Distribution of norepinephrine transporters in the non-human primate brain

Evaluation of brain activation related to resting pain using functional magnetic resonance imaging in cynomolgus macaques undergoing knee surgery

Purpose

Functional magnetic resonance imaging (fMRI) visualizes hemodynamic responses associated with brain and spinal cord activation. Various types of pain have been objectively assessed using fMRI as considerable brain activations. This study aimed to develop a pain model in cynomolgus macaques undergoing knee surgery and confirm brain activation due to resting pain after knee surgery.

Methods

An osteochondral graft surgery on the femoral condyle in the unilateral knee was performed on four cynomolgus macaques (Macaca fascicularis). Resting pain was evaluated as changes in brain fMRI findings with a 3.0-T MRI scanner preoperatively, postoperatively, and after postoperative administration of morphine. In the fMRI analysis, Z-values >1.96 were considered statistically significant.

Results

Brain activation without stimulation after surgery in the cingulate cortex (3.09) and insular cortex (3.06) on the opposite side of the surgery was significantly greater than that before surgery (1.05 and 1.03, respectively) according to fMRI. After the administration of morphine, activation due to resting pain decreased in the cingulate cortex (1.38) and insular cortex (1.21).

Conclusion

Osteochondral graft surgery on the femoral condyle can lead to postoperative resting pain. fMRI can reveal activation in pain-related brain areas and evaluate resting pain due to knee surgery.

Mesial temporal dopamine: From biology to behaviour

While colloquially recognized for its role in pleasure, reward, and affect, dopamine is also necessary for proficient action control. Many motor studies focus on dopaminergic transmission along the nigrostriatal pathway, using Parkinson's disease as a model of a dorsal striatal lesion. Less attention to the mesolimbic pathway and its role in motor control has led to an important question related to the limbic-motor network. Indeed, secondary targets of the mesolimbic pathway include the hippocampus and amygdala, and these are linked to the motor cortex through the substantia nigra and thalamus. The modulatory impact of dopamine in the hippocampus and amygdala in humans is a focus of current investigations. This review explores dopaminergic activity in the mesial temporal lobe by summarizing dopaminergic networks and transmission in these regions and examining their role in behaviour and disease.

PET reporter systems for the brain

Positron emission tomography (PET) can be used as a noninvasive method to longitudinally monitor and quantify the expression of proteins in the brain in vivo. It can be used to monitor changes in biomarkers of mental health disorders, and to assess therapeutic interventions such as stem cell and molecular genetic therapies. The utility of PET monitoring depends on the availability of a radiotracer with good central nervous system (CNS) penetration and high selectivity for the target protein. This review evaluates existing methods for the visualization of reporter proteins and/or protein function using PET imaging, focusing on engineered systems, and discusses possible approaches for future success in the development of high-sensitivity and high-specificity PET reporter systems for the brain.

Modulation of resting-state functional connectivity in default mode network is associated with the long-term treatment outcome in major depressive disorder

BACKGROUND

Treatment non-response and recurrence are the main sources of disease burden in major depressive disorder (MDD). However, little is known about its neurobiological mechanism concerning the brain network changes accompanying pharmacotherapy. The present study investigated the changes in the intrinsic brain networks during 6-month antidepressant treatment phase associated with the treatment response and recurrence in MDD.

METHODS

Resting-state functional magnetic resonance imaging was acquired from untreated patients with MDD and healthy controls at baseline. The patients' depressive symptoms were monitored by using the Hamilton Rating Scale for Depression (HAMD). After 6 months of antidepressant treatment, patients were re-scanned and followed up every 6 months over 2 years. Traditional statistical analysis as well as machine learning approaches were conducted to investigate the longitudinal changes in macro-scale resting-state functional network connectivity (rsFNC) strength and micro-scale resting-state functional connectivity (rsFC) associated with long-term treatment outcome in MDD.

RESULTS

Repeated measures of the general linear model demonstrated a significant difference in the default mode network (DMN) rsFNC change before and after the 6-month antidepressant treatment between remitters and non-remitters. The difference in the rsFNC change over the 6-month antidepressant treatment between recurring and stable MDD was also specific to DMN. Machine learning analysis results revealed that only the DMN rsFC change successfully distinguished non-remitters from the remitters at 6 months and recurring from stable MDD during the 2-year follow-up.

CONCLUSION

Our findings demonstrated that the intrinsic DMN connectivity could be a unique and important target for treatment and recurrence prevention in MDD.

Levodopa responsive freezing of gait is associated with reduced norepinephrine transporter binding in Parkinson's disease

BACKGROUND

Freezing of gait (FOG) is a major cause of falling in Parkinson's disease (PD) and can be responsive or unresponsive to levodopa. Pathophysiology is poorly understood.

OBJECTIVE

To examine the link between noradrenergic systems, the development of FOG in PD and its responsiveness to levodopa.

METHODS

We examined norepinephrine transporter (NET) binding via brain positron emission tomography (PET) to evaluate changes in NET density associated with FOG using the high affinity selective NET antagonist radioligand [¹¹C]MeNER (2S,3S)(2-[α-(2-methoxyphenoxy)benzyl]morpholine) in 52 parkinsonian patients. We used a rigorous levodopa challenge paradigm to characterize PD patients as non-freezing (NO-FOG, N = 16), levodopa responsive freezing (OFF-FOG, N = 10), and levodopa-unresponsive freezing (ONOFF-FOG, N = 21), and also included a non-PD FOG group, primary progressive freezing of gait (PP-FOG, N = 5).

RESULTS

Linear mixed models identified significant reductions in whole brain NET binding in the OFF-FOG group compared to the NO-FOG group (-16.8%, P = 0.021) and regionally in the frontal lobe, left and right thalamus, temporal lobe, and locus coeruleus, with the strongest effect in right thalamus (P = 0.038). Additional regions examined in a post hoc secondary analysis including the left and right amygdalae confirmed the contrast between OFF-FOG and NO-FOG (P = 0.003). A linear regression analysis identified an association between reduced NET binding in the right thalamus and more severe New FOG Questionnaire (N-FOG-Q) score only in the OFF-FOG group (P = 0.022).

CONCLUSION

This is the first study to examine brain noradrenergic innervation using NET-PET in PD patients with and without FOG. Based on the normal regional distribution of noradrenergic innervation and pathological studies in the thalamus of PD patients, the implications of our findings suggest that noradrenergic limbic pathways may play a key role in OFF-FOG in PD. This finding could have implications for clinical subtyping of FOG as well as development of therapies.

Mapping the primate thalamus: systematic approach to analyze the distribution of subcortical neuromodulatory afferents

Neuromodulatory afferents to thalamic nuclei are key for information transmission and thus play critical roles in sensory, motor, and limbic processes. Over the course of the last decades, diverse attempts have been made to map and describe subcortical neuromodulatory afferents to the primate thalamus, including axons using acetylcholine, serotonin, dopamine, noradrenaline, adrenaline, and histamine. Our group has been actively involved in this endeavor. The published descriptions on neuromodulatory afferents to the primate thalamus have been made in different laboratories and are not fully comparable due to methodological divergences (for example, fixation procedures, planes of cutting, techniques used to detect the afferents, different criteria for identification of thalamic nuclei…). Such variation affects the results obtained. Therefore, systematic methodological and analytical approaches are much needed. The present article proposes reproducible methodological and terminological frameworks for primate thalamic mapping. We suggest the use of standard stereotaxic planes to produce and present maps of the primate thalamus, as well as the use of the Anglo-American school terminology (vs. the German school terminology) for identification of thalamic nuclei. Finally, a public repository of the data collected under agreed-on frameworks would be a useful tool for looking up and comparing data on the structure and connections of primate thalamic nuclei. Important and agreed-on efforts are required to create, manage, and fund a unified and homogeneous resource of data on the primate thalamus. Likewise, a firm commitment of the institutions to preserve experimental brain material is much needed because neuroscience work with non-human primates is becoming increasingly rare, making earlier material still more valuable.

Transcranial Magnetic Stimulation Indices of Cortical Excitability Enhance the Prediction of Response to Pharmacotherapy in Late-Life Depression

BACKGROUND

Older adults with late-life depression (LLD) often experience incomplete or lack of response to first-line pharmacotherapy. The treatment of LLD could be improved using objective biological measures to predict response. Transcranial magnetic stimulation (TMS) can be used to measure cortical excitability, inhibition, and plasticity, which have been implicated in LLD pathophysiology and associated with brain stimulation treatment outcomes in younger adults with depression. TMS measures have not yet been investigated as predictors of treatment outcomes in LLD or pharmacotherapy outcomes in adults of any age with depression.

METHODS

We assessed whether pretreatment single-pulse and paired-pulse TMS measures, combined with clinical and demographic measures, predict venlafaxine treatment response in 76 outpatients with LLD. We compared the predictive performance of machine learning models including or excluding TMS predictors.

