Aromatase in developing sensory systems of the rat brain

Role of neuroestrogens in the regulation of social behaviors - From social recognition to mating

In this mini-review, we summarize the brain distribution of aromatase, the enzyme catalyzing the synthesis of estrogens from androgens, and the mechanisms responsible for regulating estrogen production within the brain. Understanding this local synthesis of estrogens by neurons is pivotal as it profoundly influences various facets of social behavior. Neuroestrogen action spans from the initial processing of socially pertinent sensory cues to integrating this information with an individual's internal state, ultimately resulting in the manifestation of either pro-affiliative or - aggressive behaviors. We focus here in particular on aggressive and sexual behavior as the result of correct individual recognition of intruders and potential mates. The data summarized in this review clearly point out the crucial role of locally synthesized estrogens in facilitating rapid adaptation to the social environment in rodents and birds of both sexes. These observations not only shed light on the evolutionary significance but also indicate the potential implications of these findings in the realm of human health, suggesting a compelling avenue for further investigation.

Sex steroid hormone synthesis, metabolism, and the effects on the mammalian olfactory system

Sex steroid hormones influence olfactory-mediated social behaviors, and it is generally hypothesized that these effects result from circulating hormones and/or neurosteroids synthesized in the brain. However, it is unclear whether sex steroid hormones are synthesized in the olfactory epithelium or the olfactory bulb, and if they can modulate the activity of the olfactory sensory neurons. Here, we review important discoveries related to the metabolism of sex steroids in the mouse olfactory epithelium and olfactory bulb, along with potential areas of future research. We summarize current knowledge regarding the expression, neuroanatomical distribution, and biological activity of the steroidogenic enzymes, sex steroid receptors, and proteins that are important to the metabolism of these hormones and reflect on their potential to influence early olfactory processing. We also review evidence related to the effects of sex steroid hormones on the development and activity of olfactory sensory neurons. By better understanding how these hormones are metabolized and how they act both at the periphery and olfactory bulb level, we can better appreciate the complexity of the olfactory system and discover potential similarities and differences in early olfactory processing between sexes.

Aromatase in the Human Brain

The aromatase cytochrome P450 (P450arom) enzyme, or estrogen synthase, which is coded by the CYP19A1 gene, is widely expressed in a subpopulation of excitatory and inhibitory neurons, astrocytes, and other cell types in the human brain. Experimental studies in laboratory animals indicate a prominent role of brain aromatization of androgens to estrogens in regulating different brain functions. However, the consequences of aromatase expression in the human brain remain poorly understood. Here, we summarize the current knowledge about aromatase expression in the human brain, abundant in the thalamus, amygdala, hypothalamus, cortex, and hippocampus and discuss its role in the regulation of sensory integration, body homeostasis, social behavior, cognition, language, and integrative functions. Since brain aromatase is affected by neurodegenerative conditions and may participate in sex-specific manifestations of autism spectrum disorders, major depressive disorder, multiple sclerosis, stroke, and Alzheimer's disease, we discuss future avenues for research and potential clinical and therapeutic implications of the expression of aromatase in the human brain.

Possible effects of low testosterone levels on olfactory function in males

INTRODUCTION

Functions attributed to androgens have increased, ranging from the role in hypothalamic-pituitary-gonadal axis and reproductive behaviors to modulation of cognition, mood and some other functions. Sex differences and changes in circulating sex hormones affect human sensory function. In the literature, authors reported this kind of influence for olfaction predominantly in females.

OBJECTIVE

To investigate the effects of low testosterone levels on olfactory functions in males, in this prospective clinical study.

METHODS

Male patients diagnosed with prostate cancer were included. Thirty-nine patients with prostate cancer whose testosterone levels were lower than 50ng/dL due to castration, were the study group. Thirty-one patients with prostate cancer who were not castrated with testosterone levels higher than 50ng/dL were selected as the control group. Acoustic rhinometry and peak nasal inspiratory flow tests were performed for all participants; and for evaluation of olfactory function, both groups completed the Connecticut chemosensory clinical research center olfactory test.

RESULTS

The mean ages of the patients and controls were 69.6±7.2 (57-89) and 66.3±5.8 (50-78) years, respectively (p=0.039). There was a significant difference between groups in terms of testosterone levels (p<0.0001). The multivariate logistic regression revealed testosterone level as the only predictive factor determining the difference between the groups. In terms of olfactory parameters, all scores were lower in the emasculated group (butanol threshold test p=0.019, identification p=0.059, and Connecticut center score p=0.029) There was a significant correlation between testosterone levels and olfactory parameters (p=0.023; p=0.025 for identification and Connecticut center scores, respectively).

CONCLUSION

Low testosterone levels in males have negative effects on olfactory functions. Further molecular research is required to understand the connection between testosterone and olfaction.

Neuroestradiol regulation of ventromedial hypothalamic nucleus 5'-AMP-activated protein kinase activity and counterregulatory hormone secretion in hypoglycemic male versus female rats

