Sexual dimorphism of the dopamine-beta-hydroxylase-immunoreactive neurons in the rat locus ceruleus

Heterogeneous organization of Locus coeruleus: An intrinsic mechanism for functional complexity

Locus coeruleus (LC) is a small nucleus located deep in the brainstem that contains the majority of central noradrenergic neurons, which provide the primary source of noradrenaline (NA) throughout the entire central nervous system (CNS).The release of neurotransmitter NA is considered to modulate arousal, sensory processing, attention, aversive and adaptive stress responses as well as high-order cognitive function and memory, with the highly ramified axonal arborizations of LC-NA neurons sending wide projections to the targeted brain areas. For over 30 years, LC was thought to be a homogeneous nucleus in structure and function due to the widespread uniform release of NA by LC-NA neurons and simultaneous action in several CNS regions, such as the prefrontal cortex, hippocampus, cerebellum, and spinal cord. However, recent advances in neuroscience tools have revealed that LC is probably not so homogeneous as we previous thought and exhibits heterogeneity in various aspects. Accumulating studies have shown that the functional complexity of LC may be attributed to its heterogeneity in developmental origin, projection patterns, topography distribution, morphology and molecular organization, electrophysiological properties and sex differences. This review will highlight the heterogeneity of LC and its critical role in modulating diverse behavioral outcomes.

The sex-dependent response to psychosocial stress and ischaemic heart disease

Stress is an important risk factor for modern chronic diseases, with distinct influences in males and females. The sex specificity of the mammalian stress response contributes to the sex-dependent development and impacts of coronary artery disease (CAD). Compared to men, women appear to have greater susceptibility to chronic forms of psychosocial stress, extending beyond an increased incidence of mood disorders to include a 2- to 4-fold higher risk of stress-dependent myocardial infarction in women, and up to 10-fold higher risk of Takotsubo syndrome-a stress-dependent coronary-myocardial disorder most prevalent in post-menopausal women. Sex differences arise at all levels of the stress response: from initial perception of stress to behavioural, cognitive, and affective responses and longer-term disease outcomes. These fundamental differences involve interactions between chromosomal and gonadal determinants, (mal)adaptive epigenetic modulation across the lifespan (particularly in early life), and the extrinsic influences of socio-cultural, economic, and environmental factors. Pre-clinical investigations of biological mechanisms support distinct early life programming and a heightened corticolimbic-noradrenaline-neuroinflammatory reactivity in females vs. males, among implicated determinants of the chronic stress response. Unravelling the intrinsic molecular, cellular and systems biological basis of these differences, and their interactions with external lifestyle/socio-cultural determinants, can guide preventative and therapeutic strategies to better target coronary heart disease in a tailored sex-specific manner.

Properties and modulation of excitatory inputs to the locus coeruleus

Excitatory inputs drive burst firing of locus coeruleus (LC) noradrenaline (NA) neurons in response to a variety of stimuli. Though a small number of glutamatergic LC afferents have been investigated, the overall landscape of these excitatory inputs is largely unknown. The current study used an optogenetic approach to isolate three glutamatergic afferents: the prefrontal cortex (PFC), lateral hypothalamus (LH) and periaqueductal grey (PAG). AAV5-DIO-ChR2 was injected into each region in male and female CaMKII-Cre mice and the properties of excitatory inputs on LC-NA cells were measured. Notably we found differences among these inputs. First, the pattern of axonal innervation differed between inputs such that LH afferents were concentrated in the posterior portion of the LC-NA somatic region while PFC afferents were denser in the medial dendritic region. Second, basal intrinsic properties varied for afferents, with LH inputs having the highest connectivity and the largest amplitude excitatory postsynaptic currents while PAG inputs had the lowest initial release probability. Third, while orexin and oxytocin had minimal effects on any input, dynorphin strongly inhibited excitatory inputs originating from the LH and PAG, and corticotrophin releasing factor (CRF) selectively inhibited inputs from the PAG. Overall, these results demonstrate that individual afferents to the LC have differing properties, which may contribute to the modularity of the LC and its ability to mediate various behavioural outcomes. KEY POINTS: Excitatory inputs to the locus coeruleus (LC) are important for driving noradrenaline neuron activity and downstream behaviours in response to salient stimuli, but little is known about the functional properties of different glutamate inputs that innervate these neurons We used a virus-mediated optogenetic approach to compare glutamate afferents from the prefrontal cortex (PFC), the lateral hypothalamus (LH) and the periaqueductal grey (PAG). While PFC was predicted to make synaptic inputs, we found that the LH and PAG also drove robust excitatory events in LC noradrenaline neurons. The strength, kinetics, and short-term plasticity of each input differed as did the extent of neuromodulation by both dynorphin and corticotrophin releasing factor. Thus each input displayed a unique set of basal properties and modulation by peptides. This characterization is an important step in deciphering the heterogeneity of the LC.

The Role of the Locus Coeruleus in Pain and Associated Stress-Related Disorders

The locus coeruleus (LC)-noradrenergic system is the main source of noradrenaline in the central nervous system and is involved intensively in modulating pain and stress-related disorders (e.g., major depressive disorder and anxiety) and in their comorbidity. However, the mechanisms involving the LC that underlie these effects have not been fully elucidated, in part owing to the technical difficulties inherent in exploring such a tiny nucleus. However, novel research tools are now available that have helped redefine the LC system, moving away from the traditional view of LC as a homogeneous structure that exerts a uniform influence on neural activity. Indeed, innovative techniques such as DREADDs (designer receptors exclusively activated by designer drugs) and optogenetics have demonstrated the functional heterogeneity of LC, and novel magnetic resonance imaging applications combined with pupillometry have opened the way to evaluate LC activity in vivo. This review aims to bring together the data available on the efferent activity of the LC-noradrenergic system in relation to pain and its comorbidity with anxiodepressive disorders. Acute pain triggers a robust LC stress response, producing spinal cord-mediated endogenous analgesia while promoting aversion, vigilance, and threat detection through its ascending efferents. However, this protective biological system fails in chronic pain, and LC activity produces pain facilitation, anxiety, increased aversive memory, and behavioral despair, acting at the medulla, prefrontal cortex, and amygdala levels. Thus, the activation/deactivation of specific LC projections contributes to different behavioral outcomes in the shift from acute to chronic pain.

Acute vagus nerve stimulation enhances reversal learning in rats

Cognitive flexibility is a prefrontal cortex-dependent neurocognitive process that enables behavioral adaptation in response to changes in environmental contingencies. Electrical vagus nerve stimulation (VNS) enhances several forms of learning and neuroplasticity, but its effects on cognitive flexibility have not been evaluated. In the current study, a within-subjects design was used to assess the effects of VNS on performance in a novel visual discrimination reversal learning task conducted in touchscreen operant chambers. The task design enabled simultaneous assessment of acute VNS both on reversal learning and on recall of a well-learned discrimination problem. Acute VNS delivered in conjunction with stimuli presentation during reversal learning reliably enhanced learning of new reward contingencies. Enhancement was not observed, however, if VNS was delivered during the session but was not coincident with presentation of to-be-learned stimuli. In addition, whereas VNS delivered at 30 HZ enhanced performance, the same enhancement was not observed using 10 or 50 Hz. Together, these data show that acute VNS facilitates reversal learning and indicate that the timing and frequency of the VNS are critical for these enhancing effects. In separate rats, administration of the norepinephrine reuptake inhibitor atomoxetine also enhanced reversal learning in the same task, consistent with a noradrenergic mechanism through which VNS enhances cognitive flexibility.

