The bed nucleus of the stria terminalis in the Syrian hamster: subnuclei and connections of the posterior division

Whole-brain input mapping of the lateral versus medial anterodorsal bed nucleus of the stria terminalis in the mouse

The anterior portion of the bed nucleus of the stria terminalis (BNST) modulates fear and stress responses. The anterodorsal BNST (adBNST) can be anatomically subdivided further into the lateral and medial divisions. Although output projections of BNST subregions have been studied, the local and global input connections to these subregions remain poorly understood. To further understand BNST-centered circuit operations, we have applied new viral-genetic tracing and functional circuit mapping to determine detailed synaptic circuit inputs to lateral and medial subregions of adBNST in the mouse. Monosynaptic canine adenovirus type 2 (CAV2) and rabies virus-based retrograde tracers were injected in the adBNST subregions. The amygdalar complex, hypothalamus and hippocampal formation account for the majority of overall inputs to adBNST. However, lateral versus medial adBNST subregions have distinct patterns of long-range cortical and limbic brain inputs. The lateral adBNST has more input connections from prefrontal (prelimbic, infralimbic, cingulate) and insular cortices, anterior thalamus and ectorhinal/perirhinal cortices. In contrast, the medial adBNST received biased inputs from the medial amygdala, lateral septum, hypothalamus nuclei and ventral subiculum. We confirmed long-range functional inputs from the amydalohippocampal area and basolateral amygdala to the adBNST using ChR2-assisted circuit mapping. Selected novel BNST inputs are also validated with the AAV axonal tracing data from the Allen Institute Mouse Brain Connectivity Atlas. Together, these results provide a comprehensive map of the differential afferent inputs to lateral and medial adBNST subregions, and offer new insight into the functional operations of BNST circuitry for stress and anxiety-related behaviors.

c-Fos expression following context conditioning and deep brain stimulation in the bed nucleus of the stria terminalis in rats

Deep brain stimulation (DBS) in the bed nucleus of the stria terminalis (BST), a region implicated in the expression of anxiety, shows promise in psychiatric patients, but its effects throughout the limbic system are largely unknown. In male Wistar rats, we first evaluated the neural signature of contextual fear (N = 16) and next, of the anxiolytic effects of high-frequency electrical stimulation in the BST (N = 31), by means of c-Fos protein expression. In non-operated animals, we found that the left medial anterior BST displayed increased c-Fos expression in anxious (i.e., context-conditioned) versus control subjects. Moreover, control rats showed asymmetric expression in the basolateral amygdala (BLA) (i.e., higher intensities in the right hemisphere), which was absent in anxious animals. The predominant finding in rats receiving bilateral BST stimulation was a striking increase in c-Fos expression throughout much of the left hemisphere, which was not confined to the predefined regions of interest. To conclude, we found evidence for lateralized c-Fos expression during the expression of contextual fear and anxiolytic high-frequency electrical stimulation of the BST, particularly in the medial anterior BST and BLA. In addition, we observed an extensive and unexpected left-sided c-Fos spread following bilateral stimulation in the BST.

BNST-insula structural connectivity in humans

The bed nucleus of the stria terminalis (BNST) is emerging as a critical region in multiple psychiatric disorders including anxiety, PTSD, and alcohol and substance use disorders. In conjunction with growing knowledge of the BNST, an increasing number of studies examine connections of the BNST and how those connections impact BNST function. The importance of this BNST network is highlighted by rodent studies demonstrating that projections from other brain regions regulate BNST activity and influence BNST-related behavior. While many animal and human studies replicate the components of the BNST network, to date, structural connections between the BNST and insula have only been described in rodents and have yet to be shown in humans. In this study, we used probabilistic tractography to examine BNST-insula structural connectivity in humans. We used two methods of dividing the insula: 1) anterior and posterior insula, to be consistent with much of the existing insula literature; and 2) eight subregions that represent informative cytoarchitectural divisions. We found evidence of a BNST-insula structural connection in humans, with the strongest BNST connectivity localized to the anteroventral insula, a region of agranular cortex. BNST-insula connectivity differed by hemisphere and was moderated by sex. These results translate rodent findings to humans and lay an important foundation for future studies examining the role of BNST-insula pathways in psychiatric disorders.

Aggressive behavior and brain neuronal activation in sexually naïve male Mongolian gerbils

Aggressive behavior plays an important role in animal's survival and reproductive success. Although there has been growing interests in studying neural mechanisms underlying aggressive behavior using traditional laboratory animal models, little is known about mechanisms controlling naturally occurring aggression in sexually naïve animals. In the present study, we characterized aggressive behavior displayed by sexually naïve male Mongolian gerbils (Meriones unguiculatus) and examined the subsequent neuronal activation in the brain measured by Fos-immunoreactive (Fos-ir) staining. We found that resident males initiated attacks and showed intense levels of aggression (including chase, bite, offensive sideway, lunge and on-top) towards a conspecific male intruder. Furthermore, attacks from the resident males towards the intruder produced a nonrandom distribution of bites, with the most on the rump, flank, back and tail and few on the limbs, ventrum and head. In contrast, control males that were exposed to a woodblock (control for novelty) never attacked the woodblock and showed higher levels of object/environmental investigation. Male gerbils exposed to an intruder had significantly higher levels of Fos-ir density in the medial (MeA) and anterior cortical (ACo) subnuclei of the amygdala, principal nucleus (BSTpr) and interfascicular nucleus (BSTif) of the bed nucleus of the stria terminalis, ventrolateral subdivision of the ventromedial hypothalamus (VMHvl), and paraventricular nucleus of the hypothalamus (PVN), compared to control males. Together, our results indicate that sexually naïve, group housed male gerbils naturally display aggression towards conspecific strangers, and such aggressive behavior is associated with special patterns of neuronal activation in the brain.

Ultra-High-Resolution Imaging of Amygdala Subnuclei Structural Connectivity in Major Depressive Disorder

BACKGROUND

Major depressive disorder (MDD) is an increasingly common and disabling illness. As the amygdala has been reported to have pathological involvement in mood disorders, we aimed to investigate for the first time potential changes to structural connectivity of individual amygdala subnuclei in MDD using ultra-high-field 7T diffusion magnetic resonance imaging.

METHODS

Twenty-four patients with MDD (11 women) and 24 age-matched healthy control participants (7 women) underwent diffusion-weighted imaging with a 1.05-mm isotropic resolution at 7T. Amygdala nuclei regions of interest were obtained through automated segmentation of 0.69-mm resolution T1-weighted images and 0.35-mm resolution T2-weighted images. Probabilistic tractography was performed on all subjects, with random seeding at each amygdala nucleus.

RESULTS

The right lateral, basal, central, and centrocortical amygdala nuclei exhibited significantly increased connection density to the rest of the brain, whereas the left medial nucleus demonstrated significantly lower connection density (false discovery rate p < .05). Increased connection density in the right lateral and basal nuclei was driven by the stria terminalis, and the significant difference in the right central nucleus was driven by the uncinate fasciculus. Decreased connection density at the left medial nucleus did not appear to be driven by any individual white matter tract.

CONCLUSIONS

By exploiting ultra-high-resolution magnetic resonance imaging, structural hyperconnectivity was demonstrated involving the amygdaloid nuclei in the right hemisphere in MDD. To a lesser extent, impairment of subnuclei connectivity was shown in the left hemisphere.

The Bed Nucleus of the Stria Terminalis, Homeostatic Satiety, and Compulsions: What Can We Learn From Polydipsia?

A compulsive phenotype characterizes several neuropsychiatric illnesses - including but not limited to - schizophrenia and obsessive compulsive disorder. Because of its perceived etiological heterogeneity, it is challenging to disentangle the specific neurophysiology that precipitates compulsive behaving. Using polydipsia (or non-regulatory water drinking), we describe candidate neural substrates of compulsivity. We further postulate that aberrant neuroplasticity within cortically projecting structures [i.e., the bed nucleus of the stria terminalis (BNST)] and circuits that encode homeostatic emotions (thirst, hunger, satiety, etc.) underlie compulsive drinking. By transducing an inaccurate signal that fails to represent true homeostatic state, cortical structures cannot select appropriate and adaptive actions. Additionally, augmented dopamine (DA) reactivity in striatal projections to and from the frontal cortex contribute to aberrant homeostatic signal propagation that ultimately biases cortex-dependent behavioral selection. Responding becomes rigid and corresponds with both erroneous, inflexible encoding in both bottom-up structures and in top-down pathways. How aberrant neuroplasticity in circuits that encode homeostatic emotion result in the genesis and maintenance of compulsive behaviors needs further investigation.

