Modulation of Extrasynaptic THIP Conductances by GABA_A-Receptor Modulators in Mouse Neocortex

Abstract

THIP is a hypnotic drug, which displays a unique pharmacological profile, because it activates a subset of extrasynaptic gamma-aminobutyric acid type A (GABA(A)) receptors containing delta-subunits. It is important to study the physiology and pharmacology of these extrasynaptic receptors and to determine how THIP interacts with other hypnotics and anesthetics. Here, we study the modulation of the extrasynaptic response to THIP using three classes of GABA(A)-receptor ligands. In whole cell recordings from mouse neocortical layer 2/3 pyramidal cells, THIP induced an extrasynaptic tonic current of 44 +/- 5 pA. The benzodiazepine site agonist and hypnotic zolpidem (500 nM), which displays selectivity for alpha(1/2/3)- and gamma(2)-containing receptors, did not alter the tonic current induced by THIP. The anesthetic etomidate (1 microM), which shows selectivity for beta(2)- and beta(3)-containing GABA(A) receptors, potentiated the THIP current by 126%. Etomidate also induced a small tonic GABA(A) current per se. The anesthetic propofol (1 microM), which displays broad-spectrum modulatory effects on several GABA(A)-receptor subtypes, enhanced the tonic THIP current by 117%. Finally, all three compounds modulated the function of intrasynaptic receptors activated by synaptically released GABA. Our study shows that the extrasynaptic GABA(A) receptors responsible for the tonic THIP conductance likely do not contain alpha(1)-, alpha(2)-, alpha(3)-, and gamma(2)-subunits. Thus the tonic GABAergic conductance in the neocortex is presumably mediated by alpha(4)beta(2/3)delta receptors, which are likely to play a major role for neocortical excitability. Furthermore, our study has deepened the knowledge about the cellular actions of THIP as well as THIP's interactions with other hypnotics and anesthetics.

Molecular basis for the high THIP/gaboxadol sensitivity of extrasynaptic GABAA receptors

“…1). Together, these findings demonstrate that submicromolar concentrations of THIP activate ␣6/␦ subunit-containing eGABARs on CGCs but not on MLIs and are in agreement with recent reports showing that ␣4/␦ subunit-expressing neurons in the thalamus and cortex exhibit responsiveness to submicromolar THIP (Belelli et al 2005;Chandra et al 2006;Cope et al 2005;Drasbek et al 2007;Jia et al 2005). …”

Altered Cortical Expression of GABA-Related Genes in Schizophrenia: Illness Progression vs Developmental Disturbance

“…For example, the GABA A receptor α2 subunit declines across postnatal development in monkey and human PFC 7,50 and is higher in the PFC of schizophrenia subjects, 14 and the GABA A receptor δ subunit in the PFC increases across postnatal development and is lower in schizophrenia. 51 In addition, the µ-opioid receptor, which is expressed by parvalbumin neurons [52][53][54] and regulates GABA release, 55,56 declines with postnatal development and is higher in schizophrenia. 39 Furthermore, expression patterns of alternatively spliced transcripts of the GAD1 gene are also consistent with this hypothesis; GAD67 mRNA increases and GAD25 mRNA decreases normally during early postnatal development of human PFC, but in schizophrenia subjects, the relative levels of GAD67 and GAD25 mRNAs were lower and higher, respectively, than in matched comparison subjects.…”

Phosphorylation of GABAA receptors influences receptor trafficking and neurosteroid actions

“…Tonic currents mediated by extrasynaptic GABA A Rs have been described in various brain regions including in layer III cells of the somatosensory cortex (Salin and Prince 1996), cerebellar and dentate granule cells (Brickley et al 1996; Nusser and Mody 2002), hippocampal interneurons and pyramidal cells (Bai et al 2001; Semyanov et al 2003), embryonic and developing neurons (Ge et al 2006), neocortical layer 2/3 pyramidal cells (Drasbek et al 2007; Drasbek and Jensen 2006), the spinal cord (Cronin et al 2004), and in several sub cortical structures including hypothalamic (Sarkar et al 2011; Sergeeva et al 2005), thalamocortical neurons (Belelli et al 2005; Cope et al 2005) and medium spiny neurons of the striatum (Ade et al 2008; Santhakumar et al 2010). …”