Effect of L-dopa and 6-hydroxydopamine lesioning on [11C]raclopride binding in rat striatum, quantified using PET

Role of Brain Imaging in Drug Development for Psychiatry

Background:

Over the last decades, many brain imaging studies have contributed to new insights in the pathogenesis of psychiatric disease. However, in spite of these developments, progress in the development of novel therapeutic drugs for prevalent psychiatric health conditions has been limited.

Objective:

In this review, we discuss translational, diagnostic and methodological issues that have hampered drug development in CNS disorders with a particular focus on psychiatry. The role of preclinical models is critically reviewed and opportunities for brain imaging in early stages of drug development using PET and fMRI are discussed. The role of PET and fMRI in drug development is reviewed emphasizing the need to engage in collaborations between industry, academia and phase I units.

Conclusion:

Brain imaging technology has revolutionized the study of psychiatric illnesses, and during the last decade, neuroimaging has provided valuable insights at different levels of analysis and brain organization, such as effective connectivity (anatomical), functional connectivity patterns and neurochemical information that may support both preclinical and clinical drug development. Since there is no unifying pathophysiological theory of individual psychiatric syndromes and since many symptoms cut across diagnostic boundaries, a new theoretical framework has been proposed that may help in defining new targets for treatment and thus enhance drug development in CNS diseases. In addition, it is argued that new proposals for data-mining and mathematical modelling as well as freely available databanks for neural network and neurochemical models of rodents combined with revised psychiatric classification will lead to new validated targets for drug development.

Quantitative Rodent Brain Receptor Imaging

Positron emission tomography (PET) is a non-invasive imaging technology employed to describe metabolic, physiological, and biochemical processes in vivo. These include receptor availability, metabolic changes, neurotransmitter release, and alterations of gene expression in the brain. Since the introduction of dedicated small-animal PET systems along with the development of many novel PET imaging probes, the number of PET studies using rats and mice in basic biomedical research tremendously increased over the last decade. This article reviews challenges and advances of quantitative rodent brain imaging to make the readers aware of its physical limitations, as well as to inspire them for its potential applications in preclinical research. In the first section, we briefly discuss the limitations of small-animal PET systems in terms of spatial resolution and sensitivity and point to possible improvements in detector development. In addition, different acquisition and post-processing methods used in rodent PET studies are summarized. We further discuss factors influencing the test-retest variability in small-animal PET studies, e.g., different receptor quantification methodologies which have been mainly translated from human to rodent receptor studies to determine the binding potential and changes of receptor availability and radioligand affinity. We further review different kinetic modeling approaches to obtain quantitative binding data in rodents and PET studies focusing on the quantification of endogenous neurotransmitter release using pharmacological interventions. While several studies have focused on the dopamine system due to the availability of several PET tracers which are sensitive to dopamine release, other neurotransmitter systems have become more and more into focus and are described in this review, as well. We further provide an overview of latest genome engineering technologies, including the CRISPR/Cas9 and DREADD systems that may advance our understanding of brain disorders and function and how imaging has been successfully applied to animal models of human brain disorders. Finally, we review the strengths and opportunities of simultaneous PET/magnetic resonance imaging systems to study drug-receptor interactions and challenges for the translation of PET results from bench to bedside.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

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.

[18F]fallypride-PET/CT Analysis of the Dopamine D₂/D₃ Receptor in the Hemiparkinsonian Rat Brain Following Intrastriatal Botulinum Neurotoxin A Injection

Intrastriatal injection of botulinum neurotoxin A (BoNT-A) results in improved motor behavior of hemiparkinsonian (hemi-PD) rats, an animal model for Parkinson’s disease. The caudate–putamen (CPu), as the main input nucleus of the basal ganglia loop, is fundamentally involved in motor function and directly interacts with the dopaminergic system. To determine receptor-mediated explanations for the BoNT-A effect, we analyzed the dopamine D₂/D₃ receptor (D₂/D₃R) in the CPu of 6-hydroxydopamine (6-OHDA)-induced hemi-PD rats by [¹⁸F]fallypride-PET/CT scans one, three, and six months post-BoNT-A or -sham-BoNT-A injection. Male Wistar rats were assigned to three different groups: controls, sham-injected hemi-PD rats, and BoNT-A-injected hemi-PD rats. Disease-specific motor impairment was verified by apomorphine and amphetamine rotation testing. Animal-specific magnetic resonance imaging was performed for co-registration and anatomical reference. PET quantification was achieved using PMOD software with the simplified reference tissue model 2. Hemi-PD rats exhibited a constant increase of 23% in D₂/D₃R availability in the CPu, which was almost normalized by intrastriatal application of BoNT-A. Importantly, the BoNT-A effect on striatal D₂/D₃R significantly correlated with behavioral results in the apomorphine rotation test. Our results suggest a therapeutic effect of BoNT-A on the impaired motor behavior of hemi-PD rats by reducing interhemispheric changes of striatal D₂/D₃R.

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.

Altered adenosine 2A and dopamine D2 receptor availability in the 6-hydroxydopamine-treated rats with and without levodopa-induced dyskinesia

Several lines of evidence imply alterations in adenosine signaling in Parkinson's disease (PD). Here, we investigated cerebral changes in adenosine 2A receptor (A_2AR) availability in 6-hydroxydopamine (6-OHDA)-lesioned rats with and without levodopa-induced dyskinesia (LID) using positron-emission tomography (PET) with [¹¹C]preladenant. In parallel dopamine type 2 receptor (D₂R) imaging with [¹¹C]raclopride PET and behavioral tests for motor and cognitive function were performed.

METHODS

Parametric A_2AR and D₂R binding potential (BP_ND) images were reconstructed using reference tissue models with midbrain and cerebellum as reference tissue, respectively. All images were anatomically standardized to Paxinos space and analyzed using volume-of-interest (VOI) and voxel-based approaches. The behavioral alternations were assessed with the open field test, Y-maze, novel object recognition test, cylinder test, and abnormal involuntary movement (AIM) score. In total, 28 female Wistar rats were included.

RESULTS

On the behavioral level, 6-OHDA-lesioned rats showed asymmetry in forepaw use and deficits in spatial memory and explorative behavior as compared to the sham-operated animals. 15-Days of levodopa (L-DOPA) treatment induced dyskinesia but did not alleviate motor deficits in PD rats. Intranigral 6-OHDA injection significantly increased D₂R binding in the lesioned striatum (BP_ND: 2.69 ± 0.40 6-OHDA vs. 2.31 ± 0.18 sham, + 16.6%; p = 0.03), whereas L-DOPA treatment did not affect the D₂R binding in the ipsilateral striatum of the PD rats. In addition, intranigral 6-OHDA injection tended to decrease the A_2AR availability in the lesioned striatum. The decrease became significant when data were normalized to the non-affected side (BP_ND: 4.32 ± 0.41 6-OHDA vs. 4.58 ± 0.89 sham; NS, ratio: 0.94 ± 0.03 6-OHDA vs. 1.00 ± 0.02 sham; - 6.1%; p = 0.01). L-DOPA treatment significantly increased A_2AR binding in the affected striatum (BP_ND: 6.02 ± 0.91 L-DOPA vs. 4.90 ± 0.76 saline; + 23.4%; p = 0.02). In PD rats with LID, positive correlations were found between D₂R and A_2AR BP_ND values in the ipsilateral striatum (r = 0.88, p_peak = 8.56.10^-4 uncorr), and between AIM score and the D₂R BP_ND in the contralateral striatum (r = 0.98; p_peak = 9.55.10^-5 uncorr).

