Transgenic Rat Models of Huntington's Disease.

Several animal models for Huntington's disease (HD) have been created in order to investigate mechanisms of disease, and to evaluate the potency of novel therapies. Here, we describe the characteristics of the two transgenic rat models: transgenic rat model of HD (fragment model) and the Bacterial Artificial Chromosome HD model (full-length model). We discuss their genetic, behavioural, neuropathological and neurophysiological features.

An experimental model for Huntington's chorea?

Clinically, Huntington's disease (HD) is well known for the predominant motor symptom chorea, which is a hyperkinetic motor disorder. The only experimental model currently described in the literature, as far as we are aware of, exhibiting hyperkinetic movements is the transgenic rat model of HD. We assessed and characterized these hyperkinetic movements in detail and investigated the effect of tetrabenazine (TBZ) treatment. TBZ is an effective drug in the treatment of chorea in HD patients. Our results showed that the hyperkinetic movements fulfilled the clinical-behavioral criteria of a choreiform movement. Administration of TBZ reduced the number of these hyperkinetic movements substantially. These findings suggest that the hyperkinetic movements observed in this animal model can be considered as a choreiform movement disorder. This makes these animals unique and provides opportunities for chorea-research.

Lessons learned from the transgenic Huntington's disease rats.

Huntington's disease (HD) is a fatal inherited disorder leading to selective neurodegeneration and neuropsychiatric symptoms. Currently, there is no treatment to slow down or to stop the disease. There is also no therapy to effectively reduce the symptoms. In the investigation of novel therapies, different animal models of Huntington's disease, varying from insects to nonhuman primates, have been created and used. Few years ago, the first transgenic rat model of HD, carrying a truncated huntingtin cDNA fragment with 51 CAG repeats under control of the native rat huntingtin promoter, was introduced. We have been using this animal model in our research and review here our experience with the behavioural, neurophysiological, and histopathological phenotype of the transgenic Huntington's disease rats with relevant literature.

Metabolic and electrophysiological changes in the basal ganglia of transgenic Huntington's disease rats.

Huntington's disease (HD) is characterized by neuronal loss in the striatum, ultimately leading to an 'imbalance' in the electrical activity of the basal ganglia-thalamocortical circuits. To restore this 'imbalance' in HD patients, which is held responsible for (some) of the motor symptoms, different basal ganglia nuclei have been targeted for surgical therapies, such as ablative surgery and deep brain stimulation. However, evidence to target brain nuclei for surgical therapies in HD is lacking. We reasoned that a neuronal and metabolic mapping of the basal ganglia nuclei could identify a functional substrate for therapeutic interventions. Therefore, the aim of the present study was to investigate the metabolic and neuronal activity of basal ganglia nuclei in a transgenic rat model of HD (tgHD). Subjects were 10-12 month old tgHD rats and wildtype littermates. We examined the striatum, globus pallidus, entopeduncular nucleus, the subthalamic nucleus and substantia nigra at different levels. First, we determined the overall neuronal activity at a supracellular level, by cytochrome oxidase histochemistry. Secondly, we determined the subcellular metabolic activity, by immunohistochemistry for peroxisome proliferator-activated receptor-γ transcription co-activator (PGC-1α), a key player in the mitochondrial machinery. Finally, we performed extracellular single unit recordings in the nuclei to determine the cellular activity. In tgHD rats, optical density analysis showed a significantly increased cytochrome oxidase levels in the globus pallidus and subthalamic nucleus when compared to controls. PGC-1α expression was only enhanced in the subthalamic nucleus and electrophysiological recordings revealed decreased firing frequency of the majority of the neurons in the globus pallidus and increased firing frequency of the majority of the neurons in the subthalamic nucleus. Altogether, our results suggest that the globus pallidus and subthalamic nucleus play a role in the neurobiology of HD and can be potential targets for therapeutic interventions.

Motor and non-motor behaviour in experimental Huntington's disease.

