Müller W, Connor J A
Roche Institute of Molecular Biology, Department of Neurosciences, Roche Research Center, Nutley, NJ 07110.
J Physiol Paris. 1992;86(1-3):57-66. doi: 10.1016/s0928-4257(05)80008-7.
Postsynaptic Ca2+ changes are involved in control of cellular excitability and induction of synaptic long-term changes. We monitored Ca2+ changes in dendrites and spines during synaptic and direct stimulation using high resolution microfluorometry of fura-2 injected into CA3 pyramidal neurons in guinea pig hippocampal slice. When driven by current injection from an intracellular electrode or with synaptic stimulation, postsynaptic Ca2+ accumulations were highest in the proximal dendrites with a pronounced fall-off towards the soma and some fall-off towards more distal dendrites. Muscarinic activation by low concentrations of carbachol strongly increased intradendritic Ca2+ accumulation during directly-evoked repetitive firing. This enhancement occurred in large part because muscarinic activation suppressed the normal Ca(2+)-dependent activation of K-channels that mediates adaptation of firing. Repetitive firing of cholinergic fibers in the slice reproduced the effects of carbachol. Inhibition of acetylcholine-esterase activity by eserine enhanced the effects of repetitive stimulation of chlolinergic fibers. All effects were reversible and were blocked by the muscarinic antagonist atropine. Ca2+ accumulations in postsynaptic spines might be the basis of specificity of synaptic plasticity. With selective stimulation of few associative/comissural fibers, Ca2+ accumulated in single postsynaptic spines but not in the parent dendrite. With strong stimulation, dendrite levels also increased but spine levels were considerably higher. The NMDA-receptor antagonist AP-5 blocked Ca(2+)-peaks in spines, but left Ca2+ changes in dendrite shafts largely unaffected. Sustained steep Ca2+ gradients between single spines and the parent dendrite, often lasting several minutes, developed with repeated stimulation. Our results demonstrate a spine entity that can act independent from the dendrite with respect to Ca(2+)-dependent processes. Muscarinic augmentation of dendritic Ca2+ levels might reduce diffusional loss of Ca2+ from hot spines into the parent dendrite, thus supporting cooperativity and associativity of synaptic plasticity.
突触后钙离子变化参与细胞兴奋性的调控以及突触长期变化的诱导。我们利用向豚鼠海马切片的CA3锥体神经元中注射fura-2的高分辨率显微荧光测定法，监测了突触刺激和直接刺激期间树突和棘中的钙离子变化。当由细胞内电极注入电流驱动或进行突触刺激时，突触后钙离子积累在近端树突中最高，朝着胞体明显下降，朝着更远端的树突也有一些下降。低浓度卡巴胆碱引起的毒蕈碱激活在直接诱发的重复放电期间强烈增加树突内钙离子积累。这种增强很大程度上是因为毒蕈碱激活抑制了介导放电适应性的钾通道的正常钙依赖性激活。切片中胆碱能纤维的重复放电重现了卡巴胆碱的作用。毒扁豆碱抑制乙酰胆碱酯酶活性增强了胆碱能纤维重复刺激的效果。所有效应都是可逆的，并被毒蕈碱拮抗剂阿托品阻断。突触后棘中的钙离子积累可能是突触可塑性特异性的基础。选择性刺激少数联合/连合纤维时，钙离子在单个突触后棘中积累，但不在其母树突中积累。强烈刺激时，树突水平也会升高，但棘水平要高得多。NMDA受体拮抗剂AP-5阻断了棘中的钙峰，但对树突干中的钙离子变化影响不大。单次棘与其母树突之间持续陡峭的钙离子梯度，常持续数分钟，随着重复刺激而形成。我们的结果表明，在依赖钙离子的过程中，棘能够独立于树突发挥作用。树突钙离子水平的毒蕈碱增强作用可能会减少钙离子从热点棘向母树突的扩散损失，从而支持突触可塑性的协同性和关联性。
