<p> We hypothesized that acute inhibition of the contraction-induced expansion of the muscle TCA cycle intermediate (TCAI) pool via would not adversely effect metabolism or contractile function. Forty rats were anaesthetized and the gastrocnemius muscle (GAS) from one leg was vascularly isolated and perfused with saline (CON) or a red cell media containing DL-cycloserine (CYCLO; Sigma C-7005; dose=0.05 mg/g), an inhibitor of alanine aminotransferase (AAT). After 1h of perfusion, the GAS muscle was either snap frozen (CON-Rest, n=11; CYCLO-Rest, n=9) or stimulated to contract for 10 min (1Hz, 0.3 ms, 2 V) with blood flow fixed at 30 ml min-1 100g-1 and then snap frozen (CON-Stim, n=10; CYCLO-Stim, n=10). The maximal activity of AAT was lower (P≤0.05) at both CYCLO-Rest (0.61±0.02 mmol·kg-1w.w./min; mean± SEM) and CYCLO-Stim (0.63±0.01 mmol·kg-1w.w./min) vs CON-Rest (3.56±0.16 mmol·kg-1w.w./min) and CON-Stim (3.92±0.29 mmol·kg-1w.w./min). Consistent with lower net flux through AAT, muscle [alanine] was lower (P≤0.05) after CYCLO-Stim (6.97±0.26 mmol·kg-1 dw) compared to CON-Stim (8.55±0.56 mmol·kg-1 dw) and not different vs CON-Rest (6.79±0.41 mmol·kg-1 dw). The sum of five measured TCAI (malate, fumarate, citrate, isocitrate, and 2-oxoglutarate) was higher (P≤0.05) at both CON-Rest (2.10± 0.09 mmol·kg-1 dw) and CON-Stim (2.48± 0.11 mmol·kg-1 dw) vs CYCLO-Rest (1.56± 0.11 mmol·kg-1 dw) and CYCLO-Stim (1.88± 0.15 mmol·kg-1 dw) respectively. Despite the reduction in [TCAI] following CYCLO treatment, there was no difference between conditions in muscle lactate accumulation or phosphocreatine degradation after 10 min of stimulation. Contractile function was not different (P≤0.05) between conditions at either rest or stimulation and the decline in force production over ten minutes of stimulation was identical (~60%) between CON-Stim and CYCLO-Stim respectively. We conclude that flux through AAT was reduced after cycloserine treatment, however the acute inhibition of TCAI expansion did not compromise aerobic energy provision. These data support the hypothesis that the contraction-induced increase in muscle [TCAI] is not causally linked to oxidative energy delivery.</p> / Thesis / Master of Science (MSc)

Doctor Pharmaceuticae - DPharm / Introduction: Drug-resistant tuberculosis remains a major world health problem and one of the leading cause of death worldwide. Despite adequate adherence to antituberculosis drugs by patients, the emergence of drug-resistance tuberculosis still occurs. This fact implies other factors leading to the emergence of resistant strains of Mycobacterium tuberculosis. A multidrug treatment regimen, which may consist of five to seven different drugs including terizidone, is used in the treatment of drugresistance tuberculosis. Terizidone is part of the multidrug regimen whose pharmacokinetics is scarce in literature and plasma concentration profile unknown. Two molecules of cycloserine joined by terephtalaldehyde moiety makes up a molecule of terizidone, which is thought to undergo complete metabolism into cycloserine in vivo. Additionally, the current literature report that terizidone and cycloserine can be used interchangeably as they are thought to be equivalent. The aim of this thesis was first to develop and validate bioanalytical methods for determination of terizidone and cycloserine in patients’ plasma samples. Secondly, to model population pharmacokinetics of terizidone and cycloserine. Thirdly, to determine the amount of cycloserine resulting from metabolism of terizidone.

