The neurotransmitter serotonin has been implicated in a range of complex neurological disorders linked to alterations of neuronal circuitry. Serotonin is synthesized in the developing brain before most neuronal circuits become fully functional, suggesting that serotonin might play a distinct regulatory role in shaping circuits prior to its function as a classical neurotransmitter. In this study, we asked if serotonin acts as a guidance cue by examining how serotonin alters growth cone motility of rodent sensory neurons in vitro. Using a growth cone motility assay, we found that serotonin acted as both an attractive and repulsive guidance cue through a narrow concentration range. Extracellular gradients of 50 µM serotonin elicited attraction, mediated by the serotonin 5-HT2a receptor while 100 µM serotonin elicited repulsion mediated by the 5-HT1b receptor. Importantly, high resolution imaging of growth cones indicated that these receptors signalled through their canonical pathways of endoplasmic reticulum-mediated calcium release and cAMP depletion, respectively. This novel characterisation of growth cone motility in response to serotonin gradients provides compelling evidence that secreted serotonin acts at the molecular level as an axon guidance cue to shape neuronal circuit formation during development.

神经递质血清素已牵涉到一系列与神经元回路改变有关的复杂神经系统疾病。在大多数神经元回路完全发挥功能之前，5-羟色胺是在发育中的大脑中合成的，这表明5-羟色胺在其作为经典神经递质起作用之前，可能在塑造回路中起独特的调节作用。在这项研究中，我们通过检查5-羟色胺如何在体外改变啮齿动物感觉神经元的生长锥运动来询问5-羟色胺是否作为指导提示。使用生长锥运动测定法，我们发现5-羟色胺在狭窄的浓度范围内既具有吸引力，又具有排斥性。50 µM血清素的细胞外梯度引起吸引，血清素5-HT2a受体介导的排斥反应，而100 µM血清素引起5-HT1b受体介导的排斥。重要的是，生长锥的高分辨率成像表明，这些受体分别通过内质网介导的钙释放和cAMP耗竭的经典途径发出信号。这种对5-羟色胺梯度响应的生长锥运动性的新颖表征提供了令人信服的证据，即分泌的5-羟色胺在分子水平上充当轴突指导线索，可在发育过程中塑造神经元回路的形成。