Striatum on the anxiety map: Small detours into adolescence

Adolescence is the most sensitive period for the development of pathological anxiety. Moreover, specific neural changes associated with the striatum might be related to adolescent vulnerability to anxiety. Up to now, the study of anxiety has primarily focused on the amygdala, bed nucleus of the stria terminalis (BNST), hippocampus and ventromedial prefrontal cortex (vmPFC), while the striatum has typically not been considered as part of the anxiety system. This review proposes the addition of the striatum, a complex, multi-component structure, to the anxiety network by underscoring two lines of research. First, the co-occurrence of the adolescent striatal development with the peak vulnerability of adolescents to anxiety disorders might potentially reflect a causal relationship. Second, the recognition of the role of the striatum in fundamental behavioral processes that do affect anxiety supports the putative importance of the striatum in anxiety. These behavioral processes include (1) attention, (2) conditioning/prediction error, and (3) motivation. This review proposes a simplistic schematic representation of the anxiety circuitry that includes the striatum, and aims to promote further work in this direction, as the role of the striatum in shaping an anxiety phenotype during adolescence could have critical implications for understanding and preventing the peak onset of anxiety disorders during this period. This article is part of a Special Issue entitled SI: Adolescent plasticity.

Tyrosine hydroxylase-immunoreactivity and its relations with gonadotropin-releasing hormone and neuropeptide Y in the preoptic area of the guinea pig

The present study examines the distribution of tyrosine hydroxylase (TH) immunoreactivity and its morphological relationships with neuropeptide Y (NPY)- and gonadoliberin (GnRH)-immunoreactive (IR) structures in the preoptic area (POA) of the male guinea pig. Tyrosine hydroxylase was expressed in relatively small population of perikarya and they were mostly observed in the periventricular preoptic nucleus and medial preoptic area. The tyrosine hydroxylase-immunoreactive (TH-IR) fibers were dispersed troughout the whole POA. The highest density of these fibers was observed in the median preoptic nucleus, however, in the periventricular preoptic nucleus and medial preoptic area they were only slightly less numerous. In the lateral preoptic area, the density of TH-IR fibers was moderate. Two morphological types of TH-IR fibers were distinguished: smooth and varicose. Double immunofluorescence staining showed that TH and GnRH overlapped in the guinea pig POA but they never coexisted in the same structures. TH-IR fibers often intersected with GnRH-IR structures and many of them touched the GnRH-IR perikarya or dendrites. NPY wchich was abundantly present in the POA only in fibers showed topographical proximity with TH-IR structures. Althoug TH-IR perikarya and fibers were often touched by NPY-IR fibers, colocalization of TH and NPY in the same structures was very rare. There was only a small population of fibers which contained both NPY and TH. In conclusion, the morphological evidence of contacts between TH- and GnRH-IR nerve structures may be the basis of catecholaminergic control of GnRH release in the preoptic area of the male guinea pig. Moreover, TH-IR neurons were conatcted by NPY-IR fibers and TH and NPY colocalized in some fibers, thus NPY may regulate catecholaminergic neurons in the POA.

Whole-brain mapping of afferent projections to the bed nucleus of the stria terminalis in tree shrews

The bed nucleus of the stria terminalis (BST) plays an important role in integrating and relaying input information to other brain regions in response to stress. The cytoarchitecture of the BST in tree shrews (Tupaia belangeri chinensis) has been comprehensively described in our previous publications. However, the inputs to the BST have not been described in previous reports. The aim of the present study was to investigate the sources of afferent projections to the BST throughout the brain of tree shrews using the retrograde tracer Fluoro-Gold (FG). The present results provide the first detailed whole-brain mapping of BST-projecting neurons in the tree shrew brain. The BST was densely innervated by the prefrontal cortex, entorhinal cortex, ventral subiculum, amygdala, ventral tegmental area, and parabrachial nucleus. Moreover, moderate projections to the BST originated from the medial preoptic area, supramammillary nucleus, paraventricular thalamic nucleus, pedunculopontine tegmental nucleus, dorsal raphe nucleus, locus coeruleus, and nucleus of the solitary tract. Afferent projections to the BST are identified in the ventral pallidum, nucleus of the diagonal band, ventral posteromedial thalamic nucleus, posterior complex of the thalamus, interfascicular nucleus, retrorubral field, rhabdoid nucleus, intermediate reticular nucleus, and parvicellular reticular nucleus. In addition, the different densities of BST-projecting neurons in various regions were analyzed in the tree shrew brains. In summary, whole-brain mapping of direct inputs to the BST is delineated in tree shrews. These brain circuits are implicated in the regulation of numerous physiological and behavioral processes including stress, reward, food intake, and arousal.

Sex-specific effects of relaxin-3 on food intake and body weight gain

Relaxin-3 (RLN3) is a neuropeptide that is strongly expressed in the pontine nucleus incertus (NI) and binds with high affinity to its cognate receptor RXFP3. Central administration of RLN3 in rats increases food intake and adiposity. In humans, RLN3 polymorphism has been associated with obesity and hypercholesterolaemia. Emerging evidence suggests that the effects of RLN3 may have sex-specific aspects. Thus, the RLN3 knockout female but not male mice are hypoactive. RLN3 produced stronger orexigenic and obesogenic effects in female rats compared with male rats. In addition, female rats demonstrated higher sensitivity to lower doses of RLN3. Repeated cycles of food restriction and stress were accompanied by an increase in RLN3 expression and hyperphagia in female but not in male rats. Furthermore, stress-induced binge eating in female rats was blocked by an RXFP3 receptor antagonist. RLN3 increased the expression of corticotropin releasing factor in the paraventricular hypothalamic nucleus in male but not in female rats. Conversely, in female rats, RLN3 increased the expression of orexin in the lateral hypothalamus. There is evidence that orexin directly activates the RLN3 neurons in the NI. The positive reinforcement of the RLN3 effects by orexin may intensify behavioural activation and feeding in females. Sex-specific effects of RLN3 may also depend on differential expression of RXFP3 receptors in the brain. Given the higher sensitivity of females to the orexigenic effects of RLN3 and the stress-induced activation of RLN3, the overall data suggest a possible role for RLN3 in eating disorders that show a higher propensity in women.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

High-resolution and cell-type-specific photostimulation mapping shows weak excitatory vs. strong inhibitory inputs in the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) is a key component of the extended amygdala and has been implicated in anxiety and addiction. As individual neurons function within neural circuits, it is important to understand local microcircuits and larger network connections of identified neuronal types and understand how maladaptive changes in the BNST neural networks are induced by stress and drug abuse. However, due to limitations of classic anatomical and physiological methods, the local circuit organization of synaptic inputs to specific BNST neuron types is not well understood. In this study, we report on the application of high-resolution and cell-type-specific photostimulation methodology developed in our laboratory to local circuit mapping in the BNST. Under calibrated experimental conditions, laser photostimulation via glutamate uncaging or channelrhodopsin-2 photoactivation evokes spiking of BNST neurons perisomatically, without activating spikes from axons of passage or distal dendrites. Whole cell recordings, combined with spatially restricted photostimulation of presynaptic neurons at many different locations over a large region, allow high-resolution mapping of presynaptic input sources to single recorded neurons in the BNST. We constructed maps of synaptic inputs impinging onto corticotrophin-releasing hormone-expressing (CRH+) BNST neurons in the dorsolateral BNST and found that the CRH+ neurons receive predominant local inhibitory synaptic connections with very weak excitatory connections. Through cell-type-specific optogenetic stimulation mapping, we generated maps of somatostatin-expressing neuron-specific inhibitory inputs to BNST neurons. Taken together, the photostimulation-based techniques offer us powerful tools for determining the functional organization of local circuits of specific BNST neuron types.

