Neural mechanism underlying re-depression of crayfish tailflip habituation

当对它们的尾扇施加强烈刺激时，小龙虾会通过翻转尾巴向上和向前逃跑。这种逃逸行为是由一对巨纤维——侧向巨纤维（LGs）介导的。LGs通过直接的电突触和化学突触接收来自感觉传入的机械感觉信息，并通过上行的中间神经元接收机械感觉信息。5-羟色胺（5-HT）增加了LGs对感觉刺激的EPSP振幅，但它们的静息电位和膜阻力保持一致。为了分析5-HT调节的本质，我们分析了LG逃逸回路神经元在膜透性cAMP类似物、CPT-cAMP和cAMP磷酸二酯酶抑制剂IBMX混合作用下的感觉反应。与5-HT灌注一样，cAMP模拟物灌注使LGs的EPSP振幅增加。生理盐水洗涤约30 min后，LGs反应的增强减弱。cAMP模拟物也增强了中间神经元A和C的感觉反应。提示5-羟色胺增加LG逃逸回路内禀cAMP水平。生物胺、5-羟色胺（5-HT）或章鱼胺（OA）的沐浴应用增加了LG epsp对感觉刺激的振幅。为了研究这些生物胺的作用，我们分析了膜透性cAMP类似物（CPT-cAMP）和cAMP磷酸二酯酶抑制剂（IBMX）或cGMP类似物（8-Br-cGMP）单独混合作用下LG的活性。cAMP类似物增加了LG活性，而cGMP类似物没有作用。腺苷酸环化酶抑制剂SQ22536抑制5-HT的作用，但对OA无影响。通过离子电泳将cAMP类似物Sp-cAMP直接注入LG，模拟了5-HT的调节作用。这些结果表明5-HT通过提高LG本身的内在cAMP水平来增加LG活性。

Crayfish escape up and forwards by flipping its tail when intense stimulus is applied to their tailfan. This escape behavior is mediated by a pair of giant fiber, lateral giants (LGs). The LGs receive mechanosensory information via direct electrical and chemical synapses from the sensory afferents and disynaptically via ascending interneurones. Bath application of serotonin (5-HT) increased the EPSP amplitude of LGs in response to the sensory stimulation though their resting potential and membrane resistance remained consistent. To analyze the nature of 5-HT modulation, sensory responses of LG escape circuit neurones were analyzed under the mixture of membrane permeable cAMP analog, CPT-cAMP and cAMP phosphodiesterase inhibitor IBMX. As 5-HT superfusion, EPSP amplitude of LGs was increased by cAMP analog superfusion. The increase of response of LGs was reduced after about 30 min of washing with normal saline. The sensory responses of interneurones A and C also increased by cAMP analog. These results suggest 5-HT increases intrinsic level of cAMP in LG escape circuit.Bath application of biogic amines, serotonin (5-HT) or octopamine (OA) increased amplitude of LG EPSPs in response to sensory stimulation. To investigate the action of these biogic amines, LG activity was analyzed under the mixture of membrane permeable cAMP analogue (CPT-cAMP) and cAMP phosphodiesterase inhibitor (IBMX) or cGMP analogue (8-Br-cGMP) alone. The cAMP analogue increased LG activity while cGMP analogue had no effect. Adenylate cyclase inhibitor SQ22536 inhibited the effect of 5-HT but had no effect on OA. Iontophoretic injection of cAMP analogue, Sp-cAMP, directly into LG mimicked the modulatory effect of 5-HT. These results suggest 5-HT increases LG activity by promoting intrinsic level of cAMP in LG itself.