Depression of early and late monosynaptic inhibitory postsynaptic potentials in hippocampal CA1 neurons following prolonged benzodiazepine administration: Role of a reduction in Cl driving force

Abstract

GABAergic synaptic responses were studied by direct, monosynaptic activation of GABAergic interneurons in the CA1 region of in vitro hippocampal slices from rats made tolerant to the benzodiazepine, flurazepam. Monosynaptic IPSPs were elicited in CA1 pyramidal neurons, following 1 week oral flurazepam administration, by electrical stimulation at the stratum oriens/stratum pyramidale or stratum radiatum/ stratum-lacanosum border < or = 0.5 mm from the recording electrode plane. Excitatory input to pyramidal cells and interneurons was eliminated by prior superfusion of the glutamate receptor antagonists, APV (50 microM) and DNQX (10 microM). GABAA receptor-mediated early IPSPs were further isolated by perfusion of the GABAB antagonist, CGP 35348 (25 microM) or by diffusion of Cs- from the recording electrode. GABAB receptor-mediated late IPSPs were pharmacologically isolated by perfusion of the GABAA antagonist, picrotoxin (50 microM). There was a significant decrease in the amplitude of pharmacologically isolated early and late IPSPs in FZP-treated neurons without a change in passive membrane properties. A shift of the early IPSP, but not the late IPSP, reversal potential in FZP-treated neurons suggested that a change in the driving force for anions, presumably Cl, in CA1 neurons was one important factor related to the decreased early IPSP amplitude after prolonged activation of GABAA receptors by flurazepam. A decreased early IPSP amplitude accompanied by a decreased late IPSP amplitude suggested that presynaptic GABA release onto FZP-treated pyramidal cells may also be reduced. We conclude from these data that an impairment of GABAergic transmission in CA1 pyramidal neurons associated with the development of tolerance during chronic benzodiazepine treatment may be related to the regulation of both pre- and postsynaptic mechanisms at the GABA synapse.

Progesterone Withdrawal Reduces Paired-Pulse Inhibition in Rat Hippocampus: Dependence on GABAA Receptor α4 Subunit Upregulation

“…Inhibitory pathways contributing to PPI in hippocampal slice have been well characterized and can involve both feed-forward and feed-back inhibitory circuits (Karnup and Stelzer 1999;Rogers and Hunter 1992;Turner 1990) with transmission via both GABA A and GABA B receptors. However, in the present study, the use of a 10-ms inter-pulse interval is selective for GABA A -versus GABA B -mediated events, because the latter exhibits a longer latency onset of response following an excitatory stimulus and is optimally observed with inter-pulse intervals of 200 ms (Davies et al 1990;Karlsson and Olpe 1989;Williams et al 1994;Zeng and Tietz 1997).…”

Post-hypoxic changes in rat cortical neuron GABAA receptor function require L-type voltage-gated calcium channel activation

“…Hypoxia/ischemia has been shown to increase intracellular chloride concentration in adult rat CA1 hippocampal neurons (Comerford et al, 2003; Inglefield and Schwartz-Bloom, 1998; Katchman et al, 1994; Mittmann et al, 1998) though the mechanism remains controversial. Changes in Cl− reversal potential in CA1 neurons were also seen after chronic BZ treatment, accompanied by GABA A R-mediated depolarization (Zeng et al, 1997). In our previous experiments, the resting membrane potential in cortical neurons at 13–15 days in vitro was around −65 mV (data not shown), hence the shift in chloride reversal potential from −59.5 ± 3.2 mV in control cells to −51.0 ± 3.1 mV recorded 24 h after hypoxia implies that GABA A R currents become significantly depolarizing after hypoxia and could increase neuronal excitability.…”

Benzodiazepine-mediated regulation of α1, α2, β1–3 and γ2 GABAA receptor subunit proteins in the rat brain hippocampus and cortex

“…The discrete suppression of GABA A receptor function in CA1 pyramidal cells during prolonged flurazepam administration 47,73,[75][76][77][78] and the associated regionally localized, timedependent reductions in the expression of specific GABA A receptor subunit mRNAs 65,66 warranted quantitative immunohistochemical studies of selected hippocampal and cortical GABA A receptor subunit proteins (a1, a2, b1-3 and g2). Taken together with the findings of in situ hybridization studies, 65,66 the results of these quantitative immunohistochemical studies suggested that changes in the expression of specific GABA A receptor mRNAs (a1 and b3 subunits) were mirrored by changes in their respective subunit proteins.…”

“…17,22,27,29,44,65,66,80,81 Such information may help to clarify the relationship between changes in protein expression (e.g., of a1, b2 and b3 subunits) and changes in the GABAergic function of some (CA1 pyramidal cells), but not other (CA3 pyramidal cells and dentate granule cells), hippocampal principal cell types in benzodiazepine-tolerant rats. 28,47,48,75,76,78 …”

“…The specific goal was to establish whether there is a relationship between the reduced expression of specific GABA A receptor subunit mRNAs in hippocampal and cortical neurons, 65,66 and the expression of selected GABA A receptor subunit proteins (a1, a2, b1-3 and g2). This relationship was investigated after an in vivo benzodiazepine treatment which reliably results in decreased GABA A receptor function in CA1 pyramidal cells, [73][74][75][76][77][78] but not dentate granule cells. 46 A reliable and sensitive computer-assisted image analysis method 23 was used to detect regional differences in GABA A receptor subunit protein immunostaining in the hippocampus, cortex and selected rat brain regions immediately after cessation of one-week oral flurazepam administration.…”