Imaging dopamine release with Positron Emission Tomography (PET) and 11C-raclopride in freely moving animals

Preclinical Imaging Studies: Protocols, Preparation, Anesthesia, and Animal Care

Today preclinical PET imaging connects laboratory research with clinical applications. Here PET clearly bridges the gap, as nearly identical imaging protocols can be applied to both animal and humans. However, some hurdles exist and researchers must be careful, partly because the animals are usually anesthetized during the scans, while human volunteers are awake. This review is based on our own experiences of some of the most important pitfalls and how to overcome them. This includes how studies should be designed, how to select the right anesthesia and monitoring. The choice of anesthesia is quite crucial, as it may have a greater influence on the results than the effect of the tested procedures. Monitoring is necessary, as the animals cannot fully maintain homeostasis during anesthesia, and reliable results are dependent on a stable physiology. Additionally, it is important to note that rodents, in particular, are prone to rapidly becoming hypothermic. Thus, the selection of an appropriate anesthetic and monitoring protocol is crucial for both obtaining accurate results and ensuring animal welfare. Prior to imaging, catheters for tracer administration and, if necessary, blood sampling should be implanted. The administration of tracers should be done in a manner that minimizes interference with the scans, and the same applies to any serial blood sampling. The limited blood volume and organ size of rodents should also be taken into consideration when planning experiments. Finally, if the animal needs to be awakened after the scan, proper care must be taken to ensure their welfare.

Combining CRISPR-Cas9 and brain imaging to study the link from genes to molecules to networks

Receptors, transporters, and ion channels are important targets for therapy development in neurological diseases, but their mechanistic role in pathogenesis is often poorly understood. Gene editing and in vivo imaging approaches will help to identify the molecular and functional role of these targets and the consequence of their regional dysfunction on the whole-brain level. We combine CRISPR-Cas9 gene editing with in vivo positron emission tomography (PET) and functional MRI (fMRI) to investigate the direct link between genes, molecules, and the brain connectome. The extensive knowledge of the Slc18a2 gene encoding the vesicular monoamine transporter (VMAT2), involved in the storage and release of dopamine, makes it an excellent target for studying the gene network relationships while structurally preserving neuronal integrity and function. We edited the Slc18a2 in the substantia nigra pars compacta of adult rats and used in vivo molecular imaging besides behavioral, histological, and biochemical assessments to characterize the CRISPR-Cas9-mediated VMAT2 knockdown. Simultaneous PET/fMRI was performed to investigate molecular and functional brain alterations. We found that stage-specific adaptations of brain functional connectivity follow the selective impairment of presynaptic dopamine storage and release. Our study reveals that recruiting different brain networks is an early response to the dopaminergic dysfunction preceding neuronal cell loss. Our combinatorial approach is a tool to investigate the impact of specific genes on brain molecular and functional dynamics, which will help to develop tailored therapies for normalizing brain function.

Translation of Preclinical PET Imaging Findings: Challenges and Motion Correction to Overcome the Confounding Effect of Anesthetics

Preclinical brain positron emission tomography (PET) in animals is performed using anesthesia to avoid movement during the PET scan. In contrast, brain PET scans in humans are typically performed in the awake subject. Anesthesia is therefore one of the principal limitations in the translation of preclinical brain PET to the clinic. This review summarizes the available literature supporting the confounding effect of anesthesia on several PET tracers for neuroscience in preclinical small animal scans. In a second part, we present the state-of-the-art methodologies to circumvent this limitation to increase the translational significance of preclinical research, with an emphasis on motion correction methods. Several motion tracking systems compatible with preclinical scanners have been developed, each one with its advantages and limitations. These systems and the novel experimental setups they can bring to preclinical brain PET research are reviewed here. While technical advances have been made in this field, and practical implementations have been demonstrated, the technique should become more readily available to research centers to allow for a wider adoption of the motion correction technique for brain research.

Striatal and prefrontal D2R and SERT distributions contrastingly correlate with default-mode connectivity

Although brain research has taken important strides in recent decades, the interaction and coupling of its different physiological levels is still not elucidated. Specifically, the molecular substrates of resting-state functional connectivity (rs-FC) remain poorly understood. The aim of this study was elucidating interactions between dopamine D2 receptors (D2R) and serotonin transporter (SERT) availabilities in the striatum (CPu) and medial prefrontal cortex (mPFC), two of the main dopaminergic and serotonergic projection areas, and the default-mode network. Additionally, we delineated its interaction with two other prominent resting-state networks (RSNs), the salience network (SN) and the sensorimotor network (SMN). To this extent, we performed simultaneous PET/fMRI scans in a total of 59 healthy rats using [¹¹C]raclopride and [¹¹C]DASB, two tracers used to image quantify D2R and SERT respectively. Edge, node and network-level rs-FC metrics were calculated for each subject and potential correlations with binding potentials (BP_ND) in the CPu and mPFC were evaluated. We found widespread negative associations between CPu D2R availability and all the RSNs investigated, consistent with the postulated role of the indirect basal ganglia pathway. Correlations between D2Rs in the mPFC were weaker and largely restricted to DMN connectivity. Strikingly, medial prefrontal SERT correlated both positively with anterior DMN rs-FC and negatively with rs-FC between and within the SN, SMN and the posterior DMN, underlining the complex role of serotonergic neurotransmission in this region. Here we show direct relationships between rs-FC and molecular properties of the brain as assessed by simultaneous PET/fMRI in healthy rodents. The findings in the present study contribute to the basic understanding of rs-FC by revealing associations between inter-subject variances of rs-FC and receptor and transporter availabilities. Additionally, since current therapeutic strategies typically target neurotransmitter systems with the aim of normalizing brain function, delineating associations between molecular and network-level brain properties is essential and may enhance the understanding of neuropathologies and support future drug development.

