Buzzi M, Bemelmans F F, Roubos E W, Jenks B G
Department of Cellular Animal Physiology, Nijmegen Institute for Neurosciences, Subfaculty of Biology, University of Nijmegen, The Netherlands.
Endocrinology. 1997 Jan;138(1):203-12. doi: 10.1210/endo.138.1.4886.
The melanotrope cell of Xenopus laevis is innervated by nerve terminals that contain, among other transmitter substances, the neurotransmitter gamma-aminobutyric acid (GABA). Postsynaptically the melanotrope cell possess both GABAA and GABAB receptors. Activation of either receptor type leads to an inhibition of alpha MSH release from the cell. The present study concerns the functional significance of the existence of two types of GABA receptors on the melanotrope regarding two questions: 1) do the different receptor types have different effects on the melanotrope? and 2) can the endogenous ligand GABA differentially activate these receptors? Concerning the first question, we have tested the hypothesis that the GABAA receptor (a chloride ion channel) and the GABAB receptor (a G protein-coupled receptor negatively linked to adenylyl cyclase) may have differential effects on the sensitivity of the cell to stimulation by cAMP-dependent mechanisms. We show that treatments with either isoguvacine (GABAA agonist) or baclofen (GABAB agonist) inhibit intracellular Ca2+ oscillations and peptide secretion from melanotrope cells. Treatments known to increase intracellular cAMP in the melanotrope (e.g. use of the peptide sauvagine or the cAMP analog 8-bromo-cAMP) completely overcame the inhibition induced by baclofen, but not that caused by isoguvacine. We conclude that the GABAA and GABAB receptors have different effects on the Xenopus melanotrope cell by differentially affecting the sensitivity the cell shows to stimulation by cAMP-dependent mechanisms. Concerning possible differential activation of the receptor types, we found that we could use a membrane potential probe (from the bis-oxonol family) to differentiate between GABAA and GABAB receptor activation. Using this probe we showed that low GABA concentrations (< 10(-7) M) give a response indicative of the GABAB receptor, whereas at high GABA concentrations (> 10(-7) M), the GABAA receptor response predominates. We, therefore, conclude that GABA can differentially activate the two types of GABA receptors on the Xenopus melanotrope cell.
非洲爪蟾的促黑素细胞由神经末梢支配，这些神经末梢除了含有其他递质外，还含有神经递质γ-氨基丁酸（GABA）。在突触后，促黑素细胞同时拥有GABAA和GABAB受体。激活任何一种受体类型都会导致细胞中α-MSH释放受到抑制。本研究关注促黑素细胞上两种类型的GABA受体存在的功能意义，涉及两个问题：1）不同的受体类型对促黑素细胞有不同的影响吗？2）内源性配体GABA能对这些受体产生不同的激活作用吗？关于第一个问题，我们测试了这样一个假设，即GABAA受体（一种氯离子通道）和GABAB受体（一种与腺苷酸环化酶负相关的G蛋白偶联受体）可能对细胞通过cAMP依赖性机制产生的刺激敏感性有不同影响。我们发现，用异鹅肌肽（GABAA激动剂）或巴氯芬（GABAB激动剂）处理会抑制促黑素细胞内的Ca2+振荡和肽分泌。已知能增加促黑素细胞内cAMP的处理方法（如使用肽蛙皮素或cAMP类似物8-溴-cAMP）能完全克服巴氯芬诱导的抑制作用，但不能克服异鹅肌肽引起的抑制作用。我们得出结论，GABAA和GABAB受体通过不同地影响细胞对cAMP依赖性机制刺激的敏感性，对非洲爪蟾促黑素细胞有不同的影响。关于受体类型可能的差异激活，我们发现可以使用一种膜电位探针（来自双苯甲酰亚胺家族）来区分GABAA和GABAB受体的激活。使用该探针我们表明，低浓度的GABA（<10(-7)M）会产生指示GABAB受体的反应，而在高浓度的GABA（>10(-7)M）时，GABAA受体反应占主导。因此，我们得出结论，GABA能对非洲爪蟾促黑素细胞上的两种类型的GABA受体产生不同的激活作用。