The role of medial prefrontal cortex projections to locus ceruleus in mediating the sex differences in behavior in mice with inflammatory pain

We tested the hypothesis that the cognitive impairment associated with inflammatory pain may result from dysregulation of the top-down control of locus ceruleus's (LC) activity by the medial prefrontal cortex (mPFC). Injection of complete Freund's adjuvant (CFA) served as a model for inflammatory pain. The CFA injection decreased the thermal thresholds in both sexes but only the male mice showed increased anxiety-like behavior and diminished cognition, while the females were not affected. Increased calcium fluorescence, a marker for neuronal activity, was detected by photometry in the mPFC of males but not in females with CFA. Next, while chemogenetic inhibition of the projections from the mPFC to the LC improved the object recognition memory of males with pain, the inhibition of the mPFC to LC pathway in female mice produced anxiolysis and spatial memory deficits. The behavior results prompted us to compare the reciprocal innervation of mPFC and LC between the sexes. We used an anterograde transsynaptic tagging technique, which relies on postsynaptic cre transfer, to assess the innervation of LC by mPFC efferents. The males showed a higher rate of postsynaptic cre transfer into LC neurons from mPFC efferents than the females. And vice versa, a retrograde tracing experiment demonstrated that LC to mPFC projection neurons were more numerous in females when compared to males. In conclusion, we provide evidence that subtle differences in the reciprocal neuronal circuit between the LC and mPFC may contribute to sex differences associated with the adverse cognitive effects of inflammatory pain.

Lateral hypothalamus-projecting noradrenergic locus coeruleus pathway modulates binge-like ethanol drinking in male and female TH-ires-cre mice

A growing body of literature implicates noradrenergic (NE) signaling in the modulation of ethanol consumption. However, relatively few studies have detailed specific brain pathways that mediate NE-associated binge-like ethanol consumption. To begin to fill this gap in the literature, male and female C57BL6/J and TH-ires-cre mice underwent pharmacological and chemogenetic testing, respectively, in combination with "drinking in the dark" procedures to model binge-like consumption of ethanol or sucrose solutions. First, we showed that intraperitoneal administration of the NE reuptake inhibitor, reboxetine, blunted binge-like ethanol intake in C57BL6/J mice. Chemogenetic activation of locus coeruleus (LC) tyrosine hydroxylase (TH)-expressing neurons blunted binge-like ethanol intake regardless of sex. Chemogenetic activation of LC projections to the lateral hypothalamus (LH), a region implicated in ethanol consumption, blunted binge-like ethanol drinking without altering sucrose intake in ethanol-experienced or ethanol-naïve mice. In C57BL/6 J mice, LH-targeted microinfusion of an α1-adrenergic receptor (AR) agonist blunted binge-like ethanol intake across both sexes, while LH infusion of a β-AR agonist blunted binge-like ethanol intake in females exclusively. Finally, in mice with high baseline ethanol intake both an α1- AR agonist and an α-2 AR antagonist blunted binge-like ethanol intake. The present results provide novel evidence that increased NE tone in a circuit arising from the LC and projecting to the LH reduces binge-like ethanol drinking in mice, and may represent a novel approach to treating binge or heavy drinking prior to the development of dependence. This article is part of the special Issue on "Neurocircuitry Modulating Drug and Alcohol Abuse".

Neurobiological and Hormonal Mechanisms Regulating Women's Sleep

Sleep is crucial for optimal well-being, and sex differences in sleep quality have significant implications for women's health. We review the current literature on sex differences in sleep, such as differences in objective and subjective sleep measures and their relationship with aging. We then discuss the convincing evidence for the role of ovarian hormones in regulating female sleep, and survey how these hormones act on a multitude of brain regions and neurochemicals to impact sleep. Lastly, we identify several important areas in need of future research to narrow the knowledge gap and improve the health of women and other understudied populations.

Sex Differences in the Inflammatory Consequences of Stress: Implications for Pharmacotherapy

Women are at significantly greater risk of developing stress-related disorders such as depression. The increased risk begins during puberty and continues throughout life until menopause, suggesting a role for ovarian hormones in this increased susceptibility. Importantly, inflammation has been gaining momentum in its role in the pathogenesis of depression. Herein, clinical and preclinical studies have been reviewed to better understand how sex differences within the immune system may contribute to exaggerated risk of depression in females. First, studies that investigate the ability of psychologic stress episodes to engage the inflammatory systems both in the brain and periphery are reviewed with a special focus on sex-specific effects. Moreover, studies are discussed that identify whether imbalanced inflammatory milieu contributes to the development of depression in males versus females and whether these effects are regulated by estradiol. Importantly, we propose a locus coeruleus-norepinephrine-cytokine circuit as a conduit through which stress could increase stress susceptibly in females. Finally, the anti-inflammatory capacity of traditional and nontraditional antidepressants is investigated, with the goal of providing a better understanding of pharmacotherapeutics to enhance strategies to personalize antidepressant treatments between the sexes. The studies reviewed herein strongly support the need for further studies to elucidate whether females are especially sensitive to anti-inflammatory compounds as adjuvants to traditional therapies. SIGNIFICANCE STATEMENT: Women have hve an increased risk of developing stress-related disorders such as depression. In this review, literature from clinical and preclinical studies are integrated to define sex differences in stress-induced inflammatory responses as a potential source for the etiology of sex differences in depressive disorders. Moreover, the anti-inflammatory capacity of traditional and nontraditional antidepressants is reviewed to inform on potential pharmacotherapeutic strategies to personalize antidepressant therapy in a sex-dependent manner.

Sex differences in the development of anxiodepressive-like behavior of mice subjected to sciatic nerve cuffing

We investigated the contribution of nucleus locus ceruleus (LC) to the development of pain-associated affective behavior. Mice of both sexes were subjected to sciatic nerve cuffing, a model of peripheral nerve injury, and monitored for 45 days. Although the thermal and mechanical thresholds were equally decreased in both males and females, only the male mice developed anxiodepressive-like behavior, which was complemented by suppressed hippocampal neurogenesis. Furthermore, the LC activity was lower in males when compared with females subjected to sciatic cuffing. Next, we used a chemogenetic approach to modulate the activity of LC projections to the dentate gyrus of the hippocampus in females without cuffs and in males with sciatic cuffs. Sustained inhibition of the LC projections to the dentate gyrus for 15 days induced anxiodepressive-like behavior and reduced the hippocampal neurogenesis in females. Activation of the LC projections to the dentate gyrus for 15 days prevented the development of anxiodepressive-like behavior and increased the hippocampal neurogenesis in males with cuffs. In sum, we demonstrated that the LC projections to the hippocampus link the sensory to the affective component of neuropathic injury and that the female mice are able to dissociate the nociception from affect by maintaining robust LC activity. The work provides evidence that sex differences in LC response to pain determine the sex differences in the development of pain phenotype.

Effects of Prenatal Exposure to a Low-Dose of Bisphenol A on Sex Differences in Emotional Behavior and Central Alpha2-Adrenergic Receptor Binding

Prenatal exposure to bisphenol A (BPA) influences the development of sex differences neurologically and behaviorally across many species of vertebrates. These effects are a consequence of BPA’s estrogenic activity and its ability to act as an endocrine disrupter even, at very low doses. When exposure to BPA occurs during critical periods of development, it can interfere with the normal activity of sex steroids, impacting the fate of neurons, neural connectivity and the development of brain regions sensitive to steroid activity. Among the most sensitive behavioral targets of BPA action are behaviors that are characterized by a sexual dimorphism, especially emotion and anxiety related behaviors, such as the amount of time spent investigating a novel environment, locomotive activity and arousal. Moreover, in some species of rodents, BPA exposure affected males’ sexual behaviors. Interestingly, these behaviors are at least in part modulated by the catecholaminergic system, which has been reported to be a target of BPA action. In the present study we investigated the influence of prenatal exposure of mice to a very low single dose of BPA on emotional and sexual behaviors and on the density and binding characteristics of alpha₂ adrenergic receptors. Alpha₂ adrenergic receptors are widespread in the central nervous system and they can act as autoreceptors, inhibiting the release of noradrenaline and other neurotransmitters from presynaptic terminals. BPA exposure disrupted sex differences in behavioral responses to a novel environment, but did not affect male mice sexual behavior. Importantly, BPA exposure caused a change in the binding affinity of alpha₂ adrenergic receptors in the locus coeruleus and medial preoptic area (mPOA) and it eliminated the sexual dimorphism in the density of the receptors in the mPOA.