In Vivo Receptor Visualization and Evaluation of Receptor Occupancy with Positron Emission Tomography

Positron emission tomography (PET) is useful for noninvasive in vivo visualization of disease-related receptors, for evaluation of receptor occupancy to determine an appropriate drug dosage, and for proof-of-concept of drug candidates in translational research. For these purposes, the specificity of the PET tracer for the target receptor is critical. Here, we review work in this area, focusing on the chemical structures of reported PET tracers, their K_i/K_d values, and the physical properties relevant to target receptor selectivity. Among these physical properties, such as cLogP, cLogD, molecular weight, topological polar surface area, number of hydrogen bond donors, and pK_a, we focus especially on LogD and LogP as important physical properties that can be easily compared across a range of studies. We discuss the success of PET tracers in evaluating receptor occupancy and consider likely future developments in the field.

Open-field PET: Simultaneous brain functional imaging and behavioural response measurements in freely moving small animals

A comprehensive understanding of how the brain responds to a changing environment requires techniques capable of recording functional outputs at the whole-brain level in response to external stimuli. Positron emission tomography (PET) is an exquisitely sensitive technique for imaging brain function but the need for anaesthesia to avoid motion artefacts precludes concurrent behavioural response studies. Here, we report a technique that combines motion-compensated PET with a robotically-controlled animal enclosure to enable simultaneous brain imaging and behavioural recordings in unrestrained small animals. The technique was used to measure in vivo displacement of [¹¹C]raclopride from dopamine D2 receptors (D2R) concurrently with changes in the behaviour of awake, freely moving rats following administration of unlabelled raclopride or amphetamine. The timing and magnitude of [¹¹C]raclopride displacement from D2R were reliably estimated and, in the case of amphetamine, these changes coincided with a marked increase in stereotyped behaviours and hyper-locomotion. The technique, therefore, allows simultaneous measurement of changes in brain function and behavioural responses to external stimuli in conscious unrestrained animals, giving rise to important applications in behavioural neuroscience.

Awake 18F-FDG PET Imaging of Memantine-Induced Brain Activation and Test-Retest in Freely Running Mice

PET scans of the mouse brain are usually performed with anesthesia to immobilize the animal. However, it is desirable to avoid the confounding factor of anesthesia in mouse-brain response. Methods: We developed and validated brain PET imaging of awake, freely moving mice. Head-motion tracking was performed using radioactive point-source markers, and we used the tracking information for PET-image motion correction. Regional ¹⁸F-FDG brain uptake in a test, retest, and memantine-challenge study was measured in awake (n = 8) and anesthetized (n = 8) C57BL/6 mice. An awake uptake period was considered for the anesthesia scans. Results: Awake (motion-corrected) PET images showed an ¹⁸F-FDG uptake pattern comparable to the pattern of anesthetized mice. The test–retest variability (represented by the intraclass correlation coefficient) of the regional SUV quantification in the awake animals (0.424–0.555) was marginally lower than that in the anesthetized animals (intraclass correlation coefficient, 0.491–0.629) over the different regions. The increased memantine-induced ¹⁸F-FDG uptake was more pronounced in awake (+63.6%) than in anesthetized (+24.2%) animals. Additional behavioral information, acquired for awake animals, showed increased motor activity on a memantine challenge (total distance traveled, 18.2 ± 5.28 m) compared with test–retest (6.49 ± 2.21 m). Conclusion: The present method enables brain PET imaging on awake mice, thereby avoiding the confounding effects of anesthesia on the PET reading. It allows the simultaneous measurement of behavioral information during PET acquisitions. The method does not require any additional hardware, and it can be deployed in typical high-throughput scan protocols.

Design and Mechanism of GABA Aminotransferase Inactivators. Treatments for Epilepsies and Addictions

When the brain concentration of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) diminishes below a threshold level, the excess neuronal excitation can lead to convulsions. This imbalance in neurotransmission can be corrected by inhibition of the enzyme γ-aminobutyric acid aminotransferase (GABA-AT), which catalyzes the conversion of GABA to the excitatory neurotransmitter l-glutamic acid. It also has been found that raising GABA levels can antagonize the rapid elevation and release of dopamine in the nucleus accumbens, which is responsible for the reward response in addiction. Therefore, the design of new inhibitors of GABA-AT, which increases brain GABA levels, is an important approach to new treatments for epilepsy and addiction. This review summarizes findings over the last 40 or so years of mechanism-based inactivators (unreactive compounds that require the target enzyme to catalyze their conversion to the inactivating species, which inactivate the enzyme prior to their release) of GABA-AT with emphasis on their catalytic mechanisms of inactivation, presented according to organic chemical mechanism, with minimal pharmacology, except where important for activity in epilepsy and addiction. Patents, abstracts, and conference proceedings are not covered in this review. The inactivation mechanisms described here can be applied to the inactivations of a wide variety of unrelated enzymes.

Design and Mechanism of (S)-3-Amino-4-(difluoromethylenyl)cyclopent-1-ene-1-carboxylic Acid, a Highly Potent γ-Aminobutyric Acid Aminotransferase Inactivator for the Treatment of Addiction

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system. Inhibition of GABA aminotransferase (GABA-AT), a pyridoxal 5'-phosphate (PLP)-dependent enzyme that degrades GABA, has been established as a possible strategy for the treatment of substance abuse. The raised GABA levels that occur as a consequence of this inhibition have been found to antagonize the rapid release of dopamine in the ventral striatum (nucleus accumbens) that follows an acute challenge by an addictive substance. In addition, increased GABA levels are also known to elicit an anticonvulsant effect in patients with epilepsy. We previously designed the mechanism-based inactivator (1S,3S)-3-amino-4-difluoromethylenyl-1-cyclopentanoic acid (2), now called CPP-115, that is 186 times more efficient in inactivating GABA-AT than vigabatrin, the only FDA-approved drug that is an inactivator of GABA-AT. CPP-115 was found to have high therapeutic potential for the treatment of cocaine addiction and for a variety of epilepsies, has successfully completed a Phase I safety clinical trial, and was found to be effective in the treatment of infantile spasms (West syndrome). Herein we report the design, using molecular dynamics simulations, synthesis, and biological evaluation of a new mechanism-based inactivator, (S)-3-amino-4-(difluoromethylenyl)cyclopent-1-ene-1-carboxylic acid (5), which was found to be almost 10 times more efficient as an inactivator of GABA-AT than CPP-115. We also present the unexpected crystal structure of 5 bound to GABA-AT, as well as computational analyses used to assist the structure elucidation process. Furthermore, 5 was found to have favorable pharmacokinetic properties and low off-target activities. In vivo studies in freely moving rats showed that 5 was dramatically superior to CPP-115 in suppressing the release of dopamine in the corpus striatum, which occurs subsequent to either an acute cocaine or nicotine challenge. Compound 5 also attenuated increased metabolic demands (neuronal glucose metabolism) in the hippocampus, a brain region that encodes spatial information concerning the environment in which an animal receives a reinforcing or aversive drug. This multidisciplinary computational design to preclinical efficacy approach should be applicable to the design and improvement of mechanism-based inhibitors of other enzymes whose crystal structures and inactivation mechanisms are known.

