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1 Introduction

Next to genetical testing and behavioural observations, neuroimaging studies are increasingly performed on primates and rodents to model a variety of human diseases and traits. Animal models of brain disease are available for all major neurodegenerative diseases, epilepsy, stroke, but also psychiatric diseases such as anorexia nervosa, obesity, depression and anxiety [1].

Most commonly, animal models of brain disease are used to elucidate the mechanisms underlying the human condition and to translationally evaluate pharmacological, behavioural or other treatments for the disease. There are at least two criteria that an experimental model must satisfy: (i) reliability and (ii) predictive validity. Even though also other types of validity such as construct, etiological, convergent, discriminant and face validity are relevant to animal models (for an overview see [2]), predictive validity and reliability are the only necessary and sufficient criteria that will justify the model’s use for a particular application. Briefly, (i) reliability refers to the consistency and stability with which the disease variables are observed; (ii) predictive validity incorporates specificity of responses to treatments that are effective in the human disease. Undoubtedly, the more types of validity a model satisfies; the greater its value, utility and relevance to the human disorder or condition.

Developing models that fully mimic human neurological or psychiatric disorders is in many cases troublesome. Preclinical studies often need a choice between models that reproduce cardinal pathological features of the disorders by mechanisms that may not necessarily occur in humans versus models that are based on known pathophysiological mechanisms but may not reproduce all the features seen in patients.

Neurological and psychiatric models can be subdivided in three groups on the basis of the underlying mechanism: (i) genetically engineered; (ii) pharmacology based; or (iii) produced through advanced biotechnology such as viral vector technology. They may encompass several species, both primates and rodents. The use of mice in functional neuroimaging research is limited by the small size of the brain compared to the resolution limitations of current preclinical imaging systems.

The most commonly applied neuroimaging techniques for studying in vivo disease models are positron emission tomography (PET), single-photon emission computed tomography (SPECT) and magnetic resonance imaging (MRI), each of which has its own advantages and disadvantages (see Chaps. 4, 5, and 7). In this chapter, we will focus on how functional imaging technologies (PET and SPECT) have advanced our understanding of human neurological and psychiatric diseases. In particular, we will focus on the role of these techniques in evaluating models of Parkinson’s disease, Huntington’s disease, Alzheimer, epilepsy, stroke, addiction, depression, mania and eating disorders.

In general, the development of dedicated small-animal PET/SPECT systems offers the ability to study in vivo molecular changes during (i) normal development, (ii) biological processes and responses, and (iii) disease initiation and progression. As such, they may provide surrogate markers and allow assessment of therapeutic interventions by using serial scans over an extended period in the same animal. Selective radioligands for many neurotransmitter receptors [3] and other cellular proteins [4], which act as enzyme substrates [5] or allow the interference of neuronal function [6], are available.

PET imaging offers certain advantages over SPECT including: the acquisition of relatively fast dynamic data (order of seconds) and the potential to quantify these observations [7]. The spatial resolution when compared to structural imaging techniques such as MRI, is relatively limited, with the highest resolution scanners today giving at best 1–2 mm full width at half maximum (FWHM). The sensitivity of dedicated SPECT is typically an order of magnitude lower than PET. The spatial resolution of state-of-the-art preclinical SPECT scanners can be as low as 0.35 mm, although a trade-off between sensitivity and resolution is always needed [8, 9]. However, the detection of pico- and femtomolar concentrations of radioligand is feasible with both PET and SPECT [10].

When designing or performing in vivo small-animal studies of the brain, the following methodological issues must additionally be considered:

  • Motion prevention:

    • Immobilizing the animal in the PET/SPECT scanner is the first issue that has to be taken into consideration. Although systems and procedures have been developed to use awake monkeys in PET studies [11] and dedicated instrumentation developed to scan rats in their living environment [12], anaesthesia is still the standard for ensuring immobility during in vivo imaging of small-animals. A fundamental requirement of imaging is not to disturb the biological system under investigation. However, in many instances, anaesthesia can have several effects on the studied brain parameter. These effects are not predictable since changes are often of different magnitude and direction amongst target receptor, enzyme or transporter systems. The choice of anaesthetics is therefore of great importance to investigate, as it needs to minimally interfere with the tracer dynamics and the research question to obtain biologically valid outcomes (see Chap. 18). For example, the anaesthetic isoflurane was found to alter the expression of the plasma membrane dopamine transporter (DAT) in the brain [13], excluding its use for DAT imaging. Matsumara et al. investigated the effect of six different anaesthetics agents on the uptake of [18F]-FDG in the rat brain [14] and demonstrated significant effects of the different anaesthetics agents on [18F]-FDG metabolism when administered during the uptake period. Only when anaesthesia was started after the initial uptake phase, i.e. 40 min after [18F]-FDG injection, did the small-animal PET images reflect glucose metabolism of the conscious state, as measured with ex vivo autoradiography [14]. However, delaying the induction of anaesthesia with respect to the administration of the PET ligand is not always possible. It limits imaging to a static state, delays imaging protocol and negates the opportunity for dynamic imaging protocols that are required for fully quantitative and compartmental modelling.

  • Absolute quantification:

    • Fully quantitative and compartmental modelling is a prerequisite for the accurate determination of various biological parameters, such as receptor density (B max ) and affinity (K D ) or derived combined parameters such as binding potential. Kinetic modelling generally requires arterial blood sampling to calculate the blood activity concentration over time (input function) [15]. Techniques that have been established to measure blood activity concentrations in humans need to be carefully considered for animal experiments because the vascular access is more problematic, the blood volume smaller and the heart rate much higher than in humans [16]. Arterial blood sampling in rodents is technically demanding; requires extensive animal preparation, complex catheter manipulation and physiological monitoring. Besides, it hampers serial small-animal PET/SPECT studies over weeks or months. Other techniques deriving the input function from a reference tissue devoid of receptors [17], from images of the heart [18] or from the use of an (arterial) probe [19], have also been developed and could reduce the need for blood sampling. They are, however, highly dependent on the radiotracer used and the molecular target. The last two options, for example, are only feasible when the radiotracer does not undergo significant metabolism [20], or when a standard curve describing the metabolism is usable.

  • Tracer principle:

    • Assessment of the mass effect of injection is an additional issue, especially in the case of receptor imaging [21]. In human imaging studies, only a small chemical amount of radiotracer is injected, in the range of pico- to microgram. This dose has several advantages, including conforming to true tracer kinetics and giving lower or negligible toxicity. However, in order to obtain sufficient count rates in the brain of small-animals, relatively high doses need to be injected. Indeed, in small-animal imaging, the increase in dose with increasing resolution and the decrease in dose with decreasing body weight cancel one another. Consequently, the concentration of radioligand in the animal will be higher in the ratio of human to rodent body mass, potentially saturating vulnerable systems. In the case of binding sites of low density such as receptors, the increased mass injected can lead to physiological effects and non-linear kinetics. One must therefore reduce the injected dose of small-animals as low as possible and necessary.