Hypoglycemia activates the ultra-sensitive energy gauge 5′-AMP-activated protein kinase (AMPK) in ventromedial hypothalamic nucleus (VMN) gluco-regulatory neurons. The VMN is exemplified by high levels of expression of the enzyme aromatase, which converts testosterone to estradiol. This study examined the hypothesis that neuroestradiol imposes sex-dimorphic control of VMN AMPK activity during eu- and/or hypoglycemia. VMN tissue corresponding to distinct rostro-caudal segments was obtained by micropunch dissection from testes-intact male and estradiol-replaced ovariectomized female rats that were infused intracerebroventricularly with the aromatase inhibitor letrozole (Lz) before subcutaneous insulin (INS) injection. In euglycemic rats, Lz treatment elevated (male) or decreased (female) middle VMN phosphoAMPK content, with concurrent effects on total AMPK expression. Lz prevented hypoglycemic up-regulation of the mean pAMPK/AMPK ratio in rostral and middle segments of the male VMN, and significantly inhibited this proportion throughout the VMN of hypoglycemic female rats. Lz prevented glucagon secretion in hypoglycemic rats of each sex, and abolished hypoglycemic hypercorticosteronemia in males. Results show that neuroestradiol regulation of VMN AMPK activity during euglycemia is region-specific and gender-divergent, e.g. inhibitory in males versus stimulatory in females. Intra-VMN distribution of hypoglycemia-activated AMPK varies between sexes, but in each sex, locally-generated estradiol is critical for sensor reactivity to this stimulus. Coincident Lz attenuation of VMN AMPK and counter-regulatory hormone responses to hypoglycemia infers a possible cause-and-effect association. Further effort is needed to elucidate the cellular and molecular mechanisms that underlie sex-dimorphic neuroestradiol control of VMN total AMPK and phosphoAMPK expression during distinct metabolic states.

Sex-dimorphic aromatase regulation of ventromedial hypothalamic nucleus glycogen content in euglycemic and insulin-induced hypoglycemic rats

Estrogen receptors control hypothalamic astrocyte glycogen accumulation in vitro. Glycogen metabolism impacts metabolic transmitter signaling in the ventromedial hypothalamic nucleus (VMN), a key glucoregulatory structure. Aromatase, the enzyme that converts testosterone to estradiol, is expressed at high levels in the VMN. Here, the aromatase inhibitor letrozole (Lz) was used alongside high-resolution microdissection/UPHLC-electrospray ionization-mass spectrometric methods to determine if neuroestradiol imposes sex-specific control of VMN glycogen content during glucostasis and/or glucoprivation. Testes-intact male and estradiol-replaced ovariectomized female rats were pretreated by lateral ventricular letrozole (Lz) infusion prior to subcutaneous insulin (INS) injection. Vehicle-treated female controls exhibited higher VMN glycogen content compared to males. Lz increased VMN glycogen levels in males, not females. INS-induced hypoglycemia (IIH) elevated (males) or diminished (females) rostral VMN glycogen accumulation. Induction of IIH in Lz-pretreated animals reduced male VMN glycogen mass, but augmented content in females. Data provide novel evidence for regional variation, in both sexes, in glycogen reactivity to IIH. Results highlight sex-dimorphic neuroestradiol regulation of VMN glycogen amassment during glucostasis, e.g. inhibitory in males versus insignificant in females. Locally-generated estradiol is evidently involved in hypoglycemic enhancement of male VMN glycogen, but conversely limits glycogen content in hypoglycemic females. Further research is needed to characterize mechanisms that underlie the directional shift in aromatase regulation of VMN glycogen in eu- versus hypoglycemic male rats and gain in negative impact in hypoglycemic females.

De Novo Synthesized Estradiol: A Role in Modulating the Cerebellar Function

The estrogen estradiol is a potent neuroactive steroid that may regulate brain structure and function. Although the effects of estradiol have been historically associated with gonadal secretion, the discovery that this steroid may be synthesized within the brain has expanded this traditional concept. Indeed, it is accepted that de novo synthesized estradiol in the nervous system (nE2) may modulate several aspects of neuronal physiology, including synaptic transmission and plasticity, thereby influencing a variety of behaviors. These modulations may be on a time scale of minutes via non-classical and often membrane-initiated mechanisms or hours and days by classical actions on gene transcription. Besides the high level, recent investigations in the cerebellum indicate that even a low aromatase expression can be related to the fast nE2 effect on brain functioning. These pieces of evidence point to the importance of an on-demand and localized nE2 synthesis to rapidly contribute to regulating the synaptic transmission. This review is geared at exploring a new scenario for the impact of estradiol on brain processes as it emerges from the nE2 action on cerebellar neurotransmission and cerebellum-dependent learning.

GPER1 Signaling Initiates Migration of Female V-SVZ-Derived Cells

In the rodent ventricular-subventricular zone (V-SVZ) neurons are generated throughout life. They migrate along the rostral migratory stream (RMS) into the olfactory bulb before their final differentiation into interneurons and integration into local circuits. Estrogen receptors (ERs) are steroid hormone receptors with important functions in neurogenesis and synaptic plasticity. In this study, we show that the ER GPER1 is expressed in subsets of cells within the V-SVZ of female animals and provide evidence for a potential local estrogen source from aromatase-positive astrocytes surrounding the RMS. Blocking of GPER1 in Matrigel cultures of female animals significantly impairs migration of V-SVZ-derived cells. This outgrowth is accompanied by regulation of phosphorylation of the actin-binding protein cofilin by GPER1 signaling including an involvement of the p21-Ras pathway. Our results point to a prominent role of GPER1 in the initiation of neuronal migration from the V-SVZ to the olfactory bulb.

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GPER1 is expressed within all cell types of the stem cell lineage in the V-SVZ

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Blocking of GPER1 leads to a decrease in migration of V-SVZ-derived neuroblasts

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GPER1 signaling in V-SVZ Matrigel cultures involves Ras-induced p21

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Blocking of GPER1 signaling leads to an increase in the ratio of p-cofilin/cofilin

