Genetic disruption of dopamine ß-hydroxylase dysregulates innate responses to predator odor in mice.

In rodents, exposure to predator odors such as cat urine acts as a severe stressor that engages innate defensive behaviors critical for survival in the wild. The neurotransmitters norepinephrine (NE) and dopamine (DA) modulate anxiety and predator odor responses, and we have shown previously that dopamine ß-hydroxylase knockout (Dbh -/-), which reduces NE and increases DA in mouse noradrenergic neurons, disrupts innate behaviors in response to mild stressors such as novelty. We examined the consequences of Dbh knockout on responses to predator odor (bobcat urine) and compared them to Dbh-competent littermate controls. Over the first 10 min of predator odor exposure, controls exhibited robust defensive burying behavior, whereas Dbh -/- mice showed high levels of grooming. Defensive burying was potently suppressed in controls by drugs that reduce NE transmission, while excessive grooming in Dbh -/- mice was blocked by DA receptor antagonism. In response to a cotton square scented with a novel "neutral" odor (lavender), most control mice shredded the material, built a nest, and fell asleep within 90 min. Dbh -/- mice failed to shred the lavender-scented nestlet, but still fell asleep. In contrast, controls sustained high levels of arousal throughout the predator odor test and did not build nests, while Dbh -/- mice were asleep by the 90-min time point, often in shredded bobcat urine-soaked nesting material. Compared with controls exposed to predator odor, Dbh -/- mice demonstrated decreased c-fos induction in the anterior cingulate cortex, lateral septum, periaqueductal gray, and bed nucleus of the stria terminalis, but increased c-fos in the locus coeruleus and medial amygdala. These data indicate that relative ratios of central NE and DA signaling coordinate the type and valence of responses to predator odor.

Genetic disruption of dopamine ß-hydroxylase dysregulates innate responses to predator odor in mice.

In rodents, exposure to predator odors such as cat urine acts as a severe stressor that engages innate defensive behaviors critical for survival in the wild. The neurotransmitters norepinephrine (NE) and dopamine (DA) modulate anxiety and predator odor responses, and we have shown previously that dopamine ß-hydroxylase knockout (Dbh -/-), which reduces NE and increases DA in mouse noradrenergic neurons, disrupts innate behaviors in response to mild stressors such as novelty. We examined the consequences of Dbh knockout (Dbh -/-) on responses to predator odor (bobcat urine) and compared them to Dbh-competent littermate controls. Over the first 10 min of predator odor exposure, controls exhibited robust defensive burying behavior, whereas Dbh -/- mice showed high levels of grooming. Defensive burying was potently suppressed in controls by drugs that reduce NE transmission, while excessive grooming in Dbh -/- mice was blocked by DA receptor antagonism. In response to a cotton square scented with a novel "neutral" odor (lavender), most control mice shredded the material, built a nest, and fell asleep within 90 min. Dbh -/- mice failed to shred the lavender-scented nestlet, but still fell asleep. In contrast, controls sustained high levels of arousal throughout the predator odor test and did not build nests, while Dbh -/- mice were asleep by the 90-min time point, often in shredded bobcat urine-soaked nesting material. Compared with controls exposed to predator odor, Dbh -/- mice demonstrated decreased c-fos induction in the anterior cingulate cortex, lateral septum, periaqueductal gray, and bed nucleus of the stria terminalis, but increased c-fos in the locus coeruleus and medial amygdala. These data indicate that relative ratios of central NE and DA signaling coordinate the type and valence of responses to predator odor.

Entorhinal cortex vulnerability to human APP expression promotes hyperexcitability and tau pathology.

