The vasopressin 1b receptor is prominent in the hippocampal area CA2 where it is unaffected by restraint stress or adrenalectomy

Maternal Aggression Driven by the Transient Mobilisation of a Dormant Hormone-Sensitive Circuit

Aggression, a sexually dimorphic behaviour, is prevalent in males and typically absent in virgin females. Following parturition, however, the transient expression of aggression in adult female mice protects pups from predators and infanticide by male conspecifics. While maternal hormones are known to elicit nursing, their potential role in maternal aggression remains elusive. Here, we show in mice that a molecularly defined subset of ventral premammillary (PMvDAT) neurons, instrumental for intermale aggression, switch from quiescence to a hyperexcitable state during lactation. We identify that the maternal hormones prolactin and oxytocin excite these cells through actions that include T-type Ca2+ channels. Optogenetic manipulation or genetic ablation of PMvDAT neurons profoundly affects maternal aggression, while activation of these neurons impairs the expression of non-aggression-related maternal behaviours. This work identifies a monomorphic neural substrate that can incorporate hormonal cues to enable the transient expression of a dormant behavioural program in lactating females.

Investigation of the Fasciola Cinereum, Absent in BTBR mice, and Comparison with the Hippocampal Area CA2

The arginine vasopressin 1b receptor (Avpr1b) plays an important role in social behaviors including social learning, memory, and aggression, and is known to be a specific marker for the cornu ammonis area 2 (CA2) regions of the hippocampus. The fasciola cinereum (FC) is an anatomical region in which Avpr1b expressing neurons are prominent, but the functional roles of the FC have yet to be investigated. Surprisingly, the FC is absent in the inbred BTBR T+tf/J (BTBR) mouse strain used to study core behavioral deficits of autism. Here, we characterized and compared transcriptomic expression profiles using single nucleus RNA sequencing and identified 7 different subpopulations and heterogeneity within the dorsal CA2 (dCA2) and FC. Mef2c, involved in autism spectrum disorder, is more highly expressed in the FC. Using Hiplex in situ hybridization, we examined the neuroanatomical locations of these subpopulations in the proximal and distal regions of the hippocampus. Anterograde tracing of Avpr1b neurons specific for the FC showed projections to the IG, dCA2, lacunosum molecular layer of CA1, dorsal fornix, septofibrial nuclei, and intermediate lateral septum (iLS). In contrast to the dCA2, inhibition of Avpr1b neurons in the FC by the inhibitory DREADD system during behavioral testing did not impair social memory. We performed single nucleus RNA sequencing in the dCA2 region and compared between wildtype (WT) and BTBR mice. We found that transcriptomic profiles of dCA2 neurons between BTBR and WT mice are very similar as they did not form any unique clusters; yet, we found there were differentially expressed genes between the dCA2s of BTBR and WT mice. Overall, this is a comprehensive study of the comparison of Avpr1b neuronal subpopulations between the FC and dCA2. The fact that FC is absent in BTBR mice, a mouse model for autism spectrum disorder, suggests that the FC may play a role in understanding neuropsychiatric disease.

Identification of hippocampal area CA2 in hamster and vole brain

Prairie voles (Microtus ochrogaster) and Syrian, or golden, hamsters (Mesocricetus auratus) are closely related to mice (Mus musculus) and are commonly used in studies of social behavior including social interaction, social memory, and aggression. Hippocampal area CA2 is known to play a key role in these behaviors in mice and responds to social stimuli in rats, but CA2 has yet to be characterized in hamsters or voles, which are also used in studies of social behaviors. Here, we used immunofluorescence to determine whether CA2 could be molecularly identified in tissue from voles and hamsters. We found that  staining for many CA2 markers was similar in these three species, with labeling seen in neurons at the distal end of the mossy fibers . In contrast, although perineuronal nets (PNNs) surround CA2 cells in mice, PNN staining differed across species. In voles, both CA2 and CA3 were labeled, whereas in hamsters, labeling was seen primarily in CA3. These results demonstrate that CA2 can be molecularly distinguished from neighboring CA1 and CA3 areas in voles and hamsters with several antibodies commonly used in mice. However, PNN staining is not useful for identifying CA2 in voles or hamsters, suggestive of differing roles for either PNNs or for the hippocampal subregions in social behavior. These findings reveal commonalities across species in the molecular profile of CA2 and should facilitate future studies of CA2 in these species.

Identification of hippocampal area CA2 in hamster and vole brain

Prairie voles (Microtus ochrogaster) and Syrian, or golden, hamsters (Mesocricetus auratus) are closely related to mice (Mus musculus) and rats (Rattus norvegicus, for example) and are commonly used in studies of social behavior including social interaction, social memory, and aggression. The CA2 region of the hippocampus is known to play a key role in social memory and aggression in mice and responds to social stimuli in rats, likely owing to its high expression of oxytocin and vasopressin 1b receptors. However, CA2 has yet to be identified and characterized in hamsters or voles. In this study, we sought to determine whether CA2 could be identified molecularly in vole and hamster. To do this, we used immunofluorescence with primary antibodies raised against known molecular markers of CA2 in mice and rats to stain hippocampal sections from voles and hamsters in parallel with those from mice. Here, we report that, like in mouse and rat, staining for many CA2 proteins in vole and hamster hippocampus reveals a population of neurons that express regulator of G protein signaling 14 (RGS14), Purkinje cell protein 4 (PCP4) and striatal-enriched protein tyrosine phosphatase (STEP), which together delineate the borders with CA3 and CA1. These cells were located at the distal end of the mossy fiber projections, marked by the presence of Zinc Transporter 3 (ZnT-3) and calbindin in all three species. In addition to staining the mossy fibers, calbindin also labeled a layer of CA1 pyramidal cells in mouse and hamster but not in vole. However, Wolframin ER transmembrane glycoprotein (WFS1) immunofluorescence, which marks all CA1 neurons, was present in all three species and abutted the distal end of CA2, marked by RGS14 immunofluorescence. Staining for two stress hormone receptors-the glucocorticoid (GR) and mineralocorticoid (MR) receptors-was also similar in all three species, with GR staining found primarily in CA1 and MR staining enriched in CA2. Interestingly, although perineuronal nets (PNNs) are known to surround CA2 cells in mouse and rat, we found that staining for PNNs differed across species in that both CA2 and CA3 showed staining in voles and primarily CA3 in hamsters with only some neurons in proximal CA2 showing staining. These results demonstrate that, like in mouse, CA2 in voles and hamsters can be molecularly distinguished from neighboring CA1 and CA3 areas, but PNN staining is less useful for identifying CA2 in the latter two species. These findings reveal commonalities across species in molecular profile of CA2, which will facilitate future studies of CA2 in these species. Yet to be determined is how differences in PNNs might relate to differences in social behavior across species.

