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COMMENTARY

Extrasynaptic GABAA receptors in the nucleus accumbens are necessary for drinking

Richard W. Olsen1 Department of Molecular and Medical Pharmacology, David Geffen School of Medicine, University of California, Los Angeles, CA 90095-1735

espite the fact that alcohol is ment of δ-subunit–containing GABAARs one of the most widely used (7). Third, the observation that the α4βδ D and abused of all psychoactive GABAAR subtypes are the most rapidly drugs, there is surprising lack of regulated in plastic mechanisms triggered consensus on the molecular mechanisms by high-dose EtOH or chronic exposure to of action. One particularly important as- EtOH in rats (8) is consistent with these pect of (EtOH) is its effect on the extrasynaptic GABAARs being among the reward system in the mammalian brain, first responders to EtOH in the brain. because it may help us to understand and What is not so consistent with this pic- help people afflicted with alcohol abuse ture is the phenotypes of mice lacking the and addiction better. In PNAS, Patricia GABAAR α4, α6, and δ subunits. None Janak, Dorit Ron, and their colleagues (1) of these three KO mice show seriously at the Ernest Gallo Clinic and Research Fig. 1. Reward circuit (schematic), including do- altered effects of EtOH in vivo: The α6-, Center associated with the University pamine in the VTA projecting to “reward α4-, and δ-KOs show no changes in sen- of California, San Francisco, are making neurons” in the NAc. The NAc is divided into the sitivity to EtOH effects on anxiety and progress on both aspects by using the core (C) and the shell, which is subdivided into (11–14), despite decreased sen- ventral (V), medial (M), and dorsal (D) parts. The technique of viral-mediated RNAi to sitivity to EtOH of GABAAR-mediated fi reward system is affected by most categories of knock down the expression of a speci c drugs of abuse and involves numerous trans- tonic inhibitory currents in brain neurons inhibitory receptor pro- mitters in the two main regions. The extended (7). The α4-KO shows reduced sensitiv- tein implicated in low- to moderate-dose amygdala, not shown for simplicity, is sometimes ity to the motor incoordinating effects EtOH effects on brain. This manipulation considered part of the circuit. Color code: red, and reduced enhancement of GABAAR- was carried out in vivo on rats in a speci- dopamine; violet, glutamate; blue, GABA. Acetyl- mediated inhibitory tonic currents by fic mesolimbic brain region implicated choline, 5-hydroxytryptamine (5HT), and cannabi- the GABA THIP (4,5,6,7-tetra- in the reinforcing effects of EtOH, pro- noid cells are not shown. Receptors are indicated hydroisoxazolo[5,4-c]pyridin-3-ol) (13). ducing a significant reduction in oral up- for N (brown, nicotine and nicotinic acetylcholine δ receptor), O (yellow, opioid peptides and opiate The -KO does show reduced sensitivity take of EtOH. drug receptor), and the orange ball (cocaine sites to behavioral effects of GABAergic The gene product implicated by Nie on dopamine transporter on dopamine nerve and also exhibits reduced et al. (1) using the knockdown tech- endings); BZ (sites implicated for the action of EtOH self-administration compared nique with viral-mediated RNAi (2, 3) on GABA interneurons in VTA) with WT (12). is the extrasynaptic GABAA receptor (16); and EtOH (sites implicated for action of The complexity of phenotypes produced EtOH). The author acknowledges input to this (GABAAR) δ-subunit, which has been by global KOs of genes in which an in- fi – shown to confer unique sensitivity to en- gure (17 19). dividual lacks the gene product through- hancement by concentrations of EtOH out life in all anatomical regions can make found in the blood of humans drinking one it difficult to demonstrate a suspected R100Q exhibit greater than normal sensi- or a few drinks, in recombinant heterolo- gene function in a given behavior under tivity to the motor-incoordinating actions gous cell expression, in brain slices, and study, even if the deletion is not fatal. of moderate doses of EtOH; this hyper- in vivo (4–9). The brain region implicated Effects of global KOs are often masked by sensitivity to EtOH is also seen in tonic by Nie et al. (1) where extrasynaptic compensatory changes in genes serving inhibitory currents mediated by the α6βδ- GABAARs are critical for EtOH oral in- similar functions. For example, in the type GABAARs measured by patch-clamp take is the dorsomedial shell of the nu- GABAAR α6-KO mouse, which shows cleus accumbens (NAc), an area noted for recordings from granule cells in cerebellar unimpaired sensitivity to behavioral effects dopamine-mediated reward mechanisms slices (7, 9). Recombinant GABAARs α β δ of EtOH (11), there is a total loss of the involved in virtually all drugs of abuse expressing the 6R100Q 3 in oocytes δ-subunit, whose obligatory subunit part- (e.g., cocaine, nicotine, opiates; Fig. 1). also showed higher sensitivity to EtOH ner is α6 and changes in the levels of other – This demonstration takes advantage of modulation (1 10 mM) than the already GABA R subunits; these animals show α β δ – A a unique technique for temporary reduc- sensitive WT 6R100 3 (10 30 mM) (7). compensatory up-regulation of a voltage- tion of a specific gene product by in vivo Second, the residue R100 in the GABAAR independent K+ channel (15), generating α microinjection of siRNA into a specific -subunit that affects EtOH sensitivity is a compensatory tonic inhibitory current in anatomical brain region to determine the part of the -binding cerebellar granule cells. In this abnormal α role of that gene product in a specific be- pocket on the 6-subunit, consistent not environment, it is not surprising that it is havior; here, oral EtOH intake regulated only with the discovery of a previously difficult to establish a clear role for the by GABAARs in the NAc. unappreciated benzodiazepine (BZ) site Further support for an involvement of on the δ-subunit–containing GABAARs the δ-subunit–containing GABAAR sub- but with the demonstration that these Author contributions: R.W.O. wrote the paper. types in EtOH effects has come from three unique BZ sites mediate antagonism by The author declares no conflict of interest. lines of investigation. First, rats exhib- the BZ ligand Ro15-4513 of in vivo EtOH See companion article on page 4459 in issue 11 of volume iting a naturally occurring allelic variation behaviors (10) and in vitro antagonism of 108. 