细胞外Ca^(2+)对爪蟾脑片神经元微抑制性突触后电流的调制

Modulation of extracellular calcium on miniature inhibitory postsynaptic currents of Xenopus' optic tectal neurons

应用盲法膜片钳全细胞记录技术,以爪蟾视顶盖神经元微抑制性突触后电流(miniature inhibitory postsyn-aptic currents,mIPSCs)为指标,观察了细胞外 Ca^(2+)对爪蟾脑片神经元突触后 mIPSC的调制。结果表明:用细胞外无钙或无钙含乙二醇双乙胺醚-N,N’-四乙酸(EGTA)(200 nmol/L-2mmol/L)溶液灌流,均可使mIPSCs的发放频率降低;非特异性钙离子拮抗剂氯化铬(100μmol/L)也可使mIPSCs的频率降低;内质网钙泵抑制剂thapsigargin (TG)以及内质网ryanodine受体(RyR)激动剂ryanodine均可使mIPSCs频率升高,内质网RyR拮抗剂普鲁卡因则可降低mIPSCs的频率;磷脂酶C抑制剂U73122也可降低mIPSCs的频率,对三磷酸肌醇(inositol,4,5-triphosphate,IP_3)水平有抑制作用的咖啡因亦可显著地降低mIPSCs,甚至完全抑制mIPSCs。从而表明:对突触前神经元及其末梢,细胞外钙离子可通过细胞膜上的钙通道进入细胞内,使细胞内钙浓度升高,突触前神经末梢释放出更多的神经递质,进而可能使突触后mIPSCs的频率增加;突触前细胞内钙储池上的RyR和IP_3R均可介导钙从其中释放,并也可使突触前细胞内的钙离子浓度升高,进而可能使突触后mIPSCs的发放频率增加。

Experiments were performed to study the relationship between presynaptic calcium and minia-ture inhibitory postsynaptic currents (mIPSCs) using voltage-clamp technique with whole cell mode in Xe-nopus, optic tectal slices. The results are as follows: The frequency of mIPSCs decreased from 1. 91±0. 59Hz to 0. 34±0. 09 Hz in calcium-free solution (paired t test, P =0. 019, n = 8), the value of mIPSCs fre-quency being (25. 5±4. 4) % of control. In order to rule out the effect of remaining calcium in perfusingsolution we further applied calcium-free solution containing egtazic acid, ethyleneglycol-bis (β-aminoethyle-ther) -N, N'-tetraacetic acid (EGTA) (200 nmol/L - 2 mmol/L). The mIPSCs frequency changed from1. 57±0. 57 in control to 0. 89±0. 41 Hz in calcium-free solution containing EGTA (paired t test, P =0. 002, n = 12), decreasing to (40. 0±5. 9) % of control. There is no statistical difference in the resultsbetween Ca^(2+) -free perfusion and Ca^(2+) -free solution containing EGTA (paired t test, P = 0. 74, n = 9).When cadmium chloride (CdCl_2, 100 μmol/L), a non-specific Ca^(2+) channel blocker, was applied to thebath solution, the mIPSCs' frequency decreased from 1. 15±0. 34 Hz in control to 0. 25±0. 09 Hz inCdCl_2-containing solution (paired t test, P = 0. 008, n = 11 ), reaching (29. 25±6. 1 ) % of control. How-ever, the amplitude did not change much. An endoplasmic reticulum pump inhibitor thapsigargin increased the mIPSCs frequency from 0. 93±0. 19 Hz to 1. 58±0. 28 Hz (paired t test, P =0. 002, n = 11). Thevalue in the latter is (214. 6±49. 1)% of that in the former. In order to exclude the remaining calciumfrom the bathing solution, the mIPSCs frequency was first recorded from calcium-free solution as control(0. 41±0. 08 Hz) and then from calcium-free containing TG solution (8 - 16 μmol/L) (0. 71±0. 15 Hz)(paired t test, P = 0. 026, n = 5), increasing to (175. 0±14. 6) % of control. The endoplasmic reticulumCa^(2+) store RyR agonist (ryanodine, 10 - 100 nmol/L) enhanced mIPSCs' frequency from 1. 18±0. 40 Hzto 1. 80±0. 44 Hz with increment of (261. 8±89. 5)% (paried t test, P =0. 004, n = 6). However, theendoplasmic reticulum RyR antagonist (procaine, 2 mmol/L) could inhibit mIPSCs from 1. 26±0. 35 Hz to0. 43±0. 15 Hz (paired t test, P = 0. 027, n = 6 ). U73122 (40 μmol/L), a phosphalipase C inhibitor,decreased also mIPSCs frequency from 2. 01±0. 58 Hz in control to 0. 92±0. 40 Hz in U73122-containingsolution (paired t test, P = 0. 002, n = 10). Caffeine (10 mmol/L) markedly dinimished mIPSCs frequen-cy from 3. 22±0. 64 Hz to 0. 15±0. 30 Hz (paired t test, P =0. 003, n =7), which is (4. 6±2. 9)%compared to control. Furthermore, in some cases the caffeine could abolish mIPSCs. Taken together, ourresults demonstrated that cytosolic calcium might be important for mediating the generation of mIPSCs. Thecytosolic calcium could be increased by calcium influx through membrane calcium channel on presynapticmembrane, and/or by calcium released through RyR and IP_3R in presynaptical internal store. The in-creased cytosolic Ca^(2+) both from external solution or internal Ca^(2+) stores might increase the transmitter vise-cles at the presynaptic terminal, which in turn results in the increase of the mIPSCs frequency in the posts-ynaptic neurons.