Functions of corazonin and histamine in light entrainment of the circadian pacemaker in the Madeira cockroach, Rhyparobia maderae

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Lesion and transplantation experiments located the circadian pacemaker center in the Madeira cockroach Rhyparobia (Leucophaea) maderae to the accessory medulla (AME, plural AMAE), a small, pear‐shaped neuropil at the ventromedial edge of the ME (Nishiitsutsuji‐Uwo and Pittendrigh, 1968; Page, 1982; Reischig and Stengl, 2003a). The AME is innervated by ∼240 neurons that belong to seven soma groups based on location, size, and glial sheaths (Reischig and Stengl, 2003b; Soehler et al., 2008). AME neurons express an unusually high number of partially colocalized neuropeptides (Petri et al., 1995; Hofer and Homberg, 2006b; Soehler et al., 2007; Schulze et al., 2012; Schendzielorz and Stengl, 2014). Most is known about pigment‐dispersing factor (PDF), the most important insect circadian coupling factor (for reviews see Helfrich‐Förster 2014; Stengl et al., 2015). PDF is synthesized in two soma groups in the lamina and two groups next to the AME (Reischig et al., 2004). The PDF‐immunoreactive (ir) neurons take part in photic and nonphotic input and output circuits of the circadian clock and are essential for the circadian control of locomotor activity rhythms of the Madeira cockroach (Stengl and Homberg, 1994; Reischig and Stengl, 2003a). In Drosophila melanogaster, PDF‐ir neurons serve the same conserved functions in the circadian clock network, in which the PDF receptor is expressed in about 60% of all clock neurons (Renn et al., 1999; Lin et al., 2004; Im et al., 2011). Furthermore, PDF is a functional ortholog of the mammalian neuropeptide vasoactive intestinal (poly)peptide, the most important circadian coupling factor in vertebrates (Vosko et al., 2007; Meelkop et al., 2011).
In addition to PDF, the circadian functions of the neuropeptides allatotropin (AT), members of the myoinhibitory peptides (MIPs; allatostatin‐B), and orcokinin (ORC) as well as the neurotransmitter GABA were studied in the Madeira cockroach (for review see Stengl et al., 2015). Injections of neuropeptides/neurotransmitters during different circadian times (CTs; in constant darkness [DD]) in combination with running‐wheel assays revealed CT‐dependent phase shifts of locomotor activity onset (Petri et al., 2002; Hofer and Homberg, 2006a; Schulze et al., 2013; Schendzielorz and Stengl, 2014; Schendzielorz et al., 2014). The resulting phase‐response‐curves (PRCs) allow assignment of functional roles to the applied neuroactive substances. They even provide evidence of when the respective substances play a role in the circadian network and, therefore, indicate when they might be released. In addition, clock inputs and outputs can be distinguished with this assay. Phase advances at dawn were obtained via injections of GABA, PDF, ORC‐2, and MIP‐2, whereas delays at dusk were generated via GABA, PDF, ORC‐1/2, MIP‐1, and AT.
Light is the most important phase‐shifting stimulus to the circadian clock that synchronizes it to the external rhythms of day and night. Application of light pulses to cockroaches in running wheels in DD produced a characteristic PRC that is shared among insects and mammals (for reviews see Vansteensel et al., 2008; Golombek and Rosenstein, 2010; Stengl et al., 2015). Light delays locomotor activity rhythm during the early night (dusk), whereas locomotor activity advances during the late night and early morning (dawn). Thus, we hypothesize that light entrainment of the cockroach circadian clock occurs via two distinct pathways at dusk and dawn, based on neuropeptidergic neurons that either delay or advance the clock (Stengl et al., 2015). As lesions show, light entrainment occurs in the Madeira cockroach exclusively via the ipsi‐ and contralateral compound eyes (Roberts, 1965; Loesel and Homberg, 1999). Nevertheless, light entrainment pathways are still not well understood.
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