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

Sex differences in single neuron function and proteomics profiles examined by patch-clamp and mass spectrometry in the locus coeruleus of the adult mouse

AIMS

This study aimed to characterize the properties of locus coeruleus (LC) noradrenergic neurons in male and female mice. We also sought to investigate sex-specific differences in membrane properties, action potential generation, and protein expression profiles to understand the mechanisms underlying neuronal excitability variations.

METHODS

Utilizing a genetic mouse model by crossing Dbhcre knock-in mice with tdTomato Ai14 transgenic mice, LC neurons were identified using fluorescence microscopy. Neuronal functional properties were assessed using patch-clamp recordings. Proteomic analyses of individual LC neuron soma was conducted using mass spectrometry to discern protein expression profiles. Data are available via ProteomeXchange with identifier PXD045844.

RESULTS

Female LC noradrenergic neurons displayed greater membrane capacitance than those in male mice. Male LC neurons demonstrated greater spontaneous and evoked action potential generation compared to females. Male LC neurons exhibited a lower rheobase and achieved higher peak frequencies with similar current injections. Proteomic analysis revealed differences in protein expression profiles between sexes, with male mice displaying a notably larger unique protein set compared to females. Notably, pathways pertinent to protein synthesis, degradation, and recycling, such as EIF2 and glucocorticoid receptor signaling, showed reduced expression in females.

CONCLUSIONS

Male LC noradrenergic neurons exhibit higher intrinsic excitability compared to those from females. The discernible sex-based differences in excitability could be ascribed to varying protein expression profiles, especially within pathways that regulate protein synthesis and degradation. This study lays the groundwork for future studies focusing on the interplay between proteomics and neuronal function examined in individual cells.

Sexual differences in locus coeruleus neurons and related behavior in C57BL/6J mice

BACKGROUND

In addition to social and cultural factors, sex differences in the central nervous system have a critical influence on behavior, although the neurobiology underlying these differences remains unclear. Interestingly, the Locus Coeruleus (LC), a noradrenergic nucleus that exhibits sexual dimorphism, integrates signals that are related to diverse activities, including emotions, cognition and pain. Therefore, we set-out to evaluate sex differences in behaviors related to LC nucleus, and subsequently, to assess the sex differences in LC morphology and function.

METHODS

Female and male C57BL/6J mice were studied to explore the role of the LC in anxiety, depressive-like behavior, well-being, pain, and learning and memory. We also explored the number of noradrenergic LC cells, their somatodendritic volume, as well as the electrophysiological properties of LC neurons in each sex.

RESULTS

While both male and female mice displayed similar depressive-like behavior, female mice exhibited more anxiety-related behaviors. Interestingly, females outperformed males in memory tasks that involved distinguishing objects with small differences and they also showed greater thermal pain sensitivity. Immunohistological analysis revealed that females had fewer noradrenergic cells yet they showed a larger dendritic volume than males. Patch clamp electrophysiology studies demonstrated that LC neurons in female mice had a lower capacitance and that they were more excitable than male LC neurons, albeit with similar action potential properties.

CONCLUSIONS

Overall, this study provides new insights into the sex differences related to LC nucleus and associated behaviors, which may explain the heightened emotional arousal response observed in females.

Chronic Effects of the Sympathetic Nervous System in Inflammatory Models

The immune system is embedded in a network of regulatory systems to keep homeostasis in case of an immunologic challenge. Neuroendocrine immunologic research has revealed several aspects of these interactions over the past decades, e.g., between the autonomic nervous system and the immune system. This review will focus on evidence revealing the role of the sympathetic nervous system (SNS) in chronic inflammation, like colitis, multiple sclerosis, systemic sclerosis, lupus erythematodes, and arthritis with a focus on animal models supported by human data. A theory of the contribution of the SNS in chronic inflammation will be presented that spans these disease entities. One major finding is the biphasic nature of the sympathetic contribution to inflammation, with proinflammatory effects until the point of disease outbreak and mainly anti-inflammatory influence thereafter. Since sympathetic nerve fibers are lost from sites of inflammation during inflammation, local cells and immune cells achieve the capability to endogenously produce catecholamines to fine-tune the inflammatory response independent of brain control. On a systemic level, it has been shown across models that the SNS is activated in inflammation as opposed to the parasympathetic nervous system. Permanent overactivity of the SNS contributes to many of the known disease sequelae. One goal of neuroendocrine immune research is defining new therapeutic targets. In this respect, it will be discussed that at least in arthritis, it might be beneficial to support β-adrenergic and inhibit α-adrenergic activity besides restoring autonomic balance. Overall, in the clinical setting, we now need controlled interventional studies to successfully translate the theoretical knowledge into benefits for patients.