Molecular and Functional Sex Differences of Noradrenergic Neurons in the Mouse Locus Coeruleus

Preclinical work has long focused on male animals, though biological sex clearly influences risk for certain diseases, including many psychiatric disorders. Such disorders are often treated by drugs targeting the CNS norepinephrine system. Despite roles for noradrenergic neurons in behavior and neuropsychiatric disease models, their molecular characterization has lagged. We profiled mouse noradrenergic neurons in vivo, defining over 3,000 high-confidence transcripts expressed therein, including druggable receptors. We uncovered remarkable sex differences in gene expression, including elevated expression of the EP3 receptor in females-which we leverage to illustrate the behavioral and pharmacologic relevance of these findings-and of Slc6a15 and Lin28b, both major depressive disorder (MDD)-associated genes. Broadly, we present a means of transcriptionally profiling locus coeruleus under baseline and experimental conditions. Our findings underscore the need for preclinical work to include both sexes and suggest that sex differences in noradrenergic neurons may underlie behavioral differences relevant to disease.

Copper and the brain noradrenergic system

Copper (Cu) plays an essential role in the development and function of the brain. In humans, genetic disorders of Cu metabolism may cause either severe Cu deficiency (Menkes disease) or excessive Cu accumulation (Wilson disease) in the brain tissue. In either case, the loss of Cu homeostasis results in catecholamine misbalance, abnormal myelination of neurons, loss of normal brain architecture, and a spectrum of neurologic and/or psychiatric manifestations. Several metabolic processes have been identified as particularly sensitive to Cu dis-homeostasis. This review focuses on the role of Cu in noradrenergic neurons and summarizes the current knowledge of mechanisms that maintain Cu homeostasis in these cells. The impact of Cu misbalance on catecholamine metabolism and functioning of noradrenergic system is discussed.

An age- and sex-dependent role of catecholaminergic neurons in the control of breathing and hypoxic chemoreflex during postnatal development

The respiratory system undergoes significant development during the postnatal phase. Maturation of brainstem catecholaminergic (CA) neurons is important for the control and modulation of respiratory rhythmogenesis, as well as for chemoreception in early life. We demonstrated an inhibitory role for CA neurons in CO₂ chemosensitivity in neonatal and juvenile male and female rats, but information regarding their role in the hypoxic ventilatory response (HVR) is lacking. We evaluated the contribution of brainstem CA neurons in the HVR during postnatal (P) development (P7-8, P14-15 and P20-21) in male and female rats through chemical injury with conjugated saporin anti-dopamine beta-hydroxylase (DβH-SAP, 420 ng·μL^-1) injected in the fourth ventricle. Ventilation (V̇_E) and oxygen consumption were recorded one week after the lesion in unanesthetized rats during exposure to normoxia and hypoxia. Hypoxia reduced breathing variability in P7-8 control rats of both sexes. At P7-8, the HVR for lesioned males and females increased 27% and 24%, respectively. Additionally, the lesion reduced the normoxic breathing variability in both sexes at P7-8, but hypoxia partially reverted this effect. For P14-15, the increase in V̇_E during hypoxia was 30% higher for male and 24% higher for female lesioned animals. A sex-specific difference was detected at P20-21, as lesioned males exhibited a 24% decrease in the HVR, while lesioned females experienced a 22% increase. Furthermore, the hypoxia-induced body temperature reduction was attenuated in P20-21 lesioned females. We conclude that brainstem CA neurons modulate the HRV during the postnatal phase, and possibly thermoregulation during hypoxia.

Sex differences in breathing

Breathing is a vital behavior that ensures both the adequate supply of oxygen and the elimination of CO₂, and it is influenced by many factors. Despite that most of the studies in respiratory physiology rely heavily on male subjects, there is much evidence to suggest that sex is an important factor in the respiratory control system, including the susceptibility for some diseases. These different respiratory responses in males and females may be related to the actions of sex hormones, especially in adulthood. These hormones affect neuromodulatory systems that influence the central medullary rhythm/pontine pattern generator and integrator, sensory inputs to the integrator and motor output to the respiratory muscles. In this article, we will first review the sex dependence on the prevalence of some respiratory-related diseases. Then, we will discuss the role of sex and gonadal hormones in respiratory control under resting conditions and during respiratory challenges, such as hypoxia and hypercapnia, and whether hormonal fluctuations during the estrous/menstrual cycle affect breathing control. We will then discuss the role of the locus coeruleus, a sexually dimorphic CO₂/pH-chemosensitive nucleus, on breathing regulation in males and females. Next, we will highlight the studies that exist regarding sex differences in respiratory control during development. Finally, the few existing studies regarding the influence of sex on breathing control in non-mammalian vertebrates will be discussed.

The gist and details of sex differences in cognition and the brain: How parallels in sex differences across domains are shaped by the locus coeruleus and catecholamine systems

Across three different domains, there are similar sex differences in how men and women process information. There tends to be a male advantage in attending to and remembering the gist (essential central information of a scene or situation), but a female advantage in attending to and remembering the details (non-essential peripheral information of a scene or situation). This is seen in emotional memory, where emotion enhances gist memory more for males than for females, but enhances detail memory more for females than for males. It also occurs in spatial memory, where men tend to notice and remember the gist of where they or objects are in space, allowing them to more flexibly manipulate themselves or objects within that space, whereas women tend to recall the details of the space around them, allowing them to accurately remember the locations of objects. Finally, such sex differences have also been noted in perception of stimuli such that men attend to global aspects of stimuli (such as a large letter E) more than women, whereas women attend more to the local aspects (such as the many smaller letter Ts making up the E). We review the parallel sex differences seen across these domains in this paper and how they relate to the different brain systems involved in each of these task domains. In addition, we discuss how sex differences in evolutionary pressures and in the locus coeruleus and norepinephrine system may account for why parallel sex differences occur across these different task domains.

Brainstem catecholaminergic neurones and breathing control during postnatal development in male and female rats

KEY POINTS

The brainstem catecholaminergic (CA) modulation on ventilation changes with development. We determined the role of the brainstem CA system in ventilatory control under normocapnic and hypercapnic conditions during different phases of development [postnatal day (P)7-8, P14-15 and P20-21] in male and female Wistar rats. Brainstem CA neurones produce a tonic inhibitory drive that affects breathing frequency in P7-8 rats and provide an inhibitory drive during hypercapnic conditions in both males and females at P7-8 and P14-15. In pre-pubertal rats, brainstem CA neurones become excitatory for the CO₂ ventilatory response in males but remain inhibitory in females. Diseases such as sudden infant death syndrome, congenital central hypoventilation syndrome and Rett syndrome have been associated with abnormalities in the functioning of CA neurones; therefore, the results of the present study contribute to a better understanding of this system.

ABSTRACT

The respiratory network undergoes significant development during the postnatal phase, including the maturation of the catecholaminergic (CA) system. However, postnatal development of this network and its effect on the control of pulmonary ventilation ( V̇E ) is not fully understood. We investigated the involvement of brainstem CA neurones in respiratory control during postnatal development [postnatal day (P)7-8, P14-15 and P20-21], in male and female rats, through chemical injury with conjugated saporin anti-dopamine β-hydroxylase (DβH-SAP). Thus, DβH-SAP (420 ng μL^-1 ), saporin (SAP) or phosphate buffered solution (PBS) was injected into the fourth ventricle of neonatal Wistar rats of both sexes. V̇E and oxygen consumption were recorded 1 week after the injections in unanaesthetized neonatal and juvenile rats during room air and hypercapnia. The resting ventilation was higher in both male and female P7-8 lesioned rats by 33%, with a decrease in respiratory variability being observed in males. The hypercapnic ventilatory response (HCVR) was altered in male and female lesioned rats at all postnatal ages. At P7-8, the HCVR for males and females was increased by 37% and 30%, respectively. For both sexes at P14-15 rats, the increase in V̇E during hypercapnia was 37% higher for lesioned rats. A sex-specific difference in HCRV was observed at P20-21, with lesioned males showing a 33% decrease, and lesioned females showing an increase of 33%. We conclude that brainstem CA neurones exert a tonic inhibitory effect on V̇E in the early postnatal days of the life of a rat, increase variability in P7-8 males and modulate HCRV during the postnatal phase.

