Barnstable Colin J, Wei Ji-Ye, Han Ming-Hu
Department of Ophthalmology and Visual Science, Yale University School of Medicine, 330 Cedar Street, New Haven, CT 06520-8061, USA.
Neurochem Int. 2004 Nov;45(6):875-84. doi: 10.1016/j.neuint.2004.03.018.
Cyclic nucleotide-gated cation channels have been studied intensively in the primary sensory neurons of the visual and olfactory systems. Using both anatomical and physiological methods we have shown that they have a much more widespread distribution in the nervous system. In many retinal ganglion cells cGMP, but not cAMP, activates a non-selective conductance that has many of the properties of CNG channels. As many neurons also contain cGMP-dependent protein kinases (PKGs), we have used a variety of cGMP analogues to distinguish the actions of cGMP. Sp-8-Br-PET-cGMPS is a potent non-hydrolyzable cGMP analogue that is an agonist of PKG. We found that Sp-8-Br-PET-cGMPS acts as a competitive inhibitor of at least the rod CNG channel. Rp-8-Br-cGMPS has shown the opposite effects, namely as an agonist of the rod CNG channel and an inhibitor of PKG. In dissociated cell cultures and slices of rodent visual cortex cGMP had multiple rapid and reversible effects on transmission at glutamatergic synapses. Extracellular application of 8-Br-cGMP or Sp-8-Br-PET-cGMPS reduced stimulus evoked EPSPs in cortical slices. In cortical cultures both analogs reduced the frequency of spontaneous EPSCs, but not their amplitude. The effects on both EPSPs and EPSCs were presynaptic. The effects on evoked EPSPs may be due, in part, to reduced calcium influx through voltage-gated calcium channels. The effects on spontaneous EPSCs may be due, in part, to modulation of calcium fluxes through internal stores. Similar modulations of synaptic transmission have been found at gabaergic synapses. On postsynaptic cells, PKG activation produced a dramatic enhancement of the responses to applied NMDA. No effects were detected on applied AMPA/kainate or GABA. Together the results suggest that cGMP may use multiple mechanisms to modulate synaptic efficacy and that its actions may include regulating synaptic plasticity and the relative strength of excitatory and inhibitory drive through neural pathways.
环核苷酸门控阳离子通道已在视觉和嗅觉系统的初级感觉神经元中得到深入研究。通过解剖学和生理学方法，我们发现它们在神经系统中的分布更为广泛。在许多视网膜神经节细胞中，cGMP而非cAMP可激活一种具有许多CNG通道特性的非选择性电导。由于许多神经元也含有cGMP依赖性蛋白激酶（PKG），我们使用了多种cGMP类似物来区分cGMP的作用。Sp-8-Br-PET-cGMPS是一种有效的不可水解cGMP类似物，是PKG的激动剂。我们发现Sp-8-Br-PET-cGMPS至少对视杆细胞CNG通道起竞争性抑制剂的作用。Rp-8-Br-cGMPS则表现出相反的作用，即作为视杆细胞CNG通道的激动剂和PKG的抑制剂。在啮齿动物视觉皮层的解离细胞培养物和切片中，cGMP对谷氨酸能突触传递有多种快速且可逆的影响。在皮层切片中，细胞外应用8-Br-cGMP或Sp-8-Br-PET-cGMPS可降低刺激诱发的兴奋性突触后电位（EPSP）。在皮层培养物中，这两种类似物均降低了自发兴奋性突触后电流（EPSC）的频率，但不影响其幅度。对EPSP和EPSC的影响均为突触前效应。对诱发EPSP的影响可能部分归因于通过电压门控钙通道的钙内流减少。对自发EPSC的影响可能部分归因于通过内部储存库对钙通量的调节。在GABA能突触处也发现了类似的突触传递调节。在突触后细胞上，PKG激活可显著增强对应用NMDA的反应。对应用的AMPA/海人藻酸或GABA未检测到影响。这些结果共同表明，cGMP可能利用多种机制调节突触效能，其作用可能包括调节突触可塑性以及通过神经通路调节兴奋性和抑制性驱动的相对强度。