Sex Differences in Estradiol Secretion by Trigeminal Brainstem Neurons

Estrogen status is a significant risk factor in the development of temporomandibular joint disorders (TMD). Classically, estrogen status is thought to derive mainly from ovarian sources; however, it is well known that estradiol (E2) also is synthesized by neurons in the brain. This study tested the hypothesis that E2 is produced by neurons in trigeminal subnucleus caudalis (Vc), the principal site of termination for sensory afferents that supply the temporomandibular joint (TMJ), to modify evoked responses in a model of TMJ nociception in male and female rats. Intra-TMJ injection of the small fiber excitant, allyl isothiocyanate (AIC), increased the levels of E2 collected from microdialysis probes sites at Vc of ovariectomized (OvX) female rats, ipsilateral to the stimulus, whereas males displayed no change. Dialysate levels of E2 collected from probe sites in the contralateral Vc or cerebellum in OvX rats were not affected by TMJ stimulation. Reverse dialysis of anastrozole, an aromatase (ARO) inhibitor, via the probe reduced perfusate levels of E2 in Vc. Systemic administration of letrozole, a non-steroid ARO inhibitor, for 4 days prevented TMJ-evoked increases in masseter muscle electromyography (MMemg) activity. ARO-positive neurons were distributed mainly in superficial laminae (I-III) at Vc and cell counts revealed no significant difference between OvX and male rats. Intra-TMJ injection of AIC revealed similar numbers of ARO/Fos dual-labeled neurons in OvX and male rats. By contrast, the percentage of ARO neurons co-labeled for glutamic acid decarboxylase (GAD), the biosynthetic enzyme for GABA, was greater in OvX (35%) than male rats (14%). Few ARO-positive neurons were co-labeled for estrogen receptor alpha. These data indicate that E2 is secreted continuously by Vc neurons and that acute stimulation of TMJ nociceptors evokes further secretion in a sex-dependent manner. Reduced TMJ-evoked MMemg activity after ARO inhibition suggests that locally produced E2 by Vc neurons acts via paracrine mechanisms to modify TMJ nociception in female rats.

17 beta-estradiol synthesis modulates cerebellar dependent motor memory formation in adult male rats

Neurosteroid 17 beta-estradiol (E2) is a steroid synthesized de novo in the nervous system that might influence neuronal activity and behavior. Nevertheless, the impact of E2 on the functioning of those neural systems in which it is slightly synthesized is less questioned. The vestibulo-ocular reflex (VOR) adaptation, may provide an ideal arena for investigating this issue. Indeed, E2 modulates cerebellar parallel fiber-Purkinje cell synaptic plasticity that underlies encoding of VOR adaptation. Moreover, aromatase expression in the cerebellum of adult rodents is maintained at very low levels and localized to Purkinje cells. The significance of age-related maintenance of low levels of aromatase expression in the cerebellum on behavior, however, has yet to be explored. Our aim in this study was to determine whether E2 synthesis exerts an effective and persistent modulation of VOR adaptation in adult male rats. To answer this question, we investigated the acute effect of blocking E2 synthesis on gain increases and decreases in VOR adaptation using an oral dose (2.5 mg/kg) of the aromatase inhibitor Letrozole in peri-pubertal and post-pubertal male rats. We found that Letrozole acutely impaired gain increases and decreases in VOR adaptation without altering basal ocular-motor performance and that these effects were similar in peri-pubertal and post-pubertal rats. Thus, in adult male rats neurosteroid E2 effectively modulates VOR adaptation in both of the periods studied. These findings imply that the adult cerebellum uses E2 synthesis for modulating motor memory formation and suggest that low and extremely localized E2 production may play a role in adaptive phenomena.

Sex Steroids and Adult Neurogenesis in the Ventricular-Subventricular Zone

The forebrain ventricular-subventricular zone (V-SVZ) continuously generates new neurons throughout life. Neural stem cells (type B1 cells) along the lateral ventricle become activated, self-renew, and give rise to proliferating precursors which progress along the neurogenic lineage from intermediate progenitors (type C cells) to neuroblasts (type A cells). Neuroblasts proliferate and migrate into the olfactory bulb and differentiate into different interneuronal types. Multiple factors regulate each step of this process. Newly generated olfactory bulb interneurons are an important relay station in the olfactory circuits, controlling social recognition, reproductive behavior, and parental care. Those behaviors are strongly sexually dimorphic and changes throughout life from puberty through aging and in the reproductive age during estrous cycle and gestation. Despite the key role of sex hormones in regulating those behaviors, their contribution in modulating adult neurogenesis in V-SVZ is underestimated. Here, we compare the literature highlighting the sexual dimorphism and the differences across the physiological phases of the animal for the different cell types and steps through the neurogenic lineage.

Neuronal aromatase expression in pain processing regions of the medullary and spinal cord dorsal horn

In both acute and chronic pain conditions, women tend to be more sensitive than men. This sex difference may be regulated by estrogens, such as estradiol, that are synthesized in the spinal cord and brainstem and act locally to influence pain processing. To identify a potential cellular source of local estrogen, here we examined the expression of aromatase, the enzyme that catalyzes the conversion of testosterone to estradiol. Our studies focused on primary afferent neurons and on their central targets in the spinal cord and medulla as well as in the nucleus of the solitary tract, the target of nodose ganglion-derived visceral afferents. Immunohistochemical staining in an aromatase reporter mouse revealed that many neurons in laminae I and V of the spinal cord dorsal horn and caudal spinal trigeminal nucleus and in the nucleus of the solitary tract express aromatase. The great majority of these cells also express inhibitory interneuron markers. We did not find sex differences in aromatase expression and neither the pattern nor the number of neurons changed in a sciatic nerve transection model of neuropathic pain or in the Complete Freund's adjuvant model of inflammatory pain. A few aromatase neurons express Fos after cheek injection of capsaicin, formalin, or chloroquine. In total, given their location, these aromatase neurons are poised to engage nociceptive circuits, whether it is through local estrogen synthesis or inhibitory neurotransmitter release.