In this study, we investigated motor and non-motor behaviour in the transgenic rat model of Huntington's disease (tgHD). In particular, we were interested in the development and changes of motor and non-motor features (anxiety, motivation and hedonia) of disease over time and their interactions. We found tgHD animals to be hyperkinetic in the open field test compared to their wild-type littermates at all ages tested, which was accompanied by reduced anxiety-like behaviour in the open field test and the elevated zero maze, but not in the home cage emergence test. No major changes were found in hedonia (sucrose intake test) and motivation for food (food intake test). Our data suggest that hyperkinetic features and reduced-anxiety in the tgHD rats are associated behaviours and are seen in the earlier stages of the disease.

Memory deficits in the transgenic rat model of Huntington's disease.

Memory deficits are common in patients with Huntington's disease (HD) and have a substantial impact on the quality of life of patients and their relatives. A good model resembling the human memory deficits is needed for research purposes. In this study we investigated the memory function of the transgenic rat model of Huntington's disease (tgHD) in the object location (OLT) and the object recognition task (ORT). Several studies have shown that the recent developed tgHD rat model resembles the human phenotype of HD. Impairments of spatial and object recognition memory in the OLT and ORT, however, have to our knowledge not yet been reported in this transgenic model. Our findings show that in both early and late stages of the disease the tgHD rats have clear deficits for both visuospatial and visual object memory. Since HD patients are known to be impaired in both types of memory, these results confirm the validity of this tgHD rat as a model for the human HD phenotype.

Unchanged expression of the ceramide transfer protein in the acute 6-OHDA neurodegenerative model.

Ceramides are lipids that are abundant in brain tissue where they have an important structural role in cellular membranes. Ceramides are also powerful intracellular signalling molecules controlling cell death, growth and differentiation. So far, the ceramide transfer protein (CERT), a shorter splice variant of the Goodpasture antigen-binding protein (GPBP), is the only known protein with the ability to shuttle ceramide from the endoplasmic reticulum to the Golgi apparatus. GPBP/CERT are widely distributed in the central nervous system where they act as key factors for normal brain development and homeostasis. Ceramide accumulates in neurons during acute neurodegeneration. The objective of this study was to define whether levels of the ceramide transfer protein GPBP/CERT are altered in the acute neurodegenerative process. We used design-based stereology to quantify the number of GPBP/CERT immunoreactive cells in the striatum of 6-hydroxydopamine (6-OHDA) lesioned rats as an animal model of Parkinson's disease (PD). In addition, gray value measurement was performed to quantify GPBP/CERT immunoreactivity-levels within individual cells. No difference in the striatal expression levels of GPBP/CERT proteins was found between diseased and control animals, suggesting that the expression pattern of GPBP/CERT in the striatum is not affected in the 6-OHDA rat model of PD.

Mild dopaminergic lesions are accompanied by robust changes in subthalamic nucleus activity.

The subthalamic nucleus (STN) is a major player in the input and output of the basal ganglia motor circuitry. The neuronal regular firing pattern of the STN changes into a pathological bursting mode in both advanced Parkinson's disease (PD) and in PD animals models with severe dopamine depletion. One of the current hypothesis, based on clinical and experimental evidence, is that this typical burst activity is responsible for some of the principal motor symptoms. In the current study we tested whether mild DA depletion, mimicking early stages of PD, induced deficits in motor behaviour and changes in STN neuronal activity. The present study demonstrated that rats with a mild lesion (20-40% loss of DA neurons) and a slowed motor response, but without gross motor abnormalities already have an increased number of bursty STN neurons under urethane anaesthesia. These findings indicate that the early increase in STN burst activity is a compensatory mechanism to maintain the dopamine homeostasis in the basal ganglia.

Increased plasma corticosterone levels after periaqueductal gray stimulation-induced escape reaction or panic attacks in rats.