  • Quantitative analysis:

    • By its very nature, SPECT and particularly PET are quantitative imaging modalities provided appropriate corrections for physical degrading factors are incorporated in the imaging protocol (see Chap. 17). The automated quantitative assessment of metabolic PET data is attractive and will certainly revolutionize the practice of molecular imaging since it can lower variability across facilities and may enhance the consistency of image interpretation independent of reader experience. For example, the development of tracer-specific small-animal PET probabilistic atlases [22] correlated with anatomical (e.g. MRI) templates enabled automated volume-of-interest (VOI) or voxel-based analysis of small-animal PET data with minimal end-user interaction [23]. One such software tool was developed by Kesner et al. [24] to enable the assessment of the biodistribution of PET tracers using small-animal PET data. This is achieved through non-rigid coregistration of a digital mouse anatomical model with the animal PET image followed by automated calculation of tracer concentrations in 22 predefined VOIs representing the whole body and major organs. The development of advanced anatomical models including both stylized and more realistic voxel-based mouse [25–27] and rat [28, 29] models obtained from serial cryo-sections or dedicated high resolution small-animal CT and MRI scanners will certainly help to support ongoing research in this area [30, 31]. For neuroscience applications, a high-resolution rat brain was also recently developed [32].

2 Applications in Neurology

2.1 Parkinson’s Disease

Parkinson’s disease (PD) is a progressive neurodegenerative disorder, characterized by massive degeneration of dopaminergic neurons in the substantia nigra (SN) pars compacta [33]. This nigral neuronal loss leads to a striatal dopamine deficiency, which is considered to underlie the most overt symptoms of the disease. PD affects approximately more than 2 % of the general population over the age of 65. The mean onset of the disease is around 60 years, although up to 10 % of those affected are 45 years of age or younger, referred to as young-onset PD. Clinical signs of PD are evident when about 80 % of striatal dopamine and 50 % of nigral neurons are lost [34].

Traditional models of PD fall into two major categories: (i) pharmacological, for example reserpine or amphetamine administration to deplete dopamine, a largely reversible treatment; and (ii) lesioning using neurotoxins which is permanent, for example intraparenchymal injection of 6-hydroxydopamine (6-OHDA) or systemic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), as reviewed by Hantraye et al. [35]. Newer approaches in the development of PD models have been attempted as well based on the identification of monogenic familial forms of the disease (at least 11 different linkages with 6 gene mutations have been identified) [36], resulting in transgenic and non-transgenic experimental models [37]. Each of the abovementioned models have its own strengths and weaknesses in representing the human condition, and have all been extensively characterized behaviourally and neurochemically (for an overview see [38]).

Imaging experimental PD models has largely been in the domain of PET imaging with the availability of radioligands to monitor both pre- and postsynaptic striatal dopaminergic function in order to follow the disease process and to examine compensatory mechanisms. The presynaptic nigrostriatal terminal function can be assessed with radioligands suitable for imaging at least three different functions, (i) aromatic amino acid decarboxylase activity (AADC), (ii) vesicular monoamine transporter type 2 (VMAT2) and (iii) the plasma membrane dopamine transporter (DAT). Imaging of postsynaptic dopaminergic function has focused on the dopamine D2-like receptor system.

Changes in dopamine metabolism have been widely identified in PD patients and MPTP-treated nonhuman primates using either 6-[18F]-fluoro-l-DOPA ([18F]FDOPA) [39–42] or 6-[18F]-fluoro-l-methyl-tyrosine ([18F]FMT) [43, 44], tracers of catabolism and trapping of dopamine, or its analogs. Somewhat surprisingly, [18F]FDOPA showed no significant brain uptake in the rat [45], although it can be used successfully in vivo in mice [46]. Mouse models of PD, either genetically modified or MPTP-treated, have not been extensively studied with in vivo imaging techniques so far, primarily because of the small size of the brain. However, due to the relatively large size of the striatum, a number of studies have shown that it is feasible. Sharma and colleagues have shown that striatal [18F]FDOPA uptake is reduced in the striatum of homozygous weaver mutant mice (a genetic model of PD) compared to both heterozygous and wild-type control animals [46]. Additionally, homozygous weaver mutant mice show an age-related decline in striatal [18F]FDOPA uptake [47].

Most imaging studies performed in rat PD models measure the loss of presynaptic dopamine terminals primarily using tracers for the DAT using cocaine analogs. After unilateral lesions of the nigrostriatal projection through injection in the medial forebrain bundle (MFB), decreased [11C]-CFT and [11C]RTI-121 binding have been noticed [48], as a result of decreased density of the transporter, without concomitant changes in affinity [49]. VMAT2 density changes have also been identified (Fig. 19.1b) [48], as have increased [11C]-raclopride binding to the D2 receptors [50, 51], consistent with human PET studies in early PD patients [52]. Whether the D2 receptor upregulation is a result of changes in receptor density or affinity is still unresolved, as both characteristics have been found to be altered [51, 53]. Besides, longitudinal microPET analysis of striatal D2 binding in the same animals demonstrated that the increase in D2 receptor density after unilateral 6-OHDA lesions occurs bilaterally [53]. Nikolaus and co-workers found that the D2 receptor density starts to increase in both striata within 2 days post-lesion, while a trend toward a significant increase in the contralateral striatum was noticed at day 14. The authors attributed the change in the contralateral striatum to be indicative of compensatory changes upon unilateral dopamine depletion [53]. Decreased presynaptic dopamine terminal binding have also been observed after unilateral lesions of the nigrostriatal tract through injection of 6-OHDA in the SN with [18F]-FECT (Fig. 19.1a) [54], and through lentiviral vector-based mediated overexpression of alpha-synuclein in the striatum using [123I]FPCIT [55].

