Dans le cadre du changement climatique et de l'évolution de la réglementation concernant les intrants azotés, la culture du colza risque d'être fortement pénalisée dans la mesure où c'est une culture qui nécessite d'importants apports azotés pour atteindre son potentiel de rendement. Par ailleurs, comme chez le colza un déficit hydrique induit l'accumulation de certains composés azotés, il est vraisemblable que cela conduise au détournement d'une quantité importante d'azote vers les organes végétatifs aux dépens des organes reproducteurs et donc du rendement. Chez le colza, la réponse métabolique au déficit hydrique se traduit par une très forte accumulation de proline et dans une moindre mesure une augmentation de la teneur en GABA (acide γ-aminobutyrique), deux acides aminés connus chez la plupart des plantes pour leur réponse à de nombreux stress abiotiques. L'objectif de cette thèse est de déterminer comment le métabolisme de ces deux molécules contribue à l'allocation de l'azote au cours du développement de la plante en situation normale comme en condition de stress hydrique et/ou azoté. Pour répondre à cette question nous avons fait le choix de caractériser deux voies enzymatiques majeures impliquées dans le catabolisme de la proline et du GABA : la proline déshydrogénase (ProDH) et la GABA-transaminase (GABA-T) et d'évaluer l'impact de carences hydriques et/ou azotées sur ces voies. Cette étude nécessitait d'identifier au préalable les gènes codant ces enzymes afin d'aborder une approche fonctionnelle. Les résultats montrent l'existence de multiples copies de gènes ProDH et GABA-T dans le génome du colza. L'analyse de leurs profils d'expression suggère que des processus de sub-fonctionnalisation sont en cours conduisant à l'expression spécifique, de certaines copies en réponse aux stress, et d'autres dans les processus développementaux. La comparaison des profils métaboliques avec les profils spécifiques des transcrits a permis d'élaborer des hypothèses sur le rôle de ces voies dans la gestion de l'azote. L'étude conjointe des métabolismes de la proline et du GABA suggère l'existence de régulations connexes entre les deux. Enfin, l'utilisation de plantules a permis - d'approfondir la régulation des gènes étudiés à des stades précoces de développement - et de mettre en évidence les effets délétères de l'inhibition de la GABA-T par une approche pharmacologique. En conclusion ces résultats apportent des précisions sur la régulation de ces deux enzymes et fournissent des éléments de réponse quant au rôle fonctionnel des catabolismes de la proline et du GABA dans les processus de gestion de l'eau et de l'azote chez le colza. Ces travaux constituent donc une première étape dans une démarche de validation de ces gènes comme candidats pour des programmes d'amélioration du colza visant à sélectionner des génotypes mieux adaptés aux conditions environnementales futures. / In the context of climate change and recent regulation concerning nitrogen inputs, the oilseed rape yields may be severely decreased because its crop requires significant nitrogen supply to reach high yield performance. Moreover, as water deficit induces the accumulation of some nitrogen compounds in oilseed rape, it is likely that this could lead to diversion of significant amounts of nitrogen to the vegetative organs at the expense of the reproductive ones and therefore of the yield. In oilseed rape, the metabolic response to water deficit results in a very high proline accumulation and, to a lesser extent, an increased content of GABA (γ-aminobutyric acid), both these amino acids known for their response to many environmental stresses in most species. The objective of the work presented here was to determine how the metabolism of proline and GABA contributes to the nitrogen allocation during plant development under optimal conditions and under water stress and/or nitrogen depletion. To answer this question, we have chosen to characterize two major enzymatic pathways involved in the catabolism of proline and GABA, proline dehydrogenase (ProDH) and GABA transaminase (GABA-T), and assess the impact of water and/or nitrogen deficiency on these pathways. This study has required to preliminary identify the genes encoding these enzymes in order to initiate a functional approach. The results show the presence of multiple copies of ProDH and GABA-T genes in the oilseed rape genome. Analysis of their expression profiles suggests that sub-functionalization processes are occurring, leading to the specific expression of some copies in response to stress, and some in developmental processes. Comparison of metabolic profiles with specific profiles of transcripts allows us to hypothesize about the role of these pathways in management of nitrogen. The combined study of proline and GABA metabolisms suggests the existence of relationships between them. Finally, the use of seedlings allows - further studying the regulation of genes in the early stages of development - and highlighting the deleterious effects of the inhibition of GABA-T by a pharmacological approach. In conclusion these results supply information on the regulation of these two enzymes and provide answers about the functional roles of proline and GABA catabolisms in the management processes of water and nitrogen in oilseed rape. These works constitute a first step in validation process of these genes as putative candidates for oilseed rape breeding programs to select genotypes better adapted to future environmental conditions.

