280 Chapter 33
same genes in the pups reducing stress response. The evidence against
this comes from the lab equivalent of an adoption study. If newborn
pups are brought up from birth by unrelated mothers, the pups have a
modest stress response if fostered by high-licking mothers, just as the pups
have an exaggerated stress response if fostered by low-licking mothers.
But what about reverse causality? Maybe there is something about over-
reactive pups that discourages mothers from licking or grooming them.
The evidence against this is that a rat mother’s tendency to lick and groom
her pups seems more of a fixed trait than something that varies with
each pup; for any given mother, the amount of licking and grooming
is relatively constant from one litter to another, and from one pup to
another. Thus the evidence does favour the straightforward interpretation
that the level of pup licking alters the stress response. What is more, this
seems to be a case ofearly life programming– the effect depends on what
happens to the pup in its first week of life, and then generally lasts for its
lifetime.
These findings are remarkable enough, but what is even more remark-
able is that much is now known about the underlying epigenetic mecha-
nisms. For example, when a pup receives a lot of licking and grooming in
the first week of life, one key consequence is more serotonergic activity
in the hippocampus. This triggers permanent changes to the pattern of
methylation of a key promoter region in the gene for glucocorticoid
receptors. The resultant increase of these receptors in the hippocampus
amplifies the negative feedback effect of circulating cortisol, damping
down excessive hypothalamic–pituitary–adrenal responses to stress.
In summary, the material summarised in this chapter shows that genes
and environment constantly interact in complex ways. As more is dis-
covered about these processes, it is to be hoped that more effective
interventions can be devised.
Subject review
Thapar A, Rutter M. (2008) Genetics.In: Rutter Met al.(eds)Rutter’s
Child and Adolescent Psychiatry, 5th edn. Wiley-Blackwell, Chichester, pp.
337–358.
Further reading
Bagot RC, Meaney MJ. (2010) Epigenetics and the biological basis of gene
x environment interactions.Journal of the American Academy of Child and
Adolescent Psychiatry 49 , 752–771.
Bohman M. (1996) Predisposition to criminality: Swedish adoption studies
in retrospect. In: Bock GR, Goode JA (eds)Genetics of Criminal and
Antisocial Behaviour. Wiley, Chichester, pp. 99–114.