Regulation of uterine GABA_A receptor subunit expression throughout pregnancy

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[OBJECTIVE] A new subunit of GABA_A receptor, named it, is abundant only in the uterus. Since allopregnanolone works through the GABA_A receptor and reduce uterine contraction, α subunit might change the sensitivity of GABA_A receptor to allopregnanolone and modulate the parturition. The purpose of this study was to determine the expressions of GABA_A receptor subunits in the uteri from various gestational ages, especially for the n subunit, and examine the correlation between the combination of the subunits and physiological properties.[METHODS] Uterine tissues from the timed pregnant rats (13.5 - 21.5 gestational days), non-pregnant proven breeders and postpartum day-1 rats were used to obtain plasma membranes and RNAs. Following RT-PCR and subcloning, RNase protection assay was performed to determine the mRNA expressions of GABA_A receptor π subunit and 3 HSD throughout the pregnancy. We also examined the expressions of the other GABA_A receptor subunits in the rat uteri throughout … More the pregnancy by semi-quantitative PCR. Then we performed ^3H-muscimol binding assay to the GABA_A receptor on the plasma membranes in various gestational uteri and examined the difference of the effect of allopregnanolone on the binding.[RESULTS] The expression of GABA_A π subunit mRNA was decreased at onset of labor, and 3αHSD mRNA were decreased on postpartum day-1. Semi-quantitative PCR study showed that GABA_Aα1 subunit expression was increased, while α2 and α subunits expressions were decreased during the pregnancy, and α3 subunit only appeared on postpartum day-1. GABA_A receptor in non-pregnant uterus appeared to have the high affinity binding site (Kd=1.79nM, Bmax=4.9pmole/mg protein) and the low affinity binding site (Kd=52.6nM, Bmax=12.1pmole/ mg protein) to ^3H-muscimol. And the results of the binding assay showed that the GABA_A receptor in uteri was most sensitive to allopregnanolone on 19.5 days gestation and was least sensitive in labor.[CONCLUSION] We detected that the subunits combinations of the GABA_A receptor are different in the uteri from various gestational days. The function of the each combination of the subunits is still unclear. However it was suggested that the difference of GABA_A receptor subunit expression during pregnancy may change the sensitivity of GABA_A receptor to allopregnanolone and have a regulating effect on the parturition. Less