Autoimmune processes in neurological patients are much more common than presently suspected

Autoimmune encephalitides are seldom diseases. How rare they actually are, however, is not known. The low incidence combined with the problematic identification may dampen efforts of neurologists, to identify patients with unclear symptoms as suffering from autoimmune encephalitis. Here, we aim to obtain a better estimate, how many patients with autoimmune disorders should be expected among 100 inpatients in a conventional neurological department. From a total number of 2603 non-stroke patients attended in a 2-year period (2018-2019) 460 CSFs were obtained. From this collection 187 samples (40.7%, > 500 sections) could be analyzed with our immunocytochemical technique. Autoreactive antibodies were detected in 102 (55%) of these 187 CSF samples. Certainly, the presence of autoreactive antibodies does not necessarily indicate that the patient suffers from an autoimmune disease. Our data indicate that from roughly 2000 patients during 1 year about 125 patients with autoreactive CSF antibodies should be expected in a conventional neurological department. This represents the about 35-fold value of what is generally expected at present. Being aware of this high incidence may intensify the efforts of neurologist to identify patients with any type of autoimmune encephalitis. This will be beneficial for patients, because they often profit from immunomodulatory therapy. Interestingly, some CFSs from our patients react with the CA2 subdivision of the hippocampus. While long neglected, recent research places this area into an important position to influence hippocampal network physiology. Autoreactive antibodies in the CSF may disturb the function of CA2 neurons, thereby explaining some neuropsychiatric symptoms in patients with autoimmune encephalitides.

Hyperthermia elevates brain temperature and improves behavioural signs in animal models of autism spectrum disorder

BACKGROUND

Autism spectrum disorders (ASD) are predominantly neurodevelopmental and largely genetically determined. However, there are human data supporting the idea that fever can improve symptoms in some individuals, but those data are limited and there are almost no data to support this from animal models. We aimed to test the hypothesis that elevated body temperature would improve function in two animal models of ASD.

METHODS

We used a 4 h whole-body hyperthermia (WBH) protocol and, separately, systemic inflammation induced by bacterial endotoxin (LPS) at 250 µg/kg, to dissociate temperature and inflammatory elements of fever in two ASD animal models: C58/J and Shank3B- mice. We used one- or two-way ANOVA and t-tests with normally distributed data and Kruskal-Wallis or Mann-Whitney with nonparametric data. Post hoc comparisons were made with a level of significance set at p < 0.05. For correlation analyses, data were adjusted by a linear regression model.

RESULTS

Only LPS induced inflammatory signatures in the brain while only WBH produced fever-range hyperthermia. WBH reduced repetitive behaviours and improved social interaction in C58/J mice and significantly reduced compulsive grooming in Shank3B- mice. LPS significantly suppressed most activities over 5-48 h.

LIMITATIONS

We show behavioural, cellular and molecular changes, but provide no specific mechanistic explanation for the observed behavioural improvements.

CONCLUSIONS

The data are the first, to our knowledge, to demonstrate that elevated body temperature can improve behavioural signs in 2 distinct ASD models. Given the developmental nature of ASD, evidence that symptoms may be improved by environmental perturbations indicates possibilities for improving function in these individuals. Since experimental hyperthermia in patients would carry significant risks, it is now essential to pursue molecular mechanisms through which hyperthermia might bring about the observed benefits.

An intact pituitary vasopressin system is critical for building a robust circadian clock in the suprachiasmatic nucleus

The circadian clock is a biological timekeeping system that oscillates with a circa-24-h period, reset by environmental timing cues, especially light, to the 24-h day-night cycle. In mammals, a "central" clock in the hypothalamic suprachiasmatic nucleus (SCN) synchronizes "peripheral" clocks throughout the body to regulate behavior, metabolism, and physiology. A key feature of the clock's oscillation is resistance to abrupt perturbations, but the mechanisms underlying such robustness are not well understood. Here, we probe clock robustness to unexpected photic perturbation by measuring the speed of reentrainment of the murine locomotor rhythm after an abrupt advance of the light-dark cycle. Using an intersectional genetic approach, we implicate a critical role for arginine vasopressin pathways, both central within the SCN and peripheral from the anterior pituitary.

Stimulation of median raphe terminals in dorsal CA2 reduces social investigation in male mice specifically investigating social stimulus of ovariectomized female mice

The cornu ammonis area 2 (CA2) region is essential for social behaviors, especially in social aggression and social memory. Recently, we showed that targeted CA2 stimulation of vasopressin presynaptic fibers from the paraventricular nuclei of hypothalamus strongly enhances social memory in mice. In addition, the CA2 area of the mouse hippocampus receives neuronal inputs from other regions including the septal nuclei, the diagonal bands of Broca, supramammillary nuclei, and median raphe nucleus. However, the functions of these projections have been scarcely investigated. A functional role of median raphe (MR) - CA2 projection is supported by the MR to CA2 projections and 82% reduction of hippocampal serotonin (5-HT) levels following MR lesions. Thus, we investigated the behavioral role of presynaptic fibers from the median raphe nucleus projecting to the dorsal CA2 (dCA2). Here, we demonstrate the optogenetic stimulation of 5-HT projections to dCA2 from the MR do not alter social memory, but instead reduce social interaction. We show that optical stimulation of MR fibers excites interneurons in the stratum radiatum (SR) and stratum lacunosum moleculare (SLM) of CA2 region. Consistent with these observations, we show that bath application of 5-HT increases spontaneous GABA release onto CA2 pyramidal neurons and excites presumed interneurons located in the SR/SLM. This is the first study, to our knowledge, which investigates the direct effect of 5-HT release from terminals onto dCA2 neurons on social behaviors. This highlights the different roles for these inputs (i.e., vasopressin inputs regulating social memory versus serotonin inputs regulating social interaction).

Social odor discrimination and its enhancement by associative learning in the hippocampal CA2 region

Although the hippocampus is crucial for social memory, how social sensory information is combined with contextual information to form episodic social memories remains unknown. Here, we investigated the mechanisms for social sensory information processing using two-photon calcium imaging from hippocampal CA2 pyramidal neurons (PNs)-which are crucial for social memory-in awake head-fixed mice exposed to social and non-social odors. We found that CA2 PNs represent social odors of individual conspecifics and that these representations are refined during associative social odor-reward learning to enhance the discrimination of rewarded compared with unrewarded odors. Moreover, the structure of the CA2 PN population activity enables CA2 to generalize along categories of rewarded versus unrewarded and social versus non-social odor stimuli. Finally, we found that CA2 is important for learning social but not non-social odor-reward associations. These properties of CA2 odor representations provide a likely substrate for the encoding of episodic social memory.

The Enigmatic CA2: Exploring the Understudied Region of the Hippocampus and Its Involvement in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disease that affects both motor and non-motor functions. Although motor impairment is a prominent clinical sign of PD, additional neurological symptoms may also occur, particularly in the preclinical and prodromal stages. Among these symptoms, social cognitive impairment is common and detrimental. This article aims to review non-motor symptoms in PD patients, focusing on social cognitive deficits. It also examines the specific characteristics of the CA2 region and its involvement in social behavior, highlighting recent advances and perspectives. Additionally, this review provides critical insights into and analysis of research conducted in rodents and humans, which may help improve the understanding of the current status of putative therapeutic strategies for social cognitive dysfunction in PD and potential avenues related to the function of the hippocampal CA2 region.