RESULTS

Two single-pulse TMS measures predicted venlafaxine response: cortical excitability (neuronal membrane excitability) and the variability of cortical excitability (dynamic fluctuations in excitability levels). In cross-validation, models using a combination of these TMS predictors, clinical markers of treatment resistance, and age classified patients with 73% ± 11% balanced accuracy (average correct classification rate of responders and nonresponders; permutation testing, p < .005); these models significantly outperformed (corrected t test, p = .025) models using clinical and demographic predictors alone (60% ± 10% balanced accuracy).

CONCLUSIONS

These preliminary findings suggest that single-pulse TMS measures of cortical excitability may be useful predictors of response to pharmacotherapy in LLD. Future studies are needed to confirm these findings and determine whether combining TMS predictors with other biomarkers further improves the accuracy of predicting LLD treatment outcome.

Manipulation of vocal communication and anxiety through pharmacologic modulation of norepinephrine in the Pink1-/- rat model of Parkinson disease

Vocal deficits and anxiety are common, co-occurring, and interacting signs of Parkinson Disease (PD) that have a devastating impact on quality of life. Both manifest early in the disease process. Unlike hallmark motor signs of PD, neither respond adequately to dopamine replacement therapies, suggesting that their disease-specific mechanisms are at least partially extra-dopaminergic. Because noradrenergic dysfunction is also a defining feature of PD, especially early in the disease progression, drug therapies targeting norepinephrine are being trialed for treatment of motor and non-motor impairments in PD. Research assessing the effects of noradrenergic manipulation on anxiety and vocal impairment in PD, however, is sparse. In this pre-clinical study, we quantified the influence of pharmacologic manipulation of norepinephrine on vocal impairment and anxiety in Pink1-/- rats, a translational model of PD that demonstrates both vocal deficits and anxiety. Ultrasonic vocalization acoustics, anxiety behavior, and limb motor activity were tested twice for each rat: after injection of saline and after one of three drugs. We hypothesized that norepinephrine reuptake inhibitors (atomoxetine and reboxetine) and a β receptor antagonist (propranolol) would decrease vocal impairment and anxiety compared to saline, without affecting spontaneous motor activity. Our results demonstrated that atomoxetine and reboxetine decreased anxiety behavior. Atomoxetine also modulated ultrasonic vocalization acoustics, including an increase in vocal intensity, which is almost always reduced in animal models and patients with PD. Propranolol did not affect anxiety or vocalization. Drug condition did not influence spontaneous motor activity. These studies demonstrate relationships among vocal impairment, anxiety, and noradrenergic systems in the Pink1-/- rat model of PD.

When Helping Is Risky: The Behavioral and Neurobiological Trade-off of Social and Risk Preferences

Helping other people can entail risks for the helper. For example, when treating infectious patients, medical volunteers risk their own health. In such situations, decisions to help should depend on the individual's valuation of others' well-being (social preferences) and the degree of personal risk the individual finds acceptable (risk preferences). We investigated how these distinct preferences are psychologically and neurobiologically integrated when helping is risky. We used incentivized decision-making tasks (Study 1; N = 292 adults) and manipulated dopamine and norepinephrine levels in the brain by administering methylphenidate, atomoxetine, or a placebo (Study 2; N = 154 adults). We found that social and risk preferences are independent drivers of risky helping. Methylphenidate increased risky helping by selectively altering risk preferences rather than social preferences. Atomoxetine influenced neither risk preferences nor social preferences and did not affect risky helping. This suggests that methylphenidate-altered dopamine concentrations affect helping decisions that entail a risk to the helper.

Quantification of brainstem norepinephrine relative to vocal impairment and anxiety in the Pink1-/- rat model of Parkinson disease

Vocal communication impairment and anxiety are co-occurring and interacting signs of Parkinson Disease (PD) that are common, poorly understood, and under-treated. Both vocal communication and anxiety are influenced by the noradrenergic system. In light of this shared neural substrate and considering that noradrenergic dysfunction is a defining characteristic of PD, tandem investigation of vocal impairment and anxiety in PD relative to noradrenergic mechanisms is likely to yield insights into the underlying disease-specific causes of these impairments. In order to address this gap in knowledge, we assessed vocal impairment and anxiety behavior relative to brainstem noradrenergic markers in a genetic rat model of early-onset PD (Pink1-/-) and wild type controls (WT). We hypothesized that 1) brainstem noradrenergic markers would be disrupted in Pink1-/-, and 2) brainstem noradrenergic markers would be associated with vocal acoustic changes and anxiety level. Rats underwent testing of ultrasonic vocalization and anxiety (elevated plus maze) at 4, 8, and 12 months of age. At 12 months, brainstem norepinephrine markers were quantified with immunohistochemistry. Results demonstrated that vocal impairment and anxiety were increased in Pink1-/- rats, and increased anxiety was associated with greater vocal deficit in this model of PD. Further, brainstem noradrenergic markers including TH and α1 adrenoreceptor immunoreactivity in the locus coeruleus, and β1 adrenoreceptor immunoreactivity in vagal nuclei differed by genotype, and were associated with vocalization and anxiety behavior. These findings demonstrate statistically significant relationships among vocal impairment, anxiety, and brainstem norepinephrine in the Pink1-/- rat model of PD.

Distribution of the Noradrenaline Innervation and Adrenoceptors in the Macaque Monkey Thalamus

Noradrenaline (NA) in the thalamus has important roles in physiological, pharmacological, and pathological neuromodulation. In this work, a complete characterization of NA axons and Alpha adrenoceptors distributions is provided.

NA axons, revealed by immunohistochemistry against the synthesizing enzyme and the NA transporter, are present in all thalamic nuclei. The most densely innervated ones are the midline nuclei, intralaminar nuclei (paracentral and parafascicular), and the medial sector of the mediodorsal nucleus (MDm). The ventral motor nuclei and most somatosensory relay nuclei receive a moderate NA innervation. The pulvinar complex receives a heterogeneous innervation. The lateral geniculate nucleus (GL) has the lowest NA innervation.

Alpha adrenoceptors were analyzed by in vitro quantitative autoradiography. Alpha-1 receptor densities are higher than Alpha-2 densities. Overall, axonal densities and Alpha adrenoceptor densities coincide; although some mismatches were identified. The nuclei with the highest Alpha-1 values are MDm, the parvocellular part of the ventral posterior medial nucleus, medial pulvinar, and midline nuclei. The nucleus with the lowest Alpha-1 receptor density is GL. Alpha-2 receptor densities are highest in the lateral dorsal, centromedian, medial and inferior pulvinar, and midline nuclei.

These results suggest a role for NA in modulating thalamic involvement in consciousness, limbic, cognitive, and executive functions.

Cortical inhibition, facilitation and plasticity in late-life depression: effects of venlafaxine pharmacotherapy

BACKGROUND

Late-life depression is often associated with non-response or relapse following conventional antidepressant treatment. The pathophysiology of late-life depression likely involves a complex interplay between aging and depression, and may include abnormalities in cortical inhibition and plasticity. However, the extent to which these cortical processes are modifiable by antidepressant pharmacotherapy is unknown.

METHODS

Sixty-eight patients with late-life depression received 12 weeks of treatment with open-label venlafaxine, a serotonin-norepinephrine reuptake inhibitor (≤ 300 mg/d). We combined transcranial magnetic stimulation of the left motor cortex with electromyography recordings from the right hand to measure cortical inhibition using contralateral cortical silent period and paired-pulse short-interval intracortical inhibition paradigms; cortical facilitation using a paired-pulse intracortical facilitation paradigm; and short-term cortical plasticity using a paired associative stimulation paradigm. All measures were collected at baseline, 1 week into treatment (n = 23) and after approximately 12 weeks of treatment.

RESULTS

Venlafaxine did not significantly alter cortical inhibition, facilitation or plasticity after 1 or 12 weeks of treatment. Improvements in depressive symptoms during treatment were not associated with changes in cortical physiology.

LIMITATIONS

The results presented here are specific to the motor cortex. Future work should investigate whether these findings extend to cortical areas more closely associated with depression, such as the dorsolateral prefrontal cortex.

CONCLUSION

These findings suggest that antidepressant treatment with venlafaxine does not exert meaningful changes in motor cortical inhibition or plasticity in late-life depression. The absence of changes in motor cortical physiology, alongside improvements in depressive symptoms, suggests that age-related changes may play a role in previously identified abnormalities in motor cortical processes in latelife depression, and that venlafaxine treatment does not target these abnormalities.