Sex differences in stress regulation of arousal and cognition

There are sex differences in the prevalence and presentation of many psychiatric disorders. For example, posttraumatic stress disorder (PTSD) and major depression are more common in women than men, and women with these disorders present with more hyperarousal symptoms than men. In contrast, attention deficit hyperactivity disorder (ADHD) and schizophrenia are more common in men than women, and men with these disorders have increased cognitive deficits compared to women. A shared feature of the aforementioned psychiatric disorders is the contribution of stressful events to their onset and/or severity. Here we propose that sex differences in stress responses bias females towards hyperarousal and males towards cognitive deficits. Evidence from clinical and preclinical studies is detailed. We also describe underlying neurobiological mechanisms. For example, sex differences in stress receptor signaling and trafficking in the locus coeruleus-arousal center are detailed. In learning circuits, evidence for sex differences in dendritic morphology is provided. Finally, we describe how evaluating sex-specific mechanisms for responding to stress in female and male rodents can lead to better treatments for stress-related psychiatric disorders.

Sex and estrous cycle influence diazepam effects on anxiety and memory: Possible role of progesterone

Studies with rodents and humans show the relationship between female sex hormones and cognitive/emotional tasks. However, despite the greater incidence of anxiety disorders in women, the data are still inconclusive regarding the mechanisms related to this phenomenon. We evaluated the effects of a classical anxiolytic/amnestic drug (diazepam; DZP) on female (at different estrous cycle phases) and male rats tested in the plus-maze discriminative avoidance task (PMDAT), that allows the concomitant evaluation of memory and anxiety-like behavior. Further, in order to investigate the role of progesterone and its metabolites in the effects of DZP in the PMDAT, female rats were pre-treated with the progesterone receptor antagonist mifepristone or the 5-alpha-reductase inhibitor finasteride. The main findings were: (1) DZP caused memory impairment and anxiolysis in both sexes, but only the highest dose induced the anxiolytic effect in females; (2) females in proestrus did not present the amnestic and anxiolytic effects of DZP (at 2.0 and 4.0mg/kg, respectively) and (3) the co-administration of mifepristone reestablished both amnestic and anxiolytic effects of DZP, while finasteride reinstated the amnestic effect in proestrus female rats. These results suggest that changes in the endogenous levels of progesterone and its metabolites are important in the modulation of emotional/cognitive behavior in female rats. Based on the influence on different aspects of DZP action, the mechanisms related to this modulation are probably linked to GABAergic transmission, but this point remains to be investigated. Further, the variation in therapeutic and adverse effects of DZP depending on sex and hormonal state is of great relevance considering the higher prevalence of anxiety disorders in women.

Early life stress elicits visceral hyperalgesia and functional reorganization of pain circuits in adult rats

Early life stress (ELS) is a risk factor for developing functional gastrointestinal disorders, and has been proposed to be related to a central amplification of sensory input and resultant visceral hyperalgesia. We sought to characterize ELS-related changes in functional brain responses during acute noxious visceral stimulation. Neonatal rats (males/females) were exposed to limited bedding (ELS) or standard bedding (controls) on postnatal days 2–9. Age 10–11 weeks, animals were implanted with venous cannulas and transmitters for abdominal electromyography (EMG). Cerebral blood flow (rCBF) was mapped during colorectal distension (CRD) using [¹⁴C]-iodoantipyrine autoradiography, and analyzed in three-dimensionally reconstructed brains by statistical parametric mapping and functional connectivity. EMG responses to CRD were increased after ELS, with no evidence of a sex difference. ELS rats compared to controls showed a greater significant positive correlation of EMG with amygdalar rCBF. Factorial analysis revealed a significant main effect of ‘ELS’ on functional activation of nodes within the pain pathway (somatosensory, insular, cingulate and prefrontal cortices, locus coeruleus/lateral parabrachial n. [LC/LPB], periaqueductal gray, sensory thalamus), as well as in the amygdala, hippocampus and hypothalamus. In addition, ELS resulted in an increase in the number of significant functional connections (i.e. degree centrality) between regions within the pain circuit, including the amygdala, LC/LPB, insula, anterior ventral cingulate, posterior cingulate (retrosplenium), and stria terminalis, with decreases noted in the sensory thalamus and the hippocampus. Sex differences in rCBF were less broadly expressed, with significant differences noted at the level of the cortex, amygdala, dorsal hippocampus, raphe, sensory thalamus, and caudate-putamen. ELS showed a sexually dimorphic effect (‘Sex x ELS’ interaction) at the LC/LPB complex, globus pallidus, hypothalamus, raphe, septum, caudate-putamen and cerebellum. Our results suggest that ELS alters functional activation of the thalamo-cortico-amydala pathway, as well as the emotional-arousal network (amygdala, locus coeruleus), with evidence that ELS may additionally show sexually dimorphic effects on brain function.

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Early life stress (ELS) elicits visceral hyperalgesia in adult offspring.

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ELS alters functional activation of the thalamo-cortico-amydala pathway.

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ELS shows a sexually dimorphic effects on brain function.

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Functional imaging-based endpoints promise improved animal-to-human translation.

Sex differences in the locus coeruleus-norepinephrine system and its regulation by stress

Women are more likely than men to suffer from post-traumatic stress disorder (PTSD) and major depression. In addition to their sex bias, these disorders share stress as an etiological factor and hyperarousal as a symptom. Thus, sex differences in brain arousal systems and their regulation by stress could help explain increased vulnerability to these disorders in women. Here we review preclinical studies that have identified sex differences in the locus coeruleus (LC)-norepinephrine (NE) arousal system. First, we detail how structural sex differences in the LC can bias females towards increased arousal in response to emotional events. Second, we highlight studies demonstrating that estrogen can increase NE in LC target regions by enhancing the capacity for NE synthesis, while reducing NE degradation, potentially increasing arousal in females. Third, we review data revealing how sex differences in the stress receptor, corticotropin releasing factor 1 (CRF1), can increase LC neuronal sensitivity to CRF in females compared to males. This effect could translate into hyperarousal in women under conditions of CRF hypersecretion that occur in PTSD and depression. The implications of these sex differences for the treatment of stress-related psychiatric disorders are discussed. Moreover, the value of using information regarding biological sex differences to aid in the development of novel pharmacotherapies to better treat men and women with PTSD and depression is also highlighted. This article is part of a Special Issue entitled SI: Noradrenergic System.

The noradrenergic projection from the locus coeruleus to the cochlear root neurons in rats

The cochlear root neurons (CRNs) are key components of the primary acoustic startle circuit; mediating auditory alert and escape behaviors in rats. They receive a great variety of inputs which serve to elicit and modulate the acoustic startle reflex (ASR). Recently, our group has suggested that CRNs receive inputs from the locus coeruleus (LC), a noradrenergic nucleus which participates in attention and alertness. Here, we map the efferent projection patterns of LC neurons and confirm the existence of the LC-CRN projection using both anterograde and retrograde tract tracers. Our results show that each LC projects to the CRNs of both sides with a clear ipsilateral predominance. The LC axons terminate as small endings distributed preferentially on the cell body and primary dendrites of CRNs. Using light and confocal microscopy, we show a strong immunoreactivity for tyrosine hydroxylase and dopamine β-hydroxylase in these terminals, indicating noradrenaline release. We further studied the noradrenergic system using gene expression analysis (RT-qPCR) and immunohistochemistry to detect specific noradrenergic receptor subunits in the cochlear nerve root. Our results indicate that CRNs contain a noradrenergic receptor profile sufficient to modulate the ASR, and also show important gender-specific differences in their gene expression. 3D reconstruction analysis confirms the presence of sexual dimorphism in the density and distribution of LC neurons. Our study describes a coerulean noradrenergic projection to the CRNs that might contribute to neural processes underlying sensory gating of the ASR, and also provides an explanation for the gender differences observed in the behavioral paradigm.

Early undernutrition decreases the number of neurons in the locus coeruleus of rats

The effects of perinatal undernutrition on the number of neurons and apoptotic cells of the locus coeruleus (LC) of female and male rats at postpartum days 7, 12, 20, 30 and 60 were studied. Undernutrition reduces the number of neurons in both sexes without affecting cell death, as indicated by the ratio of apoptotic cells to neurons. The data suggest that in the undernourished groups lower rates of neurogenesis and proliferation (neurogenetic/proliferation rates) might avoid these animals achieving the number of LC neurons as in the control subjects. Although food restriction in both sexes apparently provokes the loss of cells, the effect does not appear to be equal in females and males, as shown by post weaning food rehabilitation. The results suggest that severe food deprivation may interfere with the ontogenetic processes underlying neuronal differentiation of the LC. Morphological damage in the LC due to undernutrition might alter the physiology of sexual and/or feeding behaviours in which this structure is implicated.

