Multiple and plastic receptors mediate tonic GABAA receptor currents in the hippocampus - PubMed (original) (raw)

The role of α5-containing GABAA receptors in mediating tonic currents with increases in [GABA]o in pyramidal cells from control and epileptic animals. A, Time course of _I_hold in pyramidal cells from control and epileptic animals recorded in the presence of [GABA]o = 5 μ

m

. L-655,708 (50 μ

m

), an inverse agonist of α5GABAARs, reduced _I_hold in control but not in epileptic principal cells. Picrotoxin (100 μ

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) was added at the end of each experiment to estimate the total amount of GABAergic _I_hold within each cell. B, Summary histogram of the effect of L-655, 708 on _I_hold recorded in the presence of GABA (5 μ

m

). L-655, 708 (50 μ

m

) reduced by 44 and 36% the picrotoxin-sensitive _I_hold recorded from control and sham (S PC) pyramidal cells, respectively (p = 0.6 for difference). Consistent with the downregulation of α5 in epilepsy, L-655, 708 changed _I_hold by + 7% in pyramidal cells from pilocarpine epilepsy model (EP PC) and by -2% in pyramidal cells from kainic acid epilepsy model (KA PC), indicating that the increase in tonic current induced in these cells by increasing [GABA]o is not mediated by α5GABAARs. Error bars represent SEM. C, The top panel shows representative examples of the time course of _I_hold in two cultured hippocampal neurons after the application of different GABA concentrations (left, 0.1 μ

m

; right, 1 μ

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). Each dot represents the average ± SEM of three consecutive data points acquired every 4 s, normalized by the picrotoxin-sensitive tonic current measured at each [GABA]o. L-655,708 (50 μ

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) blocks a bigger fraction of the picrotoxin-sensitive _I_hold at higher GABA concentrations. The bottom panel summarizes the proportion of the picrotoxin-sensitive _I_hold mediated by α5GABAARs at five different GABA concentrations. The contribution of α5GABAARs to _I_hold becomes detectable for concentrations of GABA >0.3 μ

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and progressively increases to ∼40% when [GABA]o = 2 μ

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. Each point represents the average ± SEM of three or four cells, for a total of 18 recorded neurons. D, The fraction of the picrotoxin-sensitive _I_hold in GABA (2 μ

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) that is sensitive to L-655,708 at three different concentrations, demonstrating exquisite sensitivity of the tonic current to low concentrations of L-655,708 and no change in this sensitivity at higher concentrations, implying that the higher concentrations are not acting at additional GABAA receptor subtypes. The top panel shows a representative example of the time course of applying 0.5 n

m

L-655,708 followed by picrotoxin. *p < 0.05 for difference between Ep PC and either PC or S PC and p < 0.01 for difference between KA PC and either PC or S PC; **p < 0.01. PTX, Picrotoxin.