CONCLUSION

A_2AR availability changed in drug-naïve and in L-DOPA-treated PD rats. The observed correlations of striatal D₂R availability with A_2AR availability and with AIM score may provide new knowledge on striatal physiology and new possibilities to further unravel the functions of these targets in the pathophysiology of PD.

Relationship Between L-DOPA-Induced Reduction in Motor and Exploratory Activity and Striatal Dopamine D2 Receptor Binding in the Rat

Purpose: The present study assessed the influence of L-DOPA administration on neostriatal dopamine (DA) D₂ receptor binding in relation to motor and exploratory behaviors in the rat.

Methods: D₂ receptor binding was measured in baseline, after challenge with the aromatic L-amino acid decarboxylase inhibitor benserazide, and after challenge with either 5 or 10 mg/kg L-DOPA plus benserazide. Additional rats received injections of saline. For baseline and challenges, striatal equilibrium ratios (V3″) were computed as estimation of the binding potential. Motor and exploratory behaviors were assessed for 30 min in an open field prior to administration of [¹²³I]IBZM. D₂ receptor binding was measured with small animal SPECT 2 h after radioligand administration for 60 min.

Results: Both L-DOPA doses significantly reduced D₂ receptor binding relative to baseline and led to significantly less ambulation, less head-shoulder motility, and more sitting relative to saline. Moreover, 10 mg/kg L-DOPA induced less head-shoulder motility, more sitting, and more grooming than 5 mg/kg L-DOPA. Analysis of time-behavior curves showed that L-DOPA-treated animals relative to saline exhibited a faster rate of decrease of ambulation frequency and a slower rate of decrease of both duration and frequency of head-shoulder motility from a lower maximum level.

Conclusions: The reductions of striatal D₂ receptor binding after L-DOPA may be conceived to reflect elevated concentrations of synaptic DA. L-DOPA-treated animals showed less ambulation and less head-shoulder motility than saline-treated animals, indicating an association between less behavioral activity and increased availability of striatal DA. The faster rate of decrease of ambulation frequency and the lower maximum levels of both head-shoulder motility duration and frequency may be interpreted in terms of influence of increased DA availability on behavioral habituation to a novel environment.

In Vivo Evaluation of 11C-DASB for Quantitative SERT Imaging in Rats and Mice

Serotonin, or 5-hydroxytryptamine (5-HT), plays a key role in the central nervous system and is involved in many essential neurologic processes such as mood, social behavior, and sleep. The serotonin transporter ligand (11)C-3-amino-4(2-dimethylaminomethyl-phenylsufanyl)-benzonitrile ((11)C-DASB) has been used to determine nondisplaceable binding potential (BPND), which is defined as the quotient of the available receptor density (Bavail) and the apparent equilibrium dissociation rate constant (1/appKD) under in vivo conditions. Because of the increasing number of animal models of human diseases, there is a pressing need to evaluate the applicability of (11)C-DASB to rats and mice. Here, we assessed the feasibility of using (11)C-DASB for quantification of serotonin transporter (SERT) density and affinity in vivo in rats and mice.

METHODS

Rats and mice underwent 4 PET scans with increasing doses of the unlabeled ligand to calculate Bavail and appKD using the multiple-ligand concentration transporter assay. An additional PET scan was performed to calculate test-retest reproducibility and reliability. BPND was calculated using the simplified reference tissue model, and the results for different reference regions were compared.

RESULTS

Displaceable binding of (11)C-DASB was found in all brain regions of both rats and mice, with the highest binding being in the thalamus and the lowest in the cerebellum. In rats, displaceable binding was largely reduced in the cerebellar cortex, which in mice was spatially indistinguishable from cerebellar white matter. Use of the cerebellum with fully saturated binding sites as the reference region did not lead to reliable results. Test-retest reproducibility in the thalamus was more than 90% in both mice and rats. In rats, Bavail, appKD, and ED50 were 3.9 ± 0.4 pmol/mL, 2.2 ± 0.4 nM, and 12.0 ± 2.6 nmol/kg, respectively, whereas analysis of the mouse measurements resulted in inaccurate fits due to the high injected tracer mass.

CONCLUSION

Our data showed that in rats, (11)C-DASB can be used to quantify SERT density with good reproducibility. BPND agreed with the distribution of SERT in the rat brain. It remains difficult to estimate quantitative parameters accurately from mouse measurements because of the high injected tracer mass and underestimation of the binding parameters due to high displaceable binding in the reference region.

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.

Measuring dopaminergic function in the 6-OHDA-lesioned rat: a comparison of PET and microdialysis

BACKGROUND

[18 F]fluorodopa (FDOPA) positron emission tomography (PET) allows assessment of levodopa (LDOPA) metabolism and is widely used to study Parkinson's disease. We examined how [18 F]FDOPA PET-derived kinetic parameters relate the dopamine (DA) and DA metabolite content of extracellular fluid measured by microdialysis to aid in the interpretation of data from both techniques.

METHODS

[18 F]FDOPA PET imaging and microdialysis measurements were performed in unilaterally 6-hydroxydopamine-lesioned rats (n = 8) and normal control rats (n = 3). Microdialysis testing included baseline measurements and measurements following acute administration of LDOPA. PET imaging was also performed using [11C]dihydrotetrabenazine (DTBZ), which is a ligand for the vesicular monoamine transporter marker and allowed assessment of denervation severity.

RESULTS

The different methods provided highly correlated data. Lesioned rats had reduced DA metabolite concentrations ipsilateral to the lesion (p < 0.05 compared to controls), with the concentration being correlated with FDOPA's effective distribution volume ratio (EDVR; r = 0.86, p < 0.01) and DTBZ's binding potential (BPND; r = 0.89, p < 0.01). The DA metabolite concentration in the contralateral striatum of severely (>80%) lesioned rats was lower (p < 0.05) than that of less severely lesioned rats (<80%) and was correlated with the ipsilateral PET measures (r = 0.89, p < 0.01 for BPND) but not with the contralateral PET measures. EDVR and BPND in the contralateral striatum were not different from controls and were not correlated with the denervation severity.

CONCLUSIONS

The demonstrated strong correlations between the PET and microdialysis measures can aid in the interpretation of [18 F]FDOPA-derived kinetic parameters and help compare results from different studies. The contralateral striatum was affected by the lesioning and so cannot always serve as an unaffected control.

[11C]AF150(S), an agonist PET ligand for M1 muscarinic acetylcholine receptors

Background

The M1 muscarinic acetylcholine receptor (M1ACh-R) is a G protein-coupled receptor that can occur in interconvertible coupled and uncoupled states. It is enriched in the basal ganglia, hippocampus, olfactory bulb, and cortical areas, and plays a role in motor and cognitive functions. Muscarinic M1 agonists are potential therapeutic agents for cognitive disorders. The aim of this study was to evaluate [¹¹C]AF150(S) as a putative M1ACh-R agonist PET ligand, which, owing to its agonist properties, could provide a tool to explore the active G protein-coupled receptor.