1 in the GABAAR α6-cerebellar subunit low- to moderate-dose EtOH enhance- E-mail: [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1102818108 PNAS Early Edition | 1of2 Downloaded by guest on September 30, 2021 GABAAR δ-subunit in EtOH actions. γ-hydroxybutyrate and benzodiazepines, specific animal species) using viral vectors, Also worth noting might be the rather with drug targets located at various places and a specific behavior already linked to large differences in EtOH sensitivity for different drugs (17). The circuit is the brain region, allows a very specific among mouse strains and that the vast modulated by excitatory and inhibitory in- conclusion about gene function, certainly majority of detailed positive results impli- puts to presynaptic and postsynaptic more specific than the global KO mouse. cating extrasynaptic GABAARs in behav- receptors using glutamate, GABA, acetyl- In particular, in the current work, we have ioral alcohol action were obtained using choline (nicotinic receptors), 5-hydroxy- a totally identified location that allows a rats rather than mice (including the tryptophan, opioid peptides, and cannabi- more unambiguous demonstration of current study). noids (18). Addiction is thought to require a role for this gene product in this behav- Some success in overcoming this lack of plastic changes in the synaptic glutamate ior. Not only is the NAc implicated, but, specificity in genetic engineering has come receptor activity of this reward circuit fol- specifically, the dorsomedial shell, rather from conditional KOs, gene deletions in- lowing chronic activation by the drug of than the ventral or lateral shell or the core, duced only at a specific anatomical loca- abuse (19, 20). Alternatively, plasticity in is shown to be involved. Furthermore, the tion and specific age. Similarly, knock-ins the circuits mediating negative reinforce- ingestion of sucrose was not affected. In introduce a specific point mutation in a ment resulting from the absence of the this case, the specificity may be added to single gene that is demonstrated in vitro previously present drug of abuse (with- by the unusual extrasynaptic localization to have a functional consequence for a drawal) could contribute to addiction. of the δ-subunit–containing GABAARs, specific behavior, for example, rendering Several drugs of abuse stimulate dopa- and their unique physiology and pharma- GABAARs insensitive to a drug action, mine neurons in the VTA. Tan et al. cology. Thus, we can tentatively conclude, such as benzodiazepine or mod- (16) demonstrated that benzodiazepines, for example, because of the detailed ra- ulation (16). Alternatively, behaviors abused GABAergic drugs, potentiate tionale for the gene, tissue, and function with a well-defined anatomical localization GABA R-mediated inhibition in the VTA A analyzed, that other GABAAR subunits, and implicated gene product function can associated with induction of synaptic although they were not similarly studied be studied by knocking down the gene plasticity, and thus consistent with addic- with gene knockdown, are not really likely expression by introducing appropriate tion potential. They showed that a gene to be involved in this function just because fi fi α siRNA constructs into speci c identi ed knock-in mouse for the GABAAR 1- the δ-subunit is. This makes results ob- cells using suitable viral vectors. subunit was critical for this addictive ac- tained in this manner quite compelling. The dopamine reward circuit, including tion of the BZs in the VTA. Could this Of course, the study also unambiguously the ventral tegmental area (VTA) and same target in the VTA mediate re- fi shows that the gene in question is knocked NAc (Fig. 1), is a speci c anatomical area inforcement to EtOH? Do we need an- down and specifically in the medial shell of involved in the reinforcing effects of drugs other target for EtOH? Reinforcing ef- the NAc, using siRNA techniques already of abuse suitable for manipulation of ge- fects of many drugs of abuse have been familiar to the authors. netic expression regulation with siRNA shown to involve the NAc (17, 18). Rewal To quote Nie et al. (1): “In conclusion, using viral vectors. et al. (2) previously demonstrated a link- fi δ fi the current ndings indicate that -con- Such an opportunity has been identi ed age of EtOH effects with GABA, also in taining GABA Rs in medial NAc shell play by Nie et al. (1). Previous workers had the NAc, showing that selective reduction A α an important role in alcohol drinking be- shown that various drugs of abuse stimu- of the GABAR 4-subunit in the NAc havior, strengthening the hypothesis that late the well-known dopamine reward shell, but not in the core, reduces EtOH the α4βδ GABA R (mediating tonic in- circuit, in which the drugs stimulate do- self-administration. Jeanblanc et al. (3) A hibition) in a restricted region of the NAc pamine neurons situated in the VTA and further used the gene knock-down ap- shell is a key brain substrate for the re- projecting to the NAc and amygdala proach utilizing siRNA to implicate BDNF inforcing properties of oral alcohol.” (Fig. 1). The activity of this circuit is mod- in the NAc shell for EtOH reinforcement. ulated by EtOH but also by nicotine, The ability to influence a specific gene fi ACKNOWLEDGMENTS. I thank M. Wallner for cannabinoids, opiates, cocaine, and meth- with siRNA in a speci c anatomical region assistance with graphic art. This work was funded amphetamine as well as by the club drug and at a specific age (not to mention a by National Institutes of Health Grant AA07680.

1. Nie H, Rewal M, Gill TM, Ron D, Janak PH (2011) Extra- 7. Wallner M, Hanchar HJ, Olsen RW (2006) Low dose 14. Chandra D, et al. (2008) Normal acute behavioral re-

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