Differences in peripheral sensory input to the olfactory bulb between male and female mice

Female mammals generally have a superior sense of smell than males, but the biological basis of this difference is unknown. Here, we demonstrate sexually dimorphic neural coding of odorants by olfactory sensory neurons (OSNs), primary sensory neurons that physically contact odor molecules in the nose and provide the initial sensory input to the brain’s olfactory bulb. We performed in vivo optical neurophysiology to visualize odorant-evoked OSN synaptic output into olfactory bub glomeruli in unmanipulated (gonad-intact) adult mice from both sexes, and found that in females odorant presentation evoked more rapid OSN signaling over a broader range of OSNs than in males. These spatiotemporal differences enhanced the contrast between the neural representations of chemically related odorants in females compared to males during stimulus presentation. Removing circulating sex hormones makes these signals slower and less discriminable in females, while in males they become faster and more discriminable, suggesting opposite roles for gonadal hormones in influencing male and female olfactory function. These results demonstrate that the famous sex difference in olfactory abilities likely originates in the primary sensory neurons, and suggest that hormonal modulation of the peripheral olfactory system could underlie differences in how males and females experience the olfactory world.

Modulatory Effects of Sex Steroids Progesterone and Estradiol on Odorant Evoked Responses in Olfactory Receptor Neurons

The influence of the sex steroid hormones progesterone and estradiol on physiology and behavior during menstrual cycles and pregnancy is well known. Several studies indicate that olfactory performance changes with cyclically fluctuating steroid hormone levels in females. Knowledge of the exact mechanisms behind how female sex steroids modulate olfactory signaling is limited. A number of different known genomic and non-genomic actions that are mediated by progesterone and estradiol via interactions with different receptors may be responsible for this modulation. Next generation sequencing-based RNA-Seq transcriptome data from the murine olfactory epithelium (OE) and olfactory receptor neurons (ORNs) revealed the expression of several membrane progestin receptors and the estradiol receptor Gpr30. These receptors are known to mediate rapid non-genomic effects through interactions with G proteins. RT-PCR and immunohistochemical staining results provide evidence for progestin and estradiol receptors in the ORNs. These data support the hypothesis that steroid hormones are capable of modulating the odorant-evoked activity of ORNs. Here, we validated this hypothesis through the investigation of steroid hormone effects by submerged electro-olfactogram and whole cell patch-clamp recordings of ORNs. For the first time, we demonstrate that the sex steroid hormones progesterone and estradiol decrease odorant-evoked signals in the OE and ORNs of mice at low nanomolar concentrations. Thus, both of these sex steroids can rapidly modulate the odor responsiveness of ORNs through membrane progestin receptors and the estradiol receptor Gpr30.

Anatomically discrete sex differences in neuroplasticity in zebra finches as reflected by perineuronal nets

Large morphological sex differences in the vertebrate brain were initially identified in song control nuclei of oscines. Besides gross differences between volumes of nuclei in males and females, sex differences also concern the size and dendritic arborization of neurons and various neurochemical markers, such as the calcium-binding protein parvalbumin (PV). Perineuronal nets (PNN) of the extracellular matrix are aggregates of different compounds, mainly chondroitin sulfate proteoglycans, that surround subsets of neurons, often expressing PV. PNN develop in zebra finches song control nuclei around the end of the sensitive period for song learning and tutor deprivation, known to delay the end of the song learning sensitive period, decreases the numbers of PNN in HVC. We demonstrate here the existence in zebra finches of a major sex difference (males > females) affecting the number of PNN (especially those surrounding PV-positive cells) in HVC and to a smaller extent the robust nucleus of the arcopallium, RA, the two main nuclei controlling song production. These differences were not present in Area X and LMAN, the lateral magnocellular nucleus of the anterior nidopallium. A dense expression of material immunoreactive for chondroitin sulfate was also detected in several nuclei of the auditory and visual pathways. This material was often organized in perineuronal rings but quantification of these PNN did not reveal any sex difference with the exception that the percentage of PNN surrounding PV-ir cells in the dorsal lateral mesencephalic nucleus, MLd, was larger in females than in males, a sex difference in the opposite direction compared to what is seen in HVC and RA. These data confirm and extend previous studies demonstrating the sex difference affecting PNN in HVC-RA by showing that this sex difference is anatomically specific and does not concern visual or auditory pathways.

Estrogenic influences in pain processing

Gonadal hormones not only play a pivotal role in reproductive behavior and sexual differentiation, they also contribute to thermoregulation, feeding, memory, neuronal survival, and the perception of somatosensory stimuli. Numerous studies on both animals and human subjects have also demonstrated the potential effects of gonadal hormones, such as estrogens, on pain transmission. These effects most likely involve multiple neuroanatomical circuits as well as diverse neurochemical systems and they therefore need to be evaluated specifically to determine the localization and intrinsic characteristics of the neurons engaged. The aim of this review is to summarize the morphological as well as biochemical evidence in support for gonadal hormone modulation of nociceptive processing, with particular focus on estrogens and spinal cord mechanisms.

Mechanisms of pain modulation by sex hormones in migraine

A number of pain conditions, acute as well as chronic, are much more prevalent in women, such as temporomandibular disorder (TMD), irritable bowel syndrome, fibromyalgia, and migraine. The association of female sex steroids with these nociceptive conditions is well known, but the mechanisms of their effects on pain signaling are yet to be deciphered. We reviewed the mechanisms through which female sex steroids might influence the trigeminal nociceptive pathways with a focus on migraine. Sex steroid receptors are located in trigeminal circuits, providing the molecular substrate for direct effects. In addition to classical genomic effects, sex steroids exert rapid nongenomic actions to modulate nociceptive signaling. Although there are only a handful of studies that have directly addressed the effect of sex hormones in animal models of migraine, the putative mechanisms can be extrapolated from observations in animal models of other trigeminal pain disorders, like TMD. Sex hormones may regulate sensitization of trigeminal neurons by modulating expression of nociceptive mediator such as calcitonin gene-related peptide. Its expression is mostly positively regulated by estrogen, although a few studies also report an inverse relationship. Serotonin (5-Hydroxytryptamine [5-HT]) is a neurotransmitter implicated in migraine; its synthesis is enhanced in most parts of brain by estrogen, which increases expression of the rate-limiting enzyme tryptophan hydroxylase and decreases expression of the serotonin re-uptake transporter. Downstream signaling, including extracellular signal-regulated kinase activation, calcium-dependent mechanisms, and cAMP response element-binding activation, are thought to be the major signaling events affected by sex hormones. These findings need to be confirmed in migraine-specific animal models that may also provide clues to additional ion channels, neuropeptides, and intracellular signaling cascades that contribute to the increased prevalence of migraine in women.

