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1992; von der Ohe et al., 2006, 2007
;
Fig. 9.3
B). Synaptic dissociation could
help conserve energy (
Drew, Rice, Kuhn, & Smith, 2001
) and could also
serveaneuroprotectivepurpose(
Schwartz et al., 2013
). Recent Illumina
HiSeq sequencing of the cerebral cortex transcriptome indicates that the brain
upregulates transcripts important for plasticity and remodeling prior to hiber-
nation onset (
Schwartz et al., 2013
), suggesting that this synaptic plasticity is
circannually timed as well.
5. POTENTIAL PATHWAYS INVOLVED
IN CIRCANNUAL TIMING
As mentioned previously, the hypothalamus controls many parameters
important for seasonal events, including feeding/satiety, circadian rhythms,
reproduction, sleep, hormone release, and thermoregulation. Thus, it is likely
that this brain region is involved in circannual timing, and there are several
candidate systems potentially involved in coordinating this timing.
5.1. Thyroid hormone signaling
Thyroid hormone signaling is affected by photoperiod and is involved in the
coordination of seasonal reproduction (
Nakao, Ono, & Yoshimura, 2008
).
Light associated with long days induces conversion of thyroxine (T4) to tri-
iodothyronine (T3) in the mediobasal hypothalamus of Japanese quail
(
Coturnix japonica
) by upregulation of
DIO2
, the type 2 iodothyronine
deiodinase (
Yoshimura et al., 2003
). This increased T3 stimulates luteinizing
hormone, which initiates the expression of thyroid stimulating hormone
(TSH)
and
DIO2
in the pars tuberalis of the anterior pituitary gland
(
Fig. 9.4
;
Nakao, Ono, Yamamura, et al., 2008
). ICV injection of TSH into
quail kept in short day conditions stimulated a long day-like growth of
gonads and
DIO2
expression, triggering seasonal breeding (
Nakao, Ono,
Yamamura, et al., 2008
). Similarly in mammals, high
DIO2
expression is
found in the blood vessels surrounding the arcuate nucleus of the hypothal-
amus and in the ependymal cells of the third ventricle of Siberian hamsters
(
Phodopus sungorus
) maintained in long day conditions. This expression
decreased after melatonin injections (
Watanabe et al., 2004
). Ependymal
cells separate the brain from the cerebrospinal fluid filled ventricles, acting
as a barrier and serving a role in transport (
Bruni, 1998
).
TSH
and
DIO2
were also shown to be upregulated in mouse pars tuberalis and ependymal
cells respectively in response to increasing photoperiod (
Ono et al., 2008
).
Therefore, it appears that thyroid hormone signaling plays an important role