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Posttraumatic stress disorder (PTSD) is a severe, chronic and debilitating psychiatric disorder that can present after the experience of a life-threatening traumatic event. D-cycloserine (DCS), a partial N-methyl-D-aspartate (NMDA) receptor agonist, has been found to augment cognitive behavioural therapy by facilitating fear extinction; however, the precise mechanisms whereby DCS ameliorates fear triggered by a traumatic context remains to be fully elucidated. This study aimed to (i) identify the molecular mechanisms of intrahippocampally administered DCS in facilitating fear extinction in a rat model of PTSD by investigating gene expression profiles in the left dorsal hippocampus (LDH) of male Sprague Dawley rats and (ii) determine whether microRNA (miRNA) expression and DNA methylation mediated these gene expression changes. An adapted version of the PTSD animal model described by Siegmund and Wotjak (2007) was utilised. The total number of 120 rats were grouped into four experimental groups (of 30 rats per group) based on fear conditioning and the intrahippocampal administration of either DCS or saline: (1) fear conditioned + intrahippocampal saline administration (FS), (2) fear conditioned + intrahippocampal DCS administration (FD), (3) control + intrahippocampal saline administration (CS) and (4) control + intrahippocampal DCS administration (CD). Behavioural tests (the light/dark [L/D] avoidance test, forced swim test and open field test) were conducted to assess anxiety and PTSD-like behaviours. The L/D avoidance test was the most sensitive behavioural test of anxiety and was subsequently used to differentiate maladapted (animals that displayed anxiety-like behaviour) and well-adapted (animals that did not display anxiety-like behaviour) subgroups. In order to identify genes that were differentially expressed between FS maladapted (FSM) (n = 6) vs. FD well-adapted (FDW) (n = 6) groups, RNA sequencing was performed on the Illumina HiSeq 2000 which generated more than 60 million reads per sample. This was followed by subsequent bioinformatics analyses (using the software programs TopHat, Bowtie, Cuffdiff and Bio-Ontological Relationship Graph (BORG) database (that identifies genes that may be biologically relevant) to identify biologically relevant differentially expressed genes between the treatment groups. Epigenetic mechanisms mediating observed differences in gene expression were investigated by conducting DNA methylation and miRNAseq analyses in the FDW and FSM experimental groups. DNA methylation was investigated using real-time quantitative PCR (qPCR) amplification followed by high resolution melt analysis on the Rotor-GeneTM 6000. Differences in miRNA expression levels between the FDW and FSM groups were investigated by sequencing the miRNA fraction on the MiSeq platform. The bioinformatics pipeline used to analyse the RNAseq data identified 93 genes that were significantly downregulated in the FDW group compared to the FSM group. Forty-two of these genes were predicted to be biologically relevant (based on BORG analysis). Integrative network analyses revealed subsets of differentially expressed genes common across biological functions, pathways and disorders. The co-administration of DCS and behavioural fear extinction downregulated immune system genes and genes that transcribe proinflammatory and oxidative stress molecules. These molecules mediate neuroinflammation and subsequently cause neuronal damage. DCS also regulated genes involved in learning and memory processes. Additionally, a subset of the genes, which have been found to be associated with disorders that commonly co-occur with PTSD (such as cardiovascular disease, metabolic disease, Alzheimer‘s and Parkinson‘s disease), was downregulated by the co-administration of DCS and behavioural fear extinction. In order to determine whether real-time qPCR analysis would be sensitive enough to detect differential expression in those genes found to be differentially expressed in RNAseq analysis, the expression of nine genes was analysed using SYBR Green qPCR technology. In the LDH, six of the nine genes were found to be differentially expressed between FDW and FSM groups and one gene, matrix metallopeptidase 9 (MMP9), was observed to be differentially expressed between these two groups in the blood. Three of the nine genes for which differential expression levels were investigated using SYBR Green real-time qPCR, contained CpG islands and were used for CpG island DNA methylation analysis. Results indicated that CpG island DNA methylation did not mediate differential gene expression of TRH, NPY or MT2A. Bioinformatics analysis of miRNAseq data identified 23 miRNAs that were differentially expressed between the FDW and FSM groups. Several of these miRNAs have previously been found to be involved in brain development and behavioural measures of anxiety. Furthermore, functional luciferase