Ovarian hormone-induced reorganization of oxytocin-labeled dendrites and synapses lateral to the hypothalamic ventromedial nucleus in female rats

Progesterone and contraceptive progestin actions on the brain: A systematic review of animal studies and comparison to human neuroimaging studies

In this review we systematically summarize the effects of progesterone and synthetic progestins on neurogenesis, synaptogenesis, myelination and six neurotransmitter systems. Several parallels between progesterone and older generation progestin actions emerged, suggesting actions via progesterone receptors. However, existing results suggest a general lack of knowledge regarding the effects of currently used progestins in hormonal contraception regarding these cellular and molecular brain parameters. Human neuroimaging studies were reviewed with a focus on randomized placebo-controlled trials and cross-sectional studies controlling for progestin type. The prefrontal cortex, amygdala, salience network and hippocampus were identified as regions of interest for future preclinical studies. This review proposes a series of experiments to elucidate the cellular and molecular actions of contraceptive progestins in these areas and link these actions to behavioral markers of emotional and cognitive functioning. Emotional effects of contraceptive progestins appear to be related to 1) alterations in the serotonergic system, 2) direct/indirect modulations of inhibitory GABA-ergic signalling via effects on the allopregnanolone content of the brain, which differ between androgenic and anti-androgenic progestins. Cognitive effects of combined oral contraceptives appear to depend on the ethinylestradiol dose.

Whole-Brain Wiring Diagram of Oxytocin System in Adult Mice

Oxytocin (Oxt) neurons regulate diverse physiological responses via direct connections with different neural circuits. However, the lack of comprehensive input-output wiring diagrams of Oxt neurons and their quantitative relationship with Oxt receptor (Oxtr) expression presents challenges to understanding circuit-specific Oxt functions. Here, we establish a whole-brain distribution and anatomic connectivity map of Oxt neurons, and their relationship with Oxtr expression using high-resolution 3D mapping methods in adult male and female mice. We use a flatmap to describe Oxt neuronal expression in four hypothalamic domains including under-characterized Oxt neurons in the tuberal nucleus (TU). Oxt neurons in the paraventricular hypothalamus (PVH) broadly project to nine functional circuits that control cognition, brain state, and somatic visceral response. In contrast, Oxt neurons in the supraoptic (SO) and accessory (AN) nuclei have limited central projection to a small subset of the nine circuits. Surprisingly, quantitative comparison between Oxt output and Oxtr expression showed no significant correlation across the whole brain, suggesting abundant indirect Oxt signaling in Oxtr-expressing areas. Unlike output, Oxt neurons in both the PVH and SO receive similar monosynaptic inputs from a subset of the nine circuits mainly in the thalamic, hypothalamic, and cerebral nuclei areas. Our results suggest that PVH-Oxt neurons serve as a central modulator to integrate external and internal information via largely reciprocal connection with the nine circuits while the SO-Oxt neurons act mainly as unidirectional Oxt hormonal output. In summary, our Oxt wiring diagram provides anatomic insights about distinct behavioral functions of Oxt signaling in the brain.SIGNIFICANCE STATEMENT Oxytocin (Oxt) neurons regulate diverse physiological functions from prosocial behavior to pain sensation via central projection in the brain. Thus, understanding detailed anatomic connectivity of Oxt neurons can provide insight on circuit-specific roles of Oxt signaling in regulating different physiological functions. Here, we use high-resolution mapping methods to describe the 3D distribution, monosynaptic input and long-range output of Oxt neurons, and their relationship with Oxt receptor (Oxtr) expression across the entire mouse brain. We found Oxt connections with nine functional circuits controlling cognition, brain state, and somatic visceral response. Furthermore, we identified a quantitatively unmatched Oxt-Oxtr relationship, suggesting broad indirect Oxt signaling. Together, our comprehensive Oxt wiring diagram advances our understanding of circuit-specific roles of Oxt neurons.

The brain of the tree pangolin (Manis tricuspis). VIII. The subpallial telencephalon

The current study provides a detailed architectural analysis of the subpallial telencephalon of the tree pangolin. In the tree pangolin, the subpallial telencephalon was divided into septal and striatopallidal regions. The septal region contained the septal nuclear complex, diagonal band of Broca, and the bed nuclei of the stria terminalis. The striatopallidal region comprised of the dorsal (caudate, putamen, internal and external globus pallidus) and ventral (nucleus accumbens, olfactory tubercle, ventral pallidum, nucleus basalis, basal part of the substantia innominata, lateral stripe of the striatum, navicular nucleus, and the major island of Calleja) striatopallidal complexes. In the tree pangolin, the organization and numbers of nuclei forming these regions and complexes, their topographical relationships to each other, and the cyto‐, myelo‐, and chemoarchitecture, were found to be very similar to that observed in commonly studied mammals. Minor variations, such as less nuclear parcellation in the bed nuclei of the stria terminalis, may represent species‐specific variations, or may be the result of the limited range of stains used. Given the overall similarity across mammalian species, it appears that the subpallial telencephalon of the mammalian brain is highly conserved in terms of evolutionary changes detectable with the methods used. It is also likely that the functions associated with these nuclei in other mammals can be translated directly to the tree pangolin, albeit with the understanding that the stimuli that produce activity within these regions may be specific to the life history requirements of the tree pangolin.

The brain of the tree pangolin (Manis tricuspis). VII. The amygdaloid body

Here, we describe the cytoarchitecture and chemoarchitecture of the amygdaloid body of the tree pangolin. Our definition of the amygdaloid body includes the pallial portions of the amygdala, and the centromedial group that is a derivative of the subpallium and part of the extended amygdala. The remainder of the extended amygdala is not described herein. Within the amygdaloid body of the tree pangolin, we identified the basolateral group (composed of the lateral, basal, and accessory basal amygdaloid nuclei), the superficial, or cortical nuclei (the anterior and posterior cortical nuclei, the periamygdaloid cortex, and nuclei of the olfactory tract), the centromedial group (the central amygdaloid nucleus and the medial nuclear cluster), and other amygdaloid nuclei (the anterior amygdaloid area, the amygdalohippocampal area, the intramedullary group, and intercalated islands). The location within and relative to each other within the amygdaloid body and the internal subdivisions of these groups were very similar to that reported in other mammalian species, with no clearly derived features specific to the tree pangolin. The only variation was the lack of an insular appearance of the intercalated islands, which in the tree pangolin were observed as a continuous band of neurons located dorsomedial to the basolateral group similar in appearance to and almost continuous with the intramedullary group. In carnivores, the closest relatives of the pangolins, and laboratory rats, a similar appearance of portions of the intercalated islands has been noted.

Sexual experience in female mice involves synaptophysin-related plasticity in the accessory olfactory bulb

Sexually naïve female mice do not display high levels of sexual receptivity in their first sexual experience; they require around 4-5 sexual encounters to display the full receptive response, assessed by the lordosis reflex. In this study, we evaluated if repeated sexual stimulation with the same male is associated with changes in synaptic remodeling evaluated by synaptophysin (SYP) in brain structures involved in the control of sexual behavior such as the main and accessory olfactory bulbs (MOB and AOB, respectively), medial preoptic area (MPOA), ventromedial hypothalamus (VMH), and amygdala (AMG). Female mice were ovariectomized and hormonally primed to induce sexual receptivity. They were randomly distributed into three groups: a) sexually naïve (SN), with no prior sexual stimulation; b) sexually inexperienced (SI), with one prior mating session; and c) sexually experienced (SE), with six mating sessions. The SI group showed a significant decrease in SYP in the glomerular, mitral and granular layers of the AOB in comparison to SN and SE females. SYP expression increased in the SE group in comparison to SN and SI females in the glomerular and mitral cell layers of the AOB. No significant differences between groups were found in the other brain regions (MOB, MPOA, VMH or AMG). These changes in SYP expression in the AOB suggest that plastic modifications in this brain region can be associated with receptivity increase in sexual experience in female mice.