Noradrenergic alpha-2 receptor modulators in the ventral bed nucleus of the stria terminalis: effects on anxiety behavior in postpartum and virgin female rats

Emotional hyperreactivity can inhibit maternal responsiveness in female rats and other animals. Maternal behavior in postpartum rats is disrupted by increasing norepinephrine release in the ventral bed nucleus of the stria terminalis (BSTv) with the α2-autoreceptor antagonist, yohimbine, or the more selective α2-autoreceptor antagonist, idazoxan (Smith et al., 2012). Because high noradrenergic activity in the BSTv can also increase anxiety-related behaviors, increased anxiety may underlie the disrupted mothering of dams given yohimbine or idazoxan. To assess this possibility, anxiety-related behaviors in an elevated plus maze were assessed in postpartum rats after administration of yohimbine or idazoxan. It was further assessed if the α2-autoreceptor agonist clonidine (which decreases norepinephrine release) would, conversely, reduce dams' anxiety. Groups of diestrous virgins were also examined. It was found that peripheral or intra-BSTv yohimbine did increase anxiety-related behavior in postpartum females. However, BSTv infusion of idazoxan did not reproduce yohimbine's anxiogenic effects and anxiety was not reduced by peripheral or intra-BSTv clonidine. Because yohimbine is a weak 5HT1A receptor agonist, other groups of females received BSTv infusion of the 5HT1A receptor agonist 8OH-DPAT, but it did not alter their anxiety-related behavior. Lastly, levels of norepinephrine and serotonin in tissue punches from the BSTv did not differ between postpartum and diestrous rats, but serotonin turnover was lower in mothers. These results suggest that the impaired maternal behavior after BSTv infusion of yohimbine or idazoxan cannot both be readily explained by an increase in dams' anxiety, and that BSTv α2-autoreceptor modulation alone has little influence on anxiety-related behaviors in postpartum or diestrous rats.

Developmental expression of orphan G protein-coupled receptor 50 in the mouse brain

Mental disorders have a complex etiology resulting from interactions between multiple genetic risk factors and stressful life events. Orphan G protein-coupled receptor 50 (GPR50) has been identified as a genetic risk factor for bipolar disorder and major depression in women, and there is additional genetic and functional evidence linking GPR50 to neurite outgrowth, lipid metabolism, and adaptive thermogenesis and torpor. However, in the absence of a ligand, a specific function has not been identified. Adult GPR50 expression has previously been reported in brain regions controlling the HPA axis, but its developmental expression is unknown. In this study, we performed extensive expression analysis of GPR50 and three protein interactors using rt-PCR and immunohistochemistry in the developing and adult mouse brain. Gpr50 is expressed at embryonic day 13 (E13), peaks at E18, and is predominantly expressed by neurons. Additionally we identified novel regions of Gpr50 expression, including brain stem nuclei involved in neurotransmitter signaling: the locus coeruleus, substantia nigra, and raphe nuclei, as well as nuclei involved in metabolic homeostasis. Gpr50 colocalizes with yeast-two-hybrid interactors Nogo-A, Abca2, and Cdh8 in the hypothalamus, amygdala, cortex, and selected brain stem nuclei at E18 and in the adult. With this study, we identify a link between GPR50 and neurotransmitter signaling and strengthen a likely role in stress response and energy homeostasis.

Sexual dimorphism in locus coeruleus dendritic morphology: a structural basis for sex differences in emotional arousal

Stress-related psychiatric disorders, such as depression and anxiety, affect a disproportionate number of women. We previously demonstrated that the major brain norepinephrine (NE)-containing nucleus, locus coeruleus (LC) is more sensitive to stressors and to the stress-related neuropeptide, corticotropin-releasing factor (CRF) in female compared to male rats. Because the LC-NE system is a stress-responsive system that is thought to be dysregulated in affective disorders, sex differences in LC structure or function could play a role in female vulnerability to these diseases. The present study used different approaches to compare LC dendritic characteristics between male and female rats. Immunofluorescence labeling of tyrosine hydroxylase, the norepinephrine synthetic enzyme, revealed that LC dendrites of female rats extend further into the peri-LC region, covering a significantly greater area than those of males. Optical density measurements of dendrites in the peri-LC revealed increased dendritic density in females compared to their male counterparts. Additionally, immunoreactivity for synaptophysin, a synaptic vesicle protein, was significantly greater in the LC in female rats, suggesting an increased number of synaptic contacts onto LC processes. Individual LC neurons were juxtacellularly labeled with neurobiotin in vivo for morphological analysis. LC dendritic trees of females were longer and had more branch points and ends. Consistent with this, Sholl analysis determined that, compared to males, LC dendrites of females had a more complex pattern of branching. The greater dendritic extension and complexity seen in females predicts a higher probability of communication with diverse afferents that terminate in the peri-LC. This may be a structural basis for heightened arousal in females, an effect which may, in part, account for the sex bias in incidence of stress-related psychiatric disorders.

Varied mechanisms of oestradiol-mediated regulation of dopamine β-hydroxylase transcription

Experiments performed in vivo and in cell culture have demonstrated that oestradiol induces dopamine β-hydroxylase (DBH) gene transcription. In the present study, we examined oestrogen-responsive elements of the rat DBH gene promoter aiming to characterise the mechanisms of oestradiol-induced DBH transcription. Various mutations and deletions of DBH promoter reporter constructs were tested for responsiveness to 17β-oestradiol (E(2) ). Mutation of the half palindromic oestrogen response element (ERE) at position -759 reduced the response to E(2) in PC12 cells co-transfected with oestrogen receptor (ER) α, indicating a functional role for this motif. In cells co-transfected with ERβ, mutations at the -759 site were unresponsive to E(2) . To characterise the additional E(2) responsive elements, mediated by ERα, the DBH promoter was truncated to the proximal 249 or 200 nucleotides upstream of the transcription start site. Despite either truncation, 10 nm E(2) still elicited an approximately two-fold induction of DBH promoter activity. Mutation of a possible ERE-like sequence at -59 had no effect. The lack of a functional ERE in the proximal region of the rat DBH promoter despite E(2) -mediated DBH promoter activity, suggests regulation by a nonclassical mechanism, such as a membrane-initiated signalling pathway. Moreover, the induction of DBH promoter activity and the rise in DBH mRNA levels were observed within hours. To determine whether membrane-initiated E(2) signalling is involved in rat DBH gene transcription, a membrane impermeable E(2) conjugate, β-oestradiol-6-(O-carboxy-methyl) oxime-bovine serum albumin (E(2) BSA), was used. Incubation with E(2) -BSA induced luciferase activity and elicited a significant rise in DBH mRNA levels in the ERα transfected cells. The findings indicate two different mechanisms whereby DBH transcription is regulated by E(2) in the presence of ERα. The results implicate both genomic and membrane-initiated mechanisms, mediated by ERα, in E(2) -induced DBH gene transcription.

Neuropeptides and enzymes are targets for the action of endocrine disrupting chemicals in the vertebrate brain

Endocrine-disrupting chemicals (EDC) are molecules that interfere with endocrine signaling pathways and produce adverse consequences on animal and human physiology, such as infertility or behavioral alterations. Some EDC act through binding to androgen or/and estrogen receptors primarily operating through a genomic mechanism regulating gene expression. This mechanism of action may induce profound developmental adverse effects, and the major targets of the EDC action are the gene products, i.e., mRNAs inducing the synthesis of various peptidic molecules, which include neuropeptides and enzymes related to neurotransmitters syntheses. Available immunohistochemical data on some of the systems that are affected by EDC in lower and higher vertebrates are detailed in this review.

