Enhancement of GABAergic activity:neuropharmacological effects of benzodiazepines and therapeutic use in anaesthesiology

GABA is the major inhibitory neurotransmitter in the central nervous system (CNS). The type A GABA receptor (GABAAR) system is the primary pharmacological target for many drugs used in clinical anesthesia. The α1, β2, and γ2 subunit-containing GABAARs located in the various parts of CNS are thought to be involved in versatile effects caused by inhaled anesthetics and classic benzodiazepines (BZD), both of which are widely used in clinical anesthesiology. During the past decade, the emergence of tonic inhibitory conductance in extrasynaptic GABAARs has coincided with evidence showing that these receptors are highly sensitive to the sedatives and hypnotics used in anesthesia. Anesthetic enhancement of tonic GABAergic inhibition seems to be preferentially increased in regions shown to be important in controlling memory, awareness, and sleep. This review focuses on the physiology of the GABAARs and the pharmacological properties of clinically used BZDs. Although classic BZDs are widely used in anesthesiological practice, there is a constant need for new drugs with more favorable pharmacokinetic and pharmacodynamic effects and fewer side effects. New hypnotics are currently developed, and promising results for one of these, the GABAAR agonist remimazolam, have recently been published.</p

Stereospecific modulation of GABA(A) receptor function by urocanic acid isomers

A deamination product of histidine, urocanic acid, accumulates in the skin of mammals as trans-urocanic acid. Ultraviolet (UV) irradition converts it to the cis-isomer that is an important mediator in UV-induced immunosuppression. We have recently shown that urocanic acid interferes with the agonist binding to GABAA receptors. We now report that the effects of urocanic acid on binding of a convulsant ligand (t-butylbicyclo[35S]phosphorothionate) to GABAA receptors in brain membrane homogenates are dependent on pH of the incubation medium, the agonistic actions being enhanced at the normal pH of the skin (5.5). Using Xenopus laevis oocytes expressing recombinant rat alpha1beta1gamma2S GABAA receptors, the low pH potentiated the direct agonistic action of trans-urocanic acid under two-electrode voltage-clamp, whereas cis-urocanic acid retained its low efficacy both at pH 5.5 and 7.4. The results thus indicate clear differences between urocanic acid isomers in functional activity at one putative receptor site of immunosuppression, the GABAA receptor, the presence of which in the skin remains to be demonstrated.</p

Extrasynaptic δ‐GABAA receptors are high affinity muscimol receptors

Muscimol, the major psychoactive ingredient in the mushroom Amanita muscaria, has been regarded as a universal non‐selective GABA‐site agonist. Deletion of the GABAA receptor (GABAAR) δ subunit in mice (δKO) leads to a drastic reduction in high affinity muscimol binding in brain sections and loss of behavioral low dose muscimol effects. Here we use forebrain and cerebellar brain homogenates from WT and δKO mice to show that deletion of the δ subunit leads to a >50% loss of high affinity 5 nM [3H]muscimol binding sites despite the relatively low abundance of δ‐containing GABAARs (δ‐GABAAR) in the brain. By subtracting residual high affinity binding in δKO mice and measuring the slow association and dissociation rates we show that native δ‐GABAARs in WT mice exhibit high affinity [3H]muscimol binding sites (KD ~1.6 nM on α4βδ receptors in the forebrain and ~1 nM on α6βδ receptors in the cerebellum at room temperature). Co‐expression of the δ subunit with α6 and β2 or β3 in recombinant (HEK 293) expression leads to the appearance of a slowly dissociating [3H]muscimol component. In addition, we compared muscimol currents in recombinant α4β3δ and α4β3 receptors and show that δ subunit co‐expression leads to highly muscimol‐sensitive currents with an estimated EC50 of around 1‐2 nM and slow deactivation kinetics. These data indicate that δ subunit incorporation leads to a dramatic increase of GABAAR muscimol sensitivity. We conclude that biochemical and behavioral low dose muscimol selectivity for δ subunit‐containing receptors is due to low nanomolar binding affinity on δ‐GABAARs.</p

GABA-A Channel Subunit Expression in Human Glioma Correlates with Tumor Histology and Clinical Outcome

GABA (γ-aminobutyric acid) is the main inhibitory neurotransmitter in the CNS and is present in high concentrations in presynaptic terminals of neuronal cells. More recently, GABA has been ascribed a more widespread role in the control of cell proliferation during development where low concentrations of extrasynaptic GABA induce a tonic activation of GABA receptors. The GABA-A receptor consists of a ligand-gated chloride channel, formed by five subunits that are selected from 19 different subunit isoforms. The functional and pharmacological properties of the GABA-A channels are dictated by their subunit composition. Here we used qRT-PCR to compare mRNA levels of all 19 GABA-A channel subunits in samples of human glioma (n = 29) and peri-tumoral tissue (n = 5). All subunits except the ρ1 and ρ3 subunit were consistently detected. Lowest mRNA levels were found in glioblastoma compared to gliomas of lower malignancy, except for the θ subunit. The expression and cellular distribution of the α1, γ1, ρ2 and θ subunit proteins was investigated by immunohistochemistry on tissue microarrays containing 87 gliomas grade II. We found a strong co-expression of ρ2 and θ subunits in both astrocytomas (r = 0.86, p<0.0001) and oligodendroglial tumors (r = 0.66, p<0.0001). Kaplan-Meier analysis and Cox proportional hazards modeling to estimate the impact of GABA-A channel subunit expression on survival identified the ρ2 subunit (p = 0.043) but not the θ subunit (p = 0.64) as an independent predictor of improved survival in astrocytomas, together with established prognostic factors. Our data give support for the presence of distinct GABA-A channel subtypes in gliomas and provide the first link between specific composition of the A-channel and patient survival

Effects of lorazepam on saccadic eye movements: the role of sex, task characteristics and baseline traits

Medical Research Council; National Institute for Health Research; Mental Health Biomedical Research Centre at South London; Maudsley NHS Foundation Trust; Institute of Psychiatry, Psychology; Neuroscience, King’s College London

Morphine withdrawal increases expression of GABA(A) receptor epsilon subunit mRNA in locus coeruleus neurons

An increase in the activity of brain stem locus coeruleus noradrenergic neurons has been hypothesised to be a major factor accounting for opiate withdrawal symptoms. These neurons are under GABAergic inhibition. Their GABA(A) receptors have unique pharmacological properties, most likely due to the enriched expression of GABA(A) receptor subtypes containing novel epsilon and straight theta subunits. Using in situ hybridisation of cryostat sections, we now report a significant increase in the epsilon subunit mRNA expression after precipitation of opioid withdrawal by naloxone. Similar changes were detected in tyrosine hydroxylase mRNA expression. The results suggest increased formation of unique GABA(A) receptor subtype(s) in the locus coeruleus neurons during increased neuronal activity