3.1.4.Use of NHP in Neuroscience
Research in neuroscience collects knowledge on how the brain works in healthy human subjects and after disease and injury in humans and in experimental animals, including NHPs. Basic research is required in order to better understand how the brain works normally and in pathological conditions (Editorial, 2008). The main reasons to use NHPs in neuroscience are the close similarities between NHP and human brains in terms of overall anatomy, cellular structure and chemical communication, functional and cognitive abilities, neural circuitry, and in brain injury and diseases. This knowledge is useful, not only to understand effects and consequences of brain and spinal cord damage in humans and to devise therapies, but also help to construct new experimental models in silico and in vitro, and to develop new computational technologies. There is a continuous iteration between basic, translational, and applied medical research in neuroscience (Fitzsimmons et al., 2007; Moritz et al., 2008).
Neural injuries and diseases encompass disorders like epilepsy, cerebrovascular disease, depression, addiction, Alzheimer’s disease and other dementias, Parkinson’s disease, and multiple sclerosis. These diseases have an important impact on society in terms of number of affected humans and their relatives affected (WHO NeuroAtlas, EBC). In 2004, 127 million, or one in three, European citizens were living with a brain disorder at a total cost of €386 billion (EBC, 2008). Brain research received 8% of the life science budget in the European Commission’s Fifth Framework Programme of research (FP5, 1998-2002), and 10% of the FP6 budget (2007-2010), a proportion that is likely to grow (EBC, 2008).
The unique role of NHPs in neuroscience research
Although major advances have been made in past 50 years, our knowledge on human brain function is still limited and the use of NHPs remains crucial for a significant advancement of neurosciences (Weatherhall, 2006).
Much of our current understanding of nerve cell function is based on studies in animals such as the cat, rat and even invertebrates such as squid where brain structure and circuitry is much less complex as compared with humans. However, the organisation of nerve cells in a complex system such as the human brain is more likely be understood by studying a similarly complex primate brain. In fact, only because of recent studies in NHPs, the existence of primate-specific developmental features was discovered (Bystron et al., 2006; Dehay and Kennedy, 2007; Garcia-Cabezas et al., 2008; Letinic et al., 2002; Meyer et al., 2000; Sanchez-Gonzalez et al., 2005; Smart et al., 2002). Moreover, most current human neuroscience research is based on evidence first discovered in NHPs, e.g., the neural bases of working memory (Goldman-Rakic, 1995), dopamine-
based learning (Schultz, 2002), motor function (Georgopoulos, 2000), and mirror systems (Fabbri-Destro and Rizzolatti, 2008; Rizzolatti and Fabbri-Destro, 2008).
Experiments using invasive neurophysiological recordings in NHPs raise some ethical concerns; however, our understanding of the functional organisation of the brain areas involved in vision, sensation, hearing, motor control and cognition in primates and humans has significantly benefited from of such work. An example is the research on primate visual pathways that led to a Nobel Prize in the early 1980s which, as well as elucidating visual centres and the mechanisms in NHPs, went on to discover similar pathways in humans (Rees et al., 2000; Tootell and Taylor, 1995). NHPs currently provide the only model to systematically study the relationships between the activity of a single nerve cell and higher cognitive functions. It is relevant to note, however, that in some instances, such as during brain surgery, neurophysiological recordings can also be made on humans (Alonso-Frech et al., 2006).
Some non-invasive research techniques, such as transcranial magnetic stimulation (TMS) are used both in humans and NHPs, and provide highly relevant information on how the brain works, thus saving invasive studies (e.g. (Ellison et al., 2007). However, TMS can only be used to study human brain areas near the surface of the skull. If deeper areas need to be studied, it will require invasive methods (causing permanent and reversible lesions) in animals, including NHPs.
Motor control is another area where the use of NHPs has been instrumental for basic understanding of the production and control of arm movements (grasping and reaching) leading to brain-computer interface technologies which are of major relevance to help alleviate the consequences of brain lesions and spinal cord injuries (Moritz et al., 2008).
