title: Ectopic Expression of α6 and δ GABAA Receptor Subunits in Hilar Somatostatin Neurons Increases Tonic Inhibition and Alters Network Activity in the Dentate Gyrus.
abstract: The role of GABAA receptor (GABAAR)-mediated tonic inhibition in interneurons remains unclear and may vary among subgroups. Somatostatin (SOM) interneurons in the hilus of the dentate gyrus show negligible expression of nonsynaptic GABAAR subunits and very low tonic inhibition. To determine the effects of ectopic expression of tonic GABAAR subtypes in these neurons, Cre-dependent viral vectors were used to express GFP-tagged GABAAR subunits (α6 and δ) selectively in hilar SOM neurons in SOM-Cre mice. In single-transfected animals, immunohistochemistry demonstrated strong expression of either the α6 or δ subunit; in cotransfected animals, both subunits were consistently expressed in the same neurons. Electrophysiology revealed a robust increase of tonic current, with progressively larger increases following transfection of δ, α6, and α6/δ subunits, respectively, indicating formation of functional receptors in all conditions and likely coassembly of the subunits in the same receptor following cotransfection. An in vitro model of repetitive bursting was used to determine the effects of increased tonic inhibition in hilar SOM interneurons on circuit activity in the dentate gyrus. Upon cotransfection, the frequency of GABAAR-mediated bursting in granule cells was reduced, consistent with a reduction in synchronous firing among hilar SOM interneurons. Moreover, in vivo studies of Fos expression demonstrated reduced activation of α6/δ-cotransfected neurons following acute seizure induction by pentylenetetrazole. The findings demonstrate that increasing tonic inhibition in hilar SOM interneurons can alter dentate gyrus circuit activity during strong stimulation and suggest that tonic inhibition of interneurons could play a role in regulating excessive synchrony within the network.
  
  
  CONCLUSIONS
  In contrast to many hippocampal interneurons, somatostatin (SOM) neurons in the hilus of the dentate gyrus have very low levels of nonsynaptic GABAARs and exhibit very little tonic inhibition. In an effort to increase tonic inhibition selectively in these interneurons, we used Cre-dependent viral vectors in SOM-Cre mice to achieve interneuron-specific expression of the nonsynaptic GABAAR subunits (α6 and δ) in vivo. We show, for the first time, that such recombinant GFP-tagged GABAAR subunits are expressed robustly, assemble to form functional receptors, substantially increase tonic inhibition in SOM interneurons, and alter circuit activity within the dentate gyrus.
authors.affiliation: Departments of Neurobiology and Department of Anatomy, Histology and Embryology, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China, and.
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Tong
authors.affiliation: Departments of Neurobiology and.
authors.forename: Zechun
authors.initials: Z
authors.lastname: Peng
authors.affiliation: Departments of Neurobiology and.
authors.forename: Nianhui
authors.initials: N
authors.lastname: Zhang
authors.affiliation: Departments of Neurobiology and.
authors.forename: Yliana
authors.initials: Y
authors.lastname: Cetina
authors.affiliation: Departments of Neurobiology and.
authors.forename: Christine S
authors.initials: CS
authors.lastname: Huang
authors.affiliation: Molecular and Medical Pharmacology and Brain Research Institute, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, California 90095.
authors.forename: Martin
authors.initials: M
authors.lastname: Wallner
authors.affiliation: Departments of Neurobiology and Brain Research Institute, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, California 90095, Roche Pharmaceutical Research and Early Development, Neuroscience, Ophthalmology, and Rare Diseases Translational Area, Roche Innovation Center Basel, CH-4070, Basel, Switzerland.
authors.forename: Thomas S
authors.initials: TS
authors.lastname: Otis
authors.affiliation: Departments of Neurobiology and Brain Research Institute, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, California 90095, houser@mednet.ucla.edu.
authors.forename: Carolyn R
authors.initials: CR
authors.lastname: Houser
mesh_terms: D000200:Action Potentials / Q000187:drug effects / Q000235:genetics; D000818:Animals; D018891:Dentate Gyrus / Q000166:cytology* / Q000187:drug effects; D018755:GABA Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D005786:Gene Expression Regulation / Q000187:drug effects / Q000235:genetics; D005822:Genetic Vectors / Q000378:metabolism; D006801:Humans; D007555:Isoxazoles / Q000494:pharmacology; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D009415:Nerve Net / Q000378:metabolism*; D009433:Neural Inhibition / Q000187:drug effects / Q000235:genetics; D009474:Neurons / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D010433:Pentylenetetrazole / Q000494:pharmacology; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism; D011743:Pyrimidines / Q000494:pharmacology; D011963:Receptors, GABA-A / Q000235:genetics / Q000378:metabolism*; D013004:Somatostatin / Q000235:genetics / Q000378:metabolism*
chemical_list: D018755:GABA Agonists; D018756:GABA Antagonists; C583130:Gabrd protein, mouse; D007555:Isoxazoles; D021122:Protein Subunits; D011743:Pyrimidines; D011963:Receptors, GABA-A; D013004:Somatostatin; C034821:4-aminopyrimidine; C015542:gaboxadol; D010433:Pentylenetetrazole
keywords: dentate hilus; hippocampus; interneurons; nonsynaptic GABAA receptors; somatostatin-Cre mice