Methods

Regional kinetics of [¹¹C]AF150(S) in rat brain were measured using a high-resolution research tomograph, both under baseline conditions and following pre-treatment with various compounds or co-administration of non-radioactive AF150(S). Data were analysed by calculating standard uptake values and by applying the simplified reference tissue model (SRTM).

Results

[¹¹C]AF150(S) was rapidly taken up in the brain, followed by a rapid clearance from all brain regions. Analysis of PET data using SRTM revealed a binding potential (BP_ND) of 0.25 for the striatum, 0.20 for the hippocampus, 0.16 for the frontal cortical area and 0.15 for the posterior cortical area, all regions rich in M1ACh-R. BP_ND values were significantly reduced following pre-treatment with M1ACh-R antagonists. BP_ND values were not affected by pre-treatment with a M3ACh-R antagonist. Moreover, BP_ND was significantly reduced after pre-treatment with haloperidol, a dopamine D₂ receptor blocker that causes an increase in extracellular acetylcholine (ACh). The latter may compete with [¹¹C]AF150(S) for binding to the M1ACh-R; further pharmacological agents were applied to investigate this possibility. Upon injection of the highest dose (49.1 nmol kg⁻¹) of [¹¹C]AF150(S) diluted with non-radioactive AF150(S), brain concentration of AF150(S) reached 100 nmol L⁻¹ at peak level. At this concentration, no sign of saturation in binding to M1ACh-R was observed.

Conclusions

The agonist PET ligand [¹¹C]AF150(S) was rapidly taken up in the brain and showed an apparent specific M1ACh-R-related signal in brain areas that are rich in M1ACh-R. Moreover, binding of the agonist PET ligand [¹¹C]AF150(S) appears to be sensitive to changes in extracellular ACh levels. Further studies are needed to evaluate the full potential of [¹¹C]AF150(S) for imaging the active pool of M1ACh-R in vivo.

Fundamentals of Positron Emission Tomography and Applications in Preclinical Drug Development

Positron emission tomography (PET) is a nuclear imaging technique that can dynamically image trace amounts of positron-labeled radiopharmaceuticals in vivo. Tracer concentrations can be determined quantitatively, and by application of appropriate tracer kinetic models, the rates of a wide range of different biological processes can be measured noninvasively in humans. PET has been used as a research tool for more than 25 years and has also found clinical applications, particularly in oncology, neurological disorders, and cardiovascular disease. Recently, there has been tremendous interest in applying PET technology to in vivo small-animal imaging. Significant improvements in the imaging technology now permit a wide range of PET studies in mice and rats, using compact, relatively low-cost, dedicated small-animal PET scanners. This article reviews the fundamental basis of PET imaging and discusses the development of small-animal PET scanners and their possible application in preclinical drug development.

Evaluation of dopamine D₂/D₃ and serotonin 5-HT₂A receptor occupancy for a novel antipsychotic, lurasidone, in conscious common marmosets using small-animal positron emission tomography

RATIONALE

Lurasidone is a novel antipsychotic drug with potent binding affinity for dopamine D(2) and serotonin (5-hydroxytryptamine, 5-HT)(2A), 5-HT(7), and 5-HT(1A) receptors. Previous pharmacological studies have revealed that lurasidone exhibits a preferable profile (potent antipsychotic activity and lower incidence of catalepsy) to other antipsychotic drugs, although the contribution of receptor subtypes to this profile remains unclear.

OBJECTIVES

To compare target engagements of lurasidone with those of an atypical antipsychotic, olanzapine, we performed evaluation of dopamine D(2)/D(3) and serotonin 5-HT(2A) receptor occupancy in vivo by positron emission tomography (PET) with conscious common marmosets.

METHODS

We measured brain receptor occupancies in conscious common marmosets after oral administrations of lurasidone or olanzapine by PET with [(11)C]raclopride and [(11)C]R-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine methanol (MDL 100907) for D(2)/D(3) and 5-HT(2A) receptors, respectively.

RESULTS

Increases in brain D(2)/D(3) receptor occupancies of both lurasidone and olanzapine, which reached >80 % at maximum, were observed in the striatum with significant correlations to plasma drug levels. However, lurasidone showed lower 5-HT(2A) receptor occupancy in the frontal cortex within the same dose range, while olanzapine showed broadly comparable 5-HT(2A) and D(2)/D(3) receptor occupancies.

CONCLUSIONS

Compared with olanzapine, lurasidone preferentially binds to D(2)/D(3) receptors rather than 5-HT(2A) receptors in common marmosets. These results suggest that the contribution of in vivo 5-HT(2A) receptor blocking activity to the pharmacological profile of lurasidone might differ from olanzapine in terms of the low risk of extrapyramidal syndrome and efficacy against negative symptoms.

Characterisation of partial volume effect and region-based correction in small animal positron emission tomography (PET) of the rat brain

Accurate quantification of PET imaging data is required for a useful interpretation of the measured radioactive tracer concentrations. The partial volume effect (PVE) describes signal dilution and mixing due to spatial resolution and sampling limitations, which introduces bias in quantitative results. In the present study we investigated the magnitude of PVE for volumes of interest (VOIs) in the rat brain and the effect of positron range. In simulated (11)C-raclopride studies we examined the influence of PVE on time activity curves in striatal and cerebellar VOIs and binding potential estimation. The performance of partial volume correction (PVC) was studied using the region-based geometric transfer matrix (GTM) method including the question of whether a spatially variant point spread function (PSF) is necessary for PVC of a rat brain close to the centre of the field of view. Furthermore, we determined the effect of spillover from activity outside the brain. The results confirmed that PVE is significant in rat brain PET and showed that positron range is an important factor that needs to be included in the PSF. There was considerable bias in time activity curves for the simulated (11)C-raclopride studies and significant underestimation of binding potential even for very small centred VOIs. Good activity recovery was achieved with the GTM PVC using a spatially invariant simulated PSF when no activity was present outside the brain. PVC using a simple Gaussian fit point spread function was not sufficiently accurate. Spillover from regions outside the brain had a significant impact on measured activity concentrations and reduced the accuracy of PVC with the GTM method using rat brain regions alone, except for the smallest VOI size but at the cost of increased noise. Voxel-based partial volume correction methods which inherently compensate for spillover from outside the brain might be a more suitable choice.

Prefrontal dopaminergic receptor abnormalities and executive functions in Parkinson's disease

The main pattern of cognitive impairments seen in early to moderate stages of Parkinson's disease (PD) includes deficits of executive functions. These nonmotor complications have a significant impact on the quality of life and day-to-day activities of PD patients and are not effectively managed by current therapies, a problem which is almost certainly due to the fact that the disease extends beyond the nigrostriatal system. To investigate the role of extrastriatal dopamine in executive function in PD, PD patients and a control group were studied with positron-emission-tomography using a high-affinity dopamine D2/D3 receptor tracer, [(11) C]FLB-457. All participants were scanned twice while performing an executive task and a control task. Patients were off medication for at least 12 h. The imaging analysis revealed that parkinsonian patients had lower [(11) C]FLB-457 binding than control group independently of task conditions across different brain regions. Cognitive assessment measures were positively correlated with [(11) C]FLB-457 binding in the bilateral dorsolateral prefrontal cortex and anterior cingulate cortex only in control group, but not in PD patients. Within the control group, during the executive task (as compared to control task), there was evidence of reduced [(11) C]FLB-457 binding (indicative of increased dopamine release) in the right orbitofrontal cortex. In contrast, PD patients did not show any reduction in binding during the executive task (as compared with control task). These findings suggest that PD patients present significant abnormalities in extrastriatal dopamine associated with executive processing. These observations provide important insights on the pathophysiology of cognitive dysfunction in PD.