The Locus Coeruleus in Aging and Alzheimer's Disease: A Postmortem and Brain Imaging Review

The locus coeruleus (LC), a tiny nucleus in the brainstem and the principal site of noradrenaline synthesis, has a major role in regulating autonomic function, arousal, attention, and neuroinflammation. LC dysfunction has been linked to a range of disorders; however particular interest is given to the role it plays in Alzheimer's disease (AD). The LC undergoes significant neuronal loss in AD, thought to occur early in the disease process. While neuronal loss in the LC has also been suggested to occur in aging, this relationship is less clear as the findings have been contradictory. LC density has been suggested to be indicative of cognitive reserve and the evidence for these claims will be discussed. Recent imaging techniques allowing visualization of the LC in vivo using neuromelanin-sensitive MRI are developing our understanding of the role of LC in aging and AD. Tau pathology within the LC is evident at an early age in most individuals; however, the relationship between tau accumulation and neuronal loss and why some individuals then develop AD is not understood. Neuromelanin pigment accumulates within LC cells with age and is proposed to be toxic and inflammatory when released into the extracellular environment. This review will explore our current knowledge of the LC changes in both aging and AD from postmortem, imaging, and experimental studies. We will discuss the reasons behind the susceptibility of the LC to neuronal loss, with a focus on the role of extracellular neuromelanin and neuroinflammation caused by the dysfunction of the LC-noradrenaline pathway.

Rapidly (and Successfully) Translating Novel Brain Radiotracers From Animal Research Into Clinical Use

The advent of preclinical research scanners for in vivo imaging of small animals has added confidence into the multi-step decision-making process of radiotracer discovery and development. Furthermore, it has expanded the utility of imaging techniques available to dissect clinical questions, fostering a cyclic interaction between the clinical and the preclinical worlds. Significant efforts from medicinal chemistry have also made available several high-affinity and selective compounds amenable for radiolabeling, that target different receptors, transporters and enzymes in vivo. This substantially increased the range of applications of molecular imaging using positron emission tomography (PET) or single photon emission computed tomography (SPECT). However, the process of developing novel radiotracers for in vivo imaging of the human brain is a multi-step process that has several inherent pitfalls and technical difficulties, which often hampers the successful translation of novel imaging agents from preclinical research into clinical use. In this paper, the process of radiotracer development and its relevance in brain research is discussed; as well as, its pitfalls, technical challenges and future promises. Examples of successful and unsuccessful translation of brain radiotracers will be presented.

Background luminance effects on pupil size associated with emotion and saccade preparation

Pupil dilation is consistently evoked by affective and cognitive processing, and this dilation can result from sympathetic activation or parasympathetic inhibition. The relative contributions of the sympathetic and parasympathetic systems on the pupillary response induced by emotion and cognition may be different. Sympathetic and parasympathetic activity is regulated by global luminance level. Higher luminance levels lead to greater activation of the parasympathetic system while lower luminance levels lead to greater activation of the sympathetic system. To understand the contributions of the sympathetic and parasympathetic nervous systems to pupillary responses associated with emotion and saccade preparation, emotional auditory stimuli were presented following the fixation cue whose color indicated instruction to perform a pro- or anti-saccade while varying the background luminance level. Pupil dilation was evoked by emotional auditory stimuli and modulated by arousal level. More importantly, greater pupil dilation was observed with a dark background, compared to a bright background. In contrast, pupil dilation responses associated with saccade preparation were larger with the bright background than the dark background. Together, these results suggest that arousal-induced pupil dilation was mainly mediated by sympathetic activation, but pupil dilation related to saccade preparation was primarily mediated by parasympathetic inhibition.

Organic cation transporter 3 and the dopamine transporter differentially regulate catecholamine uptake in the basolateral amygdala and nucleus accumbens

Regional alterations in kinetics of catecholamine uptake are due in part to variations in clearance mechanisms. The rate of clearance is a critical determinant of the strength of catecholamine signaling. Catecholamine transmission in the nucleus accumbens core (NAcc) and basolateral amygdala (BLA) is of particular interest due to involvement of these regions in cognition and motivation. Previous work has shown that catecholamine clearance in the NAcc is largely mediated by the dopamine transporter (DAT), but clearance in the BLA is less DAT-dependent. A growing body of literature suggests that organic cation transporter 3 (OCT3) also contributes to catecholamine clearance in both regions. Consistent with different clearance mechanisms between regions, catecholamine clearance is more rapid in the NAcc than in the BLA, though mechanisms underlying this have not been resolved. We compared the expression of DAT and OCT3 and their contributions to catecholamine clearance in the NAcc and BLA. We found DAT protein levels were ~ 4-fold higher in the NAcc than in the BLA, while OCT3 protein expression was similar between the two regions. Immunofluorescent labeling of the two transporters in brain sections confirmed these findings. Ex vivo voltammetry demonstrated that the magnitude of catecholamine release was greater, and the clearance rate was faster in the NAcc than in the BLA. Additionally, catecholamine clearance in the BLA was more sensitive to the OCT3 inhibitor corticosterone, while clearance in the NAcc was more cocaine sensitive. These distinctions in catecholamine clearance may underlie differential effects of catecholamines on behavioral outputs mediated by these regions.

Unravelling the effects of methylphenidate on the dopaminergic and noradrenergic functional circuits

Functional magnetic resonance imaging (fMRI) can be combined with drugs to investigate the system-level functional responses in the brain to such challenges. However, most psychoactive agents act on multiple neurotransmitters, limiting the ability of fMRI to identify functional effects related to actions on discrete pharmacological targets. We recently introduced a multimodal approach, REACT (Receptor-Enriched Analysis of functional Connectivity by Targets), which offers the opportunity to disentangle effects of drugs on different neurotransmitters and clarify the biological mechanisms driving clinical efficacy and side effects of a compound. Here, we focus on methylphenidate (MPH), which binds to the dopamine transporter (DAT) and the norepinephrine transporter (NET), to unravel its effects on dopaminergic and noradrenergic functional circuits in the healthy brain at rest. We then explored the relationship between these target-enriched resting state functional connectivity (FC) maps and inter-individual variability in behavioural responses to a reinforcement-learning task encompassing a novelty manipulation to disentangle the molecular systems underlying specific cognitive/behavioural effects. Our main analysis showed a significant MPH-induced FC increase in sensorimotor areas in the functional circuit associated with DAT. In our exploratory analysis, we found that MPH-induced regional variations in the DAT and NET-enriched FC maps were significantly correlated with some of the inter-individual differences on key behavioural responses associated with the reinforcement-learning task. Our findings show that main MPH-related FC changes at rest can be understood through the distribution of DAT in the brain. Furthermore, they suggest that when compounds have mixed pharmacological profiles, REACT may be able to capture regional functional effects that are underpinned by the same cognitive mechanism but are related to engagement of distinct molecular targets.

Microglial CX3CR1 production increases in Alzheimer's disease and is regulated by noradrenaline

The loss of noradrenergic neurons and subsequent reduction of brain noradrenaline (NA) levels are associated with the progression of Alzheimer's disease (AD). This seems to be due mainly to the ability of NA to reduce the activation of microglial cells. We previously observed that NA induces the production of the chemokine Fractalkine/CX3CL1 in neurons. The activation of microglial CX3CR1, sole receptor for CX3CL1, reduces the activation of microglia, which is known to largely contribute to the neuronal damage characteristic of AD. Therefore, alterations of CX3CR1 production in microglia could translate into the enhancement or inhibition of CX3CL1 anti-inflammatory effects. In order to determine if microglial CX3CR1 production is altered in AD and if NA can control it, CX3CR1 expression and synthesis were analyzed in 5xFAD mice and human AD brain samples. In addition, the effects of NA and its reuptake inhibitor reboxetine were analyzed in microglial cultures and mice respectively. Our results indicate that in AD CX3CR1 production is increased in the brain cortex and that reboxetine administration further increases it and enhances microglial reactivity toward amyloid beta plaques. However, direct administration of NA to primary rat microglia or human HMC3 cells inhibits CX3CR1 production, suggesting that microglia responses to NA may be altered in the absence of CX3CL1-producing neurons or other nonmicroglial external factors.