Development of a population pharmacokinetic model to predict brain distribution and dopamine D2 receptor occupancy of raclopride in non-anesthetized rat

BACKGROUND

Raclopride is a selective antagonist of the dopamine D2 receptor. It is one of the most frequently used in vivo D2 tracers (at low doses) for assessing drug-induced receptor occupancy (RO) in animals and humans. It is also commonly used as a pharmacological blocker (at high doses) to occupy the available D2 receptors and antagonize the action of dopamine or drugs on D2 in preclinical studies. The aims of this study were to comprehensively evaluate its pharmacokinetic (PK) profiles in different brain compartments and to establish a PK-RO model that could predict the brain distribution and RO of raclopride in the freely moving rat using a LC-MS based approach.

METHODS

Rats (n=24) received a 10-min IV infusion of non-radiolabeled raclopride (1.61μmol/kg, i.e. 0.56mg/kg). Plasma and the brain tissues of striatum (with high density of D2 receptors) and cerebellum (with negligible amount of D2 receptors) were collected. Additional microdialysis experiments were performed in some rats (n=7) to measure the free drug concentration in the extracellular fluid of the striatum and cerebellum. Raclopride concentrations in all samples were analyzed by LC-MS. A population PK-RO model was constructed in NONMEM to describe the concentration-time profiles in the unbound plasma, brain extracellular fluid and brain tissue compartments and to estimate the RO based on raclopride-D2 receptor binding kinetics.

RESULTS

In plasma raclopride showed a rapid distribution phase followed by a slower elimination phase. The striatum tissue concentrations were consistently higher than that of cerebellum tissue throughout the whole experimental period (10-h) due to higher non-specific tissue binding and D2 receptor binding in the striatum. Model-based simulations accurately predicted the literature data on rat plasma PK, brain tissue PK and D2 RO at different time points after intravenous or subcutaneous administration of raclopride at tracer dose (RO <10%), sub-pharmacological dose (RO 10%-30%) and pharmacological dose (RO >30%).

CONCLUSION

For the first time a predictive model that could describe the quantitative in vivo relationship between dose, PK and D2 RO of raclopride in non-anesthetized rat was established. The PK-RO model could facilitate the selection of optimal dose and dosing time when raclopride is used as tracer or as pharmacological blocker in various rat studies. The LC-MS based approach, which doses and quantifies a non-radiolabeled tracer, could be useful in evaluating the systemic disposition and brain kinetics of tracers.

Sexual dimorphism in striatal dopaminergic responses promotes monogamy in social songbirds

In many songbird species, males sing to attract females and repel rivals. How can gregarious, non-territorial songbirds such as zebra finches, where females have access to numerous males, sustain monogamy? We found that the dopaminergic reward circuitry of zebra finches can simultaneously promote social cohesion and breeding boundaries. Surprisingly, in unmated males but not in females, striatal dopamine neurotransmission was elevated after hearing songs. Behaviorally too, unmated males but not females persistently exchanged mild punishments in return for songs. Song reinforcement diminished when dopamine receptors were blocked. In females, we observed song reinforcement exclusively to the mate’s song, although their striatal dopamine neurotransmission was only slightly elevated. These findings suggest that song-triggered dopaminergic activation serves a dual function in social songbirds: as low-threshold social reinforcement in males and as ultra-selective sexual reinforcement in females. Co-evolution of sexually dimorphic reinforcement systems can explain the coexistence of gregariousness and monogamy.

While monogamy is rare within the animal kingdom, some species – including humans and many birds – can be highly social and yet sustain monogamous relationships. Zebra finches, for example, are among a number of species of songbirds in which numerous males and females live closely together but maintain monogamous partnerships. Male songbirds use their songs to attract females, who do not themselves sing. But if female birds are attracted to any male song, how do they manage to remain monogamous when surrounded by potential suitors?

In songbirds, and in humans too, a region of the brain called the striatum regulates both social and sexual behaviors. It does this by modulating the release of a molecule called dopamine, which is the brain’s reward signal. Tokarev et al. show that hearing songs triggers dopamine release within the striatum of unattached male zebra finches, but has no such effect in single females. Unattached male songbirds will also put up with irritating puffs of air in exchange for being able to watch videos of singing birds, whereas unattached females will not. Female songbirds with partners will tolerate the air puffs, but only if the videos are accompanied with the songs of their own mate.

These findings suggest that song serves as a generic social stimulus for zebra finch males, helping large numbers of birds to live together. By contrast, for a female zebra finch, the song of her partner is a highly selective sexual stimulus. These sex-specific responses to the same socially-relevant stimuli may explain how gregarious animals are able to maintain monogamous pair bonds. More generally, these results are a step towards understanding how brain reward systems regulate social interactions. Studying these mechanisms in songbird species with different social and mating systems could ultimately provide insights into social and sexual behavior in people.

