Subregion-Specific Impacts of Genetic Loss of Diazepam Binding Inhibitor on Synaptic Inhibition in the Murine Hippocampus

Abstract

Benzodiazepines are commonly prescribed to treat neurological conditions including epilepsy, insomnia, and anxiety. The discovery of benzodiazepine-specific binding sites on γ-aminobutyric acid type-A receptors (GABAARs) led to the hypothesis that the brain may produce endogenous benzodiazepine-binding site ligands. An endogenous peptide, diazepam binding inhibitor (DBI), which can bind these sites, is thought to be capable of both enhancing and attenuating GABAergic transmission in different brain regions. However, the role that DBI plays in modulating GABAARs in the hippocampus remains unclear. Here, we investigated the role of DBI in modulating synaptic inhibition in the hippocampus using a constitutive DBI knockout mouse. Miniature and evoked inhibitory postsynaptic currents (mIPSCs, eIPSCs) were recorded from CA1 pyramidal cells and dentate gyrus (DG) granule cells. Loss of DBI signaling increased mIPSC frequency and amplitude in CA1 pyramidal cells from DBI knockout mice compared to wild-types. In DG granule cells, conversely, the loss of DBI decreased mIPSC amplitude and increased mIPSC decay time, indicating bidirectional modulation of GABAAR-mediated transmission in specific subregions of the hippocampus. eIPSC paired-pulse ratios were consistent across genotypes, suggesting that alterations in mIPSC frequency were not due to changes in presynaptic release probability. Furthermore, cells from DBI knockout mice did not display altered responsiveness to pharmacological applications of diazepam, a benzodiazepine, nor flumazenil, a benzodiazepine-binding site antagonist. These results provide evidence that genetic loss of DBI alters synaptic inhibition in the adult hippocampus, and that the direction of DBI-mediated modulation can vary discretely between specific subregions of the same brain structure.

Concentration dependent dual effect of the endozepine ODN on neuronal spiking activity

“…Using DBI KO (DBI -/- ) mice the authors observed that miniature IPSC frequency and amplitude in CA1 pyramidal cells are increased compared to wild-type mice, in agreement with the NAM effect of DBI and ODN described above (Courtney & Christian, 2018). However, an opposite observation was made in granule cells of the dentate gyrus where the loss of DBI decreased miniature IPSC amplitude and increases their decay time, suggesting that in this subregion of the hippocampus, DBI or its processing products act as positive allosteric modulators (PAM) of GABA A R (Courtney & Christian, 2018). The regional specificity of the action of DBI (NAM vs .…”

“…Of note, all these studies have used fairly high concentrations of ODN, in the micromolar range. Following another approach presented in a series of recent publications, Christian and colleagues reported a much more nuanced view of the role of endozepines in the modulation of GABA neurotransmission (Christian et al, 2013; Courtney & Christian, 2018). Using DBI KO (DBI -/- ) mice the authors observed that miniature IPSC frequency and amplitude in CA1 pyramidal cells are increased compared to wild-type mice, in agreement with the NAM effect of DBI and ODN described above (Courtney & Christian, 2018).…”

“…Following another approach presented in a series of recent publications, Christian and colleagues reported a much more nuanced view of the role of endozepines in the modulation of GABA neurotransmission (Christian et al, 2013; Courtney & Christian, 2018). Using DBI KO (DBI -/- ) mice the authors observed that miniature IPSC frequency and amplitude in CA1 pyramidal cells are increased compared to wild-type mice, in agreement with the NAM effect of DBI and ODN described above (Courtney & Christian, 2018). However, an opposite observation was made in granule cells of the dentate gyrus where the loss of DBI decreased miniature IPSC amplitude and increases their decay time, suggesting that in this subregion of the hippocampus, DBI or its processing products act as positive allosteric modulators (PAM) of GABA A R (Courtney & Christian, 2018).…”

Evaluating the efficacy of Optoα1AR activation in astrocytes in modulating basal hippocampal synaptic excitation and inhibition

“…Acute brain slices were prepared at ages P80-P145, with a range of 26-71 days after stereotaxic virus injection. Mice were anesthetized via intraperitoneal injection of pentobarbital (Vortech Pharmaceuticals, 55 mg/kg) as performed previously 18, 49, 50 and euthanized by decapitation. Brains were immediately dissected and placed in an ice-cold oxygenated (95% O 2 /5% CO 2 ) high-sucrose slicing solution containing (in mM) 254 sucrose, 11 glucose, 2.5 KCl, 1.25 NaH 2 PO 4 , 10 MgSO 4 , 0.5 CaCl 2 , and 26 NaHCO 3 .…”

Differential impacts on multiple forms of spatial and contextual memory in diazepam binding inhibitor knockout mice

“…In the thalamic reticular nucleus, however, DBI acts as a positive allosteric GABA A R modulator (Christian et al, 2013; Christian and Huguenard, 2013), indicating that the modulatory effects of DBI are region-specific. In this regard, our lab recently demonstrated hippocampal subregion-specific alterations in GABA A R-mediated transmission in DBI knockout (DBI −/− ) mice (Courtney and Christian, 2018), suggesting that DBI may play a role in hippocampus-dependent learning and memory processes. DBI may also modulate GABA transmission via mechanisms independent of actions at GABA A R benzodiazepine binding sites.…”