Sosa Romina, Gleason Evanna
Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, USA.
Vis Neurosci. 2004 Nov-Dec;21(6):807-16. doi: 10.1017/S0952523804216017.
In the inner plexiform layer, amacrine cells receive glutamatergic input from bipolar cells. Glutamate can depolarize amacrine cells by activation of ionotropic glutamate receptors or mediate potentially more diverse changes via activation of G protein-coupled metabotropic glutamate receptors (mGluR5). Here, we asked whether selective activation of metabotropic glutamate receptor 5 is linked to modulation of the voltage-gated Ca2+ channels expressed by cultured GABAergic amacrine cells. To address this, we performed whole-cell voltage clamp experiments, primarily in the perforated-patch configuration. We found that agonists selective for mGluR5, including (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), enhanced the amplitude of the voltage-dependent Ca2+ current. The voltage-dependent Ca2+ current and CHPG-dependent current enhancement were blocked by nifedipine, indicating that L-type Ca2+ channels, specifically, were being modulated. We have previously shown that activation of mGluR5 produces Ca2+ elevations in cultured amacrine cells (Sosa et al., 2002). Loading the cells with 5 mM BAPTA inhibited the mGluR5-dependent enhancement, suggesting that the cytosolic Ca2+ elevations are required for modulation of the current. Although activation of mGluR5 is typically linked to activation of protein kinase C, we found that direct activation of this kinase leads to inhibition of the Ca2+ current, indicating that stimulation of this enzyme is not responsible for the mGluR5-dependent enhancement. Interestingly, direct stimulation of protein kinase A produced an enhancement of the Ca2+ current similar to that observed with activation of mGluR5. Thus, activation of mGluR5 may modulate the L-type voltage-gated Ca2+ current in these GABAergic amacrine cells via activation of protein kinase A, possibly via direct activation of a Ca2(+)-dependent adenylate cyclase.
在内网状层，无长突细胞接收来自双极细胞的谷氨酸能输入。谷氨酸可通过离子型谷氨酸受体的激活使无长突细胞去极化，或通过G蛋白偶联代谢型谷氨酸受体（mGluR5）的激活介导潜在更多样化的变化。在此，我们探究了代谢型谷氨酸受体5的选择性激活是否与培养的γ-氨基丁酸能无长突细胞所表达的电压门控Ca2+通道的调节有关。为解决这一问题，我们主要采用穿孔膜片钳配置进行了全细胞电压钳实验。我们发现，对mGluR5具有选择性的激动剂，包括（RS）-2-氯-5-羟基苯甘氨酸（CHPG），增强了电压依赖性Ca2+电流的幅度。电压依赖性Ca2+电流和CHPG依赖性电流增强被硝苯地平阻断，表明特别是L型Ca2+通道受到了调节。我们之前已经表明，mGluR5的激活在培养的无长突细胞中会导致Ca2+升高（索萨等人，2002年）。用5 mM BAPTA加载细胞可抑制mGluR5依赖性增强，表明胞质Ca2+升高是电流调节所必需的。尽管mGluR5的激活通常与蛋白激酶C的激活有关，但我们发现该激酶的直接激活会导致Ca2+电流的抑制，表明该酶的刺激并非mGluR5依赖性增强的原因。有趣的是，蛋白激酶A的直接刺激产生了与mGluR5激活所观察到的类似的Ca2+电流增强。因此，mGluR5的激活可能通过蛋白激酶A的激活来调节这些γ-氨基丁酸能无长突细胞中的L型电压门控Ca2+电流，可能是通过Ca2(+)-依赖性腺苷酸环化酶的直接激活。