Sex Differences in Regional Brain Glucose Metabolism Following Opioid Withdrawal and Replacement

Methadone and buprenorphine are currently the most common pharmacological treatments for opioid dependence. Interestingly, the clinical response to these drugs appears to be sex specific. That is, females exhibit superior therapeutic efficacy, defined as extended periods of abstinence and longer time to relapse, compared with males. However, the underlying metabolic effects of opioid withdrawal and replacement have not been examined. Therefore, using ¹⁸FDG and microPET, we measured differences in regional brain glucose metabolism in males and females following morphine withdrawal and subsequent methadone or buprenorphine replacement. In both males and females, spontaneous opioid withdrawal altered glucose metabolism in regions associated with reward and drug dependence. Specifically, metabolic increases in the thalamus, as well as metabolic decreases in insular cortex and the periaqueductal gray, were noted. However, compared with males, females exhibited increased metabolism in the preoptic area, primary motor cortex, and the amygdala, and decreased metabolism in the caudate/putamen and medial geniculate nucleus. Methadone and buprenorphine initially abolished these changes uniformly, but subsequently produced their own regional metabolic alterations that varied by treatment and sex. Compared with sex-matched control animals undergoing spontaneous opioid withdrawal, male animals treated with methadone exhibited increased caudate/putamen metabolism, whereas buprenorphine produced increased ventral striatum and motor cortex metabolism in females, and increased ventral striatum and somatosensory cortex metabolism in males. Notably, when treatment effects were compared between sexes, methadone-treated females showed increased cingulate cortex metabolism, whereas buprenorphine-treated females showed decreased metabolism in cingulate cortex and increased metabolism in the globus pallidus. Perhaps the initial similarities in males and females underlie early therapeutic efficacy, whereas these posttreatment sex differences contribute to clinical treatment failure more commonly experienced by the former.

Fast and Accurate Rat Head Motion Tracking With Point Sources for Awake Brain PET

To avoid the confounding effects of anesthesia and immobilization stress in rat brain positron emission tomography (PET), motion tracking-based unrestrained awake rat brain imaging is being developed. In this paper, we propose a fast and accurate rat headmotion tracking method based on small PET point sources. PET point sources (3-4) attached to the rat's head are tracked in image space using 15-32-ms time frames. Our point source tracking (PST) method was validated using a manually moved microDerenzo phantom that was simultaneously tracked with an optical tracker (OT) for comparison. The PST method was further validated in three awake [¹⁸F]FDG rat brain scans. Compared with the OT, the PST-based correction at the same frame rate (31.2 Hz) reduced the reconstructed FWHM by 0.39-0.66 mm for the different tested rod sizes of the microDerenzo phantom. The FWHM could be further reduced by another 0.07-0.13 mm when increasing the PST frame rate (66.7 Hz). Regional brain [¹⁸F]FDG uptake in the motion corrected scan was strongly correlated ( ) with that of the anesthetized reference scan for all three cases ( ). The proposed PST method allowed excellent and reproducible motion correction in awake in vivo experiments. In addition, there is no need of specialized tracking equipment or additional calibrations to be performed, the point sources are practically imperceptible to the rat, and PST is ideally suitable for small bore scanners, where optical tracking might be challenging.

Imaging Sex Differences in Regional Brain Metabolism during Acute Opioid Withdrawal

The rate of opioid overdose continues to rise, necessitating improved treatment options. Current therapeutic approaches rely on administration of either a blocking agent, such as naloxone, or chronic treatment with replacement drugs, including methadone and/or buprenorphine. Recent findings suggest that males and females respond to these treatments uniquely. In an effort to better understand this sex-specific variation in treatment efficacy, we investigated the effects of acute opioid withdrawal in male and female rats using 18FDG and microPET. These data demonstrate that acute opioid withdrawal produces metabolic alterations in brain regions associated with reward and drug dependence, namely corpus striatum, thalamic nuclei, septum, and frontal cortex. Furthermore, certain changes are unique to males. Specifically, males demonstrated increased metabolism in the anterior cingulate cortex and the ventral hippocampus (CA3) following acute opioid withdrawal. If males and females exhibit sex-specific changes in regional brain metabolism following acute opioid withdrawal, then perhaps it is not surprising that they respond to treatment differently.

Computational Modeling of Neurotransmitter Release Evoked by Electrical Stimulation: Nonlinear Approaches to Predicting Stimulation-Evoked Dopamine Release

Neurochemical changes evoked by electrical stimulation of the nervous system have been linked to both therapeutic and undesired effects of neuromodulation therapies used to treat obsessive-compulsive disorder, depression, epilepsy, Parkinson's disease, stroke, hypertension, tinnitus, and many other indications. In fact, interest in better understanding the role of neurochemical signaling in neuromodulation therapies has been a focus of recent government- and industry-sponsored programs whose ultimate goal is to usher in an era of personalized medicine by creating neuromodulation therapies that respond to real-time changes in patient status. A key element to achieving these precision therapeutic interventions is the development of mathematical modeling approaches capable of describing the nonlinear transfer function between neuromodulation parameters and evoked neurochemical changes. Here, we propose two computational modeling frameworks, based on artificial neural networks (ANNs) and Volterra kernels, that can characterize the input/output transfer functions of stimulation-evoked neurochemical release. We evaluate the ability of these modeling frameworks to characterize subject-specific neurochemical kinetics by accurately describing stimulation-evoked dopamine release across rodent (R² = 0.83 Volterra kernel, R² = 0.86 ANN), swine (R² = 0.90 Volterra kernel, R² = 0.93 ANN), and non-human primate (R² = 0.98 Volterra kernel, R² = 0.96 ANN) models of brain stimulation. Ultimately, these models will not only improve understanding of neurochemical signaling in healthy and diseased brains but also facilitate the development of neuromodulation strategies capable of controlling neurochemical release via closed-loop strategies.

Semi-mechanistic computer simulation of psychotic symptoms in schizophrenia with a model of a humanized cortico-striatal-thalamocortical loop

Despite new insights into the pathophysiology of schizophrenia and clinical trials with highly selective drugs, no new therapeutic breakthroughs have been identified. We present a semi-mechanistic Quantitative Systems Pharmacology (QSP) computer model of a biophysically realistic cortical-striatal-thalamo-cortical loop. The model incorporates the direct, indirect and hyperdirect pathway of the basal ganglia and CNS drug targets that modulate neuronal firing, based on preclinical data about their localization and coupling to voltage-gated ion channels. Schizophrenia pathology is introduced using quantitative human imaging data on striatal hyperdopaminergic activity and cortical dysfunction. We identified an entropy measure of neuronal firing in the thalamus, related to the bandwidth of information processing that correlates well with reported historical clinical changes on PANSS Total with antipsychotics after introduction of their pharmacology (42 drug-dose combinations, r²=0.62). This entropy measure is further validated by predicting the clinical outcome of 28 other novel stand-alone interventions, 14 of them with non-dopamine D₂R pharmacology, in addition to 8 augmentation trials (correlation between actual and predicted clinical scores r²=0.61). The platform predicts that most combinations of antipsychotics have a lower efficacy over what can be achieved by either one; negative pharmacodynamical interactions are prominent for aripiprazole added to risperidone, haloperidol, quetiapine and paliperidone. The model also recapitulates the increased probability for psychotic breakdown in a supersensitive environment and the effect of ketamine in healthy volunteers. This QSP platform, combined with similar readouts for motor symptoms, negative symptoms and cognitive impairment has the potential to improve our understanding of drug effects in schizophrenia patients.

