Omer Horovitz

Dorsal periaqueductal gray-amygdala pathway conveys both innate and learned fear responses in rats

The periaqueductal gray (PAG) and amygdala are known to be important for defensive responses, and many contemporary fear-conditioning models present the PAG as downstream of the amygdala, directing the appropriate behavior (i.e., freezing or fleeing). However, empirical studies of this circuitry are inconsistent and warrant further examination. Hence, the present study investigated the functional relationship between the PAG and amygdala in two different settings, fear conditioning and naturalistic foraging, in rats. In fear conditioning, electrical stimulation of the dorsal PAG (dPAG) produced unconditional responses (URs) composed of brief activity bursts followed by freezing and 22-kHz ultrasonic vocalization. In contrast, stimulation of ventral PAG and the basolateral amygdalar complex (BLA) evoked freezing and/or ultrasonic vocalization. Whereas dPAG stimulation served as an effective unconditional stimulus for fear conditioning to tone and context conditional stimuli, neither ventral PAG nor BLA stimulation supported fear conditioning. The conditioning effect of dPAG, however, was abolished by inactivation of the BLA. In a foraging task, dPAG and BLA stimulation evoked only fleeing toward the nest. Amygdalar lesion/inactivation blocked the UR of dPAG stimulation, but dPAG lesions did not block the UR of BLA stimulation. Furthermore, in vivo recordings demonstrated that electrical priming of the dPAG can modulate plasticity of subiculum–BLA synapses, providing additional evidence that the amygdala is downstream of the dPAG. These results suggest that the dPAG conveys unconditional stimulus information to the BLA, which directs both innate and learned fear responses, and that brain stimulation-evoked behaviors are modulated by context.

Post-Weaning to Pre-Pubertal (‘Juvenile’) Stress: A Model of Induced Predisposition to Stress-Related Disorders

Human studies suggest that childhood trauma predisposes individuals to develop stress-related disorders such as depression and post-traumatic stress disorder (PTSD). Recent years have witnessed growing interest in effectively modeling in animals the long-term effects of childhood emotional trauma on stress responses in adulthood. Most studies concerned with the impact of early-life stress on subsequent stress responses in adulthood in rodents have focused on the post-natal pre-weaning period. However, psychiatric studies often refer to human childhood rather than infancy when investigating the patients’ traumatic history of stress-related psychopathologies. In accordance with that, we have examined the consequences of stress exposure at a later early-life period, the post-weaning, pre-puberty (juvenile) period, which holds greater resemblance to human childhood. This review summarizes a series of studies examining the impact of exposure of rats to stressors during ‘juvenility’ (‘juvenile stress’) on the ability of these animals to cope with stress later in life. Exposure to relatively brief but significant stress experience during juvenility was found to impair the ability of animals to cope with stressful challenges in adulthood. These behavioral manifestations were associated with lasting alterations in limbic system brain regions of neuromodulatory pathways, such as alterations in the expression of cell adhesion molecules, GABAergic system functioning and alterations in levels of circulating corticosterone. Importantly, these studies have also demonstrated considerable individual and sex differences, which call for the development of adequate analysis approaches. The juvenile stress model combined with characterization of individual profiles is presented as a useful model to study in rodents different facets of stress-related disorders and neural mechanisms of vulnerability and resilience to stress.

A rat model of pre-puberty (Juvenile) stress-induced predisposition to stress-related disorders: Sex similarities and sex differences in effects and symptoms

Abstract Objectives. This study assessed the interactive effect of two risk factors: “Juvenile stress” and sex in the long-term consequences of “Juvenile stress” in male and female rats. Methods. Rats were exposed to “Juvenile stress” and to additional stress in adulthood. Measurements of anxiety and depressive-like behaviours were assessed in relation to each stress exposure and “Sex-specific” sets of criteria in order to characterize individual profiles of altered behaviours. Results. While both male and female rats were affected by exposure to “Juvenile stress”, sex difference were evident in saccharine preference, coping with the stressful challenge of the two-way shuttle avoidance task, and on “Adult stress” induced changes in saccharine preference. “Profiling” altered behaviours revealed sex differences also in the prevalence of rats exhibiting different categories of “Affected” behaviours, indicating that female rats are more susceptible to the long-term effects of “Juvenile stress” and to the immediate effects of “Adulthood stress”. Additionally, the prevalence of “Affected” animals among “Juvenile+ Adulthood stress” was similar, yet the profile of altered behaviours was significantly different. Conclusions. The “Behavioural Profiling” approach presented here is of importance to understanding gender differences in the aetiology of predisposition to stress-related disorders, and of gender symptomatology differences in stress-related disorders.