Therapeutic Effect of the Substrate-Selective COX-2 Inhibitor IMMA in the Animal Model of Chronic Constriction Injury

Enhancement of endocannabinoid signaling has emerged as an attractive strategy for the treatment of pain. In addition to the well-characterized hydrolytic pathways, cyclooxygenase-2 (COX-2) mediated oxygenation is thought to be an alternative route for endocannabinoid metabolism and therefore provides a new avenue for drug intervention. In this study, we examined the therapeutic effect of indomethacin morpholinamide (IMMA), a novel substrate-selective COX-2 inhibitor, in the chronic constriction injury (CCI) mouse model. Treatment with IMMA significantly alleviated hyperalgesia and mechanical allodynia demonstrated by increased thermal withdrawal latency in Hargreaves test and tactile thresholds in Von Frey test. Accumulation of astrocytes and microglia in spinal cord dorsal horn and infiltration of macrophages into the dorsal root ganglion and sciatic nerve were reduced by drug treatment. Co-administration of the CB2 receptor antagonist, but not the CB1 receptor antagonist partially reversed the inhibitory effect of IMMA on pain sensitivity and inflammatory infiltrates. IMMA downregulated the mRNA expression of TNF-α and IL-1β and the production of IL-6 and MCP-1 proteins in the ipsilateral sciatic nerve. The enhanced NF-κB DNA binding activity in the CCI mouse dorsal spinal cord was also significantly reduced, suggesting that inactivation of NF-κB contributes to the anti-inflammatory property of IMMA. However, different from the previous reports showing that IMMA can increase endocannabinoids without interfering with arachidonic acid metabolism, treatment with IMMA failed to elevate the endogenous levels of AEA and 2-AG, but significantly reduced the production of prostaglandin E₂ (PGE₂). Furthermore, the mRNA expression of enzymes involved in PGE₂ production, COX-2 and prostaglandin E synthase 2 in the ipsilateral sciatic nerve was also suppressed by IMMA treatment. Taken together, these results suggested that IMMA might exert anti-nociceptive effects through multiple mechanisms which include, but are not limited to, CB2 receptor activation and reduced PGE₂ production.

Boosting Norepinephrine Transmission Triggers Flexible Reconfiguration of Brain Networks at Rest

The locus coeruleus-norepinephrine (LC-NE) system is thought to act as a reset signal allowing brain network reorganization in response to salient information in the environment. However, no direct evidence of NE-dependent whole-brain reorganization has ever been described. We used resting-state functional magnetic resonance imaging in monkeys to investigate the impact of NE-reuptake inhibition on whole-brain connectivity patterns. We found that boosting NE transmission changes functional connectivity between and within resting-state networks. It modulated the functional connectivity pattern of a brainstem network including the LC region and interactions between associative and sensory-motor networks as well as within sensory-motor networks. Among the observed changes, those involving the fronto-parietal attention network exhibited a unique pattern of uncoupling with other sensory-motor networks and correlation switching from negative to positive with the brainstem network that included the LC nucleus. These findings provide the first empirical evidence of NE-dependent large-scale brain network reorganization and further demonstrate that the fronto-parietal attention network represents a central feature within this reorganization.

Localization of the delta opioid receptor and corticotropin-releasing factor in the amygdalar complex: role in anxiety

It is well established that central nervous system norepinephrine (NE) and corticotropin-releasing factor (CRF) systems are important mediators of behavioral responses to stressors. More recent studies have defined a role for delta opioid receptors (DOPR) in maintaining emotional valence including anxiety. The amygdala plays an important role in processing emotional stimuli, and has been implicated in the development of anxiety disorders. Activation of DOPR or inhibition of CRF in the amygdala reduces baseline and stress-induced anxiety-like responses. It is not known whether CRF- and DOPR-containing amygdalar neurons interact or whether they are regulated by NE afferents. Therefore, this study sought to better define interactions between the CRF, DOPR and NE systems in the basolateral (BLA) and central nucleus of the amygdala (CeA) of the male rat using anatomical and functional approaches. Irrespective of the amygdalar subregion, dual immunofluorescence microscopy showed that DOPR was present in CRF-containing neurons. Immunoelectron microscopy confirmed that DOPR was localized to both dendritic processes and axon terminals in the BLA and CeA. Semi-quantitative dual immunoelectron microscopy analysis of gold-silver labeling for DOPR and immunoperoxidase labeling for CRF revealed that 55 % of the CRF neurons analyzed contained DOPR in the BLA while 67 % of the CRF neurons analyzed contained DOPR in the CeA. Furthermore, approximately 41 % of DOPR-labeled axon terminals targeted BLA neurons that expressed CRF while 29 % of DOPR-labeled axon terminals targeted CeA neurons that expressed CRF. Triple label immunofluorescence microscopy revealed that DOPR and CRF were co-localized in common cellular profiles that were in close proximity to NE-containing fibers in both subregions. These anatomical results indicate significant interactions between DOPR and CRF in this critical limbic region and reveal that NE is poised to regulate these peptidergic systems in the amygdala. Functional studies were performed to determine if activation of DOPR could inhibit the anxiety produced by elevation of NE in the amygdala using the pharmacological stressor yohimbine. Administration of the DOPR agonist, SNC80, significantly attenuated elevated anxiogenic behaviors produced by yohimbine as measured in the rat on the elevated zero maze. Taken together, results from this study demonstrate the convergence of three important systems, NE, CRF, and DOPR, in the amygdala and provide insight into their functional role in modulating stress and anxiety responses.

More than Meets the Eye: the Relationship between Pupil Size and Locus Coeruleus Activity

Using both direct neural recordings and electrical microstimulation, Joshi et al. (2016) show that locus coeruleus (LC) activity closely matches moment-to-moment changes in pupil size. But what causes these two measures to be related is not straightforward.

Evaluation of [11C]TAZA for amyloid β plaque imaging in postmortem human Alzheimer's disease brain region and whole body distribution in rodent PET/CT

OBJECTIVE

Alzheimer's disease (AD) is a neurodegenerative disease characterized by Aβ plaques in the brain. The aim of this study was to evaluate the effectiveness of a novel radiotracer, 4-[(11) C]methylamino-4'-N,N-dimethylaminoazobenzene ([(11)C]TAZA), for binding to Aβ plaques in postmortem human brain (AD and normal control (NC)).

METHODS

Radiosyntheses of [(11)C]TAZA, related [(11)C]Dalene ((11)C-methylamino-4'-dimethylaminostyrylbenzene), and reference [(11)C]PIB were carried out using [(11)C]methyltriflate prepared from [(11) C]CO(2) and purified using HPLC. In vitro binding affinities were carried out in human AD brain homogenate with Aβ plaques labeled with [(3) H]PIB. In vitro autoradiography studies with the three radiotracers were performed on hippocampus of AD and NC brains. PET/CT studies were carried out in normal rats to study brain and whole body distribution.

RESULTS

The three radiotracers were produced in high radiochemical yields (>40%) and had specific activities >37 GBq/μmol. TAZA had an affinity, K(i) = 0.84 nM and was five times more potent than PIB. [(11)C]TAZA bound specifically to Aβ plaques present in AD brains with gray matter to white matter ratios >20. [(11)C]TAZA was displaced by PIB (>90%), suggesting similar binding site for [(11)C]TAZA and [(11)C]PIB. [(11)C]TAZA exhibited slow kinetics of uptake in the rat brain and whole body images showed uptake in interscapular brown adipose tissue (IBAT). Binding in brain and IBAT were affected by preinjection of atomoxetine, a norepinephrine transporter blocker.

CONCLUSION

[(11)C]TAZA exhibited high binding to Aβ plaques in human AD hippocampus. Rat brain kinetics was slow and peripheral binding to IBAT needs to be further evaluated.

Effect of Pharmacological Interventions on the Fronto-Cingulo-Parietal Cognitive Control Network in Psychiatric Disorders: A Transdiagnostic Systematic Review of fMRI Studies

Executive function deficits, such as working memory, decision-making, and attention problems, are a common feature of several psychiatric disorders for which no satisfactory treatment exists. Here, we transdiagnostically investigate the effects of pharmacological interventions (other than methylphenidate) on the fronto-cingulo-parietal cognitive control network, in order to identify functional brain markers for future procognitive pharmacological interventions. Twenty-nine manuscripts investigated the effect of pharmacological treatment on executive function-related brain correlates in psychotic disorders (n = 11), depression (n = 4), bipolar disorder (n = 4), ADHD (n = 4), OCD (n = 2), smoking dependence (n = 2), alcohol dependence (n = 1), and pathological gambling (n = 1). In terms of impact on the fronto-cingulo-parietal network, the preliminary evidence for catechol-O-methyl-transferase inhibitors, nicotinic receptor agonists, and atomoxetine was relatively consistent, the data for atypical antipsychotics and anticonvulsants moderate, and interpretation of the data for antidepressants was hampered by the employed study designs. Increased activity in task-relevant areas and decreased activity in task-irrelevant areas were the most common transdiagnostic effects of pharmacological treatment. These markers showed good positive and moderate negative predictive value. It is concluded that fronto-cingulo-parietal activity changes can serve as a marker for future procognitive interventions. Future recommendations include the use of randomized double-blind designs and selective cholinergic and glutamatergic compounds.

