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