Fig. 19.1
figure 1

Overview of both pre- and postsynaptic striatal function in the 6-OHDA model of Parkinson’s disease. (a) Series of axial sections (ventral to dorsal, slice interval 1.0 mm) showing decreased DAT availability in the dorsal striatum of 6-OHDA-treated rats in comparison to controls, as measured using [18F]-FECT. Color bars indicate binding indices for the dopamine transporter. (b) Also in the same model, reductions in VMAT2 density using [11C]DTBZ with microPET (left) and autoradiography (right) have been identified. Top image: A normal control animal for comparison; middle image: an animal with a mild right unilateral lesion; bottom image: an animal with a severe right unilateral lesion. (b–d) PET measures of the integrity of the striatal dopamine function correlates to the amount of 6-OHDA injected. [11C]PE2I binding to DAT (c) and [11C]-raclopride binding to D2 receptors (d) decreased and increased, respectively, with injected dose of 6-OHDA. Reprinted from [48, 54, 56] with permission from Springer and Elsevier

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These PET measures of the integrity of the striatal dopamine system have been shown to correlate to the amount of 6-OHDA injected. One study showed that 6-OHDA lesioning causes reciprocal, dose-dependent changes in [11C]PE2I binding to DAT compared to [11C]-raclopride binding to D2 receptors, decreasing and increasing the binding respectively (Fig. 19.1c, d) [56]. Another study also intended to measure the relationship between the 6-OHDA dose and the integrity of the striatal dopamine system using [11C]DTBZ, confirmed previous correlation [48].

As functional imaging of the striatal dopaminergic system can objectively follow loss of dopamine terminal function in PD, it also provides a potential means of monitoring the efficacy of putative therapeutic strategies. Possible restorative approaches evaluated in PD models include: striatal implants of human and porcine fetal mesencephalic cells, retinal cells that release levodopa/dopamine, embryonic stem cells and neurotrophic factors.

In unilateral 6-OHDA-lesioned rats, striatal transplantation of foetal dopamine neurons restores both the regional cerebral blood volume (rCBV) response to amphetamine as measured with pharmacological MRI, as well as the [11C]CFT binding in the lesioned striatum [57]. Behavioural recovery does not occur until [11C]CFT binding is restored in the same model to 75–85 % of the intact side [58]. The logistic, practical and ethical issues associated with foetal cells, however, contributed to a shift away from this particular line of investigation to the evaluation of alternative source of cells.

Alternative cells such as retinal pigmented epithelial (RPE) cells have so far only been studied in larger animals. RPE cells produce levodopa (l-DOPA), the biochemical precursor of dopamine, as an intermediate in neuromelanin production. Implanted human RPE cells, unilaterally into the striata of bilateral MPTP-treated monkeys, results in improved motor function and increased [18F]-FDOPA uptake with a concomitant decrease in [11C]-raclopride binding 2 months after implant [59].

The potential of embryonic stem (ES) cells as other viable source of cells for transplantation in PD has been evaluated in the unilateral 6-OHDA rat model. Bjorklund et al. [60] have shown that when transplanted into the rat striatum, mouse ES cells differentiate into dopamine neurons and decrease behavioural asymmetries. In addition, [11C]CFT binding was increased in the grafted striatum and correlated with the number of TH+ neurons in the graft [60]. Also in the same model, infusion of GDNF, a neurotrophic factor, into the SN and lateral ventricles prevents the 6-OHDA-induced reduction in DAT, as measured by [11C]-RTI-121 [61, 62].

Despite both histological and PET evidence in animals, neither of the abovementioned graft techniques demonstrated clinical efficacy in controlled trials so far [63, 64]. In those trials “off” period dyskinesias were often problematic. There are suggestions, however, that less severely affected PD patients could benefit more from those intrastriatal implantations.

Apart from the dopaminergic system, another active field of research in recent years has become the study of the contribution of inflammation to the progression of neurodegenerative disorders, as it may provide alternative therapeutic opportunities. Microglia cells are the macrophages of the brain and respond to noxious stimuli by changing morphology and expression of cell-surface markers and releasing pro-inflammatory cytokines [65]. Activated microglia cells may play a major role in the extension of neuronal loss after a lesion or insult. Migration of activated microglia and macrophages towards the lesion site correlates with the secondary damage after an acute neurotoxic event [66]. Moreover, a similar mechanism might amplify and perpetuate neuronal damage in chronic neurodegenerative disorders, such as PD.

Cicchetti and coworkers have used a 11C-labeled tracer that binds to the peripheral benzodiazepine receptor ([11C]-PK11195) to study activated microglia in PD models. Unilateral striatal infusion of 6-OHDA has been shown to increase [11C]-PK11195 binding in the lesioned striatum, at 3 weeks post lesion compared to baseline [67]. In the same model, chronic treatment with a selective inhibitor of the inducible form of cyclooxygenase (COX-2) constrained this inflammatory response at 12 days postlesion and limited, to a certain extent, the progressive dopamine neuronal death in the SN, as measured using [11C]-CFT and immunohistochemistry [68].

Cerebral metabolic mapping with autoradiography has also been used in previous animal research to determine the regional alterations in brain activity related to the pathogenesis of PD [69, 70]. The technique has additionally been used to map regions responsive to levodopa in the 6-OHDA lesioned rat [71]. Nowadays, [14C]-2-deoxyglucose autoradiography is often replaced by [18F]-FDG PET imaging. [18F]-FDG is considered to be a marker of cerebral glucose consumption based on neuronal entrapment and accumulation of [18F]-FDG-6-PO4, indicating neuronal viability [6]. In PD patients, specific cortical-subcortical metabolic alterations have been described using [18F]-FDG based on direct regional analysis [72] or network analysis approaches [73]. So far, only one study in small-animals has been performed. It was recently demonstrated that the unilateral intranigral lesion produced with 6-OHDA causes a severe metabolic impairment in the ipsilateral sensory-motor cortex of 6-OHDA lesioned rats, while metabolism was relatively increased in the contralateral midbrain, comprising the SN [54]. The change in the contralateral midbrain was attributed to be indicative of compensatory changes to unilateral dopamine depletion, in line with the previous mentioned contralateral D2 receptor upregulation [53]. It was concluded that the model shows metabolically strong functional correspondence to the cortico-subcortical impairments seen in PD patients [54].

2.2 Huntington’s Disease

Huntington’s disease (HD), or Huntington’s chorea, is an autosomal dominant progressive neurodegenerative disorder affecting approximately 1 in 10,000 individuals and is characterized by involuntary movements such as chorea, emotional disturbances and cognitive impairment [74]. Typically, onset of symptoms occur in middle-age. HD is caused by a cytosine-adenine-guanine (CAG) repeat expansion within exon 1 of the HD gene (IT15) on chromosome 4 [75]. The number of CAG repeats accounts for about 60 % of the variation in age of onset, with the remainder represented by modifying genes and environment [76].