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.

Sex-Related Differential Whole-Brain Input Atlas of Locus Coeruleus Noradrenaline Neurons

As the most important organ in our bodies, the brain plays a critical role in deciding sex-related differential features; however, the underlying neural circuitry basis remains unclear. Here, we used a cell-type-specific rabies virus-mediated monosynaptic tracing system to generate a sex differences-related whole-brain input atlas of locus coeruleus noradrenaline (LC-NE) neurons. We developed custom pipelines for brain-wide comparisons of input sources in both sexes with the registration of the whole-brain data set to the Allen Mouse Brain Reference Atlas. Among 257 distinct anatomical regions, we demonstrated the differential proportions of inputs to LC-NE neurons in male and female mice at different levels. Locus coeruleus noradrenaline neurons of two sexes showed general similarity in the input patterns, but with differentiated input proportions quantitatively from major brain regions and diverse sub-regions. For instance, inputs to male LC-NE neurons were found mainly in the cerebrum, interbrain, and cerebellum, whereas inputs to female LC-NE neurons were found in the midbrain and hindbrain. We further found that specific subsets of nuclei nested within sub-regions contributed to overall sex-related differences in the input circuitry. Furthermore, among the totaled 123 anatomical regions with proportion of inputs >0.1%, we also identified 11 sub-regions with significant statistical differences of total inputs between male and female mice, and seven of them also showed such differences in ipsilateral hemispheres. Our study not only provides a structural basis to facilitate our understanding of sex differences at a circuitry level but also provides clues for future sexually differentiated functional studies related to LC-NE neurons.

Corticotropin-Releasing Factor Family: A Stress Hormone-Receptor System's Emerging Role in Mediating Sex-Specific Signaling

No organ in the body is impervious to the effects of stress, and a coordinated response from all organs is essential to deal with stressors. A dysregulated stress response that fails to bring systems back to homeostasis leads to compromised function and ultimately a diseased state. The components of the corticotropin-releasing factor (CRF) family, an ancient and evolutionarily conserved stress hormone-receptor system, helps both initiate stress responses and bring systems back to homeostasis once the stressors are removed. The mammalian CRF family comprises of four known agonists, CRF and urocortins (UCN1–3), and two known G protein-coupled receptors (GPCRs), CRF₁ and CRF₂. Evolutionarily, precursors of CRF- and urocortin-like peptides and their receptors were involved in osmoregulation/diuretic functions, in addition to nutrient sensing. Both CRF and UCN1 peptide hormones as well as their receptors appeared after a duplication event nearly 400 million years ago. All four agonists and both CRF receptors show sex-specific changes in expression and/or function, and single nucleotide polymorphisms are associated with a plethora of human diseases. CRF receptors harbor N-terminal cleavable peptide sequences, conferring biased ligand properties. CRF receptors have the ability to heteromerize with each other as well as with other GPCRs. Taken together, CRF receptors and their agonists due to their versatile functional adaptability mediate nuanced responses and are uniquely positioned to orchestrate sex-specific signaling and function in several tissues.

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.

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.

Effects of perinatal bisphenol A exposure on the volume of sexually-dimorphic nuclei of juvenile rats: A CLARITY-BPA consortium study

Bisphenol A (BPA) is a high volume endocrine disrupting chemical found in a wide variety of products including plastics and epoxy resins. Human exposure is nearly ubiquitous, and higher in children than adults. Because BPA has been reported to interfere with sex steroid hormone signaling, there is concern that developmental exposure, even at levels below the current FDA No Observed Adverse Effect Level (NOAEL) of 5mg/kg body weight (bw)/day, can disrupt brain sexual differentiation. The current studies were conducted as part of the CLARITY-BPA (Consortium Linking Academic and Regulatory Insights on BPA Toxicity) program and tested the hypothesis that perinatal BPA exposure would induce morphological changes in hormone sensitive, sexually dimorphic brain regions. Sprague-Dawley rats were randomly assigned to 5 groups: BPA (2.5, 25, or 2500μg/kgbw/day), a reference estrogen (0.5μg ethinylestradiol (EE₂)/kgbw/day), or vehicle. Exposure occurred by gavage to the dam from gestational day 6 until parturition, and then to the offspring from birth through weaning. Unbiased stereology was used to quantify the volume of the sexually dimorphic nucleus (SDN), the anteroventral periventricular nucleus (AVPV), the posterodorsal portion of the medial amygdala (MePD), and the locus coeruleus (LC) at postnatal day 28. No appreciable effects of BPA were observed on the volume of the SDN or LC. However, AVPV volume was enlarged in both sexes, even at levels below the FDA NOAEL. Collectively, these data suggest the developing brain is vulnerable to endocrine disruption by BPA at exposure levels below previous estimates by regulatory agencies.

