Barberis A, Cherubini E, Mozrzymas J W
Neuroscience Program and Istituto Nazionale Fisica della Materia Unit, International School for Advanced Studies (SISSA), 34014 Trieste, Italy.
J Neurosci. 2000 Dec 1;20(23):8618-27. doi: 10.1523/JNEUROSCI.20-23-08618.2000.
Zinc is abundantly present in the CNS, and after nerve stimulation is thought to be released in sufficient quantity to modulate the synaptic transmission. Although it is known that this divalent cation inhibits the GABAergic synaptic currents, the underlying mechanisms were not fully elucidated. Here we report that zinc reduced the amplitude, slowed the rise time, and accelerated the decay of mIPSCs in cultured hippocampal neurons. The analysis of current responses to rapid GABA applications and model simulations indicated that these effects on mIPSCs are caused by zinc modulation of GABA(A) receptor gating. In particular, zinc slowed the onset of GABA-evoked currents by decreasing both the binding (k(on)) and the transition rate from closed to open state (beta(2)). Moreover, slower onset and recovery from desensitization as well as an increased unbinding rate (k(off)) were shown to underlie the accelerated deactivation kinetics in the presence of zinc. The nonequilibrium conditions of GABA(A) receptor activation were found to strongly affect zinc modulation of this receptor. In particular, an extremely fast clearance of synaptic GABA is implicated to be responsible for a stronger zinc effect on mIPSCs than on current responses to exogenous GABA. Finally, the analysis of currents evoked by GABA coapplied with zinc indicated that the interaction between zinc and GABA(A) receptors was too slow to explain zinc effects in terms of competitive antagonism. In conclusion, our results provide evidence that inhibition of mIPSCs by zinc is attributable to the allosteric modulation of GABA(A) receptor gating.
锌大量存在于中枢神经系统中，并且在神经刺激后，据认为会释放出足够的量来调节突触传递。尽管已知这种二价阳离子会抑制GABA能突触电流，但其潜在机制尚未完全阐明。在此我们报告，锌降低了培养的海马神经元中微小抑制性突触后电流（mIPSCs）的幅度，减慢了其上升时间，并加速了其衰减。对快速施加GABA时的电流反应分析和模型模拟表明，这些对mIPSCs的影响是由锌对GABA(A)受体门控的调节引起的。特别是，锌通过降低结合速率（k(on)）和从关闭状态到开放状态的转变速率（beta(2)）来减慢GABA诱发电流的起始。此外，在锌存在的情况下，脱敏起始和恢复较慢以及解离速率（k(off)）增加被证明是加速失活动力学的基础。发现GABA(A)受体激活的非平衡条件强烈影响锌对该受体的调节。特别是，突触GABA的极快速清除被认为是锌对mIPSCs的影响比对外源GABA电流反应的影响更强的原因。最后，对与锌共同施加GABA所诱发电流的分析表明，锌与GABA(A)受体之间的相互作用太慢，无法用竞争性拮抗来解释锌的作用。总之，我们的结果提供了证据，表明锌对mIPSCs的抑制归因于GABA(A)受体门控的变构调节。