analysis indicated that the upregulation of rno-mi31a-5p could have facilitated the downregulation of interleukin 1 receptor antagonist gene (IL1RN) as detected in RNAseq. RNAseq and miRNAseq analyses in this PTSD animal model identified differentially expressed genes and miRNAs that serve to broaden our understanding of the mechanism whereby DCS facilitates fear extinction. To this end, immune system genes and genes transcribing proinflammatory and oxidative stress molecules were among the genes that were found to be differentially expressed between the FDW and FSM groups. Based on the results obtained, it can be hypothesised that DCS attenuates neuroinflammation and subsequent neuronal damage, and also regulates genes involved in learning and memory processes. Concomitantly, these gene expression alterations mediate optimal neuronal functioning, plasticity, learning and memory (such as fear extinction memory) which contribute to the fear extinction process. Furthermore, biologically relevant differentially expressed genes that were associated with DCS facilitation of fear extinction and with other chronic medical conditions, such as cardiovascular disease and metabolic diseases, might help to explain the co-occurrence of these disorders with PTSD. In conclusion, Identifying the molecular underpinnings of DCS-mediated fear extinction brings us closer to understanding the process of fear extinction and could, in future work be used to explore novel therapeutic targets to effectively treat PTSD and related disorders. / AFRIKAANSE OPSOMMING: Posttraumatiese stressindroom is 'n ernstige, kroniese aftakelende psigiatriese toestand wat kan ontwikkel na 'n lewensgevaarlike traumatiese gebeurtenis. Daar is bevind dat die gesamentlike toediening van D-sikloserien (DCS), 'n N-metiel-D-aspartaat (NMDA) reseptor agonis, en kognitiewe gedragsterapie effektief is in die bemiddeling van vrees uitwissing; maar die presiese meganisme waar deur DCS die vrees wat deur 'n traumatiese konteks ontlok word verminder, is egter onduidelik. Hierdie studie het beoog om (i) die molekulêre meganismes te identifiseer waardeur intra-hippokampaal toegediende DCS vrees uitwissing fasiliteer, in 'n rot model van posttraumatiese stressindroom, deur geen uitdrukkingsprofiele in the linker dorsale hippokampus (LDH) van manlike Sprague Dawley rotte te ondersoek en (ii) om te bepaal of mikroRNA (miRNA) uitdrukking en DNA metilering die veranderinge in geen uitdrukking bemiddel het. 'n Gewysigde weergawe van die posttraumatiese stressindroom diere model, beskryf deur Siegmund en Wotjak (2007), was gebruik tydens die studie. Rotte was in vier groepe verdeel, vrees kondisionering + soutwater (FS), vrees kondisionering + DCS (FD), kontrole + soutwater (CS) en kontrole + DCS (CD). Gedragstoetse was uitgevoer om angstige, vreesvolle en posttraumatiese stressindroom-tipe gedrag te evalueer. Gedurende die lig/donker (L/D) vermydingstoets het die FS groep aansienlik meer tyd in die donker kompartement deurgebring ('n indikasie van vreesvolle gedrag) in vergelyking met die CS en die FD groepe wat meer tyd in die verligte kompartement deurgebring het ('n indikasie van vreeslose gedrag). Die L/D toets was die mees sensitiewe gedragstoets vir angstige en vreesvolle gedrag en was gevolglik gebruik om die diere te sub-groepeer in wanaangepaste (diere wat angstige en vreesvolle gedrag vertoon het) en goedaangepaste (diere wat nie angstige en vreesvolle gedrag vertoon het nie) subgroepe. Nuwe generasie RNA volgordebepaling (RNAseq) van die LDH RNA en daaropvolgende bioinformatiese analise was uitgevoer om gene te identifiseer wat differensieel uitgedruk is tussen die twee behandelingsgroepe van belang in die betrokke studie, naamlik FS wanaangepaste (FSM) teenoor FD goedaangepaste (FDW) groepe. Epigenetiese analises was uitgevoer om te bepaal of differensieel uitgedrukte miRNAs of CpG-eiland DNA metilasie die differensiële geenuitdrukking bemiddel het. Bioinformatiese analises van die RNAseq data het 93 gene geïdentifiseer waarvan die geen uitdrukking beduidend onderdruk was in die FDW groep in vergelyking met die FSM groep; 42 van hierdie gene was voorspel om biologies relevant te wees. Geïntegreerde netwerk analise het onthul dat sekere van die differensieel uitgedrukte gene gemeenskaplik was tussen verskeie biologiese funksies, padweë en versteurings. DCS het die uitdrukking van immuun-sisteem gene en pro-inflammatoriese en oksidatiewe stres gene verlaag. Hierdie molekules medieer neuro-inflammasie wat gevolglik tot neurale skade lei. DCS het ook gene gereguleer wat betrokke is by leer en geheue prosesse. DCS het onder meer ook die geenuitdrukking verlaag van 'n sub-groep van gene wat voorheen geassosier is met komorbiede versteurings van PTSD. SYBR Green real-time qPCR (werklike tyd kwantitatiewe polimerase ketting reaksie) analise was ondersoek om te bepaal of hierdie metode sensitief genoeg sou wees om die verlaagde geen-uitdrukking van verskeie van die