Role of Oxytocin in Different Neuropsychiatric, Neurodegenerative, and Neurodevelopmental Disorders

Oxytocin has recently gained significant attention because of its role in the pathophysiology and management of dominant neuropsychiatric disorders. Oxytocin, a peptide hormone synthesized in the hypothalamus, is released into different brain regions, acting as a neurotransmitter. Receptors for oxytocin are present in many areas of the brain, including the hypothalamus, amygdala, and nucleus accumbens, which have been involved in the pathophysiology of depression, anxiety, schizophrenia, autism, Alzheimer's disease, Parkinson's disease, and attention deficit hyperactivity disorder. Animal studies have spotlighted the role of oxytocin in social, behavioral, pair bonding, and mother-infant bonding. Furthermore, oxytocin protects fetal neurons against injury during childbirth and affects various behaviors, assuming its possible neuroprotective characteristics. In this review, we discuss some of the concepts and mechanisms related to the role of oxytocin in the pathophysiology and management of some neuropsychiatric, neurodegenerative, and neurodevelopmental disorders.

Oxytocin, Erectile Function and Sexual Behavior: Last Discoveries and Possible Advances

A continuously increasing amount of research shows that oxytocin is involved in numerous central functions. Among the functions in which oxytocin is thought to be involved are those that play a role in social and sexual behaviors, and the involvement of central oxytocin in erectile function and sexual behavior was indeed one of the first to be discovered in laboratory animals in the 1980s. The first part of this review summarizes the results of studies done in laboratory animals that support a facilitatory role of oxytocin in male and female sexual behavior and reveal mechanisms through which this ancient neuropeptide participates in concert with other neurotransmitters and neuropeptides in this complex function, which is fundamental for the species reproduction. The second part summarizes the results of studies done mainly with intranasal oxytocin in men and women with the aim to translate the results found in laboratory animals to humans. Unexpectedly, the results of these studies do not appear to confirm the facilitatory role of oxytocin found in male and female sexual behavior in animals, both in men and women. Possible explanations for the failure of oxytocin to improve sexual behavior in men and women and strategies to attempt to overcome this impasse are considered.

The Structural and Electrophysiological Properties of Progesterone Receptor-Expressing Neurons Vary along the Anterior-Posterior Axis of the Ventromedial Hypothalamus and Undergo Local Changes across the Reproductive Cycle

Sex hormone levels continuously fluctuate across the reproductive cycle, changing the activity of neuronal circuits to coordinate female behavior and reproductive capacity. The ventrolateral division of the ventromedial hypothalamus (VMHvl) contains neurons expressing receptors for sex hormones and its function is intimately linked to female sexual receptivity. However, recent findings suggest that the VMHvl is functionally heterogeneous. Here, we used whole recordings and intracellular labeling to characterize the electrophysiological and morphologic properties of individual VMHvl neurons in naturally cycling females and report the existence of multiple electrophysiological phenotypes within the VMHvl. We found that the properties of progesterone receptor expressing (PR+) neurons, but not PR– neurons, depended systematically on the neuron’s location along the anterior-posterior (AP) axis of the VMHvl and the phase within the reproductive cycle. Prominent among this, the resting membrane potential of anterior PR+ neurons decreased during the receptive phase, while the excitability of medial PR+ neurons increased during the non-receptive phase. During the receptive phase of the cycle, posterior PR+ neurons simultaneously showed an increase in dendritic complexity and a decrease in spine density. These findings reveal an extensive diversity of local rules driving structural and physiological changes in response to fluctuating levels of sex hormones, supporting the anatomic and functional subdivision of the VMHvl and its possible role in the orchestration of different aspects of female socio-sexual behavior.

Central and peripheral release of oxytocin: Relevance of neuroendocrine and neurotransmitter actions for physiology and behavior

The hypothalamic neuropeptide oxytocin (OT) is critically involved in the modulation of socio-emotional behavior, sexual competence, and pain perception and anticipation. While intracellular signaling of OT and its receptor (OTR), as well as the functional connectivity of hypothalamic and extra-hypothalamic OT projections, have been recently explored, it remains elusive how one single molecule has pleotropic effects from cell proliferation all the way to modulation of complex cognitive processes. Moreover, there are astonishing species-dependent differences in the way OT regulates various sensory modalities such as touch, olfaction, and vision, which can be explained by differences in OTR expression in brain regions processing sensory information. Recent research highlights a small subpopulation of OT-synthesizing cells, namely, parvocellular cells, which merely constitute 1% of the total number of OT cells but act as "master cells' that regulate the activity of the entire OT system. In this chapter, we summarize the latest advances in the field of OT research with a particular focus on differences between rodents, monkeys and humans and highlight the main differences between OT and its "sister" peptide arginine-vasopressin, which often exerts opposite effects on physiology and behavior.

Brain oxytocin: how puzzle stones from animal studies translate into psychiatry

The neuropeptide oxytocin has attracted great attention of the general public, basic neuroscience researchers, psychologists, and psychiatrists due to its profound pro-social, anxiolytic, and "anti-stress" behavioral and physiological effects, and its potential application for treatment of mental diseases associated with altered socio-emotional competence. During the last decade, substantial progress has been achieved in understanding the complex neurobiology of the oxytocin system, including oxytocinergic pathways, local release patterns, and oxytocin receptor distribution in the brain, as well as intraneuronal oxytocin receptor signaling. However, the picture of oxytocin actions remains far from being complete, and the central question remains: "How does a single neuropeptide exert such pleotropic actions?" Although this phenomenon, typical for many of about 100 identified neuropeptides, may emerge from the anatomical divergence of oxytocin neurons, their multiple central projections, distinct oxytocin-sensitive cell types in different brain regions, and multiple intraneuronal signaling pathways determining the specific cellular response, further basic studies are required. In conjunction, numerous reports on positive effects of intranasal application of oxytocin on human brain networks controlling socio-emotional behavior in health and disease require harmonic tandems of basic researchers and clinicians. During the COVID-19 crisis in 2020, oxytocin research seems central as question of social isolation-induced inactivation of the oxytocin system, and buffering effects of either activation of the endogenous system or intranasal application of synthetic oxytocin need to be thoroughly investigated.

The brain of the African wild dog. II. The olfactory system

Employing a range of neuroanatomical stains, we detail the organization of the main and accessory olfactory systems of the African wild dog. The organization of both these systems follows that typically observed in mammals, but variations of interest were noted. Within the main olfactory bulb, the size of the glomeruli, at approximately 350 μm in diameter, are on the larger end of the range observed across mammals. In addition, we estimate that approximately 3,500 glomeruli are present in each main olfactory bulb. This larger main olfactory bulb glomerular size and number of glomeruli indicates that enhanced peripheral processing of a broad range of odorants is occurring in the main olfactory bulb of the African wild dog. Within the accessory olfactory bulb, the glomeruli did not appear distinct, rather forming a homogenous syncytia-like arrangement as seen in the domestic dog. In addition, the laminar organization of the deeper layers of the accessory olfactory bulb was indistinct, perhaps as a consequence of the altered architecture of the glomeruli. This arrangement of glomeruli indicates that rather than parcellating the processing of semiochemicals peripherally, these odorants may be processed in a more nuanced and combinatorial manner in the periphery, allowing for more rapid and precise behavioral responses as required in the highly social group structure observed in the African wild dog. While having a similar organization to that of other mammals, the olfactory system of the African wild dog has certain features that appear to correlate to their environmental niche.