Opposite long-term synaptic effects of 17β-estradiol and 5α-dihydrotestosterone and localization of their receptors in the medial vestibular nucleus of rats

In brainstem slices of male rats, we examined in single neurons of the medial vestibular nucleus (MVN) the effect of exogenous administration of estrogenic (17β-estradiol, E2) and androgenic (5α-dihydrotestosterone, DHT) steroids on the synaptic response to vestibular afferent stimulation. By whole cell patch clamp recordings we showed that E2 induced synaptic long-term potentiation (LTP) that was cancelled by the subsequent administration of DHT. Conversely, DHT induced synaptic long-term depression (LTD) that was partially reversed by E2. The electrophysiological findings were supported by immunohistochemical analysis showing the presence of estrogen (ER: α and β) and androgen receptors (AR) in the MVN neurons. We found that a large number of neurons were immunoreactive for ERα, ERβ, and AR and most of them co-localized ERβ and AR. We also showed the presence of P450-aromatase (ARO) in the MVN neurons, clearly proving that E2 can be locally synthesized in the MVN. On the whole, these results demonstrate a role of estrogenic and androgenic signals in modulating vestibular synaptic plasticity and suggest that the enhancement or depression of vestibular synaptic response may depend on the local conversion of T into E2 or DHT.

Alterations of gene expression of sodium channels in dorsal root ganglion neurons of estrogen receptor knockout (ERKO) mice induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

Estrogen receptors (ERα and ERβ) mediate the neuroprotection of estrogens against MPTP-induced striatal dopamine (DA) depletion. Pain is an important and distressing symptom in Parkinson's disease (PD). Voltage-gated sodium channels in sensory neurons are involved in the development of neuropathic pain. In this study, MPTP caused changes in nociception and alterations of gene expression of voltage-gated sodium channels in dorsal root ganglion (DRG) neurons in ER knockout (ERKO) mice were investigated. We found that administration of MPTP (11 mg/kg) to WT mice led to an extensive depletion of DA and its two metabolites, αERKO mice were observed to be more susceptible to MPTP toxicity than βERKO or WT mice. In addition, we found that the mRNA levels of TTX-S and TTX-R sodium channel subtypes were differentially affected in MPTP-treated WT animals. The MPTP-induced up-regulation of Nav1.1 and Nav1.9, down-regulation of Nav1.6 in DRG neurons may be through ERβ, up-regulation of Nav1.7 and down-regulation of Nav1.8 are dependent on both ERα and ERβ. Therefore, the MPTP-induced alterations of gene expression of sodium channels in DRG neurons could be an important mechanism to affect excitability and nociceptive thresholds, and the ERs appear to play a role in nociception in PD.

Influence of sex and estrous cycle on synaptic responses of the medial vestibular nuclei in rats: role of circulating 17β-estradiol

We investigated the possible influence of sex and estrous cycle on the synaptic responses of neurons in the medial vestibular nucleus (MVN) and their long-term modifications. In brain stem slices of male and female rats during proestrus (PE) and diestrus (DE), we evaluated the field potential evoked in the MVN by vestibular afferent stimulation. Here we find that in PE females the field potential had a lower threshold and higher amplitude than in DE females and in males and also that the stimulus-response curve was shifted to the left. Such difference is related to the level and cyclic fluctuation of circulating 17β-estradiol (E(2)). This is supported by the exogenous administration of E(2) in DE females and males, with low levels of circulating E(2) that enhanced the field potential amplitude to values close to those of PE females. Sex and estrous cycle also influence the MVN synaptic plasticity. This has been shown by investigating the effect of testosterone (T) on the induction of long-term effects, since T is the precursor for the neural synthesis of E(2) (estrogenic pathway), which is involved in the induction of fast long-term potentiation (LTP), or of 5α-dihydrotestosterone (DHT, androgenic pathway) which mediates slow LTP and long-term depression (LTD). We found that T mostly induced LTD in PE females and no effect in DE females, while it only provoked fast LTP in males. We suggest that high level of circulating E(2) may interfere with the conversion of T, by inhibiting the neural estrogenic pathway and facilitating the androgenic one. On the whole these results demonstrate an influence of circulating E(2) on vestibular synaptic transmission and plasticity that in some cases may contribute to the sex and menstrual cycle dependence of symptoms in human vestibular pathology.

Sex differences in aromatase gene expression in the medaka brain

The brain of teleost fish exhibits a significant degree of sexual plasticity, even in adulthood. This unique feature is almost certainly attributable to a teleost-specific sexual differentiation process of the brain, which remains largely unknown. To dissect the molecular basis of sexual differentiation of the teleost brain, we searched for genes differentially expressed between both sexes in the medaka brain. One gene identified in the screen, cyp19a1b, which encodes the steroidogenic enzyme aromatase, was selected for further analysis. As opposed to the situation in most vertebrates, medaka cyp19a1b is expressed at higher levels in the adult female brain than the male brain. The female-biased expression in the brain is consistent regardless of reproductive or diurnal cycle. Medaka cyp19a1b is expressed throughout the ventricular zones in wide areas of the brain, where, in most regions, females have a greater degree of expression compared to males, with the optic tectum exhibiting the most conspicuous predominance in females. Contrary to what is known in mammals, cyp19a1b expression exhibits neither a transient elevation nor a sex difference in medaka embryos. It is not until just before the onset of puberty that cyp19a1b expression in the medaka brain is sexually differentiated. Finally, cyp19a1b expression in the medaka brain is not under the direct control of sex chromosome genes but relies mostly, if not solely, on oestrogen derived from the gonad. These unique properties of aromatase expression in the brain probably contribute substantially to the less rigid sexual differentiation process, thus ensuring remarkable sexual plasticity in the teleost brain.