Effects of abstinence from chronic cocaine self-administration on nonhuman primate dorsal and ventral noradrenergic bundle terminal field structures

Repeated exposure to cocaine is known to dysregulate the norepinephrine system, and norepinephrine has also been implicated as having a role in abstinence and withdrawal. The goal of this study was to determine the effects of exposure to cocaine self-administration and subsequent abstinence on regulatory elements of the norepinephrine system in the nonhuman primate brain. Rhesus monkeys self-administered cocaine (0.3 mg/kg/injection, 30 reinforcers/session) under a fixed-interval 3-min schedule of reinforcement for 100 sessions. Animals in the abstinence group then underwent a 30-day period during which no operant responding was conducted, followed by a final session of operant responding. Control animals underwent identical schedules of food reinforcement and abstinence. This duration of cocaine self-administration has been shown previously to increase levels of norepinephrine transporters (NET) in the ventral noradrenergic bundle terminal fields. In contrast, in the current study, abstinence from chronic cocaine self-administration resulted in elevated levels of [(3)H]nisoxetine binding to the NET primarily in dorsal noradrenergic bundle terminal field structures. As compared to food reinforcement, chronic cocaine self-administration resulted in decreased binding of [(3)H]RX821002 to α2-adrenoceptors primarily in limbic-related structures innervated by both dorsal and ventral bundles, as well as elevated binding in the striatum. However, following abstinence from responding for cocaine binding to α2-adrenoceptors was not different than in control animals. These data demonstrate the dynamic nature of the regulation of norepinephrine during cocaine use and abstinence, and provide further evidence that the norepinephrine system should not be overlooked in the search for effective pharmacotherapies for cocaine dependence.

Monoamine reuptake inhibitors in Parkinson's disease

The motor manifestations of Parkinson's disease (PD) are secondary to a dopamine deficiency in the striatum. However, the degenerative process in PD is not limited to the dopaminergic system and also affects serotonergic and noradrenergic neurons. Because they can increase monoamine levels throughout the brain, monoamine reuptake inhibitors (MAUIs) represent potential therapeutic agents in PD. However, they are seldom used in clinical practice other than as antidepressants and wake-promoting agents. This review article summarises all of the available literature on use of 50 MAUIs in PD. The compounds are divided according to their relative potency for each of the monoamine transporters. Despite wide discrepancy in the methodology of the studies reviewed, the following conclusions can be drawn: (1) selective serotonin transporter (SERT), selective noradrenaline transporter (NET), and dual SERT/NET inhibitors are effective against PD depression; (2) selective dopamine transporter (DAT) and dual DAT/NET inhibitors exert an anti-Parkinsonian effect when administered as monotherapy but do not enhance the anti-Parkinsonian actions of L-3,4-dihydroxyphenylalanine (L-DOPA); (3) dual DAT/SERT inhibitors might enhance the anti-Parkinsonian actions of L-DOPA without worsening dyskinesia; (4) triple DAT/NET/SERT inhibitors might exert an anti-Parkinsonian action as monotherapy and might enhance the anti-Parkinsonian effects of L-DOPA, though at the expense of worsening dyskinesia.

Amphetamine decreases α2C-adrenoceptor binding of [11C]ORM-13070: a PET study in the primate brain

BACKGROUND

The neurotransmitter norepinephrine has been implicated in psychiatric and neurodegenerative disorders. Examination of synaptic norepinephrine concentrations in the living brain may be possible with positron emission tomography (PET), but has been hampered by the lack of suitable radioligands.

METHODS

We explored the use of the novel α2C-adrenoceptor antagonist PET tracer [(11)C]ORM-13070 for measurement of amphetamine-induced changes in synaptic norepinephrine. The effect of amphetamine on [(11)C]ORM-13070 binding was evaluated ex vivo in rat brain sections and in vivo with PET imaging in monkeys.

RESULTS

Microdialysis experiments confirmed amphetamine-induced elevations in rat striatal norepinephrine and dopamine concentrations. Regional [(11)C]ORM-13070 receptor binding was high in the striatum and low in the cerebellum. After injection of [(11)C]ORM-13070 in rats, mean striatal specific binding ratios, determined using cerebellum as a reference region, were 1.4±0.3 after vehicle pretreatment and 1.2±0.2 after amphetamine administration (0.3mg/kg, subcutaneous). Injection of [(11)C]ORM-13070 in non-human primates resulted in mean striatal binding potential (BP ND) estimates of 0.65±0.12 at baseline. Intravenous administration of amphetamine (0.5 and 1.0mg/kg, i.v.) reduced BP ND values by 31-50%. Amphetamine (0.3mg/kg, subcutaneous) increased extracellular norepinephrine (by 400%) and dopamine (by 270%) in rat striata.

CONCLUSIONS

Together, these results indicate that [(11)C]ORM-13070 may be a useful tool for evaluation of synaptic norepinephrine concentrations in vivo. Future studies are required to further understand a potential contribution of dopamine to the amphetamine-induced effect.

Synthesis and in silico evaluation of novel compounds for PET-based investigations of the norepinephrine transporter

Since the norepinephrine transporter (NET) is involved in a variety of diseases, the investigation of underlying dysregulation-mechanisms of the norepinephrine (NE) system is of major interest. Based on the previously described highly potent and selective NET ligand 1-(3-(methylamino)-1-phenylpropyl)-3-phenyl-1,3-dihydro-2H-benzimidaz- ol-2-one (Me@APPI), this paper aims at the development of several fluorinated methylamine-based analogs of this compound. The newly synthesized compounds were computationally evaluated for their interactions with the monoamine transporters and represent reference compounds for PET-based investigation of the NET.

The norepinephrine transporter in attention-deficit/hyperactivity disorder investigated with positron emission tomography

IMPORTANCE

Attention-deficit/hyperactivity disorder (ADHD) research has long focused on the dopaminergic system's contribution to pathogenesis, although the results have been inconclusive. However, a case has been made for the involvement of the noradrenergic system, which modulates cognitive processes, such as arousal, working memory, and response inhibition, all of which are typically affected in ADHD. Furthermore, the norepinephrine transporter (NET) is an important target for frequently prescribed medication in ADHD. Therefore, the NET is suggested to play a critical role in ADHD.

OBJECTIVE

To explore the differences in NET nondisplaceable binding potential (NET BPND) using positron emission tomography and the highly selective radioligand (S,S)-[18F]FMeNER-D2 [(S,S)-2-(α-(2-[18F]fluoro[2H2]methoxyphenoxy)benzyl)morpholine] between adults with ADHD and healthy volunteers serving as controls.

DESIGN, SETTING, AND PARTICIPANTS

Twenty-two medication-free patients with ADHD (mean [SD] age, 30.7 [10.4] years; 15 [68%] men) without psychiatric comorbidities and 22 age- and sex-matched healthy controls (30.9 [10.6] years; 15 [68%] men) underwent positron emission tomography once. A linear mixed model was used to compare NET BPND between groups.

MAIN OUTCOMES AND MEASURES

The NET BPND in selected regions of interest relevant for ADHD, including the hippocampus, putamen, pallidum, thalamus, midbrain with pons (comprising a region of interest that includes the locus coeruleus), and cerebellum. In addition, the NET BPND was evaluated in thalamic subnuclei (13 atlas-based regions of interest).

RESULTS

We found no significant differences in NET availability or regional distribution between patients with ADHD and healthy controls in all investigated brain regions (F1,41<0.01; P=.96). Furthermore, we identified no significant association between ADHD symptom severity and regional NET availability. Neither sex nor smoking status influenced NET availability. We determined a significant negative correlation between age and NET availability in the thalamus (R2=0.29; P<.01 corrected) and midbrain with pons, including the locus coeruleus (R2=0.18; P<.01 corrected), which corroborates prior findings of a decrease in NET availability with aging in the human brain.

CONCLUSIONS AND RELEVANCE

Our results do not indicate involvement of changes in brain NET availability or distribution in the pathogenesis of ADHD. However, the noradrenergic transmitter system may be affected on a different level, such as in cortical regions, which cannot be reliably quantified with this positron emission tomography ligand. Alternatively, different key proteins of noradrenergic neurotransmission might be affected.

Investigation of cross-species translatability of pharmacological MRI in awake nonhuman primate - a buprenorphine challenge study

BACKGROUND

Pharmacological MRI (phMRI) is a neuroimaging technique where drug-induced hemodynamic responses can represent a pharmacodynamic biomarker to delineate underlying biological consequences of drug actions. In most preclinical studies, animals are anesthetized during image acquisition to minimize movement. However, it has been demonstrated anesthesia could attenuate basal neuronal activity, which can confound interpretation of drug-induced brain activation patterns. Significant efforts have been made to establish awake imaging in rodents and nonhuman primates (NHP). Whilst various platforms have been developed for imaging awake NHP, comparison and validation of phMRI data as translational biomarkers across species remain to be explored.

