Rosario P, Barat A, Ramirez G
Centro de Biologia Molecular, CSIC-UAM, Cantoblanco, 28049 Madrid, Spain.
Neurochem Int. 1989;15(1):115-20. doi: 10.1016/0197-0186(89)90083-1.
Drosophila melanogaster membranes prepared from cephalic ganglia contain two apparently independent populations of saturable binding sites for [(3)H]GABA: the higher-affinity sites (K(d) = 23 nM; B(max = 0.97 pmol/mg)) are already labeled at 0 degrees C, while a lower-affinity population of binding sites (K(d = 258 nM); B(max = 8.51 pmol/mg)) can be additionally identified at 23 degrees C and higher temperatures. GABA and then muscimol, together with ?-aminovalerate, are very efficient displacers of the specific binding while the A-specific agonist isoguvacine, the A-specific antagonist bicuculline methiodide and the B-specific agonist baclofen have low to negligible effect. [(3)H]GABA binding is not modulated by allosteric effectors characteristic of the vertebrate A-site, except for an anomalous stimulation by DMCM. Both [(3)H]flunitrazepam and [(3)H]TBOB give "specific" binding values under different experimental conditions but, after kinetic analysis, the binding appears to be non-saturable in both cases. We conclude that there are marked differences between the Drosophila GABA receptor and the vertebrate GABA(A) receptor complex, although our results do not exclude the existence of an insect receptor complex, associated to a Cl(?) channel, with different structural and pharmacological properties.
从头部神经节制备的黑腹果蝇膜含有两类明显独立的可饱和[³H]GABA结合位点:高亲和力位点(Kd = 23 nM;Bmax = 0.97 pmol/mg)在0℃时就已被标记,而低亲和力结合位点群体(Kd = 258 nM;Bmax = 8.51 pmol/mg)在23℃及更高温度下可额外鉴定出来。GABA以及随后的蝇蕈醇,连同γ-氨基戊酸,是特异性结合的非常有效的置换剂,而A特异性激动剂异鹅膏蕈氨酸、A特异性拮抗剂荷包牡丹碱甲碘化物和B特异性激动剂巴氯芬的作用很低或可忽略不计。除了被DMCM异常刺激外,[³H]GABA结合不受脊椎动物A位点特征性变构效应剂的调节。[³H]氟硝西泮和[³H]TBOB在不同实验条件下都给出“特异性”结合值,但经过动力学分析后,两种情况下的结合似乎都是不饱和的。我们得出结论,果蝇GABA受体与脊椎动物GABA(A)受体复合物之间存在显著差异,尽管我们的结果不排除存在与氯离子通道相关的具有不同结构和药理学特性的昆虫受体复合物。