RGS14 expression in CA2 hippocampus, amygdala, and basal ganglia: Implications for human brain physiology and disease

RGS14 is a multifunctional scaffolding protein that is highly expressed within postsynaptic spines of pyramidal neurons in hippocampal area CA2. Known roles of RGS14 in CA2 include regulating G protein, H-Ras/ERK, and calcium signaling pathways to serve as a natural suppressor of synaptic plasticity and postsynaptic signaling. RGS14 also shows marked postsynaptic expression in major structures of the limbic system and basal ganglia, including the amygdala and both the ventral and dorsal subdivisions of the striatum. In this review, we discuss the signaling functions of RGS14 and its role in postsynaptic strength (long-term potentiation) and spine structural plasticity in CA2 hippocampal neurons, and how RGS14 suppression of plasticity impacts linked behaviors such as spatial learning, object memory, and fear conditioning. We also review RGS14 expression in the limbic system and basal ganglia and speculate on its possible roles in regulating plasticity in these regions, with a focus on behaviors related to emotion and motivation. Finally, we explore the functional implications of RGS14 in various brain circuits and speculate on its possible roles in certain disease states such as hippocampal seizures, addiction, and anxiety disorders.

Morphological and molecular markers of mouse area CA2 along the proximodistal and dorsoventral hippocampal axes

Hippocampal area CA2 is a molecularly and functionally distinct region of the hippocampus that has classically been defined as the area with large pyramidal neurons lacking input from the dentate gyrus and the thorny excrescences (TEs) characteristic of CA3 neurons. A modern definition of CA2, however, makes use of the expression of several molecular markers that distinguish it from neighboring CA3 and CA1. Using immunohistochemistry, we sought to characterize the staining patterns of commonly used CA2 markers along the dorsal-ventral hippocampal axis and determine how these markers align along the proximodistal axis. We used a region of CA2 that stained for both Regulator of G-protein Signaling 14 (RGS14) and Purkinje Cell Protein 4 (PCP4; "double-labeled zone" [DLZ]) as a reference. Here, we report that certain commonly used CA2 molecular markers may be better suited for drawing distinct boundaries between CA2/3 and CA2/1. For example, RGS14+ and STEP+ neurons showed minimal to no extension into area CA1 while areas stained with VGluT2 and Wisteria Floribunda agglutinin were consistently smaller than the DLZ/CA2 borders by ~100 μ on the CA1 or CA3 sides respectively. In addition, these patterns are dependent on position along the dorsal-ventral hippocampal axis such that PCP4 labeling often extended beyond the distal border of the DLZ into CA1. Finally, we found that, consistent with previous findings, mossy fibers innervate a subset of RGS14 positive neurons (~65%-70%) and that mossy fiber bouton number and relative size in CA2 are less than that of boutons in CA3. Unexpectedly, we did find evidence of some complex spines on apical dendrites in CA2, though much fewer in number than in CA3. Our results indicate that certain molecular markers may be better suited than others when defining the proximal and distal borders of area CA2 and that the presence or absence of complex spines alone may not be suitable as a distinguishing feature differentiating CA3 from CA2 neurons.

Projections of hippocampal CA2 pyramidal neurons: Distinct innervation patterns of CA2 compared to CA3 in rodents

The hippocampus is a center for spatial and episodic memory formation in rodents. Understanding the composition of subregions and circuitry maps of the hippocampus is essential for elucidating the mechanism of memory formation and recall. For decades, the trisynaptic circuit (entorhinal cortex layer II-dentate gyrus - CA3-CA1) has been considered the neural network substrate responsible for learning and memory. Recently, CA2 has emerged as an important area in the hippocampal circuitry, with distinct functions from those of CA3. In this article, we review the historical definition of the hippocampal area CA2 and the differential projection patterns between CA2 and CA3 pyramidal neurons. We provide a concise and comprehensive map of CA2 outputs by comparing (1) ipsi versus contra projections, (2) septal versus temporal projections, and (3) lamellar structures of CA2 and CA3 pyramidal neurons.

Preadolescent exposure to a sexually mature, unrelated male rat reduces postadolescent social recognition memory and CA2 c-Fos labeling

Introduction

Social memory involves social recognition: the ability to discriminate between two or more conspecifics when one has been previously encountered. The CA2 region of the hippocampus has been implicated in social memory, as lesions and dysfunction to this area lead to social memory impairments. A variety of psychogenic manipulations during postnatal sensitive developmental periods are associated with social memory impairments later in life.

Methods

In this study, we exposed preadolescent rats to a sexually, mature unrelated male and examined whether this was associated with changes in postadolescent social memory and c-Fos labeling in the CA2 region. Male and female Long-Evans rats were exposed to a male, adult rat on postnatal days 19-21 (P19-21). Social memory was measured during the postadolescent period and defined as increased interactions towards a novel age-matched rat in contrast to a previously-encountered age-matched rat. After the test, rats were euthanized and brain tissue was then collected to quantify c-Fos labeling within the CA2 region.

Results

Compared to home cage controls and controls not exposed to the adult male, male and female rats exposed to the unrelated adult during preadolescence were unable to discriminate between a novel and previously encountered conspecific during the postadolescent test showing social memory deficits. The groups that showed social recognition deficits also had significantly fewer c-Fos-positive cells within the CA2 region compared to the control groups.

Discussion

These findings indicate that threatening psychogenic encounters during preadolescence can have detrimental long-term effects on social memory potentially via disrupted activity in the CA2 hippocampal region.

Simultaneous Knockouts of the Oxytocin and Vasopressin 1b Receptors in Hippocampal CA2 Impair Social Memory

Oxytocin (Oxt) and vasopressin (Avp) are two neuropeptides with many central actions related to social cognition. The oxytocin (Oxtr) and vasopressin 1b (Avpr1b) receptors are co-expressed in the pyramidal neurons of the hippocampal subfield CA2 and are known to play a critical role in social memory formation. How the neuropeptides perform this function in this region is not fully understood. Here, we report the behavioral effects of a life-long conditional removal (knockout, KO) of either the Oxtr alone or both Avpr1b and Oxtr from the pyramidal neurons of CA2 as well as the resultant changes in synaptic transmission within the different fields of the hippocampus. Surprisingly, the removal of both receptors results in mice that are unable to habituate to a familiar female presented for short duration over short intervals but are able to recognize and discriminate females when presented for a longer duration over a longer interval. Importantly, these double KO mice were unable to discriminate between a male littermate and a novel male. Synaptic transmission between CA3 and CA2 is enhanced in these mice, suggesting a compensatory mechanism is activated to make up for the loss of the receptors. Overall, our results demonstrate that co-expression of the receptors in CA2 is necessary to allow intact social memory processing.

CA2 physiology underlying social memory

In recent years, convergent evidence has emerged in support of the idea of social brain networks, specific brain regions that are interconnected and support social behaviors. One of these regions is the CA2 area of the hippocampus, a small region strongly connected with cortical and subcortical areas implicated in social behaviors. Furthermore, CA2 area is enriched in receptors for several neuromodulators that are related to various aspects of social behaviors, suggesting that this area could be a key component of social information processing in the brain. In this review, recent findings related to the physiological mechanisms underlying the role of CA2 in social memory are discussed.

Socio-behavioral dysfunction in disorders of hypothalamic-pituitary involvement: The potential role of disease-induced oxytocin and vasopressin signaling deficits

Disorders involving hypothalamic and pituitary (HPIT) structures-including craniopharyngioma, Langerhans cell histiocytosis, and intracranial germ cell tumors-can disrupt brain and endocrine function. An area of emerging clinical concern in patients with these disorders is the co-occurring socio-behavioral dysfunction that persists after standard hormone replacement therapy. Although the two neuropeptides most implicated in mammalian social functioning (oxytocin and arginine vasopressin) are of hypothalamic origin, little is known about how disease-induced damage to HPIT structures may disrupt neuropeptide signaling and, in turn, impact patients' socio-behavioral functioning. Here we provide a clinical primer on disorders of HPIT involvement and a review of neuropeptide signaling and socio-behavioral functioning in relevant animal models and patient populations. This collective evidence suggests that neuropeptide signaling disruptions contribute to socio-behavioral deficits experienced by patients with disorders of HPIT involvement. A better understanding of the biological underpinnings of patients' socio-behavioral symptoms is now needed to enable the development of the first targeted pharmacological strategies by which to manage patients' socio-behavioral dysfunction.