Genetic cell targeting uncovers specific neuronal types and distinct subregions in the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) plays an important role in fear, stress, and anxiety. It contains a collection of subnuclei delineated by gross cytoarchitecture features; however, there has yet to be a systematic examination of specific BNST neuronal types and their associated neurochemical makeup. The present study focuses on improved characterization of the anterior BNST based on differing molecular and chemical expression aided by mouse genetics. Specific Cre driver lines crossed with a fluorescent reporter line were used for genetic cell targeting and immunochemical staining. Using this new approach, we were able to robustly identify specific excitatory and inhibitory cell types in the BNST. The presence and distribution of excitatory neurons were firmly established; glutamatergic neurons in the anterior BNST accounted for about 14% and 31% of dorsal and ventral BNST cells, respectively. GABAergic neurons expressing different isoforms of glutamic acid decarboxylase were found to have differential subregional distributions. Almost no parvalbumin-expressing cells were found in the BNST, while somatostatin-expressing cells and calretinin-expressing cells account for modest proportions of BNST cells. In addition, vasoactive intestinal peptide-expressing axonal plexuses were prominent in the oval and juxtacapsular subregions. In addition, we discovered that corticotropin-releasing hormone-expressing cells contain GABAergic and glutamatergic subpopulations. Together, this study reveals new information on excitatory and inhibitory neurons in the BNST, which will facilitate genetic dissection and functional studies of BNST subregions. J. Comp. Neurol. 524:2379-2399, 2016. © 2016 Wiley Periodicals, Inc.

Revealing the ventral amygdalofugal pathway of the human limbic system using high spatial resolution diffusion tensor tractography

The amygdala is known to have a role in core processes regulated by the limbic system such as motivation, memory, emotion, social behavior, self-awareness as well as certain primitive instincts. Several functional studies have investigated some of these brain tasks of the human limbic system. However, the underlying neuronal fiber connectivity of the amygdalo-diencephalon, as part of the limbic system, has not been delineated separately by prior diffusion-weighted imaging studies. The ability to trace the underlying fiber connections individually will be helpful in understanding the neurophysiology of these tracts in different functions. To date, few diffusion-weighted studies have focused on the amygdala, yet the fine connections of the amygdala, hypothalamus, septum or other adjacent limbic structures have yet to be elucidated by diffusion-weighted tractography studies. We therefore aimed to further investigate these fine neuronal connections using fiber tractography and high spatial resolution diffusion tensor imaging on 3T on 15 healthy right-handed male human subjects (age range 24-37 years). The ventral amygdalofugal pathway, anterior commissure and stria terminalis are the three main efferent pathways of the amygdala. We delineated the detailed trajectories of the ventral amygdalofugal tract, anterior commissure and their connections bilaterally in 15 normal adult human brains. Using a high-resolution diffusion tensor tractography technique, for the first time, we were able to demonstrate the trajectory of amygdalofugal tract and its connections to the hypothalamic and septal nuclei. We further revealed, for the first time, the close relationship of the amygdalofugal tract and anterior commissure with the fornix, stria terminalis and uncinate fasciculus bilaterally in 15 healthy adult human brains.

Mapping the trajectory of the stria terminalis of the human limbic system using high spatial resolution diffusion tensor tractography

The human limbic system is composed of gray and white matter structures which have been known to have a role in core processes such as motivation, memory, emotion, social behavior, self-awareness as well as certain primitive instincts. Multiple functional studies investigated some of these brain tasks in human brain limbic system. However, the underlying fine fiber pathways of the limbic system including the trajectory of the stria terminalis have not been delineated separately by prior diffusion weighted imaging. The ability to trace the underlying fiber anatomy noninvasively using diffusion tensor tractography (DTT) would be helpful to study the neurophysiology of these tracts in different functions in future functional studies. Few studies have focused on the stria terminalis using diffusion tensor tractography. Yet, the trajectory of the stria terminalis and some fine subtrajectories of the fornix have not been elucidated by prior DTT studies. We decided to further investigate these fine neuronal trajectory using tractography and high spatial resolution diffusion tensor imaging on 3T. Fifteen healthy right-handed men (age range 24-37 years) were studied. We delineated the detailed trajectories of the stria terminalis and fornix bilaterally in fifteen normal adult human brains. Using a high resolution DTT technique, we demonstrate for the first time, the trajectory of stria terminalis as well as detailed precommissural and postcommissural connectivity of the forniceal columns.

Descending projections from the nucleus accumbens shell excite activity of taste-responsive neurons in the nucleus of the solitary tract in the hamster

The nucleus of the solitary tract (NST) and the parabrachial nuclei (PbN) are the first and second relays in the rodent central taste pathway. A series of electrophysiological experiments revealed that spontaneous and taste-evoked activities of brain stem gustatory neurons are altered by descending input from multiple forebrain nuclei in the central taste pathway. The nucleus accumbens shell (NAcSh) is a key neural substrate of reward circuitry, but it has not been verified as a classical gustatory nucleus. A recent in vivo electrophysiological study demonstrated that the NAcSh modulates the spontaneous and gustatory activities of hamster pontine taste neurons. In the present study, we investigated whether activation of the NAcSh modulates gustatory responses of the NST neurons. Extracellular single-unit activity was recorded from medullary neurons in urethane-anesthetized hamsters. After taste response was confirmed by delivery of sucrose, NaCl, citric acid, and quinine hydrochloride to the anterior tongue, the NAcSh was stimulated bilaterally with concentric bipolar stimulating electrodes. Stimulation of the ipsilateral and contralateral NAcSh induced firings from 54 and 37 of 90 medullary taste neurons, respectively. Thirty cells were affected bilaterally. No inhibitory responses or antidromic invasion was observed after NAcSh activation. In the subset of taste cells tested, high-frequency electrical stimulation of the NAcSh during taste delivery enhanced taste-evoked neuronal firing. These results demonstrate that two-thirds of the medullary gustatory neurons are under excitatory descending influence from the NAcSh, which is a strong indication of communication between the gustatory pathway and the mesolimbic reward pathway.

Connections of the magnocellular medial preoptic nucleus (MPN mag) in male Syrian hamsters. II. The efferents

The magnocellular medial preoptic nucleus (MPN mag) plays a critical role in the regulation of male copulatory behavior in the Syrian hamster. Our study of the afferents are consistent with the hypothesis that the MPN mag receives input from areas in the chemosensory pathway and nuclear groups that contain receptors for gonadal steroids (Wang and Swann, 2006). The goal of the present study is to identify targets of the MPN mag by describing the location of labeled fibers following an injection of biotinylated dextran amine (BDA) into the MPN mag. Our results indicate that targets of the MPN mag include: (1) brainstem nuclei implicated in regulating male mating behavior in other species, such as the periaqueductal gray, deep mesencephalic nucleus, retrorubral field, ventral tegmental area and lateral paragigantocellular nucleus and (2) steroid-concentrating nuclei in the septum, preoptic area and hypothalamus. The lack of projections from the MPN mag to its chemosensory afferents indicate that the connections of the MPN mag with the posterior medial bed nucleus of the stria terminalis, medial and anterior cortical nuclei of the amygdala are unidirectional, and that chemosensory information flows from the medial amygdala and bed nucleus of the stria terminalis (BST) to the MPN mag. The bidirectional nature of the connections between the MPN mag and steroid-concentrating nuclei suggest that the MPN mag may influence the function of a steroid-concentrating network that regulates behaviors. Together these results support the hypothesis that the MPN mag regulates male mating behavior by integrating chemosensory and hormonal signals and relaying this information to brainstem areas that control motor output.

Chronic social stress in puberty alters appetitive male sexual behavior and neural metabolic activity

Repeated social subjugation in early puberty lowers testosterone levels. We used hamsters to investigate the effects of social subjugation on male sexual behavior and metabolic activity within neural systems controlling social and motivational behaviors. Subjugated animals were exposed daily to aggressive adult males in early puberty for postnatal days 28 to 42, while control animals were placed in empty clean cages. On postnatal day 45, they were tested for male sexual behavior in the presence of receptive female. Alternatively, they were tested for mate choice after placement at the base of a Y-maze containing a sexually receptive female in one tip of the maze and an ovariectomized one on the other. Social subjugation did not affect the capacity to mate with receptive females. Although control animals were fast to approach females and preferred ovariectomized individuals, subjugated animals stayed away from them and showed no preference. Cytochrome oxidase activity was reduced within the preoptic area and ventral tegmental area in subjugated hamsters. In addition, the correlation of metabolic activity of these areas with the bed nucleus of the stria terminalis and anterior parietal cortex changed significantly from positive in controls to negative in subjugated animals. These data show that at mid-puberty, while male hamsters are capable of mating, their appetitive sexual behavior is not fully mature and this aspect of male sexual behavior is responsive to social subjugation. Furthermore, metabolic activity and coordination of activity in brain areas related to sexual behavior and motivation were altered by social subjugation.