Modulation of responses to stress by estradiol benzoate and selective estrogen receptor agonists

Previously, pretreatment with estradiol benzoate (EB) was found to modulate the response of hypothalamic-pituitary-adrenal (HPA) axis and gene expression in several catecholaminergic neuronal locations in ovariectomized (OVX) rats exposed to single immobilization stress (IMO). Here, we investigated the role of estrogen receptor (ER) subtypes, using selective agonists for ERalpha (propyl pyrazole triol, PPT) or ERbeta (WAY-200070) in two major central noradrenergic systems and the HPA axis after exposure to single and repeated IMO. OVX female rats received 21 daily injections of either EB (25 mug/kg), PPT (10 mg/kg), WAY-200070 (10 mg/kg), or vehicle. Injections of EB and PPT, but not WAY-200070, elicited reduced body weight and increased uterine weight, showing their selectivity. Both EB and PPT increased corticosterone levels about two- to threefold, but prevented any further rise with either single or repeated IMO, indicating an ERalpha (ESR1)-, but not ERbeta (ESR2)-, mediated mechanism. In the locus coeruleus (LC), the rise in dopamine-beta-hydroxylase (Dbh) mRNA with both stress paradigms was abrogated in EB- or PPT-injected animals. However, WAY-200070 blocked the response of DBH mRNA to single IMO but not to repeated IMO. In the nucleus of the solitary tract (NTS), the rise in tyrosine hydroxylase and DBH mRNAs with both IMOs was absent, or greatly attenuated, in EB- or PPT-treated rats. In most cases, WAY-200070 inhibited the response to single IMO but not to repeated IMO. The results demonstrate that pretreatment with estradiol, or ER-selective agonists, modulates the stress-triggered induction of gene expression of norepinephrine biosynthetic enzymes in LC and NTS, with ER selectivity depending on duration of the stress.

Sex differences in functional brain activation during noxious visceral stimulation in rats

Studies in healthy human subjects and patients with irritable bowel syndrome suggest sex differences in cerebral nociceptive processing. Here we examine sex differences in functional brain activation in the rat during colorectal distention (CRD), a preclinical model of acute visceral pain. [(14)C]-iodoantipyrine was injected intravenously in awake, non-restrained female rats during 60- or 0-mmHg CRD while electromyographic abdominal activity (EMG) and pain behavior were recorded. Regional cerebral blood flow-related tissue radioactivity was analyzed by statistical parametric mapping from autoradiographic images of three-dimensionally reconstructed brains. Sex differences were addressed by comparing the current data with our previously published data collected from male rats. While sex differences in EMG and pain scores were modest, significant differences were noted in functional brain activation. Females showed widespread changes in limbic (amygdala, hypothalamus) and paralimbic structures (ventral striatum, nucleus accumbens, raphe), while males demonstrated broad cortical changes. Sex differences were apparent in the homeostatic afferent network (parabrachial nucleus, thalamus, insular and dorsal anterior cingulate cortices), in an emotional-arousal network (amygdala, locus coeruleus complex), and in cortical areas modulating these networks (prefrontal cortex). Greater activation of the ventromedial prefrontal cortex and broader limbic/paralimbic changes in females suggest greater engagement of affective mechanisms during visceral pain. Greater cortical activation in males is consistent with the concept of greater cortical inhibitory effects on limbic structures in males, which may relate to differences in attentional and cognitive attribution to visceral stimuli. These findings show remarkable similarities to reported sex differences in brain responses to visceral stimuli in humans.

Sex differences in stress responses: focus on ovarian hormones

Women in the reproductive age are more vulnerable to develop affective disorders than men. This difference may attribute to anatomical differences, hormonal influences and environmental factors such as stress. However, the higher prevalence in women normalizes once menopause is established, suggesting that ovarian hormones may play an important role in the development of depression in women. Ovarian hormones such as estrogen can pass the brain-blood barrier and bind to cytoplasmatic estrogen receptor (ER)-alpha and ER-beta in different areas of the limbic system. During stress, estrogen can modulate the behavioral and neurobiological response depending on the concentrations of estrogen. In this review we present evidence for disparate effects of chronic stress on neuroplasticity and brain activity in male and female rats. Furthermore, we will demonstrate that effects of social support on coping with stress can be mimicked by social housing of rats and that this model can be used for identification of underlying neurobiological mechanisms, including behavior, phosphorylation of CREB and ERK1/2, and brain activity changes as measured with fos expression. Using cyclic administration of estrogen in ovariectomized female rats we could specifically address effects of different plasma estrogen levels and antidepressants on stress-induced neuroplasticity and activity changes. In this model we also studied effects of estrogen on recovery after chronic stress. We conclude that the female brain has a different innate strategy to handle stress than the male brain and that female animal models are necessary for studying the underlying mechanisms and options for treatment.

Oestrogen receptor beta contributes to the transient sex difference in tyrosine hydroxylase expression in the mouse locus coeruleus

Oestrogen receptors (ERs) are important for sexual differentiation of the brain. Previous studies in rats have reported that the locus coeruleus (LC), a catecholaminergic nucleus in the brain stem, is sexually dimorphic such that females have more neurones than males. We hypothesised that ERs may be important for sexual differentiation of this nucleus in mice. Because previous studies reported conflicting results regarding ER protein expression in the mouse LC, we evaluated ER alpha and ER beta gene expression by in situ hybridisation and the real-time reverse transcription-polymerase chain reaction. We demonstrated that both ER alpha and ER beta mRNAs are present in tyrosine hydroxylase-immunoreactive (TH-ir) cells in the male LC. In the female LC, ER alpha mRNA is present at levels similar to males, whereas ER beta mRNA expression is significantly lower than in males. Similar to rats, male mice have fewer TH-ir cells in the LC than females at 60 days after birth, but the difference is absent at 120 days after birth when females exhibit a similar reduction in TH-ir cells. The transient sex difference is ER beta-dependent because is it absent in ER beta knockout mice, and is due to regulation of TH expression and not from death of TH-ir cells. Testicular hormones produced at adolescence are necessary for the regulation of TH expression in the male LC because orchidectomy of pre-pubertal males prevented the decrease in TH-ir cells, whereas treatment of gonadectomized males with testosterone or its metabolite, 5 alpha-androstan-3beta,17beta-diol, restored the intact male phenotype. Overall, these studies indicate that ER beta is important in regulating TH expression in the mouse LC.

Knockdown of clock genes in the suprachiasmatic nucleus blocks prolactin surges and alters FRA expression in the locus coeruleus of female rats

The nature of the circadian signal from the suprachiasmatic nucleus (SCN) required for prolactin (PRL) surges is unknown. Because the SCN neuronal circadian rhythm is determined by a feedback loop of Period (Per) 1, Per2, and circadian locomotor output cycles kaput (Clock) gene expressions, we investigated the effect of SCN rhythmicity on PRL surges by disrupting this loop. Because lesion of the locus coeruleus (LC) abolishes PRL surges and these neurons receive SCN projections, we investigated the role of SCN rhythmicity in the LC neuronal circadian rhythm as a possible component of the circadian mechanism regulating PRL surges. Cycling rats on proestrous day and estradiol-treated ovariectomized rats received injections of antisense or random-sequence deoxyoligonucleotide cocktails for clock genes (Per1, Per2, and Clock) in the SCN, and blood samples were taken for PRL measurements. The percentage of tyrosine hydroxylase-positive neurons immunoreactive to Fos-related antigen (FRA) was determined in ovariectomized rats submitted to the cocktail injections and in a 12:12-h light:dark (LD) or constant dark (DD) environment. The antisense cocktail abolished both the proestrous and the estradiol-induced PRL surges observed in the afternoon and the increase of FRA expression in the LC neurons at Zeitgeber time 14 in LD and at circadian time 14 in DD. Because SCN afferents and efferents were probably preserved, the SCN rhythmicity is essential for the magnitude of daily PRL surges in female rats as well as for LC neuronal circadian rhythm. SCN neurons therefore determine PRL secretory surges, possibly by modulating LC circadian neuronal activity.

