Synaptic inhibition by volatile ansethetics

挥发性麻醉药的麻醉作用部分是通过抑制兴奋性谷氨酸能突触传递来实现的。有证据表明，谷氨酸能突触传递抑制是由于抑制递质释放所致。然而，抑制递质释放的细胞和分子机制仍不清楚。根据我们的初步结果，我推测，挥发性麻醉剂通过两种机制抑制谷氨酸能突触传递：1)抑制突触前Na+通道，从而降低动作电位幅度，从而降低动作电位诱发的Ca~(2+)内流；2)抑制突触前钙通道。我们将在大鼠脑干切片中的梯形体内侧核的谷氨酸能突触上验证这一假说。与其他突触相比，这种突触具有显著的优势，因为它有一个大的神经末梢，可以直接记录突触前动作电位、Na+、K+和Ca~(2+)电流，以及钙离子内流的荧光记录。这些突触前的记录可以与同一突触的突触后兴奋性电流(EPSC)的记录同时进行，这使得我们能够定量地评估每种突触前离子通道类型在控制动作电位诱发的递质释放中的参与。利用这些技术，我们将研究三种常用的挥发性麻醉药--异氟烷、氟烷和七氟烷在临床相关浓度下的作用。我们将研究这些麻醉剂对突触前Na+、K+和Ca~(2+)通道的影响，以及这些影响在EPSC抑制中的作用。此外，我们还将研究这些麻醉剂是否也通过一种独立于离子通道调节的机制，即直接抑制释放机制来抑制EPSC。通过揭示挥发性麻醉剂抑制谷氨酸释放的机制，这项工作将大大有助于我们理解全身麻醉的细胞和分子机制，并最终可能有助于设计更好的全身麻醉药。

Volatile anesthetics achieve their anesthetic effects partly by depressing excitatory glutamatergic synaptic transmission. Evidence suggests that depression of glutamatergic synaptic transmission is caused by inhibition of transmitter release. However, the cellular and molecular mechanisms underlying inhibition of transmitter release remain unclear. Based on our preliminary results, I hypothesize that volatile anesthetics depress glutamatergic synaptic transmission by reducing the presynaptic Ca2+ influx by two mechanisms: 1) inhibition of presynaptic Na+ channels, which decreases the action potential amplitude and thus the action potential-evoked Ca2+ influx, and 2) inhibition of presynaptic Ca2+ channels. We will test this hypothesis at a glutamatergic synapse in the medial nucleus of the trapezoid body in rat brainstem slices. This synapse offers a significant advantage over other synapses, because it has a large nerve terminal that allows for direct recordings of presynaptic action potentials, Na+, K+ and Ca2+ currents and fluorescence recordings of Ca2+ influx. These presynaptic recordings can be performed simultaneously with recordings of the postsynaptic excitatory current (EPSC) at the same synapse, which allows us to quantitatively evaluate the involvement of each presynaptic ion channel type in controlling action potential-evoked transmitter release. With these techniques, we will study the action of three commonly used volatile anesthetics, isoflurane, halothane and sevoflurane at clinically relevant concentrations. We will characterize the effects of these anesthetics on presynaptic Na+, K+ and Ca2+ channels and the contribution of each of these effects to depression of the EPSC. In addition, we will investigate whether these anesthetics also inhibit the EPSC by a mechanism independent of modulation of ion channels, i.e., direct inhibition of the release machinery. By revealing mechanisms underlying volatile anesthetic-induced depression of glutamate release, the proposed work will significantly contribute to our understanding of the cellular and molecular mechanisms of general anesthesia, and may ultimately help to design better general anesthetics.