The hypothalamo-pituitary-adrenal (HPA) axis is involved in stress, depression and anxiety. Controversy exists on HPA axis activation during panic attacks (PAs). We examined whether the HPA axis is involved in the escape or panic-like response in an animal model of PAs induced by electrical stimulation of the dorsolateral periaqueductal gray (dlPAG) in rats. Additionally, rats were also treated with chronic administration of buspirone (BUSP) and escitalopram (ESCIT), respectively; and they were stimulated in the open-field arena for panic-like reaction. Levels of stress hormone corticosterone were measured following 30 min after escape or panic condition. Our results demonstrated that the levels of plasma corticosterone were significantly increased after the induction of escape or panic-like response in comparison with the sham animals. The levels of corticosterone were significantly decreased in the dlPAG stimulated groups after rats were treated chronically with the ESCIT but not the BUSP as compared to the saline treated animals. Importantly, the increase of corticosterone level after escape or panic-like response was paralleled by an increase of neuronal activation of c-Fos in both the parvocellular and magnocellular paraventricular nucleus of the hypothalamus. Moreover, the c-Fos data also showed a decrease in the number of positive cells particularly for the ESCIT as well as the BUSP in comparison with the saline stimulated animals. In conclusion, the present study clearly demonstrated that PA or escape response activates the HPA axis and it remains difficult to anticipate the mechanism underlying HPA axis during PAs and its relationship with 5-HT drugs.

Experimental deep brain stimulation in animal models.

DEEP BRAIN STIMULATION (DBS) as a therapy in neurological and psychiatric disorders is widely applied in the field of functional and stereotactic neurosurgery. In this respect, experimental DBS in animal models is performed to evaluate new indications and new technology. In this article, we review our experience with the concept of experimental DBS, including its development and validation. An electrode construction was developed using clinical principles to perform DBS unilaterally or bilaterally in freely moving rats. The stimulation parameters were adjusted for the rat using current density calculations. We performed validation studies in 2 animal models: a rat model of Parkinson's disease (bilateral 6-hydroxydopamine infusion in the striatum) and a rat model of Huntington's disease (transgenic rats). The effects of DBS were evaluated in different behavioral tasks measuring motor and cognitive functions. The electrode construction developed allows experimental DBS to be performed in freely moving rats. With the current setup, electrodes are placed in the target in 70% to 95% of the cases. Using a rat model, we showed that bilateral DBS of the subthalamic nucleus improves parkinsonian motor disability, but can induce behavioral side effects, similar to the clinical situation. In addition, we showed that DBS of the globus pallidus can improve motor and cognitive symptoms in a rat model of Huntington's disease. Nevertheless, during the process of the development and validation of experimental DBS, we encountered specific problems. These are discussed in detail. Experimental DBS in freely moving animals is an adequate tool to explore new indications for DBS and to refine DBS technology.

Attenuation of fear-like response by escitalopram treatment after electrical stimulation of the midbrain dorsolateral periaqueductal gray.

Electrical stimulation of the dorsolateral periaqueductal gray (dlPAG) has frequently been shown to induce escape and freezing/decreased locomotion responses which mimic panic- and fear-like behaviour. In the present study we tested whether such spontaneous fear-like behaviour could be observed in an open-field test 12 h after dlPAG stimulation. Further, we tested whether this fear-like behaviour could be attenuated by acute or chronic administration of buspirone and escitalopram. Our data demonstrate for the first time that animals showed fear-like behaviour 12 h after dlPAG stimulation, which may possibly reflect panic disorder with anticipatory anxiety/agoraphobic symptoms. Acute and chronic escitalopram, but not buspirone, treatment attenuated the fear-related behaviour. Besides, our data also showed that the stimulation intensities to evoke an escape reaction, a panicogenic response, were significantly higher after chronic buspirone and escitalopram treatment. These results suggest that the fear-like response, which was observed 12 h after dlPAG stimulation, could be considered as a relevant animal model for panic disorder with anticipatory anxiety/agoraphobic symptoms.

Hyperdopaminergic status in experimental Huntington disease.

