Hsiao C F, Del Negro C A, Trueblood P R, Chandler S H
Department of Physiological Science, University of California at Los Angeles, Los Angeles, 90095-1568, USA.
J Neurophysiol. 1998 Jun;79(6):2847-56. doi: 10.1152/jn.1998.79.6.2847.
Intracellular recordings and pharmacological manipulations were employed to investigate the ionic basis for serotonin-induced bistable membrane behaviors in guinea pig trigeminal motoneurons (TMNs). In voltage clamp, 10 microM serotonin (5-HT) induced a region of negative slope resistance (NSR) in the steady-state current-voltage (I-V) relationship at potentials less negative than -58 mV, creating the necessary conditions for membrane bistability. The contributions of sustained Na+ and Ca2+ currents to the generation of the NSR were investigated using specific ion channel antagonists and agonists. The NSR was eliminated by the L-type Ca2+ channel antagonist nifedipine (5-10 microM), indicating the contribution of L channels. In nifedipine, inward rectification was present in the I-V relationship in a similar voltage range (greater than -58 mV). This region was subsequently linearized by tetrodotoxin (TTX), indicating the presence of a persistent Na+ current. When the 5-HT-induced NSR was eliminated by perfusion in low Ca2+ solution (0.4 mM), it was restored by the Na+ channel agonist veratridine (10 microM). Commensurate with bistability, in current clamp during bath application of 5-HT, plateau potentials were elicited by transient depolarizing or hyperpolarizing stimuli. Plateau potentials evoked by depolarization were observed under control and TTX conditions, but were blocked by nifedipine, suggesting the participation of an L-type Ca2+ current. Plateau potentials initiated after release from hyperpolarization (anode break) were blocked by 300 microM Ni2+, suggesting the responses relied on deinactivation of a T-type Ca2+ current. Conditional bursting was also observed in 5-HT. Nifedipine or low Ca2+ solutions blocked bursting, and the L-channel agonist Bay K 8644 (10 microM) extended the duration of individual bursts, demonstrating the role of L-type Ca2+ currents. Interestingly, when bursting was blocked by nifedipine or low Ca2+, it could be restored by veratridine application via enhancement of the persistent Na+ current. We conclude that bistable membrane behaviors in TMNs are mediated by L-type Ca2+ and persistent Na+ currents. 5-HT is associated with enhancement of TMN activity during oral-motor activity; the induction of bistable membrane properties by 5-HT represents a cellular mechanism for this enhancement.
采用细胞内记录和药理学操作方法，研究豚鼠三叉神经运动神经元（TMNs）中5-羟色胺诱导的双稳态膜行为的离子基础。在电压钳实验中，10微摩尔5-羟色胺（5-HT）在电位低于-58 mV时，在稳态电流-电压（I-V）关系中诱导出一个负斜率电阻（NSR）区域，为膜双稳态创造了必要条件。使用特异性离子通道拮抗剂和激动剂研究持续的Na+和Ca2+电流对NSR产生的贡献。L型Ca2+通道拮抗剂硝苯地平（5-10微摩尔）消除了NSR，表明L通道的作用。在硝苯地平存在时，I-V关系在类似电压范围（大于-58 mV）内存在内向整流。该区域随后被河豚毒素（TTX）线性化，表明存在持续性Na+电流。当通过在低钙溶液（0.4 mM）中灌注消除5-HT诱导的NSR时，Na+通道激动剂藜芦碱（10微摩尔）可使其恢复。与双稳态一致，在浴加5-HT的电流钳实验中，短暂的去极化或超极化刺激可诱发平台电位。在对照和TTX条件下观察到去极化诱发的平台电位，但被硝苯地平阻断，提示L型Ca2+电流的参与。超极化释放（阳极断裂）后引发的平台电位被300微摩尔Ni2+阻断，提示反应依赖于T型Ca2+电流的去失活。在5-HT中也观察到条件性爆发。硝苯地平或低钙溶液阻断爆发，L通道激动剂Bay K 8644（10微摩尔）延长单个爆发的持续时间，证明L型Ca2+电流的作用。有趣的是，当爆发被硝苯地平或低钙阻断时，通过应用藜芦碱增强持续性Na+电流可使其恢复。我们得出结论，TMNs中的双稳态膜行为由L型Ca2+和持续性Na+电流介导。5-HT与口面部运动活动期间TMN活性增强有关；5-HT诱导双稳态膜特性代表了这种增强的细胞机制。
