Animal research has played a major role in answering fundamental questions in many areas of psychology. The need for animal testing to enhance human health research has been made evident by the work of Charles Darwin on the evolutionary link between animals and humans.
This essay will discuss whether animal research can improve our understanding of human mental health, more specifically mood disorders, and will consider both contributes and limitations of the application of animal models to study human disorders.
The evolutionary stance postulates that emotions are a universal feature developed during an evolutionary process that lasted thousands of years.
Research has shown that although humans public displays of emotions may vary depending on the social and cultural context, basic emotions such as joy and fear have a biological basis which is common to the whole human species.
This same biological basis is found in non-humans animals, especially in mammals, as evidenced by the work of Charles Darwin (Darwin, 2009 , cited in Datta, 2010), which highlighted the similarities between humans and animals in their expressions of emotions.
Animal research have greatly contributed to our understanding of the brain structures involved in perceiving emotions; on this topic, Paul MacLean (1990, cited in Datta, 2010) proposed a ‘triune brain model’ suggesting that the brain had evolved in a series of three layers, adding complexity in brain functioning, including perception of emotions. The most ancient layers in evolutionary terms, the reptilian brain (that controls the body’s vital function in response to a specific stimulus) and the limbic brain (whose main function is to record memories of experiences associated with specific emotions, and to influence our behaviour in response to these memories), are found respectively in reptiles and mammals, while the last layer, termed ‘neocortex’ (which underlies the brain’s most complex functions, such as abstract thought and language), is a unique feature of the brain of humans and of its closest relatives, apes and monkeys.
Given the biological affinity between humans and animals, it is unsurprising that animal research plays a major role in investigating the biological bases of behaviour in human mood disorders. During an experiment involving mice to test the efficacy of ADMs in treating depression and anxiety, Santarelli et al. (2003, cited in Datta, 2010) found that suppressing neurogenesis made ADMs ineffective, uncovering the crucial role of this process in the development of mood disorders.
Another experiment conducted by Mitra and Sapolsky (2008, cited in Datta, 2010) on rats has shed light on the correlation between stress and anxiety. Mitra e Sapolsky induced chronic stress in rats by injecting them with corticosterone to investigate the physiological and behavioural effects that this condition would produce. They discovered that the very structure of their neurons had changed, with more dendrites sprouting in the amygdala area (whose hyperactivity has been find to be a common trait in mood disorders); moreover, rats who received corticosterone showed increased anxiety during their performance in mazes. Mitra and Sapolsky concluded that a short-term stressful experience was sufficient to shape the structure of the amygdala, and to cause long-term anxiety. Datta (2010a) suggests that these effects are similar (and therefore could be relevant) to PTSD symptoms in humans.
Contribution of animal research is not limited to biological aspects of mood disorders. Two experiments conducted by Meaney and coll. (2001, cited in Datta, 2010) and by Nestler and coll. (Tsankova et al. 2006, cited in Datta, 2010) have helped to clarify the extent to which genetics influences the development of mood disorders.
Meaney and his team at McGill University investigated the role of early life experiences on the development of mood disorders by comparing the stress response of rats whose mothers groomed and licked them more in their first days of life, with that of rats whose mothers were less caring, discovering that nurture can be as crucial as nature in defining behaviour in adulthood. In a second experiment conducted by the same authors, the pups of the anxious, less-caring mothers were placed with the more caring, less-anxious mother, and viceversa: results showed that, regardless of their genetic propensity to anxiety and stress, maternal care played a crucial role in shaping the pups’ behaviour.
The work of Nestler and coll. focused yet on another epigenetic factor that affects the development of depression; researchers induced helplessness, a state similar to depression, in a group of mice, which as a consequence showed socially avoidant behaviour and lower levels of BDFN. Both effects were, however, reversible with ADMs treatment.
In addition, other researchers conducted on rhesus monkeys have linked the role of social hierarchies to the development of stress, which can be relevant in understanding the pressure of modern societies on individuals (Datta, 2010b).
As well as defining which factors are involved in the development of human mood disorders, animal research has greatly contributed to the development of effective pharmacological treatments (the efficacy and tolerability of ADMs on human organism are indeed assessed with experiments on animals) and behavioural therapies based on the findings of classic experiments from B. F. Skinner and other influential psychologists, which were carried out on animals.
We have considered how animal research have contributed to scientific understanding of mood disorders, but these observations should be juxtaposed with a brief reflection on its limits in terms of applications of animal models to humans.
First, while humans and animals share a biological affinity, it seems hazardous to many to blindly apply the findings obtained from experiments on rats, pigeons or other lab animals on human patients; humans are indeed extremely complex animals, whose behavior is influenced by many biological, psychological and social factors.
A second limit concerns the difficulty in obtaining a direct account from the animal of his cognitive and emotional experience.
Despite these considerations, animal research is still an essential methodological tool for modern psychological research. Much of the scientific progress in understanding mood disorders was obtained from experiments on animals that for various reasons (economic, methodological, ethical) could not have been substituted by alternative research methods such as human experimentation or computer models. Until researchers will find alternative means to investigate human brain and behaviour, it seems that, for the mentioned reasons, animal research will remain an essential part of psychological research.
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