Cyclic estrogenic fluctuation influences synaptic transmission of the medial vestibular nuclei in female rats

CONCLUSION

The estrous cycle in female rats influences the basal synaptic responsiveness and plasticity of the medial vestibular nucleus (MVN) neurons through different levels of circulating 17β-estradiol (cE(2)).

OBJECTIVE

The aim of this study was to verify, in the female rat, whether cyclic fluctuations of cE(2) influence long-term synaptic effects induced by high frequency afferent stimulation (HFS) in the MVN, since we found that HFS in the male rat induces fast long-term potentiation (fLTP), which depends on the neural synthesis of E(2) (nE(2)) from testosterone (T).

METHODS

We analyzed the field potential (FP) evoked in the MVN by vestibular afferent stimulation, under basal conditions, and after HFS, in brainstem slices of female rats during high levels (proestrus, PE) and low levels (diestrus, DE) of cE(2). Selective blocking agents of converting T enzymes were used.

RESULTS

Unlike in the male rat, HFS induced three effects: fLTP through T conversion into E(2), and slow LTP (sLTP) and long-term depression (LTD), through T conversion into DHT. The occurrence of these effects depended on the estrous cycle phase: the frequency of fLTP was higher in DE, and those of sLTP and LTD were higher in PE. Conversely, the basal FP was also higher in PE than in DE.

Influence of testosterone on synaptic transmission in the rat medial vestibular nuclei: estrogenic and androgenic effects

In brainstem slices of young male rat, we investigated the influence of the neuroactive steroid testosterone (T) on the synaptic responses by analyzing the field potential evoked in the medial vestibular nucleus (MVN) by vestibular afferent stimulation. T induced three distinct and independent long-term synaptic changes: fast long-lasting potentiation (fLP), slow long-lasting potentiation (sLP) and long-lasting depression (LD). The fLP was mediated by 17β-estradiol (E(2)) since it was abolished by blocking the estrogen receptors (ERs) or the enzyme converting T to E(2). Conversely, sLP and LD were mediated by 5α-dihydrotestosterone (DHT) since they were prevented by blocking the androgen receptors (ARs) or the enzyme converting T to DHT. Therefore, the synaptic effects of T were mediated by its androgenic or estrogenic metabolites. The pathways leading to estrogenic and androgenic conversion of T might be co-localized since, the occurrence of fLP under block of androgenic pathway, and that of sLP and LD under estrogenic block, were higher than those observed without blocks. In case of co-localization, the effect on synaptic transmission should depend on the prevailing enzymatic activity. We conclude that circulating and neuronal T can remarkably influence synaptic responses of the vestibular neurons in different and opposite ways, depending on its conversion to estrogenic or androgenic metabolites.

Effects of 17beta-estradiol on glutamate synaptic transmission and neuronal excitability in the rat medial vestibular nuclei

We investigated the effects of the neurosteroid 17beta-estradiol (E(2)) on the evoked and spontaneous activity of rat medial vestibular nucleus (MVN) neurons in brainstem slices. E(2) enhances the synaptic response to vestibular nerve stimulation in type B neurons and depresses the spontaneous discharge in both type A and B neurons. The amplitude of the field potential, as well as the excitatory post-synaptic potential (EPSP) and current (EPSC), in type B neurons, are enhanced by E(2). Both effects are long-term phenomena since they outlast the drug washout. The enhancement of synaptic response is mainly due to facilitation of glutamate release mediated by pre-synaptic N-methyl-D-aspartate receptors (NMDARs), since the reduction of paired pulse ratio (PPR) and the increase of miniature EPSC frequency after E(2) are abolished under D-(-)-2-amino-5-phosphonopentanoic acid (AP-5). E(2) also facilitates post-synaptic NMDARs, but it does not affect directly alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and group I-metabotropic glutamate receptors (mGluRs-I). In contrast, the depression of the spontaneous discharge of type A and type B neurons appears to depend on E(2) modulation of intrinsic ion conductances, as the effect remains after blockade of glutamate, GABA and glycine receptors (GlyRs). The net effect of E(2) is to enhance the signal-to-noise ratio of the synaptic response in type B neurons, relative to resting activity of all MVN neurons. These findings provide evidence for a novel potential mechanism to modulate the responsiveness of vestibular neurons to afferent inputs, and so regulate vestibular function in vivo.

Long-term potentiation in the rat medial vestibular nuclei depends on locally synthesized 17beta-estradiol

In male rat brainstem slices, we investigated the involvement of locally synthesized 17beta-estradiol (E(2)) in the induction in the medial vestibular nucleus (MVN) of long-term potentiation (LTP) by high-frequency stimulation (HFS) of the primary vestibular afferents. We demonstrated that the blockade of aromatase by letrozole or of E(2) receptors (ERalpha and ERbeta) by ICI 182,780 prevented the HFS-induced LTP of the N1 wave of the evoked field potential (FP) without affecting baseline responses. Only prolonged afferent activation could induce low LTP. In contrast, HFS applied under a combined blockade of GABA(A) receptors and aromatase or ERs was still able to induce LTP, but it was significantly lower and slower. These findings demonstrate that E(2) does not have a tonic influence on the activity of the MVN neurons and provide the first evidence of the crucial role played by local synthesis of E(2) in inducing LTP. We suggest that the synthesis of E(2) occurs after aromatase activation during HFS and facilitates the development of vestibular synaptic plasticity by influencing glutamate and GABA transmission.