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.

Sex differences in stress-related psychiatric disorders: neurobiological perspectives

Stress is associated with the onset and severity of several psychiatric disorders that occur more frequently in women than men, including posttraumatic stress disorder (PTSD) and depression. Patients with these disorders present with dysregulation of several stress response systems, including the neuroendocrine response to stress, corticolimbic responses to negatively valenced stimuli, and hyperarousal. Thus, sex differences within their underlying circuitry may explain sex biases in disease prevalence. This review describes clinical studies that identify sex differences within the activity of these circuits, as well as preclinical studies that demonstrate cellular and molecular sex differences in stress responses systems. These studies reveal sex differences from the molecular to the systems level that increase endocrine, emotional, and arousal responses to stress in females. Exploring these sex differences is critical because this research can reveal the neurobiological underpinnings of vulnerability to stress-related psychiatric disorders and guide the development of novel pharmacotherapies.

Sensitization of sexual behavior in ovariectomized rats by chronic estradiol treatment

The ovariectomized (OVX) rat treated with estradiol benzoate (EB) is used to elucidate neuroendocrine mechanisms of sexual behavior. Chronic behavioral and pharmacological manipulations can be confounded by rising baselines, since females are behaviorally more sensitive to repeated EB injections. The literature lacks a systematic examination of chronic effects of EB administered alone to the sexually experienced OVX rat. Long-Evans rats were repeatedly treated (8 tests) with s.c. injections of 2, 5, or 10 μg EB at different time intervals (4 or 8 days). Female sexual behaviors as well as receipt of mounts, intromissions and ejaculations from the male were observed in the unilevel 4-hole pacing chamber. The effects of adrenalectomy (ADX) and strain (Long-Evans vs. Wistar) were also assessed. Long-Evans OVX rats treated with 5 μg EB every 8 days showed persistently low levels of sexual behavior. Sensitization was most robust following 10 μg EB at 4-day intervals. Very few sexual behaviors were ever induced by 2 μg EB. ADX did not affect the development of behavioral sensitization by 10μg EB. Therefore, to achieve a low steady state of sexual behaviors in sexually experienced Long-Evans OVX rats 5μg of EB administered every 8days is optimal, whereas a persistently high level of sexual behaviors is induced with 10 μg EB administered every 4 days. OVX Wistar rats are behaviorally more sensitive to EB. Behavioral sensitization to EB may serve as a mechanism to optimize reproductive success.

Sex differences in molecular and cellular substrates of stress

Women are twice as likely as men to suffer from stress-related psychiatric disorders, like unipolar depression and post-traumatic stress disorder. Although the underlying neural mechanisms are not well characterized, the pivotal role of stress in the onset and severity of these diseases has led to the idea that sex differences in stress responses account for this sex bias. Corticotropin-releasing factor (CRF) orchestrates stress responses by acting both as a neurohormone to initiate the hypothalamic-pituitary-adrenal (HPA) axis and as a neuromodulator in the brain. One target of CRF modulation is the locus coeruleus (LC)-norepinephrine system, which coordinates arousal components of the stress response. Hypersecretion of CRF and dysregulation of targets downstream from CRF, such as the HPA axis and LC-norepinephrine system, are characteristic features of many stress-related psychiatric diseases, suggesting a causal role for CRF and its targets in the development of these disorders. This review will describe sex differences in CRF and the LC-norepinephrine system that can increase stress sensitivity in females, making them vulnerable to stress-related disorders. Evidence for gonadal hormone regulation of hypothalamic CRF is discussed as an effect that can lead to increased HPA axis activity in females. Sex differences in the structure of LC neurons that create the potential for hyperarousal in response to emotional stimuli are described. Finally, sex differences at the molecular level of the CRF(1) receptor that make the LC-norepinephrine system more reactive in females are reviewed. The implications of these sex differences for the treatment of stress-related psychiatric disorders also will be discussed.