METHODOLOGY

We have established an awake NHP imaging model that encompasses comprehensive acclimation procedures with a dedicated animal restrainer. Using a cerebral blood volume (CBV)-based phMRI approach, we have determined differential responses of brain activation elicited by the systemic administration of buprenorphine (0.03 mg/kg i.v.), a partial µ-opioid receptor agonist, in the same animal under awake and anesthetized conditions. Additionally, region-of-interest analyses were performed to determine regional drug-induced CBV time-course data and corresponding area-under-curve (AUC) values from brain areas with high density of µ-opioid receptors.

PRINCIPAL FINDINGS

In awake NHPs, group-level analyses revealed buprenorphine significantly activated brain regions including, thalamus, striatum, frontal and cingulate cortices (paired t-test, versus saline vehicle, p<0.05, n = 4). This observation is strikingly consistent with µ-opioid receptor distribution depicted by [6-O-[(11)C]methyl]buprenorphine ([(11)C]BPN) positron emission tomography imaging study in baboons. Furthermore, our findings are consistent with previous buprenorphine phMRI studies in humans and conscious rats which collectively demonstrate the cross-species translatability of awake imaging. Conversely, no significant change in activated brain regions was found in the same animals imaged under the anesthetized condition.

CONCLUSIONS

Our data highlight the utility and importance of awake NHP imaging as a translational imaging biomarker for drug research.

Amphetamine-induced release of dopamine in primate prefrontal cortex and striatum: striking differences in magnitude and timecourse

The psychostimulant amphetamine (AMPH) is frequently used to increase catecholamine levels in attention disorders and positron emission tomography imaging studies. Despite the fact that most radiotracers for positron emission tomography studies are characterized in non-human primates (NHPs), data on regional differences of the effect of AMPH in NHPs are very limited. This study examined the impact of AMPH on extracellular dopamine (DA) levels in the medial prefrontal cortex and the caudate of NHPs using microdialysis. In addition to differences in magnitude, we observed striking differences in the temporal profile of extracellular DA levels between these regions that can likely be attributed to differences in the regulation of dopamine uptake and biosynthesis. The present data suggest that cortical DA levels may remain elevated longer than in the caudate which may contribute to the clinical profile of the actions of AMPH. Using microdialysis probes implanted in the cortex and caudate region of non-human primate brains, we observed in vivo differences in the magnitude and temporal profile of extracellular dopamine levels in response to intravenous amphetamine administration.

Direct targeting of peptidergic amygdalar neurons by noradrenergic afferents: linking stress-integrative circuitry

Amygdalar norepinephrine (NE) plays a key role in regulating neural responses to emotionally arousing stimuli and is involved in memory consolidation of emotionally charged events. Corticotropin-releasing factor (CRF) and dynorphin (DYN), two neuropeptides that mediate the physiological and behavioral responses to stress, are abundant in the central nucleus of the amygdala (CeA), and directly innervate brainstem noradrenergic locus coeruleus (LC) neurons. Whether the CRF- and DYN-containing amygdalar neurons receive direct noradrenergic innervation has not yet been elucidated. The present study sought to define cellular substrates underlying noradrenergic modulation of CRF- and DYN-containing neurons in the CeA using immunohistochemistry and electron microscopy. Ultrastructural analysis revealed that NE-labeled axon terminals form synapses with CRF- and DYN-containing neurons in the CeA. Semi-quantitative analysis showed that approximately 31 % of NET-labeled axon terminals targeted CeA neurons that co-expressed DYN and CRF. As a major source of CRF innervation to the LC, it is also not known whether CRF-containing CeA neurons are directly targeted by noradrenergic afferents. To test this, retrograde tract tracing using FluoroGold from the LC was combined with immunocytochemical detection of CRF and NET in the CeA. Our results revealed a population of LC-projecting CRF-containing CeA neurons that are directly innervated by NE afferents. Analysis showed that approximately 34 % of NET-labeled axon terminals targeted LC-projecting CeA neurons that contain CRF. Taken together, these results indicate significant interactions between NE, CRF and DYN in this critical limbic region and reveal direct synaptic interactions of NE with amygdalar CRF that influence the LC-NE arousal system.

Short-term antidepressant administration reduces default mode and task-positive network connectivity in healthy individuals during rest

Resting-state studies in depressed patients have revealed increased connectivity within the default mode network (DMN) and task-positive network (TPN). This has been associated with heightened rumination, which is the tendency to repetitively think about symptoms of distress. Here, we performed a pharmacological neuroimaging study in healthy volunteers to investigate whether short-term antidepressant administration could reduce DMN connectivity. We recorded resting-state functional magnetic resonance imaging (fMRI) scans in twenty-three healthy volunteers after two week intake of the combined serotonin-norepinephrine reuptake inhibitor (SNRI) duloxetine in a double-blind, placebo-controlled, crossover study. Duloxetine improved mood in part as a result of increased resilience to the mood-worsening effects of scanning and reduced DMN and TPN connectivity. Within the DMN, duloxetine reduced connectivity between the medial prefrontal cortex (MPFC) and the lateral parietal cortex (LPC) and uncoupled the MPFC from the dorsolateral prefrontal cortex (DLPFC). Within the TPN, duloxetine uncoupled the intraparietal sulcus (IPS) from the inferior occipital gyrus. These results show that two-week antidepressant administration reduces DMN and TPN connectivity in healthy volunteers, which may contribute to their antidepressant effects in depression.

[18F]FMeNER-D2: reliable fully-automated synthesis for visualization of the norepinephrine transporter

PURPOSE

In neurodegenerative diseases and neuropsychiatric disorders dysregulation of the norepinephrine transporter (NET) has been reported. For visualization of NET availability and occupancy in the human brain PET imaging can be used. Therefore, selective NET-PET tracers with high affinity are required. Amongst these, [(18)F]FMeNER-D2 is showing the best results so far. Furthermore, a reliable fully automated radiosynthesis is a prerequisite for successful application of PET-tracers. The aim of this work was the automation of [(18)F]FMeNER-D2 radiolabelling for subsequent clinical use. The presented study comprises 25 automated large-scale syntheses, which were directly applied to healthy volunteers and adult patients suffering from attention deficit hyperactivity disorder (ADHD).

PROCEDURES

Synthesis of [(18)F]FMeNER-D2 was automated within a Nuclear Interface Module. Starting from 20-30 GBq [(18)F]fluoride, azeotropic drying, reaction with Br2CD2, distillation of 1-bromo-2-[(18)F]fluoromethane-D2 ([(18)F]BFM) and reaction of the pure [(18)F]BFM with unprotected precursor NER were optimized and completely automated. HPLC purification and SPE procedure were completed, formulation and sterile filtration were achieved on-line and full quality control was performed.

RESULTS

Purified product was obtained in a fully automated synthesis in clinical scale allowing maximum radiation safety and routine production under GMP-like manner. So far, more than 25 fully automated syntheses were successfully performed, yielding 1.0-2.5 GBq of formulated [(18)F]FMeNER-D2 with specific activities between 430 and 1707 GBq/μmol within 95 min total preparation time.

CONCLUSIONS

A first fully automated [(18)F]FMeNER-D2 synthesis was established, allowing routine production of this NET-PET tracer under maximum radiation safety and standardization.

Kinetic modeling and occupancy measures of the norepinephrine transporters in baboons using single photon emission computed tomography with (123)I-INER

This study aims to investigate the pharmacokinetics of a recently developed radiotracer for imaging of the norepinephrine transporter (NET) in baboon brain, (123)I-INER, using single photon emission computed tomography (SPECT). In addition, it also aims to determine NET occupancy by atomoxetine and reboxetine, two selective norepinephrine reuptake inhibitors, using (123)I-INER in baboons. Baseline and preblocking studies with a high dose of atomoxetine (0.85 mg/kg) were conducted in three baboons using SPECT with (123)I-INER administered as a bolus. Kinetic modeling analysis was investigated for different models, namely invasive and reference tissue models. Bolus plus constant infusion experiments with displacement at equilibrium using six different doses of atomoxetine (0.03-0.85 mg/kg) and four different doses of reboxetine (0.5-3.0 mg/kg) were carried out in several baboons to obtain occupancy measurements as a function of dose for the two NET selective drugs. Results showed that reference tissue models can be used to estimate binding potential values and occupancy measures of (123)I-INER in different brain regions. In addition, the apparent volume of distribution was estimated by dividing concentration in tissue by the concentration in blood at 3 hours postinjection. After administration of atomoxetine or reboxetine, a dose-dependent occupancy was observed in brain regions known to contain high densities of NET. In conclusion, pharmacokinetic properties of (123) I-INER were successfully described, and obtained results may be used to simplify future data acquisition and image processing. Dose-dependent NET occupancy for two selective norepinephrine reuptake inhibitors was successfully measured in vivo in baboon brain using SPECT and (123) I-INER.

