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

Experiments were performed to study the relationship between presynaptic calcium and miniature inhibitory postsynaptic currents (mIPSCs) using voltage-clamp technique with whole cell mode in Xenopus optic tectal slices. The results are as follows: The frequency of mIPSCs decreased from 1.91+/-0.59 Hz to 0.34+/-0.09 Hz in calcium-free solution (paired t test, P=0.019, n=8), the value of mIPSCs frequency being (25.5+/-4.4)% of control. In order to rule out the effect of remaining calcium in perfusing solution we further applied calcium-free solution containing egtazic acid, ethyleneglycol-bis (beta-aminoethylether)-N,N -tetraacetic acid (EGTA) (200 nmol/L-2 mmol/L). The mIPSCs frequency changed from 1.57+/-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 results between Ca(2+)-free perfusion and Ca(2+)-free solution containing EGTA (paired t test, P=0.74, n=9). When cadmium chloride (CdCl2, 100 micromol/L), a non-specific Ca(2+) channel blocker, was applied to the bath solution, the mIPSCs frequency decreased from 1.15+/-0.34 Hz in control to 0.25+/-0.09 Hz in CdCl2-containing solution (paired t test, P=0.008, n=11), reaching (29.25+/-6.1)% of control. However, 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). The value in the latter is (214.6 +/-49.1)% of that in the former. In order to exclude the remaining calcium from 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 micromol/L)(0.71+/-0.15 Hz)(paired t test, P=0.026, n=5), increasing to (175.0+/-14.6)% of control. The endoplasmic reticulum Ca(2+) store RyR agonist (ryanodine, 10-100 nmol/L) enhanced mIPSCs frequency from 1.18+/-0.40 Hz to 1.80+/-0.44 Hz with increment of (261.8+/-89.5)% (paired t test, P=0.004, n=6). However, the endoplasmic reticulum RyR antagonist (procaine, 2 mmol/L) could inhibit mIPSCs from 1.26+/-0.35 Hz to 0.43+/-0.15 Hz (paired t test, P=0.027, n=6). U73122 (40 micromol/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-containing solution (paired t test, P=0.002, n=10). Caffeine (10 mmol/L) markedly diminished mIPSCs frequency 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, our results demonstrated that cytosolic calcium might be important for mediating the generation of mIPSCs. The cytosolic calcium could be increased by calcium influx through membrane calcium channel on presynaptic membrane, and/or by calcium released through RyR and IP(3)R in presynaptical internal store. The increased cytosolic Ca(2+) both from external solution or internal Ca(2+) stores might increase the transmitter vesicles at the presynaptic terminal, which in turn results in the increase of the mIPSCs frequency in the postsynaptic neurons.