Social Defeat: Impact on Fear Extinction and Amygdala-Prefrontal Cortical Theta Synchrony in 5-HTT Deficient Mice

Emotions, such as fear and anxiety, can be modulated by both environmental and genetic factors. One genetic factor is for example the genetically encoded variation of the serotonin transporter (5-HTT) expression. In this context, the 5-HTT plays a key role in the regulation of central 5-HT neurotransmission, which is critically involved in the physiological regulation of emotions including fear and anxiety. However, a systematic study which examines the combined influence of environmental and genetic factors on fear-related behavior and the underlying neurophysiological basis is missing. Therefore, in this study we used the 5-HTT-deficient mouse model for studying emotional dysregulation to evaluate consequences of genotype specific disruption of 5-HTT function and repeated social defeat for fear-related behaviors and corresponding neurophysiological activities in the lateral amygdala (LA) and infralimbic region of the medial prefrontal cortex (mPFC) in male 5-HTT wild-type (+/+), homo- (−/−) and heterozygous (+/−) mice. Naive males and experienced losers (generated in a resident-intruder paradigm) of all three genotypes, unilaterally equipped with recording electrodes in LA and mPFC, underwent a Pavlovian fear conditioning. Fear memory and extinction of conditioned fear was examined while recording neuronal activity simultaneously with fear-related behavior. Compared to naive 5-HTT+/+ and +/− mice, 5-HTT−/− mice showed impaired recall of extinction. In addition, 5-HTT−/− and +/− experienced losers showed delayed extinction learning and impaired recall of extinction. Impaired behavioral responses were accompanied by increased theta synchronization between the LA and mPFC during extinction learning in 5-HTT-/− and +/− losers. Furthermore, impaired extinction recall was accompanied with increased theta synchronization in 5-HTT−/− naive and in 5-HTT−/− and +/− loser mice. In conclusion, extinction learning and memory of conditioned fear can be modulated by both the 5-HTT gene activity and social experiences in adulthood, accompanied by corresponding alterations of the theta activity in the amygdala-prefrontal cortex network

Patterns of Coupled Theta Activity in Amygdala-Hippocampal-Prefrontal Cortical Circuits during Fear Extinction

Signals related to fear memory and extinction are processed within brain pathways involving the lateral amygdala (LA) for formation of aversive stimulus associations, the CA1 area of the hippocampus for context-dependent modulation of these associations, and the infralimbic region of the medial prefrontal cortex (mPFC) for extinction processes. While many studies have addressed the contribution of each of these modules individually, little is known about their interactions and how they function as an integrated system. Here we show, by combining multiple site local field potential (LFP) and unit recordings in freely behaving mice in a fear conditioning paradigm, that theta oscillations may provide a means for temporally and functionally connecting these modules. Theta oscillations occurred with high specificity in the CA1-LA-mPFC network. Theta coupling increased between all areas during retrieval of conditioned fear, and declined during extinction learning. During extinction recall, theta coupling partly rebounded in LA-mPFC and CA1-mPFC, and remained at a low level in CA1-LA. Interfering with theta coupling through local electrical microstimulation in CA1-LA affected conditioned fear and extinction recall depending on theta phase. These results support the hypothesis that theta coupling provides a means for inter-areal coordination in conditioned behavioral responsiveness. More specifically, theta oscillations seem to contribute to a population code indicating conditioned stimuli during recall of fear memory before and after extinction

Adaptive Reifung von Dopamin und Serotonin im Nucleus accumbens, der integrativen Schnittebene zwischen Emotion und Bewegung : Isolationsaufzucht und Methamphetamin-Intoxikation als Induktoren einer gestörten Reifung bei Meriones unguiculatus

