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larval
Rhodnius
brains with anti-PTTH and anti-PERIOD (or anti-
TIMELESS) antibodies revealed that dendrites originating from PERIOD
and TIMELESS-positive ventral-lateral and dorsal neurons extended very
close to the axons of the PTTH-positive neurons (
Vafopoulou, Steel, &
Terry, 2005
). These observations suggested that a subset of neurons in
the brain of
Rhodnius
larvae, thought to be the equivalent to the
Drosophila
lateral clock neurons, harbor photosensitive clocks and establish synaptic
interactions with the PTTH-producing neurons, thereby controlling the
periodicity of PTTH release.
Since PG cells possess their own local photosensitive clock and rhythmi-
cally secrete ecdysone as well, it may seem that the role of PTTH in stim-
ulating ecdysone synthesis is redundant. However, abolishing PTTH
function, either by genetic ablation of PTTH neurons in
Drosophila
(
McBrayer et al., 2007
), or, in other insects, by decapitation and neurotoxic
treatments with tetrodotoxin (TTX) all negatively affected ecdysteroid syn-
thesis. Rather, it appears that PTTH is required for entraining the PG clock
(
Vafopoulou & Steel, 2006
). The experiments described above provided the
first molecular and physiological evidence for the existence of a peripheral
clock in the PG. While the PG maintains its own circadian rhythm, the
clock has to be calibrated with external cues that act as zeitgebers, such
as environmental signals (light) and neurosecretory input (PTTH)
(
Fig. 1.1
B). Next, we will discuss how these daily ecdysone rhythms are
transduced in the developing organism.
7. CIRCADIAN OSCILLATIONS OF EcR
In insects, the heterodimer of the nuclear receptors EcR and Usp
binds to 20E, the principal step in transducing the ecdysteroid signal in tar-
get tissues (
Riddiford, 1993; Russel, 1996
). In the section above, we have
discussed daily oscillations of ecdysone production, which leads us to the
question as to whether the ecdysteroid receptor (EcR/Usp) would also
exhibit some oscillatory behavior. Indeed, in
Rhodnius
,EcRexhibitscir-
cadian shuttling in specific tissues and thus appears to respond to or antic-
ipate the daily peaks of ecdysone. Specifically, immunostains with EcR
antibodies in epidermis cells showed that during daytime, EcR protein
levels increase and tend to accumulate in the cytoplasm until early night,
while around midnight, when ecdysone levels are high, EcR protein local-
izes to the nucleus (
Vafopoulou & Steel, 2006, 2012a
). Similar
nucleocytoplasmic shuttling of EcR protein was observed in the fat body,