Preventative treatment for Alzheimer's Disease is of dire importance, and yet, cellular mechanisms underlying early regional vulnerability in Alzheimer's Disease remain unknown. In human patients with Alzheimer's Disease, one of the earliest observed pathophysiological correlates to cognitive decline is hyperexcitability1. In mouse models, early hyperexcitability has been shown in the entorhinal cortex, the first cortical region impacted by Alzheimer's Disease2-4. The origin of hyperexcitability in early-stage disease and why it preferentially emerges in specific regions is unclear. Using cortical-region and cell-type- specific proteomics and patch-clamp electrophysiology, we uncovered differential susceptibility to human-specific amyloid precursor protein (hAPP) in a model of sporadic Alzheimer's. Unexpectedly, our findings reveal that early entorhinal hyperexcitability may result from intrinsic vulnerability of parvalbumin interneurons, rather than the suspected layer II excitatory neurons. This vulnerability of entorhinal PV interneurons is specific to hAPP, as it could not be recapitulated with increased murine APP expression. Furthermore, the Somatosensory Cortex showed no such vulnerability to adult-onset hAPP expression, likely resulting from PV-interneuron variability between the two regions based on physiological and proteomic evaluations. Interestingly, entorhinal hAPP-induced hyperexcitability was quelled by co-expression of human Tau at the expense of increased pathological tau species. This study suggests early disease interventions targeting non-excitatory cell types may protect regions with early vulnerability to pathological symptoms of Alzheimer's Disease and downstream cognitive decline.

Age-dependent dysregulation of locus coeruleus firing in a transgenic rat model of Alzheimer's disease.

Hyperphosphorylated tau in the locus coeruleus (LC) is ubiquitous in prodromal Alzheimer's disease (AD), and LC neurons degenerate as AD progresses. Hyperphosphorylated tau alters firing rates in other brain regions, but its effects on LC neurons are unknown. We assessed single unit LC activity in anesthetized wild-type (WT) and TgF344-AD rats at 6 months, which represents a prodromal stage when LC neurons are the only cells containing hyperphosphorylated tau in TgF344-AD animals, and at 15 months when amyloid-ß (Aß) and tau pathology are both abundant in the forebrain. At baseline, LC neurons from TgF344-AD rats were hypoactive at both ages compared to WT littermates but showed elevated spontaneous bursting properties. Differences in footshock-evoked LC firing depended on age, with 6-month TgF344-AD rats demonstrating aspects of hyperactivity, and 15-month transgenic rats showing hypoactivity. Early LC hyperactivity is consistent with appearance of prodromal neuropsychiatric symptoms and is followed by LC hypoactivity which contributes to cognitive impairment. These results support further investigation into disease stage-dependent noradrenergic interventions for AD.

Tyrosinase-induced neuromelanin accumulation triggers rapid dysregulation and degeneration of the mouse locus coeruleus.

Motor symptoms in Parkinson's disease (PD) are caused by degeneration of dopamine (DA) neurons of the substantia nigra (SN), while early non-motor symptoms such as anxiety and sleep disturbances are likely mediated by dysfunction of locus coeruleus (LC) norepinephrine (NE) neurons. The LC develops α-synuclein pathology prior to SN DA neurons in PD, and later undergoes degeneration, but the mechanisms responsible for its vulnerability are unknown. The SN and LC are the only structures in the brain that produces appreciable amounts of neuromelanin (NM), a dark cytoplasmic pigment. It has been proposed that NM initially plays a protective role by sequestering toxic catecholamine metabolites and heavy metals, but may become harmful during aging and PD as they overwhelm cellular machinery and are released during neurodegeneration. Rodents do not naturally produce NM, limiting the study of causal relationships between NM and PD-associated LC pathology. Adapting a viral-mediated approach for expression of human tyrosinase, the enzyme responsible for peripheral melanin production, we successfully promoted pigmentation in mouse LC neurons that recapitulates key features of endogenous NM found in primates, including eumelanin and pheomelanin, lipid droplets, and a double-membrane encasement. Pigment expression results in mild neurodegeneration, reduced NE levels, transcriptional changes, and novelty-induced anxiety phenotypes as early as 1-week post-injection. By 6-weeks, NM accumulation is associated with severe LC neurodegeneration and a robust neuroinflammatory response. These phenotypes are reminiscent of LC dysfunction in PD, validating this model for studying the consequences of pigment accumulation in the LC as it relates to neurodegenerative disease.