biologies relevante differensieel uitgedrukte gene te identifiseer, in dieselfde LDH komplementêre DNA (cDNA) monsters as wat in die RNAseq gebruik is, asook in die bloed cDNA monsters. SYBR Green real-time qPCR was in staat om ses, van die nege, differensieel uitgedrukte gene in die LDH cDNA monsters en een geen, matriks metallopeptidase 9 (MMP9), in die bloed cDNA monsters op te tel. Drie van die gene waarvoor SYBR Green real-time qPCR gebruik is om differensiële geenuitdrukking te toets, het CpG eilande bevat en was gevolglik gebruik in CpG eiland DNA metilering analises. Resultate het getoon dat CpG eiland DNA metilering nie die differensiële geenuitdrukking van TRH, NPY of MT2A gedryf het nie. Bioinformatiese analises van die miRNAseq data het 23 miRNAs geïdentifiseer wat differensieël uitgedruk was tussen die FDW en FSM groepe. Verskeie van hierdie miRNAs is reeds voorheen beskryf om betrokke te wees in brein ontwikkeling en angs gedrags metings. Funksionele luciferase analises het verder aangedui dat die verhoogde uitdrukking van rno-mi31a-5p moontlik die verlaagde geen uitdrukking van IL1RN, soos waargeneem in die RNAseq data, kon bewerkstellig het. RNAseq en miRNAseq analises in hierdie posttraumatiese stressindroom dieremodel het differensieël uitgedrukte gene en miRNAs geïdentifiseer wat dien om die verstaanswyse te verbreed van hoe DCS die vrees uitwissings proses fasiliteer. Die meganismes waardeur DCS vrees uitwissings bewerkstellig het sluit die verlaging van immuun-sisteem geen-uitdrukking in, sowel as verlaagde uitdrukking van gene wat pro-inflammatoriese en oksidatiewe stress gene transkribeer. DCS het daardeur neuro-inflammasie en gevolglike neurale skade voorkom. DCS het daarmee saam ook gene gereguleer wat betrokke is by leer en geheue prosesse. Hierdie gesamentlike veranderings in geen uitdrukking het gelei tot die uiteindelike bewerkstelling van optimale neurale funksionering, plastisiteit, leer en geheue prosesse wat uiteindelik bygedra het tot vrees uitwissing. Biologies relevante differensieël uitgedrukte gene wat ook geassosieer was met ander kondisies, soos middel verwante versteurings en metaboliese versteurings, kan help om die komorbiditeit met posttraumatiese stressindroom te verklaar. Identifisering van die molekulêre grondslae van DCS bemiddelde vrees uitwissing verbreed ons begrip en verstaan van vrees uitwissing en kan moontlik, in toekomstige navorsing gebruik word om nuwe innoverende terapeutiese teikens te verken om sodoende posttraumatiese stressindroom meer effektief te kan behandel.

Burkholderia cepacia complex (Bcc) species are opportunistic pathogens in patients with cystic fibrosis (CF), which are able to cause lethal infections. The Bcc are inherently resistant to most classes of antibiotics, which makes successful treatment problematic. Lipid A (also known as endotoxin), the hydrophobic anchor of lipopolysaccaride (LPS), is the bio-active component of LPS. One of several unique characteristics of the lipid A of the Bcc, is the permanent attachment of 4-amino-4-deoxy-L-arabinose (L-Ara4N) to the lipid A molecule. Also, the genes involved in L-Ara4N biosynthesis are necessary for viability in B. cenocepacia. Here we present research on lipid A biosynthesis, modi cation, and transport in the Bcc and highlight promising antimicrobial targets. The synthetic antibiotic CHIR-090 is an inhibitor of LpxC, an enzyme involved in the lipid A biosynthetic pathway. I investigated the activity of CHIR-090 against the Bcc and found that sensitivity to this antibiotic was both species- and strain-specific. CHIR-090 displayed MICs between 0.1 and 12.5 μg/ml against a panel of B. multivorans, the most prevalent Burkholderia species in CF. The species- and strain-specific sensitivity towards CHIR-090 was further explored and a strong correlation was found between the presence of a unique open reading frame, named LpxC2, in resistant species. To address the problem of multiple drug-resistance of the Bcc, we investigated the activity of the pyridoxal 50-phosphate (PLP)-dependent enzyme inhibitor cycloserine (CS) against the Bcc. CS is used as a second line of defense against M. tuberculosis. The activity of the D-enantiomer of CS (DCS) against the Bcc was tested and displayed MICs between 2 and 128 μg/ml and acted bactericidal towards the Bcc. Additionally, DCS inhibition of recombinant ArnB from B. cenocepacia J2315, a PLP-dependent enzyme necessary for viability in the Bcc, was studied. ArnB was inhibited reversibly by DCS. ArnB was further explored as a promising drug-target in the Bcc, but only CS has been identified as an inhibitor so far. In this thesis it was attempted to find the reason why is L-Ara4N modification of lipid A necessary for viability in B. cenocepacia. Therefore, two proteins were characterised, which are involved in lipid A transport: LptA, the periplasmic lipid A binding protein, and LptB, the cytoplasmic ATP-ase. LptA was found to be able to bind both modified and unmodified lipid A in vitro and therefore is not L-Ara4N specific. Furthermore, LptA could bind deep-rough-, rough-, and smooth- LPS, similar to that described for Escherichia coli LptA. The kinetic parameters of LptB were determined in vitro (kcat = 5.71 min-1 and KM = 0.88 mM), and were comparable to E. coli LptB. The ATP-ase activity of LptB was not influenced by the presence of any forms of LPS (modified or non-modified). Therefore, we concluded that both B. cenocepacia J2315 LptA and LptB are not L-Ara4N specific.