The earliest animal models of HD are based on the selective vulnerability of striatal neurons to excitotoxic amino acids [77]. Intrastriatal injection of quinolinic acid (QA) [78] or systemic administration of 3-nitropropionic acid (3-NP) [79] results in ‘pathogenic models’ of the disease. Since the recent discovery of the gene mutation for the disease, new transgenic models are also being developed. Transgenic animal models of HD were first created in mice [80] and subsequently in Drosophila [81]; later on, a transgenic rat model of HD was reported [82].

Small-animal imaging data on HD are largely PET-based using similar markers described for Parkinson’s disease with more experimental data utilizing fluorodeoxyglucose to map regions of reduced metabolism, indicative of cell loss [6]. Imaging of HD models has been used extensively to evaluate both the validity of the models and the effects of various interventions.

In the QA-lesioned rat striatum, [18F]-FDG uptake is substantially decreased at 1 week post lesioning, and decreases further at 5 and 7 weeks postlesion [83]. In contrast, lesion-induced effects on dopamine D2 receptor binding were more progressive, with an initial upregulation of [18F]-FESP binding apparent 1 week postlesion followed by a decline 5 and 7 weeks thereafter [83]. Additional experiments revealed that the marked upregulation of dopamine D2 receptors consequent to QA injections could be detected as early as 3 days after the initial insult [83]. Using [11C]KFI8446, Ishiwata et al. have shown that adenosine A2A receptor binding is decreased to a similar extent as [11C]-raclopride binding to D2 receptors in the lesioned striatum, but to a greater extent than [11C]SCH23390 binding to D1 receptors [84]. Also in the same model, the QA injury did not affect [11C]flumazenil binding to benzodiazepine receptors at day 5 postlesion [85]. In addition, Moresco et al. have shown that the loss of adenosine A2A and dopamine D2 receptors are paralleled by an increase of microglia activation, as measured using [11C]PK11195 [86]. At the very first time point investigated, from QA administration (24 h), only a slight non-significant increase in [11C]PK11195 binding was observed. At 8, 30 and 60 days, however, [11C]PK11195 binding values were on average three times higher than the controls.

Compared to intrastriatal QA, repeated systemic administration of 3-NP results in more widespread striatal lesions. A longitudinal [18F]-FDG study aimed to explore the acute and chronic effects of systemic 3-NP administration, showed a significant interanimal variation in response to the toxin [87]. Rats that developed large striatal lesions had decreased glucose utilization in the striatum and cortex 1 day after starting 3-NP injections. Rats that did not develop lesions showed reversible enhancement in cortical glucose utilization and no changes in striatal glucose metabolism. Progressive degeneration was observed by a decrease in glucose metabolism in the striatum [87].

Transgenic rodents expressing a mutant form of the huntingtin gene are used in the study of disease progression and the effects of treatments. A recent longitudinal study, following up R6/2 mice with [18F]-FDG, demonstrated an exponential decrease in glucose metabolism, starting at the age of 8 weeks and continuing through the 6 weeks follow-up time in the striatum, cortex and cerebellum [88]. Treatment with the transglutaminase inhibitor cystamine in these animals has been shown to have a neuroprotective effect in a dose-dependent manner, attenuating the decrease in striatal, cortical and cerebellar [18F]-FDG [88].

2.3 Alzheimer’s Disease

Alzheimer’s disease (AD) is the most common cause of progressive cognitive decline in aged humans, comprising 50–70 % of all cases [89]. AD begins with great difficulty in new learning and memory, which leads to forgetfulness of recent events. AD worsens over time, usually over many years, leading to problems in word finding and reasoning, difficulty completing daily activities of living and ultimately death. The most severe neuropathological changes occur in the hippocampus, followed by the association cortices and some subcortical structures, such as the amygdala [90]. The neuropathological changes are characterized by massive neuronal cell and synapse loss [91], as well as beta-amyloid plaques and neurofibrillary lesions [90]. The major protein component of plaques is the polypeptide Abeta that is derived from amyloid precursor protein (APP). The neurofibrillary lesions contain hyperphosphorylated aggregates of the microtubule-associated protein tau and are found in cell bodies and apical dendrites as neurofibrillary tangles (NFT).

Less than 1 % of the total number of AD cases are caused by autosomal dominant mutations in three genes, among which APP [92]. Mutations in the gene encoding tau have also been linked to neurodegeneration and dementia [93]. The fact that these genes encode for proteins that are deposited in plagues and NFTs further confirms their causal role in the disease and led to the generation of transgenic animal models [94]. Most widely used animal models comprise both mouse and invertebrate; rat models of AD are rare. As far as neuro-anatomy, memory and motor functions are concerned, the transgenic mouse models are superior to the invertebrate ones in resembling the human condition [95]. Many different strains exist, but most of these overexpress human APP.

Not all PET studies of transgenic mice have been very successful so far because of the small size of the mouse brain and the inherent limitations in resolution of the first generation of small-animal PET scanners [96]. Considering that [18F]-FDG is becoming one of the most widely used diagnostic adjuncts for AD [97], the expectation that this technique would also be widely used in transgenic mice has for this reason not been met. Whereas autoradiographic studies show decreased cerebral glucose metabolism in the posterior cingulate cortex, in vivo imaging does not allow the identification of this change [98].

Visualizing Abeta deposition in vivo is another novel diagnostic tool, which might contribute to a definite diagnosis of AD and to monitor the success of treatments. The earliest probe was a dye called BSB (trans,trans)-1-bromo-2,5-bis-(3-hydroxycarbonyl-4-hydroxy)styrylbenzene, which was used to label Abeta-plaques in Tg2576 mice [99]. In recent years, the novel PET tracer 11C-labeled Pittsburgh Compound-B (PIB) has gained significant attention [100]. PIB was shown to enter the brain quickly and label plaques within minutes [101]. It was used as a PET tracer in APP transgenic mice but initially failed to reflect the amount of Abeta [102]. In APP23 mice, an age-dependent increase in radioligand binding was found to be consistent with progressive Abeta accumulation [103]. Importantly, Abeta reductions upon vaccination with an anti-Abeta-antibody were reflected by reduced binding of [11C]-PIB. The use of transgenic mice in preclinical studies is however limited as species-differences in Abeta accumulation have been reported [103].

In order to diminish the spatial resolution limitations of PET, high-field strength MRI can be combined with preclinical PET imaging for more accurate anatomical localization. In a study combining a transgenic mouse model of AD and the use of the toxin N-(2-chloroethyl)-N-ethyl-bromo-benzylamine (dsp4), which specifically targets the noradrenergic neurons of the locus coeruleus, Heneka et al. demonstrated an interaction effect between beta-amyloid deposition and noradrenergic neurotransmission [104]. Only transgenic mice who were also treated with dsp4 showed decreased cortex/cerebellum ratios in [18F]-FDG uptake, [11C]flumazenil binding and [11C]N-methyl-4-piperidyl-acetate trapping, indicative of reduced cerebral glucose metabolism, decreased neuronal integrity and attenuated acetylcholinesterase activity, respectively.