Markerless rat head motion tracking using structured light for brain PET imaging of unrestrained awake small animals

Preclinical positron emission tomography (PET) imaging in small animals is generally performed under anesthesia to immobilize the animal during scanning. More recently, for rat brain PET studies, methods to perform scans of unrestrained awake rats are being developed in order to avoid the unwanted effects of anesthesia on the brain response. Here, we investigate the use of a projected structure stereo camera to track the motion of the rat head during the PET scan. The motion information is then used to correct the PET data. The stereo camera calculates a 3D point cloud representation of the scene and the tracking is performed by point cloud matching using the iterative closest point algorithm. The main advantage of the proposed motion tracking is that no intervention, e.g. for marker attachment, is needed. A manually moved microDerenzo phantom experiment and 3 awake rat [¹⁸F]FDG experiments were performed to evaluate the proposed tracking method. The tracking accuracy was 0.33 mm rms. After motion correction image reconstruction, the microDerenzo phantom was recovered albeit with some loss of resolution. The reconstructed FWHM of the 2.5 and 3 mm rods increased with 0.94 and 0.51 mm respectively in comparison with the motion-free case. In the rat experiments, the average tracking success rate was 64.7%. The correlation of relative brain regional [¹⁸F]FDG uptake between the anesthesia and awake scan reconstructions was increased from on average 0.291 (not significant) before correction to 0.909 (p  <  0.0001) after motion correction. Markerless motion tracking using structured light can be successfully used for tracking of the rat head for motion correction in awake rat PET scans.

The effect of isoflurane on 18F-FDG uptake in the rat brain: a fully conscious dynamic PET study using motion compensation

BACKGROUND

In preclinical positron emission tomography (PET) studies an anaesthetic is used to ensure that the animal does not move during the scan. However, anaesthesia may have confounding effects on the drug or tracer kinetics under study, and the nature of these effects is usually not known.

METHOD

We have implemented a protocol for tracking the rigid motion of the head of a fully conscious rat during a PET scan and performing a motion compensated list-mode reconstruction of the data. Using this technique we have conducted eight rat studies to investigate the effect of isoflurane on the uptake of ¹⁸F-FDG in the brain, by comparing conscious and unconscious scans.

RESULTS

Our results indicate that isoflurane significantly decreases the whole brain uptake, as well as decreasing the relative regional FDG uptake in the cortex, diencephalon, and inferior colliculi, while increasing it in the vestibular nuclei. No statistically significant changes in FDG uptake were observed in the cerebellum and striata.

CONCLUSION

The applied event-based motion compensation technique allowed for the investigation of the effect of isoflurane on FDG uptake in the rat brain using fully awake and unrestrained rats, scanned dynamically from the moment of injection. A significant effect of the anaesthesia was observed in various regions of the brain.

Development of a simultaneous optical/PET imaging system for awake mice

Simultaneous measurements of multiple physiological parameters are essential for the study of brain disease mechanisms and the development of suitable therapies to treat them. In this study, we developed a measurement system for simultaneous optical imaging and PET for awake mice. The key elements of this system are the OpenPET, optical imaging and fixation apparatus for an awake mouse. The OpenPET is our original open-type PET geometry, which can be used in combination with another device because of the easily accessible open space of the former. A small prototype of the axial shift single-ring OpenPET was used. The objective lens for optical imaging with a mounted charge-coupled device camera was placed inside the open space of the AS-SROP. Our original fixation apparatus to hold an awake mouse was also applied. As a first application of this system, simultaneous measurements of cerebral blood flow (CBF) by laser speckle imaging (LSI) and [(11)C]raclopride-PET were performed under control and 5% CO2 inhalation (hypercapnia) conditions. Our system successfully obtained the CBF and [(11)C]raclopride radioactivity concentration simultaneously. Accumulation of [(11)C]raclopride was observed in the striatum where the density of dopamine D2 receptors is high. LSI measurements could be stably performed for more than 60 minutes. Increased CBF induced by hypercapnia was observed while CBF under the control condition was stable. We concluded that our imaging system should be useful for investigating the mechanisms of brain diseases in awake animal models.

[(11)C]Raclopride binding in the striatum of minimally restrained and free-walking awake mice in a positron emission tomography study

Anesthesia and restraint stress have profound impacts on brain functions, including neural activity and cerebrovascular function, possibly influencing functional and neurochemical positron emission tomography (PET) imaging data. For circumventing this effect, we developed an experimental system enabling PET imaging of free-walking awake mice with minimal restraints by fixing the head to a holder. The applicability of this system was investigated by performing PET imaging of D2 dopamine receptors with [(11)C]raclopride under the following three different conditions: (1) free-walking awake state; (2) 1.5% isoflurane anesthesia; and (3) whole-body restraint without anesthesia. [(11)C]raclopride binding potential (BP(ND)) values under isoflurane anesthesia and restrained awake state were significantly lower than under free-walking awake state (P < 0.01). Heart rates in restrained awake mice were significantly higher than those in free-walking awake mice (P < 0.01), suggesting that free-walking awake state minimized restraint stress during the PET scan. [(11)C] raclopride-PET with methamphetamine (METH) injection was also performed in awake and anesthetized mice. METH-induced reduction of [(11)C]raclopride BP(ND) in anesthetized mice showed a trend to be less than that in free-walking awake mice, implying that pharmacological modulation of dopaminergic transmissions could be sensitively captured by PET imaging of free-walking awake mice. We concluded that our system is of utility as an in vivo assaying platform for studies of brain functions and neurotransmission elements in small animals, such as those modeling neuropsychiatric disorders.

