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and
BBS9
, which are all elevated during hibernation (both torpor and IBA)
(
Schwartz et al., 2013
). Additionally, intraflagellar transcripts
IFT80
,
IFT81
,
and
IFT88
are also upregulated during torpor in the ground squirrel hypo-
thalamus. Ground squirrels will not eat during IBAs even if food is provided
artificially in the lab (
Torke & Twente, 1977
), most likely due to strong
circannual satiety signals that are in place to prevent foraging during periods
of time when food is scarce and conditions are unfavorable. The expression
of BBS and IFT transcripts during hibernation could be playing a role in
orchestrating this satiety through modulation of neuronal cilia.
5.4. Retinoic acid signaling
Retinoic acid is a derivative of retinol (vitamin A) which has a wide variety of
effects on development, differentiation, and homeostasis (
Chambon, 1996
).
Retinoic acid has two receptor families, RXR and RAR, each with three
subtypes (
a
,
b
,
g
). Retinoic acid signaling is affected by photoperiod and
could play an important role in circannual timing. Lengthening photoperiod
was shown to enhance retinoic acid signaling in mouse hypothalamus
(
Shearer et al., 2010
). Cellular retinoic acid binding protein 1 (
CRBP1
)is
downregulated during short photoperiods (
Barrett et al., 2006
). Retinoic acid
receptors (
RXR
,
RAR
),
CRBP1
, and
CRBP2
are upregulated in the arcuate
nucleus of the hypothalamus in Siberian hamsters when they are switched
from short to long photoperiods (
Ross et al., 2005
).
RXR
appears to be spe-
cifically associated with the changes in body weight (
Ross et al., 2004
).
Retinoic acid signaling is also important for synaptic plasticity. Retinol
deficiency resulted in a loss of LTP and LTD in mouse hippocampus, which
was reversed by direct application of retinol or retinoic acid (
Misner et al.,
2001
). Deficiency of
RARb
eliminates LTP and LTD in mouse hippocam-
pus, while
RXRg
deficiency results in a loss of LTD but not LTP (
Chiang
et al., 1998
), similar to the
VGF
knockout mice discussed above. Retinoic
acid and RAR
a
have also been shown to play a role in enhancing synaptic
scaling, a method of homeostatic plasticity employed by individual neurons
to stabilize their firing rates (
Aoto, Nam, Poon, Ting, & Chen, 2008
). Syn-
aptic plasticity is a characteristic of several circannual rhythms discussed here,
including hibernation and seasonal birdsong.
In the thirteen-lined ground squirrel hypothalamus, several retinoic acid
signaling-related genes are differentially expressed across the hibernation
season (
Schwartz et al., 2013
).
RXRa
and
RXRg
are both elevated during
the prehibernation October collection point.
CRBP1
is elevated in April