Only small aspects of the complex interactions in the brain can be studied using in vitro techniques. More complex interactions can be studied using brain slices, but this requires the sacrifice of animals. Cell cultures can be used to study synaptic mechanisms and cellular events occurring in single cells. For example, to study the role of neuromodulatory molecules (like dopamine) on a neural network requires the use of intact tissue because the location of receptors on the different cells at different stages of a local network is one of the important keys to understand how the neuromodulation will act on information processing by the network. Such mechanisms have been studied on rodents, in particular rat, brain slices. However, the characteristics of e.g. cortical arrangement of neural networks and distribution of the different dopaminergic receptors within the cortical layers differ greatly from rodents to primates. Although there are evolutionary trends of variation in receptor localisations between primate species, there are drastic changes between rodents and primates. Therefore, conclusions based on in vitro data are limited.
NHP models and treatment of diseases
The use of animals in pain studies for the basic mechanistic understanding and for the development of therapies is one of the most controversial areas of research, but it has been estimated that chronic pain of moderate to severe intensity occurs in 19% of adult Europeans, seriously affecting the quality of their social and working lives (Breivik et al., 2006). A workshop was recently held to review and discuss the potential and challenges of using ethically conducted studies in human patients and volunteers to replace animals in certain areas in pain research and in the development of new therapies (Langley et al., 2008), and it appeared possible in some areas.
It should be noted that pain research is mainly performed in rodents and very rarely with NHPs. Research on neuropathic (chronic) pain is still a major issue since neither animals nor healthy humans are good models.
The study of the neurotransmitters (chemicals transmitting information between neurons), their receptors, transporters and enzyme systems in the human brain is key to understanding normal and pathological brain function, as well as to reveal possible targets for treatment. Basic mechanisms on neurotransmission and neuromodulation can be studied in vitro and in non-primate species (such as rodents and rabbits) (Carlsson, 1993; Vandecasteele et al., 2008). However, due to the large evolutionary distance between humans and rodents, the study of neurotransmitters and related molecules at the level of the entire brain requires research on NHPs. In fact, data on rodents may be misleading as their brain physiology and biochemistry is different from those of NHPs and humans (Bjorklund and Dunnett, 2007; Garrick and Murphy, 1980; Howell and Wilcox, 2002).
Brain disorders such as depression, schizophrenia, attention deficit hyperactivity disorder, autism, drug addiction and obsessive compulsive disorders, all involve malfunctioning of the highly developed primate frontal lobes and their interactions with other parts of the brain. This is also true of conditions such as traumatic head injury, Huntington's disease, stroke and some types of dementia, which also involve interactions between multiple systems in the brain (Gil and Rego, 2008; Van Hoesen et al., 2000). Such conditions cannot, or only partially, be reproduced in non-primate species. Even if NHP are not ideal models for the above conditions, their complex brains make them more suitable to further advance knowledge than non-primate species (Yang et al., 2008).
NHPs have been used to investigate neurological diseases occurring in human neonates. For example, rhesus can be a surrogate model for asphyxia insults at birth and accurately reflects the mechanisms and neuropathology seen in human newborns suffering from this condition. More recently, a premature baboon model developed for evaluation of bronchopulmonary dysplasia has been applied to the investigation of cerebral development and injury, revealing a high similarity in neuropathology to the premature human infant (Inder et al., 2005).
Stroke research does not use large numbers of NHPs, but the importance of their availability as a research tool is significant. An example of NHP use in stroke research was the development of the drug NXY-059 (Marshall et al., 2003) that had significant effects and reduce functional disability following ischemic stroke. The NXY-059 studies in NHPs were regarded as crucial in designing the SAINT trials in humans. While the SAINT I clinical trials showed some promise, the SAINT II trials revealed no significant effects. The review procedures from animal trials to clinical studies have been published and suggested a need, not to change the model, but the criteria for publication, evaluation and use of animal testing (Macleod et al., 2008).