Endogenous opioid release in the human brain reward system induced by acute amphetamine administration

BACKGROUND

We aimed to demonstrate a pharmacologically stimulated endogenous opioid release in the living human brain by evaluating the effects of amphetamine administration on [(11)C]carfentanil binding with positron emission tomography (PET).

METHODS

Twelve healthy male volunteers underwent [(11)C]carfentanil PET before and 3 hours after a single oral dose of d-amphetamine (either a "high" dose, .5 mg/kg, or a sub-pharmacological "ultra-low" dose, 1.25 mg total dose or approximately .017 mg/kg). Reductions in [(11)C]carfentanil binding from baseline to post-amphetamine scans (ΔBP(ND)) after the "high" and "ultra-low" amphetamine doses were assessed in 10 regions of interest.

RESULTS

[(11)C]carfentanil binding was reduced after the "high" but not the "ultra-low" amphetamine dose in the frontal cortex, putamen, caudate, thalamus, anterior cingulate, and insula.

CONCLUSIONS

Our findings indicate that oral amphetamine administration induces endogenous opioid release in different areas of human brain, including basal ganglia, frontal cortex areas, and thalamus. The combination of an amphetamine challenge and [(11)C]carfentanil PET is a practical and robust method to probe the opioid system in the living human brain.

Iodine-123 labeled reboxetine analogues for imaging of noradrenaline transporter in brain using single photon emission computed tomography

Preliminary investigation of the radioiodinated (S,S)-reboxetine analogue, (123) I-INER, in baboons showed this tracer to have promise for imaging the noradrenaline transporter (NAT) using single photon emission computed tomography (SPECT). More recently, the radioiodinated (R,S)-stereoisomer of (123) I-INER, (123) I-NKJ64, has been synthesized and preliminary evaluation in rats has been reported. This article reports the brain distribution and pharmacokinetic properties of (123) I-NKJ64 in baboons and compares results with (123) I-INER data in the same species. SPECT studies were conducted in two ovariectomized adult female baboons using two different protocols: (1) bolus of (123) I-INER or (123) I-NKJ64; and (2) bolus plus constant infusion of (123) I-NKJ64 with reboxetine (2.0 mg/kg) administration at equilibrium. Following bolus injection, both radiotracers rapidly and avidly entered the baboon brain. The regional brain accumulation of (123) I-NKJ64 did not match the known distribution of NAT in baboon brain, contrasting with previous results obtained in rats. Conversely, the regional distribution of (123) I-INER was consistent with known distribution of NAT in baboon brain. No displacement of (123) I-NKJ64 was observed following administration of reboxetine. This contrasts with previous data obtained for (123) I-INER, where 60% of specific binding was displaced by a lower dose of reboxetine. These data suggest that (123) I-NKJ64 may lack affinity and selectivity for NAT in baboon brain and (123) I-INER is the most promising iodinated reboxetine analogue developed to date for in vivo imaging of NAT in brain using SPECT. This study highlights the importance of species differences during radiotracer development and the stereochemical configuration of analogues of reboxetine in vivo.

Where and what is the paralaminar nucleus? A review on a unique and frequently overlooked area of the primate amygdala

The primate amygdala is composed of multiple subnuclei that play distinct roles in amygdala function. While some nuclei have been areas of focused investigation, others remain virtually unknown. One of the more obscure regions of the amygdala is the paralaminar nucleus (PL). The PL in humans and non-human primates is relatively expanded compared to lower species. Long considered to be part of the basal nucleus, the PL has several interesting features that make it unique. These features include a dense concentration of small cells, high concentrations of receptors for corticotropin releasing hormone and benzodiazepines, and dense innervation of serotonergic fibers. More recently, high concentrations of immature-appearing cells have been noted in the primate PL, suggesting special mechanisms of neural plasticity. Following a brief overview of amygdala structure and function, this review will provide an introduction to the history, embryology, anatomical connectivity, immunohistochemical and cytoarchitectural properties of the PL. Our conclusion is that the PL is a unique subregion of the amygdala that may yield important clues about the normal growth and function of the amygdala, particularly in higher species.

Evaluation of [(11)C]MRB for assessment of occupancy of norepinephrine transporters: Studies with atomoxetine in non-human primates

[(11)C]MRB is one of the most promising radioligands used to measure brain norepinephrine transporters (NET) with positron emission tomography (PET). The objective of this study was to evaluate the suitability of [(11)C]MRB for drug occupancy studies of NET using atomoxetine (ATX), a NET uptake inhibitor used in the treatment of depression and attention-deficit hyperactivity disorder (ADHD). A second goal of the study was identification of a suitable reference region. Ten PET studies were performed in three anesthetized rhesus monkeys following an infusion of ATX or placebo. [(11)C]MRB arterial input functions and ATX plasma levels were also measured. A dose-dependent reduction of [(11)C]MRB volume of distribution was observed after correction for [(11)C]MRB plasma free fraction. ATX IC(50) was estimated to be 31 ± 10ng/mL plasma. This corresponds to an effective dose (ED(50)) of 0.13mg/kg, which is much lower than the therapeutic dose of ATX in ADHD (1.0-1.5mg/kg). [(11)C]MRB binding potential BP(ND) in the thalamus was estimated to be 1.8 ± 0.3. Defining a reference region for a NET radiotracer is challenging due to the widespread and relatively uniform distribution of NET in the brain. Three regions were evaluated for use as reference region: caudate, putamen and occipital cortex. Caudate was found to be the most suitable for preclinical drug occupancy studies in rhesus monkeys. The IC(50) estimate obtained using MRTM2 BP(ND) without arterial blood sampling was 21 ± 3ng/mL (using caudate as the reference region). This study demonstrated that [(11)C]MRB is suitable for drug occupancy studies of NET.

Differential effects of serotonergic and noradrenergic antidepressants on brain activity during a cognitive control task and neurofunctional prediction of treatment outcome in patients with depression

BACKGROUND

We investigated the differential effects of serotonergic and noradrenergic antidepressants on brain activation in patients with major depressive disorder during a Stroop task. We predicted that pretreatment hyperactivity in the rostral anterior cingulate cortex would predict better treatment outcomes.

METHODS

In total, 20 patients underwent naturalistic open-label clinical treatment with citalopram (n = 12) or reboxetine (n = 8). We performed functional magnetic resonance imaging at baseline and after 6 weeks of treatment.

RESULTS

There were no significant group differences in clinical characteristics, treatment outcomes or baseline fMRI activations. The group by time interaction revealed significant voxels in the right amygdala-hippocampus complex (p < 0.05, family-wise error corrected by use of the bilateral amygdala and hippocampus mask image as a small volume), indicating a posttreatment blood oxygen level- dependent signal decrease in the citalopram group. Pretreatment hyperactivity in the rostral anterior cingulate cortex was not related to symptom improvement.

LIMITATIONS

Our study was a nonrandomized clinical trial.

CONCLUSION

These results indicate that serotonergic and noradrenergic antidepressants have a differential effect on brain activity, especially in the amygdala and hippocampus.

Serotonergic and noradrenergic modulation of emotion processing by single dose antidepressants

Serotonergic and noradrenergic pathways are the main targets of antidepressants. Their differential effects on emotion processing-related brain activation are, however, to be further characterized. We aimed at elucidating the neural sites of action of an acute differential serotonergic and noradrenergic influence on an emotion-processing task, which was earlier shown to be associated with depressiveness. In a single-blind pseudo-randomized crossover study, 21 healthy subjects (16 subjects finally included in the analysis) participated to ingest a single dose at three time points of either 40 mg citalopram, a selective serotonin-reuptake inhibitor, 8 mg reboxetine, a selective noradrenaline-reuptake inhibitor, or placebo 2-3 h before functional magnetic resonance imaging (fMRI). During fMRI, subjects performed a task comprising the anticipation and perception of pictures of either 'known' (positive, negative, neutral) or 'unknown' valence (randomly 50% positive or negative). In direct comparison with citalopram and with placebo, reboxetine increased brain activity in the medial thalamus. Citalopram modulated certain prefrontal and insular areas more prominently. Other frontal and parieto-occipital areas were modulated by both drugs. In conclusion, the functional network involved in emotional information processing could be modulated by the acute application of selective noradrenergic and serotonergic drugs revealing a noradrenergic effect in thalamic and frontal areas, and a prefrontal and insular focus of serotonergic modulation. These findings could have implications for future selection criteria concerning personalized antidepressant medication in depression.