BNST neurocircuitry in humans

Anxiety and addiction disorders are two of the most common mental disorders in the United States, and are typically chronic, disabling, and comorbid. Emerging evidence suggests the bed nucleus of the stria terminalis (BNST) mediates both anxiety and addiction through connections with other brain regions, including the amygdala and nucleus accumbens. Although BNST structural connections have been identified in rodents and a limited number of structural connections have been verified in non-human primates, BNST connections have yet to be described in humans. Neuroimaging is a powerful tool for identifying structural and functional circuits in vivo. In this study, we examined BNST structural and functional connectivity in a large sample of humans. The BNST showed structural and functional connections with multiple subcortical regions, including limbic, thalamic, and basal ganglia structures, confirming structural findings in rodents. We describe two novel connections in the human brain that have not been previously reported in rodents or non-human primates, including a structural connection with the temporal pole, and a functional connection with the paracingulate gyrus. The findings of this study provide a map of the BNST's structural and functional connectivity across the brain in healthy humans. In large part, the BNST neurocircuitry in humans is similar to the findings from rodents and non-human primates; however, several connections are unique to humans. Future explorations of BNST neurocircuitry in anxiety and addiction disorders have the potential to reveal novel mechanisms underlying these disabling psychiatric illnesses.

Chemosignals and hormones in the neural control of mammalian sexual behavior

Males and females of most mammalian species depend on chemosignals to find, attract and evaluate mates and, in most cases, these appetitive sexual behaviors are strongly modulated by activational and organizational effects of sex steroids. The neural circuit underlying chemosensory-mediated pre- and peri-copulatory behavior involves the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), medial preoptic area (MPOA) and ventromedial hypothalamus (VMH), each area being subdivided into interconnected chemoreceptive and hormone-sensitive zones. For males, MA-BNST connections mediate chemoinvestigation whereas the MA-MPOA pathway regulates copulatory initiation. For females, MA-MPOA/BNST connections also control aspects of precopulatory behavior whereas MA-VMH projections control both precopulatory and copulatory behavior. Significant gaps in understanding remain, including the role of VMH in male behavior and MPOA in female appetitive behavior, the function of cortical amygdala, the underlying chemical architecture of this circuit and sex differences in hormonal and neurochemical regulation of precopulatory behavior.

Endogenous oxytocin is necessary for preferential Fos expression to male odors in the bed nucleus of the stria terminalis in female Syrian hamsters

Successful reproduction in mammals depends on proceptive or solicitational behaviors that enhance the probability of encountering potential mates. In female Syrian hamsters, one such behavior is vaginal scent marking. Recent evidence suggests that the neuropeptide oxytocin (OT) may be critical for regulating this behavior. Blockade of OT receptors in the bed nucleus of the stria terminalis (BNST) or the medial preoptic area (MPOA) decreases vaginal marking responses to male odors; lesion data suggest that BNST, rather than MPOA, mediates this effect. However, how OT interacts with sexual odor processing to drive preferential solicitation is not known. To address this issue, intact female Syrian hamsters were exposed to male or female odors and their brains processed for immunohistochemistry for Fos, a marker of recent neuronal activation, and OT. Additional females were injected intracerebroventricularly (ICV) with an oxytocin receptor antagonist (OTA) or vehicle, and then tested for vaginal marking and Fos responses to sexual odors. Colocalization of OT and Fos in the paraventricular nucleus of the hypothalamus was unchanged following exposure to male odors, but decreased following exposure to female odors. Following injections of OTA, Fos expression to male odors was decreased in BNST, but not in MPOA or the medial amygdala (MA). Fos expression in BNST may be functionally relevant for vaginal marking, given that there was a positive correlation between Fos expression and vaginal marking for BNST, but not MPOA or MA. Together, these data suggest that OT facilitation of neuronal activity in BNST underlies the facilitative effects of OT on solicitational responses to male odors.

Curvilinear relationships between mu-opioid receptor labeling and undirected song in male European starlings (Sturnus vulgaris)

Female-directed communication in male songbirds has been reasonably well studied; yet, relatively little is known about communication in other social contexts. Songbirds also produce song that is not clearly directed towards another individual (undirected song) when alone or in flocks. Although the precise functions of undirected song may differ across species, this type of song is considered important for flock maintenance, song learning or practice. Past studies show that undirected song is tightly coupled to analgesia and positive affective state, which are both mediated by opioid activity. Furthermore, labeling for the opioid met-enkephalin in the medial preoptic nucleus (POM) correlates positively with undirected song production. We propose that undirected song is facilitated and maintained by opioid receptor activity in the POM and other brain regions involved in affective state, analgesia, and social behavior. To provide insight into this hypothesis, we used immunohistochemistry to examine relationships between undirected song and mu-opioid receptors in male starlings. Polynomial regression analyses revealed significant inverted-U shaped relationships between measures of undirected song and mu-opioid receptor labeling in the POM, medial bed nucleus of the stria terminalis (BSTm), and periaqueductal gray (PAG). These results suggest that low rates of undirected song may stimulate and/or be maintained by mu-opioid receptor activity; however, it may be that sustained levels of mu-opioid receptor activity associated with high rates of undirected song cause mu-opioid receptor down-regulation. The results indicate that mu-opioid receptor activity in POM, BSTm, and PAG may underlie previous links identified between undirected song, analgesia, and affective state.

Chemosignals, hormones and mammalian reproduction

Many mammalian species use chemosignals to coordinate reproduction by altering the physiology and behavior of both sexes. Chemosignals prime reproductive physiology so that individuals become sexually mature and active at times when mating is most probable and suppress it when it is not. Once in reproductive condition, odors produced and deposited by both males and females are used to find and select individuals for mating. The production, dissemination and appropriate responses to these cues are modulated heavily by organizational and activational effects of gonadal sex steroids and thereby intrinsically link chemical communication to the broader reproductive context. Many compounds have been identified as "pheromones" but very few have met the expectations of that term: a unitary, species-typical substance that is both necessary and sufficient for an experience-independent behavioral or physiological response. In contrast, most responses to chemosignals are dependent or heavily modulated by experience, either in adulthood or during development. Mechanistically, chemosignals are perceived by both main and accessory (vomeronasal) olfactory systems with the importance of each system tied strongly to the nature of the stimulus rather than to the response. In the central nervous system, the vast majority of responses to chemosignals are mediated by cortical and medial amygdala connections with hypothalamic and other forebrain structures. Despite the importance of chemosignals in mammals, many details of chemical communication differ even among closely related species and defy clear categorization. Although generating much research and public interest, strong evidence for the existence of a robust chemical communication among humans is lacking.

The medial preoptic area is necessary for sexual odor preference, but not sexual solicitation, in female Syrian hamsters

Precopulatory behaviors that are preferentially directed towards opposite-sex conspecifics are critical for successful reproduction, particularly in species wherein the sexes live in isolation, such as Syrian hamsters (Mesocricetus auratus). In females, these behaviors include sexual odor preference and vaginal scent marking. The neural regulation of precopulatory behaviors is thought to involve a network of forebrain areas that includes the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), and the medial preoptic area (MPOA). Although MA and BNST are necessary for sexual odor preference and preferential vaginal marking to male odors, respectively, the role of MPOA in odor-guided female precopulatory behaviors is not well understood. To address this issue, female Syrian hamsters with bilateral, excitotoxic lesions of MPOA (MPOA-X) or sham lesions (SHAM) were tested for sexual odor investigation, scent marking, and lordosis. MPOA-X females did not investigate male odors more than female odors in an odor preference test, indicating that MPOA may be necessary for normal sexual odor preference in female hamsters. This loss of preference cannot be attributed to a sensory deficit, since MPOA-X females successfully discriminated male odors from female odors during an odor discrimination test. Surprisingly, no deficits in vaginal scent marking were observed in MPOA-X females, although these females did exhibit decreased overall levels of flank marking compared to SHAM females. Finally, all MPOA-X females exhibited lordosis appropriately. These results suggest that MPOA plays a critical role in the neural regulation of certain aspects of odor-guided precopulatory behaviors in female Syrian hamsters.

Hyper-volume of eye-contact perception and social anxiety traits

Eye-contact facilitates effective interpersonal exchange during social interactions, but can be a considerable source of anxiety for individuals with social phobia. However, the relationship between the fundamental spatial range of eye-contact perception and psychiatric traits is, to date, unknown. In this study, I analyzed the eye-contact spatial response bias and the associated pupil response, and how they relate to traits of social interaction disorders. In a face-to-face situation, 21 pairs of subjects were randomly assigned to be either viewers or perceivers. The viewer was instructed to gaze either at the perceiver's eyes, or at a predetermined point, and the perceiver was asked to indicate whether eye-contact had been established or not. I found that the perceptual volume is much larger than the actual volume of eye-contact, and that the subjective judgment of eye-contact elicited greater pupil dilation in the perceiver. Furthermore, the relationship between behavioral performance and social anxiety traits was identified. These findings provide new indications that internal traits related to lower social anxiety are potentially related to a restriction of spatial response bias for eye-contact.