MeCP2 est une protéine multifonctionnelle agissant à de nombreux niveaux de contrôle des programmes génétiques. Un mauvais dosage de MeCP2 cause un groupe de maladies neurologiques dont le point commun est une déficience intellectuelle sévère. Des mutations ou une délétion de MECP2 causent le syndrome de Rett chez les filles, alors que sa surexpression cause chez les garçons le syndrome de duplication de MECP2. Plusieurs modèles murins de Mecp2-pathies ont été générés qui permettent d’expliciter les mécanismes qui sous-tendent l’apparition des symptômes dans ces différentes maladies. Dans notre laboratoire, deux modèles murins sont utilisés: le modèle Mecp2tm1Bird qui présente une déficience en Mecp2 et le modèle Mecp2Tg1 présentant une surexpression de Mecp2. Ce travail de thèse a permis de caractériser l’évolution postnatale des déficits moteurs et physiologique affectant la souris Mecp2Tg1. Nos résultats montrent que la surexpression de Mecp2 conduit à l’apparition de problèmes moteurs, et des convulsions chez la souris. En parallèle, nous avons étudié les déficits neuronaux affectants la voie GABAergique et glutamatergique chez la souris déficiente en Mecp2. Nous avons montré que la déficience en Mecp2 cause une dérégulation de la transmission synaptique dépendante du ‘territoire’ et de l’âge de la maladie. Ces dérégulations sous-tendent vraisemblablement des différences neurophysiologiques importantes entre les régions du cerveau qu’il nous reste encore à découvrir. Par ailleurs, nous avons constaté que la stimulation pharmacologique du système GABAergique par la Tiagabine, permet d’augmenter la survie des animaux Mecp2-déficients. / MeCP2 is a multifunctional protein acting on many levels of control of genetic programs. Thus, an abnormal dosage of MeCP2 protein causes a group of neurological disorders with a common feature of severe intellectual disability. Mutations or deletions in MECP2 gene cause Rett Syndrome in females, whereas in boys its overexpression causes the MECP2-duplication Syndrome. Several mouse models of MECP2-pathologies were generated. The use of these models is crucial for understanding the mechanisms underlying the onset of symptoms related to the pathology. In our laboratory, two mouse models are under study: The Mecp2tm1Bird model with an Mecp2 deficiency and the transgenic Mecp2Tg1 model with a double expression of Mecp2. My thesis work enabled the characterization of the postnatal physiological and motor deficits affecting Mecp2Tg1 mice. My work led to a better understanding of the gene dosage effect. Our results showed that overexpression of Mecp2 in mice, led to the occurrence of motor problems, and seizures. In parallel, we studied the neural deficits affecting the GABA and the glutamate pathway in several structures of the Mecp2 deficient brain (Mecp2tm1bird). We showed that Mecp2-deficiency causes deregulation of the synaptic transmission, which is dependent on the area, and the age of the study. These deregulations underlie significant neurophysiological differences between the different regions of the brain that we still have to uncover. Furthermore, we found that pharmacological stimulation of the GABA system with Tiagabine, a molecule capable of acting on GABA transporters to prevent its uptake, increases the survival of Mecp2-deficients animals.

