The study of epigenetics has been a central focus of research in fields such as psychobiology and cognitive neuroscience, which aim to explain behavior based on physiological and biological mechanism, for years. Epigenetics is related to the processes and modifications which cause phenotypic (observable) changes in an organism without any modifications in the DNA. These processes influence the expression of genes, thus the way genes “operate” using the genetic information stored in them, causing typically an increase or decrease in the production of specific proteins.
The influence on the expression of genes comes from mechanism such as methylation or demethylation of DNA, that consist in the addition or removal of a methyl group to the cytosine, in the nucleotidic sequence (genes). This mechanism causes influences the access of the transcription factor to the sequence itself. The gene, therefore, may be more or less expressed (usually, methylation blocks the access, not allowing the expression, while demethylation improves the expression). This concept is very important because the theory of evolution, coined by Darwin, is centered on the presence of DNA mutations.
To explain the importance of epigenetics, I’ll take a step back to summarize the most important revelations of Darwin’s theory. In his theory, Darwin has revealed to us that any trait, characteristic, behaviour of any living organism is “modelled” in a long time (and thus through different generation of “individuals”) by a process called natural selection. This process works by selecting those characteristics that improve the chances of survival and reproduction of an individual/organism which has them. Darwin (and the other evolutionists that followed him) assumed that the first appearance of a trait, which would afterwards be favored on unfavored by the process of natural selection, must have been caused by a random mutation in the DNA of the individual that showed that trait or characteristic.
Epigenetics is thus important because, by finding specific mechanism that cause changes in the phenotype of an organism (for example in his behavior, as I’m to show next) without un underlying DNA mutation, researches have been able to integrate the assumption made by Darwin about the appearance of the traits. We now know that there are modifications in the expressions of genes activated or caused by experience, thus by the environment, and not by random mutations. These modifications can also be inherited through several generations of individuals. Some interesting evidence about the effect and influence of experience and environment on behaviour comes from the study of maternal behaviour, and the influence of said behaviour on the offspring.
A very important study by Weaver et al (2004) has shown that parental care, in this case the degree to which mothers (the experimental models used in this case are rats) had physical contact with the offspring, induces a profound effect in terms of behavior (thus in terms of the cerebral activity) in the latter. The authors divided the offspring it two groups, based on the frequency of physical contact with the mother: a group formed by the individual who received “high care” and a group of individuals who received “low care”. The effects observed in the groups are related to anxiety and stress, in fact signals of stress and anxiety in the behaviour of the offspring of “less caring” mothers are significantly more frequent than in the offspring of “caring” mothers. These phenotypic differences have been related to specific epigenetic mechanism found by the authors, which cause, among the other effects, a higher response of the HPA axis (Hypothalamic – pituitary – adrenal axis, responsible for the release of glucocorticoids and the consequent stress reaction) in the offspring of less caring mothers. Between the epigenetic effects caused by the lack of care, the most relevant is the methylation of the gene for the glucocorticoid receptor (GR) in the hyppocampus, which is linked to the HPA axis. This process causes a reduced expression of the GR gene, which is consequently related to a lack of GRs in the said area. Lack of glucocorticoid receptors in the hyppocampus is associated with a release and spread of substances such as cortisol in the the periphery of the organism (because this substances cannot bind to the GR in the hyppocampus) after the activation of the HPA axis. The behavioral effects of this complex mechanism are sustained during the life of the offspring.
Other studies have found that the influences of experience in the phenotype of an organism or individual are not restricted to infancy, but can be extended also after the weaning or even back in the foetal period. Furthemore, these changes have been shown to persist through generations (they are not only limited on the individual that “lived” that experience, but can be passed to his/her offspring too). All this evidence brought to the conclusion that development is a dinamic process during which there is a constant and reciprocal interaction between organism and environment. Epigenetic mechanism may be a way through which experience during the lifetime causes sustained changes in the behaviour of an individual, which are furthermore inherited by the subsequent generations. As said before, this mechanism constitues an integration of the assumption of Darwin, because it allows to add a cause for the development of a trait or a characteristics that is later favored or unfavored by natural selection. The appearance of those characteristics are thus not ony caused by random mutations, but also by the experience (and the environment) during the lifetime.