Huntington disease has been linked to increased dopaminergic neurotransmission in the striatum, and clinical studies have demonstrated that the associated chorea can be treated with dopamine antagonist or dopamine-depleting drugs. The origin of this hyperdopaminergic status is unknown. Because substantia nigra pars compacta and the ventral tegmental area are the main sources of striatal dopamine input, we hypothesized that changes in these regions relate to striatal dopaminergic alterations. Here, in a recently generated transgenic rat Huntington disease model that shows progressive striatal neurodegeneration and chorea, we found evidence of increased dopamine levels in the striatum. We also demonstrate more dopaminergic cells in the substantia nigra pars compacta and ventral tegmental area in these rats. These results suggest that increased striatal dopamine comes from these 2 main nuclei, and that it is not necessarily related to shrinkage of the striatum. The findings implicate increased dopamine input from these nuclei in the pathogenesis of chorea in Huntington disease.

Deep brain stimulation of the nucleus accumbens shell increases impulsive behavior and tissue levels of dopamine and serotonin.

The nucleus accumbens (NAc) is gaining interest as a target for deep brain stimulation (DBS) in refractory neuropsychiatric disorders with impulsivity as core symptom. The nucleus accumbens is composed of two subterritories, core and shell, which have different anatomical connections. In animal models, it has been shown that DBS of the NAc changes impulsive action. Here, we tested the hypothesis that a change in impulsive action by DBS of the NAc is associated with changes in dopamine levels. Rats received stimulating electrodes either in the NAc core or shell, and underwent behavioral testing in a reaction time task. In addition, in a second experiment, the effect of DBS of the NAc core and shell on extracellular dopamine and serotonin levels was assessed in the NAc and medial prefrontal cortex. Control subjects received sham surgery. We have found that DBS of the NAc shell stimulation induced more impulsive action but less perseverative checking. These effects were associated with increased levels of dopamine and serotonin in the NAc, but not in the medial prefrontal cortex. DBS of the NAc core had no effect on impulsive action, but decreased perseverative responses indicative of a better impulse control. In these subjects, no effects were found on neurotransmitter levels. Our data point out that DBS of the NAc shell has negative effects on impulsive action which is accompanied by increases of dopamine and serotonin levels in the NAc, whereas DBS of the NAc core has beneficial behavioral effects.

Bilateral high frequency stimulation of the subthalamic nucleus normalizes COX activity in the substantia nigra of Parkinsonian rats.

Part of the mechanism underlying the therapeutic effect of subthalamic nucleus (STN) high frequency stimulation (HFS) involves the activity of the basal ganglia output structures. The general idea is that STN's burst activity leads to an increased activity of the basal ganglia output nuclei and that HFS reverses this. However, the published data sets conflict. Here, we addressed the question which effect STN HFS had on the overall substantia nigra pars reticulata (SNr; one of the basal ganglia output nuclei) activity, in rats rendered Parkinsonian by 6-hydroxydopamine (6-OHDA) injections in the striatum. We used a marker for metabolic activity, cytochrome C oxidase (COX). The expression of COX in the SNr of Parkinsonian rats was significantly higher as compared to sham-operated rats. However, in Parkinsonian rats with STN HFS, expression of COX was significantly lower as compared to non-stimulated Parkinsonian rats, and was comparable to the level of expression in sham-operated rats. These results show that STN HFS decreased the overall activity of the basal ganglia output nucleus in dopamine-depleted rats.

Cognitive and limbic effects of deep brain stimulation in preclinical studies.