Au niveau du canal central (CC) du tronc cérébral et de la moelle épinière, on trouve des neurones au contact avec le liquide céphalorachidien (Nc-LCR). Les Nc-LCR sont GABAergiques et projettent une seule dendrite dans le CC qui se termine par une large protrusion. Ils expriment sélectivement le canal PKD2L1 pour lequel des fonctions de chimio- et mécanorécepteur ont été démontrées. Compte tenu de leur localisation, de leur morphologie et de l’expression sélective de PKD2L1, les Nc-LCR représenteraient une nouvelle population de neurones sensoriels dans le SNC. Au cours de ma thèse, je me suis concentrée sur la caractérisation des canaux Ca2+ et les mécanismes de signalisation Ca2+ dans les Nc-LCR bulbo-spinaux de souris. Je rapporte que les Nc-LCR expriment des canaux Ca2+ qui sont modulés par les récepteurs métabotropiques GABAB et muscarinique. Je montre aussi l'implication des stocks intracellulaires dans la régulation du Ca2+ intracellulaire. Ensuite, je démontre pour la première fois la relation fonctionnelle entre la protrusion et le soma et indique que la protrusion serait dépourvu de conductance ionique active. Enfin, pour aborder le rôle des Nc-LCR, j'ai développé des modèles chimiogénétiques (DREADDs) et optogénétiques (channelrhodopsin) chez la souris afin de manipuler sélectivement l'activité Nc-LCR. Dans l'ensemble, les résultats de mon étude de doctorat contribuent à mieux comprendre les Nc-LCR bulbo-spinaux des mammifères en en contribuant à la caractérisation de leur physiologie et modulation. Ils ouvrent également la voie à de futures études qui permettront de démontrer le rôle de cette population neuronale dans la régulation de l'activité du SNC. / Cerebrospinal fluid-contacting neurons (CSF-cNs) located in the ependymal region around the central canal (CC) in the brainstem and the spinal cord are GABAergic neurons that project a single dendrite to the CSF and ends with a large protrusion. They selectively express PKD2L1 channel suggested to act as chemo- and mechanoreceptor. Considering their localization, morphology and selective expression of PKD2L1 channel, CSF-cNs would represent a novel population of sensory neurons within the CNS. To better understand the role of CSF-cNs in mammals, it is necessary to describe the physiological properties and modulation of CFS-cNs. In the present study, I focused on Ca2+ channels and Ca2+ signaling mechanisms in mouse medullo-spinal CSF-cNs. I report that Ca2+ channels in CSF-cNs undergo modulation by metabotropic GABAB and muscarinic acetylcholine receptors. I further show the involvement of intracellular Ca2+ stores in the regulation of intracellular Ca2+. Next, I demonstrate for the first time functional relationship between bud and soma and indicate that the bud would be devoid of active ionic conductance. Finally, to address the role of CSF-cNs, I developed chemogenetic (DREADDs) and optogenetic (channelrhodopsin) mice models to be able to selectively manipulate CSF-cN activity. Altogether, the results of my PhD study contribute to better understanding mammalian medullo-spinal CSF-cNs by providing valuable information on their physiology and modulation. They also set ground for further studies carried out in ex-vivo preparation or in vivo models to demonstrate their role in the regulation of CNS activity.

