the anteroventral thalamus, lateral geniculate body, frontal cortex, subiculum, and cerebellum. Results: The rapid uptake and binding of 18F-nifene in nAChR-rich regions of the brain was appropriately modeled using the 1TCM. No evidence for specific binding of 18F-nifene in the cerebellum was detected on the basis of the coinjection studies, suggesting the suitability of the cerebellum as a reference region. Total distribution volumes in the cerebellum were 6.91 ± 0.61 mL/cm3. BPND values calculated with the 1TCM were 1.60 ± 0.17, 1.35 ± 0.16, 0.26 ± 0.08, and 0.30 ± 0.07 in the anteroventral thalamus, lateral geniculate body, frontal cortex, and subiculum, respectively. For all brain regions, there was a less than 0.04 absolute difference in the average BPND values calculated with each of the 1TCM, multilinear reference tissue model, and Logan methods. Conclusion: The fast kinetic properties and specific regional binding of 18F-nifene promote extension of the radioligand into preclinical animal models and human subjects.

Serotonin Transporter Genotype Affects Serotonin 5-HT1A Binding in Primates

Disruption of the serotonin system has been implicated in anxiety and depression and a related genetic variation has been identified that may predispose individuals for these illnesses. The relationship of a functional variation of the serotonin transporter promoter gene (5-HTTLPR) on serotonin transporter binding using in vivo imaging techniques have yielded inconsistent findings when comparing variants for short (s) and long (l) alleles. However, a significant 5-HTTLPR effect on receptor binding at the 5-HT1A receptor site has been reported in humans, suggesting the 5-HTTLPR polymorphism may play a role in serotonin (5-HT) function. Rhesus monkeys possess a 5-HTTLPR length polymorphism similar to humans and serve as an excellent model for studying the effects of this orthologous genetic variation on behaviors and neurochemical functions related to the 5-HT system. In this study, PET imaging of [18F]mefway was performed on 58 rhesus monkeys (33 l/l, 25 s-carriers) to examine the relation between 5-HT1A receptor-specific binding and 5-HTTLPR genotypes. Significantly lower 5-HT1A binding was found in s-carrier subjects throughout both cortical brain regions and the raphe nuclei. These results demonstrate that the underlying 5-HT neurochemical system is influenced by this functional polymorphism and illustrate the strong potential for extending the nonhuman primate model into investigating the role of this genetic variant on behavior and gene–environment interactions.

Prenatal Stress Induces Increased Striatal Dopamine Transporter Binding in Adult Nonhuman Primates

BACKGROUND To determine the effects in adult offspring of maternal exposure to stress and alcohol during pregnancy, we imaged striatal and midbrain dopamine transporter (DAT) binding by positron emission tomography in rhesus monkeys (Macaca mulatta). We also evaluated the relationship between DAT binding and behavioral responses previously found to relate to dopamine D2 receptor density (responsivity to tactile stimuli, performance on a learning task, and behavior during a learning task). METHODS Subjects were adult offspring derived from a 2 × 2 experiment in which pregnant monkeys were randomly assigned to control, daily mild stress exposure (acoustic startle), voluntary consumption of moderate-level alcohol, or both daily stress and alcohol. Adult offspring (n = 38) were imaged by positron emission tomography with the DAT ligand [(18)F]2β-carbomethoxy-3β-(4-chlorophenyl)-8-(2-fluoroethyl)-nortropane ([(18)F]FECNT). RESULTS Results showed that prenatal stress yielded an overall increase of 15% in [(18)F]FECNT binding in the striatum (p = .016), 17% greater binding in the putamen (p = .012), and 13% greater binding in the head of the caudate (p = .028) relative to animals not exposed to prenatal stress. Striatal [(18)F]FECNT binding correlated negatively with habituation to repeated tactile stimulation and positively with tactile responsivity. There were no significant effects of prenatal alcohol exposure on [(18)F]FECNT binding. CONCLUSIONS Maternal exposure to mild daily stress during pregnancy yielded increases in striatal DAT availability that were apparent in adult offspring and were associated with behavioral characteristics reflecting tactile hyperresponsivity, a condition associated with problem behaviors in children.

