Parfitt K D, Hoffer B J, Browning M D
University of Colorado Health Sciences Center, Department of Pharmacology, Denver 80262.
Proc Natl Acad Sci U S A. 1991 Mar 15;88(6):2361-5. doi: 10.1073/pnas.88.6.2361.
A number of recent reports have suggested that norepinephrine (NE) produces a form of synaptic enhancement that resembles long-term potentiation (LTP). LTP, thought to be an electrophysiological correlate of memory, in part involves an augmentation of transmitter release. Although the effects of NE have not been unequivocally linked to LTP, it is clear that NE can produce increased transmitter release in the dentate gyrus of the hippocampus. The purpose of this study was to determine whether NE was capable of enhancing the phosphorylation of synapsin I and synapsin II, two homologous phosphoproteins thought to be involved in modulation of neurotransmitter release. NE (10 microM) and isoproterenol (250 nM) produced an increase in the phosphorylation of synapsin I and synapsin II in dentate slices from young rats. Phosphorylation site analysis of synapsin I, performed by limited proteolysis, indicated that NE and isoproterenol increased the phosphorylation of synapsin I at sites modified by Ca2+/calmodulin-dependent protein kinase II as well as cAMP-dependent protein kinase. These data demonstrate that NE stimulates the phosphorylation of synapsin I at its Ca2+/calmodulin-dependent protein kinase II site, which is a site that has been shown to regulate the effect of synapsin I on neurotransmitter release. We have also examined the effects of NE and isoproterenol on synapsin phosphorylation in dentate slices prepared from aged animals. Such animals have previously been shown to exhibit deficits in NE sensitivity as well as significant impairment in their ability to exhibit LTP. Neither NE nor isoproterenol stimulated synapsin phosphorylation in slices prepared from aged animals. Interestingly, the basal level of phosphorylation of the synapsin proteins was higher in slices prepared from aged animals. This higher basal level of phosphorylation may underlie the failure of aged animals to exhibit NE-stimulated increases in phosphorylation of the synapsin proteins. We hypothesize that the beta-adrenergic agonist-stimulated phosphorylation of synapsin I and synapsin II in young rats plays a role in the increase in transmitter release produced by NE in the dentate. Thus, the failure of the aged rats to show such phosphorylation may underlie, in part, their failure to exhibit normal responsiveness to NE. Moreover, these deficits in synapsin phosphorylation may also play some role in the deficits in plasticity seen in aged rats.
最近的一些报告表明，去甲肾上腺素（NE）可产生一种类似于长时程增强（LTP）的突触增强形式。LTP被认为是记忆的电生理相关指标，部分涉及递质释放的增强。尽管NE的作用尚未明确与LTP相关联，但很明显NE可使海马齿状回中的递质释放增加。本研究的目的是确定NE是否能够增强突触素I和突触素II的磷酸化，这两种同源磷蛋白被认为参与神经递质释放的调节。NE（10微摩尔）和异丙肾上腺素（250纳摩尔）可使幼鼠齿状切片中突触素I和突触素II的磷酸化增加。通过有限蛋白酶解对突触素I进行的磷酸化位点分析表明，NE和异丙肾上腺素可增加突触素I在由Ca2+/钙调蛋白依赖性蛋白激酶II以及cAMP依赖性蛋白激酶修饰的位点的磷酸化。这些数据表明，NE刺激突触素I在其Ca2+/钙调蛋白依赖性蛋白激酶II位点的磷酸化，该位点已被证明可调节突触素I对神经递质释放的作用。我们还研究了NE和异丙肾上腺素对老年动物制备的齿状切片中突触素磷酸化的影响。此前已表明，此类动物表现出NE敏感性缺陷以及其表现出LTP的能力显著受损。在老年动物制备的切片中，NE和异丙肾上腺素均未刺激突触素磷酸化。有趣的是，老年动物制备的切片中突触素蛋白的基础磷酸化水平较高。这种较高的基础磷酸化水平可能是老年动物未能表现出NE刺激的突触素蛋白磷酸化增加的原因。我们推测，β-肾上腺素能激动剂刺激幼鼠突触素I和突触素II的磷酸化在NE在齿状回中产生的递质释放增加中起作用。因此，老年大鼠未能表现出这种磷酸化可能部分是其对NE表现出正常反应性失败的原因。此外，突触素磷酸化的这些缺陷也可能在老年大鼠中观察到的可塑性缺陷中起一定作用。
