Olianas M C, Onali P
Department of Neurosciences, University of Cagliari, Italy.
Br J Pharmacol. 1999 Feb;126(3):657-64. doi: 10.1038/sj.bjp.0702349.
Previous studies have shown that GABA(B) receptors facilitate cyclic AMP formation in brain slices likely through an indirect mechanism involving intracellular second messengers. In the present study, we have investigated whether a positive coupling of GABA(B) receptors to adenylyl cyclase could be detected in a cell-free preparation of rat olfactory bulb, a brain region where other Gi/Go-coupled neurotransmitter receptors have been found to stimulate the cyclase activity. The GABA(B) receptor agonist (-)-baclofen significantly increased basal adenylyl cyclase activity in membranes of the granule cell and external plexiform layers, but not in the olfactory nerve-glomerular layer. The adenylyl cyclase stimulation was therefore examined in granule cell layer membranes. The (-)-baclofen stimulation (pD2=4.53) was mimicked by 3-aminopropylphosphinic acid (pD2=4.60) and GABA (pD2=3.56), but not by (+)-baclofen, 3-aminopropylphosphonic acid, muscimol and isoguvacine. The stimulatory effect was counteracted by the GABA(B) receptor antagonists CGP 35348 (pA2=4.31), CGP 55845 A (pA2=7.0) and 2-hydroxysaclofen (pKi=4.22). Phaclofen (1 mM) was inactive. The (-)-baclofen stimulation was not affected by quinacrine, indomethacin, nordihydroguaiaretic acid and staurosporine, but was completely prevented by pertussis toxin and significantly reduced by the alpha subunit of transducin, a betagamma scavenger. The betagamma subunits of transducin stimulated the cyclase activity and this effect was not additive with that produced by (-)-baclofen. In the external plexiform and granule cell layers, but not in the olfactory nerve-glomerular layer, (-)-baclofen enhanced the adenylyl cyclase stimulation elicited by the neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP) 38. Conversely, the adenylyl cyclase activity stimulated by either forskolin or Ca2+/calmodulin-(Ca2+/CaM) was inhibited by (-)-baclofen in all the olfactory bulb layers examined. These data demonstrate that in specific layers of rat olfactory bulb activation of GABA(B) receptors enhances basal and neurotransmitter-stimulated adenylyl cyclase activities by a mechanism involving betagamma subunits of Gi/Go. This positive coupling is associated with a widespread inhibitory effect on forskolin- and Ca2+/CaM-stimulated cyclic AMP formation.
以往的研究表明，GABA(B)受体可能通过涉及细胞内第二信使的间接机制促进脑片中环磷酸腺苷（cAMP）的形成。在本研究中，我们调查了在大鼠嗅球的无细胞制剂中是否能检测到GABA(B)受体与腺苷酸环化酶的正向偶联，嗅球是一个已发现其他与Gi/Go偶联的神经递质受体可刺激环化酶活性的脑区。GABA(B)受体激动剂(-)-巴氯芬显著增加了颗粒细胞层和外丛状层膜中的基础腺苷酸环化酶活性，但在嗅神经 - 肾小球层中未增加。因此，我们在颗粒细胞层膜中检测了腺苷酸环化酶的刺激作用。(-)-巴氯芬的刺激作用（pD2 = 4.53）可被3 - 氨基丙基次膦酸（pD2 = 4.60）和GABA（pD2 = 3.56）模拟，但不能被(+)-巴氯芬、3 - 氨基丙基膦酸、蝇蕈醇和异谷氨酰胺模拟。GABA(B)受体拮抗剂CGP 35348（pA2 = 4.31）、CGP 55845 A（pA2 = 7.0）和2 - 羟基巴氯芬（pKi = 4.22）可抵消这种刺激作用。苯氯芬（1 mM）无活性。(-)-巴氯芬的刺激作用不受奎纳克林、吲哚美辛、去甲二氢愈创木酸和星形孢菌素的影响，但百日咳毒素可完全阻断，转导素的α亚基（一种βγ清除剂）可使其显著降低。转导素的βγ亚基刺激环化酶活性，且这种作用与(-)-巴氯芬产生的作用无相加性。在嗅球的外丛状层和颗粒细胞层中，但不在嗅神经 - 肾小球层中，(-)-巴氯芬增强了神经肽垂体腺苷酸环化酶激活多肽（PACAP）38引起的腺苷酸环化酶刺激作用。相反，在所有检测的嗅球层中，(-)-巴氯芬抑制了福斯高林或Ca2+/钙调蛋白 - （Ca2+/CaM）刺激的腺苷酸环化酶活性。这些数据表明，在大鼠嗅球的特定层中，GABA(B)受体的激活通过涉及Gi/Go的βγ亚基的机制增强基础和神经递质刺激的腺苷酸环化酶活性。这种正向偶联与对福斯高林和Ca2+/CaM刺激的cAMP形成的广泛抑制作用相关。
