Monkeys and apes are our closest relatives in the animal kingdom. Since insights into human disease may be obtained from lower life forms including yeasts, nematode worms and fruit flies, it is no surprise that studies using primates are especially valuable. This is particularly the case in the quest to understand and treat infections and diseases associated with human physiological processes such as ageing, reproduction, endocrine function, metabolism, and neurology. Nevertheless, primates’ high cognitive abilities and complex social behaviour mean that biomedical research using these animals requires additional justification and high welfare standards
The order Primates can be divided into 11 families. Man belongs to the family Pongidae, which is divided into four Genera: Pongo (orangutan), Pan (chimp and Bonobo), Gorilla and Homo. Excluding humans and the great and lesser apes (gorillas, chimpanzees, bonobos, orangutans, gibbons and siamangs), the members of the other 10 families can be roughly divided into prosimians (eg lemurs) and monkeys. Old World monkeys (baboons, macaques), also called true monkeys, are more closely related to humans than New World monkeys (marmosets, capuchins).
Due to the high degree of genetic, anatomical and physiological conservation, primates can be the best models for understanding human biological processes. Primates may be used to understand normal or abnormal structure and function or determine the efficacy of treatments where no other suitable animal models exist.1 Their use has led to a number of valuable medicines and treatments. The chimpanzee, which shares over 98% of its genes with humans, is the most closely related primate, but the majority of biomedical research studies that require primates use the macaque monkeys.
Whilst genetic similarity to humans is high in non-human primates, it is also high in less developed species; for instance, we share 96% of our DNA with mice, 70% with fruit flies, and indeed 50% with crops such as bananas. In different species the same gene may be expressed in different ways or interact in different ways with other genes. Having genes in common may help with comparing and understanding some biological processes but is of limited relevance with respect to assessing welfare, social needs etc.
Despite their close relatedness, research with primates is not widespread and is only undertaken when other mammals are clearly inadequate. Primates are used in a small number of essential studies where only they share a particular biochemical or metabolic pathway with humans or where they model a human disease particularly well. For example atherosclerosis, osteoporosis and hypertension occur naturally in primates, which make them ideal animal models for those diseases. Without the use of primates, it would have been impossible to quickly identify the coronavirus responsible for the SARs outbreaks. Scientists then developed potential SARs vaccines that have provided protection against the disease in animals. Until we can develop other mammals with ‘near-human’ immune systems, primates are invaluable in safety testing potential human vaccines.15
The majority of primates are used in the safety testing of medicines. Except in exceptional circumstances, new medicines must be tested in two species, a rodent and a non-rodent, before human clinical trials can take place.The purpose of these studies is not to prove a new medicine or vaccine is absolutely safe but rather to permit research to move on to human volunteers and patients. It is only after human clinical trials and licensing that a doctor can prescribe a new medicine.
The non-rodent species used in toxicology is usually the dog, but the type of the medicine being tested dictates the final choice. This is based on biological and pharmacological information eg the presence of a particular receptor. A test compound may elicit an immune response in one species but not in others, and differences in metabolic pathways may exist. For example, dogs are particularly sensitive to some test compounds (eg non steroidal anti-inflammatory drugs) and some drug vehicles (eg cremaphor, PVP). Other options for the second/non-rodent species include pigs and ferrets. The main reason why primates are considered is to assess safety of new vaccines or biologicals. In the development of specific vaccines, the interactions between parasites, viruses and their host are so specific that they must be studied in species closely related to humans to forecast the chance of unexpected or hyper-immune reactions. The science involved in selecting the second species is not always exact - a judgment is made on the balance of probabilities, and the question of what should count as sufficient/acceptable data to enable a properly informed choice about whether or not the drug should enter human clinical trials.2
Due to the complexity of the brain, it is not possible to replicate its function in a test tube or rely on computer models. Thus, to develop new treatments for neurological disease it is necessary to use animals. Neuroscience research continues to produce important insights into the function of the human brain and associated disease states. Although there are some aspects of cognition that may be unique to humans, there is very strong evidence for structural, functional, behavioural and neurobiological commonalities that extend across species. The advantage of studying the monkey brain is that its connectivity, size, functional areas (reflected in its motor and behavioural capacities) and ageing processes are similar to ours. Nevertheless, there are gross anatomical differences between human and monkey brains - the gyri and sulci (ridges and valleys) of the human brain are much more pronounced; but the way the neurones themselves grow, develop, and send messages is common to all mammals. In fact, some of these more basic studies can be and are done in rodents, eg studying the effect of different concentrations of receptor agonists and antagonists on neuronal cell signalling in mouse brain slices by electrophysiology.
Some of the studies carried out on primates are behavioural studies, as monkeys are perceived to share similar emotions and are capable of carrying out similar actions. But to study how nerve cells work to produce behaviour it is necessary to examine their firing activity patterns using microelectrodes that are inserted painlessly into specific regions of the brain. This technique does not in any way incapacitate the animal and only causes minimal discomfort. The techniques are very similar to those used in certain human disorders, such as epilepsy and Parkinson’s disease, where it is necessary to record brain activity. The vocal behaviour of primates and its underlying neural processes is another area of scientific investigation. Similarly, advances in the understanding of how the primate auditory cortex functions are leading to new hypotheses for the cause of deafness. Thus, researchers can learn about both normal and disordered brain functions underpinning higher cognitive and motor performance by using primates.
