Circadian light-input pathways in Drosophila

Abstract

Light is the most important environmental cue to entrain the circadian clock in most animals. In the fruit fly Drosophila melanogaster, the light entrainment mechanisms of the clock have been well-studied. The Drosophila brain contains approximately 150 neurons that rhythmically express circadian clock genes. These neurons are called "clock neurons" and control behavioral activity rhythms. Many clock neurons express the Cryptochrome (CRY) protein, which is sensitive to UV and blue light, and thus enables clock neurons deep in the brain to directly perceive light. In addition to the CRY protein, external photoreceptors in the Drosophila eyes play an important role in circadian light-input pathways. Recent studies have provided new insights into the mechanisms that integrate these light inputs into the circadian network of the brain. In this review, we will summarize the current knowledge on the light entrainment pathways in the Drosophila circadian clock.

Keywords: Drosophila melanogaster; circadian clock; circadian rhythm; cryptochrome; light entrainment; photoreceptor; pigment-dispersing factor.

Figure 1.

The circadian clock network in the Drosophila brain. The left hemisphere shows the distribution of the Cryptochrome (CRY)-positive (blue) and CRY-negative (white) clock neurons. The right hemisphere shows the compound eye and the Hofbauer-Buchner (HB) eyelet. The ocelli are located on top of the brain. The small and large lateral neuron (s-LN_v and l-LN_v) clusters express the Pigment-dispersing factor (PDF) protein and are regarded as the clock neurons controlling the morning (M) activity peak. The lateral dorsal neurons (LN_d) and the 5^th s-LN_v are the evening (E) clock neurons. The dorsal neuron (DN) clusters are not well characterized. The HB eyelet is directly connected to the s-LN_v neurons through projections to the surrounding area. In contrast, the connections between the compound eyes and the clock neurons have not yet been elucidated.

Figure 2.

Three light-input pathways to the Drosophila clock. CRY is expressed in many clock neurons, which transmit light information to CRY-negative clock neurons. Histamine, dopamine, and serotonin have been suggested as neurotransmitters that convey light inputs from the compound eyes. The HB eyelets use acetylcholine and histamine as neurotransmitters and directly target the PDF neurons. PDF plays a role in intercellular communication between the PDF neurons and other clock neurons that express the PDF receptor.