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[摘要]：Cerebellar granule cells are particularly sensitive to inhibition by methylmercury (MeHg) on GABA A receptor function. This is manifested as a more rapid block of inhibitory postsynaptic currents/inhibitory postsynaptic potentials than for Purkinje cells. The underlying mechanism(s) for differential sensitivity of GABAergic transmission to MeHg in cerebellar neurons is unknown. Differential expression of alpha(6) subunit-containing GABA A receptors in cerebellar granule and Purkinje neurons could partially explain this. GABA-evoked currents (I-GABA) were recorded in response to MeHg in alpha(6) subunit-containing cerebellar granule cells and alpha(6) subunit-deficient cerebral cortical cells in culture. Cortical cells were substituted for Purkinje cells, which do not express alpha(6) subunits. They express the same alpha(1)-containing GABA(A) receptor as Purkinje cells but lack characteristics that enhance Purkinje cell resistance to MeHg. I-GABA were obtained using whole-cell recording and symmetrical [Cl-]. MeHg reduced I-GABA to complete block in both cell types in a timeand concentrationdependent manner. This effect was faster in granule cells than cortical cells. Effects of MeHg on I-GABA were recorded in granule cells at various developmental stages (days in vitro 4, 6, and 8) to alter the expression level of alpha(6) subunitcontaining GABA(A) receptors. Effects of MeHg on I-GABA were similar in cells at all days. In human embryonic kidney 293 cells expressing either alpha(6) or alpha(1) subunitcontaining GABA(A) receptors, time to block of I-GABA by MeHg was comparable. Thus, the presence of the alpha(6) subunit alone may not underlie the differential effects of MeHg on I-GABA observed in cerebellar granule and cortical neurons; other factors are likely to be involved as well. |
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