Regunathan S, Feinstein D L, Reis D J
Division of Neurobiology, Cornell University Medical College, New York, NY 10021.
J Neurosci Res. 1993 Apr 15;34(6):681-8. doi: 10.1002/jnr.490340611.
Clonidine and related imidazoline agents, beside binding to alpha 2-adrenergic receptors, have been shown to bind to a non-adrenergic site (imidazoline sites) in brain and peripheral tissues. However, which cell types in brain, namely neurons or glia, express this binding site and the cellular effects of activation of this site are not known. We investigated the cellular localization of imidazoline binding sites in cultured rat cortical astrocytes and neurons. Membranes prepared from cultured astrocytes showed specific, high affinity binding (KD: 4 nM) for 3H-idazoxan with about tenfold higher number of binding sites than alpha 2-adrenergic sites (Bmax: 220 vs. 20 fmol/mg protein). Displacement studies exhibited the rank order of potency: cirazoline > idazoxan > amiloride > clonidine >>> epinephrine = ruawolscine defining this site as I-2a subtype of imidazoline binding sites. Moreover, the binding was inhibited by K+ but not by Na+, another characteristic of imidazoline binding sites. In contrast, membranes prepared from cultured neurons showed fewer binding sites for 3H-idazoxan that were completely displayed by adrenergic agents. Incubation of astrocytes with idazoxan, but not rauwolscine, resulted in a concentration-dependent increase in the levels of mRNA for the astrocyte specific molecule glial fibrillary acidic protein. We conclude that (a) the non-adrenergic imidazoline binding sites are expressed in astrocytes but not in neurons in rat cerebral cortex and (b) these "receptors" may influence astrocyte physiology by regulating the levels of GFAP.
可乐定及相关咪唑啉类药物，除了能与α2 -肾上腺素能受体结合外，还被证明能与脑和外周组织中的一个非肾上腺素能位点（咪唑啉位点）结合。然而，脑中哪些细胞类型，即神经元还是神经胶质细胞，表达这个结合位点以及该位点激活后的细胞效应尚不清楚。我们研究了培养的大鼠皮质星形胶质细胞和神经元中咪唑啉结合位点的细胞定位。从培养的星形胶质细胞制备的膜对3H - 伊达唑胺显示出特异性、高亲和力结合（解离常数KD：4 nM），其结合位点数量比α2 -肾上腺素能位点高约十倍（最大结合量Bmax：220对20 fmol/mg蛋白质）。置换研究显示效价顺序为：西拉唑啉＞伊达唑胺＞阿米洛利＞可乐定 >>> 肾上腺素 = 育亨宾，将该位点定义为咪唑啉结合位点的I - 2a亚型。此外，该结合受钾离子抑制但不受钠离子抑制，这是咪唑啉结合位点的另一个特征。相比之下，从培养的神经元制备的膜对3H - 伊达唑胺的结合位点较少，且完全被肾上腺素能药物所占据。用伊达唑胺而非育亨宾孵育星形胶质细胞，导致星形胶质细胞特异性分子胶质纤维酸性蛋白的mRNA水平呈浓度依赖性增加。我们得出结论：（a）非肾上腺素能咪唑啉结合位点在大鼠大脑皮质的星形胶质细胞中表达，而在神经元中不表达；（b）这些“受体”可能通过调节胶质纤维酸性蛋白的水平来影响星形胶质细胞的生理功能。
