Tonically Active GABAA Receptors in Hippocampal Pyramidal Neurons Exhibit Constitutive GABA-Independent Gating

Abstract

Phasic and tonic inhibitory currents of hippocampal pyramidal neurons exhibit distinct pharmacological properties. Picrotoxin and bicuculline methiodide inhibited both components, consistent with a role for GABA A receptors; however, gabazine, at a concentration that abolished miniature GABAergic inhibitory postsynaptic currents and responses to exogenous GABA, had no effect on tonic currents. Because all GABA-activated GABA A receptors in pyramidal neurons are gabazine-sensitive, it follows that tonic currents are not GABA-activated. Furthermore, picrotoxin-sensitive spontaneous single-channel events recorded from outside-out patches had the same chord conductance as GABA-activated channels and were gabazineresistant. Therefore, we hypothesize that GABA A receptors, constitutively active in the absence of GABA, mediate tonic current; the failure of gabazine to block tonic current reflects a lack of negative intrinsic efficacy of the antagonist. We compared the negative efficacies of bicuculline and gabazine using the general anesthetic propofol to directly activate GABA A receptors native to pyramidal neurons or ␣1␤3␥2 receptors recombinantly expressed in human embryonic kidney 293 cells. Propofol activated gabazine-resistant, bicuculline-sensitive currents when applied to either preparation. Although gabazine had negligible efficacy as an inhibitor of propofol-activated currents, it prevented inhibition by bicuculline, which acts as an inverse agonist inhibiting GABA-independent gating. Recombinant ␣1␤1/3␥2 receptors also mediated agonist-independent tonic currents that were resistant to gabazine and inhibited by bicuculline. Thus, gabazine is a competitive antagonist with negligible negative efficacy and is therefore unable to inhibit GABA A receptors that are active in the absence of GABA because of either anesthetic or spontaneous gating. Moreover, spontaneously active GABA A receptors mediate gabazine-resistant tonic currents in pyramidal neurons.

GABAAReceptor-Mediated Signaling Alters the Structure of Spontaneous Activity in the Developing Retina

“…3E, Tables 1, 2), indicating that ambient GABA tonically activates GABA A receptors on SACs. Whether RGCs have a gabazine-insensitive, picrotoxin-sensitive GABA A receptor as recently observed in hippocampus (McCartney et al, 2007) remains to be determined. Furthermore, GABA concentrations in vivo may be higher than in the isolated retina, so we cannot rule out the possibility that GABA A receptors are tonically active on RGCs as well as on SACs.…”

“…For example, in cerebellum (Mody and Pearce, 2004) and some regions of thalamus (Cope et al, 2005), the ␦ subunit is found preferentially in extrasynaptic receptors, in which it increases the receptors' affinity for GABA, thereby making the receptors sensitive to ambient levels of GABA (Mody, 2001;Farrant and Nusser, 2005). In contrast, GABA A receptors found in hippocampus mediate a tonic conductance that is not blocked by gabazine but is blocked by picrotoxin (McCartney et al, 2007). It has been proposed that these extrasynaptic GABA A receptors are active in the absence of GABA.…”

A new meaning for “Gin & Tonic”: tonic inhibition as the target for ethanol action in the brain

“…These include the activity of GABA uptake mechanisms, the level of spontaneous activity of GABAergic neurons, possible leakage of GABA from neurons or glial cells, the rate of perfusion of the slices, oxygenation levels, etc. It is also possible to record tonic current in the total absence of agonist-dependent activation of GABA A Rs, when just the spontaneous openings of the channels is responsible for a steady conductance (Birnir et al, 2000;McCartney et al, 2006). Interestingly the spontaneously opening receptors can be blocked by bicuculline but not gabazine (McCartney et al, 2006) thus introducing an added cautionary note when using various antagonists to reveal the magnitude of tonic inhibition (Bai et al, 2001).…”

Which GABAAReceptor Subunits Are Necessary for Tonic Inhibition in the Hippocampus?

“…Yet, in the presence of 5 M GABA, mice lacking ␣5 subunits (Gabra5 Ϫ/Ϫ ) showed a residual tonic inhibition in CA1/CA3 PCs mediated by an upregulation of ␦ subunits (Glykys and Mody, 2006), and mice lacking ␦ subunits (Gabrd Ϫ/Ϫ ) expressed small residual tonic inhibitory currents in DGGC and molecular layer (ML) interneurons (Stell et al, 2003; mediated by a yet unidentified GABA A R subunit. GABA A Rs containing subunits have been proposed to contribute to tonic inhibition in CA3 PCs through openings that do not always require the presence of an agonist (McCartney et al, 2007), and GABA A Rs formed solely of ␣ and ␤ subunits have been shown to mediate a small fraction of tonic inhibition in CA1 PCs (Mortensen and Smart, 2006).…”