Descending projections from the nucleus accumbens shell suppress activity of taste-responsive neurons in the hamster parabrachial nuclei

The parabrachial nuclei (PbN), the second central relay for the gustatory pathway, transfers taste information to various forebrain gustatory nuclei and to the gustatory cortex. The nucleus accumbens is one of the critical neural substrates of the reward system, and the nucleus accumbens shell region (NAcSh) is associated with feeding behavior. Taste-evoked neuronal responses of PbN neurons are modulated by descending projections from the gustatory nuclei in the forebrain. In the present study, we investigated whether taste-responsive neurons in the PbN project to the NAcSh and whether pontine gustatory neurons are subject to modulatory influence from the NAcSh in urethane-anesthetized hamsters. Extracellular single-unit activity was recorded in the PbN, and taste responses were confirmed by the delivery of 32 mM sucrose, NaCl, quinine hydrochloride, and 3.2 mM citric acid to the anterior tongue. The NAcSh was then stimulated (0.5 ms, ≤100 μA) bilaterally using concentric bipolar stimulating electrodes. A total of 98 taste neurons were recorded from the PbN. Eighteen neurons were antidromically invaded from the NAcSh, mostly the ipsilateral NAcSh ( n = 16). Stimulation of the ipsilateral and contralateral NAcSh suppressed the neuronal activity of 88 and 55 neurons, respectively; 52 cells were affected bilaterally. In a subset of pontine neurons tested, electrical stimulation of the NAcSh during taste stimulation also suppressed taste-evoked neuronal firing. These results demonstrated that taste-responsive neurons in the PbN not only project to the NAcSh but also are under substantial descending inhibitory influence from the bilateral NAcSh.

Dissociated functional pathways for appetitive and consummatory reproductive behaviors in male Syrian hamsters

In many species, including Syrian hamsters, the generation of male reproductive behavior depends critically on the perception of female odor cues from conspecifics in the environment. The behavioral response to these odors is mediated by a network of steroid-sensitive ventral forebrain nuclei including the medial amygdala (MA), posterior bed nucleus of the stria terminalis (BNST) and medial preoptic area (MPOA). Previous studies have demonstrated that each of these three nuclei is required for appropriate sexual behavior and that MA preferentially sends female odor information directly to BNST and MPOA. It is unknown, however, how the functional connections between MA and BNST and/or MPOA are organized to generate different aspects of reproductive behavior. Therefore, the following experiments used the asymmetrical pathway lesion technique to test the role of the functional connections between MA and BNST and/or MPOA in odor preference and copulatory behaviors. Lesions that functionally disconnected MA from MPOA eliminated copulatory behavior but did not affect odor preference. In contrast, lesions that functionally disconnected MA from BNST eliminated preference for volatile female odors but did not affect preference for directly contacted odors or copulatory behavior. These results therefore demonstrate a double dissociation in the functional connections required for attraction to volatile sexual odors and copulation and, more broadly, suggest that appetitive and consummatory reproductive behaviors are mediated by distinct neural pathways.

L-amino acid decarboxylase- and tyrosine hydroxylase-immunoreactive cells in the extended olfactory amygdala and elsewhere in the adult prairie vole brain

Neurons synthesizing dopamine (DA) are widely distributed in the brain and implicated in a tremendous number of physiological and behavioral functions, including socioreproductive behaviors in rodents. We have recently been investigating the possible involvement of sex- and species-specific TH-immunoreactive (TH-ir) cells in the male prairie vole (Microtus ochrogaster) principal bed nucleus of the stria terminalis (pBST) and posterodorsal medial amygdala (MeApd) in the chemosensory control of their monogamous pairbonding and parenting behaviors. These TH-ir cells are not immunoreactive for dopamine-beta-hydroxylase (DBH), suggesting they are not noradrenergic but possibly DAergic. A DAergic phenotype would require them to contain aromatic L-amino acid decarboxylase (AADC) and here we examined the existence of cells immunoreactive for both TH and AADC in the pBST and MeApd of adult virgin male and female prairie voles. We also investigated the presence of TH/AADC cells in the anteroventral periventricular nucleus (AVPV), medial preoptic area (MPO), arcuate nucleus (ARH), zona incerta (ZI), substantia nigra (SN) and ventral tegmental area (VTA). Among our findings were: (1) the pBST and MeApd each contained completely non-overlapping distributions of TH-ir and AADC-ir cells, (2) the AVPV contained surprisingly few AADC-ir cells and almost no TH-ir cells contained AADC-ir, (3) approximately 60% of the TH-ir cells in the MPO, ARH, and ZI also contained AADC-ir, (4) unexpectedly, only about half of TH-ir cells in the SN and VTA contained AADC-ir, and (5) notable populations of AADC-ir cells were found outside traditional monoamine-synthesizing regions, including some sites that do not contain AADC-ir cells in adult laboratory rats or cats (medial septum and cerebral cortex). In the absence of the chemical requirements to produce DA, monoenzymatic TH-ir cells in the virgin adult prairie vole pBST, MeApd, and elsewhere in their brain may instead produce L-DOPA as an end product and use it as a neurotransmitter or neuromodulator, similar to what has been observed for monoenzymatic TH-synthesizing cells in the laboratory rat brain.

The bed nucleus of the stria terminalis is critical for sexual solicitation, but not for opposite-sex odor preference, in female Syrian hamsters

Successful reproduction in vertebrates depends critically upon a suite of precopulatory behaviors that occur prior to mating. In Syrian hamsters (Mesocricetus auratus), these behaviors include vaginal scent marking and preferential investigation of male odors. The neural regulation of vaginal marking and opposite-sex odor preference likely involves an interconnected set of steroid-sensitive nuclei that includes the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), and the medial preoptic area (MPOA). For example, lesions of MA eliminate opposite-sex odor preference and reduce overall levels of vaginal marking, whereas lesions of MPOA decrease vaginal marking in response to male odors. Although BNST is densely interconnected with both MA and MPOA, little is known about the role of BNST in female precopulatory behaviors. To address this question, females received either bilateral, excitotoxic lesions of BNST (BNST-X) or sham lesions (SHAM), and were tested for scent marking and for investigatory responses to male and female odors. Whereas SHAM females vaginal marked more to male odors than female odors on two days of the estrous cycle, BNST-X females marked at equivalent levels to both odors. This deficit is not due to alterations in social odor investigation, as both BNST-X and SHAM females investigated male odors more than female odors. Finally, BNST lesions did not generally disrupt the cyclic changes in reproductive behaviors that occur across the estrous cycle. Taken together, these results demonstrate that BNST is critical for the normal expression of solicitational behaviors by females in response to male odor stimuli.

Neuroanatomical projections of the species-specific tyrosine hydroxylase-immunoreactive cells of the male prairie vole bed nucleus of the stria terminalis and medial amygdala

The principal nucleus of the bed nucleus of the stria terminalis (BSTpr) and posterodorsal part of the medial amygdalar nucleus (MEApd) are densely interconnected sites transmitting olfactory information to brain areas mediating sociosexual behaviors. In male prairie voles (Microtus ochrogaster), the BSTpr and MEApd contain hundreds of cells densely immunoreactive for tyrosine hydroxylase (TH). Such tremendous numbers of TH-immunoreactive (TH-ir) cells do not exist in other rodents examined, and studies from our laboratory suggest these cells may be part of a unique chemical network necessary for monogamous behaviors in prairie voles. To obtain information about how these TH-ir cells communicate with other sites involved in social behaviors, we first used biotinylated dextran amine (BDA) to determine sites that receive BSTpr efferents and also contain TH-ir fibers. Only in the medial preoptic area (MPO) and MEApd did we find considerable comingling of BDA-containing and TH-ir fibers. To examine if these sites receive input specifically from BSTpr TH-ir cells, the retrograde tracer Fluorogold was infused into the MPO or MEApd. Almost 80% of TH-ir projections to the MPO originated from the BSTpr or MEApd, involving about 40% of all TH-ir cells in these sites. In contrast, the MEApd received almost no input from TH-ir cells in the BSTpr, and received it primarily from the ventral tegmental area. Retrograde tracing from the BSTpr itself revealed substantial input from MEApd TH-ir cells. Thus, the male prairie vole brain contains a species-specific TH-ir network involving the BSTpr, MEApd, and MPO. By connecting brain sites involved in olfaction, sociality and motivation, this network may be essential for monogamous behaviors in this species.

