A culmination of temporal adaptations to cyclic environment in living organisms is circadian oscillation. The clock consists of three functional units: input, clock per se and output system. The general view of circadian oscillation is based on the assumption that rhythmicity is generated by negative feedback of clock gene transcription by the protein product. This model was originally proposed for Drosophila melanogaster but recent data support that a negative feedback system constitutes the core mechanism for circadian oscillation in other groups of organisms. Even though different organisms possess the same set of genes, the components are assembled in unique ways, or otherwise the respective genes and their products serve different functions. The differences may be related to post-translational modifications, subcellular localization and specific role of the protein within them. The focus of this review is not to provide an extensive bibliography but rather attempts to present data suggestive of possible output regulatory pathway of the clock in insects. We put special emphasis on the indolamine pathway leading to melatonin synthesis and the penultimate enzyme for melatonin synthesis, arylalkylamine N-acetyltransferase (NAT) in circadian responses.
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