Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense, 28040, Madrid, Spain.
Instituto de Investigación Sanitaria del Hospital Clínico San Carlos, 28040, Madrid, Spain.
J Physiol. 2018 Mar 1;596(5):921-940. doi: 10.1113/JP275371. Epub 2018 Jan 25.
Neurotransmitter release is inhibited by metabotropic glutamate type 7 (mGlu ) receptors that reduce Ca influx, yet synapses lacking this receptor also produce weaker release, suggesting that mGlu receptors may also prime synaptic vesicles for release. Prolonged activation of mGlu receptors with the agonist l-AP4 first reduces and then enhances the amplitude of EPSCs through a presynaptic effect. The inhibitory response is blocked by pertussis toxin, while the potentiating response is prevented by a phospholipase C inhibitor (U73122) and an inhibitor of diacylglycerol (DAG) binding (calphostin C), suggesting that this receptor also couples to pathways that generate DAG. Release potentiation is associated with an increase in the number of synaptic vesicles close to the plasma membrane, which was dependent on the Munc13-2 and RIM1α proteins. The Glu7 receptors activated by the glutamate released following high frequency stimulation provoke a bidirectional modulation of synaptic transmission.
Neurotransmitter release is driven by Ca influx at synaptic boutons that acts on synaptic vesicles ready to undergo exocytosis. Neurotransmitter release is inhibited when metabotropic glutamate type 7 (mGlu ) receptors provoke a reduction in Ca influx, although the reduced release from synapses lacking this receptor suggests that they may also prime synaptic vesicles for release. These mGlu receptors activate phospholipase C (PLC) and generate inositol trisphosphate, which in turn releases Ca from intracellular stores and produces diacylglycerol (DAG), an activator of proteins containing DAG-binding domains such as Munc13 and protein kinase C (PKC). However, the full effects of mGlu receptor signalling on synaptic transmission are unclear. We found that prolonged activation of mGlu receptors with the agonist l-AP4 first reduces and then enhances the amplitude of EPSCs, a presynaptic effect that changes the frequency but not the amplitude of the mEPSCs and the paired pulse ratio. Pertussis toxin blocks the inhibitory response, while the PLC inhibitor U73122, and the inhibitor of DAG binding calphostin C, prevent receptor mediated potentiation. Moreover, this DAG-dependent potentiation of the release machinery brings more synaptic vesicles closer to the active zone plasma membrane in a Munc13-2- and RIM1α-dependent manner. Electrically evoked release of glutamate that activates mGlu receptors also bidirectionally modulates synaptic transmission. In these conditions, potentiation now occurs rapidly and it overcomes any inhibition, such that potentiation prevails unless it is suppressed with the PLC inhibitor U73122.
代谢型谷氨酸受体 7（mGlu7）通过减少 Ca2+内流抑制神经递质释放，但缺乏这种受体的突触也会产生较弱的释放，这表明 mGlu7 受体也可能使突触囊泡为释放做好准备。激动剂 l-AP4 对 mGlu7 受体的长期激活首先通过突触前效应减少然后增强 EPSC 的幅度。抑制反应被百日咳毒素阻断，而增强反应被 PLC 抑制剂（U73122）和二酰基甘油（DAG）结合抑制剂（钙调蛋白 C）阻止，这表明该受体还与生成 DAG 的途径偶联。释放增强与靠近质膜的突触囊泡数量增加有关，这依赖于 Munc13-2 和 RIM1α 蛋白。高频刺激后释放的谷氨酸激活的 Glu7 受体引起突触传递的双向调制。
神经递质释放由突触末梢的 Ca2+内流驱动，该 Ca2+内流作用于准备胞吐的突触囊泡。代谢型谷氨酸受体 7（mGlu7）引发 Ca2+内流减少，从而抑制神经递质释放，但缺乏这种受体的突触释放减少表明，它们也可能使突触囊泡为释放做好准备。这些 mGlu7 受体激活磷脂酶 C（PLC）并产生三磷酸肌醇，后者反过来从细胞内储存库中释放 Ca2+并产生二酰基甘油（DAG），DAG 是含有 DAG 结合域的蛋白（如 Munc13 和蛋白激酶 C（PKC））的激活剂。然而，mGlu 受体信号对突触传递的完全影响尚不清楚。我们发现，用激动剂 l-AP4 长时间激活 mGlu 受体首先减少，然后增强 EPSC 的幅度，这是一种改变频率但不改变 mEPSC 幅度和成对脉冲比的突触前效应。百日咳毒素阻断抑制反应，而 PLC 抑制剂 U73122 和 DAG 结合抑制剂钙调蛋白 C 可防止受体介导的增强。此外，这种依赖 DAG 的释放机制增强以 Munc13-2 和 RIM1α 依赖的方式使更多的突触囊泡更靠近活性区质膜。激活 mGlu 受体的谷氨酸的电诱发释放也双向调节突触传递。在这些条件下，增强现在迅速发生，并且克服任何抑制，使得增强占主导地位，除非用 PLC 抑制剂 U73122 抑制。