Brain aromatization: classic roles and new perspectives

Aromatization of testosterone to estradiol by neural tissue has classically been associated with the regulation of sexual differentiation, gonadotropin secretion, and copulatory behavior. However, new data indicate that the capacity for aromatization is not restricted to the endocrine brain and demonstrate roles for locally formed estrogens in neurogenesis and in responses of brain tissue to injury. This article summaries our current understanding of the distribution and regulation of aromatase in the brain and describes the classic and novel roles it plays. A better understanding of brain aromatization could shed new light on its physiologic and pathologic functions and someday lead to new, centrally acting drug therapies.

Effects of 17beta-estradiol on synaptic plasticity in the rat medial vestibular nuclei

CONCLUSION

This study shows that 17beta-estradiol (E(2)) can amplify the long-term potentiation (LTP) induced in the vestibular nuclei by high frequency stimulation (HFS), while potentiation induced by E(2) alone, which is unrelated to synaptic high frequency activation, is reversed by HFS.

OBJECTIVE

Like HFS, exogenous E(2) induces long-lasting enhancement of synaptic responses to vestibular afferent stimulation in the medial vestibular nuclei (MVN), through NMDA receptor activation. The aim of this study was to verify the possible interaction of E(2) and HFS in inducing LTP.

MATERIALS AND METHODS

In rat brainstem slices, we analysed the modifications induced in the field potential evoked in the MVN by: 1) HFS delivered after induction of E(2) effect and 2) E(2) applied after induction of HFS-LTP.

RESULTS

HFS reversed the E(2)-induced potentiation in most cases, while E(2) was able to increase the magnitude of potentiation induced by HFS.

Organizational influence of the postnatal testosterone surge on the circadian rhythm of core body temperature of adult male rats

The suprachiasmatic nucleus (SCN) of the hypothalamus coordinates physiological and behavioral circadian rhythms such as activity, body temperature, and hormone secretion. Circadian rhythms coordinated by the SCN often show sex differences arising from both organizational and activational effects of gonadal hormones. In males, little is known about the organizational role of testosterone on the circadian regulation of core body temperature (CBT) in adulthood. To explore this, we castrated or sham-operated male rats on the day of birth, and at 4 months of age, implanted them with transmitters that measured CBT rhythms under a 12:12 light/dark cycle. This study revealed a significantly earlier rise in CBT during the light phase in neonatally castrated males. Subsequently, we found that treating neonatally castrated males with testosterone propionate (TP) in adulthood did not reverse the effect of neonatal castration, thus indicating an organizational role for testosterone. In contrast, a single injection of TP at the time of neonatal surgery, to mimic the postnatal surge of testosterone, coupled with TP treatment in adulthood, normalized the circadian rise in CBT. In a final study we examined CBT circadian rhythms in intact adult male and female rats and detected no differences in the rise of CBT during the light phase, although there was a greater overall elevation in female CBT. Together, results of these studies reveal an early organizational role of testosterone in males on the timing of the circadian rise of CBT, a difference that does not appear to reflect "defeminization".

Oestrogen receptor alpha and beta immunoreactive cells in the suprachiasmatic nucleus of mice: distribution, sex differences and regulation by gonadal hormones

Oestrogen regulates various aspects of circadian rhythm physiology. The presence of oestrogen receptors within the suprachiasmatic nucleus (SCN), the principal circadian oscillator, indicates that some actions of oestrogen on circadian functions may be exerted at that site. The present study analysed sex differences, topographic distribution, and neurochemical phenotype of neurones expressing the alpha and beta subtypes of oestrogen receptors (ERalpha and ERbeta) in the mouse SCN. We found that relatively few neurones in the SCN are immunoreactive (IR) for ERalpha (approximately 4.5% in females and 3% in males), but five- to six-fold more SCN neurones express ERbeta. ER-IR neurones are primarily in the shell subdivision of the nucleus and show differences between the sexes, significantly greater numbers being found in females. Treatment of male or female gonadectomised mice with oestradiol benzoate for 24 h substantially reduced the number of ERbeta-IR neurones, but not ERalpha-IR neurones. Double-labelling immunocytochemical experiments to characterise the phenotype of the oestrogen-receptive neurones showed the presence of the calcium-binding proteins calretinin or calbindin D28K in approximately 12% and 10%, respectively, of ERalpha-IR neurones. A higher proportion (approximately 38%) of ERbeta-IR neurones contains calbindin D28K; a few (approximately 2%) express calretinin or vasopressin. These double-labelled cells appear primarily in the shell subdivision of the SCN. Neither vasoactive intestinal polypeptide- nor gastrin releasing peptide-immunoreactivity was observed in ER-IR neurones. These data indicate that the primary target cells for oestrogen are in the shell subdivision of the nucleus. The sexually differentiated expression and distribution of ERalpha and ERbeta in various cell populations of the SCN suggest multiple modes of oestrogen signalling within this nucleus, which may modulate circadian functions.

Gonadectomy reveals sex differences in circadian rhythms and suprachiasmatic nucleus androgen receptors in mice

In mammals, it is well established that circadian rhythms in physiology and behavior, including the rhythmic secretion of hormones, are regulated by a brain clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. While SCN regulation of gonadal hormone secretion has been amply studied, the mechanisms whereby steroid hormones affect circadian functions are less well known. This is surprising considering substantial evidence that sex hormones affect many aspects of circadian responses, and that there are significant sex differences in rhythmicity. Our previous finding that "core" and "shell" regions of the SCN differ in their expression of clock genes prompted us to examine the possibility that steroid receptors are localized to a specific compartment of the brain clock, with the discovery that the androgen receptor (AR) is concentrated in the SCN core in male mice. In the present study, we compare AR expression in female and male mice using Western blots and immunochemistry. Both of these methods indicate that ARs are more highly expressed in males than in females; gonadectomy eliminates and androgen treatment restores these sex differences. At the behavioral level, gonadectomy produces a dramatic loss of the evening activity onset bout in males, but has no such effect in females. Treatment with testosterone, or with the non-aromatizable androgen dihydrotestosterone, restores male locomotor activity and eliminates sex differences in the behavioral response. The results indicate that androgenic hormones regulate circadian responses, and suggest an SCN site of action.

