Identifying Specific Light Inputs for Each Subgroup of Brain Clock Neurons inDrosophilaLarvae

Dec 2, 2011The Journal of neuroscience : the official journal of the Society for Neuroscience

Finding which light signals affect different groups of brain clock neurons in fruit fly larvae

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Abstract

In the absence of functional cryptochrome (CRY), synchronization of the DN1 neurons in Drosophila larvae requires the pigment-dispersing factor (PDF).

Opsin visual photopigments and blue-light-sensitive cryptochrome contribute to the synchronization of circadian clocks in Drosophila.
Five lateral neurons (LNs) express PDF, while CRY is present only in PDF-expressing LNs and DN1s.
Synchronization of DN2 neurons requires PDF signaling when CRY is not functional.
The fifth LN, which does not express PDF, can still receive visual input independently of PDF-expressing cells.
Light-dark cycles entrain all clock neurons in specific mutant combinations, but a triple mutant lacks circadian responses.
Constant activation of the visual system suppresses molecular oscillations in PDF-expressing LNs, a response that is CRY-dependent in adults.

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In Drosophila, opsin visual photopigments as well as blue-light-sensitive cryptochrome (CRY) contribute to the synchronization of circadian clocks. We focused on the relatively simple larval brain, with nine clock neurons per hemisphere: five lateral neurons (LNs), four of which express the pigment-dispersing factor (PDF) neuropeptide, and two pairs of dorsal neurons (DN1s and DN2s). CRY is present only in the PDF-expressing LNs and the DN1s. The larval visual organ expresses only two rhodopsins (RH5 and RH6) and projects onto the LNs. We recently showed that PDF signaling is required for light to synchronize the CRY(-) larval DN2s. We now show that, in the absence of functional CRY, synchronization of the DN1s also requires PDF, suggesting that these neurons have no direct connection with the visual system. In contrast, the fifth (PDF(-)) LN does not require the PDF-expressing cells to receive visual system inputs. All clock neurons are light-entrained by light-dark cycles in the rh5(2);cry(b), rh6(1) cry(b), and rh5(2);rh6(1) double mutants, whereas the triple mutant is circadianly blind. Thus, any one of the three photosensitive molecules is sufficient, and there is no other light input for the larval clock. Finally, we show that constant activation of the visual system can suppress molecular oscillations in the four PDF-expressing LNs, whereas, in the adult, this effect of constant light requires CRY. A surprising diversity and specificity of light input combinations thus exists even for this simple clock network.

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Identifying Specific Light Inputs for Each Subgroup of Brain Clock Neurons inDrosophilaLarvae

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