2.4 Epilepsy

Epilepsy is a common chronic neurological disorder that is characterized by recurrent, unprovoked seizures [105] and affects approximately 3 % of the population during their life-time [106]. Seizures arise from excessive abnormal hypersynchronized firing of a population of cortical neurons [107]. About 60–70 % of patients experience focal or partial seizures, and about 30–40 % generalized seizures [108]. Epileptic seizures are controlled with medication in approximately 70 % of the cases. When seizures are medically intractable, resection of the epileptogenic cortex may be considered.

Many epilepsy syndromes, particularly temporal lobe epilepsy (TLE), are associated with structural and functional abnormalities of the brain. The relationship of these abnormalities to the development (i.e. epileptogenesis), progression and prognosis of epilepsy are still incompletely understood.

Animal models in epilepsy neuroimaging research have long been important for the investigation of the neurobiology, consequences and treatment of seizures and epilepsy. The use of animal models particularly enables the investigation of the neurobiological changes during epileptogenesis, which is difficult to study in humans, given that in most patients, the chronic rather than the early epileptogenic stage is represented. Besides, prospective studies, in which patients at risk are followed up until the onset of the first seizure, are time-consuming, costly and practically difficult to undertake.

The animal models used in epilepsy research can be divided into models of seizures and models of epilepsy itself [109]. In the case of the former, the seizures are induced by the application of an acute brain insult, usually electrical and chemical, while in the latter the seizures occur spontaneously as in human epilepsy. In these chronic models, epileptogenesis is generally induced by a precipitating insult (genetic or acquired) that initiates a cascade of processes that transform a normal to a hyperexcitable epileptic brain, resulting in the occurrence of recurrent spontaneous seizures after a latent or silent period.

In 2000, Kornblum et al. were the first to publish in vivo [18F]-FDG findings during acute seizures using small-animal PET [110]. [18F]-FDG is one of the most commonly used radiotracers for PET imaging in clinical epilepsy practice and research, and is used in patients with refractory epilepsy to presurgically localize the functional deficit zone [111]. In the model of kainic-acid induced seizures, during status epilepticus (SE) a dramatic enhancement in glucose metabolism in several brain regions was demonstrated, most notably in hippocampus and entorhinal cortex [110]. Rats displaying moderate to severe seizures demonstrated 1.6- and 2.3-fold increases in [18F]-FDG uptake in the hippocampus, respectively. This correlation between [18F]-FDG uptake and seizure activity was also confirmed in C57BI/6 mice [112]. Using pilocarpine induced SE in these animals, [18F]-FDG uptake in the hippocampus (+33.2 %) was directly correlated with seizure activity during the uptake period.

Wang et al. studied a mouse model for GLUT-1 haploinsufficiency using [18F]-FDG small-animal PET [113]. GLUT-1 is the predominant glucose transporter expressed in the blood–brain barrier and is responsible for glucose entry into the brain [114]. GLUT-1 haploinsufficient mice were found to display among others spontaneous epileptiform discharges on electroencephalography, paralleled by a diffuse hypometabolism in the brain as compared to the wild-type strain [113].

Recently, longitudinal small-animal PET imaging was used to study brain glucose metabolism during epileptogenesis in two chronic epilepsy models of TLE [115]. In the amygdala kindling model, rats were found to have decreased glucose uptake in the ipsilateral hippocampus at the end of the kindling period that persisted at the 2 weeks post-kindling scan. In the post kainic acid induced SE model, a global hypometabolism was presented as early as 24 h after the kainic acid treatment. This hypometabolism remained persistently decreased for the following 6 weeks and was not affected by the onset of spontaneous seizures.

More recently, the relationship between brain glucose metabolism and epileptogenesis was further investigated in the rat lithium-pilocarpine model of epilepsy using a voxel-based analysis approach [116]. Early in the silent phase of epileptogenesis (day 3), rats displayed a severely hypometabolism in the entire cerebrum, although no electro-encephalographic or behavioural seizure activity was present at that time. This hypometabolism was most pronounced in the hippocampus, entorhinal cortex and thalamus bilaterally (Fig. 19.2); regions all characterized by the highest [18F]-FDG uptake during SE. During the chronic epileptic phase, a normalization of the glucose metabolism was seen.

Fig. 19.2
figure 2

Comparison of brain glucose metabolism between SE and control animals on day 3. Differences for the brain regions have been color-coded and are superimposed on a MRI template. Axial (top 2 rows) and coronal (bottom 3 rows) brain sections showing significantly decreased [18F]-FDG uptake on day 3 in SE rats compared to controls, most pronounced in hippocampus (HC), entorhinal cortex (ERC) and thalamus (THAL) bilaterally (white arrows). Significance at the voxel level is shown with a T statistic color scale. Reprinted from [116], with permission from Elsevier

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Apart from [18F]-FDG, as marker of neuronal activity, specific neurochemical changes can be measured by the use of specific receptor ligands. Several of these receptor systems represent existing or possible therapeutic targets.

The receptor that has most commonly been imaged using PET in human epilepsy studies has been the central benzodiazepine (cBZ) receptor using radiolabeled flumazenil (FMZ). Changes in expression and function of the GABAA/cBZ receptor complex are well described in human focal epilepsy [111]. Liefaard et al. used a population pharmacokinetic model to study the changes in GABAa/cBZ complex in the kindling model of TLE [117]. After injection of an excess amount of [11C]FMZ to fully saturate the receptors, the concentration-time curves of [11C]FMZ in blood and brain were measured, from which K D and B max were estimated. After kindling, K D was unaffected, but B max in epileptic rats decreased to 64 % of controls. Also in fully kindled rats, the brain volume of distribution was found to be increased with 180 %, indicating, in these animals, reductions in transport of [11C]FMZ outside the brain [117].

