Xiao Cheng, Zhou Chunyi, Li Keyong, Ye Jiang-Hong
Department of Anesthesiology, Pharmacology and Physiology, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, 185 South Orange Avenue, Newark, NJ 07103, USA.
J Physiol. 2007 May 1;580(Pt.3):731-43. doi: 10.1113/jphysiol.2006.124099. Epub 2007 Feb 15.
Gamma-aminobutyric acid A receptor (GABA(A)R)-mediated postsynaptic currents (IPSCs) were recorded from dopaminergic neurons of the ventral tegmental area of young rats in acute brain slices and from mechanically dissociated neurons. Low concentrations (0.1-0.3 microm) of muscimol, a selective GABA(A)R agonist, increased the amplitude, and reduced the paired pulse ratio of evoked IPSCs. Moreover, muscimol increased the frequency but not the amplitude of spontaneous IPSCs (sIPSCs). These data point to a presynaptic locus of muscimol action. It is interesting that 1 microm muscimol caused an inhibition of sIPSCs, which was reversed to potentiation by the GABA(B) receptor antagonist CGP52432. Isoguvacine, a selective GABA(A)R agonist that belongs to a different class, mimicked the effects of muscimol on sIPSCs: it increased them at low (<or= 0.5 microm), and decreased them at a higher concentration (1 microm). Hence, the activation of presynaptic GABA(A)Rs facilitates GABA release, which is limited by presynaptic GABA(B)Rs. Furthermore, facilitation of sIPSCs by muscimol was eliminated in a medium containing tetrodotoxin or cadmium. It is noteworthy that sIPSC frequency was greatly increased by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol(gaboxadol, or THIP), an agonist with preferential effects on extrasynaptic GABA(A)Rs containing alpha4betadelta subunits, or by guvacine, a GABA transport blocker, which increases ambient GABA levels. In addition, sIPSC frequency was attenuated by furosemide, a selective antagonist of alpha6 subunits. Thus, the presynaptic GABA(A)Rs may be situated at extrasynaptic sites and may contain alpha4/6betadelta subunits. Given the marked sensitivity of extrasynaptic GABA(A)Rs to ambient GABA, alcohols and anaesthetics, these receptors may present a critical site for regulating synaptic function in the developing brain in both physiological and pathological situations.
在急性脑片的幼鼠腹侧被盖区多巴胺能神经元以及机械分离的神经元中记录到γ-氨基丁酸A受体（GABA(A)R）介导的突触后电流（IPSCs）。低浓度（0.1 - 0.3微摩尔）的蝇蕈醇，一种选择性GABA(A)R激动剂，增加了诱发IPSCs的幅度，并降低了其配对脉冲比率。此外，蝇蕈醇增加了自发IPSCs（sIPSCs）的频率，但未改变其幅度。这些数据表明蝇蕈醇的作用位点在突触前。有趣的是，1微摩尔的蝇蕈醇引起了sIPSCs的抑制，而GABA(B)受体拮抗剂CGP52432可将其逆转至增强作用。异鹅膏蕈氨酸，一种属于不同类别的选择性GABA(A)R激动剂，模拟了蝇蕈醇对sIPSCs的作用：在低浓度（≤0.5微摩尔）时增加sIPSCs，在高浓度（1微摩尔）时降低sIPSCs。因此，突触前GABA(A)Rs的激活促进了GABA释放，而这受到突触前GABA(B)Rs的限制。此外，在含有河豚毒素或镉的培养基中，蝇蕈醇对sIPSCs的促进作用被消除。值得注意的是，4,5,6,7 - 四氢异恶唑并[5,4 - c]吡啶 - 3 - 醇（加波沙朵，或THIP），一种对含有α4βδ亚基的突触外GABA(A)Rs有优先作用的激动剂，或通过增加周围GABA水平的GABA转运阻滞剂胍氨酸，可使sIPSC频率大幅增加。此外，速尿，一种α6亚基的选择性拮抗剂，可使sIPSC频率降低。因此，突触前GABA(A)Rs可能位于突触外位点，并且可能含有α4/6βδ亚基。鉴于突触外GABA(A)Rs对周围GABA、酒精和麻醉剂具有显著敏感性，这些受体可能是在生理和病理情况下调节发育中大脑突触功能的关键位点。