The use of primates in cognitive neuroscience research is covered further in Dick Passingham's article.
Alzheimer’s disease affects more than 18 million people worldwide. It is a chronic debilitating disease that leads to irreversible memory loss, due to selective neuronal cell death. Accurate diagnosis, by autopsy, has revealed that the clinical features of Alzheimer’s are the presence of beta-amyloid plaques and tau protein tangles in specific parts of the brain. Studies in macaque monkeys in the early 1990s led to the identification of the critical regions of the brain that are essential for cognition and memory and, like humans, ageing monkeys may show evidence of beta-amyloid plaques and lose neurones as they age.3
Partial models of Alzheimer’s may also be created by priming monkeys with small amounts of human amyloid – they will develop plaques later whilst still reasonably young. As primates can be trained to perform memory-related tasks that permit the evaluation of changes in cognitive memory and emotional behaviour during ageing, they can be used to evaluate various treatment and prevention strategies. Recently, Bard and colleagues showed that they could prevent the build up of plaques and eliminate pre-existing plaques by treating mice with a beta-amyloid vaccine.4 Beta-amyloid vaccines have been tested for tolerability in monkeys and humans and it is hoped that their use will lead to the alleviation of Alzheimer’s symptoms.
The therapeutic techniques that are currently used for Parkinson’s disease and Essential Tremor would not have been possible without fundamental research on monkeys. The cause of Parkinson’s disease was elucidated following the chance finding that Californian drug addicts who injected a home-made compound containing MPTP developed Parkinson’s-like symptoms.5 The suicide and subsequent post-mortem of one of the addicts revealed that the changes in the brain were identical to that of true PD patients. Shortly after, scientists showed that they could model the disease by giving MPTP to large primates. This enabled them to study how the symptoms manifested and to test new therapies.
Researchers in the UK found that in the primates with Parkinson’s-like symptoms there is overactivity in a part of the brain that controls movement – namely the subthalamic nucleus – and that the overactivity is due to the selective loss of neurones in the substantia nigra that manufacture the chemical messenger dopamine.6
They were thus able to understand why the administration of L-dopa, a precursor of dopamine, was an effective treatment. To date, all the dopaminergic therapies that have been tested in the MPTP-treated primate have proven to be highly predictive of their clinical action in man.7 However, L-dopa and related antiparkinsonian agents have side effects and their effectiveness wears off over long-term treatment.
Alim Benabid and colleagues in Grenoble, France were the first to find that by implanting an electrode into the subthalamic nucleus tremors could be controlled and normal movement restored.8 This surgical technique, known as Deep Brain Stimulation (DBS), has been approved in Canada, Europe and Australia since 1998 for the treatment of PD and some tremor-like disorders. The procedure involves implanting electrodes into the patients’ skull whilst they are awake. A battery-operated pacemaker that sends continuous electrical pulses is also placed beneath the skin. The patient can turn off the generator, eg at night, with the use of a special magnet. The high frequency stimulation ‘paralyses’ the overactive nerve cells. Indeed, two thirds of patients have a significant reduction in their tremor. So far, worldwide, around 40,000 patients have been treated with this technique which often reduces or eliminates the need for antitremor medication.9
Another intervention derived directly from primate research is constraint-induced movement therapy (CI therapy), which effectively strengthens weak limbs. This form of rehabilitation for stroke patients arose from the finding that if the sensory nerves supplying one arm in an adult monkey were severed, the brain would undergo long-term massive reorganisation of neuronal circuits.10, 11(review) Many years later non-invasive techniques showed that something similar happens in the stroke-damaged brains of humans.
This fundamental observation in the primate led to the development of CI therapy, which involves restricting movement of the less affected arm while intensively training the more affected arm. Over the two years of their randomised controlled CI therapy studies, Taub and Uswatte showed large improvements in upper limb motor function after stroke.11
Initial hopes that primates could be used in the development of a Human Immunodeficiency Virus (HIV) vaccine were dashed when it was found that the virus did not cause disease in chimpanzees. However, primates do have their own species-specific immunodeficiency virus, Simian Immunodeficiency Virus (SIV), and they develop an AIDS-like condition when infected with SIV ie they have similar nervous system changes, develop dementia and exhibit behavioural changes similar to those seen in HIV-infected patients.12 This is not surprising given that HIV and SIV have similar genes and properties, and both attack T helper (CD4) immune system cells.