MPTP-induced neuroblast apoptosis in the subventricular zone is not regulated by dopamine or other monoamine transporters

For 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to exert neurotoxicity on dopaminergic neurons, 1-methyl-4-phenylpyridinium (MPP+), a metabolite of MPTP, must be taken up into the dopaminergic neuron via the dopamine transporter (DAT). Previous reports have shown that MPTP also causes neuroblast apoptosis in the subventricular zone (SVZ) of adult mice. The aim of this study is to elucidate the role of DAT and other monoamine transporters including vesicular monoamine transporter 2 (VMAT2), the serotonin transporter (SERT), and the norepinephrine transporter (NET) on the neuroblast apoptosis induced by MPTP administration. There were no DAT-positive neuroblasts in the SVZ, whereas some neuroblasts were immunopositive for VMAT2 and SERT. To examine whether these transporters are involved in MPTP-induced neuroblast apoptosis in the SVZ, terminal deoxynucleotidyl transferase-mediated dUTP endlabeling (TUNEL)-positive cells were semiquantitatively analyzed after the injection of GBR12909 (GBR), a DAT inhibitor; tetrabenazine (TBZ), a VMAT2 inhibitor; fluoxetine (FLU), a SERT inhibitor, or desipramine (DES), a NET inhibitor, prior to MPTP injection. However, the injection of these transporter inhibitors had no influence on the MPTP-induced neuroblast apoptosis in the SVZ. It is likely that neither DAT nor other monoamine transporters are involved in MPTP-induced neuroblast apoptosis. The present findings suggest that the neurotoxicity of MPTP to neuroblasts in the SVZ does not require DAT or other monoamine transporters, and the apoptosis it induces may be executed through other unknown pathways.

Amygdala projections to central amygdaloid nucleus subdivisions and transition zones in the primate

In rats and primates, the central nucleus of the amygdala (CeN) is most known for its role in responses to fear stimuli. Recent evidence also shows that the CeN is required for directing attention and behaviors when the salience of competing stimuli is in flux. To examine how information flows through this key output region of the primate amygdala, we first placed small injections of retrograde tracers into the subdivisions of the central nucleus in Old world primates, and examined inputs from specific amygdaloid nuclei. The amygdalostriatal area and interstitial nucleus of the posterior limb of the anterior commissure (IPAC) were distinguished from the CeN using histochemical markers, and projections to these regions were also described. As expected, the basal nucleus and accessory basal nucleus are the main afferent connections of the central nucleus and transition zones. The medial subdivision of the central nucleus (CeM) receives a significantly stronger input from all regions compared to the lateral core subdivision (CeLcn). The corticoamygdaloid transition zone (a zone of confluence of the medial parvicellular basal nucleus, paralaminar nucleus, and the sulcal periamygdaloid cortex) provides the main input to the CeLcn. The IPAC and amygdalostriatal area can be divided in medial and lateral subregions, and receive input from the basal and accessory basal nucleus, with differential inputs according to subdivision. The piriform cortex and lateral nucleus, two important sensory interfaces, send projections to the transition zones. In sum, the CeM receives broad inputs from the entire amygdala, whereas the CeLcn receives more restricted inputs from the relatively undifferentiated corticoamygdaloid transition region. Like the CeN, the transition zones receive most of their input from the basal nucleus and accessory basal nucleus, however, inputs from the piriform cortex and lateral nucleus, and a lack of input from the parvicellular accessory basal nucleus, are distinguishing afferent features.

Monoaminergic changes in locus coeruleus and dorsal raphe nucleus following noradrenaline depletion

The goal of our study was to assess the monoaminergic changes in locus coeruleus (LC) and dorsal raphe nucleus (DRN) following noradrenaline (NA) depletion. Seven days after a single N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) intraperitoneal administration in mice, we observed a decrease of NA in both the LC and DRN, as well as in prefrontal cortex (PFC) and hippocampus (HIPP). Moreover, an increase of serotonin (5-HT) and 5-hydroxyindolacetic acid (5-HIAA) was detected at LC level, while no change was found in DRN. DSP-4 also caused a significant decrease of dopamine (DA) tissue content in HIPP and DRN, without affecting the LC and the PFC. A decrease of DA metabolite, homovanillic acid (HVA), was found in the DRN of NA-depleted mice. These results highlight that the neurotoxic action of DSP-4 is not restricted to LC terminal projections but also involves NA depletion at the cell body level, where it is paralleled by adaptive changes in both serotonergic and dopaminergic systems.

Investigation of the metabolites of (S,S)-[(11)C]MeNER in humans, monkeys and rats

INTRODUCTION

(S,S)-[(11)C]MeNER ((S,S)-2-(alpha-(2-[(11)C]methoxyphenoxy)benzyl)morpholine) is a positron emission tomography (PET) radioligand recently applied in clinical studies of norepinephrine transporters (NETs) in the human brain in vivo. In view of further assessment of the suitability of (S,S)-[(11)C]MeNER as a NET radioligand, its metabolism and the identity of the in vivo radiometabolites of (S,S)-[(11)C]MeNER are of great interest.

MATERIALS AND METHODS

Thus, PET studies were used to measure brain dynamics of (S,S)-[(11)C]MeNER, and plasma reverse-phase radiochromatographic analysis was performed to monitor and quantify its rate of metabolism. Eighteen healthy human volunteers, five cynomolgus monkeys, and five rats were studied.

RESULTS AND DISCUSSION

In human subjects, the plasma radioactivity representing (S,S)-[(11)C]MeNER decreased from 88 +/- 5% at 4 min after injection to 82 +/- 7% at 40 min, while a polar radiometabolite increased from 3 +/- 3% to 16 +/- 7% at the same time-points, respectively. A more lipophilic radiometabolite than (S,S)-[(11)C]MeNER decreased from 9 +/- 5% at 4 min to 1 +/- 2% at 40 min. In monkeys, plasma radioactivity representing (S,S)-[(11)C]MeNER decreased from 97 +/- 2% at 4 min to 74 +/- 7% at 45 min, with a polar fraction as the major radiometabolite. A more lipophilic radiometabolite than (S,S)-[(11)C]MeNER, constituted 3 +/- 2% of radioactivity at 4 min and was not detectable later on. In rats, 17 +/- 4% of plasma radioactivity was parent radioligand at 30 min with the remainder comprising mainly a polar radiometabolite. (S,S)-[(11)C]MeNER in rat brain and urine at 30 min after injection were 90% and 4%, respectively. On a brain regional level, parent radioligand ranged from 87.5 +/- 3.9% (57.2 +/- 14.2% SUV [standard uptake values, %injected radioactivity per mL multiplied with animal weight (in g)]; cerebellum) to 92.9 +/- 1.8% (36.1 +/- 4.7% SUV; striatum), with differential distribution of the radiometabolite in the cerebellum (6.7 +/- 0.3% SUV) and the striatum (2.5 +/- 0.3% SUV). Liquid chromatography-mass spectrometry analysis of rat urine identified a hydroxylation product of the methoxyphenoxy ring of (S,S)-MeNER as the main metabolite. In the brain, the corresponding main metabolite was the product from O-de-methylation of (S,S)-MeNER. PET measurements were performed in rats as well as in wild-type and P-gp-knock-out mice. In rats, the brain peak level of radioactivity was found to be very low (65%SUV). In mice, there was only a small difference in peak brain accumulation between P-gp knock-out and wild-type mice (145 vs. 125%SUV) with the following rank order of regional brain radioactivity: cerebellum x thalamus > cortical regions > striatum.

CONCLUSION

It can be concluded that radiometabolites of (S,S)-[(11)C]MeNER are of minor importance in rat and monkey brain imaging. The presence of a transient lipophilic radiometabolite in peripheral human plasma may induce complications with brain imaging, but its kinetics appear favorable in relation to the slow kinetics of (S,S)-[(11)C]MeNER in humans.

The comparative distributions of the monoamine transporters in the rodent, monkey, and human amygdala

The monoamines in the amygdala modulate multiple aspects of emotional processing in the mammalian brain, and organic or pharmacological dysregulation of these systems can result in affective pathologies. Knowledge of the normal distribution of these neurotransmitters, therefore, is central to our understanding of both the normal processes regulated by the amygdala and the pathological conditions associated with monoaminergic dysregulation. The monoaminergic transporters have proven to be accurate and reliable markers of the distributions of their substrates. The purpose of this review was twofold: First, to briefly recount the functional relevance of dopamine, serotonin, and norepinephrine transmission in the amygdala, and second, to describe and compare the distributions of the monoamine transporters in the rodent, monkey, and human brain. The transporters were found to be heterogeneously distributed in the amygdala. The dopamine transporter (DAT) is consistently found to be extremely sparsely distributed, however the various accounts of its subregional topography are inconsistent, making any cross-species comparisons difficult. The serotonin transporter (SERT) had the greatest overall degree of labeling of the three markers, and was characterized by substantial inter-species variability in its relative distribution. The norepinephrine transporter (NET) was shown to possess an intermediate level of labeling, and like the SERT, its distribution is not consistent across the three species. The results of these comparisons indicate that caution should be exercised when using animal models to investigate the complex processes modulated by the monoamines in the amygdala, as their relative contributions to these functions may differ across species.