Important efforts are being made to develop better models of psychiatric disease by screening large populations of animals to detect the presence of a similar disease in NHPs. In the study of depression, simple tests for anti-depressant-like activity of pharmacological substances are often based on known classes of therapeutically successful existing anti-depressant agents, but investigators have also endeavoured to reproduce factors in the laboratory that are believed to initiate depression in humans. Studies on large macaque colonies show that some individuals, in particular those of low social rank, express behavioural signs comparable with some human depression syndromes. Neuroimaging has shown activity deficits in NHPs, comparable with those in humans, thereby supporting their use for further neurobiological characterisation and modelling (O'Neil and Moore, 2003).
A model of Parkinson’s disease (PD) in NHPs has been highly valuable in studying its pathophysiology (Beal, 2001; Boulet et al., 2008; Emborg, 2004; Hamani and Lozano, 2003; Mounayar et al., 2007). The application of deep-brain stimulation (DBS) in humans with Parkinson’s disease derives from experiments in a NHP model showing that destruction or high-frequency stimulation of certain areas in the brain reversed Parkinsonian symptoms. Over 40,000 patients have now been treated with DBS worldwide, and there are 160 DBS centres in Western Europe. In addition, DBS is showing promise in other brain conditions such as drug resistant cases of depression, obsessive compulsive disorders, and Tourette’s syndrome. However, there are still significant cases of unpredicted adverse effects, and numerous patients who are not suitable for DBS show there is still a need for further research such as advancements in DBS, local delivery of neural active substances, the application of neurotrophins and stem cells, gene therapy, and molecular neurosurgery (Hamani and Lozano, 2003).
Stem cell technology, like induced pluripotent stem cells, opens the possibility to use somatic cells of an individual to repair his own tissue, thereby removing some of the ethical issues concerning embryos and the problem of rejection. These technologies are being currently developed for the repair of brain tissue in Parkinsons, Huntingtons, in stroke and in spinal cord and brain injuries, but will likely require safety and efficacy testing in NHPs (Brundin et al., 2008; Chen and Palmer, 2008; Roh et al., 2008).
Research on sensory and motor systems has led to the new field of neuroprosthetics to restore the severe loss of sensory abilities or movement capabilities in paralysed patients (Weatherhall, 2006). The development of brain machine interfaces (BMI) has much benefited from neurophysiological experiments in NHPs showing that it is possible to use natural brain cortical neural activity to drive computers, robots, and artificial limbs, to restore volitional control of movement to paralysed limbs, and/or to compensate for perceptual deficits (Fitzsimmons et al., 2007; Moritz et al., 2008).
New fields in neurosciences
New fields and techniques to assess brain structure and function, such as Magnetic Resonance Imaging (MRI, now routinely used for diagnosis in humans), and functional MRI (fMRI), are rapidly developing and increasingly employed in human clinical and experimental studies. Even though they are powerful means to study brain function, these techniques also have important limitations as they only measure blood flow, which is an indirect measure of neuronal activity (Logothetis, 2008; Vanzetta, 2006; Vanzetta and Grinvald, 2008). Other valuable non-invasive techniques, that provide useful information on how the brain works, include multichannel electroencephalography (EEG) and magneto-encephalography (MEG), both of which have a better temporal resolution than fMRI, but still have poor spatial resolution. Thus, fundamental research, at the level of single neuron activity (that may be invasive), is still needed to obtain information about how the brain works to improve understanding of pathological changes. Additional non-invasive novel technologies, like magnetic resonance spectroscopy (MRS), provide information on the chemical composition of the brain. They represent potentially powerful tools to gather data on brain biochemistry; however, at present the information they can provide is much cruder than that provided by more precise methods, like positron emission tomography(PET),ligandbinding, micro-dialysis and immunochemical techniques.
Improvement of molecular and cell biology has reached a point where genetically modified NHP models of brain diseases are becoming available (Yang et al., 2008). Such genetically modified models will, potentially, be of even higher predictability power for human outcomes than rodent genetically modified models.