Lesting J. Adaptive Reifung von Dopamin und Serotonin im Nucleus accumbens, der integrativen Schnittebene zwischen Emotion und Bewegung : Isolationsaufzucht und Methamphetamin-Intoxikation als Induktoren einer gestörten Reifung bei Meriones unguiculatus. Bielefeld (Germany): Bielefeld University; 2005.Arbeiten an unserem Tiermodell (Meriones unguiculatus) zu den neurobiologischen Grundlagen der Psychose zeigen massive Veränderungen in der Reifung der Monoamine Dopamin (DA) und Serotonin (5-HT) in limbischen (Busche et al., 2004) und präfrontalen Gehirnarealen. Diese entwicklungsbedingten Veränderungen sind Folge einer chronischen Schädigung durch (1) restriktive Isolationsaufzucht (IR) und (2) einmalige frühkindliche Methamphetamin-Behandlung (MA-Intoxikation) am postnatalen Tag (P) 14. Ziel der vorliegenden Arbeit war es, den Einfluss der beiden nicht-invasiven experimentellen Induktoren - getrennt beziehungsweise in Kombination - auf die postnatale DA und 5-HT Reifung in Core und Shell des Nucleus accumbens (NAC), dem so genannten "limbic-motor interface", quantitativ zu untersuchen. Darüber hinaus wurde die Reifung der DAergen Innervation von Core und Shell analysiert, um mögliche kritische Phasen in der Entwicklung des NAC zu detektieren. - Core und Shell des NAC weisen unterschiedliche Reifungsverläufe der DAergen Innervation auf. Die DAerge Faserdichte des Core fällt zwischen P 14 und 30 ab, danach steigt sie stetig bis P 180 an und bleibt auf stabilem Niveau bis P 720. Im Shell erfolgt ein leichter Anstieg der DAergen Faserdichte bis P 70, dann folgt ein hoch signifikanter Anstieg bis P 90 mit anschließender Angleichung an die Core Werte (Lesting et al., 2005a, submitted). - IR führt zu einer signifikanten exzessiven Reifung der DAergen Faserdichte in Core und Shell bis P 90. Stattdessen hat die einmalige MA-Intoxikation bei restriktiv aufgewachsenen Tieren eine suppressive Reifung der DAergen Faserdichte in Core und Shell der linken und im Core der rechten Hemisphäre zur Folge. Keine Wirkung zeigt die MA-Intoxikation bei semi-natürlich aufgewachsenen Tieren (Lesting et al., 2005b). - Die 5-HTerge Faserdichte des NAC wird durch IR nicht beeinflusst (Lehmann et al., 2003; Lesting et al., 2005b). Die einmalige MA-Intoxikation zeigt eine auf die rechte Hemisphäre beschränkte Anhebung der 5-HTergen Innervation in Core und Shell für beide Aufzuchtbedingungen (semi-natürlich und restriktiv). Einen signifikanten Lateralisierungseffekt zeigt die Kombination von IR und einmaliger MA-Intoxikation in Core und Shell des NAC (Lesting et al., 2005b). Die unterschiedliche Reifung der DAergen Innervation von Core und Shell spiegelt die zeitlich differenzierte funktionelle Einbindung der accumbalen Subregionen in motorische und limbische Reifungsgeschehnisse wider. Darüber hinaus zeigt die Reifungsstudie, dass die zwei nicht-invasiven Induktoren unseres Tiermodells in der erkannten kritischen Reifungsphase (P14-70) auf unterschiedliche Weise auf den NAC einwirken. Damit unterstützt und ergänzt die Pathologie der Transmitterreifung im NAC die bereits in kortikalen und limbischen Regionen erhobenen Daten zu einem "Zwei-Stufen-Modell" auf dem Weg in die Psychose: Im ersten Schritt wird durch die chronische Deprivation lediglich die DAerge Faserdichte angehoben. Erst durch die zusätzliche pharmakologische Traumatisierung wird eine funktionale Diskonnektion zwischen DA und 5-HT in Core und Shell des NAC ausgelöst

Impact of Life History on Fear Memory and Extinction

Behavioral profiles are strongly shaped by an individual’s whole life experience. The accumulation of negative experiences over lifetime is thought to promote anxiety-like behavior in adulthood (‘allostatic load hypothesis’). In contrast, the ‘mismatch hypothesis’ of psychiatric disease suggests that high levels of anxiety-like behavior are the result of a discrepancy between early and late environment. The aim of the present study was to investigate how different life histories shape the expression of anxiety-like behavior and modulate fear memory. In addition, we aimed to clarify which of the two hypotheses can better explain the modulation of anxiety and fear. For this purpose, male mice grew up under either adverse or beneficial conditions during early phase of life. In adulthood they were further subdivided in groups that either matched or mismatched the condition experienced before, resulting in four different life histories. The main results were: (i) Early life benefit followed by late life adversity caused decreased levels of anxiety-like behavior. (ii) Accumulation of adversity throughout life history led to impaired fear extinction learning. Late life adversity as compared to late life benefit mainly affected extinction training, while early life adversity as compared to early life benefit interfered with extinction recall. Concerning anxiety-like behavior, the results do neither support the allostatic load nor the mismatch hypothesis, but rather indicate an anxiolytic effect of a mismatched early beneficial and later adverse life history. In contrast, fear memory was strongly affected by the accumulation of adverse experiences over the lifetime, therefore supporting allostatic load hypothesis. In summary, this study highlights that anxiety-like behavior and fear memory are differently affected by specific combinations of adverse or beneficial events experienced throughout life

