Protective mechanism of fetal brain against hypoxia: in vivo optical recordings

众所周知，胎儿大脑对缺氧有很强的抵抗力。分娩过程中，胎儿间歇性严重缺氧持续时间长。然而，几乎胎儿出生时没有脑损伤。因此，认为胎脑可能具有抗缺氧的保护机制。本实验采用光学成像系统，通过对多巴胺能神经元突触传递的影响，研究了其保护机制。孕鼠（SpragueDawly大鼠，3月龄）用乌拉坦（1.2- 1.4g/kg，i. P.）。将胎儿与母体通过脐带连接，固定在立体定位仪上。通过剖腹产暴露子宫，并部分切割以暴露胎儿。电刺激胎儿上级丘（SC）时，将双极电极（不锈钢丝）插入SC的喙部，以1-3 mA的电流，用单脉冲或串脉冲（10个脉冲）进行SC的局灶性刺激。荧光钙敏感染料（Fura-2/AM）w ...更多信息 通过光学成像系统（ARGUS/HiSCA）记录Ca 2+敏感染料荧光的分数变化。带着母体的胎儿被放在显微镜下。1）脐带阻断过程中SC内[Ca ~（2+）]I升高，脐带阻断解除后逐渐恢复。用非NMDA受体拮抗剂（CNQX）预处理后，NMDA受体拮抗剂（AP-5）可抑制SC区[Ca ~（2+）] i的增加。刺激电流从1 mA增加到3 mA，[Ca 2 +]i增加幅度更大。串脉冲刺激诱导延长的反应（>10秒）（迟发反应）。早期反应被CNQX和AP-5拮抗。AP-5可拮抗迟发反应。3）阻断脐带后3 h，刺激SC可诱发早期反应，但迟发反应明显减弱。CNQX能拮抗早期反应，AP-5几乎不能拮抗早期反应。提示局灶性SC刺激和缺氧引起的胎鼠Ca ~（2+）内流是由多巴胺能突触传递介导的。此外，在接受缺氧后，NMDA受体可能随着时间的推移而失活。这可能有助于保护胎脑免受过量的Ca 2+内流。少

It is known that fetal brain is strong against hypoxia. During delivery, intermittent severe hypoxia on fetus lasts for a long time. However, Almost fetus was born without the brain damage. Therefore, it is thought the fetal brain may have protective mechanism against hypoxia. We studied the protective mechanism by forcusing on glutaminergic synaptic transmission, using the optic imaging system. The termed pregnant rat (Sprague Dawly rat, 3 months old) was anesthetized with uretane (1.2-1.4 g/kg, i. P.). The fetus which was connected with dam by umbilical cord, was fixed on a stereotaxic instrument. The uterus was exposed by caesarean section and partially cut to exposure of the fetus. For electrical stimulation of the fetal superior colliculus (SC), a bipolar electrode (stainless steel wire) was inserted into the rostral part of the SC. Focal stimulation of the SC was made by a single pulse or train pulse (10 pulses) at currents of 1-3 mA. Fluorescent Ca2+ sensitive dye (Fura-2/AM ) w … More as loaded into the SC regions for 90 min. The fractional changes in fluorescence of Ca2+ sensitive dye were recorded by an optical imaging system (ARGUS/HiSCA). The fetus with the dam was set under the microscope. The acute hypoxia was performed with occluding the umbilical cord by a fine clip.1) [Ca2+]I in SC was increased during with the occlusion of the umbilical cord and recovered gradually after the release of the umbilical cord occlusion. The increased [Ca2+]I was attenuated by NMDA receptor antagonist (AP-5) after pretreatment with non-NMDA receptor antagonist (CNQX ).2) [Ca2+]I in the SC regions was steeply increased by the focal SC stimulation of the train pulses (the early response). Increasing the stimulus current from 1 to 3 mA produced greater increases in [Ca2+]i. The train pulse stimulation induced a prolonged response (>10 sec) (the late response). The early response was antagonized with CNQX and AP-5. The late response was antagonized by AP-5.3) At 3hrs after the occlusion of umbilical cord, the early response was induced with the focal SC stimulation, but the late response was markedly reduced. The early response was antagonized by CNQX but not almost by AP-5.These results suggest that the Ca2+influx following focal SC stimulation and hypoxia in fetal rats is mediated by glutaminergic synapse transmission. Further, after receiving hypoxia, NMDA receptor may be inactivated with a lapse of time. This may contribute to protect fetal brain from an excess of Ca2+influx. Less