Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 200

dominant males attack. However, upon perceiving a social opportunity, the subordi-
nate individual switches to the dominant males' reproductive and behavioral physiol-
ogy. This is associated with rapid changes in expression of genes in the three levels
of the hypothalamic-pituitary-gonadal (HPG) axis, which is typical for the dominant
males. Because of this social ascent, it expresses the immediate-early gene (IEG)
Egr-1 , also known as zenk , in neurons of the POA and in GnRH1 neurons of the
hypothalamus, in 20-30min. At the same time, the fish increases secretion of the
neurohormones kisspeptin and GnRH1. The changes in gene expression as a result of
the change in social status extend farther down the axis to the pituitary and gonads.
In the pituitary, the social ascent increases the expression of gonadotropin-releasing
hormone (GnRH) receptors to the level of dominant males and induces the expres-
sion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) genes. In
ovaries and testes, social information leads to the expression of FSH and androgen
receptors, stimulating reproductive activity. Changes in gene expression and behav-
ior are followed by morphological changes ( Maruska and Fernald, 2011 ).
In another example, the adipose tissue in mammals is composed of white adi-
pose tissue (WAT) and brown adipose tissue (BAT). The latter consists of brown
adipose cells characterized by numerous lipid droplets and a greater number of
mitochondria. BAT is the main source of nonshivering heat in hibernating animals
and in human newborns. In the latter, BAT represents about 25% of a newborn's
weight. Activation of BAT prevents hypothermia in neonates. Production of the
uncoupling protein-1 (UCP-1), also known as thermogenin, triggers mitochondrial
thermogenesis ( Figure 4.1 ) in adipocytes. BAT thermogenesis is activated by a long
signal cascade starting in the CNS, after information on the cold thermosensory
nerve endings reaches the dorsal root ganglion (DRG) ( Morrison, 2004; Bartness and
Song, 2005 ).
The simplified neural cascade for nonshivering thermogenesis looks as follows:
Cold temperature → cold thermoreceptors → DRG → DH (dorsal horn) →
GABAergic neurons in a still unidentified region of the brain → neurons of the
POA (hypothalamic preoptic area) → neurons of the DMH (hypothalamic dorso-
medial nucleus) → sympathetic motor neurons in rostral raphe pallidus nucleus of
the brain stem → autonomic motor neurons of the IML (intermediolateral cell col-
umn) → epinephrin released by postganglionic sympathetic nerves → a signal
transduction pathway in brown adipocytes → expression of UCP-1 gene in brown
adipocytes → adipocyte mitochondrial thermogenesis.
Both approaches, from the expression of nonhousekeeping genes up and from the
environmental stimuli down, show that the epigenetic information used to express
these genes is generated by processing stimuli in neural circuits. The odyssey of the
encoded stimulus from the sensory receptors through various centers of the spinal
cord and brain is necessary for computing the adaptive response. The output of the
stimulus processing in the neural circuit is a specific chemical, which activates a spe-
cific signal cascade and ultimately expresses one or a number of genes, rather than
roaming aimlessly. Evolution is parsimonious. Natural selection does not allow the
wasteful expenditure of energy that is not outweighed by a benefit.