The Neurotoxin DSP-4 Dysregulates the Locus Coeruleus-Norepinephrine System and Recapitulates Molecular and Behavioral Aspects of Prodromal Neurodegenerative Disease.

The noradrenergic locus coeruleus (LC) is among the earliest sites of tau and α-synuclein pathology in Alzheimer's disease (AD) and Parkinson's disease (PD), respectively. The onset of these pathologies coincides with loss of noradrenergic fibers in LC target regions and the emergence of prodromal symptoms including sleep disturbances and anxiety. Paradoxically, these prodromal symptoms are indicative of a noradrenergic hyperactivity phenotype, rather than the predicted loss of norepinephrine (NE) transmission following LC damage, suggesting the engagement of complex compensatory mechanisms. Because current therapeutic efforts are targeting early disease, interest in the LC has grown, and it is critical to identify the links between pathology and dysfunction. We employed the LC-specific neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), which preferentially damages LC axons, to model early changes in the LC-NE system pertinent to AD and PD in male and female mice. DSP-4 (two doses of 50 mg/kg, one week apart) induced LC axon degeneration, triggered neuroinflammation and oxidative stress, and reduced tissue NE levels. There was no LC cell death or changes to LC firing, but transcriptomics revealed reduced expression of genes that define noradrenergic identity and other changes relevant to neurodegenerative disease. Despite the dramatic loss of LC fibers, NE turnover and signaling were elevated in terminal regions and were associated with anxiogenic phenotypes in multiple behavioral tests. These results represent a comprehensive analysis of how the LC-NE system responds to axon/terminal damage reminiscent of early AD and PD at the molecular, cellular, systems, and behavioral levels, and provides potential mechanisms underlying prodromal neuropsychiatric symptoms.

Norepinephrine and dopamine contribute to distinct repetitive behaviors induced by novel odorant stress in male and female mice.

Exposure to unfamiliar odorants induces an array of repetitive defensive and non-defensive behaviors in rodents which likely reflect adaptive stress responses to the uncertain valence of novel stimuli. Mice genetically deficient for dopamine ß-hydroxylase (Dbh-/-) lack the enzyme required to convert dopamine (DA) into norepinephrine (NE), resulting in globally undetectable NE and supranormal DA levels. Because catecholamines modulate novelty detection and reactivity, we investigated the effects of novel plant-derived odorants on repetitive behaviors in Dbh-/- mice and Dbh+/- littermate controls, which have catecholamine levels comparable to wild-type mice. Unlike Dbh+/- controls, which exhibited vigorous digging in response to novel odorants, Dbh-/- mice displayed excessive grooming. Drugs that block NE synthesis or neurotransmission suppressed odorant-induced digging in Dbh+/- mice, while a DA receptor antagonist attenuated grooming in Dbh-/- mice. The testing paradigm elicited high circulating levels of corticosterone regardless of Dbh genotype, indicating that NE is dispensable for this systemic stress response. Odorant exposure increased NE and DA abundance in the prefrontal cortex (PFC) of Dbh+/- mice, while Dbh-/- animals lacked NE and had elevated PFC DA levels that were unaffected by novel smells. Together, these findings suggest that novel odorant-induced increases in central NE tone contribute to repetitive digging and reflect psychological stress, while central DA signaling contributes to repetitive grooming. Further, we have established a simple method for repeated assessment of stress-induced repetitive behaviors in mice, which may be relevant for modeling neuropsychiatric disorders like Tourette syndrome or obsessive-compulsive disorder that are characterized by stress-induced exacerbation of compulsive symptoms.

CRISPR-Cas9 editing of the arginine-vasopressin V1a receptor produces paradoxical changes in social behavior in Syrian hamsters.