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.

Pharmacological challenge and synaptic response - assessing dopaminergic function in the rat striatum with small animal single-photon emission computed tomography (SPECT) and positron emission tomography (PET)

Disturbances of dopaminergic neurotransmission may be caused by changes in concentrations of synaptic dopamine (DA) and/or availabilities of pre- and post-synaptic transporter and receptor binding sites. We present a series of experiments which focus on the regulatory mechanisms of the dopamin(DA)ergic synapse in the rat striatum. In these studies, DA transporter (DAT) and/or D(2) receptor binding were assessed with either small animal single-photon emission computed tomography (SPECT) or positron emission tomography (PET) after pharmacological challenge with haloperidol, L-DOPA and methylphenidate, and after nigrostriatal 6-hydroxydopamine lesion. Investigations of DAT binding were performed with [(123)I]N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)nortropane ([(123)I]FP-CIT). D(2) receptor bindingd was assessed with either [(123)I](S)-2-hydroxy-3-iodo-6-methoxy-N-[(1-ethyl-2-pyrrolidinyl)methyl]benzamide ([(123)I]IBZM) or [(18)F]1[3-(4'fluorobenzoyl)propyl]-4-(2-keto-3-methyl-1-benzimidazolinyl)piperidine ([(18)F]FMB). Findings demonstrate that in vivo investigations of transporter and/or receptor binding are feasible with small animal SPECT and PET. Therefore, tracers that are radiolabeled with isotopes of comparatively long half-lives such as (123)I may be employed. Our approach to quantify DAT and/or D(2) receptor binding at baseline and after pharmacological interventions inducing DAT blockade, D(2) receptor blockade, and increases or decreases of endogenous DA concentrations holds promise for the in vivo assessment of synaptic function. This pertains to animal models of diseases associated with pre- or postsynaptic DAergic deficiencies such as Parkinson's disease, Huntington's disease, attention-deficit/hyperactivity disorder, schizophrenia or drug abuse.

L-amino acid decarboxylase- and tyrosine hydroxylase-immunoreactive cells in the extended olfactory amygdala and elsewhere in the adult prairie vole brain

Neurons synthesizing dopamine (DA) are widely distributed in the brain and implicated in a tremendous number of physiological and behavioral functions, including socioreproductive behaviors in rodents. We have recently been investigating the possible involvement of sex- and species-specific TH-immunoreactive (TH-ir) cells in the male prairie vole (Microtus ochrogaster) principal bed nucleus of the stria terminalis (pBST) and posterodorsal medial amygdala (MeApd) in the chemosensory control of their monogamous pairbonding and parenting behaviors. These TH-ir cells are not immunoreactive for dopamine-beta-hydroxylase (DBH), suggesting they are not noradrenergic but possibly DAergic. A DAergic phenotype would require them to contain aromatic L-amino acid decarboxylase (AADC) and here we examined the existence of cells immunoreactive for both TH and AADC in the pBST and MeApd of adult virgin male and female prairie voles. We also investigated the presence of TH/AADC cells in the anteroventral periventricular nucleus (AVPV), medial preoptic area (MPO), arcuate nucleus (ARH), zona incerta (ZI), substantia nigra (SN) and ventral tegmental area (VTA). Among our findings were: (1) the pBST and MeApd each contained completely non-overlapping distributions of TH-ir and AADC-ir cells, (2) the AVPV contained surprisingly few AADC-ir cells and almost no TH-ir cells contained AADC-ir, (3) approximately 60% of the TH-ir cells in the MPO, ARH, and ZI also contained AADC-ir, (4) unexpectedly, only about half of TH-ir cells in the SN and VTA contained AADC-ir, and (5) notable populations of AADC-ir cells were found outside traditional monoamine-synthesizing regions, including some sites that do not contain AADC-ir cells in adult laboratory rats or cats (medial septum and cerebral cortex). In the absence of the chemical requirements to produce DA, monoenzymatic TH-ir cells in the virgin adult prairie vole pBST, MeApd, and elsewhere in their brain may instead produce L-DOPA as an end product and use it as a neurotransmitter or neuromodulator, similar to what has been observed for monoenzymatic TH-synthesizing cells in the laboratory rat brain.

Neuroanatomical projections of the species-specific tyrosine hydroxylase-immunoreactive cells of the male prairie vole bed nucleus of the stria terminalis and medial amygdala

The principal nucleus of the bed nucleus of the stria terminalis (BSTpr) and posterodorsal part of the medial amygdalar nucleus (MEApd) are densely interconnected sites transmitting olfactory information to brain areas mediating sociosexual behaviors. In male prairie voles (Microtus ochrogaster), the BSTpr and MEApd contain hundreds of cells densely immunoreactive for tyrosine hydroxylase (TH). Such tremendous numbers of TH-immunoreactive (TH-ir) cells do not exist in other rodents examined, and studies from our laboratory suggest these cells may be part of a unique chemical network necessary for monogamous behaviors in prairie voles. To obtain information about how these TH-ir cells communicate with other sites involved in social behaviors, we first used biotinylated dextran amine (BDA) to determine sites that receive BSTpr efferents and also contain TH-ir fibers. Only in the medial preoptic area (MPO) and MEApd did we find considerable comingling of BDA-containing and TH-ir fibers. To examine if these sites receive input specifically from BSTpr TH-ir cells, the retrograde tracer Fluorogold was infused into the MPO or MEApd. Almost 80% of TH-ir projections to the MPO originated from the BSTpr or MEApd, involving about 40% of all TH-ir cells in these sites. In contrast, the MEApd received almost no input from TH-ir cells in the BSTpr, and received it primarily from the ventral tegmental area. Retrograde tracing from the BSTpr itself revealed substantial input from MEApd TH-ir cells. Thus, the male prairie vole brain contains a species-specific TH-ir network involving the BSTpr, MEApd, and MPO. By connecting brain sites involved in olfaction, sociality and motivation, this network may be essential for monogamous behaviors in this species.

In vivo imaging neurotransmitter function. The rat 6-hydroxydopamine model and its relevance for human Parkinson's disease

This article gives an overview of those small animal imaging studies which have been conducted on neurotransmitter function in the rat 6-hydoxydopamine (6-OHDA) model of Parkinson's disease, and discusses findings with respect to the outcome of clinical studies on Parkinsonian patients.