Another receptor radiotracer with potential for small-animal epilepsy models is (N-[2-(3-cyano-phenyl)-3-(4-(2-[18F]fluorethoxy)phenyl)-1-methylpropyl]-2-(5-methyl-2-pyridyloxy)-2-methyl proponamide) ([18F]MK-9470), which labels brain type 1 cannabinoid (CB1) receptors [3]. Cerebral CB1 receptors belong to the endocannabinoid system (ECS), together with a family of naturally occurring lipids, the endocannabinoids, and with transport and degradation proteins [118]. The endocannabinoid system would provide an ‘on-demand’ protection against acute excitotoxicity in neurons of the central nervous system and would contribute to a signaling system that protects neurons against the consequences of abnormal discharge activity [119]. In rodent models of epilepsy, administration of cannabinoids is protective against seizures [120, 121] and affects seizures’ frequency and duration. Endocannabinoids and CB1 receptor levels are increased in these epileptic animals [122]. Using small-animal PET, the involvement of the ECS was recently demonstrated, more precisely the CB1 receptor, in the mechanism-of-action of the anti-epileptic drug valproate (VPA) [123]. A significant increase (+32 %) was found in global cerebral [18F]MK-9470 binding after 2-week chronic VPA administration compared to sham-treated animals. As VPA does not exhibit high affinity for the CB1 receptor, such upregulation is likely caused by an indirect effect on the ECS, since, as mentioned before, activation of the CB1 receptor has been shown to decrease excitability and excitotoxicity on demand [119].

At present, none of the abovementioned neurobiological changes have been shown to be a biological marker, predictive for seizures outcome.

2.5 Cerebral Ischaemia

Stroke is the second most common cause of death and major cause of disability worldwide [124]. Stroke results from a transient or permanent reduction in cerebral blood flow (CBF), usually secondary to thromboembolic occlusion of one or more cerebral arteries. The compromised blood supply leads to functional impairment, followed by structural disintegration of neurons in the absence of reperfusion. The initial phase of dysfunction is potentially reversible, prior to subsequent cell death. While some brain tissue may be irreversibly damaged, other hypoperfused areas may be at risk but are potentially salvageable. These latter areas are called the penumbra [125]. If people reach their life expectancy, one in four men will have had a disabling stroke by age 80, and one in five women by age 85 [126]. The burden of stroke on patients, their families, and society in general is enormous.

The use of animal models in neuroimaging stroke research has focused on a number of areas which include: (i) defining and understanding the concept of the penumbra; (ii) development of new imaging techniques for the diagnosis and prognosis of stroke; and (iii) the investigation both of the underlying processes that lead to cell death, and of possible therapies for the treatment of stroke.

Over the last decade, comprehensive reviews have described the numerous possible animal models of cerebral ischaemia and their relevance to the human disease [127]. Experimental models may be broadly classified by the reduction in cerebral blood flow (CBF), as either global or focal models, which may in turn be permanent or reversible in nature. Models of focal ischemia most often involve unilateral, transient, or permanent occlusion of the middle cerebral artery (MCA-O), leading to ischemic damage in the neocortex and/or caudate-putamen [128]. Models of global ischemia are usually transient, since persistent global cerebral ischemia readily leads to death. Transient global ischemia can be induced with hypoxic ventilation and/or multi-vessel occlusions [129].

The penumbra is the most important target for acute stroke therapy. Since the penumbra can be considered as a temporary phase of potential viability through which ischemic tissue progresses into infarction, the therapeutic time-window is limited, possibly to a few hours; therefore early detection is essential. In humans, multitracer PET measuring blood flow and metabolism is still the current gold-standard technique for penumbral identification [130]. Diffusion/perfusion-weighted MRI and perfusion CT are more commonly used because of their simplicity and suitability for repeat studies; however, they may not differentiate infarct, penumbra and oligemia (i.e. deficiency in blood volume) reliably after stroke [130]. Following occlusion of the MCA in male spontaneously hypertensive and male normotensive Wistar Kyoto rats, CBF was decreased 1 h after the occlusion to <30 % of the control hemisphere in both groups, cerebral metabolic rate of oxygen consumption (CMRO2) was diminished to a similar extent and oxygen extraction fraction (OEF) was increased, indicating misery perfusion [131]. During permanent occlusion, the underlying physiological disturbances were greater in spontaneously hypertensive rats as compared to the normotensive ones, revealing that hypertension is a risk factor for the onset of stroke as well as for poorer outcome after stroke [131]. In the same models, a collapse of the compensatory OEF mechanism was found 24 h after occlusion [132].

In rats undergoing distal MCA occlusion surgery, sequential [18F]-FDG PET studies demonstrated decreased glucose metabolism in the stroke area delineated by MRI [133]. The [18F]-FDG uptake in the stroke area was about 0.5 % of the injected dose per gram (ID/g) at days 1, 15 and 22. At day 8, the stroke area appeared to be smaller and the uptake was higher, likely attributable to inflammation.

Other PET studies have started to validate new ways of identifying infarction and penumbra in experimental models. [18F]-FMISO PET was evaluated in rats undergoing permanent and temporary MCA occlusion [134]. Nitroimidazole compounds such as FMISO are trapped in hypoxic cells [135], but not in necrotic tissue [136], thereby providing a simple direct image of the penumbra. In the hyperacute phase (until 30 min) after permanent MCA occlusion, there was increased [18F]-FMISO binding in the entire ipsilateral MCA territory, which normalized 48 h later, in line with a nearly complete MCA territory infarct. In contrast, there was no demonstrable tracer retention in temporary MCA occlusion models, which histopathologically showed ischemic changes only, supporting the validity of [18F]-FMISO as a marker of the penumbra after stroke.

Recently, Reshef et al. reported on 18F-labeled-5-fluoropentyl-2-methyl-malonic acid ([18F]-ML-10) as potential radioligand for imaging apoptosis among others in cerebral stroke [137]. Although necrosis predominates as the mode of cell death during the hyperacute stage of stroke, apoptosis plays an important role in ensuing staged of active disease [138]. Following permanent MCA occlusion in mice, increased [18F]-ML-10 uptake was observed selectively in the ischemic MCA territory, and correlated with the histological evidence of cell death. The degree, however, to which a defective blood–brain-barrier (BBB) contribute to the specific uptake of [18F]-ML-10 has not been determined.

No treatment currently exists to restore the lost neurological function after stroke. Increased vascularisation in the stroke border zone within a few days after stroke is associated with neurological recovery [139], and may be valuable not only as prognostic factor but also as measurement of success to help guide proangiogenic therapies. Using [64Cu]-DOTA-VGEF121 in rats undergoing distal MCA occlusion surgery, Cai et al. found that angiogenesis appeared very rapidly after stroke (i.e. 2 days post occlusion) [133]. [64Cu]-DOTA-VGEF121 uptake peaked in the stroke border zone ~10 days after surgery, confirmed by histology and autoradiography, after which it decreases. No correlation of [64Cu]-DOTA-VGEF121 uptake with long term stroke outcome has been performed yet to evaluate the potential prognostic value.