Anti-stress Effect of Octopus Cephalotocin in Rats

Cephalotocin is a bioactivity-regulating peptide expressed in octopus (Octopus vulgaris). The peptide sequence of cephalotocin is very similar to the peptide sequence of mammalian vasopressin, and cephalotocin has been proposed to mainly activate arginine vasopressin 1b receptor (Avpr1b) in the brain. However, the effects of cephalotocin on mammalian behavior have not been studied. In the current study, cephalotocin significantly reduced both the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) recorded from not only cultured neuronal cells from postnatal Sprague–Dawley (SD) rats but also hippocampal slices from 4-week-old male C57BL/6 mice. Intraperitoneal (IP) injection did not affect the open field behaviors of C57BL/6 mice. Cephalotocin was directly infused into the hippocampus because the normalized Avpr1b staining intensity divided by the DAPI staining intensity indicated that Avpr1b expression tended to be high in the hippocampus. A hippocampal infusion of 1 mg/kg cephalotocin via an implanted cannula exerted an anti-stress effect, significantly reducing the immobility time in the tail suspension test (TST). The present results provide evidence that the effects of cephalotocin on the activity of hippocampal neurons are related to ameliorating stress, suggesting that cephalotocin may be developed as an anti-stress biomodulator that functions by affecting the brain.

Mineralocorticoid receptor and glucocorticoid receptor work alone and together in cell-type-specific manner: Implications for resilience prediction and targeted therapy

'You can't roll the clock back and reverse the effects of experiences' Bruce McEwen used to say when explaining how allostasis labels the adaptive process. Here we will for once roll the clock back to the times that the science of the glucocorticoid hormone was honored with a Nobel prize and highlight the discovery of their receptors in the hippocampus as inroad to its current status as master regulator in control of stress coping and adaptation. Glucocorticoids operate in concert with numerous neurotransmitters, neuropeptides, and other hormones with the aim to facilitate processing of information in the neurocircuitry of stress, from anticipation and perception of a novel experience to behavioral adaptation and memory storage. This action, exerted by the glucocorticoids, is guided by two complementary receptor systems, mineralocorticoid receptors (MR) and glucocorticoid receptors (GR), that need to be balanced for a healthy stress response pattern. Here we discuss the cellular, neuroendocrine, and behavioral studies underlying the MR:GR balance concept, highlight the relevance of hypothalamic-pituitary-adrenal (HPA) -axis patterns and note the limited understanding yet of sexual dimorphism in glucocorticoid actions. We conclude with the prospect that (i) genetically and epigenetically regulated receptor variants dictate cell-type-specific transcriptome signatures of stress-related neuropsychiatric symptoms and (ii) selective receptor modulators are becoming available for more targeted treatment. These two new developments may help to 'restart the clock' with the prospect to support resilience.

Exocrine scent marking: Coordinative role of arginine vasopressin in the systemic regulation of social signaling behaviors

Arginine vasopressin (AVP) is a neurohypophysial hormone that coordinatively regulates central socio-emotional behavior and peripheral control of antidiuretic fluid homeostasis. Most mammals, including rodents, utilize exocrine or urine-contained scent marking as a social signaling tool that facilitates social adaptation. The exocrine scent marking behavior is postulated to fine-tune sensory and cognitive abilities to recognize key social features via exocrine/urinary olfactory cues and subsequently control exocrine deposition or urinary marking through the mediation of osmotic fluid balance. AVP is implicated as a major player in controlling both recognition and signaling responses. This review provides constructive hypotheses on the coordinative processes of the AVP neurohypophysial circuits in the systemic regulations of fluid control and social-communicative behavior, via the expression of exocrine scent marking, and further emphasizes a potential role of AVP in a common mechanism underlying social communication in rodents.

Regulation of Social Recognition Memory in the Hippocampal Circuits

Social recognition memory reflects the ability of the social animals to recognize and remember familiar individuals of the same species. The unique ability for mammals to recognize conspecifics is essential and beneficial when animals conduct daily social activities. This brief review summarizes a brain network underlying social recognition memory and explores the possible relationships between social isolation and social recognition memory. Finally, we introduce some possible related molecular mechanisms underlying social recognition memory. These findings help us explore potential targeting brain areas or circuits of social communication disorder.

The Paraventricular Nucleus of the Hypothalamus in Control of Blood Pressure and Blood Pressure Variability

The paraventricular nucleus (PVN) is a highly organized structure of the hypothalamus that has a key role in regulating cardiovascular and osmotic homeostasis. Functionally, the PVN is divided into autonomic and neuroendocrine (neurosecretory) compartments, both equally important for maintaining blood pressure (BP) and body fluids in the physiological range. Neurosecretory magnocellular neurons (MCNs) of the PVN are the main source of the hormones vasopressin (VP), responsible for water conservation and hydromineral balance, and oxytocin (OT), involved in parturition and milk ejection during lactation. Further, neurosecretory parvocellular neurons (PCNs) take part in modulation of the hypothalamic–pituitary–adrenal axis and stress responses. Additionally, the PVN takes central place in autonomic adjustment of BP to environmental challenges and contributes to its variability (BPV), underpinning the PVN as an autonomic master controller of cardiovascular function. Autonomic PCNs of the PVN modulate sympathetic outflow toward heart, blood vessels and kidneys. These pre-autonomic neurons send projections to the vasomotor nucleus of rostral ventrolateral medulla and to intermediolateral column of the spinal cord, where postganglionic fibers toward target organs arise. Also, PVN PCNs synapse with NTS neurons which are the end-point of baroreceptor primary afferents, thus, enabling the PVN to modify the function of baroreflex. Neuroendocrine and autonomic parts of the PVN are segregated morphologically but they work in concert when the organism is exposed to environmental challenges via somatodendritically released VP and OT by MCNs. The purpose of this overview is to address both neuroendocrine and autonomic PVN roles in BP and BPV regulation.

Oxytocin and vasopressin in the hippocampus

Oxytocin (OXT) and vasopressin (AVP) are related neuropeptides that exert a wide range of effects on general health, homeostasis, development, reproduction, adaptability, cognition, social and nonsocial behaviors. The two peptides are mainly of hypothalamic origin and execute their peripheral and central physiological roles via OXT and AVP receptors, which are members of the G protein-coupled receptor family. These receptors, largely distributed in the body, are abundantly expressed in the hippocampus, a brain region particularly vulnerable to stress exposure and various lesions. OXT and AVP have important roles in the hippocampus, by modulating important processes like neuronal excitability, network oscillatory activity, synaptic plasticity, and social recognition memory. This chapter includes an overview regarding OXT and AVP structure, synthesis, receptor distribution, and functions, focusing on their relationship with the hippocampus and mechanisms by which they influence hippocampal activity. Brief information regarding hippocampal structure and susceptibility to lesions is also provided. The roles of OXT and AVP in neurodevelopment and adult central nervous system function and disorders are highlighted, discussing their potential use as targeted therapeutic tools in neuropsychiatric diseases.