Indiana University-Purdue University Indianapolis (IUPUI) / Stroke is the second leading cause of mortality worldwide, accounting for 5.5 million deaths annually. In addition to its high mortality rate, stroke is the most common cause of acquired epilepsy. Three to thirty percent of stroke survivors develop post-stroke epilepsy. Although currently available therapies such as thrombolytics and mechanical thrombectomy prevent immediate mortality by restoring blood flow after stroke, these treatments do not target the cellular and molecular mechanisms that lead to post-stroke epileptogenesis. With the increasing number of stroke survivors, there is an urgent need for therapies that prevent epilepsy development in this population. Here, we showed that homeostatic plasticity is involved in the development of hyperexcitability after stroke and can be targeted to prevent the development of post-stroke epilepsy. Using two-photon calcium imaging, we found that homeostatic regulation leads to cortical hyperexcitability after stroke. We also found that activity enhancement by optogenetic and pharmacological approaches can target homeostatic plasticity to prevent post-stroke epilepsy. This study demonstrates the high translational potential of activity enhancement as a novel strategy to prevent post-stroke epilepsy through regulating cortical homeostatic plasticity. Sudden premature death is a leading cause of death in patients with medically refractory epilepsy. This unanticipated death of a relatively healthy person with epilepsy in which no structural or toxicological cause of death can be identified after postmortem analysis is referred to as sudden unexpected death in epilepsy patients (SUDEP). Respiratory failure during seizures is an important underlying mechanism of SUDEP. Here, we showed that LPS-induced peripheral inflammation is protective against SUDEP. This protection is mediated at least in part via enhancing serotonergic function in the brain stem. To the best of our knowledge, this is the first study demonstrating the relationship between peripheral inflammation and SUDEP prevention. / 2024-11-01

Approximately 22% of the United States population suffers from a chronic orofacial pain condition. One such condition is known as trigeminal neuropathic pain frequently reported as continuous aching and burning pain, often accompanied by intermittent electrical shock-like sensations. Dental procedures or trauma are known causes of peripheral trigeminal nerve injury and inflammation. Patients who have this type of facial pain also suffer from emotional distress. For these reasons, trigeminal neuropathic pain needs to be studied in more detail to improve the understanding of the etiology and maintenance of this condition, as well as to develop effective treatment strategies. The first experiment was focused on characterizing the behavioral aspects of the Trigeminal Inflammatory Compression (TIC) mouse model. The findings determined that the TIC injury model induced mechanical and cold hypersensitivity that persist at least 21 weeks. This orofacial, neuropathic pain condition was accompanied by anxiety- and depressive-like behaviors at week 8 post injury. The TIC injury mouse model’s chronicity and development of psychosocial impairments demonstrated its usefulness as a facial pain model. The second experiment used the mouse TIC injury model to test the ability of pioglitazone (PIO), a PPARγ agonist used clinically for treatment of diabetes, on alleviating trigeminal pain. A single low dose of PIO had no effect, but a higher dose attenuated facial pain. The third experiment determined that combining ineffective low doses of PIO and D-cycloserine (DCS) produced a potentiated anti-allodynic response of these drugs and attenuated the anxiety associated with the TIC injury. Ex vivo studies revealed that cortical mitochondrial dysfunction occurred after the TIC injury but could be reversed by the combination of DCS/PIO which improves mitochondrial function. Overall, the present studies determined that the novel mouse TIC injury model is a clinically relevant facial neuropathic pain model. The results suggest that PPARγ and brain mitochondria may represent new molecular targets for the treatment of trigeminal neuropathic pain. These studies support the future “repurposing” of PIO and DCS as well as the combination of the two drugs for this new use in patients with trigeminal neuropathic pain.