In a SPECT study, 99mTC-HYNIC-annexin V, another radioligand for imaging apoptosis, was used to monitor the response of neuroprotective therapy with monoclonal antibody raised against FasL in a rodent model of transient MCA occlusion [140]. FasL is the cognate ligand for the Fas death receptor, a member of the tumour necrosis receptor subfamily [141]. FasL rapidly increased its expression within the neurons of the penumbra following ischemic injury, thereby inducing pro-apoptotic mechanisms [142]. Blankenberg et al. demonstrated that radiolabeled annexin V detects the early phases of neuronal ischemic injury and its response to anti-FasL therapy. Anti-FasL treatment significantly reduced annexin uptake by 92 % with a 60 % decrease in the number of apoptotic neurons on day 1. On day 6, treated rat had an 80 % reduction in tracer uptake with a 75 % decrease in infarct size as compared to controls. Annexin V uptake was in both controls and treated animals linearly correlated with infarct size and the number of apoptotic nuclei.

The same radioligand was also used in sequential SPECT studies to demonstrate the neuroprotective potential of minocycline, an antibiotic with antiapoptotic properties [143], in CB6/F1 mice undergoing unilateral distal MCA occlusion [144]. Seven-day minocycline treatment was found to significantly reduce annexin V uptake between 1 and 30 days after injury, in line with the infarct size. Annexin V uptake was in control and treated-animal maximal on days 1 and 7, followed by a decline at 30 days.

3 Applications in Psychiatry

The application of small-animal PET/SPECT in the field of biological psychiatry is hampered by the limited presence of suitable animal models. Despite a significant number of human PET/SPECT studies, there is a paucity of functional imaging studies in animals. It is indeed more difficult to reproduce human psychiatric diseases using animals in view of the large symptomatic heterogeneity of psychiatric illness, even inside the same clinical definition (endophenotypes). For example, it is particularly difficult to model specific human emotions and to evaluate this in rodents. Moreover, the aberrant behaviours symptomatic of human mental illness are mostly uniquely human, particularly those that are mediated by brain pathways without homology in rodents, e.g. the expanded prefrontal cortex of the human brain.

Despite these difficulties inherent in modelling human psychiatric phenotypes, there has been some recent success, mainly in mice, identifying genetic mutations that give rise to some of the characteristic features of anxiety, depression, schizophrenia, autism, obsessive-compulsive disorder and bipolar disorder [145]. Also here, the application of mice models in small-animal PET/SPECT research has been limited by the small size of the mouse brain. We give an overview of the various neurochemical systems that may provide possible molecular targets for small-animal PET/SPECT research.

Among the various neurotransmitter systems, the dopaminergic system is of particular interest in drug abuse and addiction. Research into brain mechanisms of additive behaviour have traditionally focused on the nucleus accumbens and dopamine inputs to this region from the ventral tegmental area [146]. The potentially significant involvements of the oribitofrontal cortex, anterior cingulate cortex, and other limbic cortical areas have more recently been recognized and investigated [147–149]. Although the precise aetiology of drug addiction is poorly understood, it is widely known to be linked to certain personality traits (e.g. risk-takers, sensation- or novelty-seekers) [150, 151] and individuals diagnosed with particular brain disorders such as attention-deficit hyperactivity disorder (ADHD) [152].

In a recent small-animal PET study, Dalley et al. investigated dopamine D2/3 receptor availability in the dorsal and ventral striatum of high impulsive rats compared with non-impulsive rats to evaluate the causal relationship between impulsivity and drug abuse vulnerability [153]. Rats were scanned using the radioligand [18F]fallypride prior to drug exposure. The authors found that [18F]fallypride binding was significantly reduced in the nucleus accumbens but not the dorsal striatum of high impulsive rats compared with non-impulsive subjects. Extracellular dopamine levels were also measured in the nucleus accumbens using in vivo microdialysis [153]; there was no difference between both groups suggesting that dopamine D2/3 receptors were likely fewer in number in the nucleus accumbens of high impulsive rats. The authors hypothesized from their PET findings that low dopamine D2/3 receptors associated with certain personality traits may be a susceptible neurobiological marker that confers vulnerability to drug experimentation and encourage excessive drug use.

In addition, the origin of inter-individual variability in impulsive behaviour is still unknown, but potentially involves both genetic and environmental influences [154, 155]. The importance of environment on dopamine D2 receptor availability was demonstrated in the monkey striatum [156]. Dopamine D2 receptor availability measured by [18F]fluoroclebopride was found to be lower in the subordinate monkeys than dominant monkeys. Crucially, this difference was present only when the monkeys were housed together—not before—suggesting this effect to be related to social context rather than to traits variables. The authors also demonstrated a link between low dopamine D2 receptor availability in the striatum and cocaine self-administration. This finding was substantiated in a recent longitudinal PET study in the monkey brain, demonstrating that baseline dopamine D2 receptor availability inversely predicts cocaine self-administration and that cocaine itself further reduce the dopamine D2 receptor availability in this region [157].

This latter finding was also observed in rats, self-administering the psychostimulant d-amphetamine [158]. Drug-exposed and saline control rats were scanned 24 h after the discontinuation of self-administration using PET and [11C]-raclopride. Consistent with the abovementioned study in monkey, chronic administration of amphetamine significantly decreased [11C]-raclopride binding potential in the dorsal striatum (Fig. 19.3).

Fig. 19.3
figure 3

Reduced binding potential of the dopamine D2/3 receptor antagonist [11C]-raclopride in the dorsal striatum of rats exposed to intravenous d-amphetamine self-administration compared with control rats receiving yoked infusions of saline (left hand graph). Shown also are co-registered [11C]-raclopride binding potential maps and MRI images for a saline control rat and an amphetamine-exposed rat. **p < 0.01 (Student’s unpaired t-test). Reprinted from [158], with permission from Elsevier

Full size image

Besides drug abuse and addiction, molecular neuroimaging studies are also consistent with the notion that dopaminergic dysregulation is a key pathological feature of schizophrenia. Schizophrenia affects 1 % of the population [159] and usually begins in late adolescence or early adulthood. It is characterized by positive psychotic symptoms, such as delusions and hallucinations and disorganized speech, and negative psychotic symptoms, such as emotional blunting and loss of drive. PET and SPECT human imaging studies have shown that schizophrenia is associated with increased presynaptic striatal dopamine synthesis and storage [160, 161], and increased striatal release of dopamine following amphetamine administration [162, 163]. Treatment consequently involves the use of antipsychotic drug, all of which act as antagonists at central dopamine D2 receptors [164].

