Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 209

In response to social stimuli, the puppy's brain translates afferent information of
the maternal LG behavior into efferent information for demethylation of specific
genes in hippocampal neurons and for modifying the function of the HPA axis and
the stress response. Figure 4.6 reveals the main features of the methylation/demeth-
ylation in hippocampal machinery that translates social stimuli into instructions for
inducing changes in gene expression and producing phenotypic (behavioral) changes
without changes in genes.
Early life separation of baby mice from their mothers leads to lifelong stress,
characterized by increased activity of the HPA axis. The hyperactivity of the HPA
axis is related to increased production of the neuropeptides CRH and arginin-vaso-
pressin (AVP), with the latter strengthening the action of the former. Experimental
evidence shows that stress induces hypomethylation of a specific region of the AV P
gene enhancer in hypothalamic paraventricular nucleus (PVN) neurons. The hypo-
methylation reduces the binding of the phosphorylated methyl-CpG-binding pro-
tein 2 (MeCP2) to the hypomethylated region of the enhancer of the AV P gene. The
release of the enhancer of AV P gene from MeCP2 occupancy allows its transcription
( Dulac, 2010 ), lifelong increased HPA activity, and stress condition.
[T]his vicious circle does not only uncouple MeCP2 occupancy from the initial
stimulus but ultimately leads to the hard-coding of the early life experience at the
level of DNA methylation. The mice are permanently caught by their adverse early
life experience and their latent scars become easily unmasked by further stress
exposure.
Murgatroyd et al. (2010)
Recently, the adaptive regulation of the methylation of genes in response to social
stimuli is also observed in honey bees ( Apis mellifera ). A correlation is observed to
exist between the task (nursing or foraging) and the level of methylation of genes in
the brain. So, e.g., the protein kinase C binding protein-1 ( PKCbp1 ) gene is consist-
ently higher in the brains of foragers than nurses. Correlations between the methyla-
tion level and the task are also identified for other genes in the honey bee brains. It
is believed that higher methylation in foraging honey bees is related to the higher
cognitive demands of foraging and to production of task-specific protein isoforms
through manipulative splicing ( Lockett et al., 2012 ).
In the 1960s, Altman (1963) published experimental evidence to “support the
possibility that new neurons may be formed in forebrain structures, both in rodents
and carnivores.” It took a number of other investigations during the rest of the twen-
tieth century before the dogma that no new neurons produced in adult animals were
invalidated. Today, it is known that adult neurogenesis may be induced by electri-
cally stimulating particular brain regions. In adult rodents, for example, deep brain
electrical stimulation induces a group of cells in the subventricular and the dentate
gyrus of the hippocampal formation to differentiate into neurons ( Toda et al., 2008 ).
Social stimuli in the form of an enriched environment and learning also stimulate
adult neurogenesis in mammals ( Abrous et al., 2005; Zhong et al., 2007 ).