The brain of the African wild dog. III. The auditory system

The large external pinnae and extensive vocal repertoire of the African wild dog (Lycaon pictus) has led to the assumption that the auditory system of this unique canid may be specialized. Here, using cytoarchitecture, myeloarchitecture, and a range of immunohistochemical stains, we describe the systems-level anatomy of the auditory system of the African wild dog. We observed the cochlear nuclear complex, superior olivary nuclear complex, lateral lemniscus, inferior colliculus, medial geniculate body, and auditory cortex all being in their expected locations, and exhibiting the standard subdivisions of this system. While located in the ectosylvian gyri, the auditory cortex includes several areas, resembling the parcellation observed in cats and ferrets, although not all of the auditory areas known from these species could be identified in the African wild dog. These observations suggest that, broadly speaking, the systems-level anatomy of the auditory system, and by extension the processing of auditory information, within the brain of the African wild dog closely resembles that observed in other carnivores. Our findings indicate that it is likely that the extraction of the semantic content of the vocalizations of African wild dogs, and the behaviors generated, occurs beyond the classically defined auditory system, in limbic or association neocortical regions involved in cognitive functions. Thus, to obtain a deeper understanding of how auditory stimuli are processed, and how communication is achieved, in the African wild dog compared to other canids, cortical regions beyond the primary sensory areas will need to be examined in detail.

Mapping Central Projection of Oxytocin Neurons in Unmated Mice Using Cre and Alkaline Phosphatase Reporter

Oxytocin, a neuropeptide and peptide hormone, is produced by neurons in the hypothalamus and released by the posterior pituitary to control breastfeeding and labor. Recent studies have revealed that oxytocin in the central nervous system is also involved in modulating social interaction. To understand the potential role and innervation pattern of oxytocin neurons before sexual interaction, here we used transgenic mice which have the Cre recombinase under the control of an endogenous oxytocin promoter and Cre-dependent human placental alkaline phosphatase (AP) reporter to label the oxytocin neurons in the naive mouse brain. Since AP is located on the membrane of oxytocin neurons, AP histochemistry staining enabled us to observe the fine axonal terminals and the innervation pattern of oxytocin neurons in the thick serial coronal brain slices. Here we show that the number of AP-labeled cells varies with staining reaction time and ranges from 30% of the oxytocin immune-positive cell count to slightly higher than the oxytocin immune-positive cell count. Using AP staining with extended reaction time, which may not label all oxytocin neurons, we confirmed many innervation targets of oxytocin neurons from the anterior olfactory nucleus, some cortex regions, the limbic system, the hypothalamus, and the hindbrain, while the cell bodies were exclusively located in the hypothalamus and the bed nucleus of the stria terminalis. Finally, we observe some individual variance at the olfactory area, isocortex, striatum, paraventricular nucleus of thalamus, locus coeruleus, and Barrington's nucleus.

The hippocampal formation of two carnivore species: The feliform banded mongoose and the caniform domestic ferret

Employing cyto-, myelo-, and chemoarchitectural staining techniques, we analyzed the structure of the hippocampal formation in the banded mongoose and domestic ferret, species belonging to the two carnivoran superfamilies, which have had independent evolutionary trajectories for the past 55 million years. Our observations indicate that, despite the time since sharing a last common ancestor, these species show extensive similarities. The four major portions of the hippocampal formation (cornu Ammonis, dentate gyrus, subicular complex, and entorhinal cortex) were readily observed, contained the same internal subdivisions, and maintained the topological relationships of these subdivisions that could be considered typically mammalian. In addition, adult hippocampal neurogenesis was observed in both species, occurring at a rate similar to that observed in other mammals. Despite the overall similarities, several differences to each other, and to other mammalian species, were observed. We could not find evidence for the presence of the CA2 and CA4 fields of the cornu Ammonis region. In the banded mongoose the dentate gyrus appears to be comprised of up to seven lamina, through the sublamination of the molecular and granule cell layers, which is not observed in the domestic ferret. In addition, numerous subtle variations in chemoarchitecture between the two species were observed. These differences may contribute to an overall variation in the functionality of the hippocampal formation between the species, and in comparison to other mammalian species. These similarities and variations are important to understanding to what extent phylogenetic affinities and constraints affect potential adaptive evolutionary plasticity of the hippocampal formation.

The diencephalon of two carnivore species: The feliform banded mongoose and the caniform domestic ferret

This study provides an analysis of the cytoarchitecture, myeloarchitecture, and chemoarchitecture of the diencephalon (dorsal thalamus, ventral thalamus, and epithalamus) of the banded mongoose (Mungos mungo) and domestic ferret (Mustela putorius furo). Using architectural and immunohistochemical stains, we observe that the nuclear organization of the diencephalon is very similar in the two species, and similar to that reported in other carnivores, such as the domestic cat and dog. The same complement of putatively homologous nuclei were identified in both species, with only one variance, that being the presence of the perireticular nucleus in the domestic ferret, that was not observed in the banded mongoose. The chemoarchitecture was also mostly consistent between species, although there were a number of minor variations across a range of nuclei in the density of structures expressing the calcium-binding proteins parvalbumin, calbindin, and calretinin. Thus, despite almost 53 million years since these two species of carnivores shared a common ancestor, strong phylogenetic constraints appear to limit the potential for adaptive evolutionary plasticity within the carnivore order. Apart from the presence of the perireticular nucleus, the most notable difference between the species studied was the physical inversion of the dorsal lateral geniculate nucleus, as well as the lateral posterior and pulvinar nuclei in the domestic ferret compared to the banded mongoose and other carnivores, although this inversion appears to be a feature of the Mustelidae family. While no functional sequelae are suggested, this inversion is likely to result from the altricial birth of Mustelidae species.

The amygdaloid body of two carnivore species: The feliform banded mongoose and the caniform domestic ferret

The current study provides an analysis of the cytoarchitecture, myeloarchitecture, and chemoarchitecture of the amygdaloid body of the banded mongoose (Mungos mungo) and domestic ferret (Mustela putorius furo). Using architectural and immunohistochemical stains, we observe that the organization of the nuclear and cortical portions of the amygdaloid complex is very similar in both species. The one major difference is the presence of a cortex-amygdala transition zone observed in the domestic ferret that is absent in the banded mongoose. In addition, the chemoarchitecture is, for the most part, quite similar in the two species, but several variances, such as differing densities of neurons expressing the calcium-binding proteins in specific nuclei are noted. Despite this, certain aspects of the chemoarchitecture, such as the cholinergic innervation of the magnocellular division of the basal nuclear cluster and the presence of doublecortin expressing neurons in the shell division of the accessory basal nuclear cluster, appear to be consistent features of the Eutherian mammal amygdala. The domestic ferret presented with an overall lower myelin density throughout the amygdaloid body than the banded mongoose, a feature that may reflect artificial selection in the process of domestication for increased juvenile-like behavior in the adult domestic ferret, such as a muted fear response. The shared, but temporally distant, ancestry of the banded mongoose and domestic ferret allows us to generate observations relevant to understanding the relative influence that phylogenetic constraints, adaptive evolutionary plasticity, and the domestication process may play in the organization and chemoarchitecture of the amygdaloid body.