Hypoxia/Ischemia-Induced Rod Microglia Phenotype in CA1 Hippocampal Slices

The complexity of microglia phenotypes and their related functions compels the continuous study of microglia in diseases animal models. We demonstrated that oxygen-glucose deprivation (OGD) induced rapid, time- and space-dependent phenotypic microglia modifications in CA1 stratum pyramidalis (SP) and stratum radiatum (SR) of rat organotypic hippocampal slices as well as the degeneration of pyramidal neurons, especially in the outer layer of SP. Twenty-four h following OGD, many rod microglia formed trains of elongated cells spanning from the SR throughout the CA1, reaching the SP outer layer where they acquired a round-shaped amoeboid phagocytic head and phagocytosed most of the pyknotic, damaged neurons. NIR-laser treatment, known to preserve neuronal viability after OGD, prevented rod microglia formation. In CA3 SP, pyramidal neurons were less damaged, no rod microglia were found. Thirty-six h after OGD, neuronal damage was more pronounced in SP outer and inner layers of CA1, rod microglia cells were no longer detectable, and most microglia were amoeboid/phagocytic. Damaged neurons, more numerous 36 h after OGD, were phagocytosed by amoeboid microglia in both inner and outer layers of CA1. In response to OGD, microglia can acquire different morphofunctional phenotypes which depend on the time after the insult and on the subregion where microglia are located.

CA2 inhibition reduces the precision of hippocampal assembly reactivation

The structured reactivation of hippocampal neuronal ensembles during fast synchronous oscillatory events, termed sharp-wave ripples (SWRs), has been suggested to play a crucial role in the storage and use of memory. Activity in both the CA2 and CA3 subregions can precede this population activity in CA1, and chronic inhibition of either region alters SWR oscillations. However, the precise contribution of CA2 to the oscillation, as well as to the reactivation of CA1 neurons within it, remains unclear. Here, we employ chemogenetics to transiently silence CA2 pyramidal cells in mice, and we observe that although SWRs still occur, the reactivation of CA1 pyramidal cell ensembles within the events lose both temporal and informational precision. These observations suggest that CA2 activity contributes to the fidelity of experience-dependent hippocampal replay.

Metaplastic Reinforcement of Long-Term Potentiation in Hippocampal Area CA2 by Cholinergic Receptor Activation

Hippocampal CA2, an inconspicuously positioned area between the well-studied CA1 and CA3 subfields, has captured research interest in recent years because of its role in social memory formation. However, the role of cholinergic inputs to the CA2 area for the regulation of synaptic plasticity remains to be fully understood. We show that cholinergic receptor activation with the nonselective cholinergic agonist, carbachol (CCh), triggers a protein synthesis-dependent and NMDAR-independent long-term synaptic depression (CCh-LTD) at entorhinal cortical (EC)-CA2 and Schaffer collateral (SC)-CA2 synapses in the hippocampus of adult male Wistar rats. The activation of muscarinic acetylcholine receptors (mAChRs) is critical for the induction of CCh-LTD with the results suggesting an involvement of M3 and M1 mAChRs in the early facilitation of CCh-LTD, while nicotinic AChR activation plays a role in the late maintenance of CCh-LTD at CA2 synapses. Remarkably, we find that CCh priming lowers the threshold for the subsequent induction of persistent long-term potentiation (LTP) of synaptic transmission at EC-CA2 and the plasticity-resistant SC-CA2 pathways. The effects of such a cholinergic-dependent synaptic depression on subsequent LTP at EC-CA2 and SC-CA2 synapses have not been previously explored. Collectively, the results demonstrate that CA2 synaptic learning rules are regulated in a metaplastic manner, whereby modifications triggered by prior cholinergic stimulation can dictate the outcome of future plasticity events. Moreover, the reinforcement of LTP at EC inputs to CA2 following the priming stimulus coexists with concurrent sustained CCh-LTD at the SC-CA2 pathway and is dynamically scaled by modulation of SC-CA2 synaptic transmission.SIGNIFICANCE STATEMENT The release of the neuromodulator acetylcholine is critically involved in processes of hippocampus-dependent memory formation. Cholinergic afferents originating in the medial septum and diagonal bands of Broca terminating in the hippocampal area CA2 might play an important role in the modulation of area-specific synaptic plasticity. Our findings demonstrate that cholinergic receptor activation induces an LTD of synaptic transmission at entorhinal cortical- and Schaffer collateral-CA2 synapses. This cholinergic activation-mediated LTD displays a bidirectional metaplastic switch to LTP on a future timescale. This suggests that such bidirectional synaptic modifications triggered by the dynamic modulation of tonic cholinergic receptor activation may support the formation of CA2-dependent memories given the increased hippocampal cholinergic tone during active wakefulness observed in exploratory behavior.

Complementary Role of Oxytocin and Vasopressin in Cardiovascular Regulation

The neurons secreting oxytocin (OXY) and vasopressin (AVP) are located mainly in the supraoptic, paraventricular, and suprachiasmatic nucleus of the brain. Oxytocinergic and vasopressinergic projections reach several regions of the brain and the spinal cord. Both peptides are released from axons, soma, and dendrites and modulate the excitability of other neuroregulatory pathways. The synthesis and action of OXY and AVP in the peripheral organs (eye, heart, gastrointestinal system) is being investigated. The secretion of OXY and AVP is influenced by changes in body fluid osmolality, blood volume, blood pressure, hypoxia, and stress. Vasopressin interacts with three subtypes of receptors: V1aR, V1bR, and V2R whereas oxytocin activates its own OXTR and V1aR receptors. AVP and OXY receptors are present in several regions of the brain (cortex, hypothalamus, pons, medulla, and cerebellum) and in the peripheral organs (heart, lungs, carotid bodies, kidneys, adrenal glands, pancreas, gastrointestinal tract, ovaries, uterus, thymus). Hypertension, myocardial infarction, and coexisting factors, such as pain and stress, have a significant impact on the secretion of oxytocin and vasopressin and on the expression of their receptors. The inappropriate regulation of oxytocin and vasopressin secretion during ischemia, hypoxia/hypercapnia, inflammation, pain, and stress may play a significant role in the pathogenesis of cardiovascular diseases.

Oxytocin and Fear Memory Extinction: Possible Implications for the Therapy of Fear Disorders?

Several psychiatric conditions such as phobias, generalized anxiety, and post-traumatic stress disorder (PTSD) are characterized by pathological fear and anxiety. The main therapeutic approach used in the management of these disorders is exposure-based therapy, which is conceptually based upon fear extinction with the formation of a new safe memory association, allowing the reduction in behavioral conditioned fear responses. Nevertheless, this approach is only partially resolutive, since many patients have difficulty following the demanding and long process, and relapses are frequently observed over time. One strategy to improve the efficacy of the cognitive therapy is the combination with pharmacological agents. Therefore, the identification of compounds able to strengthen the formation and persistence of the inhibitory associations is a key goal. Recently, growing interest has been aroused by the neuropeptide oxytocin (OXT), which has been shown to have anxiolytic effects. Furthermore, OXT receptors and binding sites have been found in the critical brain structures involved in fear extinction. In this review, the recent literature addressing the complex effects of OXT on fear extinction at preclinical and clinical levels is discussed. These studies suggest that the OXT roles in fear behavior are due to its local effects in several brain regions, most notably, distinct amygdaloid regions.

