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.

Chronic benzodiazepine-induced reduction in GABAA receptor-mediated synaptic currents in hippocampal CA1 pyramidal neurons prevented by prior nimodipine injection

“…1A and Table 1). As previously reported (Poisbeau et al , 1997; Zeng and Tietz, 1999), there was a significant decrease in mIPSC amplitude (43.0%, p<0.05) in CA1 neurons from 2-day FZP-withdrawn rats (FZP/VEH: 12.9 ±1.6 pA, n=8) in comparison to those isolated from control rats (CON/VEH: 22.7 ± 1.1 pA, n=9). Prior vehicle injection had no effect on basal GABA A receptor mIPSC amplitude in control neurons or the decrease in mIPSC amplitude in 2-day FZP-withdrawn rats.…”

“…The absence of an effect of nimodipine on in vitro tolerance to zolpidem in contrast to its effects to prevent the reduction in GABA mIPSC amplitude suggests that L-type VGCC-mediated Ca 2+ influx might mediate some, but not other measures of GABA dysfunction in rats chronically administered benzodiazepines. As expected from previous recordings carried out at room temperature (Perrais and Ropert, 1999; Zeng and Tietz, 1999) 1 μM zolpidem also increased the amplitude of mIPSCs in control and FZP-treated cells from rats injected with both vehicle and nimodipine (CON-VEH: 106.3 ± 3.1%, n=7; FZP-VEH: 105.1 ± 2.7%, n=7; CON-NIM: 110.4 ± 6.4%, N=6; FZP-NIM: 107.9 ± 0.2%, N=7, p>0.05).…”

“…1C and Table 1). As reported previously (Poisbeau et al , 1997; Zeng and Tietz, 1999), there were also no significant differences (p>0.05) in resting membrane potential, mIPSC frequency or rise time in neurons from control vs. FZP-treated rats.…”

“…These initial single-channel studies also further validated the decrease in GABA A receptor-mediated chloride conductance, estimated by NSFA (Fig. 1 and 2, Zeng and Tietz, 1999). Nonetheless, it is uncertain whether such a mechanism results in non-functional membrane bound receptors, receptor internalization or a rapid switch in subunit composition related to a change in membrane conductance (De Koninck and Mody, 1996; Smart, 1997; Churn and DeLorenzo, 1998; Stelzer et al , 1998; Poisbeau et al , 1999; Sanchez et al , 2005; Houston et al , 2006).…”

Zolpidem Reduces Hippocampal Neuronal Activity in Freely Behaving Mice: A Large Scale Calcium Imaging Study with Miniaturized Fluorescence Microscope

“…The alignment between the effects observed in anesthetized and freely behaving animals addresses the concern that the previously observed Zolpidem effects could be a by-product of the interaction between Zolpidem and anesthetics [60] – [61] . The tendency of Zolpidem to suppress neuronal activity in vivo is in agreement with the findings in hippocampal slices where Zolpidem was consistently found to potentiate inhibitory currents [34] – [40] , [63] . It is generally assumed that increased inhibitory currents detected in hippocampal slices would translate into decreased activity of hippocampal cells in behaving animals; our study provides direct evidence in support of this hypothesis.…”