Studies from a variety of species indicate that arginine­vasopressin (AVP) and its V1a receptor (Avpr1a) play a critical role in the regulation of a range of social behaviors by their actions in the social behavior neural network. To further investigate the role of AVPRs in social behavior, we performed CRISPR-Cas9­mediated editing at the Avpr1a gene via pronuclear microinjections in Syrian hamsters (Mesocricetus auratus), a species used extensively in behavioral neuroendocrinology because they produce a rich suite of social behaviors. Using this germ-line gene-editing approach, we generated a stable line of hamsters with a frame-shift mutation in the Avpr1a gene resulting in the null expression of functional Avpr1as. Avpr1a knockout (KO) hamsters exhibited a complete lack of Avpr1a-specific autoradiographic binding throughout the brain, behavioral insensitivity to centrally administered AVP, and no pressor response to a peripherally injected Avpr1a-specific agonist, thus confirming the absence of functional Avpr1as in the brain and periphery. Contradictory to expectations, Avpr1a KO hamsters exhibited substantially higher levels of conspecific social communication (i.e., odor-stimulated flank marking) than their wild-type (WT) littermates. Furthermore, sex differences in aggression were absent, as both male and female KOs exhibited more aggression toward same-sex conspecifics than did their WT littermates. Taken together, these data emphasize the importance of comparative studies employing gene-editing approaches and suggest the startling possibility that Avpr1a-specific modulation of the social behavior neural network may be more inhibitory than permissive.

Consequences of Hyperphosphorylated Tau in the Locus Coeruleus on Behavior and Cognition in a Rat Model of Alzheimer's Disease.

BACKGROUND: The locus coeruleus (LC) is one of the earliest brain regions to accumulate hyperphosphorylated tau, but a lack of animal models that recapitulate this pathology has hampered our understanding of its contributions to Alzheimer's disease (AD) pathophysiology. OBJECTIVE: We previously reported that TgF344-AD rats, which overexpress mutant human amyloid precursor protein and presenilin-1, accumulate early endogenous hyperphosphorylated tau in the LC. Here, we used TgF344-AD rats and a wild-type (WT) human tau virus to interrogate the effects of endogenous hyperphosphorylated rat tau and human tau in the LC on AD-related neuropathology and behavior. METHODS: Two-month-old TgF344-AD and WT rats received bilateral LC infusions of full-length WT human tau or mCherry control virus driven by the noradrenergic-specific PRSx8 promoter. Rats were subsequently assessed at 6 and 12 months for arousal (sleep latency), anxiety-like behavior (open field, elevated plus maze, novelty-suppressed feeding), passive coping (forced swim task), and learning and memory (Morris water maze and fear conditioning). Hippocampal microglia, astrocyte, and AD pathology were evaluated using immunohistochemistry. RESULTS: In general, the effects of age were more pronounced than genotype or treatment; older rats displayed greater hippocampal pathology, took longer to fall asleep, had reduced locomotor activity, floated more, and had impaired cognition compared to younger animals. TgF344-AD rats showed increased anxiety-like behavior and impaired learning and memory. The tau virus had negligible influence on most measures. CONCLUSION: Effects of hyperphosphorylated tau on AD-like neuropathology and behavioral symptoms were subtle. Further investigation of different forms of tau is warranted.

Measuring Behavior in the Home Cage: Study Design, Applications, Challenges, and Perspectives.

The reproducibility crisis (or replication crisis) in biomedical research is a particularly existential and under-addressed issue in the field of behavioral neuroscience, where, in spite of efforts to standardize testing and assay protocols, several known and unknown sources of confounding environmental factors add to variance. Human interference is a major contributor to variability both within and across laboratories, as well as novelty-induced anxiety. Attempts to reduce human interference and to measure more "natural" behaviors in subjects has led to the development of automated home-cage monitoring systems. These systems enable prolonged and longitudinal recordings, and provide large continuous measures of spontaneous behavior that can be analyzed across multiple time scales. In this review, a diverse team of neuroscientists and product developers share their experiences using such an automated monitoring system that combines Noldus PhenoTyper® home-cages and the video-based tracking software, EthoVision® XT, to extract digital biomarkers of motor, emotional, social and cognitive behavior. After presenting our working definition of a "home-cage", we compare home-cage testing with more conventional out-of-cage tests (e.g., the open field) and outline the various advantages of the former, including opportunities for within-subject analyses and assessments of circadian and ultradian activity. Next, we address technical issues pertaining to the acquisition of behavioral data, such as the fine-tuning of the tracking software and the potential for integration with biotelemetry and optogenetics. Finally, we provide guidance on which behavioral measures to emphasize, how to filter, segment, and analyze behavior, and how to use analysis scripts. We summarize how the PhenoTyper has applications to study neuropharmacology as well as animal models of neurodegenerative and neuropsychiatric illness. Looking forward, we examine current challenges and the impact of new developments. Examples include the automated recognition of specific behaviors, unambiguous tracking of individuals in a social context, the development of more animal-centered measures of behavior and ways of dealing with large datasets. Together, we advocate that by embracing standardized home-cage monitoring platforms like the PhenoTyper, we are poised to directly assess issues pertaining to reproducibility, and more importantly, measure features of rodent behavior under more ethologically relevant scenarios.