Dopamine-induced interactions of female mouse hypothalamic proteins with progestin receptor-A in the absence of hormone

Neural progestin receptors (PR) function in reproduction, neural development, neuroprotection, learning, memory and the anxiety response. In the absence of progestins, PR can be activated by dopamine (DA) in the rodent hypothalamus to elicit female sexual behaviour. The present study investigated mechanisms of DA activation of PR by testing the hypothesis that proteins from DA-treated hypothalami interact with PR in the absence of progestins. Ovariectomised, oestradiol-primed mice were infused with a D1-receptor agonist, SKF38393 (SKF), into the third ventricle 30 minutes prior to death. Proteins from SKF-treated hypothalami were pulled-down with glutathione S-transferase-tagged mouse PR-A or PR-B and the interactomes were analysed by mass spectrometry. The largest functional group to interact with PR-A in a DA-dependent manner was synaptic proteins. To test the hypothesis that DA activation of PR regulates synaptic proteins, we developed oestradiol-induced PR-expressing hypothalamic-like neurones derived from human-induced pluripotent stem cells (hiPSCs). Similar to progesterone (P4), SKF treatment of hiPSCs increased synapsin1/2 expression. This SKF-dependent effect was blocked by the PR antagonist RU486, suggesting that PR are necessary for this DA-induced increase. The second largest DA-dependent PR-A protein interactome comprised metabolic regulators involved in glucose metabolism, lipid synthesis and mitochondrial energy production. Interestingly, hypothalamic proteins interacted with PR-A, but not PR-B, in an SKF-dependent manner, suggesting that DA promotes the interaction of multiple hypothalamic proteins with PR-A. These in vivo and in vitro results indicate novel mechanisms by which DA can differentially activate PR isoforms in the absence of P4 and provide a better understanding of ligand-independent PR activation in reproductive, metabolic and mental health disorders in women.

The brain of the African wild dog. IV. The visual system

The variegated pelage and social complexity of the African wild dog (Lycaon pictus) hint at the possibility of specializations of the visual system. Here, using a range of architectural and immunohistochemical stains, we describe the systems-level organization of the image-forming, nonimage forming, oculomotor, and accessory optic, vision-associated systems in the brain of one representative individual of the African wild dog. For all of these systems, the organization, in terms of location, parcellation and topology (internal and external), is very similar to that reported in other carnivores. The image-forming visual system consists of the superior colliculus, visual dorsal thalamus (dorsal lateral geniculate nucleus, pulvinar and lateral posterior nucleus) and visual cortex (occipital, parietal, suprasylvian, temporal and splenial visual regions). The nonimage forming visual system comprises the suprachiasmatic nucleus, ventral lateral geniculate nucleus, pretectal nuclear complex and the Edinger-Westphal nucleus. The oculomotor system incorporates the oculomotor, trochlear and abducens cranial nerve nuclei as well as the parabigeminal nucleus, while the accessory optic system includes the dorsal, lateral and medial terminal nuclei. The extent of similarity to other carnivores in the systems-level organization of these systems indicates that the manner in which these systems process visual information is likely to be consistent with that found, for example, in the well-studied domestic cat. It would appear that the sociality of the African wild dog is dependent upon the processing of information extracted from the visual system in the higher-order cognitive and affective neural systems.

The brain of the tree pangolin (Manis tricuspis). IV. The hippocampal formation

Employing a range of standard and immunohistochemical stains we provide a description of the hippocampal formation in the brain of the tree pangolin. For the most part, the architecture, chemical neuroanatomy, and topological relationships of the component parts of the hippocampal formation of the tree pangolin were consistent with that observed in other mammalian species. Within the hippocampus proper fields CA1, 3, and 4 could be identified with certainty, while CA2 was tentatively identified as a small transitional zone between the CA1 and CA3 fields. Within the dentate gyrus evidence for adult hippocampal neurogenesis at a rate comparable to other mammals was observed. The subicular complex and entorhinal cortex also exhibited divisions typically observed in other mammalian species. In contrast to many other mammals, an architecturally and neurochemically distinct CA4 field was observed, supporting Lorente de Nó's proposed CA4 field, at least in some mammalian species. In addition, up to seven laminae were evident in the dentate gyrus. Calretinin immunostaining revealed the three sublamina of the molecular layer, while immunostaining for vesicular glutamate transporter 2 and neurofilament H indicate that the granule cell layer was composed of two sublamina. The similarities and differences observed in the tree pangolin indicate that the hippocampal formation is an anatomically and neurochemically conserved neural unit in mammalian evolution, but minor changes may relate to specific life history features and habits of species.

Cross-talk among oxytocin and arginine-vasopressin receptors: Relevance for basic and clinical studies of the brain and periphery

Oxytocin (OT) and arginine-vasopressin (AVP) act in the brain to regulate social cognition/social behavior and in the periphery to influence a variety of physiological processes. Although the chemical structures of OT and AVP as well as their receptors are quite similar, OT and AVP can have distinct or even opposing actions. Here, we review the increasing body of evidence that exogenously administered and endogenously released OT and AVP can activate each other's canonical receptors (i.e., cross-talk) and examine the possibility that receptor cross-talk following the synaptic and non-synaptic release of OT and AVP contributes to their distinct roles in the brain and periphery. Understanding the consequences of cross-talk between OT and AVP receptors will be important in identifying how these peptides control social cognition and behavior and for the development of drugs to treat a variety of psychiatric disorders.

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

The Progestin Receptor Interactome in the Female Mouse Hypothalamus: Interactions with Synaptic Proteins Are Isoform Specific and Ligand Dependent

Progestins bind to the progestin receptor (PR) isoforms, PR-A and PR-B, in brain to influence development, female reproduction, anxiety, and stress. Hormone-activated PRs associate with multiple proteins to form functional complexes. In the present study, proteins from female mouse hypothalamus that associate with PR were isolated using affinity pull-down assays with glutathione S-transferase–tagged mouse PR-A and PR-B. Using complementary proteomics approaches, reverse phase protein array (RPPA) and mass spectrometry, we identified hypothalamic proteins that interact with PR in a ligand-dependent and isoform-specific manner and were confirmed by Western blot. Synaptic proteins, including synapsin-I and synapsin-II, interacted with agonist-bound PR isoforms, suggesting that both isoforms function in synaptic plasticity. In further support, synaptogyrin-III and synapsin-III associated with PR-A and PR-B, respectively. PR also interacted with kinases, including c-Src, mTOR, and MAPK1, confirming phosphorylation as an integral process in rapid effects of PR in the brain. Consistent with a role in transcriptional regulation, PR associated with transcription factors and coactivators in a ligand-specific and isoform-dependent manner. Interestingly, both PR isoforms associated with a key regulator of energy homeostasis, FoxO1, suggesting a novel role for PR in energy metabolism. Because many identified proteins in this PR interactome are synaptic proteins, we tested the hypothesis that progestins function in synaptic plasticity. Indeed, progesterone enhanced synaptic density, by increasing synapsin-I–positive synapses, in rat primary cortical neuronal cultures. This novel combination of RPPA and mass spectrometry allowed identification of PR action in synaptic remodeling and energy homeostasis and reveals unique roles for progestins in brain function and disease.