Chemosensory and hormone information are relayed directly between the medial amygdala, posterior bed nucleus of the stria terminalis, and medial preoptic area in male Syrian hamsters

In many rodent species, including Syrian hamsters, the expression of appropriate social behavior depends critically on the perception and identification of conspecific odors. The behavioral response to these odors is mediated by a network of steroid-sensitive ventral forebrain nuclei including the medial amygdala (Me), posterior bed nucleus of the stria terminalis (BNST), and medial preoptic area (MPOA). Although it is well-known that Me, BNST, and MPOA are densely interconnected and each uniquely modulates odor-guided social behaviors, the degree to which conspecific odor information and steroid hormone cues are directly relayed between these nuclei is unknown. To answer this question, we injected the retrograde tracer, cholera toxin B (CTB), into the BNST or MPOA of male subjects and identified whether retrogradely-labeled cells in Me and BNST 1) expressed immediate early genes (IEGs) following exposure to male and/or female odors or 2) expressed androgen receptor (AR). Although few retrogradely-labeled cells co-localized with IEGs, a higher percentage of BNST- and MPOA-projecting cells in the posterior Me (MeP) expressed IEGs in response to female odors than to male odors. The percentage of retrogradely-labeled cells that expressed IEGs did not, however, differ between and female and male odor-exposed groups in the anterior Me (MeA), posterointermediate BNST (BNSTpi), or posteromedial BNST (BNSTpm). Many retrogradely-labeled cells co-localized with AR, and a higher percentage of retrogradely-labeled MeP and BNSTpm cells expressed AR than retrogradely-labeled MeA and BNSTpi cells, respectively. Together, these data demonstrate that Me, BNST, and MPOA interact as a functional circuit to process sex-specific odor cues and hormone information in male Syrian hamsters.

Glutamate-vasopressin interactions and the neurobiology of anabolic steroid-induced offensive aggression

In the latero-anterior hypothalamus (LAH) increased glutamate and vasopressin (AVP) activity facilitate anabolic androgenic steroid (AAS)-induced offensive aggression. In addition, adolescent AAS treatment increases the strength of glutamate-mediated connections between the LAH and the brain nucleus of stria terminalis (BNST). The current set of studies used male Syrian hamsters exposed to AAS during adolescence to examine whether increased glutamate-mediated stimulation of the BNST is dependent on LAH-AVP signaling and whether this neural pathway modulates adolescent AAS-induced offensive aggression. In the first set of AAS-treated animals offensive aggression was measured following blockade of glutamate activity within the BNST using NBQX. Then, in a second group of AAS-treated animals aggression levels were examined following simultaneous blockade of LAH-AVP activity using Manning compound and stimulation of BNST glutamate using AMPA. Lastly, the number of AVP fibers in apposition to glutamate cells was examined in AAS and control animals, using double-label immunofluorescence. The results showed that administration of NBQX into the BNST dose-dependently reduced aggressive behavior in AAS-treated animals. Further, the current results replicated previous findings showing that blockade of LAH-AVP significantly reduces aggressive behavior in AAS-treated animals. In these animals stimulation of BNST-AMPA receptors had a linear effect on aggression, where the smallest dose exacerbated the inhibitory effect of the V1a antagonist, the medium dose had no effect and the highest dose recuperated aggression to control levels. Finally when compared with control animals, AAS treatment produced a significant increase in the number of AVP fibers in apposition to LAH-glutamate cells. Overall, these results identify the BNST as a key brain region involved in aggression control and provide strong evidence suggesting that AVPergic-mediated stimulation of BNST-glutamate is a possible mechanism that facilitates aggression expression in adolescent AAS-treated animals.

Appetitive and consummatory sexual and agonistic behaviour elicits FOS expression in aromatase and vasotocin neurones within the preoptic area and bed nucleus of the stria terminalis of male domestic chickens

Some components of male sexual and agonistic behaviours are considered to be regulated by the same neurocircuitry in the medial preoptic nucleus (POM) and the medial portion of bed nucleus of the stria terminalis (BSTM). To better understand this neurocircuitry, numbers of aromatase- (ARO) or arginine vasotocin- (AVT) immunoreactive (ir) neurones expressing immediate early gene protein FOS were compared in the POM and BSTM of male chickens following sexual or agonistic behaviours. Observations were made on males showing: (i) appetitive (courtship) and consummatory (copulation) sexual behaviours; (ii) only appetitive sexual behaviour, or (iii) displaying agonistic behaviour toward other males. Control males were placed on their own in the observation pen, or only handled. In the POM, appetitive sexual behaviour increased ARO+FOS colocalisation, whereas agonistic behaviour decreased the number of visible ARO-ir cells. In the dorsolateral subdivision of BSTM (BSTM1), appetitive sexual behaviour also increased ARO+FOS colocalisation, although the numbers of visible ARO-ir and AVT-ir cells were not altered by sexual or agonistic behaviours. In the ventromedial BSTM (BSTM2), appetitive sexual behaviour increased ARO+FOS and AVT+FOS colocalisation, and all behaviours decreased the number of visible ARO-ir cells, particularly in males expressing consummatory sexual behaviour. Positive correlations were found between numbers of cells with ARO+FOS and AVT+FOS colocalisation in both subdivisions of the BSTM. Waltzing frequency was positively correlated with ARO+FOS colocalisation in the lateral POM, and in both subdivisions of the BSTM in males expressing sexual behaviour. Waltzing frequency in males expressing agonistic behaviour was negatively correlated with the total number of visible ARO-ir cells in the lateral POM and BSTM2. These observations suggest a key role for ARO and AVT neurones in BSTM2 in the expression of appetitive sexual behaviour, and differential roles for ARO cells in the POM and BSTM in the regulation of components of sexual and agonistic behaviours.

The role of the medial preoptic area in appetitive and consummatory reproductive behaviors depends on sexual experience and odor volatility in male Syrian hamsters

In Syrian hamsters (Mesocricetus auratus), the expression of reproductive behavior requires the perception and discrimination of sexual odors. The behavioral response to these odors is mediated by a network of ventral forebrain nuclei, including the medial preoptic area (MPOA). The role of MPOA in male copulatory behavior has been well-studied, but less is known about the role of MPOA in appetitive aspects of male reproductive behavior. Furthermore, many previous studies that examined the role of MPOA in reproductive behavior have used large lesions that damaged other nuclei near MPOA or fibers of passage within MPOA, making it difficult to attribute post-lesion deficits in reproductive behavior to MPOA specifically. Thus, the current study used discrete, excitotoxic lesions of MPOA to test the role of this nucleus in opposite-sex odor preference and copulatory behavior in both sexually-naïve and sexually-experienced males. Lesions of MPOA eliminated preference for volatile, opposite-sex odors in sexually-naïve, but not sexually-experienced, males. When males were allowed to contact the sexual odors, however, preference for female odors remained intact. Surprisingly, lesions of MPOA caused severe copulatory deficits only in sexually-naïve males, suggesting previous reports of copulatory deficits following MPOA lesions in sexually-experienced males were not due to damage to MPOA itself. Together, these results demonstrate that the role of MPOA in appetitive and consummatory aspects of reproductive behavior varies with the volatility of the sexual odors and the sexual experience of the male.

Blocking oxytocin receptors inhibits vaginal marking to male odors in female Syrian hamsters

In Syrian hamsters (Mesocricetus auratus), precopulatory behaviors such as vaginal scent marking are essential for attracting a suitable mate. Vaginal marking is dependent on forebrain areas implicated in the neural regulation of reproductive behaviors in rodents, including the medial preoptic/anterior hypothalamus (MPOA-AH). Within MPOA-AH, the neuropeptide oxytocin (OT) acts to facilitate copulation (lordosis), as well as ultrasonic vocalizations towards males. It is not known, however, if OT in this area also facilitates vaginal marking. In the present study, a specific oxytocin receptor antagonist (OTA) was injected into MPOA-AH of intact female Syrian hamsters to determine if oxytocin receptor-dependent signaling is critical for the normal expression of vaginal marking elicited by male, female, and clean odors. OTA injections significantly inhibited vaginal marking in response to male odors compared with vehicle injections. There was no effect of OTA on marking in response to either female or clean odors. When injected into the bed nucleus of the stria terminalis (BNST), a nearby region to MPOA-AH, OTA was equally effective in decreasing marking. Finally, the effects of OTA appear to be specific to vaginal marking, as OTA injections in MPOA-AH or BNST did not alter general locomotor activity, flank marking, or social odor investigation. Considered together, these results suggest that OT in MPOA-AH and/or BNST normally facilitates male odor-induced vaginal marking, providing further evidence that OT generally supports prosocial interactions among conspecifics.