Perineuronal net degradation rescues CA2 plasticity in a mouse model of Rett syndrome.

Perineuronal nets (PNNs), a specialized form of extracellular matrix, are abnormal in the brains of people with Rett syndrome (RTT). We previously reported that PNNs function to restrict synaptic plasticity in hippocampal area CA2, which is unusually resistant to long-term potentiation (LTP) and has been linked to social learning in mice. Here we report that PNNs appear elevated in area CA2 of the hippocampus of an individual with RTT and that PNNs develop precociously and remain elevated in area CA2 of a mouse model of RTT (Mecp2-null). Further, we provide evidence that LTP could be induced at CA2 synapses prior to PNN maturation (postnatal day 8-11) in wild-type mice and that this window of plasticity was prematurely restricted at CA2 synapses in Mecp2-null mice. Degrading PNNs in Mecp2-null hippocampus was sufficient to rescue the premature disruption of CA2 plasticity. We identified several molecular targets that were altered in the developing Mecp2-null hippocampus that may explain aberrant PNNs and CA2 plasticity, and we discovered that CA2 PNNs are negatively regulated by neuronal activity. Collectively, our findings demonstrate that CA2 PNN development is regulated by Mecp2 and identify a window of hippocampal plasticity that is disrupted in a mouse model of RTT.

Mineralocorticoid receptors dampen glucocorticoid receptor sensitivity to stress via regulation of FKBP5.

Responding to different dynamic levels of stress is critical for mammalian survival. Disruption of mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) signaling is proposed to underlie hypothalamic-pituitary-adrenal (HPA) axis dysregulation observed in stress-related psychiatric disorders. In this study, we show that FK506-binding protein 51 (FKBP5) plays a critical role in fine-tuning MR:GR balance in the hippocampus. Biotinylated-oligonucleotide immunoprecipitation in primary hippocampal neurons reveals that MR binding, rather than GR binding, to the Fkbp5 gene regulates FKBP5 expression during baseline activity of glucocorticoids. Notably, FKBP5 and MR exhibit similar hippocampal expression patterns in mice and humans, which are distinct from that of the GR. Pharmacological inhibition and region- and cell type-specific receptor deletion in mice further demonstrate that lack of MR decreases hippocampal Fkbp5 levels and dampens the stress-induced increase in glucocorticoid levels. Overall, our findings demonstrate that MR-dependent changes in baseline Fkbp5 expression modify GR sensitivity to glucocorticoids, providing insight into mechanisms of stress homeostasis.

Co-released norepinephrine and galanin act on different timescales to promote stress-induced anxiety-like behavior.