GABAergic Synapses at the Axon Initial Segment of Basolateral Amygdala Projection Neurons Modulate Fear Extinction

Inhibitory synaptic transmission in the amygdala has a pivotal role in fear learning and its extinction. However, the local circuits formed by GABAergic inhibitory interneurons within the amygdala and their detailed function in shaping these behaviors are not well understood. Here we used lentiviral-mediated knockdown of the cell adhesion molecule neurofascin in the basolateral amygdala (BLA) to specifically remove inhibitory synapses at the axon initial segment (AIS) of BLA projection neurons. Quantitative analysis of GABAergic synapse markers and measurement of miniature inhibitory postsynaptic currents in BLA projection neurons after neurofascin knockdown ex vivo confirmed the loss of GABAergic input. We then studied the impact of this manipulation on anxiety-like behavior and auditory cued fear conditioning and its extinction as BLA related behavioral paradigms, as well as on long-term potentiation (LTP) in the ventral subiculum–BLA pathway in vivo. BLA knockdown of neurofascin impaired ventral subiculum–BLA–LTP. While this manipulation did not affect anxiety-like behavior and fear memory acquisition and consolidation, it specifically impaired extinction. Our findings indicate that modification of inhibitory synapses at the AIS of BLA projection neurons is sufficient to selectively impair extinction behavior. A better understanding of the role of distinct GABAergic synapses may provide novel and more specific targets for therapeutic interventions in extinction-based therapies.

Receptor tyrosine kinase EphA7 is required for interneuron connectivity at specific subcellular compartments of granule cells

Neuronal transmission is regulated by the local circuitry which is composed of principal neurons targeted at different subcellular compartments by a variety of interneurons. However, mechanisms that contribute to the subcellular localisation and maintenance of GABAergic interneuron terminals are poorly understood. Stabilization of GABAergic synapses depends on clustering of the postsynaptic scaffolding protein gephyrin and its interaction with the guanine nucleotide exchange factor collybistin. Lentiviral knockdown experiments in adult rats indicated that the receptor tyrosine kinase EphA7 is required for the stabilisation of basket cell terminals on proximal dendritic and somatic compartments of granular cells of the dentate gyrus. EphA7 deficiency and concomitant destabilisation of GABAergic synapses correlated with impaired long-term potentiation and reduced hippocampal learning. Reduced GABAergic innervation may be explained by an impact of EphA7 on gephyrin clustering. Overexpression or ephrin stimulation of EphA7 induced gephyrin clustering dependent on the mechanistic target of rapamycin (mTOR) which is an interaction partner of gephyrin. Gephyrin interactions with mTOR become released after mTOR activation while enhanced interaction with the guanine nucleotide exchange factor collybistin was observed in parallel. In conclusion, EphA7 regulates gephyrin clustering and the maintenance of inhibitory synaptic connectivity via mTOR signalling.

Dorsal periaqueductal gray simultaneously modulates ventral subiculum induced-plasticity in the basolateral amygdala and the nucleus accumbens.

The ventral subiculum of the hippocampus projects both to the basolateral amygdala (BLA), which is typically, associated with a response to aversive stimuli, as well as to the nucleus accumbens (NAcc), which is typically associated with a response to appetitive stimuli. Traditionally, studies of the responses to emotional events focus on either negative or positive affect-related processes, however, emotional experiences often affect both. The ability of high-level processing brain regions (e.g., medial prefrontal cortex) to modulate the balance between negative and positive affect-related regions was examined extensively. In contrast, the ability of low-level processing areas (e.g., periaqueductal gray—PAG) to do so, has not been sufficiently studied. To address whether midbrain structures have the ability to modulate limbic regions, we first examined the ventral subiculum stimulation’s (vSub) ability to induce plasticity in the BLA and NAcc simultaneously in rats. Further, dorsal PAG (dPAG) priming ability to differentially modulate vSub stimulation induced plasticity in the BLA and the NAcc was subsequently examined. vSub stimulation resulted in plasticity in both the BLA and the NAcc simultaneously. Moreover, depending on stimulus intensity, differential dPAG priming effects on LTP in these two regions were observed. The results demonstrate that negative and positive affect-related processes may be simultaneously modulated. Furthermore, under some conditions lower-level processing areas, such as the dPAG, may differentially modulate plasticity in these regions and thus affect the long-term emotional outcome of the experience.