Bakgrund: Smärta definieras som en subjektiv upplevelse där upplevelsen till viss del formas i hjärnan genom komplexa processer. Långvarig stress har visat sig leda till förändring i smärtkänsligheten vilket kanske förklaras av att akut stress förändrar aktivitet i främre delar av hjärnan. Anterior cingulate cortex (ACC) är ett frontalt hjärnområde med en roll i den emotionella upplevelsen av smärta. Det finns indikationer på förändringar i ACC av den hämmande transmittorsubstansen GABA, och den stimulerande transmittorsubstansen glutamat, vid vissa smärttillstånd.  Motiv: I dagsläget saknas kunskap om hur stress påverkar smärtkänslighet samt hur det påverkar grundnivåerna av transmittorsubstanserna GABA och glutamat.  Syfte: Att studera samband mellan skattad stress och skattad smärta, samt studera samband mellan skattad stress, smärta, GABA och glutamat i två ACC regioner.  Metod: En experimentell pilotstudie med kvantitativ design utfördes med 10 friska deltagare. Percieved Stress Questionnaire användes för skattning av stressnivåer före smärtprovokation och Numeric Rating Scale användes för att skatta smärta vid smärtprovokation. GABA- och glutamatnivåer i två ACC regioner mättes med hjälp av Magnetic Resonans Spectroscopi.  Resultat: Spearmans test visade att det fanns ett signifikant positivt samband mellan skattad stress och skattad smärta (r=.86, p=.001). Det fanns även signifikant positivt samband mellan skattad stress och glutamat (r=.778, p=.008), skattad smärta och glutamat (r=.729, p=.017). Samt ett signifikant negativt samband mellan skattad stress och GABA (r= -.687, p=.028), skattad smärta och GABA (r=-.667, p=.035) i två ACC regioner.  Konklusion: Resultatet indikerar att sambandet mellan stress och smärta kan förklaras av förändringar i transmittorsubstansnivåer i två ACC regioner. Denna kunskap kan bidra till ökad förståelse hos vårdpersonalen för individuella skillnader i smärtkänslighet och hur komplex situationen kring patienter med smärta är, vilket motiverar till omvårdnadsåtgärder som minskar stress och smärta. / Background: Pain is defined as a subjective experience that some extent is formed through complex processes in the brain. Prolonged stress can affect pain sensitivity, which may be explained by changes in frontal parts of the brain. The anterior cingulate cortex (ACC) is a frontal brain area that plays a role in the emotional experience of pain. In certain pain conditions, there are indications of changes in the neurotransmitter level GABA and glutamate in ACC. Motive: At present, there is a lack of knowledge about how stress affects pain sensitivity and how it affects the basic levels of the neurotransmitter GABA and glutamate in ACC.  Aim: To study the correlation between rated stress and pain, and study the correlation between rated stress, pain, GABA, and glutamate in ACC.  Methods: An experimental pilot study was performed with 10 healthy participants. The Perceived Stress Questionnaire was used to rate stress levels before pain provocation and the Numeric Rating Scale was used to rate pain during pain provocation. GABA and glutamate levels were measured in ACC by Magnetic Resonance Spectroscopy.  Result: Spearman´s test showed a significant positive correlation between rated stress and rated pain (r = .86, p = .001). There was a significant positive relationship between rated stress and glutamate (r = .778, p = .008), rated pain and glutamate (r = .729, p = .017) and a significant negative correlation between rated stress and GABA (r = -.687, p = .028), rated pain and GABA (r = -.667, p = .035) in ACC.  Conclusion: The results indicate that the correlation between stress and pain can be explained by changes in transmitter levels in the ACC region. This knowledge can contribute to increased understanding among healthcare professionals for individual differences in pain sensitivity and the complexity of pain conditions.