Mouse models in neurological disorders: applications of non-invasive imaging

Neuroimaging techniques represent powerful tools to assess disease-specific cellular, biochemical and molecular processes non-invasively in vivo. Besides providing precise anatomical localisation and quantification, the most exciting advantage of non-invasive imaging techniques is the opportunity to investigate the spatial and temporal dynamics of disease-specific functional and molecular events longitudinally in intact living organisms, so called molecular imaging (MI). Combining neuroimaging technologies with in vivo models of neurological disorders provides unique opportunities to understand the aetiology and pathophysiology of human neurological disorders. In this way, neuroimaging in mouse models of neurological disorders not only can be used for phenotyping specific diseases and monitoring disease progression but also plays an essential role in the development and evaluation of disease-specific treatment approaches. In this way MI is a key technology in translational research, helping to design improved disease models as well as experimental treatment protocols that may afterwards be implemented into clinical routine. The most widely used imaging modalities in animal models to assess in vivo anatomical, functional and molecular events are positron emission tomography (PET), magnetic resonance imaging (MRI) and optical imaging (OI). Here, we review the application of neuroimaging in mouse models of neurodegeneration (Parkinson's disease, PD, and Alzheimer's disease, AD) and brain cancer (glioma).

Lower level of endogenous dopamine in patients with cocaine dependence: findings from PET imaging of D(2)/D(3) receptors following acute dopamine depletion

OBJECTIVE

Previous positron emission tomography (PET) imaging studies have demonstrated that cocaine dependence is associated with a decrease in dopamine type 2 and 3 (D(2)/D(3)) receptor binding in cocaine-dependent individuals relative to healthy comparison subjects. However, given the nature of PET imaging, it is possible that the measured decrease in radiotracer binding results from an increase in baseline dopamine levels. The purpose of this study was to measure D(2)/D(3) receptors following acute dopamine depletion in cocaine-dependent volunteers relative to healthy comparison subjects.

METHOD

Cocaine-dependent volunteers (N=15) and healthy matched comparison subjects (N=15) were scanned using PET, with the dopamine receptor radiotracer [(11)C]raclopride, at baseline and again following acute depletion of endogenous dopamine via alpha-methyl-para-tyrosine (AMPT) administration. Changes in radiotracer binding were measured in the subdivisions of the striatum (caudate, putamen, and ventral striatum) in addition to the striatum as a whole.

RESULTS

Findings revealed that cocaine-dependent volunteers exhibited lower levels of endogenous dopamine relative to comparison subjects, which was measured as an increase in [(11)C]raclopride binding following AMPT administration. The increase in [(11)C]raclopride binding in the striatum was 11.1% (SD=4.4%) in healthy comparison subjects and 5.7% (SD=5.9%) in cocaine-dependent volunteers. Similar differences were seen in the subdivisions of the striatum.

CONCLUSIONS

The decrease in striatal D(2)/D(3 )receptors associated with cocaine dependence cannot be attributed to higher levels of endogenous dopamine.

In vivo multimodal imaging of stem cell transplantation in a rodent model of Parkinson's disease

Stem cell therapy in the nervous system aims to replace the lost neurons and provide functional recovery. However, it is imperative that we understand the in vivo behaviour of these cells post-implantation. We report visualisation of iron oxide labelled bone marrow-derived stem cells (BMSCs) implanted into the striatum of hemi-parkinsonian rats by magnetic resonance imaging (MRI). Functional efficacy of the donor cells was monitored in vivo using the positron emission tomography (PET) radioligand [11C]raclopride. The cells were visible for 28 days by in vivo MRI. BMSCs provided functional recovery demonstrated by a decreased binding of [11C]raclopride. Although, histology confirmed the persistence of donor cells, no tyrosine hydroxylase positive cells were present. This suggests that BMSCs may have a limited paracrine effect and influence functional recovery. We demonstrate, using multimodal imaging, that we can not only track BMSCs but also establish their effects in a pre-clinical model of Parkinson's disease.

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.

Applying neuroimaging ligands to study major depressive disorder

The recent increase in radioligands available for neuroimaging major depressive disorder has led to advancements in our understanding of the pathophysiology of this illness and improved antidepressant development. Major depressive disorder can be defined as an illness of recurrent major depressive episodes of persistently low mood, dysregulated sleep, appetite and weight, anhedonia, cognitive impairment, and suicidality. The main target sites investigated with radioligand neuroimaging include receptor sites that regulate in response to lowered monoamine levels, targets related to removal of monoamines, uptake of ligands related to regional brain function, and target sites of antidepressants.

In-vivo PET imaging of implanted human retinal pigment epithelium cells in a Parkinson's disease rat model

BACKGROUND AND AIM

Researchers find that monitoring the differentiation of implanted cells in vivo is difficult. This study was designed to show that it is possible to track the efficacy of transplanted human retinal pigment epithelial cells (RPE cells) in a rat model of Parkinson's disease by using positron emission tomography (PET).

METHODS

RPE cells or normal saline were injected into striatum of the injured side of the rat model in treated and control groups, respectively. PET imaging of both groups was undertaken before transplantation and at intervals afterwards, using C-raclopride and C-beta-CFT as the markers. Observation of the rats' behaviour and immunofluorescence confocal microscopy were also used to prove the PET results.

RESULTS

PET studies showed increased accumulation of C-raclopride and decreased C-beta-CFT in the injured side of striatum in both groups. C-raclopride decreased along with a concomitant increase of C-beta-CFT after transplantation in the treated group. The changes shown by the PET studies paralleled the behavioural states and confocal microscopy observations in the treated animals.

CONCLUSION

These results suggest that even a clinical PET scanner could, to a certain extent, provide some information on the existence and in-vivo differentiation of RPE cells in a rat model of Parkinson's disease.

Intrastriatal inhibition of aromatic amino acid decarboxylase prevents l-DOPA-induced dyskinesia: a bilateral reverse in vivo microdialysis study in 6-hydroxydopamine lesioned rats

l-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesia consists of involuntary choreiform and dystonic movements. Here we report whether intrastriatal l-DOPA itself is able to trigger dyskinetic behavior and which role the neurotransmitter dopamine (DA) and its metabolites play. Intrastriatal l-DOPA as well as DA administration at the 6-hydroxydopamine (6-OHDA) lesioned side led to a significant appearance of dyskinetic behavior, whereas DA metabolites were ineffective. Intrastriatal inhibition of the enzyme aromatic amino acid decarboxylase (AADC) by benserazide prevented the appearance of l-DOPA-induced dyskinetic movements at the lesioned side. Principle component analysis of DA and DA metabolite levels with dyskinesia scores after l-DOPA/benserazide (6/15 mg/kg) administration indicated a significant correlation only for DA, whereas DA metabolites did not show any significant correlation with the occurrence of dyskinetic behavior. We conclude that intrastriatal l-DOPA itself is not able to induce dyskinetic movements, whereas the increase of intrastriatal DA levels is instrumental for l-DOPA- and DA-induced dyskinetic behavior.

Animal Models of Neurodegenerative Disease: Insights from In vivo Imaging Studies

Animal models have been used extensively to understand the etiology and pathophysiology of human neurodegenerative diseases, and are an essential component in the development of therapeutic interventions for these disorders. In recent years, technical advances in imaging modalities such as positron emission tomography (PET) and magnetic resonance imaging (MRI) have allowed the use of these techniques for the evaluation of functional, neurochemical, and anatomical changes in the brains of animals. Combining animal models of neurodegenerative disorders with neuroimaging provides a powerful tool to follow the disease process, to examine compensatory mechanisms, and to investigate the effects of potential treatments preclinically to derive knowledge that will ultimately inform our clinical decisions. This article reviews the literature on the use of PET and MRI in animal models of Parkinson's disease, Huntington's disease, and Alzheimer's disease, and evaluates the strengths and limitations of brain imaging in animal models of neurodegenerative diseases.

