, 2003). ETH-R is alternatively spliced into two variant protein isoforms and, in Drosophila, the two proteins display largely nonoverlapping patterns of expression in the CNS. ETH-RA has been most extensively described: remarkably it is largely confined to diverse sets of peptidergic (DIMM-positive) neuroendocrine neurons ( Park et al., 2008). These ETH targets include identified neurons that (differentially) express large amounts of the peptides dFMRFa, leukokinins, CAPA peptides and EH ( Kim et al., 2006a). Thus, rather than targeting selleck chemicals llc the motor, or even the immediate

premotor elements of the CPG that drives rhythmic ecdysial muscle activity, ETH modulation focuses on a collection of peptidergic elements as immediate targets. ETH modulation represents the temporal orchestration among these diverse peptidergic elements ( Figure 2). To visualize such orchestration, Kim et al. (2006b) monitored GCAmP fluorescence intensity as a proxy for cellular activity in ETH-RA expressing neurons following exposure to ETH in vitro. They described a remarkable and highly predictable order by which the different ETH peptidergic targets displayed transient activation over the course of Selleckchem Nintedanib the ∼30 min subsequent to ETH exposure. Thus, despite the fact that each group visualized expresses ETH-RA, there must exist parallel inhibitory interactions established within the target network to assure an

orderly temporal activation and thus, a proper sequencing of target peptide actions (Figure 2). To support their hypothesis, Kim et al. through (2006b) showed that RNAi knockdown of individual peptide targets (e.g., knockdown of dFMRFa or of leukokinin) produced partial deficits in ecdysial

behavior, deficits entirely consistent with a presumption of their sequential recruitment by the command chemical ETH. Thus in this highly detailed model of peptide modulation underlying the release of a complex innate behavior, the following model is put forth. The command chemical (neuropeptide ETH) directly activates a series of secondary neuropeptide messengers, and does so reliant on circuit interactions among targets that assure their proper temporal ordering. With the strong evidence that numerous different neuropeptides act as command signals to trigger innate behaviors, an important emerging question becomes—under what conditions are such critical factors released? From the study of insect ecdysis, there is excellent cellular, genetic, and endocrinological evidence to suggest that positive feedback loops are employed for a stepwise, incremental approach to reach a threshold for peptide release. The best evidence is found in the control of ecdysis behavior in Lepidoptera. As mentioned above, in Manduca, injection of either EH or ETH can elicit complete ecdysis routines (albeit with different latencies). In part this reflects the fact that both peptides are positive regulators of release of the other.