Baines Richard A
Neuroscience Group, Department of Biological Sciences, University of Warwick, Coventry, CV4 7AL, United Kingdom.
J Neurosci. 2003 Sep 24;23(25):8664-72. doi: 10.1523/JNEUROSCI.23-25-08664.2003.
Previous work has identified a role for synaptic activity in the development of excitable properties of motoneurons in the Drosophila embryo. In this study the underlying mechanism that enables two such neurons, termed aCC and RP2, to respond to increased exposure to synaptic excitation is characterized. Synaptic excitation is increased in genetic backgrounds that lack either a cAMP-specific phosphodiesterase (EC:3.1.4, dunce) or acetylcholinesterase (EC:3.1.1.7, ace), the enzyme that terminates the endogenous cholinergic excitation of these motoneurons. Analysis of membrane excitability in aCC/RP2, in either background, shows that these neurons have a significantly reduced capability to fire action potentials (APs) in response to injection of depolarizing current. Analysis of underlying voltage-gated currents show that this effect is associated with a marked reduction in magnitude of the voltage-dependent inward Na+ current (INa). Partially blocking INa in these motoneurons, using low concentrations of TTX, demonstrates that a reduction of INa is, by itself, sufficient to reduce membrane excitability. An analysis of firing implicates an increased AP threshold to underlie the reduction in membrane excitability observed because of heightened exposure to synaptic excitation. Genetic or pharmacological manipulations that either elevate cAMP or increase protein kinase A (PKA) activity in wild-type aCC/RP2 mimic both the reductions in membrane excitability and INa. In comparison, increasing cAMP catabolism or inhibition of PKA activity is sufficient to block the induction of these activity-dependent changes. The induced changes in excitability can be rapid, occurring within 5 min of exposure to a membrane-permeable cAMP analog, indicative that threshold can be regulated in these neurons by a post-translational mechanism that is dependent on phosphorylation.
先前的研究已经确定了突触活动在果蝇胚胎运动神经元兴奋性特性发育中的作用。在本研究中，对使两个这样的神经元（称为aCC和RP2）能够对增加的突触兴奋暴露做出反应的潜在机制进行了表征。在缺乏环磷酸腺苷特异性磷酸二酯酶（EC：3.1.4，傻瓜）或乙酰胆碱酯酶（EC：3.1.1.7，王牌）的遗传背景中，突触兴奋增加，乙酰胆碱酯酶是终止这些运动神经元内源性胆碱能兴奋的酶。在任何一种背景下对aCC/RP2中的膜兴奋性进行分析表明，这些神经元在响应去极化电流注入时产生动作电位（AP）的能力显著降低。对潜在电压门控电流的分析表明，这种效应与电压依赖性内向钠电流（INa）幅度的显著降低有关。使用低浓度的河豚毒素（TTX）部分阻断这些运动神经元中的INa表明，INa的减少本身就足以降低膜兴奋性。对放电的分析表明，AP阈值升高是由于暴露于突触兴奋增强而观察到的膜兴奋性降低的基础。在野生型aCC/RP2中，提高cAMP或增加蛋白激酶A（PKA）活性的基因或药理学操作模拟了膜兴奋性和INa的降低。相比之下，增加cAMP分解代谢或抑制PKA活性足以阻断这些活动依赖性变化的诱导。兴奋性的诱导变化可以很快发生，在暴露于膜通透性cAMP类似物后5分钟内就会出现，这表明阈值可以通过依赖于磷酸化的翻译后机制在这些神经元中得到调节。