Le but de ce travail était la préparation de dérivés cyclobutaniques du GABA optiquement purs et leur utilisation dans la préparation de γ/α-peptides pouvant adopter une structure tridimensionnelle bien définie. Pour cela, deux stratégies ont été développées. La première consistait en l’utilisation de la β-Cyclodextrine comme hôte supramoléculaire chirale lors de cyclizations photochimiques énantiosélectives. La tentative de cyclisation [2+2] intramoléculaire du N-allyl-N-(4-methoxyphenyl)acrylamide n’a conduit qu’à un δ-lactame issu d’une électrocyclisation 6π. L’électrocyclisation de la 1,3-dihydro-2H-azepin-2-one nous a permis d’obtenir le γ-latame bicyclique précurseur du (+)-cis-3,4CB-GABA avec un excès énantiomérique de 45%. La deuxième stratégie était basée sur une synthèse racémique du N-Boc-cis-3,4CB-GABA suivi d’une séparation des deux énantiomères par CLHP semi-préparative avec une colonne chirale. Les (-) et (+)-cis-3,4CB-GABA optiquement purs ont ainsi été obtenu à l’échelle du gramme. Ces deux énantiomères (-) et (+)-cis-3,4CB-GABA ont ensuite été utilisés pour la préparation de deux séries de peptides mixtes γ/α, diastéréoisomères [(S,S/R) et (R,R/R)] à courtes chaines contenant alternativement le cis-3,4CB-GABA et le D-Alanine. L'analyse des conformations des dipeptides des deux séries par Diffraction des Rayons X, n'a montré aucune interaction intramoléculaire mais plutôt un assemblage de liaisons d'hydrogène intermoléculaires entre les molécules du dipeptide. D'autre part, les études RMN 1D et 2D (en solution) ont montré que le tétrapeptide des séries (S,S/R) pourraient avoir une structure hélicoïdale 12/10, tandis que son analogue diastéréoisomères des séries (R,R/R), a montré, en solution, une nouvelle structure sous forme de ruban 7/9. / This work revolves around the synthesis of ennatiomerically pure cyclobutane derivatives of GABA, and their use in the preparation of hybrid γ/α-peptides that could adopt a well-defined three dimensional secondary structure. In this aim we developed two strategies. The first one employed native β-Cyclodextrin as a supramolecular chiral host to achieve enantiodifferentiating photochemical cyclizations. Attempting to perform an intramolecular [2+2] cyclization of N-allyl-N-(4-methoxyphenyl)acrylamide, we only obtained a δ-lactam resulting from a 6π electrocyclization, whereas the electrocyclization of 1,3-Dihydro‑2H‑azepin-2-one allowed access to a 45% enantiomerically enriched bicyclic γ-lactam precursor of (+)-cis-3,4CB-GABA. The second strategy was based on a racemic synthesis of N-Boc-cis-3,4CB-GABA followed by a separation of the two enantiomers using a semi-preparative HPLC fitted with a chiral column. This allowed access to optically pure (-) and (+)-cis-3,4CB-GABA, on a gram scale. Furthermore, the enantiomerically pure (-) and (+)-cis-3,4CB-GABA, were used to synthesize, and fully characterize two series [the (S,S/R) and the (R,R/R)] of short diasteriomeric hybrid γ/α-peptides composed of alternating cis-3,4CB-GABA and D-Alanine. Analysis of the conformational behavior of the dipeptides from both series by X-Ray diffraction on a single crystal, showed no intramolecular interactions but rather an array of intermolecular hydrogen bonding between the dipeptide molecules. On the other hand, a series of 1D and 2D NMR experiments showed that the tetrapeptide of the (S,S/R)-series could attain a 12/10 helical structuration, whereas its diasteriomeric analog of the (R,R/R)-series, displayed evidence of an unprecedented 7/9 folding pattern in solution.

