Matheson Thomas, Rogers Stephen M, Krapp Holger G
Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, United Kingdom.
J Neurophysiol. 2004 Jan;91(1):1-12. doi: 10.1152/jn.00795.2003. Epub 2003 Sep 17.
We demonstrate pronounced differences in the visual system of a polyphenic locust species that can change reversibly between two forms (phases), which vary in morphology and behavior. At low population densities, individuals of Schistocerca gregaria develop into the solitarious phase, are cryptic, and tend to avoid other locusts. At high densities, individuals develop instead into the swarm-forming gregarious phase. We analyzed in both phases the responses of an identified visual interneuron, the descending contralateral movement detector (DCMD), which responds to approaching objects. We demonstrate that habituation of DCMD is fivefold stronger in solitarious locusts. In both phases, the mean time of peak firing relative to the time to collision nevertheless occurs with a similar characteristic delay after an approaching object reaches a particular angular extent on the retina. Variation in the time of peak firing is greater in solitarious locusts, which have lower firing rates. Threshold angle and delay are therefore conserved despite changes in habituation or behavioral phase state. The different rates of habituation should contribute to different predator escape strategies or flight control for locusts living either in a swarm or as isolated individuals. For example, increased variability in the habituated responses of solitarious locusts should render their escape behaviors less predictable. Relative resistance to habituation in gregarious locusts should permit the continued responsiveness required to avoid colliding with other locusts in a swarm. These results will permit us to analyze neuronal plasticity in a model system with a well-defined and controllable behavioral context.
我们证明了一种多型蝗虫视觉系统中存在显著差异，这种蝗虫能够在两种形态（阶段）之间可逆地变化，这两种形态在形态和行为上各不相同。在低密度种群中，沙漠蝗个体发育为独居阶段，体色隐密，且倾向于避开其他蝗虫。在高密度时，个体则发育为形成群体的群居阶段。我们分析了在这两个阶段中一个已识别的视觉中间神经元——下行对侧运动检测器（DCMD）对接近物体的反应。我们证明，独居蝗虫中DCMD的习惯化程度要强五倍。然而，在这两个阶段中，相对于碰撞时间，峰值放电的平均时间在接近物体在视网膜上达到特定角度范围后，都会以相似的特征延迟出现。峰值放电时间的变化在放电率较低的独居蝗虫中更大。因此，尽管习惯化或行为阶段状态发生了变化，阈值角度和延迟仍然保持不变。不同的习惯化速率应该有助于群居或独居蝗虫采取不同的捕食者逃避策略或飞行控制。例如，独居蝗虫习惯化反应中增加的变异性应该会使它们的逃避行为更难预测。群居蝗虫对习惯化的相对抗性应该能使它们保持继续响应的能力，以避免在群体中与其他蝗虫碰撞。这些结果将使我们能够在一个具有明确且可控行为背景的模型系统中分析神经元可塑性。