Kinetic modeling in pre-clinical positron emission tomography

Pre-clinical positron emission tomography (PET) has evolved in the last few years from pure visualization of radiotracer uptake and distribution towards quantification of the physiological parameters. For reliable and reproducible quantification the kinetic modeling methods used to obtain relevant parameters of radiotracer tissue interaction are important. Here we present different kinetic modeling techniques with a focus on compartmental models including plasma input models and reference tissue input models. The experimental challenges off deriving the plasma input function in rodents and the effect of anesthesia are discussed. Finally, in vivo application of kinetic modeling in various areas of pre-clinical research is presented and compared to human data.

Motion compensation for brain PET imaging using wireless MR active markers in simultaneous PET-MR: phantom and non-human primate studies

Brain PET scanning plays an important role in the diagnosis, prognostication and monitoring of many brain diseases. Motion artifacts from head motion are one of the major hurdles in brain PET. In this work, we propose to use wireless MR active markers to track head motion in real time during a simultaneous PET-MR brain scan and incorporate the motion measured by the markers in the listmode PET reconstruction. Several wireless MR active markers and a dedicated fast MR tracking pulse sequence module were built. Data were acquired on an ACR Flangeless PET phantom with multiple spheres and a non-human primate with and without motion. Motions of the phantom and monkey's head were measured with the wireless markers using a dedicated MR tracking sequence module. The motion PET data were reconstructed using list-mode reconstruction with and without motion correction. Static reference was used as gold standard for quantitative analysis. The motion artifacts, which were prominent on the images without motion correction, were eliminated by the wireless marker based motion correction in both the phantom and monkey experiments. Quantitative analysis was performed on the phantom motion data from 24 independent noise realizations. The reduction of bias of sphere-to-background PET contrast by active marker based motion correction ranges from 26% to 64% and 17% to 25% for hot (i.e., radioactive) and cold (i.e., non-radioactive) spheres, respectively. The motion correction improved the channelized Hotelling observer signal-to-noise ratio of the spheres by 1.2 to 6.9 depending on their locations and sizes. The proposed wireless MR active marker based motion correction technique removes the motion artifacts in the reconstructed PET images and yields accurate quantitative values.

Visualizing epigenetics: current advances and advantages in HDAC PET imaging techniques

Abnormal gene regulation as a consequence of flawed epigenetic mechanisms may be central to the initiation and persistence of many human diseases. However, the association of epigenetic dysfunction with disease and the development of therapeutic agents for treatment are slow. Developing new methodologies used to visualize chromatin-modifying enzymes and their function in the human brain would be valuable for the diagnosis of brain disorders and drug discovery. We provide an overview of current invasive and noninvasive techniques for measuring expression and functions of chromatin-modifying enzymes in the brain, emphasizing tools applicable to histone deacetylase (HDAC) enzymes as a leading example. The majority of current techniques are invasive and difficult to translate to what is happening within a human brain in vivo. However, recent progress in molecular imaging provides new, noninvasive ways to visualize epigenetics in the human brain. Neuroimaging tool development presents a unique set of challenges in order to identify and validate CNS radiotracers for HDACs and other histone-modifying enzymes. We summarize advances in the effort to image HDACs and HDAC inhibitory effects in the brain using positron emission tomography (PET) and highlight generalizable techniques that can be adapted to investigate other specific components of epigenetic machinery. Translational tools like neuroimaging by PET and magnetic resonance imaging provide the best way to link our current understanding of epigenetic changes with in vivo function in normal and diseased brains. These tools will be a critical addition to ex vivo methods to evaluate - and intervene - in CNS dysfunction.

Effects of anesthesia and species on the uptake or binding of radioligands in vivo in the Göttingen minipig

Progress in neuroscience research often involves animals, as no adequate alternatives exist to animal models of living systems. However, both the physiological characteristics of the species used and the effects of anesthesia raise questions of common concern. Here, we demonstrate the confounding influences of these effects on tracer binding in positron emission tomography (PET). We determined the effects of two routinely used anesthetics (isoflurane and propofol) on the binding of two tracers of monoamine function, [¹¹C]SCH23390, a tracer of the dopamine D1 and D5 receptors, and the alpha2-adrenoceptor antagonist, [¹¹C]yohimbine, in Göttingen minipigs. The kinetics of SCH23390 in the pigs differed from those of our earlier studies in primates. With two different graphical analyses of uptake of SCH23390, the initial clearance values of this tracer were higher with isoflurane than with propofol anesthesia, indicative of differences in blood flow, whereas no significant differences were observed for the volumes of distribution of yohimbine. The study underscores the importance of differences of anesthesia and species when the properties of radioligands are evaluated under different circumstances that may affect blood flow and tracer uptake. These differences must be considered in the choice of a particular animal species and mode of anesthesia for a particular application.

A wireless beta-microprobe based on pixelated silicon for in vivo brain studies in freely moving rats

The investigation of neurophysiological mechanisms underlying the functional specificity of brain regions requires the development of technologies that are well adjusted to in vivo studies in small animals. An exciting challenge remains the combination of brain imaging and behavioural studies, which associates molecular processes of neuronal communications to their related actions. A pixelated intracerebral probe (PIXSIC) presents a novel strategy using a submillimetric probe for beta(+) radiotracer detection based on a pixelated silicon diode that can be stereotaxically implanted in the brain region of interest. This fully autonomous detection system permits time-resolved high sensitivity measurements of radiotracers with additional imaging features in freely moving rats. An application-specific integrated circuit (ASIC) allows for parallel signal processing of each pixel and enables the wireless operation. All components of the detector were tested and characterized. The beta(+) sensitivity of the system was determined with the probe dipped into radiotracer solutions. Monte Carlo simulations served to validate the experimental values and assess the contribution of gamma noise. Preliminary implantation tests on anaesthetized rats proved PIXSIC's functionality in brain tissue. High spatial resolution allows for the visualization of radiotracer concentration in different brain regions with high temporal resolution.