Pathophysiologische und therapeutische Bedeutung der a1- und a2-Untereinheiten des GABAA-Rezeptors für Dystonien: Untersuchungen im dtsz Hamstermodell

Structural biology has become an important part of researching various diseases and drug development. In this thesis, I provide details on how I worked with approaches to structural characterization of a heteromeric GABA(A)R. These pentameric ligand gated ion channels take part in regulating inhibition of action potentials in nerve cells by allowing the passage of Cl- ions when bound by gamma-aminobutyric acid (GABA). They are formed by the assembly of five subunits which can be of various different types, denoted by greek letters and a number. Much is still unknown about how GABA and several other ligands bind to these ion channels and how that impacts function. Obtaining a structure of these proteins can aid in closing those knowledge gaps. It is reasonable to screen the proteins you have before you study their structures by Cryo-EM in order to get the best result, a methodology for which is described here. I have followed this methodology to screen two heteromeric GABA$_A$R that we wish to determine the structure of, alpha 5 beta 3 and rho 1 gamma 2. Neither of the combinations of genes we used to express these proteins proved to produce the desired fully assembled heteromeric protein. In the case of alpha 5 beta 3, we only witnessed building blocks, with no fully assembled channels. In rho 1 gamma 2, we instead only witnessed fully formed homomers of the rho 1 subunit. These findings then exclude the gene constructs used from further structural study, and the methodology described will inform the next steps to be taken. / Strukturbiologi har blivit en viktig del av forskningen kring många sjukdomar samt utveckling av läkemedel. I denna uppsats delger jag hur jag arbetat med metoder för strukturell karakterisering av en heteromerisk GABA(A)R. Dessa pentameriska ligandstyrda jonkanaler deltar i regleringen av hämning av aktionspotentialer i nervceller genom att tillåta passagen av Cl- joner när gamma-aminosmörsyra (GABA) binder. Dessa består av fem subenheter som kan vara en av flera olika typer, vilka anges med en grekisk bokstav och en siffra. Mycket om hur GABA och andra ligander binder till dessa jonkanaler och hur det påverkar dess funktion är fortfarande okänt. Att hitta en struktur av dessa proteiner kan hjälpa oss att stänga kunskapsgapen. Det är klokt att undersöka om genen man ska använda för att uttrycka det sökta proteinet ger det man söker innan man sen börjar studera strukturen. Jag har beskrivit en metodologi för detta och följt den för två heteromeriska proteiner, alpha 5 beta 3 och rho 1 gamma 2. Ingen av kombinationerna av gener vi använt för att uttrycka dessa proteiner har producerat de sökta, fullt ihoppbyggda proteinerna. I fallet för alpha 5 \beta 3 så ser vi endast byggstenar och inga kompletta proteiner, och för rho 1 gamma 2 så ser vi endast homomeriska proteiner av rho 1. Dessa slutsatser exkluderar de genkonstruktioner vi använt från vidare strukturella studier, och stegen som bör tas härnäst beskrivs av den använda metodologin.

Alcohol use disorder (AUD) is a relapsing–remitting condition characterized by excessive and/or continued alcohol consumption despite harmful consequences. New adjuvant tools, such as noninvasive brain stimulation techniques, might be helpful additions to conventional treatment approaches or even provide an alternative option for patients who fail to respond adequately to other treatment options. Here, we discuss the potential use of auricular transcutaneous vagus nerve stimulation (atVNS) as an ADD-ON intervention in AUD. Compared with other techniques, atVNS has the advantage of directly stimulating nuclei that synthesize GABA and catecholamines, both of which are functionally altered by alcohol intake in AUD patients. Pharmacological options targeting those neurotransmitters are widely available, but have relatively limited beneficial effects on cognition, even though restoring normal cognitive functioning, especially cognitive control, is key to maintaining abstinence. Against this background, atVNS could be a particularly useful add-on because there is substantial meta-analytic evidence based on studies in healthy individuals that atVNS can enhance cognitive control processes that are crucial to regaining control over drug intake. We discuss essential future research on using atVNS as an ADD-ON intervention in AUD to enhance clinical and cognitive outcomes by providing a translational application. Given that this novel technique can be worn like an earpiece and can be employed without medical supervision/outside the clinical settings, atVNS could be well integratable into the daily life of the patients, where the task of regaining control over drug intake is most challenging.