Distribution and chemical composition of estrogen receptor β neurons in the paraventricular nucleus of the female and male mouse hypothalamus

Activation of estrogen receptor beta (ERβ)-expressing neurons regulates the mammalian stress response via the hypothalamic-pituitary-adrenal (HPA) axis. These neurons densely populate the paraventricular nucleus of the hypothalamus (PVN). Recent research has revealed striking differences between rat and mouse PVN cytochemistry, but careful exploration of PVN ERβ neurons in mice has been hindered by a lack of specific ERβ antisera. Therefore, we used male and female transgenic mice expressing EGFP under the control of the mouse ERβ promoter (ERβ-EGFP) to examine the chemical architecture of PVN ERβ cells. Using immunohistochemistry, we found that 90% of ERβ-immunoreactivity (-ir) colocalized with EGFP. Cellular colocalization of EGFP with neuropeptides, transcription modulators, and neuronal tracers was examined throughout the PVN. ERβ-EGFP cells expressed oxytocin more abundantly in the rostral (71 ± 3%) than caudal (33 ± 8%) PVN. Arginine vasopressin colocalized with EGFP more often in females (18 ± 3%) than males (4 ± 1%). Moreover, estrogen receptor α-ir colocalized with ERβ-EGFP at low levels (15 ± 3%). Using a corticotropin releasing hormone-cre driver X tdTomato reporter mouse, we found a moderate colocalization with ERβ-ir (48 ± 16%) in the middle PVN. Peripheral injection of fluorogold revealed that the rostral PVN ERβ-EGFP cells are neuroendocrine neurons whereas non-neuroendocrine (presumably pre-autonomic) ERβ-EGFP neurons predominated in the posterior PVN. These data demonstrate chemoarchitectural differences in ERβ neurons of the mouse PVN that are different from that previously described for the rat, thus, elucidating potential neuronal pathways involved in the regulation of the HPA axis in mice.

The stability of the transcriptome during the estrous cycle in four regions of the mouse brain

We analyzed the transcriptome of the C57BL/6J mouse hypothalamus, hippocampus, neocortex, and cerebellum to determine estrous cycle-specific changes in these four brain regions. We found almost 16,000 genes are present in one or more of the brain areas but only 210 genes, ∼1.3%, are significantly changed as a result of the estrous cycle. The hippocampus has the largest number of differentially expressed genes (DEGs) (82), followed by the neocortex (76), hypothalamus (63), and cerebellum (26). Most of these DEGs (186/210) are differentially expressed in only one of the four brain regions. A key finding is the unique expression pattern of growth hormone (Gh) and prolactin (Prl). Gh and Prl are the only DEGs to be expressed during only one stage of the estrous cycle (metestrus). To gain insight into the function of the DEGs, we examined gene ontology and phenotype enrichment and found significant enrichment for genes associated with myelination, hormone stimulus, and abnormal hormone levels. Additionally, 61 of the 210 DEGs are known to change in response to estrogen in the brain. 50 of the 210 genes differentially expressed as a result of the estrous cycle are related to myelin and oligodendrocytes and 12 of the 63 DEGs in the hypothalamus are oligodendrocyte- and myelin-specific genes. This transcriptomic analysis reveals that gene expression in the female mouse brain is remarkably stable during the estrous cycle and demonstrates that the genes that do fluctuate are functionally related.

The ventromedial hypothalamus oxytocin induces locomotor behavior regulated by estrogen

Our previous studies demonstrated that excitation of neurons in the rat ventromedial hypothalamus (VMH) induced locomotor activity. An oxytocin receptor (Oxtr) exists in the VMH and plays a role in regulating sexual behavior. However, the role of Oxtr in the VMH in locomotor activity is not clear. In this study we examined the roles of oxytocin in the VMH in running behavior, and also investigated the involvement of estrogen in this behavioral change. Microinjection of oxytocin into the VMH induced a dose-dependent increase in the running behavior in male rats. The oxytocin-induced running activity was inhibited by simultaneous injection of Oxtr-antagonist, (d(CH2)5(1), Try(Me)(2), Orn(8))-oxytocin. Oxytocin injection also induced running behavior in ovariectomized (OVX) female rats. Pretreatment of the OVX rats with estrogen augmented the oxytocin-induced running activity twofold, and increased the Oxtr mRNA in the VMH threefold. During the estrus cycle locomotor activity spontaneously increased in the dark period of proestrus. The Oxtr mRNA was up-regulated in the proestrus afternoon. Blockade of oxytocin neurotransmission by its antagonist before the onset of the dark period of proestrus decreased the following nocturnal locomotor activity. These findings demonstrate that Oxtr in the VMH is involved in the induction of running behavior and that estrogen facilitates this effect by means of Oxtr up-regulation, suggesting the involvement of oxytocin in the locomotor activity of proestrus female rats.

Assembling the Puzzle: Pathways of Oxytocin Signaling in the Brain

Oxytocin (OT) is a neuropeptide, which can be seen to be one of the molecules of the decade due to its profound prosocial effects in nonvertebrate and vertebrate species, including humans. Although OT can be detected in various physiological fluids (blood, saliva, urine, cerebrospinal fluid) and brain tissue, it is unclear whether peripheral and central OT releases match and synergize. Moreover, the pathways of OT delivery to brain regions involved in specific behaviors are far from clear. Here, we discuss the evolutionarily and ontogenetically determined pathways of OT delivery and OT signaling, which orchestrate activity of the mesolimbic social decision-making network. Furthermore, we speculate that both the alteration in OT delivery and OT receptor expression may cause behavioral abnormalities in patients afflicted with psychosocial diseases.

The "metabolic sensor" function of rat supraoptic oxytocin and vasopressin neurons is attenuated during lactation but not in diet-induced obesity

The oxytocin (OT) and vasopressin (VP) neurons of the supraoptic nucleus (SON) demonstrate characteristics of "metabolic sensors". They express insulin receptors and glucokinase (GK). They respond to an increase in glucose and insulin with an increase in intracellular [Ca(2+)] and increased OT and VP release that is GK dependent. Although this is consistent with the established role of OT as an anorectic agent, how these molecules function relative to the important role of OT during lactation and whether deficits in this metabolic sensor function contribute to obesity remain to be examined. Thus, we evaluated whether insulin and glucose-induced OT and VP secretion from perifused explants of the hypothalamo-neurohypophyseal system are altered during lactation and by diet-induced obesity (DIO). In explants from female day 8 lactating rats, increasing glucose (Glu, 5 mM) did not alter OT or VP release. However, insulin (Ins; 3 ng/ml) increased OT release, and increasing the glucose concentration in the presence of insulin (Ins+Glu) resulted in a sustained elevation in both OT and VP release that was not prevented by alloxan, a GK inhibitor. Explants from male DIO rats also responded to Ins+Glu with an increase in OT and VP regardless of whether obesity had been induced by feeding a high-fat diet (HFD). The HFD-DIO rats had elevated body weight, plasma Ins, Glu, leptin, and triglycerides. These findings suggest that the role of SON neurons as metabolic sensors is diminished during lactation, but not in this animal model of obesity.

Enhanced male-evoked responses in the ventromedial hypothalamus of sexually receptive female mice

Social encounters often start with routine investigatory behaviors before developing into distinct outcomes, such as affiliative or aggressive actions. For example, a female mouse will initially engage in investigatory behavior with a male but will then show copulation or rejection, depending on her reproductive state. To promote adaptive social behavior, her brain must combine internal ovarian signals and external social stimuli, but little is known about how socially evoked neural activity is modulated across the reproductive cycle [1]. To investigate this, we performed single-unit recordings in the ventrolateral region of the ventromedial hypothalamus (VMHvl) in freely behaving, naturally cycling, female mice interacting with conspecifics of both genders. The VMHvl has been implicated in rodent sociosexual behavior [2, 3]: it has access to social sensory stimuli [4-8] and is involved in aggression and mating [9-11]. Furthermore, many VMHvl neurons express ovarian hormone receptors [12, 13], which play a central role in female sociosexual behavior [14-16]. We found that a large fraction of VMHvl neurons was activated in the presence of conspecifics with preference to male stimuli and that the activity of most VMHvl neurons was modulated throughout social interactions rather than in response to specific social events. Furthermore, neuronal responses to male, but not female, conspecifics in the VMHvl were enhanced during the sexually receptive state. Thus, male-evoked VMHvl responses are modulated by the reproductive state, and VMHvl neural activity could drive gender-specific and reproductive state-dependent sociosexual behavior.

