Sessler F M, Mouradian R D, Cheng J T, Yeh H H, Liu W M, Waterhouse B D
Department of Physiology and Biophysics, Hahnemann University, Philadelphia, PA 19102.
Brain Res. 1989 Oct 9;499(1):27-38. doi: 10.1016/0006-8993(89)91132-3.
Previous in vivo studies from our laboratory have consistently shown that iontophoretically applied norepinephrine (NE) can potentiate gamma-aminobutyric acid (GABA)-induced depressant responses of cerebrocortical, cerebellar and hypothalamic neurons. Additional experiments have further suggested that this noradrenergic facilitating action is specific for GABA and results from the activation of a beta-type adrenoceptor. The goal of the present studies was to determine if the cAMP second messenger system might also be a component of the mechanism responsible for this NE modulatory action on GABA-mediated inhibition. In one set of in vitro experiments, we examined cerebellar neuronal responses to GABA before, during and after iontophoretic application of NE, 8-bromo-3',5'-cyclic AMP (BcAMP) or 3-isobutyl-1-methyl xanthine (IBMX) or bath application of forskolin (10-30 microM). In a second group of in vivo studies, extracellularly recorded responses of individual cerebellar Purkinje (P) cells to iontophoretic pulses of GABA or beta-alanine were examined before, during and after NE or BcAMP microiontophoresis. In 20 of 25 cerebellar cells recorded from tissue slices, iontophoretically applied NE markedly enhanced responses to GABA in a manner similar to that observed previously in vivo. In these in vitro preparations, bath application of forskolin was also capable of potentiating GABA-induced inhibition in each of 4 cases tested whereas dideoxy-forskolin was not. Iontophoretic application of IBMX further enhanced the facilitating effects of NE on GABA-induced inhibition in 10 of 11 cases tested. Furthermore, under in vitro conditions, BcAMP augmented inhibitory responses to GABA in all cerebellar neurons tested. In the intact rat brain, iontophoretic administration of BcAMP caused a marked NE-like augmentation of P-cell responses to GABA in 73% of the cells tested. As with NE, BcAMP was ineffective in enhancing P-cell inhibitory responses to beta-alanine, an agent which like GABA causes hyperpolarization, by increasing Cl conductance. In summary, these results indicate that a membrane permeant analog of cAMP, a phosphodiesterase inhibitor and an agent which directly activates adenyl cyclase can mimic the previously observed GABA-potentiating actions of NE. Thus, these findings provide further support for the contention that noradrenergic enhancement of GABA inhibition results from a cascade of transmembrane events which includes beta-receptor activation, adenyl cyclase stimulation and increased intracellular production of cAMP.
我们实验室之前的体内研究一直表明，离子电渗法应用去甲肾上腺素（NE）可增强γ-氨基丁酸（GABA）诱导的大脑皮质、小脑和下丘脑神经元的抑制反应。额外的实验进一步表明，这种去甲肾上腺素能促进作用对GABA具有特异性，且是由β型肾上腺素能受体的激活所导致。本研究的目的是确定环磷酸腺苷（cAMP）第二信使系统是否也是NE对GABA介导的抑制作用进行调节的机制的组成部分。在一组体外实验中，我们在离子电渗法应用NE、8-溴-3',5'-环磷酸腺苷（BcAMP）或3-异丁基-1-甲基黄嘌呤（IBMX）之前、期间和之后，以及在浴槽中应用福司可林（10 - 30 microM）时，检测了小脑神经元对GABA的反应。在第二组体内研究中，在NE或BcAMP微离子电渗法之前、期间和之后，检测了细胞外记录的单个小脑浦肯野（P）细胞对GABA或β-丙氨酸离子电渗脉冲的反应。在从组织切片记录的25个小脑细胞中的20个中，离子电渗法应用的NE以类似于之前在体内观察到的方式显著增强了对GABA的反应。在这些体外制剂中，在4个测试案例中的每一个中，浴槽中应用福司可林也能够增强GABA诱导的抑制作用，而双脱氧福司可林则不能。在11个测试案例中的10个中，离子电渗法应用IBMX进一步增强了NE对GABA诱导的抑制作用的促进效果。此外，在体外条件下，BcAMP增强了所有测试的小脑神经元对GABA的抑制反应。在完整的大鼠脑中，离子电渗法给予BcAMP在73%的测试细胞中引起了P细胞对GABA的反应出现明显的类似NE的增强。与NE一样，BcAMP在增强P细胞对β-丙氨酸的抑制反应方面无效，β-丙氨酸与GABA一样，通过增加Cl电导导致超极化。总之，这些结果表明，cAMP的膜通透性类似物、磷酸二酯酶抑制剂以及直接激活腺苷酸环化酶的药物能够模拟之前观察到的NE对GABA的增强作用。因此，这些发现为以下论点提供了进一步的支持，即去甲肾上腺素能增强GABA抑制作用是由一系列跨膜事件导致的，这些事件包括β受体激活、腺苷酸环化酶刺激以及细胞内cAMP生成增加。
