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Animal Defenders International

The Primate Nations: Species differences, page 2

Posted: 1 September 2006

There has long been criticism of the application of research on monkey brains for humans. Researchers at the Salk Institute and the University of California wrote: “What is known about the neuroanatomy of the human brain? Do we have a human cortical map corresponding to that for the macaque? And what does the human equivalent of the connectional map look like? The shameful answer is that we do not have such detailed maps because, for obvious reasons, most of the experimental methods used on the macaque brain cannot be used on humans. For other cortical regions, such as the language areas, we cannot use the macaque brain even as a rough guide as it probably lacks comparable regions”(65).

Human brains have a folded cerebral cortex (a gyrencephalic brain) whereas smaller primates, such as the marmoset, have a smooth cerebral cortex (a lissencephalic brain). Not only are there anatomical differences between the two types of brain but evidence suggests that there are functional differences, too(66).

This gained further weight in 2004, when scientists discovered that entire brain areas of the rhesus monkey were “very different” to our own(67). As mentioned earlier, in 2005 London researchers blamed “remarkable species differences” for the failure to apply primate findings to human brains(46).

Researchers in Denmark and the USA have highlighted the need to reconsider the use of primates in research. The team compared genes found in humans to their equivalent genes in chimpanzees. They found that the genes which differ the most between humans and chimpanzees are those related to immune defence and cancer development. They also identified that the genes which are most similar between humans and chimpanzees are those which are expressed in the brain. This work has drawn attention to the misleading nature of primate research, as results obtained from tests on chimpanzees and other primates for drugs, toxicity, disease and cancer would be inaccurate when applied to humans(68).

Other researchers have reported that often, areas in the brain that appear to have a function in monkeys do not have the same role in humans(69). Lower and higher primates differ from one another by a number of structural features in their nervous systems and sense organs. The brains of lower primates are much smaller in relation to body size than those of the higher primates. The association areas, which govern the transfer of information between the different brain centres, differ in development between brains of higher and lower primates(70).

Reliance on studies of monkey brains has been misleading: The role of the hippocampus in human memory was complicated for a time by findings from behavioural studies of monkeys and other animals with hippocampal lesions, until its role was established in 1986, from human study(71).

The processes involved in behavioural responses in humans are known to be more complex than in other species(78), and no animal species reacts to behaviour altering drugs in the same way as a human being would(79). For example caffeine, which can induce panic attacks in people, has conflicting results in animal models of anxiety(80).

Parkinson’s disease is unique to humans(82), slowly progressing, whereas in the artificial lab disease ‘model’, using the drug MPTP, the disease is rapid in its course. There are differences in nerve degeneration and the transmission of nerve impulses in naturally-occurring human Parkinson’s disease and MPTP-induced Parkinson’s disease in animals(83). Also, there are major differences at both the behavioural and neurochemical (nerve chemistry) levels between marmosets and cynomolgus monkeys when administered MPTP, making it impossible to predict with any certainty how results of macaque and marmoset experiments can be used in humans(84).

Gout is caused by excess uric acid which is produced in monkeys, apes and humans; but only humans get gout(85).

Although HIV can infect chimpanzees and rabbits, it does not induce disease in them. Human beings are the only species to have been found to be susceptible to HIV(86).

Several species of non-human primates have been experimentally infected with hepatitis E virus; the infection is less severe in these animals than it is in humans(88).

Herpes B virus in monkeys may cause lesions on the face, lips, mouth, body. Monkeys can carry the virus without suffering the disease; in humans, the disease is rare but is almost always fatal(90).

How drugs react differently in different species:

• An anti-Parkinson’s disease drug, tolcapone (Tasmar); withdrawn from the market in 1998 after links to deaths from liver disease(72). Similarly, the antidepressant Seroxat was also linked to liver damage, in 1997(73).
• The safety of donepazil for Alzheimer’s came under review in 1999 resulting in updating of product information(74), and clinical trials of a potential Alzheimer’s vaccine were suspended when participating patients began experiencing side-effects to the nervous system(75),(76). The vaccine had been hailed as “revolutionary ..following encouraging tests on animals” (77).
• The therapeutic effects of the appetite suppressant fenfluramine for autism and its potential to reduce suicidal tendencies were discovered in people and could not have been predicted in animal experiments(81).
• The way drugs break down and are excreted are similar in monkeys and humans, but metabolism rates differ radically(87).
• Chloramphenicol does not have the adverse effect in monkeys and dogs that it has in humans(89).
• The drug azauracil caused no apparent toxic effects in monkeys; in humans, it produced unpleasant effects that caused its use to be stopped(91).

Next: Primate supply, transport, conservation issues, page 1

References

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