Both the noradrenergic and galaninergic systems have been implicated in stress-related neuropsychiatric disorders, and these two neuromodulators are co-released from the stress-responsive locus coeruleus (LC); however, the individual contributions of LC-derived norepinephrine (NE) and galanin to behavioral stress responses are unclear. Here we aimed to disentangle the functional roles of co-released NE and galanin in stress-induced behavior. We used foot shock, optogenetics, and behavioral pharmacology in wild-type (WT) mice and mice lacking either NE (Dbh-/-) or galanin (GalcKO-Dbh) specifically in noradrenergic neurons to isolate the roles of these co-transmitters in regulating anxiety-like behavior in the elevated zero maze (EZM) either immediately or 24 h following stress. Foot shock and optogenetic LC stimulation produced immediate anxiety-like behavior in WT mice, and the effects of foot shock persisted for 24 h. NE-deficient mice were resistant to the anxiogenic effects of acute stress and optogenetic LC stimulation, while mice lacking noradrenergic-derived galanin displayed typical increases in anxiety-like behavior. However, when tested 24 h after foot shock, both Dbh-/- and GalcKO-Dbh mice lacked normal expression of anxiety-like behavior. Pharmacological rescue of NE, but not galanin, in knockout mice during EZM testing was anxiogenic. In contrast, restoring galanin, but not NE, signaling during foot shock normalized stress-induced anxiety-like behavior 24 h later. These results indicate that NE and noradrenergic-derived galanin play complementary, but distinguishable roles in behavioral responses to stress. NE is required for the expression of acute stress-induced anxiety, while noradrenergic-derived galanin mediates the development of more persistent responses following a stressor.

Novel role for mineralocorticoid receptors in control of a neuronal phenotype.

Mineralocorticoid receptors (MRs) in the brain play a role in learning and memory, neuronal differentiation, and regulation of the stress response. Within the hippocampus, the highest expression of MRs is in area CA2. CA2 pyramidal neurons have a distinct molecular makeup resulting in a plasticity-resistant phenotype, distinguishing them from neurons in CA1 and CA3. Thus, we asked whether MRs regulate CA2 neuron properties and CA2-related behaviors. Using three conditional knockout methods at different stages of development, we found a striking decrease in multiple molecular markers for CA2, an effect mimicked by chronic antagonism of MRs. Furthermore, embryonic deletion of MRs disrupted afferent inputs to CA2 and enabled synaptic potentiation of the normally LTP-resistant synaptic currents in CA2. We also found that CA2-targeted MR knockout was sufficient to disrupt social behavior and alter behavioral responses to novelty. Altogether, these results demonstrate an unappreciated role for MRs in controlling CA2 pyramidal cell identity and in facilitating CA2-dependent behaviors.

Binding affinities of oxytocin, vasopressin and Manning compound at oxytocin and V1a receptors in male Syrian hamster brains.

Oxytocin (OT) and arginine vasopressin (AVP), as well as synthetic ligands targeting their receptors (OTR, V1aR), are used in a wide variety of research contexts, although their pharmacological properties are determined in only a few species. Syrian hamsters (Mesocricetus auratus) have a long history of use as a behavioural and biomedical model for the study of OT and AVP and, more recently, hamsters have been used to investigate behavioural consequences of OT-mediated activation of V1aR. We aimed to determine the binding affinities of OT, AVP and the selective V1aR antagonist, Manning compound, for OTR and V1aR in hamster brains. We performed saturation binding assays to determine the Kd values for the selective OTR and V1aR radioligands, [125 I]ornithine vasotocin analogue and [125 I]linear vasopressin antagonist. We then performed competition binding assays to determine Ki values for OT, AVP and Manning compound at both the OTR and V1aR. We found that OT and AVP each had the highest affinity for their canonical receptors (OT-OTR Ki = 4.28 [95% confidence interval (CI) = 2.9-6.3] nmol L-1 ; AVP-V1ar Ki = 4.70 [95% CI = 1.5-14.1] nmol L-1 ) and had the lowest affinity for their non-canonical ligands (OT-V1aR = 495.2 [95% CI = 198.5-1276] nmol L-1 ; AVP-OTR Ki = 36.1 [95% CI = 12.4-97.0] nmol L-1 ). Manning compound had the highest affinity for the V1aR (MC-V1aR Ki = 6.87 [95% CI = 4.0-11.9] nmol L-1 ; MC-OTR Ki = 213.8 [95% CI = 117.3-392.7] nmol L-1 ), although Manning compound was not as selective for the V1aR in hamsters as has been reported for the receptor in rats. When comparing these data with previously published work, we found that the promiscuity of the V1aR in hamsters with respect to OT and AVP binding is more similar to the promiscuity of the human V1aR than to the rat V1aR receptor. Moreover, the selectivity of OT at hamster receptors is more similar to the selectivity of OT at human receptors than the selectivity of OT at rat receptors. These data highlight the importance of determining the pharmacological properties of behaviourally relevant compounds in diverse model species.