Persistent dopamine functions of neurons derived from embryonic stem cells in a rodent model of Parkinson disease

The derivation of dopamine neurons is one of the best examples of the clinical potential of embryonic stem (ES) cells, but the long-term function of the grafted neurons has not been established. Here, we show that, after transplantation into an animal model, neurons derived from mouse ES cells survived for over 32 weeks, maintained midbrain markers, and had sustained behavioral effects. Microdialysis in grafted animals showed that dopamine (DA) release was induced by depolarization and pharmacological stimulants. Positron emission tomography measured the expression of presynaptic dopamine transporters in the graft and also showed that the number of postsynaptic DA D(2) receptors was normalized in the host striatum. These data suggest that ES cell-derived neurons show DA release and reuptake and stimulate appropriate postsynaptic responses for long periods after implantation. This work supports continued interest in ES cells as a source of functional DA neurons.

Mass effect of injected dose in small rodent imaging by SPECT and PET

This paper discusses the effect of mass (chemical quantity) of injected dose on positron emission tomography (PET) and single-photon emission computed tomography (SPECT). Commonly, PET or SPECT imaging study uses a "no-carrier added" dose, which contains a small amount of radioactive imaging agent (in picogram to microgram). For small animal (rodent) imaging studies, specifically targeting binding sites or biological processes, the mass (chemical quantity) in the dose may significantly modify the binding, pharmacokinetics and, ultimately, the imaging outcome. Due to differences in size and other physiological factors between humans and rodents, there is a dramatic divergence of mass effect between small animal and human imaging study. In small animal imaging studies, the mass, or effective dose (ED(50)), a dose required for 50% of receptor or binding site occupancy, is usually not directly related to binding potential (B(max)/K(d)) (measured by in vitro binding assay). It is likely that dynamic interplays between specific and nonspecific binding in blood circulation, transient lung retention, kidney excretion, liver-gallbladder flow, soft tissue retention as well as metabolism could each play a significant role in determining the concentration of the tracer in the target regions. When using small animal imaging for studying drug occupancy (either by a pretreatment, coinjection or chasing dose), the mass effects on imaging outcome are important factors for consideration.

Correlation between quantitative imaging and behavior in unilaterally 6-OHDA-lesioned rats

We evaluated correlation between neurochemical and functional alterations of the nigrostriatal dopaminergic system in rat brains lesioned with 6-hydroxydopamine (6-OHDA), that model hemi-Parkinson's disease (PD), by using three different quantitative in vivo and in vitro methods. Rats unilaterally lesioned with different doses of 6-OHDA underwent two types of in vivo experiments: (1) a rotational behavioral study with methamphetamine (MAP) or apomorphine (APO); and (2) a positron emission tomography (PET) study with [11C]PE2I (radioligand for dopamine transporters) or [11C]raclopride (radioligand for dopamine D2 receptors). An in vitro autoradiographic study with the same radioligands was also conducted. The number of rotations after the MAP or APO injection increased with increased doses of 6-OHDA. The in vitro and in vivo binding of [11C]PE2I dose-dependently decreased in response to the 6-OHDA injections, while that of [11C]raclopride dose-dependently increased. There was a significant negative hyperbolic correlation between the number of rotations after MAP injection and the binding of [11C]PE2I. In contrast, there was a significant positive linear correlation between the number of rotations after APO injections and the binding of [11C]raclopride. These results robustly reveal a molecular pharmacological basis of parkinsonian symptoms in animal models of PD, and indicate the utility and validity of in vivo PET measurements in assessing pre- and post-synaptic dopaminergic functions.

Evaluation of [123I]IBZM pinhole SPECT for the detection of striatal dopamine D2 receptor availability in rats

Single photon emission computed tomography (SPECT) and MRI coregistration have been assessed to characterize striatal dopamine D2/D3 receptor (D2/D3R) availability in rats following injection of the D2 and D3R radioligand [123I] iodobenzamide ([123I]IBZM). High-resolution SPECT data were obtained with a pinhole collimator. In order to precisely estimate brain regions of low radioligand uptake, SPECT images were coregistered onto a MRI template with high accuracy (maximum mismatch 1.1 mm). To evaluate an adequate dose of radioligand to be administered without exceeding the radioligand-to-receptor occupancy >5% and to define an appropriate time period for image acquisition, three untreated groups of animals received 29.6, 37, and 44.4 MBq of [123I]IBZM and underwent five consecutive SPECT acquisitions lasting 64 min each. Ratio calculations between specific striatal radioligand uptake and nondisplaceable cerebellar uptake revealed a secular equilibrium between 75 and 355 min post-tracer application in all three animal groups. Consequently, since the highest regional uptake values were obtained in the animal group receiving 44.4 MBq [123I]IBZM, this injection dose was considered to be appropriate. Finally, the capacity of the imaging method to detect distinct severity levels of striatal dopamine D2/D3 receptor loss was tested in a low, medium, and high dose quinolinic acid (QA) animal model of Huntington's disease. Motor impairment indicative of striatal dysfunction was monitored using amphetamine-induced rotational behavior and locomotor activity. Loss of striatal D2/D3R bearing medium-sized spiny neurons was assessed by DARPP-32 immunohistochemistry and compared to [123I]IBZM binding. Optical density measures of DARPP-32 immunohistochemistry demonstrated QA dose-dependent mild to subtotal unilateral striatal lesions ranging from 29.4% to 96.9% when compared to the nonlesioned side. Linear regression analysis showed that measurements of striatal DARPP-32 optical density and striatal [123I]IBZM uptake of the lesioned side were highly correlated (r2=0.83; P<0.001) whereas correlation with locomotor activity was less tight (r2=0.23; P<0.05; amphetamine-induced rotational behavior was not significantly correlated). This is the first study to demonstrate that in vivo [123I]IBZM SPECT and MRI coregistration are highly sensitive and, in contrast to behavioral measures, accurately detect mild to subtotal striatal lesions by measuring loss of D2/D3R availability. SPECT-MRI-based estimation of regional [123I]IBZM uptake provides a cost effective and widely available in vivo imaging technique for assessing striatal integrity in animal studies.

Development of a simultaneous PET/microdialysis method to identify the optimal dose of 11C-raclopride for small animal imaging

In the field of small animal positron emission tomography (PET), the assumptions underlying human and primate kinetic models may not be sustained in rodents. That is, the threshold dose at which a pharmacologic response occurs may be lower in small animals. In order to define this relationship, we combined microPET imaging using 11C-raclopride with microdialysis measures of extracellular fluid (ECF) dopamine (DA). In addition, we performed a series of studies in which a known mass of raclopride was microinfused into one striatum prior to a high specific activity (SA) systemic injection of 11C-raclopride. This single-injection approach provided a high and low SA region of radiotracer binding in the same animal during the same scanning session. Our data demonstrate that the binding potential (BP) declines above 3.5 pmol/ml (0.35 microg), with an ED50 of 8.55+/-5.62 pmol/ml. These data also provide evidence that BP may be compromised by masses of raclopride below 2.0 pmol/ml (0.326 microg). Increases in ECF DA were produced by mass doses of raclopride over 3.9 pmol/ml (0.329 microg) with an ED50 of 8.53+/-2.48 pmol/ml. Taken together, it appears that an optimal range of raclopride mass exists between 2.0 and 3.5 pmol/ml, around which the measured BP can be compromised by system sensitivity, endogenous DA, or excessive competition with unlabeled compound.

