A key role for nectin-1 in the ventral hippocampus in contextual fear memory

Nectins are cell adhesion molecules that are widely expressed in the brain. Nectin expression shows a dynamic spatiotemporal regulation, playing a role in neural migratory processes during development. Nectin-1 and nectin-3 and their heterophilic trans-interactions are important for the proper formation of synapses. In the hippocampus, nectin-1 and nectin-3 localize at puncta adherentia junctions and may play a role in synaptic plasticity, a mechanism essential for memory and learning. We evaluated the potential involvement of nectin-1 and nectin-3 in memory consolidation using an emotional learning paradigm. Rats trained for contextual fear conditioning showed transient nectin-1—but not nectin-3—protein upregulation in synapse-enriched hippocampal fractions at about 2 h posttraining. The upregulation of nectin-1 was found exclusively in the ventral hippocampus and was apparent in the synaptoneurosomal fraction. This upregulation was induced by contextual fear conditioning but not by exposure to context or shock alone. When an antibody against nectin-1, R165, was infused in the ventral-hippocampus immediately after training, contextual fear memory was impaired. However, treatment with the antibody in the dorsal hippocampus had no effect in contextual fear memory formation. Similarly, treatment with the antibody in the ventral hippocampus did not interfere with acoustic memory formation. Further control experiments indicated that the effects of ventral hippocampal infusion of the nectin-1 antibody in contextual fear memory cannot be ascribed to memory non-specific effects such as changes in anxiety-like behavior or locomotor behavior. Therefore, we conclude that nectin-1 recruitment to the perisynaptic environment in the ventral hippocampus plays an important role in the formation of contextual fear memories. Our results suggest that these mechanisms could be involved in the connection of emotional and contextual information processed in the amygdala and dorsal hippocampus, respectively, thus opening new venues for the development of treatments to psychopathological alterations linked to impaired contextualization of emotions

Hippocampal neuroligin-2 overexpression leads to reduced aggression and inhibited novelty reactivity in rats

Disturbances of the excitation/inhibition (E/I) balance in the brain were recently suggested as potential factors underlying disorders like autism and schizophrenia resulting in associated behavioral alterations including changes in social and emotional behavior as well as abnormal aggression. Neuronal cell adhesion molecules (nCAMs) and mutations in these genes were found to be strongly implicated in the pathophysiology of these disorders. Neuroligin2 (nlgn2) is a postsynaptic cell adhesion molecule, which is predominantly expressed at inhibitory synapses and required for synapse specification and stabilization. Changes in the expression of nlgn2 were shown to result in alterations of social behavior as well as altered inhibitory synaptic transmission, hence modifying the E/I balance. In our study, we focused on the role of nlgn2 in the dorsal hippocampus in the regulation of emotional and social behaviors. To this purpose, we injected an AAV construct overexpressing nlgn2 in the hippocampus of rats and investigated the effects on behavior and on markers for the E/I ratio. We could show an increase in GAD65, a GABA-synthesizing protein in neuronal terminals, and furthermore, reduced exploration of novel stimuli and less offensive behavior. Our data suggest nlgn2 in the hippocampus to be strongly implicated in maintaining the E/I balance in the brain and thereby modulating social and emotional behavior

The interplay of conditional NCAM-knockout and chronic unpredictable stress leads to increased aggression in mice

Abstract The neural cell adhesion molecule (NCAM) is a key regulator of brain plasticity. Substantial evidence indicates that NCAM is down-regulated by exposure to sustained stress and chronic stress seems to lead to increased aggression. In addition, constitutional NCAM deletion in mice has been shown to lead to increased intermale aggression and altered emotionality Forebrain-specific postnatal NCAM knockout was previously shown to impair cognitive function, particularly when animals were exposed to subchronic stress, but the effects on emotional and social behavior remain unclear. In this study, we investigated the potential interplay of a forebrain-specific postnatal NCAM deletion and exposure to different lengths of repeated stress (i.e., subchronic: 14 days; chronic: 29 days) on aggressive and emotional behavior. Our results show that postnatal deletion of NCAM in the forebrain leads to increased aggression and altered emotionality depending on the duration of stress, whereas conditional NCAM knockout has no basal impact on these behaviors. These findings support the involvement of NCAM in the regulation of emotional and aggressive behaviors, suggesting that diminished NCAM expression might be a critical vulnerability factor for the development of these behavioral alterations under repeated exposure to stres

