Sanchez R M, Surkis A, Leonard C S
Department of Physiology, New York Medical College, Valhalla, New York 10595, USA.
J Neurophysiol. 1998 Jun;79(6):3111-26. doi: 10.1152/jn.1998.79.6.3111.
Increased firing of cholinergic neurons of the laterodorsal tegmental nucleus (LDT) plays a critical role in generating the behavioral states of arousal and rapid eye movement sleep. The majority of these neurons exhibit a prominent transient potassium current (IA) that shapes firing but the properties of which have not been examined in detail. Although IA has been reported to be blocked by intracellular cesium, the IA in LDT neurons appeared resistant to intracellular cesium. The present study compared the properties of this cesium-resistant current to those typically ascribed to IA. Whole cell recordings were obtained from LDT neurons (n = 67) in brain slices with potassium- or cesium-containing pipette solutions. A transient current was observed in cells dialyzed with each solution (KGluc-85%; CsGluc-79%). However, in cesium-dialyzed neurons, the transient current was inward at test potentials negative to about -35 mV. Extracellular 4-aminopyridine (4-AP; 2-5 mM) blocked both inward and outward current, suggesting the inward current was reversed IA rather than an unmasked transient calcium current as previously suggested. This conclusion was supported by increasing [K]o from 5 to 15 mM, which shifted the reversal potential positively for both inward and outward current (+17.89 +/- 0.41 mV; mean +/- SE). Moreover, recovery from inactivation was rapid (tau = 15.5 +/- 4 ms; n = 4), as reported for IA, and both inward and outward transient current persisted in calcium-free solution [0 calcium/4 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N', N'-tetraacetic acid; n = 4] and during cadmium-blockade of calcium currents (n = 3). Finally, the transient current was blocked by intracellular 4-AP indicating that adequate dialysis occurred during the recordings. Thus the Cs-resistant current is a subthreshold IA. We also estimated the voltage-dependence of activation (V1/2 = -45.8 +/- 2 mV, k = 5.21 +/- 0.62 mV, n = 6) and inactivation (V1/2 = -59. 0 +/- 2.38 mV, k = -5.4 +/- 0.49 mV, n = 3) of this current. Computer simulations using a morphologically accurate model cell indicated that except for the extreme case of only distal A-channels and a high intracellular resistivity, our parameter estimates were good approximations. In conclusion, guinea pig LDT neurons express subthreshold A-channels that are resistant to intracellular cesium ions. This suggests that these channels differ fundamentally in their ion permeation mechanism from those previously studied. It remains to be determined if Cs+ resistance is common among brain A-channels or if this property is conferred by known A-channel subunits.
外侧背盖核（LDT）胆碱能神经元放电增加在产生觉醒和快速眼动睡眠的行为状态中起关键作用。这些神经元中的大多数表现出显著的瞬时钾电流（IA），该电流塑造放电，但尚未对其特性进行详细研究。尽管据报道IA可被细胞内铯阻断，但LDT神经元中的IA似乎对细胞内铯具有抗性。本研究比较了这种耐铯电流与通常归因于IA的电流特性。使用含钾或含铯的移液管溶液从脑片中的LDT神经元（n = 67）获得全细胞记录。在用每种溶液透析的细胞中观察到瞬时电流（葡萄糖酸钾 - 85％；葡萄糖酸铯 - 79％）。然而，在铯透析的神经元中，在测试电位负于约 - 35 mV时瞬时电流是内向的。细胞外4 - 氨基吡啶（4 - AP；2 - 5 mM）阻断内向和外向电流，表明内向电流是反转的IA，而不是先前提出的未掩盖的瞬时钙电流。将[K]o从5 mM增加到15 mM支持了这一结论，这使内向和外向电流的反转电位正向移动（+17.89 +/- 0.41 mV；平均值 +/- 标准误）。此外，失活后的恢复很快（τ = 15.5 +/- 4 ms；n = 4），如IA的报道，并且内向和外向瞬时电流在无钙溶液[0钙/4 mM乙二醇双（β - 氨基乙基醚） - N，N，N'，N' - 四乙酸；n = 4]中以及在镉阻断钙电流期间（n = 3）持续存在。最后，瞬时电流被细胞内4 - AP阻断，表明在记录期间发生了充分的透析。因此，耐铯电流是阈下IA。我们还估计了该电流激活（V1/2 = - 45.8 +/- 2 mV，k = 5.21 +/- 0.62 mV，n = 6）和失活（V1/2 = - 59.0 +/- 2.38 mV，k = - 5.4 +/- 0.49 mV，n = 3）的电压依赖性。使用形态学精确的模型细胞进行的计算机模拟表明，除了仅远端A通道和高细胞内电阻率的极端情况外，我们的参数估计是良好的近似值。总之，豚鼠LDT神经元表达对细胞内铯离子具有抗性的阈下A通道。这表明这些通道的离子渗透机制与先前研究的通道有根本不同。Cs + 抗性是否在脑A通道中常见，或者该特性是否由已知的A通道亚基赋予，仍有待确定。
