Biology Reference
In-Depth Information
compared to all other collection points. The gene stimulated by retinoic acid
gene 6 homolog (
STRA6
) is also elevated in April, while the gene encoding
for retinoic acid induced 1 (
RAI1
) is upregulated in both April and October.
There are also several transcripts associated with retinol, the precursor to ret-
inoic acid, including retinol binding protein 1 (
RBP1
), and retinol dehydro-
genase 11 (
RDH11
).
RBP1
is involved in the transport of retinol and is
elevated in April.
RDH11
is elevated during hibernation, and interestingly
is involved in adaptation to dark in the retina (
Kasus-Jacobi et al., 2005
),
suggesting a potential role in transducing photoperiod.
5.5. Melatonin and clock genes
Melatonin is produced and released by the pineal gland at night and is
inhibited during the day by light via the SCN, which receives a direct photic
input from the retina by the RHT. This photoperiodic input entrains the
circadian clock to the surrounding environment. A suite of clock genes
in the SCN (
Per
,
Cry
,
Bmal1
,
Clk
,
Rev-Erba
,
RORA
,
DEC1
) form
autoregulatory feedback loops that generate the 24-h expression patterns
that drive circadian rhythms in all cells (
Fig. 9.7
;
Kawamoto et al., 2004;
Ko & Takahashi, 2006
). Because the nightly duration of melatonin is rep-
resentative of the length of the night, melatonin is an excellent molecular
indicator for time of year. Additionally, light input to the SCN also varies
according to seasonal time, so the circadian clock could also be playing a role
in interpreting circannual time.
Melatonin receptors are found throughout the brain, but the most con-
centrated areas are the pars tuberalis and parts of the hypothalamus (
Morgan,
Barrett, Howell, &Helliwell, 1994
), supporting the role of melatonin in sea-
sonal behavior. Melatonin receptors are also found in the song control sys-
tem of songbirds (
Whitfield-Rucker & Cassone, 1996
), and as mentioned
previously, exogenous melatonin can stimulate or inhibit singing behavior
and the associated neural changes (
Cassone et al., 2008
). Untreated
pinealectomized pied flycatchers (
Ficedula hypoleuca pallas
) failed to orient
in the correct direction for migration, but pinealectomized flycatchers that
had received nightly melatonin injections exhibited the correct migratory
orientation (
Schneider, Thalau, Semm, & Wiltschko, 1994
). Binding of
radiolabeled melatonin was decreased in the pars tuberalis of hibernating
ground squirrels (
Stanton, Siuciak, Dubocovich, & Krause, 1991
), most
likely due to a decrease in receptor number, suggesting that the melatonin
system is seasonally regulated.
