Singer J H, Talley E M, Bayliss D A, Berger A J
Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle, Washington 98195-7290, USA.
J Neurophysiol. 1998 Nov;80(5):2608-20. doi: 10.1152/jn.1998.80.5.2608.
Using an in vitro rat brain stem slice preparation, we examined the postnatal changes in glycinergic inhibitory postsynaptic currents (IPSCs) and passive membrane properties that underlie a developmental change in inhibitory postsynaptic potentials (IPSPs) recorded in hypoglossal motoneurons (HMs). Motoneurons were placed in three age groups: neonate (P0-3), intermediate (P5-8), and juvenile (P10-18). During the first two postnatal weeks, the decay time course of both unitary evoked IPSCs [mean decay time constant, taudecay = 17.0 +/- 1.6 (SE) ms in neonates and 5.5 +/- 0.4 ms in juveniles] and spontaneous miniature IPSCs (taudecay = 14.2 +/- 2.4 ms in neonates and 6.3 +/- 0.7 ms in juveniles) became faster. As glycine uptake does not influence IPSC time course at any postnatal age, this change most likely results from a developmental alteration in glycine receptor (GlyR) subunit composition. We found that expression of fetal (alpha2) GlyR subunit mRNA decreased, whereas expression of adult (alpha1) GlyR subunit mRNA increased postnatally. Single GlyR-channels recorded in outside-out patches excised from neonate motoneurons had longer mean burst durations than those from juveniles (18.3 vs. 11.1 ms). Concurrently, HM input resistance (RN) and membrane time constant (taum) decreased (RN from 153 +/- 12 MOmega to 63 +/- 7 MOmega and taum from 21.5 +/- 2.7 ms to 9.1 +/- 1.0 ms, neonates and juveniles, respectively), and the time course of unitary evoked IPSPs also became faster (taudecay = 22.4 +/- 1.8 and 7.7 +/- 0.9 ms, neonates vs. juveniles, respectively). Simulated synaptic currents were used to probe more closely the interaction between IPSC time course and taum, and these simulations demonstrated that IPSP duration was reduced as a consequence of postnatal changes in both the kinetics of the underlying GlyR channel and the membrane properties that transform the IPSC into a postsynaptic potential. Additionally, gramicidin perforated-patch recordings of glycine-evoked currents reveal a postnatal change in reversal potential, which is shifted from -37 to -73 mV during this same period. Glycinergic PSPs are therefore depolarizing and prolonged in neonate HMs and become faster and hyperpolarizing during the first two postnatal weeks.
利用体外大鼠脑干切片制备技术，我们研究了甘氨酸能抑制性突触后电流（IPSCs）的出生后变化以及被动膜特性，这些特性构成了舌下运动神经元（HMs）中记录到的抑制性突触后电位（IPSPs）发育变化的基础。运动神经元被分为三个年龄组：新生儿组（P0 - 3）、中间组（P5 - 8）和幼年组（P10 - 18）。在出生后的前两周，单突触诱发IPSCs的衰减时间进程[平均衰减时间常数，新生儿组的τdecay = 17.0 ± 1.6（SE）ms，幼年组为5.5 ± 0.4 ms]以及自发微小IPSCs（新生儿组的τdecay = 14.2 ± 2.4 ms，幼年组为6.3 ± 0.7 ms）都变快了。由于甘氨酸摄取在任何出生后年龄都不影响IPSC时间进程，这种变化很可能是由于甘氨酸受体（GlyR）亚基组成的发育性改变所致。我们发现，胎儿（α2）GlyR亚基mRNA的表达在出生后下降，而成年（α1）GlyR亚基mRNA的表达在出生后增加。从新生儿运动神经元上切下的外侧向外膜片上记录的单个GlyR通道的平均爆发持续时间比幼年组的更长（18.3对11.1 ms）。同时，HM的输入电阻（RN）和膜时间常数（τm）下降（新生儿组的RN从153 ± 12 MΩ降至63 ± 7 MΩ，τm从21.5 ± 2.7 ms降至9.1 ± 1.0 ms，幼年组的情况分别对应），单突触诱发IPSPs的时间进程也变快了（τdecay = 22.4 ± 1.8和7.7 ± 0.9 ms，分别为新生儿组与幼年组）。利用模拟突触电流更深入地探究IPSC时间进程与τm之间的相互作用，这些模拟表明，由于基础GlyR通道动力学和将IPSC转化为突触后电位的膜特性的出生后变化，IPSP持续时间缩短。此外，短杆菌肽穿孔膜片钳记录的甘氨酸诱发电流显示出出生后反转电位的变化，在此期间从 - 37 mV变为 - 73 mV。因此，在新生儿HMs中，甘氨酸能PSPs是去极化且持续时间延长的，在出生后的前两周变得更快且超极化。
