Li Ying, van den Pol Anthony N
Department of Neurosurgery, Yale University School of Medicine, New Haven, Connecticut 06520, USA.
J Neurosci. 2005 Jan 5;25(1):173-83. doi: 10.1523/JNEUROSCI.4015-04.2005.
Hypothalamic hypocretin enhances arousal, similar to the actions of norepinephrine (NE). The physiological actions of NE were examined in hypocretin neurons identified by selective green fluorescent protein expression in transgenic mouse hypothalamic slices using whole-cell recording. NE induced an outward current, inhibited spike frequency, and hyperpolarized hypocretin neurons dose dependently. Similar actions were evoked by the selective alpha2 adrenergic agonist clonidine. The alpha2 antagonist idazoxan increased spike frequency, suggesting tonic NE-mediated inhibition. The NE-induced current was inwardly rectified, and the reversal potential was dependent on external potassium concentration; it was blocked by barium in the bath and by GTP-gamma-S in the pipette, suggesting activation of a G-protein inward rectifying K+ (GIRK) current. NE and clonidine decreased calcium currents evoked by depolarizing voltage steps. The selective alpha1 adrenergic agonist phenylephrine had no effect on membrane potential but did increase IPSC frequency; miniature IPSC frequency was also increased, in some cells without any effect on amplitude, suggesting a facilitative presynaptic action at alpha1 receptors on GABAergic axons that innervate hypocretin neurons. NE therefore inhibits hypocretin neurons directly through two mechanisms: activation of a GIRK current, depression of calcium currents, and indirectly through increased inhibitory GABA input. Similar to NE, dopamine and epinephrine reduced or blocked spikes and, in the presence of TTX, showed direct hyperpolarizing actions. The action of dopamine was blocked by the D2 receptor antagonist eticlopride, whereas a D1/5 antagonist had no effect. These data suggest that catecholamines evoke strong inhibitory actions on hypocretin neurons and suggest negative feedback from catecholamine cells that may be excited by hypocretin.
下丘脑泌素增强觉醒，类似于去甲肾上腺素（NE）的作用。利用全细胞膜片钳记录技术，在转基因小鼠下丘脑切片中通过选择性绿色荧光蛋白表达鉴定的下丘脑泌素神经元中，研究了NE的生理作用。NE诱发外向电流，抑制放电频率，并使下丘脑泌素神经元超极化，呈剂量依赖性。选择性α2肾上腺素能激动剂可乐定也诱发了类似的作用。α2拮抗剂咪唑克生增加了放电频率，提示NE存在紧张性介导的抑制作用。NE诱发的电流呈内向整流，反转电位依赖于细胞外钾离子浓度；该电流在浴槽中被钡离子阻断，在电极内液中被GTP-γ-S阻断，提示激活了一种G蛋白内向整流钾离子（GIRK）电流。NE和可乐定降低了去极化电压阶跃诱发的钙电流。选择性α1肾上腺素能激动剂去氧肾上腺素对膜电位无影响，但增加了抑制性突触后电流（IPSC）频率；微小IPSC频率也增加，在一些细胞中对幅度无任何影响，提示在支配下丘脑泌素神经元的GABA能轴突的α1受体上存在突触前易化作用。因此NE通过两种机制直接抑制下丘脑泌素神经元：激活GIRK电流，抑制钙电流，并通过增加抑制性GABA输入间接发挥作用。与NE类似，多巴胺和肾上腺素减少或阻断放电，并且在存在河豚毒素（TTX）的情况下，表现出直接的超极化作用。多巴胺 的作用被D2受体拮抗剂依替必利阻断，而D1/5拮抗剂则无作用。这些数据表明儿茶酚胺对下丘脑泌素神经元产生强烈的抑制作用，并提示可能被下丘脑泌素兴奋的儿茶酚胺能细胞存在负反馈。