Involvement of CRFR1 in the basolateral amygdala in the immediate fear extinction deficit

Several animal and clinical studies have highlighted the ineffectiveness of fear extinction sessions delivered shortly after trauma exposure. This phenomenon, termed the immediate extinction deficit, refers to situations in which extinction programs applied shortly after fear conditioning may result in the reduction of fear behaviors (in rodents frequently measured as freezing responses to the conditioned cue/s) during extinction training, but failure to consolidate this reduction in the long-term. The molecular mechanisms driving this immediate extinction resistance remain unclear. Here we present evidence for the involvement of the corticotropin releasing factor (CRF) system in the basolateral amygdala (BLA) in male Wistar rats. Intra-BLA micro-infusion of the CRFR1 antagonist NBI30775 enhances extinction recall, while administering the CRF agonist CRF6-33 before delayed extinction disrupts recall of extinction. We link the immediate fear extinction deficit with dephosphorylation of GluA1 glutamate receptors at Ser845 and enhanced activity of the protein phosphatase calcineurin in the BLA. Their reversal following treatment with the CRFR1 antagonist indicates their dependency on CRFR1 actions. These findings can have important implications for the improvement of therapeutic approaches to trauma, as well as furthering our understanding of the neurobiological mechanisms underlying fear-related disorders. Significance Statement Trauma-related disorders are costly, highlighting the need to understand the reduction of fear through extinction learning for the development of better therapies. When extinction programs are applied too soon after the traumatic event, numerous studies have found it to be ineffective, though the underlying mechanisms were unclear. Here we confirm that futility of immediate extinction and provide a mechanistic explanation. Using a pharmacological approach, we show evidence for the involvement of the corticotropin releasing factor (CRF) system in the basolateral amygdala in this extinction deficit. We link this involvement with downstream molecular targets of the CRF system that are critical in synaptic plasticity, thus explaining the futility of immediate extinction and providing further insight into fear-related disorders

Low vagal tone in two rat models of psychopathology involving high or low corticosterone stress responses

The two stress-responsive physiological systems, autonomic nervous system (ANS) and hypothalamus-pituitary-adrenal (HPA) axis exert complementary and interrelated actions in the organism. Individuals that suffer stress-related psychopathologies frequently present simultaneous alterations –i.e., either low or high- responsiveness– in both systems. However, there is scarce evidence establishing whether a priori alterations in these systems –i.e., independent of previous stress exposure– may predispose to the development of psychopathologies possibly due to the lack of animal models simultaneously involving aberrant HPA and SNS responses. In this study, we describe two animal models selectively bred according to their differential (either high, ‘High’ or low, ‘Low’) glucocorticoid responsiveness to stress, in comparison to a third line of rats that displays intermediate (‘Inter’) glucocorticoid responses. The two extreme lines may be considered distinct models of psychopathology; the High line representing a model of constitutive mood alterations while the Low line a model of vulnerability to develop stress-induced psychopathologies. We recorded the electrocardiogram in rats from the three lines and quantified heart rate variability and vagal tone indexes during rest and stress challenges. Rats from both High and Low lines displayed higher heart rate and lower basal vagal tone than the Inter group, both at resting and following stress exposure. Specific pharmacological manipulations probing the relative contribution of sympathetic and parasympathetic components on HR modulation confirmed a relative lower vagal tone in High and Low lines and discarded differences in the sympathetic regulation of heart rate between the lines. Therefore, the two genetically-selected High and Low glucocorticoid rat lines emerge as two valuable preclinical models of psychopathology involving two key risk factors for psychiatric and cardiovascular disorders, namely dysregulations in the HPA axis and cardiac vagal functioning

Peripubertal stress increases play fighting at adolescence and modulates nucleus accumbens CB1 receptor expression and mitochondrial function in the amygdala