Directional Theta Coherence in Prefrontal Cortical to Amygdalo-Hippocampal Pathways Signals Fear Extinction

<div><p>Theta oscillations are considered crucial mechanisms in neuronal communication across brain areas, required for consolidation and retrieval of fear memories. One form of inhibitory learning allowing adaptive control of fear memory is extinction, a deficit of which leads to maladaptive fear expression potentially leading to anxiety disorders. Behavioral responses after extinction training are thought to reflect a balance of recall from extinction memory and initial fear memory traces. Therefore, we hypothesized that the initial fear memory circuits impact behavioral fear after extinction, and more specifically, that the dynamics of theta synchrony in these pathways signal the individual fear response. Simultaneous multi-channel local field and unit recordings were obtained from the infralimbic prefrontal cortex, the hippocampal CA1 and the lateral amygdala in mice. Data revealed that the pattern of theta coherence and directionality within and across regions correlated with individual behavioral responses. Upon conditioned freezing, units were phase-locked to synchronized theta oscillations in these pathways, characterizing states of fear memory retrieval. When the conditioned stimulus evoked no fear during extinction recall, theta interactions were directional with prefrontal cortical spike firing leading hippocampal and amygdalar theta oscillations. These results indicate that the directional dynamics of theta-entrained activity across these areas guide changes in appraisal of threatening stimuli during fear memory and extinction retrieval. Given that exposure therapy involves procedures and pathways similar to those during extinction of conditioned fear, one therapeutical extension might be useful that imposes artificial theta activity to prefrontal cortical-amygdalo-hippocampal pathways that mimics the directionality signaling successful extinction recall.</p></div

Phase-locking of single unit activity to LFP theta <i>across</i> regions.

<p>(A) Summary of phase-locking analysis during freezing states in R1 and non-freezing states in E. Each identified unit in CA1 (upper), LA (middle), and IL-PFC (bottom) was assigned a LFP theta phase recorded simultaneously in the other two brain areas (grey, blue, green diagrams indicating LFP in CA1, LA, IL-PFC, respectively). Plotted are mean time shifts between unit activity and the LFP theta phase calculated from MRLs in each area. Note the significant lead of unit activity in IL-PFC related to the LFP theta in CA1 and LA during non-freezing in E (highlighted by red lines). (B) Mean MRL of all identified theta phase-locked IL-PFC units during freezing in R1 and non-freezing in E related to LFP theta oscillations recorded simultaneously in CA1 (upper diagram) and LA (lower diagram). Note the peak MRL (indicated by red arrows) signaling a lead of unit activity in IL-PFC related to LFP theta in CA1 and LA during non-freezing in E. (C) Schematic representation of phase-locking of single unit activity to LFP theta across regions, during freezing at R1 and non-freezing at E. Black lines with two arrowheads indicate high theta synchrony (with no significant time lead or lag), red lines indicate theta directionality. Values are mean±SEM; asterisks indicate the significance level revealed by Wilcoxon Signed Rank test : *,p<0.05. MRL, mean resultant length vector; f, freezing; nf, non-freezing.</p

Summary of directional unit phase-locking analysis.

<p>Each identified unit (CA1, LA, IL-PFC) was assigned a theta phase of LFPs recorded in the same and the other two brain areas (unit CA1 - LFP CA1, LA and IL-PFC, unit LA - LFP LA, CA1 and IL-PFC, unit IL-PFC – LFP IL-PFC, CA1 and LA), and lag or lead of the respective unit was determined by the peak MRL value. N [unit/animal] describes the total number of identified phase-locked units and the number of animals in which these units were identified. Results obtained at freezing during retrieval of fear memory (R1), and non-freezing during recall of fear extinction (E). A negative mean [ms] describes a preceding (lead) of the unit, phase-locked to the respective LFP, a positive mean circumscribes the opposite (lag). The p value determines the significance of directionality during R1 and E by use of paired Wilcoxon’s signed rank non-parametric test.</p

Conditioned freezing in response to presentation of the first CS+ during R1, R6 and E.

<p>The fraction of time spent freezing during presentation of the first CS+ significantly declined comparing R1 with R6 and E. Values are mean±SEM; asterisks indicate the significance level: ***,p<0.001 [One-way ANOVA (F_2,36 = 16.03, p<0.001), followed by Tukey’s post hoc test for multiple comparison], n = 13.</p