Silent GABA_ASynapses during Flurazepam Withdrawal Are Region-Specific in the Hippocampal Formation

Abstract

Whole-cell patch-clamp recordings were made from CA1 pyramidal and dentate gyrus granule cells (GCs) in hippocampal slices to assess the effects of withdrawal from chronic flurazepam (FRZ) treatment on the function of synaptic GABA A receptors. In slices from control rats, acute perfusion of FRZ (30 M) increased the monoexponential decay time constant of miniature IPSCs (mIPSCs) in CA1 and GCs (from 3.4 Ϯ 0.6 to 7.6 Ϯ 2.1 msec and from 4.2 Ϯ 0.6 to 7.1 Ϯ 1.8 msec, respectively) but did not change their mean conductance, 10 -90% rise time, or frequency of occurrence. Withdrawal (2-5 d) from chronic in vivo FRZ treatment (40 -110 mg/kg per day, per os) resulted in a dramatic loss of mIPSCs in CA1 neurons. On day 5 of withdrawal, no mIPSCs could be recorded in 40% of CA1 pyramidal cells. In the remaining 60% of the neurons, mIPSCs had a reduced mean conductance (from 0.78 Ϯ 0.12 nS in vehicle-treated controls to 0.31 Ϯ 0.05 nS) and a diminished frequency of occurrence (from 20.7 Ϯ 7.9 to 4.1 Ϯ 0.6 Hz). We have estimated that Ͼ80% of GABA A synapses on CA1 pyramidal cells had become silent, whereas at still-active synapses the number of functional GABA A receptor channels decreased by 60%. This reduction rapidly reverted to control levels on day 6 of withdrawal. FRZ withdrawal did not alter mIPSC properties in GCs. Our results are consistent with the hypothesis that chronic benzodiazepine treatment leads to a reduced number of functional synaptic GABA A receptors in a region-specific manner that may stem from differences in the subunit composition of synaptic GABA A receptors. Key words: withdrawal; inhibitory synapses; drug dependence; benzodiazepines; GABA A ; IPSCs; hippocampusBenzodiazepines (BZ) commonly are used as anxiolytic, antiepileptic, sedative hypnotic, and muscle relaxant agents. However, their usefulness in chronic therapy is limited by a combination of tolerance, side effects, abuse potential, and interaction with ethanol (File, 1993). Furthermore, withdrawal symptoms develop after prolonged exposure to BZs, and these symptoms may occur even after low to moderate doses (Gallager and Primus, 1993;Klein and Harris, 1996).During prolonged BZ treatment tolerance develops and is associated with the progressive loss of drug effectiveness. It implies an increase of the dose required to obtain therapeutic effects. Cessation of drug treatment produces a behavioral syndrome called withdrawal, which is characterized by agitation, anxiety, tremor, insomnia, or convulsions (Schoch et al., 1993); these symptoms can be relieved by readministering the drug, and therefore they define a state of drug dependence. Animal studies have revealed that withdrawal symptoms after the termination of chronic BZ treatment may be blocked by the application of BZ receptor antagonists Gallager, 1985, 1988).The loss of anticonvulsant activity after chronic BZ treatment is thought to be associated with an uncoupling of BZ and GABA binding at the level of GABA A receptors (Gallager et al., 1984;Marley and Gallager, 1989). In some stud...

Reduced Inhibition and Sensitivity to Neurosteroids in Hippocampus of Mice Lacking the GABAAReceptor δ Subunit

“…4) that were slower than those reported previously for rat dentate granule neurons (Buhl et al 1996;De Koninck and Mody 1996;Poisbeau et al 1997). The slower kinetics could be explained by using lower than physiological temperature (32°C), by use of higher tip resistance recording electrodes (8 -15 M⍀) and incomplete series resistance compensation (ϳ70%) during our recordings.…”

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

“…Since hippocampal glutamatergic afferents to septal nucleus can regulate expression of anxiety in the elevated plus-maze (Menard and Treit, 2001), hippocampal hyperexcitability, as observed in the present study (Figure 5), may provide a neurophysiological mechanism for expression of anxiety after drug withdrawal (Figure 4). Indeed, spectral analysis of hippocampal electrical activity during withdrawal from 1-week FZP reveals increased power of a 7 Hz (theta) peak (Poisbeau et al, 1997). Future study using direct injection of glutamate antagonists into hippocampus, amygdala, and septal nucleus during FZP withdrawal may help to clarify the specific role of hippocampal glutamate receptors in expression of FZP withdrawal-anxiety.…”

GABA A receptor α4 subunits mediate extrasynaptic inhibition in thalamus and dentate gyrus and the action of gaboxadol

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