Anaesthesia for positron emission tomography scanning of animal brains

Positron emission tomography (PET) provides a means of studying physiological and pharmacological processes as they occur in the living brain. Mice, rats, dogs, cats, pigs and non-human primates are often used in studies using PET. They are commonly anaesthetized with ketamine, propofol or isoflurane in order to prevent them from moving during the imaging procedure. The use of anaesthesia in PET studies suffers, however, from the drawback of possibly altering central neuromolecular mechanisms. As a result, PET findings obtained in anaesthetized animals may fail to correctly represent normal properties of the awake brain. Here, we review findings of PET studies carried out either in both awake and anaesthetized animals or in animals given at least two different anaesthetics. Such studies provide a means of estimating the extent to which anaesthesia affects the outcome of PET neuroimaging in animals. While no final conclusion can be drawn concerning the 'best' general anaesthetic for PET neuroimaging in laboratory animals, such studies provide findings that can enhance an understanding of neurobiological mechanisms in the living brain.

Tracking and characterizing the head motion of unanaesthetized rats in positron emission tomography

Positron emission tomography (PET) is an important in vivo molecular imaging technique for translational research. Imaging unanaesthetized rats using motion-compensated PET avoids the confounding impact of anaesthetic drugs and enables animals to be imaged during normal or evoked behaviour. However, there is little published data on the nature of rat head motion to inform the design of suitable marker-based motion-tracking set-ups for brain imaging-specifically, set-ups that afford close to uninterrupted tracking. We performed a systematic study of rat head motion parameters for unanaesthetized tube-bound and freely moving rats with a view to designing suitable motion-tracking set-ups in each case. For tube-bound rats, using a single appropriately placed binocular tracker, uninterrupted tracking was possible greater than 95 per cent of the time. For freely moving rats, simulations and measurements of a live subject indicated that two opposed binocular trackers are sufficient (less than 10% interruption to tracking) for a wide variety of behaviour types. We conclude that reliable tracking of head pose can be achieved with marker-based optical-motion-tracking systems for both tube-bound and freely moving rats undergoing PET studies without sedation.

Experimental protocols for behavioral imaging: seeing animal models of drug abuse in a new light

Behavioral neuroimaging is a rapidly evolving discipline that represents a marriage between the fields of behavioral neuroscience and preclinical molecular imaging. This union highlights the changing role of imaging in translational research. Techniques developed for humans are now widely applied in the study of animal models of brain disorders such as drug addiction. Small animal or preclinical imaging allows us to interrogate core features of addiction from both behavioral and biological endpoints. Snapshots of brain activity allow us to better understand changes in brain function and behavior associated with initial drug exposure, the emergence of drug escalation, and repeated bouts of drug withdrawal and relapse. Here we review the development and validation of new behavioral imaging paradigms and several clinically relevant radiotracers used to capture dynamic molecular events in behaving animals. We will discuss ways in which behavioral imaging protocols can be optimized to increase throughput and quantitative methods. Finally, we discuss our experience with the practical aspects of behavioral neuroimaging, so investigators can utilize effective animal models to better understand the addicted brain and behavior.

Striatal and extrastriatal microPET imaging of D2/D3 dopamine receptors in rat brain with [¹⁸F]fallypride and [¹⁸F]desmethoxyfallypride

In this study, we compared two different D(2/3) receptor ligands, [¹⁸F]fallypride and [¹⁸F]desmethoxyfallypride ([¹⁸F]DMFP) with respect to the duration of the scan, visualization of extrastriatal receptors, and binding potentials (BP(ND) ) in the rat brain. In addition, we studied the feasibility of using these tracers following a period of awake tracer uptake, during which the animal may perform a behavioral task. Male Sprague-Dawley rats were imaged with [¹⁸F]fallypride and with [¹⁸F]DMFP in four different studies using microPET. All scans were performed under isoflurane anesthesia. The first (test) and second (retest) study were 150-min baseline scans. No retest scans were performed with [¹⁸F]DMFP. A third study was a 60-min awake uptake of radiotracer followed by a 90-min scan. A fourth study was a 150-min competition scan with haloperidol (0.2 mg/kg) administered via tail vein at 90-min post-[¹⁸F]fallypride injection and 60-min post-[¹⁸F]DMFP. For the test-retest studies, BP(ND) was measured using both Logan noninvasive (LNI) method and the interval ratios (ITR) method. Cerebellum was used as a reference region. For the third study, the binding was measured only with the ITR method, and the results were compared to the baseline results. Studies showed that the average transient equilibrium time in the dorsal striatum (DSTR) was at 90 min for [¹⁸F]fallypride and 30 min for [¹⁸F]DMFP. The average BP(ND) for [¹⁸F]fallypride was 14.4 in DSTR, 6.8 in ventral striatum (VSTR), 1.3 in substantia nigra/ventral tegmental area (SN/VTA), 1.4 in colliculi (COL), and 1.5 in central gray area. In the case of [¹⁸F]DMFP, the average BP(ND) values were 2.2 in DSTR, 2.7 in VSTR, and 0.8 in SN/VTA. The haloperidol blockade showed detectable decrease in binding of both tracers in striatal regions with a faster displacement of [¹⁸F]DMFP. No significant changes in BP(ND) of [¹⁸F]fallypride due to the initial awake state of the animal were found, whereas BP(ND) of [¹⁸F]DMFP was significantly higher in the awake state compared to baseline. We were able to demonstrate that dynamic PET using MicroPET Inveon allows quantification of both striatal and extrastriatal [¹⁸F]fallypride binding in rats in vivo. Quantification of the striatal regions could be achieved with [¹⁸F]DMFP.

Differential effects of stress on [¹¹C]raclopride and [¹¹C]MNPA binding to striatal D₂/D₃ dopamine receptors: a PET study in conscious monkeys

It has been reported that stress and facilitation of dopamine neuronal system are closely related. In the present study, the effects of stress on the binding of antagonist-based [¹¹C]raclopride and agonist-based (R)-2-CH3O-N-n- propylnorapomorphine ([¹¹C]MNPA) to D₂/D₃ receptors were evaluated in the striatum of conscious monkey brain. The stress state assessed from plasma cortisol level was negatively correlated with [¹¹C]raclopride binding as expected. It was noteworthy that [¹¹C]MNPA binding exhibited a positive correlation with stress state; thus, the animals with higher cortisol levels showed higher binding to D₂/D₃ receptors.

