Synaptic inhibition by volatile ansethetics

挥发性麻醉剂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，抑制谷氨酸能突触传递是由抑制递质释放引起的。然而，抑制递质释放的细胞和分子机制尚不清楚。基于我们的初步结果，我假设挥发性麻醉剂通过两种机制减少突触前Ca2+内流来抑制谷氨酸能突触传递：1)抑制突触前Na+通道，从而降低动作电位振幅，从而降低动作电位诱发的Ca2+内流；2)抑制突触前Ca2+通道。我们将在大鼠脑干切片的梯形体内侧核的谷氨酸能突触上验证这一假设。与其他突触相比，该突触具有显著的优势，因为它有一个大的神经终端，可以直接记录突触前动作电位、Na+、K+和Ca2+电流以及Ca2+内流的荧光记录。这些突触前记录可以与同一突触的突触后兴奋电流（EPSC）记录同时进行，这使我们能够定量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作用。利用这些技术，我们将研究三种常用的挥发性麻醉剂，异氟烷、氟烷和七氟烷在临床相关浓度下的作用。我们将描述这些麻醉药对突触前Na+、K+和Ca2+通道的影响，以及这些影响对EPSC抑制的贡献。此外，我们将研究这些麻醉药是否也通过独立于离子通道调节的机制抑制EPSC，即直接抑制释放机制。通过揭示挥发性麻醉药诱导谷氨酸释放抑制的机制，本研究将有助于我们理解全身麻醉的细胞和分子机制，并最终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.