Biology Reference
In-Depth Information
luciferase
reporter gene. To test whether theCNS influences these oscillations,
ex vivo
organ cultures of isolated RGs were compared to RGs that were still
attached to the CNS (CNS-RG). In either case, the oscillation of
period
transcript levels was stable for many days. Interestingly, in CNS-RGs,
period
oscillations started in the proximal part of the RG, corresponding to where
projections from the brain innervate theRG. Subsequently, these oscillations
then propagate in a wave-like manner to the distal part of the RG. However,
when RGs were not attached to the CNS, this spatial phasing was lost, and
instead oscillations occurred uniformly and synchronized across PG cells.
These findings indicate that
period
transcript oscillations in PGs are self-
sustained and that signals from the brain determine how these transcriptional
rhythms are spatially synchronized across PG cells.
In the same report, the CNS was also shown to be required for relaying a
light-dark transition response to the prothoracic glands. When CNS-RGs
were moved from LD to DD conditions, the first 12-h cycle resulted in a
high amplitude of
period
transcript levels, which then rapidly declined to a
lower but stable level under constant darkness (DD). This light-dark tran-
sition response did not occur when brain-derived signals where blocked
either by administering the Na
þ
channel inhibitor TTX or the L-type
Ca
2
þ
channel inhibitor nimodipine, or when RGs were cultured in isolation
from the CNS. Clearly, exposing CNS-RGs to light relayed photic infor-
mation to the RG, and indeed, a mutation (
cry
b
) in the blue light photore-
ceptor gene
cryptochrome
also abolished the light-dark transition response.
To further characterize the oscillatory response of
period
to light, the
authors exposed CNS-RGs to 30 min light pulses either shortly before a
peak of
period
expression, or shortly after such a peak. This treatment affected
the phase of the following period peaks, resulting in delayed or advanced
peaks, respectively. Similar to the above series of experiments, the effect
of the light pulse was abolished using pharmacological inhibitors of synaptic
transmission, or treating RGs in isolation. Likewise, removing
cryptochrome
function resulted in strongly diminished phase shifts, demonstrating that the
CNS plays a key role in receiving and processing light information that is
relayed to the PG. However, this does not appear to generally true for all
peripheral clocks, since the authors go on to show that the Malpighian
tubules are in fact photoresponsive and thus independent from the CNS.
How do these findings relate to developmental timing? The authors
argue that the phase shifts observed after administering light pulses corre-
spond well to earlier findings that showed light-induced shifts in eclosion
rhythms of
Drosophila
populations (
Pittendrigh, 1964; Winfree, 1970
). As