Social experience and sex-dependent regulation of aggression in the lateral septum by extrasynaptic δGABAA receptors.

RATIONALE: Understanding the neurobiological mechanisms mediating dominance and competitive aggression is essential to understanding the development and treatment of various psychiatric disorders. Previous research suggests that these mechanisms are both sexually differentiated and influenced substantially by social experience. In numerous species, GABAA receptors in the lateral septum have been shown to play a significant role in aggression in males. However, very little is known about the role of these GABAA receptors in female aggression, the role of social experience on GABAA receptor-mediated aggression, or the roles of different GABAA subtypes in regulating aggression. OBJECTIVES: Thus, in the following set of experiments, we determined the role of social experience in modulating GABAA receptor-induced aggression in both male and female Syrian hamsters, with a particular focus on the GABAA receptor subtype mediating these effects. RESULTS: Activation of GABAA receptors in the dorsal lateral septum increased aggression in both males and females. Social housing, however, significantly decreased the ability of GABAA receptor activation to induce aggression in males but not females. No significant differences were observed in the effects of GABAA receptor activation in dominant versus subordinate group-housed hamsters. Finally, examination of potential GABAA receptor subtype specificity revealed that social housing decreased the ratio of δ extrasynaptic to γ2 synaptic subunit GABAA receptor mRNA expression in the anterior dorsal lateral septum, while activation of δ extrasynaptic, but not γ2 synaptic, GABAA receptors in the dorsal lateral septum increased aggression. CONCLUSIONS: These data suggest that social experience can have profound effects on the neuronal mechanisms mediating aggression, especially in males, and that δ extrasynaptic GABAA receptors may be an important therapeutic target in disorders characterized by high levels of aggression.

Sex-dependent effects of social isolation on the regulation of arginine-vasopressin (AVP) V1a, oxytocin (OT) and serotonin (5HT) 1a receptor binding and aggression.

It is widely held that social isolation produces higher rates of mortality and morbidity and has deleterious effects on an individual's sociality. Relatedly, it is widely observed that socially isolated adult rodents display significantly higher levels of aggression when placed in a social situation than do their conspecifics living in social groups. In the following study, we investigated the effects of social isolation on several neurochemical signals that play key roles in the regulation of social behavior in adults. More specifically, we examined the effects of social isolation on vasopressin (AVP) V1a, oxytocin (OT) and serotonin (5-HT)1a receptor binding within the neural circuit controlling social behavior. Male and female Syrian hamsters were housed individually or with two other hamsters for four weeks and were then tested with a same-sex nonaggressive intruder in a neutral arena for 5 min. Social isolation significantly increased aggression in both males and females and altered receptor binding in several brain regions in a sex-dependent manner. For example, V1a receptor binding was greater in socially isolated males in the anterior hypothalamus than it was in any other group. Taken together, these data provide substantial new support for the proposition that the social environment can have a significant impact on the structural and neurochemical mechanisms regulating social behavior and that the amount and type of social interactions can produce differential effects on the circuit regulating social behavior in a sex-dependent manner.

Modulation of CA2 neuronal activity increases behavioral responses to fear conditioning in female mice.

