Distinct Functional and Pharmacological Properties of Tonic and Quantal Inhibitory Postsynaptic Currents Mediated by γ-Aminobutyric Acid_A Receptors in Hippocampal Neurons

Abstract

gamma-Aminobutyric acid (GABA), the principal inhibitory neurotransmitter, activates a persistent low amplitude tonic current in several brain regions in addition to conventional synaptic currents. Here we demonstrate that GABA(A) receptors mediating the tonic current in hippocampal neurons exhibit functional and pharmacological properties different from those of quantal synaptic currents. Patch-clamp techniques were used to characterize miniature inhibitory postsynaptic currents (mIPSCs) and the tonic GABAergic current recorded in CA1 pyramidal neurons in rat hippocampal slices and in dissociated neurons grown in culture. The competitive GABA(A) receptor antagonists, bicuculline and picrotoxin, blocked both the mIPSCs and the tonic current. In contrast, mIPSCs but not the tonic current were inhibited by gabazine (SR-95531). Coapplication experiments and computer simulations revealed that gabazine bound to the receptors responsible for the tonic current but did not prevent channel activation. However, gabazine competitively inhibited bicuculline blockade. The unitary conductance of the GABA(A) receptors underlying the tonic current (approximately 6 pS) was less than the main conductance of channels activated during quantal synaptic transmission (approximately 15--30 pS). Furthermore, compounds that potentiate GABA(A) receptor function including the benzodiazepine, midazolam, and anesthetic, propofol, prolonged the duration of mIPSCs and increased tonic current amplitude in cultured neurons to different extents. Clinically-relevant concentrations of midazolam and propofol caused a greater increase in tonic current compared with mIPSCs, as measured by total charge transfer. In summary, the receptors underlying the tonic current are functionally and pharmacologically distinct from quantally activated synaptic receptors and these receptors represent a novel target for neurodepressive drugs.

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

“…Similar response patterns for propofol actions (from 1 µM) on GABAergic miniature and/or spontaneous IPSCs occur in brainstem cardiac projection neurons (Wang et al, 2004). Propofol likewise affected GABA A IPSCs in supramedullary neurons from embryonic to early postnatal brain slices from the hippocampus (from 0.2 µM) (Bai et al, 2001), paraventricular nucleus (from 10 µM, decay time (Shirasaka et al, 2004)) and cortex (primary culture, from 1 µM (Kitamura et al, 2003)). Note that experimental design factors such as Cl − gradient and driving force influence discrimination of such threshold values particularly regarding GABA A mediated currents.…”

“…Certainly the strongest evidence links general anesthetic actions to GABA A ionotropic receptors and this is true for propofol in particular (Rudolph & Antkowiak, 2004;Franks, 2006). 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).…”

“…Strong mechanistic links have been established, mostly in forebrain regions, between several of its clinical features and underlying cellular-molecular actions (Rudolph & Antkowiak, 2004;Franks, 2006). 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).…”

“…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).…”

Pre‐ and postsynaptic volatile anaesthetic actions on glycinergic transmission to spinal cord motor neurons

“…Intravenous anaesthetic agents increase a tonic GABAergic inhibition in hippocampal neurons pharmacologically distinct from mIPSCs (Bai et al, 2001). This inhibition is seen as baseline noise.…”