The role of oxytocin in male and female reproductive behavior

Oxytocin (OT) is a nonapeptide with an impressive variety of physiological functions. Among them, the 'prosocial' effects have been discussed in several recent reviews, but the direct effects on male and female sexual behavior did receive much less attention so far. As our contribution to honor the lifelong interest of Berend Olivier in the control mechanisms of sexual behavior, we decided to explore the role of OT in the present review. In the successive sections, some physiological mechanisms and the 'pair-bonding' effects of OT will be discussed, followed by sections about desire, female appetitive and copulatory behavior, including lordosis and orgasm. At the male side, the effects on erection and ejaculation are reviewed, followed by a section about 'premature ejaculation' and a possible role of OT in its treatment. In addition to OT, serotonin receives some attention as one of the main mechanisms controlling the effects of OT. In the succeeding sections, the importance of OT for 'the fruits of labor' is discussed, as it plays an important role in both maternal and paternal behavior. Finally, we pay attention to an intriguing brain area, the ventrolateral part of the ventromedial hypothalamic nucleus (VMHvl), apparently functioning in both sexual and aggressive behavior, which are at first view completely opposite behavioral systems.

Glutamatergic phenotype of glucagon-like peptide 1 neurons in the caudal nucleus of the solitary tract in rats

The expression of a vesicular glutamate transporter (VGLUT) suffices to assign a glutamatergic phenotype to neurons and other secretory cells. For example, intestinal L cells express VGLUT2 and secrete glutamate along with glucagon-like peptide 1 (GLP1). We hypothesized that GLP1-positive neurons within the caudal (visceral) nucleus of the solitary tract (cNST) also are glutamatergic. To test this, the axonal projections of GLP1 and other neurons within the cNST were labeled in rats via iontophoretic delivery of anterograde tracer. Dual immunofluorescence and confocal microscopy was used to visualize tracer-, GLP1-, and VGLUT2-positive fibers within brainstem, hypothalamic, and limbic forebrain nuclei that receive input from the cNST. Electron microscopy was used to confirm GLP1 and VGLUT2 immunolabeling within the same axon varicosities, and fluorescent in situ hybridization was used to examine VGLUT2 mRNA expression by GLP1-positive neurons. Most anterograde tracer-labeled fibers displayed VGLUT2-positive varicosities, providing new evidence that ascending axonal projections from the cNST are primarily glutamatergic. Virtually all GLP1-positive varicosities also were VGLUT2-positive. Electron microscopy confirmed the colocalization of GLP1 and VGLUT2 immunolabeling in axon terminals that formed asymmetric (excitatory-type) synapses with unlabeled dendrites in the hypothalamus. Finally, in situ hybridization confirmed that GLP1-positive cNST neurons express VGLUT2 mRNA. Thus, hindbrain GLP1 neurons in rats are equipped to store glutamate in synaptic vesicles, and likely co-release both glutamate and GLP1 from axon varicosities and terminals in the hypothalamus and other brain regions.

The role of the hypothalamic paraventricular nucleus and the organum vasculosum lateral terminalis in the control of sodium appetite in male rats

Angiotensin II (AngII) and aldosterone cooperate centrally to produce a robust sodium appetite. The intracellular signaling and circuitry that underlie this interaction remain unspecified. Male rats pretreated with both deoxycorticosterone (DOC; a synthetic precursor of aldosterone) and central AngII exhibited a marked sodium intake, as classically described. Disruption of inositol trisphosphate signaling, but not extracellular-regulated receptor kinase 1 and 2 signaling, prevented the cooperativity of DOC and AngII on sodium intake. The pattern of expression of the immediate early gene product cFos was used to identify key brain regions that may underlie this behavior. In the paraventricular nuclei (PVN) of the hypothalamus, DOC pretreatment diminished both AngII-induced cFos induction and neurosecretion of oxytocin, a peptide expressed in the PVN. Conversely, in the organum vasculosum lateral terminalis (OVLT), DOC pretreatment augmented cFos expression. Immunohistochemistry identified a substantial presence of oxytocin fibers in the OVLT. In addition, when action potentials in the PVN were inhibited with intraparenchymal lidocaine, AngII-induced sodium ingestion was exaggerated. Intriguingly, this treatment also increased the number of neurons in the OVLT expressing AngII-induced cFos. Collectively, these results suggest that the behavioral cooperativity between DOC and AngII involves the alleviation of an inhibitory oxytocin signal, possibly relayed directly from the PVN to the OVLT.

Oxytocin in the ventromedial hypothalamic nucleus reduces feeding and acutely increases energy expenditure

Central oxytocin reduces food intake and increases energy expenditure. The ventromedial hypothalamic nucleus (VMN) is associated with energy balance and contains a high density of oxytocin receptors. We hypothesized that oxytocin in the VMN is a negative regulator of energy balance acting to reduce feeding and increase energy expenditure. To test this idea, oxytocin or vehicle was injected directly into the VMN of Sprague-Dawley rats during fasted and nonfasted conditions. Energy expenditure (via indirect calorimetry) and spontaneous physical activity (SPA) were recorded simultaneously. Animals were also exposed to a conditioned taste aversion test, to determine whether oxytocin's effects on food intake were associated with malaise. When food was available during testing, oxytocin-induced elevations in energy expenditure lasted for 1 h, after which overall energy expenditure was reduced. In the absence of food during the testing period, oxytocin similarly increased energy expenditure during the first hour, but differences in 12-h energy expenditure were eliminated, implying that the differences may have been due to the thermic effects of feeding (digestion, absorption, and metabolic processing). Oxytocin acutely elevated SPA and reduced feeding at doses that did not cause a conditioned taste aversion during both the fed and fasted states. Together, these data suggest that oxytocin in the VMN promotes satiety and acutely elevates energy expenditure and SPA and implicates the VMN as a relevant site for the antiobesity effects of oxytocin.

Estrogenic regulation of histamine receptor subtype H1 expression in the ventromedial nucleus of the hypothalamus in female rats

Female sexual behavior is controlled by central estrogenic action in the ventromedial nucleus of the hypothalamus (VMN). This region plays a pivotal role in facilitating sex-related behavior in response to estrogen stimulation via neural activation by several neurotransmitters, including histamine, which participates in this mechanism through its strong neural potentiating action. However, the mechanism through which estrogen signaling is linked to the histamine system in the VMN is unclear. This study was undertaken to investigate the relationship between estrogen and histamine receptor subtype H1 (H1R), which is a potent subtype among histamine receptors in the brain. We show localization of H1R exclusively in the ventrolateral subregion of the female VMN (vl VMN), and not in the dorsomedial subregion. In the vl VMN, abundantly expressed H1R were mostly colocalized with estrogen receptor α. Intriguingly, H1R mRNA levels in the vl VMN were significantly elevated in ovariectomized female rats treated with estrogen benzoate. These data suggest that estrogen can amplify histamine signaling by enhancing H1R expression in the vl VMN. This enhancement of histamine signaling might be functionally important for allowing neural excitation in response to estrogen stimulation of the neural circuit and may serve as an accelerator of female sexual arousal.