Positron emission tomography (PET) of radiotracer uptake and distribution in living plants: methodological aspects

The use of radioisotopes as tracers in biology was pioneered in 1923 by Hevesy in the study of the uptake and distribution of lead in the bean plant. This seminal work has spawned applications of nuclear analytical methods in the life sciences. In medicine, nuclear imaging has become a standard part of patient care. Positron emission tomography (PET) is a nuclear medicine imaging modality that allows assessment of biochemical processes in vivo, to diagnose and stage diseases and monitor their treatment. Going full circle to Hevesy’s original biological study, PET imaging can be applied to the assessment of radiotracer uptake and distribution in living plants non-invasively. In order to apply PET imaging to the study of plants, several methodological aspects were examined in this study with the goal of establishing protocols that would be useful to investigators with access to commercial small animal PET scanners. The methodological aspects examined in this study included (a) the selection of suitable plant models, (b) standardization of scanning strategies such as PET tracer “injection” into plants and (c) imaging protocols to optimize detection of annihilation gammas.

Initial investigation of three selective and potent small molecule oxytocin receptor PET ligands in New World monkeys

The neuropeptide oxytocin is part of a neuroendocrine system that has physiological effects ranging from ensuring uterine myometrial contractions at parturition and post-partum mammary gland milk ejection to the modulation of neural control of social relationships. This initial study was performed to investigate the potential use of positron emission tomography (PET) for localizing oxytocin receptors in two New World primates. Three biomarkers for PET (1-3) that are known to have high affinity and selectivity for the human oxytocin receptor were investigated in the common marmoset (Callithrix jacchus) via PET imaging. Brain penetration, and uptake in the salivary gland area were both observed with biomarkers 2 and 3. No brain penetration was observed with 1, but uptake was observed more specifically in several peripheral endocrine glands compared to 2 or 3. Biomarker 2, which displayed the best brain penetration of the three biomarkers in the marmoset, was then investigated in the monogamous coppery titi monkey (Callicebus cupreus) in a brain scan and a limited full body scan. No significant brain penetration of 2 was observed in the titi monkey, but significant uptake was observed in various locations throughout the periphery. Metabolism of 2 was suspected to have been significant based upon HPLC analysis of blood draws, but parent compound was still present near the end of the scan. Follow-up investigations will focus on next generation biomarkers bearing improved binding characteristics and brain penetrability as well as investigating tissue in regions where biomarker uptake was observed.

Measuring α4β2∗ Nicotinic Acetylcholine Receptor Density in Vivo with [18F]nifene PET in the Nonhuman Primate

[18F]Nifene is an agonist PET radioligand developed to image α4β2∗ nicotinic acetylcholine receptors (nAChRs). This work aims to quantify the receptor density (Bmax) of α4β2∗ nAChRs and the in vivo (apparent) dissociation constant (KDapp) of [18F]nifene. Multiple-injection [18F]nifene experiments with varying cold nifene masses were conducted on four rhesus monkeys with a microPET P4 scanner. Compartment modeling techniques were used to estimate regional Bmax values and a global value of KDapp. The fast kinetic properties of [18F]nifene also permitted alternative estimates of Bmax and KDapp at transient equilibrium with the same experimental data using Scatchard-like methodologies. Averaged across subjects, the compartment modeling analysis yielded Bmax values of 4.8 ± 1.4, 4.3 ±1.0, 1.2 ± 0.4, and 1.2 ± 0.3 pmol/mL in the regions of antereoventral thalamus, lateral geniculate, frontal cortex, and subiculum, respectively. The KDapp of nifene was 2.4 ± 0.3 pmol/mL. The Scatchard analysis based on graphical evaluation of the data after transient equilibrium yielded Bmax estimations comparable to the modeling results with a positive bias of 28%. These findings show the utility of [18F]nifene for measuring α4β2∗ nAChR Bmax in vivo in the rhesus monkey with a single PET experiment.