Quantitative validation of an intracerebral ?-sensitive microprobe system to determine in vivo drug-induced receptor occupancy using [11C]raclopride in rats

In this study, we evaluated the potential of using a new beta-sensitive microprobe system for in vivo quantification of [11C]raclopride binding and for in vivo determination of drug-induced receptor occupancy in the rat striatum. To validate this system, an ex vivo tissue dissection method was used to corroborate in vivo beta-microprobe measurements. Our data showed that the beta-microprobe-derived [11C]raclopride binding kinetics in striatum could be quantified using a tissue compartmental model with a cerebellar reference region. Haloperidol (0.001-0.1 mg/kg; i.v.) induced a dose-dependent decrease in [11C]raclopride binding in striatum as measured using the beta-microprobe with an ED50 value of 0.013 mg/kg. Highly significant relationships (P < 0.0001) were observed, within the same animals, between in vivo and ex vivo measures of haloperidol-induced D2-receptor occupancy (r = 0.98) as well as between in vivo and ex vivo measures of [11C]raclopride binding potentials (r = 0.99). Results from pretreatment and displacement studies with unlabeled raclopride and amphetamine conformed to the effect of these drugs as observed in humans using [11C]raclopride and PET and allowed estimation of the in vivo k(off) value of raclopride to 0.025 +/- 0.004 min(-1). However, allowing the system to stabilize before measurements and shielding the photomultiplier tubes were critical for obtaining these consistent results. This study demonstrates that the beta-microprobe provides reliable measurements of [11C]raclopride binding kinetics in rodents, allows for quantitative in vivo measurements of antipsychotic drug action in brain, and represents a valid and cost-effective alternative to positron emission tomography imaging in small animals.

Bilateral increase in striatal dopamine D2 receptor density in the 6-hydroxydopamine-lesioned rat: a serial in vivo investigation with small animal PET

Unilateral destruction of the substantia nigra by local application of 6-hydroxydopamine (6-OHDA) serves as an animal model for Parkinson's disease. In this study, the changes in neostriatal dopamine D(2) receptor density were investigated with a small animal positron emission tomograph (PET) before and after 6-OHDA lesion. PET scans were performed in 14 rats after injection of the D(2) receptor radioligand [(18)F] N-methylbenperidol. After the first scan (day 0), nigrostriatal pathways were lesioned by unilateral injections of 6-OHDA. Further PET scans were performed on days 2 and 14 post-lesion. For both striata, B(max) values were determined from saturation binding curves with non-linear regression analysis. In the striatum ipsilateral to the lesion, B(max) initially amounted to 19.3+/-1. 9 fmol/mg (mean+/-SD) and increased to 19.7+/-2.2 and 29.9+/-5.7 fmol/mg on days 2 and 14 post-lesion, respectively. Contralateral B(max) values increased from 19.2+/-2 fmol/mg prior to the lesion to 21.2+/-2.9 and 28.6+/-5.7 fmol/mg on days 2 and 14, respectively. On day 14, the ipsilateral saturation binding curve differed from the ipsilateral pre-lesion curve (P=0.04; F test). When the contralateral pre-lesion saturation binding curve was compared with the contralateral post-lesion curve on day 14, a P value of 0.08 was obtained. This first serial in vivo imaging study of 6-OHDA-lesioned rats showed a time-dependent increase in striatal D(2) receptor density on both sides, the increase being more pronounced ipsilateral to the lesion. This result implies that compensatory mechanisms in the intact hemisphere contribute to regenerative processes following nigrostriatal dopaminergic denervation. Overall, our findings show the feasibility of repetitive in vivo studies of striatal receptor density with a small animal tomograph. Moreover, the applied in vivo saturation binding technique provides a versatile method for the quantification of time-dependent changes in the concentration of receptor binding sites.

L-3,4-Dihydroxyphenylalanine as a neurotransmitter candidate in the central nervous system

Historically, 3,4-dihydroxyphenylalanine (DOPA) has been believed to be an inert amino acid that alleviates the symptoms of Parkinson's disease by its conversion to dopamine via the enzyme aromatic L-amino acid decarboxylase. In contrast to this generally accepted idea, we propose that DOPA itself is a neurotransmitter and/or neuromodulator, in addition to being a precursor of dopamine. Several criteria, such as synthesis, metabolism, active transport, existence, physiological release, competitive antagonism, and physiological or pharmacological responses, must be satisfied before a compound is accepted as a neurotransmitter. Recent evidence suggests that DOPA fulfills these criteria in its involvement mainly in baroreflex neurotransmission in the lower brainstem and in delayed neuronal death by transient ischemia in the striatum and the hippocampal CA1 region of rats.

Neuroimaging of animal models of brain disease

The main aim of this review is to describe some of the many animal models that have proved to be valuable from a neuroimaging perspective. This paper complements other articles in this volume, with a focus on animal models of the pathology of human brain disorders for investigations with modern non-invasive neuroimaging techniques. The use of animal model systems forms a fundamental part of neuroscience research efforts to improve the prevention, diagnosis, understanding and treatment of neurological conditions. Without such models it would be impossible to investigate such topics as the underlying mechanisms of neuronal cell damage and death, or to screen compounds for possible anticonvulsant properties. The adequacy of any one particular model depends on the suitability of information gained during experimental conditions. It is important, therefore, to understand the various types of animal model available and choose an appropriate model for the research question.

Tyrosine-free amino acid mixture attenuates amphetamine-induced displacement of [11C]raclopride in striatum in vivo: A rat PET study

Previous neurochemical and behavioural studies show that tyrosine depletion using a nutritionally balanced tyrosine-free amino acid mixture attenuates the dopamine-releasing and psychostimulant properties of amphetamine. Here we investigate the effect of a tyrosine-free amino acid mixture on striatal binding of [(11)C]raclopride, and amphetamine-induced [(11)C]raclopride displacement, using positron emission tomography in the rat. Rats were scanned for 60 min after an i.v. injection of approximately 11 MBq [(11)C]raclopride using a quad-HIDAC system. Amphetamine (2 mg/kg i.p., 30 min prior to scan) caused a 12% reduction in [(11)C]raclopride distribution volume ratio (DVR) compared to saline-injected controls. The tyrosine-free amino acid mixture (1 g/kg i.p.) caused a small (+7%) but statistically insignificant increase in [(11)C]raclopride DVR and attenuated, although it did not fully block, the amphetamine-induced reduction. These data are in keeping with previous neurochemical, immunocytochemical, and behavioural studies showing that tyrosine-free amino acid mixtures reduce dopamine function and offer promise for future PET studies testing the effect of tyrosine-depleting paradigms on dopamine release in humans.