Evolution of oxytocin pathways in the brain of vertebrates

The central oxytocin system transformed tremendously during the evolution, thereby adapting to the expanding properties of species. In more basal vertebrates (paraphyletic taxon Anamnia, which includes agnathans, fish and amphibians), magnocellular neurosecretory neurons producing homologs of oxytocin reside in the wall of the third ventricle of the hypothalamus composing a single hypothalamic structure, the preoptic nucleus. This nucleus further diverged in advanced vertebrates (monophyletic taxon Amniota, which includes reptiles, birds, and mammals) into the paraventricular and supraoptic nuclei with accessory nuclei (AN) between them. The individual magnocellular neurons underwent a process of transformation from primitive uni- or bipolar neurons into highly differentiated neurons. Due to these microanatomical and cytological changes, the ancient release modes of oxytocin into the cerebrospinal fluid were largely replaced by vascular release. However, the most fascinating feature of the progressive transformations of the oxytocin system has been the expansion of oxytocin axonal projections to forebrain regions. In the present review we provide a background on these evolutionary advancements. Furthermore, we draw attention to the non-synaptic axonal release in small and defined brain regions with the aim to clearly distinguish this way of oxytocin action from the classical synaptic transmission on one side and from dendritic release followed by a global diffusion on the other side. Finally, we will summarize the effects of oxytocin and its homologs on pro-social reproductive behaviors in representatives of the phylogenetic tree and will propose anatomically plausible pathways of oxytocin release contributing to these behaviors in basal vertebrates and amniots.

Neurochemical characterization of neurons expressing melanin-concentrating hormone receptor 1 in the mouse hypothalamus

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that acts via MCH receptor 1 (MCHR1) in the mouse. It promotes positive energy balance; thus, mice lacking MCH or MCHR1 are lean, hyperactive, and resistant to diet-induced obesity. Identifying the cellular targets of MCH is an important step to understanding the mechanisms underlying MCH actions. We generated the Mchr1-cre mouse that expresses cre recombinase driven by the MCHR1 promoter and crossed it with a tdTomato reporter mouse. The resulting Mchr1-cre/tdTomato progeny expressed easily detectable tdTomato fluorescence in MCHR1 neurons, which were found throughout the olfactory system, striatum, and hypothalamus. To chemically identify MCH-targeted cell populations that play a role in energy balance, MCHR1 hypothalamic neurons were characterized by colabeling select hypothalamic neuropeptides with tdTomato fluorescence. TdTomato fluorescence colocalized with dynorphin, oxytocin, vasopressin, enkephalin, thyrothropin-releasing hormone, and corticotropin-releasing factor immunoreactive cells in the paraventricular nucleus. In the lateral hypothalamus, neurotensin, but neither orexin nor MCH neurons, expressed tdTomato. In the arcuate nucleus, both Neuropeptide Y and proopiomelanocortin cells expressed tdTomato. We further demonstrated that some of these arcuate neurons were also targets of leptin action. Interestingly, MCHR1 was expressed in the vast majority of leptin-sensitive proopiomelanocortin neurons, highlighting their importance for the orexigenic actions of MCH. Taken together, this study supports the use of the Mchr1-cre mouse for outlining the neuroanatomical distribution and neurochemical phenotype of MCHR1 neurons.

Neuropeptides and central control of sexual behaviour from the past to the present: a review

Of the numerous neuropeptides identified in the central nervous system, only a few are involved in the control of sexual behaviour. Among these, the most studied are oxytocin, adrenocorticotropin, α-melanocyte stimulating hormone and opioid peptides. While opioid peptides inhibit sexual performance, the others facilitate sexual behaviour in most of the species studied so far (rats, mice, monkeys and humans). However, evidence for a sexual role of gonadotropin-releasing hormone, corticotropin releasing factor, neuropeptide Y, galanin and galanin-like peptide, cholecystokinin, substance P, vasoactive intestinal peptide, vasopressin, angiotensin II, hypocretins/orexins and VGF-derived peptides are also available. Corticotropin releasing factor, neuropeptide Y, cholecystokinin, vasopressin and angiotensin II inhibit, while substance P, vasoactive intestinal peptide, hypocretins/orexins and some VGF-derived peptide facilitate sexual behaviour. Neuropeptides influence sexual behaviour by acting mainly in the hypothalamic nuclei (i.e., lateral hypothalamus, paraventricular nucleus, ventromedial nucleus, arcuate nucleus), in the medial preoptic area and in the spinal cord. However, it is often unclear whether neuropeptides influence the anticipatory phase (sexual arousal and/or motivation) or the consummatory phase (performance) of sexual behaviour, except in a few cases (e.g., opioid peptides and oxytocin). Unfortunately, scarce information has been added in the last 15 years on the neural mechanisms by which neuropeptides influence sexual behaviour, most studied neuropeptides apart. This may be due to a decreased interest of researchers on neuropeptides and sexual behaviour or on sexual behaviour in general. Such a decrease may be related to the discovery of orally effective, locally acting type V phosphodiesterase inhibitors for the therapy of erectile dysfunction.

Molecular characterization of the subnuclei in rat habenula

The mammalian habenula is involved in regulating the activities of serotonergic and dopaminergic neurons. It consists of the medial and lateral habenulae, with each subregion having distinct neural connectivity. Despite the functional significance, manipulating neural activity in a subset of habenular pathways remains difficult because of the poor availability of molecular markers that delineate the subnuclear structures. Thus, we examined the molecular nature of neurons in the habenular subnuclei by analyzing the gene expressions of neurotransmitter markers. The results showed that different subregions of the medial habenula (MHb) use different combinations of neurotransmitter systems and could be categorized as either exclusively glutamatergic (superior part of MHb), both substance P-ergic and glutamatergic (dorsal region of central part of MHb), or both cholinergic and glutamatergic (inferior part, ventral region of central part, and lateral part of MHb). The superior part of the MHb strongly expressed interleukin-18 and was innervated by noradrenergic fibers. In contrast, the inferior part, ventral region of the central part, and lateral part of the MHb were peculiar in that acetylcholine and glutamate were cotransmitted from the axonal terminals. In contrast, neurons in the lateral habenula (LHb) were almost uniformly glutamatergic. Finally, the expressions of Htr2c and Drd2 seemed complementary in the medial LHb division, whereas they coincided in the lateral division, suggesting that the medial and lateral divisions of LHb show strong heterogeneity with respect to monoamine receptor expression. These analyses clarify molecular differences between subnuclei in the mammalian habenula that support their respective functional implications.

DNA methyltransferases1 (DNMT1) and 3a (DNMT3a) colocalize with GAD67-positive neurons in the GAD67-GFP mouse brain

DNA methylation is an epigenetic regulatory mechanism commonly associated with transcriptional silencing. DNA methyltransferases (DNMTs) are a family of related proteins that both catalyze the de novo formation of 5-methylcytosine and maintain these methylation marks in cell-specific patterns in virtually all mitotic cells of the body. In the adult brain, methylation occurs in progenitor cells of the neurogenic zones and in postmitotic neurons. Of the DNMTs, DNMT1 and DNMT3a are most highly expressed in postmitotic neurons. While it has been commonly thought all postmitotic neurons and glia express DNMTs at comparable levels, the coexpression of selected DNMTs with markers of distinct neurotransmitter phenotypes has not been previously examined in detail in the mouse. To this end, we analyzed the expression of DNMT1 and DNMT3a along with GAD67 in the brains of the glutamic acid decarboxylase67-enhanced green fluorescent protein (GAD67-GFP) knockin mice. After first confirming that GFP-immunopositive neurons were also GAD67-positive, we showed that in the motor cortex, piriform cortex, striatum, CA1 region of the hippocampus, dentate gyrus, and basolateral amygdala (BLA), GFP immunofluorescence coincided with the signal corresponding to DNMT1 and DNMT3a. A detailed examination of cortical neurons, showed that ≈30% of NeuN-immunopositive neurons were also DNMT1-positive. These data do not exclude the expression of DNMT1 or DNMT3a in glutamatergic neurons and glia. However, they suggest that their expression is low compared with the levels present in GABAergic neurons.

