Role Of Hco3- Ions In Depolarizing Gaba-a Receptor-mediated Responses In Pyramidal Cells Of Rat Hippocampus
Creator
Grover L M; Lambert N A; Schwartzkroin P A; Teyler T J
Publisher
Journal of Neurophysiology
Date
1993
1993-05
Description
1. Activation of GABA(A) receptors can produce both hyperpolarizing and depolarizing responses in CA1 pyramidal cells. The hyperpolarizing response is mediated by a Cl- conductance, but the ionic basis of the depolarizing response is not clear. We compared the GABA(A) receptor-mediated depolarizations induced by synaptically released gamma-aminobutyric acid [GABA; depolarizing inhibitory postsynaptic potentials (dIPSPs)] with those produced by exogenous GABA (depolarizing GABA responses). Short trains of high-frequency (200 Hz) stimuli were used to generate dIPSPs. We found that dIPSPs generated by trains of stimuli and depolarizing responses to exogenous GABA were accompanied by a conductance increase and had a similar reversal potential, indicating a similar ionic basis for both responses. 2. We wished to determine whether an HCO3- current contributed to the GABA(A)-mediated depolarizations. We found that dIPSPs and depolarizing GABA responses were sensitive to perfusion with HCO3--free medium. Interpretation of these data was complicated by the mixed nature of the responses: dIPSPs were invariably accompanied by conventional, Cl--mediated fast hyperpolarizing IPSPs (fIPSPs), and response to exogenous GABA usually consisted of biphasic hyperpolarizing and depolarizing responses. However, it was sometimes possible to elicit responses to GABA that appeared purely depolarizing (monophasic depolarizing GABA responses). 3. We analyzed monophasic depolarizing GABA responses and found no change in reversal potential when slices were perfused with HCO3--free medium. We also made whole-cell recordings from CA1 pyramidal cells, attempting to reduce [HCO3-]i, and compared the reversal potential for monophasic depolarizing GABA responses with similar responses recorded with fine intracellular microelectrodes. We found no difference in reversal potential. We also examined effects of the carbonic anhydrase inhibitor acetazolamide (ACTZ) on depolarizing GABA responses. ACTZ reduced these responses but did not change their reversal potential. 4. Effects of HCO3--free medium were not specific to GABA(A) receptor-mediated responses. GABA(B) receptor-mediated slow IPSPs (sIPSPs) were also reduced, as were excitatory postsynaptic potentials (EPSPs). Analyses of field potentials and spontaneous fIPSPs suggested a decrease in presynaptic excitability during perfusion with HCO3--free medium. In addition, pyramidal cells showed decreased input resistance when perfused with HCO 3--free medium. 5. The sensitivity of GABA(A) receptor-mediated depolarizations to HCO3--free medium can be explained by a decrease in presynaptic excitability and an increased resting conductance in postsynaptic neurons. Reduced presynaptic excitability and resting input resistance are also likely causes of the reduction in fast IPSPs, slow IPSPs, and EPSPs in HCO3--free medium. We suggest that these nonspecific effects of HCO3--free medium may be a consequence of an extracellular acidification. These data do not provide convincing evidence for involvement of an HCO3- conductance in the generation of dIPSPs and depolarizing GABA responses.
Nuclear receptor-mediated repression of human cholesterol 7alpha-hydroxylase gene transcription by bile acids.
Creator
Chen W; Owsley E; Yang Y; Stroup D; Chiang J Y
Publisher
Journal of lipid research
Date
2001
2001-09
Description
Hydrophobic bile acids strongly repressed transcription of the human cholesterol 7alpha-hydroxylase gene (CYP7A1) in the bile acid biosynthetic pathway in the liver. Farnesoid X receptor (FXR) repressed CYP7A1/Luc reporter activity in a transfection assay in human liver-derived HepG2 cells, but not in human embryonic kidney (HEK) 293 cells. FXR-binding activity was required for bile acid repression of CYP7A1 transcription despite the fact that FXR did not bind to the CYP7A1 promoter. FXR-induced liver-specific factors must be required for mediating bile acid repression. Bile acids and FXR repressed endogenous CYP7A1 but stimulated alpha-fetoprotein transcription factor (FTF) and small heterodimer partner (SHP) mRNA expression in HepG2 cells. Feeding of rats with chenodeoxycholic acid repressed CYP7A1, induced FTF, but had no effect on SHP mRNA expression in the liver. FTF strongly repressed CYP7A1 transcription in a dose-dependent manner, and SHP further inhibited CYP7A1 in HepG2 cells, but not in HEK 293 cells. FXR only moderately stimulated SHP transcription, whereas FTF strongly inhibited SHP transcription in HepG2 cells. Results revealed that FTF was a dominant negative factor that was induced by bile acid-activated FXR to inhibit both CYP7A1 and SHP transcription. Differential regulation of FTF and SHP expression by bile acids may explain the wide variation in CYP7A1 expression and the rate of bile acid synthesis and regulation in different species.