The use of deep brain stimulation (DBS) to control severely disabling neurological and psychiatric conditions is an exciting and fast emerging area of neuroscience. Deep brain stimulation has generally the same clinical effects as a lesion with respect to the improvement of clinical disability, but has more advantages such as its adjustability and reversibility. To this day, fundamental knowledge regarding the application of electrical currents to deep brain structures is far from complete. Despite improving key symptoms in movement disorders, DBS can be associated with the occurrence of a variety of changes in cognitive and limbic functions both in humans and animals. Furthermore, in psychiatric disorders, DBS is primarily used to evoke cognitive and limbic changes to reduce the psychiatric disability. Preclinical DBS experiments have been carried out to investigate the mechanisms underlying the clinical effects of DBS for at least three (interrelated) reasons: to increase our scientific knowledge, to optimize/refine the technology, or to prevent/reduce side-effects. In this review, we will discuss the limbic and cognitive effects of DBS in preclinical studies.

Increased electrical and metabolic activity in the dorsal raphe nucleus of Parkinsonian rats.

Serotonin (5-HT) containing neurons in the dorsal raphe nucleus (DRN) may play important roles in Parkinson's disease (PD). This study investigated neural and metabolic activity of the DRN in animal PD models based on dopamine depletion. The data show both increased firing rate of DRN 5-HT neurons and increased cytochrome oxidase activity in dopamine-depleted rats, as compared to controls. These data support the hypothesis that the DRN 5-HT system is hyperactive in the dopamine-depleted brain.

High-frequency stimulation of the dorsolateral periaqueductal gray and ventromedial hypothalamus fails to inhibit panic-like behaviour.

Electrical stimulation of the dorsolateral periaqueductal gray (dlPAG) and one of its target structures, the ventromedial hypothalamus (VMH), produces a typical behaviour in rats consisting of vigorous running and jumping which is known as "escape behaviour". Escape behaviour in rodents closely mimics panic attacks in humans. Since electrical stimulation at higher frequencies generally inhibits the stimulated region, we tested in this study the hypothesis that deep brain stimulation (DBS) of the dlPAG and VMH at higher frequencies (> 100 Hz) would not induce escape behaviour. More specifically, we evaluated whether experimental DBS could be used to inhibit panic-like behaviour. Rats underwent implantation of DBS-electrodes at the level of the dlPAG and VMH and the effects of various stimulation parameters were assessed. In addition, we studied the neural activation pattern resulting from DBS of the dlPAG and VMH using c-Fos immunohistochemistry. We found that stimulation amplitude is the most important stimulation parameter in the induction of escape behaviour. Remarkably, stimulation frequency (1-300 Hz) had no effect on stimulation-induced escape behaviour and therefore it was not possible to prevent the induction of escape behaviour with higher frequencies. The neuronal activation pattern resulting from dlPAG and VMH DBS was similar. These findings suggest that DBS of the dlPAG and VMH induces panic-related behaviours even at higher frequencies.

Motor and cognitive improvement by deep brain stimulation in a transgenic rat model of Huntington's disease.

Altered activity of the globus pallidus externus (GPe) is responsible for at least part of the cognitive and motor symptoms of Huntington's disease (HD). In this study, we tested the hypothesis that bilateral globus pallidus (GP; equivalent of GPe in primates) deep brain stimulation (DBS) improves cognitive and motor symptoms in the first transgenic rat model of HD (tgHD rats). GP DBS with clinically relevant stimulation parameters resulted in a significant improvement of cognitive dysfunction and reduced the number of choreiform movements. This data indicate that GPe DBS can be used to treat cognitive and motor dysfunction in HD.

Progressive deterioration of reaction time performance and choreiform symptoms in a new Huntington's disease transgenic ratmodel.

We tested the hypothesis that a recently developed transgenic rat model of Huntington's disease (tgHD rats) showed an age-and genotype-dependent change in psychomotor performance and in the frequency of choreiform movements similar to HD patients. Wild type and tgHD (homozygotic and heterozygotic) rats were behaviorally tested at an age of 15 and 20 months. Our results show that tgHD rats exhibit an age-, and genotype-dependent deterioration of the psychomotor performance and choreiform symptoms, closely mimicking the clinical time course changes of psychomotor symptoms of HD patients. These data provide further experimental evidence that the tgHD rat can be considered as a relevant animal model of HD.