Benzodiazepine tolerance at GABAergic synapses on hippocampal CA1 pyramidal cells

Abstract

Modulation of GABA function following 1 week oral administration of flurazepam (FZP) was investigated in chloride-loaded, rat hippocampal CA1 pyramidal neurons. Rats were sacrificed 2 or 7 days after ending drug treatment, when anticonvulsant tolerance was present or absent in vivo, respectively. Spontaneous (s)IPSCs and miniature (m)IPSCs were recorded using whole-cell voltage-clamp techniques. s/mIPSCs were bicuculline-sensitive, voltage-dependent, and reversed their polarity at 0 mV, the predicted E(Cl-). Comparisons of s/mIPSCs between FZP-treated and control groups were made at Vh = -90, -70, and -50 mV. The frequency of sIPSCs, but not mIPSCs, was significantly decreased in FZP-treated neurons 2 days, but not 7 days, after FZP treatment, suggesting a decrease in interneuron activity. These conclusions were supported by the negative findings of additional studies of [3H]GABA release from hippocampal slices and [3H]GABA uptake from hippocampal synaptosomes. The lack of change in the paired-pulse depression of GABA(B)-mediated IPSPs suggested that autoreceptor function was also not impaired following chronic FZP treatment. A large reduction in both sIPSC and mIPSC amplitude (60%) in FZP-treated neurons, the absence of mIPSCs in one-third of FZP-treated cells, and a measurable reduction in synaptic and unitary conductance confirmed that postsynaptic GABA(A) receptor function was profoundly impaired in FZP-treated CA1 neurons. Zolpidem, an alpha1-selective benzodiazepine receptor ligand, enhanced mIPSC amplitude and decay, but its ability to prolong mIPSC decay was reduced in FZP-treated neurons. Several pre- and postsynaptic changes at GABAergic synapses on CA1 pyramidal cells might be related to the decreased tonic GABA inhibition in FZP-treated CA1 neurons associated with the expression of benzodiazepine anticonvulsant tolerance.

Transient Plasticity of Hippocampal CA1 Neuron Glutamate Receptors Contributes to Benzodiazepine Withdrawal-Anxiety

“…Increases in a4 subunit protein were implicated in withdrawal from ethanol (Matthews et al, 1998;Sanna et al, 2003) and BZs (Follesa et al, 2001), while neurosteroid treatment was associated with withdrawal-anxiety, upregulation of GABAR a4 subunits and a reduction in mIPSC decay (Smith, 2002). Although a4 subunit protein was upregulated immediately after 1-week FZP treatment (Chen et al, 2000), mIPSC decay was unaltered on days 1 and 2 of withdrawal (Table 2; Zeng and Tietz, 1999). The lack of change in mIPSC amplitude or decay when withdrawal-anxiety is present weakens support for GABAR-mediated mechanisms in the expression of BZ withdrawal-anxiety and suggests a more prominent role for glutamatergic-mediated mechanisms.…”

“…Chronic BZ receptor activation modifies the GABAergic system in brain regions with key roles in modulating anxiety states: frontal cortex, hippocampus, and amygdala Millan, 2003;Tietz et al, 1986;Zeng and Tietz, 1999), and BZ withdrawal may provide the necessary stimulus for local homeostatic regulation of excitationinhibition in these brain areas. The septo-hippocampal system, for example, is a primary neural substrate of anxiety in that all clinically effective anxiolytics, including BZs, have the capacity to raise the septal stimulus intensity needed to drive hippocampal theta activity (McNaughton and Gray, 2000).…”

“…To assess the status of GABAR-mediated inhibition at the time when anxiety was present, GABAR-mediated miniature inhibitory postsynaptic currents (mIPSCs) were recorded in symmetrical Cl À solutions in hippocampal slices from 1-day FZPwithdrawn rats in which AMPA or NMDA responses were also recorded. As detailed previously Zeng and Tietz, 1999), patch pipettes for mIPSC recording were filled with (in mM): CsCl, 130; HEPES, 10; EGTA, 1; CaCl 2 , 0.5; MgCl 2 , 2; Mg-ATP, 2; QX-314, 2; pH 7.2 adjusted with CsOH. Slices were superfused with 1 mM TTX, 50 mM APV, 10 mM DNQX and 25 mM CGP-35348.…”

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

“…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.…”

Aniracetam Reversed Learning and Memory Deficits Following Prenatal Ethanol Exposure by Modulating Functions of Synaptic AMPA Receptors

“…Slices were visualized with Nomarski differential interference contrast optics using an Olympus BX51WI (Olympus, USA) equipped with a water immersion lens. In the presence of channel blockers (1 mM TTX, 100 mM APV, and 50 mM picrotoxin), we recorded the AMPAR-mediated, action potential independent mEPSCs in CA1 pyramidal neurons, using whole-cell configuration as described previously (Zeng and Tietz, 1999). At the end of each experiment, 30 mM CNQX was added to block AMPA currents.…”