Lesions of the posterior bed nucleus of the stria terminalis eliminate opposite-sex odor preference and delay copulation in male Syrian hamsters: role of odor volatility and sexual experience

In Syrian hamsters (Mesocricetus auratus), the expression of reproductive behavior requires the perception of social odors. The behavioral response to these odors is mediated by a network of ventral forebrain nuclei, including the posterior bed nucleus of the stria terminalis (pBNST). Previous studies have tested the role of the pBNST in reproductive behavior, but the use of large, fiber-damaging lesions in these studies make it difficult to attribute post-lesion deficits to the pBNST specifically. Thus, the current study used discrete, excitotoxic lesions of the pBNST to test the role of the pBNST in opposite-sex odor preference and copulatory behavior in both sexually-naive and sexually-experienced males. Lesions of the pBNST decreased sexually-naive males' investigation of volatile female odors, resulting in an elimination of opposite-sex odor preference. This elimination of preference was not due to a sensory deficit, as males with pBNST lesions were able to discriminate between odors. When, however, subjects were given sexual experience prior to pBNST lesions, their preference for volatile opposite-sex odors remained intact post-lesion. Similarly, when sexually-naive or sexually-experienced subjects were allowed to contact the social odors during the preference test, lesions of the pBNST decreased males' investigation of female odors but did not eliminate preference for opposite-sex odors, regardless of sexual experience. Finally, lesions of the pBNST delayed the copulatory sequence in sexually-naive, but not sexually-experienced, males such that they took longer to mount, intromit, ejaculate and display long intromissions. Together, these results demonstrate that the pBNST plays a unique and critical role in both appetitive and consummatory aspects of male reproductive behaviors.

Social novelty increases tyrosine hydroxylase immunoreactivity in the extended olfactory amygdala of female prairie voles

The monogamous social behaviors of prairie voles (Microtus ochrogaster) require olfactory inputs, which are processed by the posterodorsal medial amygdala (MeApd) and principal bed nucleus of the stria terminalis (pBST). The male prairie vole MeApd and pBST contain hundreds of cells densely immunoreactive for tyrosine hydroxylase (TH-ir). Female prairie voles have relatively few of these cells, but we previously found that the number of these TH-ir cells is greatly increased in females by exogenous estradiol. We here hypothesized that the number of TH-ir cells in their MeApd and pBST would also increase during the natural hormone surges associated with females' induced estrus. We found that the number of TH-ir cells in both sites did significantly increase after females cohabitated for two days with an unfamiliar male. However, this increase did not require the presence of ovaries and even tended to occur in the pBST of females cohabitating for two days with unfamiliar females. We then determined if the greater number of TH-ir cells after heterosexual pairing was transient by examining two groups of long-term pairbonded females (primiparous and multiparous), and found these females also had significantly more TH-ir cells in the pBST and/or MeApd compared to unmated controls. Thus, social novelty arising from cohabitation with unfamiliar conspecifics produces a reoccurring increase in the number of TH-ir cells in the female prairie vole extended olfactory amygdala. Ovarian hormones are not necessarily required. This increase in catecholaminergic cells may facilitate acquisition and retention of olfactory memories necessary for social recognition in this species.

Lesions that functionally disconnect the anterior and posterodorsal sub-regions of the medial amygdala eliminate opposite-sex odor preference in male Syrian hamsters (Mesocricetus auratus)

In many rodent species, such as Syrian hamsters, reproductive behavior requires neural integration of chemosensory information and steroid hormone cues. The medial amygdala (MA) processes both of these signals through anatomically distinct sub-regions; the anterior region (MeA) receives substantial chemosensory input, but contains few steroid receptor-labeled neurons, whereas the posterodorsal region (MePD) receives less chemosensory input, but contains a dense population of steroid receptors. Importantly, these sub-regions have considerable reciprocal connections, and the goal of this experiment was therefore to determine whether interactions between MeA and MePD are required for male hamsters' preference to investigate female over male odors. To functionally disconnect MeA and MePD, males received unilateral lesions of MeA and MePD within opposite brain hemispheres. Control males received either unilateral lesions of MeA and MePD within the same hemisphere or sham surgery. Odor preferences were measured using a 3-choice apparatus, which simultaneously presented female, male and clean odor stimuli; all tests were done under conditions that either prevented or allowed contact with the odor sources. Under non-contact conditions, males with asymmetrical lesions investigated female and male odors equally, whereas males in both control groups preferred to investigate female odors. Under contact conditions, all groups investigated female odors longer than male odors, although males with asymmetrical lesions displayed decreased investigation of female odors compared to sham males. These data suggest that MeA-MePD interactions are critical for processing primarily the volatile components of social odors and highlight the importance of input from the main olfactory system (MOS) to these nuclei in the regulation of reproductive behavior. More broadly, these results support the role of the MA in integrating chemosensory and hormone information, a process that may underlie social odor processing in a variety of behavioral contexts.

The organization of feedback projections in a pathway important for processing pheromonal signals

In most of the mammalian sensory systems there are massive cortical feedback projections to early processing stations. The mammalian accessory olfactory system is considered unique in several aspects. It is specialized for processing pheromonal signals and plays a critical role in regulating sociosexual behaviors. Furthermore, pheromonal signals are believed to bypass cortex and reach the hypothalamic behavioral centers after merely three forward projections. Because the organization of the feedback projections in the accessory olfactory system remains largely unclear, the importance of the feedback projections in the processing of pheromonal signals has been ignored. Here we show that in mice the feedback projections from the bed nucleus of stria terminalis (BST) and the vomeronasal amygdala to the accessory olfactory bulb (AOB) are topographically organized and use different neurotransmitters. By retrograde and anterograde tracing, we find that the feedback projection from the BST terminates in the AOB mitral cell layer, whereas that from the amygdala terminates in the AOB granule cell layer. By combining tracing, genetic labeling of GABAergic neurons, and immunostaining against a marker of glutamatergic synapses, we observe that the BST-to-AOB projection is GABAergic whereas the amygdala-to-AOB projection is glutamatergic. In addition, a substantial number of feedback neurons in the amygdala and BST express estrogen receptors. Thus, the accessory olfactory system, like other sensory systems, possesses extensive feedback projections. Moreover, our results suggest that central hormonal cues may modulate the processing of pheromonal signals at early stations through the precisely organized feedback projections.

The posteromedial cortical amygdala regulates copulatory behavior, but not sexual odor preference, in the male Syrian hamster (Mesocricetus auratus)

In rodent species, the expression of reproductive behavior relies heavily on the perception of social odors, as well as the presence of circulating steroid hormones. In the Syrian hamster, chemosensory and hormonal cues are processed within an interconnected network of ventral forebrain nuclei that regulates many aspects of social behavior. Within this network, the posteromedial cortical amygdala (PMCo) receives direct projections from the accessory olfactory bulbs and contains a dense population of steroid receptor-containing neurons. Consequently, the PMCo may be important for generating odor-guided aspects of reproductive behavior, yet little is known regarding the role of this nucleus in regulating these behaviors. Thus, the present study tested male hamsters with site-specific electrolytic lesions of the PMCo for their (a) sexual odor preference in a Y-maze apparatus, (b) sexual odor discrimination in a habituation-dishabituation task, and (c) copulatory behavior when paired with a sexually receptive female. PMCo-lesioned males preferred to investigate female odors over male odors and were able to discriminate between these odor sources. However, PMCo lesions were associated with several alterations in the male copulatory pattern. First, PMCo-lesioned males displayed increased investigation of the female's non-anogenital region, suggesting that the PMCo may be involved in directing appropriate chemosensory investigation during mating. Second, PMCo lesions altered the temporal pattern of the mating sequence, as PMCo-lesioned males took longer than Sham-lesioned males to reach sexual satiety, as indicated by the delayed expression of long intromissions. This delayed onset of satiety was associated with an increased number of ejaculations compared with Sham-lesioned males. Importantly, these data provide the first direct evidence for a functional role of the PMCo in regulating male reproductive behavior.