Sex-dependent changes induced by prenatal stress in cortical and hippocampal morphology and behaviour in rats: an update

Recent prospective studies have shown that gestational stress in humans is more likely to cause cognitive and emotional problems in the offspring if it occurs during weeks 12-20 of pregnancy. There are also suggestions that such problems may be gender dependent. This review describes recent studies that found sex differences in the behaviour and brain morphology of rats stressed prenatally during the equivalent period of neuronal development in humans. Learning deficits are more prevalent in males and anxious behaviour in females but their appearance depends also on the timing and intensity of the stress and the age when the offspring were tested. Cognitive deficits and anxiety are linked to a sex-dependent reduction in neurogenesis and in measures of dendritic morphology in the prefrontal cortex and hippocampal formation. Maternal adrenalectomy prior to the stress prevents the anxiety in both sexes and learning deficits in males. Corticosterone administration to the dam to mimic levels induced by stress reinstates only the anxiety, indicating that it arises from foetal exposure to corticosterone from the maternal circulation. Learning deficits in males may result from a combination of a reduction in testosterone and in aromatase activity, together with the action of other adrenal hormones.

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.

Effects of BNST lesions in female rats on forced swimming and navigational learning

The bed nucleus of the stria terminalis (BNST) in the forebrain shows sexual dimorphism in its neuroanatomical connectivity and neurochemical characteristics. The structure is involved in many behavioral and motivational phenomena particularly related to coping with stress. Female rats differ from males in responding to stressful situations such as forced swimming and navigational learning in the water maze. It was previously shown that bilateral damage to the BNST in male Wistar rats aggravated depression as measured by forced swim tests, but did not impair navigational learning in the water maze. The present study extended the findings to female rats demonstrating that bilateral electrolytic lesions of the BNST increased immobility and decreased climbing compared to sham-operated controls, but failed to affect performance in the water maze. Additionally, lesions did not alter behavior in the open field and the elevated plus-maze tests suggesting not only that the modulation of depression by BNST lesions is specific, but also providing support for the view that the BNST may not necessarily be critically involved in anxiety.

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.

Sex differences in the human olfactory system

The olfactory system (accessory) implicated in reproductive physiology and behavior in mammals is sexually dimorphic. These brain sex differences present two main characteristics: they are seen in neural circuits related to sexual behavior and sexual physiology and they take one of two opposite morphological patterns (male>female or female>male). The present work reports sex differences in the olfactory system in a large homogeneous sample of men (40) and women (51) using of voxel-based morphology. Gray matter concentration showed sexual dimorphism in several olfactory regions. Women have a higher concentration in the orbitofrontal cortex involving Brodmann's areas 10, 11 and 25 and temporomedial cortex (bilateral hippocampus and right amygdala), as well as their left basal insular cortex. In contrast, men show a higher gray matter concentration in the left entorhinal cortex (Brodmann's area 28), right ventral pallidum, dorsal left insular cortex and a region of the orbitofrontal cortex (Brodmann's area 25). This study supports the hypothesis that the mammalian olfactory system is a sexually dimorphic network and provides a theoretical framework for the morphofunctional approach to sex differences in the human brain.

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.

Learning deficits expressed as delayed extinction of a conditioned running response following perinatal exposure to vinclozolin

Vinclozolin (Vz) is one member of a group of fungicides whose metabolites are androgen receptor antagonists. These fungicides have been shown to block androgen-driven development and compromise reproductive function. The current study sought to determine if Vz also affects learning following exposure to low doses during the perinatal period. To test this, an androgen-dependent behavior was examined, the extinction of a previously reinforced running response. Pregnant Long-Evans rats were administered a daily oral dose of 0, 1.5, 3, 6 or 12 mg/kg Vz from the 14th day of gestation through postnatal day 3. After reaching adulthood, male and female offspring were trained to run through a short alleyway for food reinforcement. Acquisition of the response was not affected by Vz exposure. However, males required more trials than females for response extinction once food was no longer available in the apparatus. Males exposed to 6 or 12 mg/kg Vz failed to show any extinction by the end of the procedure, while the lowest dose of Vz appeared to facilitate extinction in both male and female offspring. These results demonstrate that endocrine disrupting antiandrogens can alter nervous system development in addition to the reproductive system.