Activity of hippocampal pyramidal cells is critical for certain forms of learning and memory, and work from our lab and others has shown that CA2 neuronal activity is required for social cognition and behavior. Silencing of CA2 neurons in mice impairs social memory, and mice lacking Regulator of G-Protein Signaling 14 (RGS14), a protein that is highly enriched in CA2 neurons, learn faster than wild types in the Morris water maze spatial memory test. Although the enhanced spatial learning abilities of the RGS14 KO mice suggest a role for CA2 neurons in at least one hippocampus-dependent behavior, the role of CA2 neurons in fear conditioning, which requires activity of hippocampus, amygdala, and possibly prefrontal cortex is unknown. In this study, we expressed excitatory or inhibitory DREADDs in CA2 neurons and administered CNO before the shock-tone-context pairing. On subsequent days, we measured freezing behavior in the same context but without the tone (contextual fear) or in a new context but in the presence of the tone (cued fear). We found that increasing CA2 neuronal activity with excitatory DREADDs during training resulted in increased freezing during the cued fear tests in males and females. Surprisingly, we found that only females showed increased freezing during the contextual fear memory tests. Using inhibitory DREADDs, we found that inhibiting CA2 neuronal activity during the training phase also resulted in increased freezing in females during the subsequent contextual fear memory test. Finally, we tested fear conditioning in RGS14 KO mice and found that female KO mice had increased freezing on the cued fear memory test. These three separate lines of evidence suggest that CA2 neurons are actively involved in both intra- and extra-hippocampal brain processes and function to influence fear memory. Finally, the intriguing and consistent findings of enhanced fear conditioning only among females is strongly suggestive of a sexual dimorphism in CA2-linked circuits.

Transcriptomic Analysis Reveals Sex-Dependent Expression Patterns in the Basolateral Amygdala of Dominant and Subordinate Animals After Acute Social Conflict.

The basolateral amygdala (BLA) is a critical nucleus mediating behavioral responses after exposure to acute social conflict. Male and female Syrian hamsters both readily establish a stable dominant-subordinate relationship among same-sex conspecifics, and the goal of the current study was to determine potential underlying genetic mechanisms in the BLA facilitating the establishment of social hierarchy. We sequenced the BLA transcriptomes of dominant, subordinate, and socially neutral males and females, and using de novo assembly techniques and gene network analyses, we compared these transcriptomes across social status within each sex. Our results revealed 499 transcripts that were differentially expressed in the BLA across both males and females and 138 distinct gene networks. Surprisingly, we found that there was virtually no overlap in the transcript changes or in gene network patterns in males and females of the same social status. These results suggest that, although males and females reliably engage in similar social behaviors to establish social dominance, the molecular mechanisms in the BLA by which these statuses are obtained and maintained are distinct.

Brain-derived neurotrophic factor signaling mitigates the impact of acute social stress.

Brain-derived neurotrophic factor (BDNF) is known to promote fear learning as well as avoidant behavioral responses to chronic social defeat stress, but, conversely, this peptide can also have antidepressant effects and can reduce depressant-like symptoms such as social avoidance. The purpose of this study was to use a variety of approaches to determine whether BDNF acting on tropomyosin receptor kinase B (TrkB) promotes or prevents avoidant phenotypes in hamsters and mice that have experienced acute social defeat stress. We utilized systemic and brain region-dependent manipulation of BDNF signaling before or immediately following social defeat stress in Syrian hamsters, TrkBF616A knock-in mice, and C57Bl/6J mice and measured the subsequent behavioral response to a novel opponent. Systemic TrkB receptor agonists reduced, and TrkB receptor antagonists enhanced, behavioral responses to social defeat in hamsters and mice. In the neural circuit that we have shown mediates defeat-induced behavioral responses, BDNF in the basolateral amygdala, but not the nucleus accumbens, also reduced social avoidant phenotypes. Conversely, knockdown in the basolateral amygdala of TrkB signaling in TrkBF616A mice enhanced defeat-induced social avoidance. These data demonstrate that systemic administration of BDNF-TrkB drugs at the time of social defeat alters the behavioral response to the defeat stressor. These drugs appear to act, at least in part, in the basolateral amygdala and not the nucleus accumbens. These findings were generalizable to two rodent species with very different social structures and, within mice, to a variety of strains providing converging evidence that BDNF-TrkB signaling reduces anxiety- and depression-like symptoms following short-term social stress.