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.

Complete Elimination of Peripheral Auditory Input Before Onset of Hearing Causes Long-Lasting Impaired Social Memory in Mice

Hearing is one of the most important senses needed for survival, and its loss is an independent risk factor for dementia. Hearing loss (HL) can lead to communication difficulties, social isolation, and cognitive dysfunction. The hippocampus is a critical brain region being greatly involved in the formation of learning and memory and is critical not only for declarative memory but also for social memory. However, until today, whether HL can affect learning and memory is poorly understood. This study aimed to identify the relationship between HL and hippocampal-associated cognitive function. Mice with complete auditory input elimination before the onset of hearing were used as the animal model. They were first examined via auditory brainstem response (ABR) to confirm hearing elimination, and behavior estimations were applied to detect social memory capacity. We found significant impairment of social memory in mice with HL compared with the controls (p < 0.05); however, no significant differences were seen in the tests of novel object recognition, Morris water maze (MWM), and locomotion in the open field (p > 0.05). Therefore, our study firstly demonstrates that hearing input is required for the formation of social memory, and hearing stimuli play an important role in the development of normal cognitive ability.

The mechanisms shaping CA2 pyramidal neuron action potential bursting induced by muscarinic acetylcholine receptor activation

Recent studies have revealed that hippocampal area CA2 plays an important role in hippocampal network function. Disruption of this region has been implicated in neuropsychiatric disorders. It is well appreciated that cholinergic input to the hippocampus plays an important role in learning and memory. While the effect of elevated cholinergic tone has been well studied in areas CA1 and CA3, it remains unclear how changes in cholinergic tone impact synaptic transmission and the intrinsic properties of neurons in area CA2. In this study, we applied the cholinergic agonist carbachol and performed on-cell, whole-cell, and extracellular recordings in area CA2. We observed that under conditions of high cholinergic tone, CA2 pyramidal neurons depolarized and rhythmically fired bursts of action potentials. This depolarization depended on the activation of M1 and M3 cholinergic receptors. Furthermore, we examined how the intrinsic properties and action-potential firing were altered in CA2 pyramidal neurons treated with 10 µM carbachol. While this intrinsic burst firing persisted in the absence of synaptic transmission, bursts were shaped by synaptic inputs in the intact network. We found that both excitatory and inhibitory synaptic transmission were reduced upon carbachol treatment. Finally, we examined the contribution of different channels to the cholinergic-induced changes in neuronal properties. We found that a conductance from K_v7 channels partially contributed to carbachol-induced changes in resting membrane potential and membrane resistance. We also found that D-type potassium currents contributed to controlling several properties of the bursts, including firing rate and burst kinetics. Furthermore, we determined that T-type calcium channels and small conductance calcium-activated potassium channels play a role in regulating bursting activity.

Crucial role for CA2 inputs in the sequential organization of CA1 time cells supporting memory

There is considerable evidence for hippocampal time cells that briefly activate in succession to represent the temporal structure of memories. Previous studies have shown that time cells can be disrupted while leaving place cells intact, indicating that spatial and temporal information can be coded in parallel. However, the circuits in which spatial and temporal information are coded have not been clearly identified. Here we investigated temporal and spatial coding by dorsal hippocampal CA1 (dCA1) neurons in mice trained on a classic spatial working-memory task. On each trial, the mice approached the same choice point on a maze but were trained to alternate between traversing one of two distinct spatial routes (spatial coding phase). In between trials, there was a 10-s mnemonic delay during which the mouse continuously ran in a fixed location (temporal coding phase). Using cell-type-specific optogenetic methods, we found that inhibiting dorsal CA2 (dCA2) inputs into dCA1 degraded time cell coding during the mnemonic delay and impaired the mouse's subsequent memory-guided choice. Conversely, inhibiting dCA2 inputs during the spatial coding phase had a negligible effect on place cell activity in dCA1 and no effect on behavior. Collectively, our work demonstrates that spatial and temporal coding in dCA1 is largely segregated with respect to the dCA2-dCA1 circuit and suggests that CA2 plays a critical role in representing the flow of time in memory within the hippocampal network.

CA2 beyond social memory: Evidence for a fundamental role in hippocampal information processing

Hippocampal region CA2 has received increased attention due to its importance in social recognition memory. While its specific function remains to be identified, there are indications that CA2 plays a major role in a variety of situations, widely extending beyond social memory. In this targeted review, we highlight lines of research which have begun to converge on a more fundamental role for CA2 in hippocampus-dependent memory processing. We discuss recent proposals that speak to the computations CA2 may perform within the hippocampal circuit.

The neural circuitry of social homeostasis: Consequences of acute versus chronic social isolation

Social homeostasis is the ability of individuals to detect the quantity and quality of social contact, compare it to an established set-point in a command center, and adjust the effort expended to seek the optimal social contact expressed via an effector system. Social contact becomes a positive or negative valence stimulus when it is deficient or in excess, respectively. Chronic deficits lead to set-point adaptations such that reintroduction to the previous optimum is experienced as a surplus. Here, we build upon previous models for social homeostasis to include adaptations to lasting changes in environmental conditions, such as with chronic isolation.

SorCS2 is required for social memory and trafficking of the NMDA receptor

Social memory processing requires functional CA2 neurons, however the specific mechanisms that regulate their activity are poorly understood. Here, we document that SorCS2, a member of the family of the Vps10 family of sorting receptors, is highly expressed in pyramidal neurons of CA2, as well as ventral CA1, a circuit implicated in social memory. SorCS2 specifically localizes to the postsynaptic density and endosomes within dendritic spines of CA2 neurons. We have discovered that SorCS2 is a selective regulator of NMDA receptor surface trafficking in hippocampal neurons, without altering AMPA receptor trafficking. In addition, SorCS2 regulates dendritic spine density in CA2 neurons where SorCS2 expression is enriched, but not in dorsal CA1 neurons, which normally express very low levels of this protein. To specifically test the role of SorCS2 in behavior, we generated a novel SorCS2-deficient mouse, and identify a significant social memory deficit, with no change in sociability, olfaction, anxiety, or several hippocampal-dependent behaviors. Mutations in sorCS2 have been associated with bipolar disease, schizophrenia, and attention deficient-hyperactivity disorder, and abnormalities in social memory are core components of these neuropsychiatric conditions. Thus, our findings provide a new mechanism for social memory formation, through regulating synaptic receptor trafficking in pyramidal neurons by SorCS2.

A local circadian clock for memory?

The master clock, suprachiasmatic nucleus, is believed to control peripheral circadian oscillators throughout the brain and body. However, recent data suggest there is a circadian clock involved in learning and memory, potentially housed in the hippocampus, which is capable of acting independently of the master clock. Curiously, the hippocampal clock appears to be influenced by the master clock and by hippocampal dependent learning, while under certain conditions it may also revert to its endogenous circadian rhythm. Here we propose a mechanism by which the hippocampal clock could locally determine the nature of its entrainment. We introduce a novel theoretical framework, inspired by but extending beyond the hippocampal memory clock, which provides a new perspective on how circadian clocks throughout the brain coordinate their rhythms. Importantly, a local clock for memory would suggest that hippocampal-dependent learning at the same time every day should improve memory, opening up a range of possibilities for non-invasive therapies to alleviate the detrimental effects of circadian rhythm disruption on human health.