Oxytocin and dendrite remodeling in the hypothalamus

For most people, their quality of life depends on their successful interdependence with others, which requires sophisticated social cognition, communication, and emotional bonds. Across the lifespan, new bonds must be forged and maintained, and conspecific menaces must be managed. The dynamic nature of the human social landscape suggests ongoing specific alterations in neural circuitry across several brain systems to subserve social behavior. To discover the biological mechanisms that contribute to normal social activities, animal models of social behavior have been developed. One valuable model system has been female rat sexual behavior, which is governed by cyclic variation of ovarian hormones. This behavior is modulated by the neuropeptide oxytocin (OT) through its actions in the hypothalamic ventromedial nucleus (VMH). The fluctuation of this behavior is associated with dendrite remodeling, like several other examples of behavioral plasticity. This review compares hormone-induced plasticity in the VMH with other examples of dendrite plasticity across the mammalian nervous system, namely the neurobehavioral paradigms of environmental enrichment, chronic stress, and incentive sensitization, which affect the neocortex, hippocampal formation, and ventral striatum, respectively. This comparison suggests that the effects of ovarian hormones on VMH neurons in rats, given the simple dendritic arbor and short time course for dendrite remodeling, provide a dual opportunity for mechanistic and functional studies that will shed light on i) the neural actions of OT that regulate social behavior and, ii) behaviorally relevant dendrite regulation in a variety of brain structures. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Toward understanding how early-life social experiences alter oxytocin- and vasopressin-regulated social behaviors

The early-life social environment has profound effects on brain development and subsequent expression of social behavior. Oxytocin and vasopressin are expressed and released in the brain and are important regulators of social behavior. Accordingly, the early social environment may alter social behaviors via changes in the oxytocin and/or vasopressin systems. To test this hypothesis, and to gain mechanistic insights, rodent models mimicking either a deprived (e.g. maternal separation) or enriched (e.g. neonatal handling) early social environment have been utilized. Findings indeed show that differences in the quality of the early social environment are associated with brain region-specific alterations in oxytocin and vasopressin expression and oxytocin receptor and vasopressin 1a receptor binding. Early social environment-induced changes in oxytocin and vasopressin systems were associated with changes in several forms of social behavior, including maternal care, aggression, play-fighting, and social recognition. First studies provide evidence for a causal link between altered vasopressin responsiveness and impairments in social recognition in rats exposed to maternal separation and a role for epigenetic mechanisms to explain persistent increases in vasopressin expression in mice exposed to maternal separation. Overall, initial findings suggest that oxytocin and vasopressin systems may mediate early social environment-induced alterations in social behavior. Additional comprehensive studies will be necessary to advance our understanding to what extent changes in oxytocin and vasopressin underlie early social environment-induced alterations in social behavior. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Orphanin FQ in the mediobasal hypothalamus facilitates sexual receptivity through the deactivation of medial preoptic nucleus mu-opioid receptors

Sexual receptivity, lordosis, can be induced by sequential estradiol and progesterone or extended exposure to high levels of estradiol in the female rat. In both cases estradiol initially inhibits lordosis through activation of β-endorphin (β-END) neurons of the arcuate nucleus of the hypothalamus (ARH) that activate μ-opioid receptors (MOP) in the medial preoptic nucleus (MPN). Subsequent progesterone or extended estradiol exposure deactivates MPN MOP to facilitate lordosis. Opioid receptor-like receptor-1 (ORL-1) is expressed in ARH and ventromedial hypothalamus (VMH). Infusions of its endogenous ligand, orphanin FQ (OFQ/N, aka nociceptin), into VMH-ARH region facilitate lordosis. Whether OFQ/N acts in ARH and/or VMH and whether OFQ/N is necessary for steroid facilitation of lordosis are unclear. In Exp I, OFQ/N infusions in VMH and ARH that facilitated lordosis also deactivated MPN MOP indicating that OFQ/N facilitation of lordosis requires deactivation of ascending ARH-MPN projections by directly inhibiting ARH β-END neurons and/or through inhibition of excitatory VMH-ARH pathways to proopiomelanocortin neurons. It is unclear whether OFQ/N activates the VMH output motor pathways directly or via the deactivation of MPN MOP. In Exp II we tested whether ORL-1 activation is necessary for estradiol-only or estradiol+progesterone lordosis facilitation. Blocking ORL-1 with UFP-101 inhibited estradiol-only lordosis and MPN MOP deactivation but had no effect on estradiol+progesterone facilitation of lordosis and MOP deactivation. In conclusion, steroid facilitation of lordosis inhibits ARH β-END neurons to deactivate MPN MOP, but estradiol-only and estradiol+progesterone treatments appear to use different neurotransmitter systems to inhibit ARH-MPN signaling.

Ovarian hormone action in the hypothalamic ventromedial nucleus: remodelling to regulate reproduction

The ventromedial nucleus of the hypothalamus (VMH) is a major site for the control of female sexual behaviour by ovarian steroid hormones. This review explores recent details that have emerged regarding the ovarian hormone-induced remodelling of neural circuits within the VMH in adult female rats, with the goal of refining the model of the VMH neural circuit. VMH neurones exhibit simple dendritic arbours, with a single long primary dendrite (LPD) and several short primary dendrites. We recently found that ovarian hormones have unanticipated differential effects on the length of the LPDs, suggesting an intricate synaptic reorganisation. LPDs extend into the lateral fibre plexus where they contact oxytocin-labelled terminals. Oestradiol treatment rearranges this oxytocin innervation, in particular by withdrawing some of the LPDs and intensifying the oxytocin input to the remaining dendrites. These changes are reversed with concomitant progesterone treatment. Incorporating these new results, we have updated our working model of hormone-induced synaptic reorganisation in the VMH, emphasising the rebalancing of local versus extrinsic connectivity. The new working model synthesises the recent evidence for rewiring with insights from electrophysiological and behavioural pharmacological studies that pertain to the roles of oxytocin and glutamate in VMH neural activity and mating behaviour.

Sex differences in the neural circuit that mediates female sexual receptivity

Female sexual behavior in rodents, typified by the lordosis posture, is hormone-dependent and sex-specific. Ovarian hormones control this behavior via receptors in the hypothalamic ventromedial nucleus (VMH). This review considers the sex differences in the morphology, neurochemistry and neural circuitry of the VMH to gain insights into the mechanisms that control lordosis. The VMH is larger in males compared with females, due to more synaptic connections. Another sex difference is the responsiveness to estradiol, with males exhibiting muted, and in some cases reverse, effects compared with females. The lack of lordosis in males may be explained by differences in synaptic organization or estrogen responsiveness, or both, in the VMH. However, given that damage to other brain regions unmasks lordosis behavior in males, a male-typical VMH is unlikely the main factor that prevents lordosis. In females, key questions remain regarding the mechanisms whereby ovarian hormones modulate VMH function to promote lordosis.