It takes just months for SIV infection to progress to simian AIDS as opposed to the many years usually seen in HIV-infected humans. Human studies have shown that the majority of HIV infections occur when the virus crosses mucosal membranes, typically during sex or birth. Further studies, in female primates, led to the identification of the mucosal cells that are initially infected during heterosexual transmission of the virus. The SIV model also confirmed that the virus could be transmitted to newborns that swallow amniotic fluids or breast milk from infected mothers.13 These discoveries open new opportunities for blocking HIV transmission with drugs, vaccines, or other precautions. For example, a humanised monoclonal antibody-based therapeutic approach, that in vitro inhibits HIV and SIV replication, has been shown to be safe when administered to rhesus monkeys.14 For ethical and legal reasons, therapeutics such as these can not be directly tested in humans in case they provoke a severe reaction, hence they are first administered to primates that have a similar immune system.
Primates were and continue to be essential for the development and testing of the oral polio virus vaccine (OPV), also known as the Sabin vaccine, and the Salk vaccine. The OPV consists of several strains of live attenuated virus and the Salk vaccine of ‘killed virus’. Until recently, each lot of vaccines had to be tested on monkeys to ensure that they were safe. However, in the last couple of years the WHO has approved and recommended that a transgenic mouse test for OPV be implemented as an alternative to the monkey neurovirulence test.15 Interestingly, humans are not the only mammals to have benefited from the development of a polio vaccine - it has also been used to protect a wild colony of East African colony of chimpanzees from a potential epidemic.16
Primates are extremely valuable models for understanding malaria pathogenesis, screening anti-malarial drugs and vaccine development. Malaria is caused by a protozoan parasite that is carried by mosquitoes. Interestingly, primates do not die from malaria although they may harbour the parasite. The reason why primates resist disease when infected, whereas humans do not, is an important question for researchers to answer. Additionally, the fact that primates can harbour an infection without becoming seriously ill makes them ideal for research into vaccine and drug development.17
Some female primates menstruate and undergo menopause in the same way as women.18 Specifically, the manner in which pregnancy is maintained following fertilisation and implantation of the embryo into the uterus is shared by all primates. During the first trimester, the corpus luteum (which develops after ovulation from the residual follicules) is responsible for synthesising progesterone, a hormone without which a pregnancy would not continue. Therefore, the corpus luteum has to be switched on at the right time and just as importantly switched off. This is key to understanding how the body maintains a pregnancy. Another example relates to the hormone prolactin, which, if locally active in primate uteri, has an immunoprotective effect. If the key to pregnancy loss is to be found, one must therefore use the primate model alongside normal cultured human uterine tissue. This human tissue is quite difficult to obtain as samples from healthy young women who are not on the contraceptive pill are needed. Tissue from women with a pre-existing pathology who have had a hysterectomy is not appropriate for these studies. These are just two examples of studies in the area of in vitro fertilisation (IVF) area which could not advance without using primates.
Not all primate experiments involve the whole animal. Some studies make use of primate tissue and cells in culture, eg primate stem cells, but while these in vitro studies do reduce the number of animals needed in a study, they by no means replace them. This is partly because in the laboratory the properties of cells and tissues change over time. The conditions in a glass dish do not replicate the conditions in the 3D normal body environment where cells are exposed to a mass of circulating hormones and other substances. Indeed, many of the proteins in our body have not yet been identified. Thus the cell and tissue culture methods of research should be viewed as complementary approaches not exclusive ones. These in vitro methods will not, in the foreseeable future, replace the need for whole animal experiments whether on rodents or primates.
1. The need for non-human primates in biomedical research. European Commission Health and Consumer Protection Directorate-General. Statement of the scientific steering committee adopted at its meeting of 4-5 April 2002.
4. Bard F, Barbour R, Cannon C, Carretto R, Fox M, Games D, Guido T, Hoenow K, Hu K, Johnson-Wood K, Khan K, Kholodenko D, Lee C, Lee M, Motter R, Nguyen M, Reed A, Schenk D, Tang P, Vasquez N, Seubert P, Yednock T (2003) Epitope and isotype specificities of antibodies to beta -amyloid peptide for protection against Alzheimer's disease-like neuropathology. Proc Natl Acad Sci USA 100(4):2023-8.
6. Mitchell IJ, Clarke CE, Boyce S, Robertson RG, Peggs D, Sambrook MA, Crossman AR (1989) Neuralmechanisms underlying parkinsonian symptoms based upon regional uptake of 2-deoxyglucose in monkeys exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Neuroscience 32(1):213-26.
8. Limousin P, Pollak P, Benazzouz A, Hoffmann D, Le Bas JF, Broussolle E, Perret JE, Benabid AL (1995) Effect of parkinsonian signs and symptoms of bilateral subthalamic nucleus stimulation. Lancet 345(8942):91-5.
10. Taub E (1980) Somatosensory deafferentation research with monkeys: implications for rehabilitation medicine. In: Behavioural psychology in rehabilitation medicine: clinical applications pp 371-401. Williams and Wilkins, New York
14. Reimann KA, Khunkhun R, Lin W, Gordon W, Fung M (2002) A humanized, nondepleting anti-CD4 antibody that blocks virus entry inhibits virus replication in rhesus monkeys chronically infected with simian immunodeficiency virus. AIDS Res Hum Retroviruses 18 (11):747-55.