Evidence for Inhibition Mediated by Coassembly of GABA_Aand GABA_CReceptor Subunits in Native Central Neurons

Abstract

Fast inhibition in the nervous system is commonly mediated by GABA A receptors comprised of 2␣/2␤/1␥ subunits. In contrast, GABA C receptors containing only subunits (1-3) have been predominantly detected in the retina. However, here using reverse transcription-PCR and in situ hybridization we show that mRNA encoding the 1 subunit is highly expressed in brainstem neurons. Immunohistochemistry localized the 1 subunit to neurons at light and electron microscopic levels, where it was detected at synaptic junctions. Application of the GABA C receptor agonist cis-4-aminocrotonic acid (100 -800 M) requires the 1 subunit to elicit responses, which surprisingly are blocked independently by antagonists to GABA A (bicuculline, 10 M) and GABA C [(1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA); 40 -160 M] receptors. Responses to GABA C agonists were also enhanced by the GABA A receptor modulator pentobarbitone (300 M). Spontaneous and evoked IPSPs were reduced in amplitude but never abolished by TPMPA, but were completely blocked by bicuculline. We therefore tested the hypothesis that GABA A and GABA C subunits formed a heteromeric receptor. Immunohistochemistry indicated that 1 and ␣1 subunits were colocalized at light and electron microscopic levels. Electrophysiology revealed that responses to GABA C receptor agonists were enhanced by the GABA A receptor modulator zolpidem (500 nM), which acts on the ␣1 subunit when the ␥2 subunit is also present. Finally, coimmunoprecipitation indicated that the 1 subunit formed complexes that also contained ␣1 and ␥2 subunits. Taken together these separate lines of evidence suggest that the effects of GABA in central neurons can be mediated by heteromeric complexes of GABA A and GABA C receptor subunits.

Insect GABA Receptors: Splicing, Editing, and Targeting by Antiparasitics and Insecticides

“…Vertebrate ionotropic GABARs may be divided into two pharmacological categories: bicuculline-sensitive GABA A receptors (composed of ␣ 1-6 , ␤ 1-3 , ␥ 1-3 , ␦, ⑀, , and 1-3 subunits and allosterically modulated by benzodiazepines and barbiturates as well as pregnane steroids) and bicuculline-insensitive GABA C receptors (composed of the three known isoforms of the subunit and insensitive to the majority of modulators of GABA A receptors) (Mustafa, 1995;Sieghart, 1995;McKernan and Whiting, 1996;Whiting, 2003;Connolly and Wafford, 2004;Rudolph and Mohler, 2004). This division into A and C subtypes is difficult to reconcile with recent findings on GABARs in brainstem neurons, which are composed of 1 subunits coexpressed with ␣1 and ␥2 subunits (Milligan et al, 2004) to yield receptors with properties of both GABA A and GABA C subtypes. Furthermore, GABA A and GABA B receptors, which differ structurally and in their signaling mechanisms, may be closely functionally coupled.…”

Mechanisms of neurosteroid interactions with GABAA receptors

“…This channel underlies GABA-C responses in the retina. Although homomeric receptors containing the ρ1 subunit are thought to be most physiologically relevant, there is evidence for heteromeric combinations among ρ subunits 1-3 (Pan et al, 2006), and even among ρ subunits and GABA A subunits (Milligan et al, 2004). Because the GABA-C receptor can function as a homo-oligomer, this in principle could greatly simplify efforts to find steroid binding sites.…”

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

“…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). In intact animals, ongoing activity of GABA pathways was suggested by observations that GABA antagonists increased spontaneous firing (Bennett et al, 1987).…”