Einige psychische Störungen, darunter Angst- und Suchterkrankungen, zeichnen sich durch eine abnorme Beteiligung basaler assoziativer Lernprozesse aus. Pawlow’sche Rückfallphänomene den langfristigen Erfolg extinktionsbasierter Therapien. Damit kommt der Untersuchung pharmakologischer Interventionen zur Unterstützung des Extinktionslernens bzw. -abrufs eine zentrale Bedeutung zu. Die vorliegende Dissertation umfasst vier Studien und bedient sich translationaler Pawlow’scher Lernmodelle, um (i) behaviorale und neuronale Mechanismen appetitiver Pawlow’scher Rückfallphänomene beim Menschen zu untersuchen (Studien I und II) sowie (ii) den Effekt des partiellen NMDA Rezeptor Agonisten D-Cycloserin (DCS) zur Unterstützung des Extinktionslernens appetitiver und aversiver Stimuli zu testen (Studien III und IV). Studie I demonstriert, dass appetitive Pawlow’sche Rückfalleffekte im Labor untersucht werden können und lieferte Evidenz für differenzielle Einflüsse der Amygdala und des vmPFC beim Wiederauftreten der konditionierten Reaktion. Studie II belegt die Sensitivität verschiedener, teilweise neuer okularer Reaktionsmaße für die appetitive Konditionierungsforschung. Studie III zeigte, dass DCS mit einer attenuierten BOLD-Antwort in der Amygdala und einer gesteigerten funktionellen Amygdala-vmPFC Konnektivität während des appetitiven Extinktionsabrufs assoziiert war. Studie IV ergab, dass Probanden der DCS- Gruppe attenuierte Arousal Ratings wie auch neuronale Aktivierungen in der Amygdala und dem posterioren Hippocampus im Vergleich zur Placebo-Gruppe aufwiesen. Die vorliegende Arbeit erweitert unser Verständnis appetitiver Pawlow’scher Rückfallphänomene und weist dem vmPFC eine bedeutsame Rolle beim Extinktionsabruf zu. Weiterhin unterstützt sie die Hypothese, dass DCS das Extinktionslernen unterstützt und damit Rückfallphänomene reduziert. / Pavlovian learning mechanisms play an important role in the development, maintenance, and relapse of psychiatric conditions like drug addiction and anxiety disorders. Pavlovian relapse phenomena challenge the long-term success of extinction-based exposure treatments. As such, investigating pharmacological adjuncts that could help to improve extinction learning or long- term retention are of great clinical importance. This dissertation comprises four studies applying translational human laboratory models of Pavlovian learning (i) to characterize the behavioral and neural mechanisms of appetitive Pavlovian relapse (Studies I and II), and (ii) to investigate D-cycloserine (DCS), a partial NMDA receptor agonist, as a pharmacological adjunct to augment Pavlovian extinction learning of appetitive and aversive stimuli (Studies III and IV). In Study I, we showed that appetitive Pavlovian relapse can be successfully modeled in the laboratory and provided evidence for opposing roles of amygdala and vmPFC in mediating the return of conditioned responding. Study II showed the usefulness of different and partly novel ocular response measures for appetitive conditioning research. Finally, we found DCS to attenuate amygdala reactivity during appetitive extinction recall and enhance amygdala-vmPFC coupling (Study III). Corroborating these results, Study IV showed DCS to reduce return of fear on behavioral arousal ratings and in brain areas associated with defense reactions like amygdala and posterior hippocampus. Overall, the present work extends evidence on experimentally induced return of fear to the appetitive research domain and suggests an overarching regulatory role of the vmPFC during extinction recall. Finally, it supports the hypothesis that DCS can augment extinction learning, thereby reducing the risk of relapse phenomena.