Rescheduling Behavioral Subunits of a Fixed Action Pattern by Genetic Manipulation of Peptidergic Signaling

The ecdysis behavioral sequence in insects is a classic fixed action pattern (FAP) initiated by hormonal signaling. Ecdysis triggering hormones (ETHs) release the FAP through direct actions on the CNS. Here we present evidence implicating two groups of central ETH receptor (ETHR) neurons in scheduling the first two steps of the FAP: kinin (aka drosokinin, leucokinin) neurons regulate pre-ecdysis behavior and CAMB neurons (CCAP, AstCC, MIP, and Bursicon) initiate the switch to ecdysis behavior. Ablation of kinin neurons or altering levels of ETH receptor (ETHR) expression in these neurons modifies timing and intensity of pre-ecdysis behavior. Cell ablation or ETHR knockdown in CAMB neurons delays the switch to ecdysis, whereas overexpression of ETHR or expression of pertussis toxin in these neurons accelerates timing of the switch. Calcium dynamics in kinin neurons are temporally aligned with pre-ecdysis behavior, whereas activity of CAMB neurons coincides with the switch from pre-ecdysis to ecdysis behavior. Activation of CCAP or CAMB neurons through temperature-sensitive TRPM8 gating is sufficient to trigger ecdysis behavior. Our findings demonstrate that kinin and CAMB neurons are direct targets of ETH and play critical roles in scheduling successive behavioral steps in the ecdysis FAP. Moreover, temporal organization of the FAP is likely a function of ETH receptor density in target neurons.

昆虫蜕皮行为序列是一类经典的固定动作模式（Fixed Action Pattern, FAP），其启动依赖于激素信号传导。蜕皮触发激素（Ecdysis Triggering Hormone, ETHs）通过直接作用于中枢神经系统（Central Nervous System, CNS）触发该固定动作模式的发生。在此，我们提供证据表明，两类中枢ETH受体（Ecdysis Triggering Hormone Receptor, ETHR）神经元参与调控该固定动作模式前两步的时序安排：激肽（又称果蝇激肽、亮激肽）神经元调控蜕皮前行为，而CAMB神经元（包含CCAP、AstCC、MIP及Bursicon相关神经元）则启动向蜕皮行为的转换。消融激肽神经元或改变此类神经元中ETH受体的表达水平，均可改变蜕皮前行为的时序与强度。对CAMB神经元进行细胞消融或ETH受体敲低，会延迟向蜕皮行为的转换；而在这类神经元中过表达ETH受体或表达百日咳毒素，则会加快转换的时序。激肽神经元内的钙动态与蜕皮前行为在时间上完全契合，而CAMB神经元的活动则与从蜕皮前行为向蜕皮行为的转换同步。通过温度敏感型TRPM8门控通道激活CCAP神经元或CAMB神经元，足以触发蜕皮行为。本研究结果表明，激肽神经元与CAMB神经元是ETH的直接作用靶点，并在调控蜕皮固定动作模式的连续行为步骤时序中发挥关键作用。此外，该固定动作模式的时间组织架构，可能取决于靶神经元中ETH受体的表达密度。