Sexual dimorphism in hybrids rats

Laboratory rat strains descend from Wistar rats as a consequence of artificial selection. Previously we reported that the medial posterior division of the bed nucleus of the stria terminalis (BSTMP) was sexually dimorphic in Wistar and Long-Evans strains while the medial anterior division (BSTMA) and the locus coeruleus (LC) only showed sex differences in the ancestor Wistar strain. The lateral posterior division (BSTLP) was isomorphic in both strains. The present work studies the number of neurons in the BSTMP, BSTMA, BSTLP and LC of male and female Wistar and Long-Evans rats (F(0)) and their hybrid F(1) and F(2) generations. The BSTMP is sexually dimorphic in the F(0), F(1) and F(2) generations while sex differences in the LC are only seen in F(0) Wistar rats but not in the F(0) Long-Evans or the F(1) and F(2) hybrid generations. Sex differences in the BSTMA are seen in F(0) Wistar but not in F(0) Long-Evans rats and completely disappear in the F(2) generations. The number of neurons in the LC of both males and females decreased in heterozygotic individuals (F(1)) but increased in homozygotic (F(2)). However, the number of neurons in the BSTMP changes significantly over the generations, although the ratio of neurons (female/male) is stable and unaffected in homo- or heterozygosis. Thus, the mechanism that regulates the neuronal female/male ratio would be different from the one that controls the number of neurons. The facts that sex differences in the BSTMP are not affected by homo- or heterozygosis and that they are seen in several mammalian orders suggest the existence of a "fixed" type of brain sex differences in the Mammalia Class.

Sexual dimorphism in the vomeronasal system of the rabbit

Studies have shown that the vomeronasal system (VNS), an olfactory neural network that participates in the control of reproductive physiology and behavior, is sexually dimorphic in the rat. These works have also shown two main characteristics of brain sexual dimorphism: (a) dimorphism appears in neural networks related to reproduction and (b) it can present two morphological patterns: one in which males present greater morphological measures than females (male > female) and another in which the opposite is true (female > male). The present work extends the hypothesis to the rabbit, as a representative species of Lagomorpha. In addition, the locus coeruleus (LC), which is known to send rich noradrenergic projections to VNS structures, was also studied. Sex differences were found in: (a) the number of mitral, and dark and light granule cells (female > male) of the accessory olfactory bulb (AOB); (b) the medial amygdala (Me) and its dorsal (Med) and ventral (Mev) subdivisions, males showing greater values than females in volume and number of neurons, while in the posteromedial cortical amygdala (PMCo or C(3)), females show greater density of neurons than males and (c) the posteromedial division of the bed nucleus of the stria terminalis (BSTMP) in which males have more neurons than females. No sex differences were seen in the bed nucleus of the accessory olfactory tract (BAOT) and the LC. These results evidence that, as it was observed in rodents, sex differences are also seen in the VNS of Lagomorpha and that these sex differences present the two morphological patterns seen in Rodentia. Differences between orders are discussed with respect to the species-specific physiological and behavioral peculiarities.

Estrogen alters c-Fos response to immobilization stress in the brain of ovariectomized rats

Estrogen receptors are widely expressed in the brain, where estrogen modulates central nervous function. In this study, we investigated the effect of estrogen on the emotional stress response in the brain by comparing the CNS patterns of c-Fos expression in response to immobilization stress (IMO) in ovariectomized rats with placebo treatment (OVX + Pla) vs. ovariectomized rats supplemented with 17beta-estradiol (OVX + E2). Increased c-Fos immunoreactive neurons in response to IMO were observed in cerebral cortex, septum, thalamus, hypothalamus, midbrain, pons and medulla oblongata in accordance with previous findings. When OVX + E2/Stress were compared with OVX + Pla/Stress, the numbers of c-Fos immunoreactive cells were significantly lower in the lateral septum, paraventricular hypothalamic nucleus, dorsomedial hypothalamic nucleus, medial amygdaloid nucleus, lateral periaqueductal gray, laterodorsal tegmental nucleus and locus coeruleus, while they were significantly higher in paraventricular thalamic nucleus and nucleus of the solitary tract. These data suggest that neuronal activities in these areas are influenced bidirectionally by systemic estrogen level.

Sexually dimorphic responses of the brain norepinephrine system to stress and corticotropin-releasing factor

Stress-related psychiatric disorders are more prevalent in females than males, and this has been attributed to differences in stress sensitivity. As activation of the locus coeruleus (LC)-norepinephrine (NE) system is an important component of the stress response, this study compared LC responses to stress in female and male rats under different hormonal conditions in the halothane-anesthetized state. The mean basal LC discharge rate was similar between groups. However, the magnitude of LC activation elicited by hypotensive stress was substantially greater in females, regardless of hormonal status. The difference in stress sensitivity could be attributed to the differential postsynaptic sensitivity of LC neurons to corticotropin-releasing factor (CRF), which mediates LC activation by hypotension. CRF was 10-30 times more potent in activating LC neurons in female vs male rats. Interestingly, previous exposure to swim stress differentially regulated LC responses to CRF by sensitizing LC neurons of male, but not female, rats to CRF. The net effect of this was to abolish sex differences in LC sensitivity. Finally, CRF receptor (CRF-R) protein levels in the LC were greater in ovarectomized female vs male rats. This is the first study to demonstrate sex differences in the stress responsiveness of the brain noradrenergic system. Substantial sex differences were apparent in postsynaptic sensitivity to CRF and stress-induced regulation of postsynaptic sensitivity to CRF. These sex differences in the CRF regulation of the LC-NE system translate to a differential response to stress and may play a role in the increased vulnerability of females to stress-related psychiatric disorders.

The expression of brain sexual dimorphism in artificial selection of rat strains

Central nervous system sex differences have two morphological patterns. In one pattern, males show larger measurements (volume, number of neurons) than females (male > female; m > f) and, in the other, the opposite is true (female > male; f > m). The bed nucleus of the stria terminalis (BST) is a unique model for the study of sex differences because it has dimorphic and isomorphic subdivisions, with the former showing the two sexually differentiated morphological patterns. Meanwhile, other CNS structures, like the locus coeruleus (LC), present the f > m pattern. The philogenetic maintenance of the two patterns of sexual differentiation can help to disentangle the functional meaning of sex differences. Laboratory rat strains, whether albino or pigmented, descend from the Wistar strain through artificial selection. The present work compares the BST and LC of Wistar and Long-Evans rats. The medial posterior subdivision of the BST (BSTMP) is sexually dimorphic (m > f pattern) in the original (Wistar) and derived (Long-Evans) strains, while the lateral anterior and medial anterior subdivisions of the BST and the LC only present sex differences (f > m pattern) in the ancestor Wistar strain. Isomorphic BST regions are the same in both strains. The fact that the BSTMP, which is implicated in male copulatory behavior, is sexually dimorphic in both strains, as well as in other species, including humans, indicates the relevance of this structure in male sexual behavior in mammals.

The role of the androgen receptor in CNS masculinization

The medial posterior region of the bed nucleus of the stria terminalis (BSTMP) and the locus coeruleus (LC) show opposite patterns of sexual dimorphism. The BSTMP in males is greater in volume and number of neurons than in females (male > female) while in the LC, the opposite is true (female > male). To investigate the possible role of the androgen receptor (AR) in the masculinization of these two structures, males with the testicular feminization mutation (Tfm) were compared to their control littermate males. No differences were seen in the number of neurons of the BSTMP between Tfm and their control littermate males, while in the LC, Tfm males have a greater number of neurons than their control littermate males. These results show that the AR is involved in the control of neuron number in the LC but not in the BSTMP. Results based on the LC suggest that when females have a larger brain area than males, masculinization in males may be achieved through the AR, with androgens perhaps decreasing cell survival.

Estrogen modifies stress response of catecholamine biosynthetic enzyme genes and cardiovascular system in ovariectomized female rats

Estrogen is likely involved in the gender specific differences in coping with stress. Activation of catecholamine (CA) biosynthetic enzyme gene expression in central and peripheral CA systems plays a key role in response to stress and in regulation of the cardiovascular system. Here we examined whether estradiol can modulate response of hypothalamic-pituitary-adrenal axis (HPA), gene expression of enzymes related to CA biosynthesis in several noradrenergic locations, tetrahydrobiopterin (BH4) concentration and blood pressure (BP) in response to immobilization stress (IMO) of ovariectomized female rats. Rats were injected with 25 mug/kg estradiol benzoate (EB) or sesame oil once daily for 16 days and subsequently exposed to two hours of IMO. The IMO triggered elevation in plasma ACTH was lessened in EB-pretreated animals. However, estradiol did not alter the IMO-elicited rise of tyrosine hydroxylase mRNA levels in adrenal medulla (AM) and in the nucleus of solitary track (NTS) compared with controls. The response of GTP cyclohydrolase I (GTPCH) mRNA in AM to IMO was also similar in both groups. Several responses to IMO in EB-treated rats were reversed. Instead of IMO-elicited elevation in dopamine beta-hydroxylase mRNA levels in the locus coeruleus, GTPCH mRNA and BH4 levels in the NTS, they were reduced by IMO. In a parallel experiment, BP was monitored during restraint stress. The elevation of BP in response to single or repeated restraint stress was sustained during 2 h in controls and reduced after 70 min stress in EB treated rats. One month after withdrawal of EB treatment, the BP response to restraint was similar to that of rats which never received EB. The results demonstrate that estrogen can modulate responses to stress affecting HPA axis, CA biosynthesis, in central and peripheral noradrenergic systems, and BP.

