Klink R, Alonso A
Department of Neurology and Neurosurgery, Montreal Neurological Institute and McGill University, Quebec, Canada.
J Neurophysiol. 1997 Apr;77(4):1829-43. doi: 10.1152/jn.1997.77.4.1829.
The mechanisms underlying direct muscarinic depolarizing responses in the stellate cells (SCs) and non-SCs of medial entorhinal cortex layer II were investigated in tissue slices by intracellular recording and pressure-pulse applications of carbachol (CCh). Subthreshold CCh depolarizations were largely potentiated in amplitude and duration when paired with a short DC depolarization that triggered cell firing. During Na+ conductance block, CCh depolarizations were also potentiated by a brief DC depolarization that allowed Ca2+ influx and the potentiation was more robust in non-SCs than in SCs. Also, in non-SCs, CCh depolarizations could be accompanied by spikelike voltage oscillations at a slow frequency. In both SCs and non-SCs, the voltage-current (V-I) relations were similarly affected by CCh, which caused a shift to the left of the steady-state V-I relations over the entire voltage range and an increase in apparent slope input resistance at potentials positive to about -70 mV. CCh responses potentiated by Ca2+ influx demonstrated a selective increase in slope input resistance at potentials positive to about -75 mV in relation to the nonpotentiated responses. K+ conductance block with intracellular injection of Cs+ (3 M) and extracellular Ba2+ (1 mM) neither abolished CCh depolarizations nor resulted in any qualitatively distinct effect of CCh on the V-I relations. CCh depolarizations were also undiminished by block of the time-dependent inward rectifier Ih, with extracellular Cs . However, CCh depolarizations were abolished during Ca2+ conductance block with low-Ca2+ (0.5 mM) solutions containing Cd2+, Co2+, or Mn2+, as well as by intracellular Ca2+ chelation with bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid. Inhibition of the Na+-K+ ATPase with strophanthidin resulted in larger CCh depolarizations. On the other hand, when NaCl was replaced by N-methyl-D-glucamine, CCh depolarizations were largely diminished. CCh responses were blocked by 0.8 microM pirenzepine, whereas hexahydro-sila-difenidolhydrochloride,p-fluoroanalog (p-F-HHSiD) and himbacine were only effective antagonists at 5- to 10-fold larger concentrations. Our data are consistent with CCh depolarizations being mediated in both SCs and non-SCs by m1 receptor activation of a Ca2+-dependent cationic conductance largely permeable to Na+. Activation of this conductance is potentiated in a voltage-dependent manner by activity triggering Ca2+ influx. This property implements a Hebbian-like mechanism whereby muscarinic receptor activation may only be translated into substantial membrane depolarization if coupled to postsynaptic cell activity. Such a mechanism could be highly significant in light of the role of the entorhinal cortex in learning and memory as well as in pathologies such as temporal lobe epilepsy.
通过细胞内记录和压力脉冲施加卡巴胆碱（CCh），在组织切片中研究了内嗅皮层II层星状细胞（SCs）和非星状细胞（non-SCs）中直接毒蕈碱去极化反应的潜在机制。当与触发细胞放电的短直流去极化配对时，阈下CCh去极化在幅度和持续时间上大多得到增强。在Na⁺电导阻断期间，短暂的直流去极化也增强了CCh去极化，该直流去极化允许Ca²⁺内流，并且在非星状细胞中这种增强比在星状细胞中更显著。此外，在非星状细胞中，CCh去极化可能伴随着低频的尖峰状电压振荡。在星状细胞和非星状细胞中，电压-电流（V-I）关系受到CCh的类似影响，这导致在整个电压范围内稳态V-I关系向左移动，并且在正于约-70 mV的电位下表观斜率输入电阻增加。由Ca²⁺内流增强的CCh反应相对于未增强的反应，在正于约-75 mV的电位下显示出斜率输入电阻的选择性增加。通过细胞内注射Cs⁺（3 M）和细胞外Ba²⁺（1 mM）阻断K⁺电导，既没有消除CCh去极化，也没有导致CCh对V-I关系产生任何质的不同影响。用细胞外Cs阻断时间依赖性内向整流Ih时，CCh去极化也没有减弱。然而，在用含有Cd²⁺、Co²⁺或Mn²⁺的低Ca²⁺（0.5 mM）溶液阻断Ca²⁺电导期间，以及通过用双（邻氨基苯氧基）-N,N,N',N'-四乙酸进行细胞内Ca²⁺螯合时，CCh去极化被消除。用毒毛花苷抑制Na⁺-K⁺ ATP酶导致更大的CCh去极化。另一方面，当用N-甲基-D-葡萄糖胺替代NaCl时，CCh去极化大大减弱。CCh反应被0.8 μM哌仑西平阻断，而六氢硅二苯胺盐酸盐、对氟类似物（p-F-HHSiD）和海巴辛仅在浓度高5至10倍时才是有效的拮抗剂。我们的数据与CCh去极化在星状细胞和非星状细胞中均由m1受体激活主要对Na⁺通透的Ca²⁺依赖性阳离子电导介导一致。这种电导的激活通过触发Ca²⁺内流的活动以电压依赖性方式增强。这种特性实现了一种类似赫布机制，即毒蕈碱受体激活可能仅在与突触后细胞活动耦合时才转化为显著的膜去极化。鉴于内嗅皮层在学习和记忆以及在诸如颞叶癫痫等病理中的作用，这样一种机制可能非常重要。