Play fighting is a highly rewarding behavior that helps individuals to develop social skills. Early-life stress has been shown to alter play fighting in rats and hamsters as well as to increase aggressive behaviors at adulthood. However, it is not known whether individual differences in stress-induced play fighting are related to differential developmental trajectories towards adult aggression. To address this question, we used a rat model of peripubertal stress (PPS)-induced psychopathology that involves increased aggression at adulthood. We report that, indeed, PPS leads to enhanced play fighting at adolescence. Using a stratification approach, we identify individuals with heightened levels of play fighting as the ones that show abnormal forms of aggression at adulthood. These animals showed as well a rapid habituation of their corticosterone responsiveness to repeated stressor exposure at peripuberty. They also showed a striking increase in mitochondrial function in the amygdala—but not nucleus accumbens—when tested ex vivo. Conversely, low, but not high players, displayed increased expression of the CB1 cannabinoid receptor in the nucleus accumbens shell. Our results highlight adolescence as a potential critical period in which aberrant play fighting is linked to the emergence of adult aggression. They also point at brain energy metabolism during adolescence as a possible target to prevent adult aggression

The effects of stress during early postnatal periods on behavior and hippocampal neuroplasticity markers in adult male mice

Infancy is a critical period for brain development. Emerging evidence indicates that stress experienced during that period can have long-term programming effects on the brain and behavior. However, whether different time periods represent different vulnerabilities to the programming of different neurobehavioral domains is not yet known. Disrupted maternal care is known to interfere with neurodevelopmental processes and may lead to the manifestation of behavioral abnormalities in adulthood. Mouse dams confronted with insufficient bedding/nesting material have been shown to provide fragmented maternal care to their offspring. Here, we compared the impact of this model of early life stress (ELS) during different developmental periods comprising either postnatal days (PND) 2-9 (ELS-early) or PND 10-17 (ELS-late) on behavior and hippocampal cell adhesion molecules in male mice in adulthood. ELS-early treatment caused a permanent reduction in bodyweight, whereas this reduction only occurred transiently during juvenility in ELS-late mice. Anxiety was only affected in ELS-late mice, while cognition and sociability were equally impaired in both ELS-treated groups. We analyzed hippocampal gene expression of the γ2 subunit of the GABAa receptor (Gabrg2) and of genes encoding cell adhesion molecules. Gabrg2 expression was increased in the ventral hippocampus in ELS-late-treated animals and was correlated with anxiety-like behavior in the open-field test. ELS-early-treated animals exhibited an increase in nectin-1 expression in the dorsal hippocampus, and this increase was associated with the social deficits seen in these animals. Our findings highlight the relevance of developmental age on stress-induced long-term behavioral alterations. They also suggest potential links between early stress-induced alterations in hippocampal Gabrg2 expression and the developmental programming of anxiety and between changes in hippocampal nectin-1 expression and stress-induced social impairment

Increased brain glucocorticoid actions following social defeat in rats facilitates the long-term establishment of social subordination

Social rank is frequently established through aggressive encounters between new conspecifics. Despite increasing evidence suggesting that social rank is critical for the well-being of both humans and animals, knowledge about the factors influencing social rank remain scarce. Stress was previously shown to affect the establishment and maintenance of social hierarchies in rats. Likewise, increasing systemic corticosterone levels post-encounter in the emerging subordinate rat facilitates the long-term establishment of social subordination. Here, we investigated whether central corticosterone actions are sufficient to mediate this effect. Our data shows that, indeed, an intracerebroventricular corticosterone injection given to the emerging subordinate rat facilitates the long-term maintenance of the subordinate rank. Next, we attempted to identify a particular brain region in which enhancement of corticosterone actions could be sufficient to exert the facilitation of a long-term maintenance in the emerging subordinate brain. However, post-encounter administration of corticosterone into the basolateral amygdala, medial amygdala, lateral septum and the nucleus accumbens, brain regions selected for their implication in social rank establishment and emotional modulation of memory, did not affect long-term social subordination. Our study highlights the involvement of intracerebral corticosterone actions on the facilitation of long-lasting subordinate behavior, likely by having a modulatory role in the neurobehavioral plasticity engaged in the shaping of social subordination