Wall M J, Dale N
School of Biological Sciences, Bristol University, United Kingdom.
J Neurosci. 1994 Oct;14(10):6248-55. doi: 10.1523/JNEUROSCI.14-10-06248.1994.
Activation of GABAB receptors in the Xenopus embryo, a simple vertebrate, causes presynaptic inhibition of transmitter release from glycinergic spinal neurons and an increase in action potential threshold. To investigate the underlying mechanisms of GABAB receptor action, we have made whole-cell voltage-clamp recordings from acutely isolated Xenopus embryo spinal neurons. The GABAB receptor agonist baclofen caused a reversible reduction in the amplitude of Ca2+ currents. This reduction of Ca2+ currents appeared to be voltage dependent as it was removed at very positive potentials. Since the specific GABAB antagonists CGP35348, phaclofen, and 2-hydroxysaclofen all blocked the reduction in Ca2+ currents, we concluded that the modulation of the Ca2+ current was mediated by GABAB receptors. We have investigated the pharmacological identity of the Ca2+ current modulated by baclofen using the selective blocker omega-conotoxin, fraction GVIA (omega-CgTX). omega-CgTX selectively blocked voltage-gated Ca2+ currents without affecting the voltage-gated Na+ current. omega-CgTX substantially occluded the action of baclofen, suggesting that GABAB receptors modulate an omega-CgTX-sensitive Ca2+ current. Since GABAB receptors mediate presynaptic inhibition, we have studied the involvement of the omega-CgTX-sensitive Ca2+ current in synaptic transmission in the intact spinal cord. Inhibitory interneuron axons were stimulated to evoke monosynaptic IPSPs in motoneurons, and recorded intracellularly. Since omega-CgTX blocked inhibitory transmission, we concluded that the omega-CgTX-sensitive Ca2+ current plays an essential role in transmitter release. If modulation of this current were to occur in nerve terminals, it could contribute to the GABAB receptor-mediated presynaptic inhibition of transmitter release.(ABSTRACT TRUNCATED AT 250 WORDS)
在非洲爪蟾胚胎（一种简单的脊椎动物）中，GABAB受体的激活会导致甘氨酸能脊髓神经元的递质释放出现突触前抑制，并使动作电位阈值升高。为了研究GABAB受体作用的潜在机制，我们对急性分离的非洲爪蟾胚胎脊髓神经元进行了全细胞电压钳记录。GABAB受体激动剂巴氯芬导致Ca2+电流幅度可逆性降低。这种Ca2+电流的降低似乎依赖于电压，因为在非常正的电位时它会消失。由于特异性GABAB拮抗剂CGP35348、法氯芬和2-羟基巴氯芬均能阻断Ca2+电流的降低，我们得出结论，Ca2+电流的调节是由GABAB受体介导的。我们使用选择性阻滞剂ω-芋螺毒素GVIA（ω-CgTX）研究了巴氯芬调节的Ca2+电流的药理学特性。ω-CgTX选择性地阻断电压门控Ca2+电流，而不影响电压门控Na+电流。ω-CgTX基本上消除了巴氯芬的作用，表明GABAB受体调节一种对ω-CgTX敏感的Ca2+电流。由于GABAB受体介导突触前抑制，我们研究了对ω-CgTX敏感的Ca2+电流在完整脊髓突触传递中的作用。刺激抑制性中间神经元轴突以诱发运动神经元中的单突触抑制性突触后电位，并进行细胞内记录。由于ω-CgTX阻断了抑制性传递，我们得出结论，对ω-CgTX敏感的Ca2+电流在递质释放中起重要作用。如果这种电流的调节发生在神经末梢，它可能有助于GABAB受体介导的递质释放的突触前抑制。（摘要截短于250字）