Numerous previous studies of GABAAR ligands have suggested that GABAAR agonists must be zwitterionic and feature an intercharge separation similar to that of GABA (approx. 4.7-6.0 Ã ). We have demonstrated that monomeric, homodimeric and heterodimeric non-zwitterionic GABA amides are partial, full, or superagonists at the murine GABAA receptor (GABAAR). The agonism of these GABA amides is comparable to that of THIP, as shown by in vitro assay results. The assay data indicate that the agonism of GABA amides is tether length-dependent. Optimum agonism is achieved with a tether length of four methylenes in GABA amide dimers and in GABA amides bearing pendant amide or amino groups. We have further investigated the structure-activity relationship for GABA amides on the GABAAR by performing structural modifications to both the superagonist 2c and the agonist 6c. Synergism and [3H]muscimol binding experiments show that 2c binds to the same sites as GABA. Structural modification of 2c demonstrated that partial rigidification of the tether eliminated agonism and caused ligands to behave as weak competitive antagonists. We have also investigated the agonism of four ZAPA derivatives in 36Cl- uptake functional assay. Two of them are found to be as potent as GABA. In our studies of 1,4-benzodiazepines, our goal was to synthesize three different subtypes of quaternary 1,4-benzodiazepines by use of the memory of chirality (MOC) strategy. Disappointingly, most of the deprotonation/alkylations failed, due to various reasons. The failure of the reactions of (S)-alanine-derived tetrahydro-1,4-benzodiazepin-3-ones was probably due to either the unexpected side reactions or the steric hindrance of enolate alkylation. In the case of tetrahydro-1,4-benzodiazepin-2-ones, computational studies suggested that steric hindrance by both the benzo ring and N4-allyl group might retard deprotonation at C3 by bulky bases like KHMDS or LDA. Finally, (S)-serine-derived 1,4-benzodiazepin-2-ones and their elimination products (ï ¡-methylene benzodiazepines) were prepared. These proved unreactive towards deprotonation/alkylations and conjugate additions, respectively. The low reactivity of the ï ¡-methylene benzodiazepines towards nucleophiles was attributed to highly delocalized LUMOs that failed to direct nucleophiles to the ï ¢-carbons. / Ph. D.

The mammalian central nervous system maintains a delicate balance between neuronal excitation and inhibition. Conventional synaptic inhibition is mediated through the transient activity of postsynaptic γ-aminobutyric acid (GABA) at type A GABA (GABAA) receptors. A subset of GABAA receptors is also located outside of inhibitory synapses. These extrasynaptic receptors generate a tonic inhibitory conductance in response to low concentrations of extracellular GABA. Tonic inhibition broadly suppresses neuronal activity and regulates many vital processes such as sleep, consciousness and memory formation. This thesis examines the physiological effects of tonic inhibition at the cellular level and in the behaving animal. This thesis also explores whether gabapentin, a commonly used sedative, anxiolytic, and analgesic drug, enhances tonic GABAergic inhibition. I hypothesize that: (1) tonic GABAA receptor activity reduces the intrinsic excitability of neurons; (2) the activity of tonically active GABAA receptors in spinal pain pathways attenuates nociception; and (3) tonic inhibition can be upregulated by gabapentin. The results show that a tonic inhibitory current generated by α5 subunit-containing GABAA (α5GABAA) receptors reduces the excitability of hippocampal pyramidal neurons excitability by increasing the rheobase, but does not change the gain of action potential firing. A similar shunting inhibition is present in spinal cord lamina II neurons that is generated by δ subunit-containing GABAA receptors. The activity of these receptors in spinal nociceptive pathways reduces acute thermal nociception and may constrain central sensitization in a behavioural model of persistent pain. Finally, gabapentin increases a tonic inhibitory current in cultured hippocampal neurons independent from changes in the expression of α5GABAA receptors or in the concentration of GABAA receptor ligands. The results of this thesis demonstrate that tonically active GABAA receptors play an important role in the regulation of neuronal activity and nociception, and that tonic inhibition represents a novel target of therapeutic drugs.