Vasopressin and oxytocin beyond the pituitary in the human brain

Vasopressin and oxytocin are primarily synthesized in the magnocellular supraoptic and paraventricular nuclei of the hypothalamus and transported to the posterior pituitary. In the human, an extensive accessory magnocellular neuroendocrine system is present with contact to the posterior pituitary and blood vessels in the hypothalamus itself. Vasopressin and oxytocin are involved in social and behavioral functions. However, only few neocortical areas are targeted by vasopressinergic and oxytocinergic nerve fibers, which mostly project to limbic areas in the forebrain, where also their receptors are located. Vasopressinergic/oxytocinergic perikarya in the forebrain project to the brain stem and spinal cord targeting nuclei and areas involved in autonomic functions. Parvocellular neurons containing vasopressin are located in the suprachiasmatic nucleus and synchronize the activity of the pacemaker in this nucleus. From the suprachiasmatic nucleus fibers project to the parvocellular part of the paraventricular nucleus, where preautonomic neurons project to the intermediolateral nucleus in the thoracic spinal cord, from where the superior cervical ganglion is reached whose noradrenergic fibers terminate in the pineal gland to stimulate melatonin secretion at night. The pineal gland is also innervated by vasopressin- and oxytocin-containing fibers reaching the gland via the "central innervation" in the pineal stalk, which might be involve in an annual regulation of melatonin secretion.

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.

P11 deficiency increases stress reactivity along with HPA axis and autonomic hyperresponsiveness

Patients suffering from mood disorders and anxiety commonly exhibit hypothalamic-pituitary-adrenocortical (HPA) axis and autonomic hyperresponsiveness. A wealth of data using preclinical animal models and human patient samples indicate that p11 deficiency is implicated in depression-like phenotypes. In the present study, we used p11-deficient (p11KO) mice to study potential roles of p11 in stress responsiveness. We measured stress response using behavioral, endocrine, and physiological readouts across early postnatal and adult life. Our data show that p11KO pups respond more strongly to maternal separation than wild-type pups, even though their mothers show no deficits in maternal behavior. Adult p11KO mice display hyperactivity of the HPA axis, which is paralleled by depression- and anxiety-like behaviors. p11 was found to be highly enriched in vasopressinergic cells of the paraventricular nucleus and regulates HPA hyperactivity in a V_1B receptor-dependent manner. Moreover, p11KO mice display sympathetic-adrenal-medullary (SAM) axis hyperactivity, with elevated adrenal norepinephrine and epinephrine levels. Using conditional p11KO mice, we demonstrate that this SAM hyperactivity is partially regulated by the loss of p11 in serotonergic neurons of the raphe nuclei. Telemetric electrocardiogram measurements show delayed heart rate recovery in p11KO mice in response to novelty exposure and during expression of fear following auditory trace fear conditioning. Furthermore, p11KO mice have elevated basal heart rate in fear conditioning tests indicating increased autonomic responsiveness. This set of experiments provide strong and versatile evidence that p11 deficiency leads to HPA and SAM axes hyperresponsiveness along with increased stress reactivity.

Coding of social novelty in the hippocampal CA2 region and its disruption and rescue in a 22q11.2 microdeletion mouse model

The hippocampal CA2 region is essential for social memory. To determine whether CA2 activity encodes social interactions, we recorded from CA2 pyramidal neurons in male mice during social behavior. While CA2 neuronal firing showed only weak spatial selectivity, it accurately encoded contextual changes and distinguished between a novel and familiar mouse. In the Df(16)A^+/− mouse model of the human 22q11.2 microdeletion, which confers a 30-fold increased risk of schizophrenia, CA2 social coding was impaired, consistent with the social memory deficit observed in these mice; in contrast, spatial coding accuracy was greatly enhanced. CA2 pyramidal neurons were previously found to be hyperpolarized in Df(16)A^+/− mice, likely due to upregulation of TREK-1 K⁺ current. We found that TREK-1 blockade rescued social memory and CA2 social coding in Df(16)A^+/− mice, supporting a crucial role for CA2 in the normal encoding of social stimuli and in social behavioral dysfunction in disease.

Characterization of Oxytocin Receptor Expression Within Various Neuronal Populations of the Mouse Dorsal Hippocampus

Oxytocin, acting through the oxytocin receptor (Oxtr) in the periphery, is best known for its roles in regulating parturition and lactation. However, it is also now known to possess a number of important social functions within the central nervous system, including social preference, memory and aggression, that vary to different degrees in different species. The Oxtr is found in both excitatory and inhibitory neurons within the brain and research is focusing on how, for example, activation of the receptor in interneurons can enhance the signal-to-noise of neuronal transmission. It is important to understand which neurons in the mouse dorsal hippocampus might be activated during memory formation. Therefore, we examined the colocalization of transcripts in over 5,000 neurons for Oxtr with those for nine different markers often found in interneurons using hairpin chain reaction in situ hybridization on hippocampal sections. Most pyramidal cell neurons of CA2 and many in the CA3 express Oxtr. Outside of those excitatory neurons, over 90% of Oxtr-expressing neurons co-express glutamic acid decarboxylase-1 (Gad-1) with progressively decreasing numbers of co-expressing cholecystokinin, somatostatin, parvalbumin, neuronal nitric oxide synthase, the serotonin 3a receptor, the vesicular glutamate transporter 3, calbindin 2 (calretinin), and vasoactive intestinal polypeptide neurons. Distributions were analyzed within hippocampal layers and regions as well. These findings indicate that Oxtr activation will modulate the activity of ~30% of the Gad-1 interneurons and the majority of the diverse population of those, mostly, interneuron types specifically examined in the mouse hippocampus.

CA2: A Highly Connected Intrahippocampal Relay

Although Lorente de No' recognized the anatomical distinction of the hippocampal Cornu Ammonis (CA) 2 region, it had, until recently, been assigned no unique function. Its location between the key players of the circuit, CA3 and CA1, which along with the entorhinal cortex and dentate gyrus compose the classic trisynaptic circuit, further distracted research interest. However, the connectivity of CA2 pyramidal cells, together with unique patterns of gene expression, hints at a much larger contribution to hippocampal information processing than has been ascribed. Here we review recent advances that have identified new roles for CA2 in hippocampal centric processing, together with specialized functions in social memory and, potentially, as a broadcaster of novelty. These new data, together with CA2's role in disease, justify a closer look at how this small region exerts its influence and how it might best be exploited to understand and treat disease-related circuit dysfunctions.

NMDA Receptor in Vasopressin 1b Neurons Is Not Required for Short-Term Social Memory, Object Memory or Aggression

The arginine vasopressin 1b receptor (Avpr1b) plays an important role in social behaviors including aggression, social learning and memory. Genetic removal of Avpr1b from mouse models results in deficits in aggression and short-term social recognition in adults. Avpr1b gene expression is highly enriched in the pyramidal neurons of the hippocampal cornu ammonis 2 (CA2) region. Activity of the hippocampal CA2 has been shown to be required for normal short-term social recognition and aggressive behaviors. Vasopressin acts to enhance synaptic responses of CA2 neurons through a NMDA-receptor dependent mechanism. Genetic removal of the obligatory subunit of the NMDA receptor (Grin1) within distinct hippocampal regions impairs non-social learning and memory. However, the question of a direct role for NMDA receptor activity in Avpr1b neurons to modulate social behavior remains unclear. To answer this question, we first created a novel transgenic mouse line with Cre recombinase knocked into the Avpr1b coding region to genetically target Avpr1b neurons. We confirmed this line has dense Cre expression throughout the dorsal and ventral CA2 regions of the hippocampus, along with scattered expression within the caudate-putamen and olfactory bulb (OB). Conditional removal of the NMDA receptor was achieved by crossing our line to an available floxed Grin1 line. The resulting mice were measured on a battery of social and memory behavioral tests. Surprisingly, we did not observe any differences between Avpr1b-Grin1 knockout mice and their wildtype siblings. We conclude that mice without typical NMDA receptor function in Avpr1b neurons can develop normal aggression as well as short-term social and object memory performance.

