Tonically Activated GABA_A Receptors in Hippocampal Neurons Are High-Affinity, Low-Conductance Sensors for Extracellular GABA

Abstract

In the hippocampus, two distinct forms of GABAergic inhibition have been identified, phasic inhibitory postsynaptic currents that are the consequence of the vesicular release of GABA and a tonic conductance that is activated by low ambient concentrations of extracellular GABA. It is not known what accounts for the distinct properties of receptors that mediate the phasic and tonic inhibitory conductances. Moreover, the physiological role of the tonic inhibitory conductance remains uncertain because pharmacological tools that clearly distinguish tonic and phasic receptors are lacking. Here, we demonstrate that GABA A receptors that generate a tonic conductance in cultured hippocampal neurons from embryonic mice have different pharmacological properties than those in cerebellar granule neurons or pyramidal neurons in the dentate gyrus. The tonic conductance in cultured hippocampal neurons is enhanced by the benzodiazepine, midazolam, and is insensitive to the inhibitory effects of the competitive antagonist, gabazine (Յ10 M). We also identify penicillin as an uncompetitive antagonist that selectively inhibits the synaptic but not tonic conductance. GABA was applied to hippocampal neurons to investigate the properties of synaptic and extrasynaptic receptors. GABA-evoked current was composed of two components: a rapidly desensitizing current that was blocked by penicillin and a nondesensitizing current that was insensitive to penicillin blockade. The potency of GABA was greater for the penicillin-insensitive nondesensitizing current. Single-channel studies show that the gabazine-insensitive GABA A receptors have a lower unitary conductance (12 pS) than that estimated for synaptic receptors. Thus, specialized GABA A receptors with an apparent higher affinity for GABA that do not readily desensitize mediate the persistent tonic conductance in hippocampal neurons. The receptors underlying tonic and phasic inhibitory conductances in hippocampal neurons are pharmacologically and biophysically distinct, suggesting that they serve different physiological roles.

Propofol enhances both tonic and phasic inhibitory currents in second-order neurons of the solitary tract nucleus (NTS)

“…Propofol acts at GABA A receptors where it increases both decay times and the open channel probability (Kitamura et al, 2004) and slows desensitization (Bai et al, 1999;Bai et al, 2001). Diverse expression and the multitude of naturally occurring sub-unit combinations of GABA A receptors make it difficult to predict anesthetic sensitivity and mechanisms (Yeung et al, 2003). This receptor diversity differs prominently across brain regions and is thought to be responsible for pharmacological differences between phasic and tonic GABA receptor mediated currents within and across neurons (Yeung et al, 2003).…”

“…Diverse expression and the multitude of naturally occurring sub-unit combinations of GABA A receptors make it difficult to predict anesthetic sensitivity and mechanisms (Yeung et al, 2003). This receptor diversity differs prominently across brain regions and is thought to be responsible for pharmacological differences between phasic and tonic GABA receptor mediated currents within and across neurons (Yeung et al, 2003). In this context, our goal in the present studies was to try to identify the potential targets for anesthetic actions in adult NTS neurons with fully mature development.…”

“…The relative insensitivity of tonic currents in NTS neurons for propofol contrasts with the high sensitivity for tonic GABA A currents in hippocampal and neocortical neurons to propofol and isoflurane (Bai et al, 2001;Caraiscos et al, 2004a;Caraiscos et al, 2004b;Drasbek et al, 2007). This contrasting pattern of NTS compared to forebrain regions may represent differences in GABA A receptor subunit composition (Feng & Macdonald, 2004;Caraiscos et al, 2004a) and/or extrasynaptic cellular loci (Yeung et al, 2003). Indeed, a unique possibility might include actions of propofol on GABA A receptors containing ρ 1 subunits which are expressed in NTS (Milligan et al, 2004).…”

“…The hypnotic effects of propofol are primarily attributed to the enhancement of A-type γ-aminobutyric acid (GABA A ) receptor function by inducing extended GABA A channel open times (Kitamura et al, 2004) and slowing desensitization (Bai et al, 1999;Bai et al, 2001), both attributes of phasic GABAergic transmission. Increasing interest has focused on propofol actions to induce an inhibitory tonic current that is pharmacologically and biophysically separable from phasic GABAergic currents (Hales & Lambert, 1991;Bai et al, 2001;Yeung et al, 2003). In hippocampal neurons, this tonic, "extrasynaptic" GABA A component is more sensitive to anesthetics than actions on phasic currents in the same neurons (Hemmings, Jr. et al, 2005) and seemingly plays a substantial role in suppressing neuronal excitability (Bieda & MacIver, 2004).…”

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

“…During the last decade, several studies have shown that GABA A receptor activation can also mediate tonic inhibition; these receptors are extrasynaptically and perisynaptically localized and contain a distinct, high affinity subunit composition made of δ and α4, α5 & α6 subunits (Nusser et al, 1998;Yeung et al, 2003;Semyanov et al, 2003Semyanov et al, , 2004Cope et al, 2005;Scimemi et al, 2005;Glykys et al, 2008). Hence, GABA A receptors can generate two types of current, depending on their location and subunit composition: the first type is the classic synaptic phasic (or 'transient') current that results from the release of GABA from synaptic vesicles in the synaptic cleft; the second type is a tonic ('always on') current that is caused by GABA A receptors responding to low levels of ambient GABA (Farrant and Nusser, 2005).…”