Effects of sex and hormonal status on astrocytic basic fibroblast growth factor-2 and tyrosine hydroxylase immunoreactivity after medial forebrain bundle 6-hydroxydopamine lesions of the midbrain dopamine neurons

We examined astrocytic basic fibroblast growth factor immunoreactivity (FGF-2-IR) and tyrosine hydroxylase immunoreactivity (TH-IR) in the cell-body region of midbrain dopaminergic neurons after unilateral infusions of the neurotoxin 6-hydroxydopamine into the medial forebrain bundle in male and female rats. In addition, to determine whether neonatal exposure to gonadal hormones has consequences on the expression of astrocytic FGF-2 and cell loss in response to injury in adulthood, we studied the effects of these lesions in adult male and female rats that had been exposed or not to testosterone in the neonatal period. In both males and females there was a progressive loss of TH-expressing cells that peaked 5 weeks after the lesions. Females showed less loss of TH-expressing cells than males, but this effect was not estrogen dependent. Lesions led to an increase in expression of astrocytic FGF-2 that was greater in males than in females. Finally, it was found that, regardless of genetic sex, rats exposed to testosterone neonatally showed greater astrocytic FGF-2 expression after lesions than those not exposed, and that among those not exposed to testosterone, estrogen treatment had a modest protective effect. Analysis of behavior and striatal dopamine content showed that the percent of striatal dopamine depletion 14 days after the lesion correlated with the amount of behavioral asymmetry displayed by animals on all tests conducted after lesioning. In groups killed 2 and 5 weeks after the lesion, the amount of behavioral asymmetry correlated with the percent loss of TH-IR cells and with the percent increase in FGF-2-IR cells in the midbrain. These relationships were not evident in groups killed 3 and 7 days after the lesion, possibly because the changes in the number of FGF-2- and TH-IR cells were not fully manifested. The present findings show that hormonal events early in life can alter the response of midbrain dopamine neurons to insult and injury in adult life and suggest that the slow degeneration of these neurons may release signals triggering a sustained activation of adjacent astrocytes which, in turn, may lead to induction of astrocytic FGF-2.

Sex-related differences in IBS patients: central processing of visceral stimuli

BACKGROUND & AIMS

Women have a higher prevalence of irritable bowel syndrome (IBS) and possible differences in response to treatment, suggesting sex-related differences in underlying pathophysiology. The aim of this study was to determine possible sex-related differences in brain responses to a visceral and a psychological stressor in IBS.

METHODS

Regional cerebral blood flow measurements using H(2)(15)O positron emission tomography were compared across 23 female and 19 male nonconstipated patients with IBS during a visceral stimulus (moderate rectal inflation) and a psychological stimulus (anticipation of a visceral stimulus).

RESULTS

In response to the visceral stimulus, women showed greater activation in the ventromedial prefrontal cortex, right anterior cingulate cortex, and left amygdala, whereas men showed greater activation of the right dorsolateral prefrontal cortex, insula, and dorsal pons/periaqueductal gray. Similar differences were observed during the anticipation condition. Men also reported higher arousal and lower fatigue.

CONCLUSIONS

Male and female patients with IBS differ in activation of brain networks concerned with cognitive, autonomic, and antinociceptive responses to delivered and anticipated aversive visceral stimuli.

Sex differences in the regulation of tyrosine hydroxylase gene transcription by estrogen in the locus coeruleus of TH9-LacZ transgenic mice

Although estrogen is recognized increasingly as having an important role in modulating extrahypothalamic brain function, the mechanisms through which this occur are not well established. The norepinephrine (NE) neurons of the locus coeruleus provide an important neuromodulatory influence upon multiple neural networks throughout the brain and estrogen has been implicated in their regulation. Using a tyrosine hydroxylase (TH) promoter-LacZ transgenic mouse model, which enables rates of TH gene transcription to be examined in vivo, we have examined here whether estrogen regulates expression of the TH gene in the locus coeruleus of males and females. Optical area measurements of Xgal reaction product in the locus coeruleus revealed that gonadectomy exerted opposite effects on TH gene transcription in males and females; transgene expression was increased in males (P<0.01) but reduced in females (P<0.05). Estrogen reversed these effects in both sexes by suppressing gene expression in males (P<0.05) but elevating it in the female (P<0.05). These studies reveal a marked and unexpected sex difference in the regulation of TH gene activity in the mouse. While estrogen in the male, synthesized from circulating testosterone, suppresses TH gene transcription, estrogen in the female enhances TH promoter activity. The present results indicate that estrogen may exert very different sex-dependent effects upon the biosynthesis of NE within the locus coeruleus.

Steroid-induced plasticity in the sexually dimorphic vasotocinergic innervation of the avian brain: behavioral implications

Vasotocin (VT, the antidiuretic hormone of birds) is synthesized by diencephalic magnocellular neurons projecting to the neurohypophysis. In addition, in male quail and in other oscine and non-oscine birds, a sexually dimorphic group of VT-immunoreactive (ir) parvocellular neurons is located in a region homologous to the mammalian nucleus of the stria terminalis, pars medialis (BSTm) and in the medial preoptic nucleus (POM). These cells are not visible in females. VT-ir fibers are present in many diencephalic and extradiencephalic locations. Quantitative morphometric analyses demonstrate that, in quail, these elements are expressed in a sexually dimorphic manner (males>females) in regions involved in the control of different aspects of reproduction: i.e., the POM (copulatory behavior), the lateral septum (secretion of gonadotropin-releasing hormone [GnRH]), the nucleus intercollicularis (control of vocalizations), and the locus coeruleus (the main noradrenergic center of the avian brain). In many of these regions, VT-ir fibers are closely related to aromatase-ir, GnRH-ir, or estrogen receptor-expressing neurons. This dimorphism has an organizational nature: administration of estradiol-benzoate to quail embryos (a treatment that abolishes male sexual behavior) results in a dramatic decrease of the VT-immunoreactivity in all sexually dimorphic regions of the male quail brain. Conversely, the inhibition of estradiol (E2) synthesis during embryonic life (a treatment that stimulates the expression of male copulatory behavior in adult testosterone (T)-treated females) results in a male-like distribution of VT-ir cells and fibers. Castration markedly decreases the immunoreactivity in both the VT-immunopositive elements of the BSTm and the innervation of the SL and POM, whereas T-replacement therapy restores the VT immunoreactivity to a level typical of intact birds. These changes reflect modifications of VT mRNA concentrations (and probably synthesis) as demonstrated by in situ hybridization and they are paralleled by similar changes in male copulatory behavior (absent in castrated male quail, fully expressed in CX+T males). The aromatization of T into estradiol (E2) also controls VT expression and, in parallel limits the activation of male sexual behavior by T. In castrated male quail, the restoration by T of the VT immunoreactivity in POM, BSTm and lateral septum could be fully mimicked by a treatment with E2, but the androgen 5alpha-dihydrotestosterone (DHT) had absolutely no effect on the VT immunoreactivity in these conditions. At the doses used in this study, DHT also did not synergize with E2 to enhance the density of VT immunoreactive structures. Systemic or i.c.v. injections of VT markedly inhibit the expression of all aspects of male sexual behavior. VT, presumably, does not simply represent one step in the biochemical cascade of events that is induced by T in the brain and leads to the expression of male sexual behavior. Androgens and estrogens presumably affect reproductive behavior both directly, by acting on steroid-sensitive neurons in the preoptic area, and indirectly, by modulating peptidergic (specifically vasotocinergic) inputs to this and other areas. The respective contribution of these two types of actions and their interaction deserves further analysis.