A unified circuit for social behavior

Recent advances in circuit manipulation technologies have enabled the association of distinct neural circuits with complex social behaviors. The brain areas identified through historical anatomical characterizations as mediators of sexual and parental behaviors can now be functionally linked to adult social behaviors within a unified circuit. In vivo electrophysiology, optogenetics and chemogenetics have been used to follow the processing of social sensory stimuli from perception by the olfactory system to valence detection by the amygdala and mesolimbic dopamine system to integration by the cerebral and cerebellar cortices under modulation of hypothalamic neuropeptides. Further, these techniques have been able to identify the distinct functional changes induced by social as opposed to non-social stimuli. Together this evidence suggests that there is a distinct, functionally coupled circuit that is selectively activated by social stimuli. A unified social circuit provides a new framework against which synaptopathic autism related mutations can be considered and novel pharmacotherapeutic strategies can be developed.

Social Memory and the Role of the Hippocampal CA2 Region

The CA2 region of the hippocampus is a somewhat obscure area lacking in an understanding of its form and function. Until recently, the CA2 has been thought of as merely an extension of the CA3, with some referring to it as the CA3a region. Recent investigations into this area have not only delineated the CA2, but also defined it as a region distinct from both CA1 and CA3, with a unique set of cortical inputs and outputs and contributions to cognitive processes. One such process that has been shown to depend on the CA2 is the ability to recognize a novel or familiar conspecific, known as social recognition memory. Here, we review these findings and discuss the parallels between CA2 dysfunction and social impairments.

Is AD a Stress-Related Disorder? Focus on the HPA Axis and Its Promising Therapeutic Targets

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that has important health and economic impacts in the elderly. Despite a better understanding of the molecular mechanisms leading to the appearance of major pathological hallmarks (senile plaques and neurofibrillary tangles), effective treatments are still lacking. Sporadic AD forms (98% of all cases) are multifactorial, and a panoply of risk factors have been identified. While the major risk factor is aging, growing evidence suggests that chronic stress or stress-related disorders increase the probability to develop AD. An early dysregulation of the hypothalamic-pituitary-adrenal axis (HPA axis or stress axis) has been observed in patients. The direct consequence of such perturbation is an oversecretion of glucocorticoids (GC) associated with an impairment of its receptors (glucocorticoid receptors, GR). These steroids hormones easily penetrate the brain and act in synergy with excitatory amino acids. An overexposure could be highly toxic in limbic structures (prefrontal cortex and hippocampus) and contribute in the cognitive decline occurring in AD. GC and GR dysregulations seem to be involved in lots of functions disturbed in AD and a vicious cycle appears, where AD induces HPA axis dysregulation, which in turn potentiates the pathology. This review article presents some preclinical and clinical studies focusing on the HPA axis hormones and their receptors to fight AD. Due to its primordial role in the maintenance of homeostasis, the HPA axis appears as a key-actor in the etiology of AD and a prime target to tackle AD by offering multiple angles of action.

Amygdala Arginine Vasopressin Modulates Chronic Ethanol Withdrawal Anxiety-Like Behavior in the Social Interaction Task

BACKGROUND

Chronic ethanol (EtOH) exposure induces neurobehavioral maladaptations in the brain though the precise changes have not been fully explored. The central nucleus of the amygdala (CEA) regulates anxiety-like behavior induced by withdrawal from chronic intermittent EtOH (CIE) exposure, and the arginine vasopressin (AVP) system within the CEA regulates many anxiety-like behaviors. Thus, adaptations occur in the CEA AVP system due to chronic EtOH exposure, which lead to anxiety-like behaviors in rats.

METHODS

Chronic exposure to a low-dose EtOH (4.5% wt/vol) induces anxiety-like behavior in rats. Wistar or Sprague Dawley rats were exposed to a modified CIE or CIE, while intra-CEA microinjections of AVP or a V1b receptor antagonist were used to elicit or block withdrawal-induced anxiety. Additionally, AVP microinjections into the CEA were given 24 hours following 15 days of continuous high-dose EtOH (7% wt/vol), a time period when rats no longer express anxiety. Chemogenetics was also used to activate the basolateral amygdala (BLA) or deactivate the dorsal periaqueductal gray=(dm/dlPAG) therefore PAG=periaqueductal gray to elicit or block withdrawal-induced anxiety.

RESULTS

AVP microinjected into the CEA in lieu of exposure to the first 2 cycles of CIE was sufficient to induce anxiety-like behavior in these commonly used rat strains. The V1b receptor antagonist, but not an oxytocin receptor agonist, into the CEA during the first 2 withdrawal cycles suppressed anxiety. However, activation of the BLA in lieu of exposure to the first 2 cycles of CIE was insufficient to induce anxiety-like behavior. AVP microinjection into the CEA 24 hours into withdrawal reelicited anxiety-like behavior, and deactivation of the dm/dlPAG reduced this effect of CEA AVP.

CONCLUSIONS

Taken together, this study demonstrates a role of CEA AVP and a CEA-dm/dlPAG circuit in the development of anxiety induced by CIE. Such information is valuable for identifying novel therapeutic targets for alcohol- and anxiety-associated disorders.

A phase 2 clinical trial of a vasopressin V1a receptor antagonist shows improved adaptive behaviors in men with autism spectrum disorder

There are no approved pharmacological therapies to address the core symptoms of autism spectrum disorder (ASD), namely, persistent deficits in social communication and social interaction and the presence of restricted, repetitive patterns of behaviors, interests, or activities. The neuropeptide vasopressin has been implicated in the regulation of social behaviors, and its modulation has emerged as a therapeutic target for ASD. The phase 2 VANILLA clinical trial reported here evaluated balovaptan, an orally administered selective vasopressin V1a receptor antagonist, in 223 men with ASD and intelligence quotient ≥70. The drug was administered daily for 12 weeks and was compared with placebo. Participants were randomized to placebo (n = 75) or one of three balovaptan dose arms (1.5 mg, n = 32; 4 mg, n = 77; 10 mg, n = 39). Balovaptan treatment was not associated with a change from baseline compared with placebo at 12 weeks in the primary efficacy endpoint (Social Responsiveness Scale, 2nd Edition). However, dose-dependent and clinically meaningful improvements on the Vineland-II Adaptive Behavior Scales composite score were observed for participants treated with balovaptan 4 or 10 mg compared with placebo. This was driven principally by improvements in the Vineland-II socialization and communication scores. Balovaptan was well tolerated across all doses, and no drug-related safety concerns were identified. These results support further study of balovaptan as a potential treatment for the socialization and communication deficits in ASD.