Molecular imaging in neuroscience research with small-animal PET in rodents

Cognitive neuroscience, which studies the biological basis of mental processes, widely uses neuroimaging technologies like functional magnetic resonance imaging and positron emission tomography (PET) to study the human brain. Small laboratory animals, like rodents, are commonly used in brain research and provide abundant models of human brain diseases. The development of high-resolution small-animal PET and various radiotracers together with sophisticated methods for analyzing functional brain imaging data have accelerated research on brain function and neurotransmitter release during behavioral tasks in rodents. In this review, we first summarize advances in the methodology of cognitive research brought about by the development of sophisticated methods for whole-brain imaging analysis and improvements in neuroimaging protocols. Then, we discuss basic mechanisms related to metabolic changes and the expression of neurotransmitters in various brain areas during task-induced neural activity. In particular, we discuss glucose metabolism imaging and brain receptor imaging for various receptor systems. Finally, we discuss the current status and future perspectives. Mechanisms of neurotransmitter expression will probably become an increasingly important field of study in the future, leading to more collaboration between investigators in fields such as computational and theoretical neuroscience.

Assessment of i.p. injection of [18F]fallypride for behavioral neuroimaging in rats

Great progress has been made toward using small animal PET to assess neurochemical changes during behavior. [(18)F]fallypride (FAL) is a D(2)/D(3) antagonist that is sensitive to changes in endogenous dopamine, and, in theory, could be used to assess changes in dopamine during behavioral paradigms. Tail vein injections of tracer require restraint in awake animals, and catheter implantation is invasive and can cause logistical problems. Thus, administering tracer with i.p. injections (which are well-tolerated by rodents) would be preferable. The purpose of this study was to determine whether i.p. injection of FAL would produce striatal uptake similar to that seen with traditional i.v. tail vein injection protocols. Four male Sprague-Dawley rats underwent i.p. injection of FAL, followed by a 30-min uptake and subsequent dynamic image acquisition on the IndyPET III small animal scanner. Three of these rats also received traditional dynamic scanning with i.v. FAL injection via a tail vein. Two rats that received i.p. injection had moderate striatal uptake, with striatum/cerebellum ratios (SUVR) that were only ∼20% lower than ratios from i.v. scans. Two other rats had little to no uptake; SUVR values were ∼70% lower than i.v. SUVR. These latter two animals showed heavy bone uptake, evidence of defluorination of FAL. The results of this pilot study suggest that it may be possible to achieve striatal uptake of FAL after i.p. injection. However, this was not seen consistently across animals. Future studies are needed to validate, and then to optimize, the use of i.p. FAL for behavioral imaging protocols.

The dopaminergic basis of human behaviors: A review of molecular imaging studies

This systematic review describes human molecular imaging studies which have investigated alterations in extracellular DA levels during performance of behavioral tasks. Whilst heterogeneity in experimental methods limits meta-analysis, we describe the advantages and limitations of different methodological approaches. Interpretation of experimental results may be limited by regional cerebral blood flow (rCBF) changes, head movement and choice of control conditions. We revisit our original study of striatal DA release during video-game playing [Koepp, M.J., Gunn, R.N., Lawrence, A.D., Cunningham, V.J., Dagher, A., Jones, T., Brooks, D.J., Bench, C.J., Grasby, P.M., 1998. Evidence for striatal dopamine release during a video game. Nature 393, 266-268] to illustrate the potentially confounding influences of head movement and alterations in rCBF. Changes in [(11)C]raclopride binding may be detected in extrastriatal as well as striatal brain regions-however we review evidence which suggests that extrastriatal changes may not be clearly interpreted in terms of DA release. Whilst several investigations have detected increases in striatal extracellular DA concentrations during task components such as motor learning and execution, reward-related processes, stress and cognitive performance, the presence of potentially biasing factors should be carefully considered (and, where possible, accounted for) when designing and interpreting future studies.

Cue-induced dopamine release predicts cocaine preference: positron emission tomography studies in freely moving rodents

Positron emission tomography studies in drug-addicted patients have shown that exposure to drug-related cues increases striatal dopamine, which displaces binding of the D(2) ligand, [(11)C]-raclopride. However, it is not known if animals will also show cue-induced displacement of [(11)C]-raclopride binding. In this study, we use [(11)C]-raclopride imaging in awake rodents to capture cue-induced changes in dopamine release associated with the conditioned place preference model of drug craving. Ten animals were conditioned to receive cocaine in a contextually distinct environment from where they received saline. Following conditioning, each animal was tested for preference and then received two separate [(11)C]-raclopride scans. For each scan, animals were confined to the cocaine and/or the saline-paired environment for the first 25 min of uptake, after which they were anesthetized and scanned. [(11)C]-raclopride uptake in the saline-paired environment served as a within-animal control for uptake in the cocaine-paired environment. Cocaine produced a significant place preference (p = 0.004) and exposure to the cocaine-paired environment decreased [(11)C]-raclopride binding relative to the saline-paired environment in both the dorsal (20%; p < 0.002) and ventral striatum (22%; p < 0.05). The change in [(11)C]-raclopride binding correlated with preference in the ventral striatum (R(2) = -0.87; p = 0.003). In this region, animals who showed little or no preference exhibited little or no change in [(11)C]-raclopride binding in the cocaine-paired environment. This noninvasive procedure of monitoring neurochemical events in freely moving, behaving animals advances preclinical molecular imaging by interrogating the degree to which animal models reflect the human condition on multiple dimensions, both biological and behavioral.

Developing new molecular imaging probes for PET

Positron emission tomography (PET) is a fully translational molecular imaging technique that requires specific probes radiolabelled with short-lived positron emitting radionuclides. This review discusses relevant methods which are applied throughout the different steps in the development of new PET probes for in vivo visualization of specific molecular targets related to diagnosis or important for drug development.