Imaging transgenic animals

Transgenic and eugenic animals as small as 30 g can be studied non-invasively by radionuclides with resolutions of 1-2 mm, by MRI with resolution of 100 microns and by light fluorescence and bioluminescence with high sensitivities. The technologies of radionuclide emission, magnetic resonance imaging, magnetic resonance spectroscopy, optical tomography, optical fluorescence and optical bioluminescence are currently being applied to small-animal studies. These technologies and examples of their applications are reviewed in this chapter.

Evaluation of neurotoxicity of TIQ and MPTP and of parkinsonism-preventing effect of 1-MeTIQ by in vivo measurement of pre-synaptic dopamine transporters and post-synaptic dopamine D(2) receptors in the mouse striatum

Parkinsonism-inducing neurotoxicity of 1,2,3,4-tetrahydroisoquinoline (TIQ), as contrasted to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and parkinsonism-preventing effect of 1-methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTIQ) have been investigated in mice by measuring their effects on the in vivo binding of radioligand to pre-synaptic dopamine transporters (DATs) or to dopamine D(2) receptors (D2R) in the striatum. A significant reduction of the ligand-DATs binding was found in the mice treated with MPTP, but not with TIQ, under the dosage inducing behavioral abnormality and loss of tyrosine hydroxylase-positive cells in the substantia nigra. A slight decrease in the ligand-DATs binding was observed in the mice given a larger dose of TIQ. Compensatory up-regulation in the post-synaptic D2Rs was found in the MPTP-treated mice. Pre-treatment with (S)-enantiomer, but not (R)-enantiomer, of 1-MeTIQ prevented the degeneration of DATs to some extent. We concluded that the TIQ-induced parkinsonism model is different from the MPTP-induced model as evaluated by the radioligand-DATs binding and that (S)-1-MeTIQ has a preventing effect for the degeneration of the DATs to a certain extent.

Apomorphine-induced changes in synaptic dopamine levels: positron emission tomography evidence for presynaptic inhibition

The authors developed a novel positron emission tomography method to estimate changes in the synaptic level of dopamine ([DA]) induced by direct dopamine agonists (for example, apomorphine) in patients with Parkinson disease. The method is based on the typical asymmetry of the nigrostriatal lesion that often occurs in Parkinson disease. Using the between-side difference (ipsilateral (I) and contralateral (C) putamen to the more affected body side) of the inverse of the putamen [11C]raclopride binding potential (BP), the authors obtained [equation: see text] at baseline (that is, before apomorphine administration) and [equation: see text] after apomorphine administration (assuming the concentration of apomorphine is equal in both putamina). The between-side difference in the estimated synaptic concentration of dopamine (diff[DA]) should remain constant unless apomorphine affects dopamine release differently between the two sides. The authors found that apomorphine given subcutaneously at doses of 0.03 and 0.06 mg/kg induced significant changes in their estimate of diff[DA] (P < 0.05). Such changes were more pronounced when only patients with a stable response to levodopa were considered (P < 0.01). These findings provide in vivo evidence that direct dopamine agonists can inhibit the release of endogenous dopamine. The authors propose that this effect is mainly mediated by the activation of presynaptic D2/D3 dopamine receptors.

Age-related changes of dopamine D1-like and D2-like receptor binding in the F344/N rat striatum revealed by positron emission tomography and in vitro receptor autoradiography

To clarify age-related changes in dopamine D1-like and D2-like receptor binding in the striatum, positron emission tomography (PET) and in vitro receptor autoradiography (in vitro ARG) were performed using F344/N rats of various ages (6, 12, 18, and 24 months). In the PET study, [11C]SCH23390 and [11C]raclopride were used to image dopamine D1-like receptors and dopamine D2-like receptors, respectively, while [3H]SCH23390 and [3H]raclopride were used for the in vitro ARG study. With PET, we calculated the binding potential (= k3/k4, Bmax/Kd) of [11C]SCH23390 and [11C]raclopride in the striatum according to the curve fitting (CF) and the Logan plot (LP) methods. The binding potential of [11C]SCH23390 in the striatum demonstrated significant decrease as a function of age (max. decrease -26%) by the LP method, while this was not observed in the data analyzed by the CF method. In contrast, the binding potential of [11C]raclopride in the striatum decreased significantly with age by both the CF (max. decrease -28%) and the LP (max. decrease -36%) methods. However, no significant difference by either method was observed in rats between 6 and 12 months old using either ligand. In the in vitro ARG study, the specific binding (fmol/mg tissue) of [3H]SCH23390 and [3H]raclopride in the striatum were determined. Both [3H]SCH23390 and [3H]raclopride binding declined considerably with age as noted by comparing 12, 18, and 24-month-old rats against those 6 months old (max. decrease -29% and -31%, respectively). The substantial difference in binding shown in 12-month-olds in comparison with 6-month-olds using either ligand with in vitro ARG was in contrast with the PET results. These distinctions between the PET and the in vitro ARG studies may be attributed to the receptor microenvironment created under these experimental conditions. The results indicate that PET with LP analysis is useful in obtaining age-related changes of D1-like and D2-like receptor binding in the striatum of living rats.

The central aromatic amino acid DOPA decarboxylase inhibitor, NSD-1015, does not inhibit L-DOPA-induced circling in unilateral 6-OHDA-lesioned-rats

The centrally acting aromatic amino acid dopa decarboxylase (AADC) inhibitor, 3-hydroxybenzyl hydrazine (NSD-1015), is widely used to study the neurotransmitter-like actions of L-DOPA. However, the effects of NSD-1015 on L-DOPA-induced motor activity are unclear as both increases and decreases have been reported. We now investigate the effects of NSD-1015 on L-DOPA-induced contralateral circling behaviour in 6-OHDA-lesioned rats and on striatal levels of L-DOPA, 3-O-methyl-DOPA (3-OMD), dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) using microdialysis techniques. NSD-1015 (50-200 mg/kg i.p.) inhibited AADC activity both in the liver and striatum of normal rats. Administration of NSD-1015 (50-200 mg/kg i.p.), delayed the onset of circling produced by administration of L-DOPA (25 mg/kg i.p.) and carbidopa (12.5 mg/kg i. p.), suggesting blockade of central AADC activity. However, the duration of the L-DOPA-induced circling was prolonged and overall no inhibition of circling behaviour occurred. L-DOPA (25 mg/kg i.p.) plus carbidopa (12.5 mg/kg i.p.) increased extracellular levels of L-DOPA, 3-OMD, dopamine, DOPAC and HVA in the 6-OHDA-lesioned striatum. Pretreatment of rats with the central AADC inhibitor, NSD-1015 (100 mg/kg i.p.), potentiated the increase in dialysate levels of L-DOPA and 3-OMD. However, it did not reduce striatal dopamine levels in the 6-OHDA-lesioned hemisphere, which were elevated following L-DOPA administration. The increases in DOPAC and HVA levels were abolished by NSD-1015 pretreatment. These results suggest that, while NSD-1015 blocks central AADC activity, it also acts as a monoamine oxidase inhibitor so maintaining striatal dopamine concentration by reducing dopamine metabolism. NSD-1015, therefore, may not be an appropriate tool for the study of brain AADC activity and for assessing the neuromodulatory role of L-DOPA.