Organization of the Circadian System in Insects

Abstract

The circadian systems of different insect groups are summarized and compared. Emphasis is placed on the anatomical identification and characterization of circadian pacemakers, as well as on their entrainment, coupling, and output pathways. Cockroaches, crickets, beetles, and flies possess bilaterally organized pacemakers in the optic lobes that appear to be located in the accessory medulla, a small neuropil between the medulla and the lobula. Neurons that are immunoreactive for the peptide pigment-dispersing hormone (PDH) arborize in the accessory medulla and appear to be important components of the optic lobe pacemakers. The neuronal architecture of the accessory medulla with associated PDH-immunoreactive neurons is best characterized in cockroaches, while the molecular machinery of rhythm generation is best understood in fruit flies. One essential component of the circadian clock is the period protein (PER), which colocalizes with PDH in about half of the fruit fly's presumptive pacemaker neurons. PER is also found in the presumptive pacemaker neurons of beetles and moths, but appears to have different functions in these insects. In moths, the pacemakers are situated in the central brain and are closely associated with neuroendocrine functions. In the other insects, neurons associated with neuroendocrine functions also appear to be closely coupled to the optic lobe pacemakers. Some crickets and flies seem to possess central brain pacemakers in addition to their optic lobe pacemakers. With respect to neuronal organization, the circadian systems of insects show striking similarities to the vertebrate circadian system.

Polarization-Sensitive Descending Neurons in the Locust: Connecting the Brain to Thoracic Ganglia

“…El Jundi and Homberg (2010) provided evidence for the existence of a second polarization vision pathway in locusts that connects the accessory medulla to the central complex. In the fruit fly Drosophila melanogaster and the cockroach Leucophaea maderae, the accessory medulla is the site of the circadian clock controlling circadian changes in locomotion and other behaviors (Helfrich-Förster et al, 1998;Homberg et al, 2003, Helfrich-Förster, 2004, but whether this is also true for locusts remains to be seen.…”

Circadian clocks in crustaceans: identified neuronal and cellular systems

“…57,[316][317][318]. Notwithstanding the differences that exist between insect species with regard to the distinct cellular locations and distributions of neuronal clock elements and regulatory clock genes or their products in the brain and/or the optic lobe (57,(316)(317)(318)(319), commonalities have perhaps best been documented for the locations of PDH/PDF-ir neurons close to the optic lobes and the role of PDF as a clock output factor (57,(316)(317)(318). The bearings on homologies and functional similarity of these neurons are discussed below (see 4.2.…”

“…The Drosophila clock neuron clusters of a few morphologically and functionally distinct s-/l-LN v s and LN d s are likely a highly adapted clock system. However, PDH/PDF-ir neurons located next to the optic ganglia usually occur in most other insects (57,(316)(317)(318) and in diverse crustaceans (138,140,141) in much larger numbers. They could, thus, represent a clock organisation less specialised than that in Drosophila.…”

Circadian Organization in Hemimetabolous Insects

“…This general scheme led studies for elucidating the localization of the constituents and their interaction to understand the underlying neural mechanism. Hemimetabolous insects such as cockroaches and crickets have been served as good experimental animals to analyze those physiological mechanisms at a neural level (Page, 1985a;Helfrich-Förster et al, 1998;Tomioka et al, 2001;Homberg et al, 2003). Their relatively large nervous system with identifiable neurons allowed the search for constituents of the circadian system in the CNS.…”