Biology Reference
In-Depth Information
in seasonal timing in both mammals and birds. Similarly, thyroid hormone
signaling also plays a role in the timing of amphibian metamorphosis, where
individuals progress through specific developmental stages, discussed in
chapter 10
.
Thyroid hormone signaling also appears to play a role in hibernation.
Levels of thyroxine-binding globulin (
TBH
) are increased during hiberna-
tion in the liver of the golden-mantled ground squirrel (
Callospermophilus
lateralis
)(
Epperson & Martin, 2002
).
TBH
binds both T3 and T4. In the
hypothalamus of the thirteen-lined ground squirrel, thyrotrophin releasing
hormone (
TRH
) expression peaks in October during the prehibernation
period (
Schwartz et al., 2013
). Additionally,
DIO2
expression peaks in April
after completion of hibernation, a time when the squirrels are becoming
reproductively active.
DIO2
expression is very low in every other time
period observed.
CGA
, the alpha subunit of TSH, increases greater than
fourfold in April compared to any other collection point.
CRYM
, a thyroid
hormone binding protein, also shows seasonal differential expression with
decreased expression in October compared to all other points.
TTR
shows
minimal expression in the hypothalamus, but is differentially expressed in
cerebral cortex, with increased expression in the hibernation collection
points compared to outside hibernation. As mentioned previously,
CGA
,
CRYM
, and
TTR
are all differentially expressed in song sparrow hypothal-
amus according to season as well (
Mukai et al., 2009
).
5.2. VGF signaling
VGF (nonacronymic) is a protein involved in energy balance and reproduc-
tion (
Jethwa & Ebling, 2008
). It is processed into several small peptides by
prohormone convertases and is expressed throughout the brain, although it
is most abundant in the hypothalamus.
VGF
mRNA is induced in the arcu-
ate nucleus of the hypothalamus (
Fig. 9.4
) in Siberian and Syrian hamsters
(
Mesocricetus auratus
) switched from a long day photoperiod to short days
(
Barrett et al., 2005
).
VGF
knockout mice are lean and hypermetabolic,
and resisted obesity even with genetic predisposition (
Hahm et al., 2002
).
These mice exhibited cold intolerance, decreased fat storage in both white
and brown adipose tissue, upregulation of brown adipose uncoupling pro-
teins 1 and 2 (
UCP1
-
2
), and an increase in the overall amount of mitochon-
dria and in the density of mitochondrial cristae (
Watson et al., 2009
).
VGF
also could play a role in synaptic plasticity. Long-term depression
(LTD) could not be induced in hippocampal slices taken from
VGF
knock-
out mice, although there were no deficits in long-term potentiation (LTP)