Periaqueductal Grey differential modulation of Nucleus Accumbens and Basolateral Amygdala plasticity under controllable and uncontrollable stress

Resilience has been conceptualized in part as a dynamic process that includes the ability to adapt to stressful conditions. As such it encompasses the extent to which neural plasticity may be promoted. The current study examined metaplasticity by referring to the “plasticity of synaptic plasticity” in a neural circuit composed of the basolateral amygdala (BLA) and the nucleus accumbens (NAcc), using behavioural stress controllability with or without preceding stimulation of the dorsal periaqueductal gray (i.e. dPAG priming). A tendency for increased plasticity in the controllable versus the uncontrollable group was found in both the BLA and NAcc. dPAG priming suppressed NAcc LTP in all groups, but it suppressed BLA LTP only in the uncontrollable group, demonstrating dissociation between either controllable and uncontrollable groups or the NAcc and BLA. Thus, metaplasticity in the dPAG-BLA-NAcc circuit regulated differentially by controllable or uncontrollable stress may underlie stress coping, and thus contribute to stress-related psychopathologies.

Juvenile adversity and adult threat controllability in translational models of stress-related disorders

Only some individuals exposed to a traumatic experience eventually develop stress-related disorders such as anxiety and PTSD, indicating that the development and course of such disorders are influenced considerably by different risk factors. Understanding the way such risk factors contribute to the development of pathology is thus a key issue in understanding the neurobiology of stress-related disorders. Here we review behavioral approaches and evidence from recent studies which utilized innovative fear conditioning procedures in rats aiming to model pre- and peri-exposure risk factors, including pre-exposure to pre-pubertal adversities, and the level of controllability over the stressful experience during the exposure to the trauma. Furthermore, the importance of taking into consideration individual variability in post-exposure stress-related behaviors in order to differentiate between exposed-affected and exposed-unaffected individuals is demonstrated.

Developmental effects of stimulus gender and the social context in which it appears on threat detection

This study used a hands-free eye-tracking visual search (VS) task to examine possible developmental differences in target detection. Thirty-two young adults and 27 youth were asked to detect a fearful face (male or female) among a crowd of either neutral or happy faces. Fearful male faces were detected faster than fearful female faces, but only by young adults and only when displayed among neutral faces. Additionally, young adults had shorter scanpath lengths prior to the target detection. Finally, a strong negative correlation emerged between age and detection speed for a male target in a neutral crowd. Using this age-matched VS task, the study found age differences in the way individuals detect a threat in a social-related contextual environment, pointing to subtle differences in the emotion-attention interplay during the course of development. Statement of contribution What is already known on this subject? Visual search of threat detection is critical for survival, specifically regarding expressive faces. Visual search efficiency is affected by both stimulus-driven and higher goal-directed processes. Stimuli and contextual features affect threat speed detection. What does this study add? A novel task was designed to examine age-related differences in visual search. Specific stimuli gender and contextual features yielded age-related differences in threat detection. The study further demonstrates the subtle developmental differences in attention-emotion interaction.

The Early Adolescent or “Juvenile Stress” Translational Animal Model of Posttraumatic Stress Disorder

The diagnosis of posttraumatic stress disorder (PTSD) requires exposure to a major traumatic event. This criterion has influenced the thinking about causes of the disorder, approaches to treatment, and also how to establish an effective animal model of the disorder. Accordingly, the focus in animal models of PTSD is on what would establish an effective traumatic exposure that would lead to PTSD-related symptoms in the studied animal. However, the prevalence of PTSD among individuals exposed to a traumatic event suggests that the exposure to the trauma is not sufficient to induce PTSD, since most exposed individuals will not develop the disorder. The emphasis in translational animal models of PTSD should thus be on inclusion of relevant risk factors, that together with the exposure to a traumatic event will lead to the development of PTSD. Epidemiological studies indicate that childhood trauma predisposes individuals to develop stress-related disorders later in life. As a result of this finding, we developed an animal model in which exposing rats to early adolescence (postweaning, prepubertal, juvenile) stress was also found to induce a predisposition to develop PTSD following an exposure to an additional stress in adulthood. Within these pre-exposed animals, dissociation could be caused among individuals that exhibited more anxious or more depressive symptoms. This dissociation, which has been recognized also in human patients, required the development of a behavioral profiling approach that enabled grouping animals according to their clusters of symptoms. Interestingly, this approach also enabled the identification of new data on the influence of sex on long-term consequences of “Juvenile Stress.” These data reveal that whilst both male and female rats showed behavioral changes following exposure to a stressor at juvenility, the profile of effects differed between the sexes. Collectively, these findings indicate that the model presented here is an effective translational model for understanding of the etiology of trauma-related disorders and of relevant predisposing factors.