The application of small-animal PET/SPECT in schizophrenia research is mainly focused on the development and evaluation of these antipsychotic drugs for the treatment of this disorder. Despite extensive clinical experience with antipsychotics, there has long been no broad consensus on the doses of these substances that should be administered. Formerly, most antipsychotics were administered empirically according to clinical dose-finding studies, in which arbitrarily selected doses were tested to find the “most efficient” dose range in a patient population, with no regard for the molecular effects of the tested drug. Brain PET imaging studies in healthy rats have indicated that occupancy of at least 65 % of dopamine D2 receptors is needed for clinical response to antipsychotics, and that occupancy rates exceeding 72 and 78 % are associated with a high risk for motor adverse effects, providing a rationale for the use of relatively low doses of typical antipsychotics [165].

PET studies in patients with schizophrenia that assessed the level of dopamine D1 receptors in the prefrontal cortex using [11C]SCH23390 and [11C]NCC112 [166–168], have generated contradictory findings. Small-animal PET in healthy rats provided evidence to elucidate this inconsistency [169]. Sprague–Dawley rats subjected to acute dopamine depletion did not show alterations in [11C]NCC112 in the striatum and prefrontal cortex, while paradoxically striatal [11C]SCH23390 binding was decreased. On the other hand, subchronic dopamine depletion was associated with increased [11C]NCC112 binding and decreased [11C]SCH23390 binding in these regions, suggesting that the binding of these tracers in the prefrontal cortex of patients with schizophrenia might reflect changes in dopamine D1 receptors secondary to sustained deficit in the prefrontal dopamine function.

Apart from the dopaminergic system, the central serotonergic system has received great interest in depression research [170]. Modifications of serotonergic activity contribute to many of the symptoms, for example, mood, appetite, sleep, sexual and cognitive dysfunction. Depression has the highest prevalence of all psychiatric disorders and occurs twice as frequently in women as in men. It can begin at any age, but has its average age of onset in the mid-20s [171].

The involvement of the serotonergic system in depression is also based partly on the observation that selective serotonin reuptake inhibitors (SSRIs) exert antidepressant effects and that most antidepressant drugs either directly or indirectly enhance serotonin (5-HT) transmission [172]. The most promising evidence for a deficit in central serotonin neurotransmission that is compensated by antidepressant pharmacotherapy involves postsynaptic serotonin-1A receptors (5-HT1A). In human PET studies, changes in expression and function of the 5-HT1A receptor are well described, particularly in the mesiotemporal cortex and raphe nucleus (for review see [173]). Currently, most of the radiopharmaceuticals developed to image 5-HT1A receptors have also been validated in rodents, but not yet applied in depression research. 5-HT1A receptor subtypes have been extensively visualized with small-animal PET using [11C]WAY-100635 [174]. A new promising tool for the in vivo imaging of this receptor subtype in rodents is recently presented by [18F]MPPF [175]. Aznavour and colleagues demonstrated using [18F]MPPF in rats that values of binding potential for hippocampus (1.2), entorhinal cortex (1.1), medial prefrontal cortex (1.0) and raphe nuclei (0.6) were comparable to those previously measured with PET in cats, non-human primates or humans [175]. Test-retest variability was also in the order of 10 % in the larger brain regions and less than 20 % in small nuclei.

Individuals affected by depression may also suffer from mania in bipolar disorder, which affects approximately 1 % of the world’s population [176]. Hougland et al. performed in vivo [18F]-FDG imaging in a rat model of mania [177]. Following ICV ouabain administration, brain [18F]-FDG uptake was reduced compared to those animals receiving equal volumes of artificial cerebrospinal fluid. Pretreatment with lithium, a standard treatment for mania, normalized this [18F]-FDG uptake. Imaging of ill bipolar patients has revealed consistent findings [178, 179].

The dopaminergic and serotonergic neurotransmitter systems have also been central in functional imaging research of patients with eating disorders. Patients with anorexia nervosa (AN) and bulimia nervosa (BN) display anxiety, depression, and suicide or have symptoms related to altered reward and excessive motor activity [180]. In human imaging studies of AN, involvement of the parietal, frontal and cingulate cortices in the pathophysiology have been well demonstrated using radioligands for the serotonergic pathways [181], as well as [18F]-FDG [182].

In an animal model of AN, Barabarich-Marsteller et al. investigated using [18F]-FDG whether the psychobiological changes that occur following ‘voluntary’ starvation in individuals are comparable to the changes that result from involuntary food restriction (i.e., an experimental procedure leading to activity-based anorexia or ABA) [183]. Briefly, rats were restricted to 40 % of their baseline daily food intake until a 30 % weight loss occurred. Combining the food restriction with access to a running wheel induced hyperactivity and a spontaneous restriction of food intake, as seen in humans [184]. Only the food-restricted rats displayed an increase in [18F]-FDG uptake in the cerebellum, while a decrease was observed in the hippocampus and striatum; the latter is in line with previous reports in the clinical condition [182].

More recently, these cerebral metabolic changes in the same ABA model were monitored using a voxel-based analysis approach [185]. In line with Barabarich-Marsteller et al., a higher regional metabolism in the cerebellum and hypometabolism was found in the ventral striatum. In addition, relative [18F]-FDG uptake was increased in the mediodorsal thalamus and ventral pontine nuclei, while body weight loss was positively correlated with cerebral metabolism in the cingulate cortex and the adjacent somato-sensory cortex. It was concluded that the activity-based rat model of AN share indeed several characteristics with the human disease, encompassing complex interplays between different circuitries involving motor activity, food-related behaviour and somato-sensory regions, thus adding proof to the validity of this model.

Despite the abovementioned body of evidence indicating a prominent role of dopamine and serotonin neurotransmission in various psychiatric disorders, there is a limited base of PET/SPECT studies investigating the role of other neurochemical systems in patients or models for psychiatry. Human studies have suggested that GABAergic abnormalities are associated with stress, anxiety, and depression [186–188]. Glutamate neurotransmission, on the other hand, may be involved in obsessive compulsive behaviour and schizophrenia [189, 190]. Although radioligands to image both systems have been validated in small-animals [191, 192], none of them have yet been utilized in psychiatric animal research.

4 Conclusion

Preclinical in vivo imaging techniques such as PET and SPECT have played a role in validating animal models of neurological and psychiatric diseases at a basic science level, and therefore contributed to improve our understanding of human diseases. In particular, the key advantages of these research tools is that subjects can be followed longitudinally, over time, thus allowing investigation of the disease process, the development of compensatory changes, and the long-term evaluation of the safety and efficacy of drug-based, surgical, cellular or gene-therapy based interventions.

The preclinical evaluation of potential therapies using small-animal imaging has mainly been applied in the field of neurology. The use of small-animal PET/SPECT imaging in psychiatry is hampered by the lack of suitable animal models, as preclinical neuroimaging research is only as good as the animal model being employed.