Neural circuitry of play fighting in golden hamsters

In hamsters, play fighting matures gradually into adult aggression. As these two behaviors share many similarities in this species, we predicted that a single neural circuitry controls their offensive component. The goal of the present study was to identify neural systems associated with offensive play fighting in male juvenile golden hamsters. The neural circuitry related to this behavior was identified through quantification of c-Fos immunolabeling. We also looked for vasopressin cells possibly associated with play fighting. We found that areas previously associated with offensive aggression in adult hamsters, including the ventrolateral hypothalamus, the medial amygdala, and the bed nucleus of the stria terminalis, also showed enhanced c-Fos expression after play fighting. In addition, vasopressin neurons in the nucleus circularis and the medial division of the supraoptic nucleus expressed enhanced c-Fos immunolabeling in juveniles after play fighting, as previously reported in adult hamsters after aggression. Finally, enhanced c-Fos expression associated with play fighting was also found in areas previously unexplored in adult hamsters, such as the prefrontal cortex. Together, our results support the hypothesis of a single core neural circuitry controlling the offensive components of play fighting and adult aggression throughout puberty in hamsters.

Anabolic androgenic steroids differentially affect social behaviors in adolescent and adult male Syrian hamsters

Anabolic androgenic steroids (AAS) are synthetic derivatives of testosterone used by over half a million adolescents in the United States for their tissue-building potency and performance-enhancing effects. AAS also affect behavior, including reports of heightened aggression and changes in sexual libido. The expression of sexual and aggressive behaviors is a function of complex interactions among hormones, social context, and the brain, which is extensively remodeled during adolescence. Thus, AAS may have different consequences on behavior during adolescence and adulthood. Using a rodent model, these studies directly compared the effects of AAS on the expression of male sexual and aggressive behaviors in adolescents and adults. Male Syrian hamsters were injected daily for 14 days with either vehicle or an AAS cocktail containing testosterone cypionate (2 mg/kg), nandrolone decanoate (2 mg/kg), and boldenone undecylenate (1 mg/kg), either during adolescence (27-41 days of age) or in adulthood (63-77 days of age). The day after the last injection, males were tested for either sexual behavior with a receptive female or agonistic behavior with a male intruder. Adolescent males treated with AAS showed significant increases in sexual and aggressive behaviors relative to vehicle-treated adolescents. In contrast, AAS-treated adults showed significantly lower levels of sexual behavior compared with vehicle-treated adults and did not show heightened aggression. Thus, adolescents, but not adults, displayed significantly higher behavioral responses to AAS, suggesting that the still-developing adolescent brain is more vulnerable than the adult brain to the adverse consequences of AAS on the nervous system and behavior.

Segregated pathways to the vomeronasal amygdala: differential projections from the anterior and posterior divisions of the accessory olfactory bulb

Apically and basally located receptor neurons in the vomeronasal sensory epithelium express G(i2 alpha)- and G(o alpha)-proteins, V1R and V2R vomeronasal receptors, project to the anterior and posterior accessory olfactory bulb and respond to different stimuli, respectively. The extent to which secondary projections from the two portions of the accessory olfactory bulb are convergent in the vomeronasal amygdala is controversial. This issue is addressed by using anterograde and retrograde tract-tracing methods in rats including electron microscopy. Injections of dextran-amines, Fluoro Gold, cholera toxin-B subunit and Fast Blue were delivered to the anterior and posterior accessory olfactory bulb, bed nucleus of the stria terminalis, dorsal anterior amygdala and bed nucleus of the accessory olfactory tract/anteroventral medial amygdaloid nucleus. We have demonstrated that, apart from common vomeronasal-recipient areas, only the anterior accessory olfactory bulb projects to the bed nucleus of the stria terminalis, medial division, posteromedial part, and only the posterior accessory olfactory bulb projects to the dorsal anterior amygdala and deep cell layers of the bed nucleus of the accessory olfactory tract and the anteroventral medial amygdaloid nucleus. These results provide evidence that, excluding areas of convergence, the V1R and V2R vomeronasal pathways project to specific areas of the amygdala. These two vomeronasal subsystems are therefore anatomically and functionally separated in the telencephalon.

Stereological sex difference during development of the magnocelluar subdivision of the medial preoptic nucleus (MPN mag)

In Syrian hamsters, reproductive behaviors are initiated in the presence of appropriate hormonal and chemosensory cues. These cues are detected and integrated within a highly conserved pathway that converges on a small nuclear group in the lateral aspect of the medial preoptic area, the magnocellular subdivision of the medial preoptic nucleus (MPN mag). The MPN mag plays a critical role in the regulation of male mating behavior--bilateral ablation of the MPN mag eliminates copulation. The MPN mag is sexually differentiated in both neuron number and density, but not in overall volume or volume of individual neurons. The current study used unbiased stereological methods to determine when the MPN mag becomes sexually differentiated. Our data indicate that the MPN mag becomes sexually dimorphic in volume and cell number after the critical period when steroid treatment induces male sexual behavior.

Sex and species differences in tyrosine hydroxylase-synthesizing cells of the rodent olfactory extended amygdala

The bed nucleus of the stria terminalis (BST) and the medial amygdala (MeA) are anatomically connected sites necessary for chemosensory regulation of social behaviors in rodents. Prairie voles (Microtus ochrogaster) are a valuable model for studying the neural regulation of social behaviors because, unlike many other rodents, they are gregarious, pair bond after copulating, and are biparental. We herein describe sex and species differences in immunoreactivity for tyrosine hydroxylase (TH), the rate-limiting enzyme for catecholamine synthesis, in the BST and MeA. Virgin male prairie voles had a large number of TH-immunoreactive cells in areas analogous to the rat principal nucleus of the BST (pBST) and the posterodorsal medial amygdala (MeAPd). Virgin female prairie voles had far fewer TH-immunoreactive cells in these sites ( approximately 17% of the number of cells as males in the pBST, approximately 35% of the number of cells in the MeAPd). A few TH-immunoreactive cells were found in the BST of male and female hamsters and meadow voles, but not in rats. The MeApd also contained a few TH-immunoreactive cells in male and female hamsters and male meadow voles, but not rats. Castration greatly reduced the number of TH-immunoreactive cells in the male prairie vole pBST and MeAPd, an effect that could be reversed with testosterone. Furthermore, treating ovariectomized females with testosterone substantially increased TH-immunoreactive cells in both sites. Therefore, a species-specific sex difference in TH expression is found in a chemosensory pathway in prairie voles. Expression of TH in these sites is influenced by circulating gonadal hormones in adults, which may be related to changes in their display of social behaviors across the reproductive cycle.

The magnocellular medial preoptic nucleus I. Sources of afferent input

The magnocellular medial preoptic nucleus plays a crucial role in the regulation of male sexual behavior in Syrian hamsters. Histological and behavioral studies suggest that the magnocellular medial preoptic nucleus regulates male mating behavior by integrating chemosensory and hormonal signals. The present study is the first to systematically identify the afferent connections of the magnocellular medial preoptic nucleus by tracing the uptake of cholera toxin B from deposits in the magnocellular medial preoptic nucleus of adult male Syrian hamsters. Our findings indicate that the magnocellular medial preoptic nucleus receives 1) chemosensory input from areas in the main and accessory olfactory pathways including the posterior medial bed nucleus of the stria terminalis, anterior medial, anterior cortical and posterior cortical nuclei of the amygdala; 2) input from steroid responsive structures such as the posterior medial nucleus of the amygdala, bed nucleus of the stria terminalis, lateral septum, anteroventral periventricular nucleus, medial preoptic nucleus, ventromedial nucleus of the hypothalamus and arcuate nucleus; 3) input from structures in the brainstem such as the subparafascicular thalamic nucleus, peripeduncular nucleus and the premamillary nucleus in the hypothalamus that carry sensory information from the genitalia. The major afferent input to the magnocellular medial preoptic nucleus was confirmed by injecting anterograde tracer biotinylated dextran amine into the anterior medical nucleus of the amygdala, the posterodorsal part of the medial nucleus of the amygdala, the posteromedial part of the bed nucleus of the stria terminalis and the posterointermediate part of the bed nucleus of the stria terminalis. Our results support the hypothesis that the magnocellular medial preoptic nucleus is part of the chemosensory pathway that receives chemosensory and hormonal input to regulate mating behavior and suggest that the magnocellular medial preoptic nucleus may utilize information from the genitalia to regulate male mating behavior.