Changes in the α4β2* nicotinic acetylcholine system during chronic controlled alcohol exposure in nonhuman primates

BACKGROUND The precise nature of modifications to the nicotinic acetylcholine receptor (nAChR) system in response to chronic ethanol exposure is poorly understood. The present work used PET imaging to assay α4β2* nAChR binding levels of eight rhesus monkeys before and during controlled chronic ethanol intake. METHODS [(18)F]Nifene PET scans were conducted prior to alcohol exposure, and then again after at least 8 months controlled ethanol exposure, including 6 months at 1.5 g/kg/day following a dose escalation period. Receptor binding levels were quantified with binding potentials (BPND) using the cerebellum as a reference region. Alcohol self-administration was assessed as average daily alcohol intake during a 2 month free drinking period immediately following controlled alcohol. RESULTS Significant decreases in α4β2* nAChR binding were observed in both frontal and insular cortex in response to chronic ethanol exposure. During chronic alcohol exposure, BPND in the lateral geniculate region correlated positively with the amount of alcohol consumed during free drinking. CONCLUSIONS The observed decreases in nAChR availability following chronic alcohol consumption suggest alterations to this receptor system in response to repeated alcohol administration, making this an important target for further study in alcohol abuse and alcohol and nicotine codependence.

Measurement of 5-HT1A Receptor Density and in-vivo Binding Parameters of [18F]mefway in the Nonhuman Primate

The goal of this work was to characterize the in-vivo behavior of [18F]mefway as a suitable positron emission tomography (PET) radiotracer for the assay of 5-hydroxytryptamine1A (5-HT1A) receptor density (Bmax). Six rhesus monkeys were studied using a multiple-injection (M-I) protocol consisting of three sequential bolus injections of [18F]mefway. Injection times and amounts of unlabeled mefway were optimized for the precise measurement of Bmax and specific binding parameters koff and kon for estimation of apparent KD. The PET time series were acquired for 180 minutes with arterial sampling performed throughout. Compartmental analysis using the arterial input function was performed to obtain estimates for K1, k2, koff, Bmax, and KDapp in the cerebral cortex and raphe nuclei (RN) using a model that accounted for nontracer doses of mefway. Averaged over subjects, highest binding was seen in the mesial temporal and dorsal anterior cingulate cortices with Bmax values of 42±8 and 36±8 pmol/mL, respectively, and lower values in the superior temporal cortex, RN, and parietal cortex of 24±4, 19±4, and 13±2 pmol/mL, respectively. The KDapp of mefway for the 5-HT1A receptor sites was 4.3±1.3 nmol/L. In conclusion, these results show that M-I [18F]mefway PET experiments can be used for the in-vivo measurement of 5-HT1A receptor density.

5-HT1A sex based differences in Bmax, in vivo KD, and BPND in the nonhuman primate.

UNLABELLED Serotonin (5-HT) dysfunction has been implicated in neuropsychiatric illnesses and may play a pivotal role in the differential prevalence of depression between the sexes. Previous PET studies have revealed sex-based differences in 5-HT1A binding potential (BPND). The binding potential is a function of the radioligand-receptor affinity (1/KDapp), and receptor density (Bmax). In this work, we use a multiple-injection (MI) PET protocol and the 5-HT1A receptor antagonist, [(18)F]mefway, to compare sex-based differences of in vivo affinity, Bmax, and BPND in rhesus monkeys. METHODS PET [(18)F]mefway studies were performed on 17 (6m, 11f) rhesus monkeys using a 3-injection protocol that included partial saturation injections of mefway. Compartmental modeling was performed using a model to account for non-tracer doses of mefway for the estimation of KDapp and Bmax. BPND estimates were also acquired from the first injection (high specific activity [(18)F]mefway, 90-minute duration) for comparison using the cerebellum (CB) as a reference region. Regions of interest were selected in 5-HT1A binding regions of the hippocampus (Hp), dorsal anterior cingulate cortex (dACC), amygdala (Am), and raphe nuclei (RN). RESULTS Female subjects displayed significantly (*p<0.05) lower KDapp in the Hp (-32%), Am (-38%), and RN (-37%). Only the Hp displayed significant differences in Bmax with females having a Bmax of -29% compared to males. Male subjects demonstrated significantly lower BPND measurements in the Am (14%) and RN (29%). CONCLUSION These results suggest that the higher BPND values found in females are the result of lower [(18)F]mefway KDapp. Although a more experimentally complex measurement, separate assay of KDapp and Bmax provides a more sensitive measure than BPND to identify the underlying differences between females and males in 5-HT1A function.