Nature's food anticipatory experiment: entrainment of locomotor behavior, suprachiasmatic and dorsomedial hypothalamic nuclei by suckling in rabbit pups

In nature and under laboratory conditions, dams nurse rabbit pups once daily for a duration of fewer than 5 min. The present study explored neural mechanisms mediating the timing of nursing in this natural model of food anticipatory activity, focussing on the suprachiasmatic nucleus (SCN), the locus of the master circadian clock and on the dorsomedial hypothalamic nucleus (DMH), a region implicated in timing of food-entrained behavior. Rabbit pups are born in the dark, with eyelids closed. Nursing visits to the litters also occurs during the dark phase. To explore the effect of the timing of feeding, pups were maintained in constant darkness, while females housed in a light-dark cycle were permitted to nurse their pups either during the night (night-fed group) or day (day-fed group). All pups exhibited anticipatory locomotor activity before daily nursing. In the SCN, PER1 and FOS peaked during the night in both groups, with a longer duration of elevated protein expression in the night-fed group. In contrast, DMH peak PER1 expression occurred 8 h after pups were fed, corresponding to the shift in timing of nursing. Comparison of nursed and 48 h fasted pups indicates that the timing of PER1 expression was similar in the SCN and DMH, with fewer PER1-positive cells in the latter group. The results indicate that rabbit pups show food anticipatory activity, and that timing of nursing differentially affects PER1 expression in the SCN and DMH.

Brain aromatase: roles in reproduction and neuroprotection

It is well established that aromatization constitutes an essential part of testosterone's signaling pathway in brain and that estrogen metabolites, often together with testosterone, organize and activate masculine neural circuits. This paper summarizes the current understanding regarding the distribution, regulation and function of brain aromatase in mammals. Data from our laboratory are presented that highlight the important function of aromatase in the regulation of androgen feedback sensitivity in non-human primates and the possible role that aromatase plays in determining the brain structure and sexual partner preferences of rams. In addition, new data is presented indicating that the capacity for aromatization in cortical astrocytes is associated with cell survival and may be important for neuroprotection. It is anticipated that a better appreciation of the physiological and pathophysiological functions of aromatase will lead to important clinical insights.

Distribution of estrogen receptor alpha mRNA in the brain and inner ear of a vocal fish with comparisons to sites of aromatase expression

Among vertebrates, teleost fish have the greatest capacity for estrogen production in the brain. Previously, we characterized the distribution of the estrogen-synthesizing enzyme aromatase in the brain of the midshipman fish. Here, we investigated the distribution of estrogen receptor alpha (ERalpha). A partial cDNA of ERalpha was cloned and used to generate midshipman-specific primers for RT and real-time PCR which identified transcripts in liver and ovary, the CNS, and the sensory epithelium of the main auditory endorgan (sacculus). In situ hybridization revealed abundant expression throughout the preoptic area, a vocal-acoustic site in the hypothalamus, amygdala homologs of the dorsal pallium, the pineal organ, the inner ear, the pituitary, and the ovary. Weaker expression was found in the midbrain's nucleus of the medial longitudinal fasciculus and in the dimorphic vocal motor nucleus. ERalpha expression in the pineal, gonad, and pituitary axis may function to time seasonal abiotic cues to reproductive state, while expression in the vocal motor and auditory systems support neurophysiological evidence for estrogen as a modulator of vocal motor and auditory encoding mechanisms in midshipman fish. While ERalpha is restricted to specific nuclei, aromatase expression is abundant in glial cells throughout the entire forebrain, and high in midbrain and hindbrain - spinal vocal regions. The only site of aromatase-containing neurons is in the peripheral auditory system, where it is localized to ganglion cells in the auditory nerve. Estrogen production proximal to ERalpha-positive neurons may provide for focal sites of estrogen effects on reproductive-, vocal-, and auditory-related neurons.

Estradiol enhances light-induced expression of transcription factors in the SCN

The suprachiasmatic nucleus of the hypothalamus (SCN) is the master clock that regulates circadian and seasonal rhythms. Among these, the SCN regulates the phasic release of hormones and provides for the timing of the preovulatory luteinizing hormone (LH) surge necessary for ovulation in females. There is little evidence, however, of sex hormone effects on mechanisms underlying SCN function. This study examined the effects of exogenous administration of estradiol on the light-induced expression of transcription factors in the SCN of female rats. Ovariectomized (OVX) female rats were given estradiol or cholesterol implants and perfused 48 h later. Half of the animals were sacrificed 1 h after the regular onset of light within the colony. The rest had the lights go on 2 h prior to the regular time and perfused 1 h later. Collected brains were sliced and sets of SCN sections were processed for immunoreactivity (ir) detecting the Fos, pCREB, egr-1, CREB binding protein (CBP), and calbindin-D (28K) proteins. Following quantification, statistical analyses demonstrated that estradiol enhanced Fos and p-CREB-ir in the SCN of females that experienced a 2-h phase advance. The phase advance also enhanced calbindin and egr-1-ir, but the expression of these proteins was not affected by estradiol. These results demonstrate that estradiol enhances the levels of transcription factors that precede the expression of clock gene proteins in the SCN in response to advances in the onset of environmental light. These data support the hypothesis that steroid hormones play an important role in the fine tuning of the clock in the face of environmental changes in daylight.

Sex differences in adult suprachiasmatic nucleus neurons emerging late prenatally in rats

The suprachiasmatic nucleus (SCN) is implicated in the control of circadian rhythms of gonadal function. Although several structures surrounding the SCN are sensitive to the effects of gonadal steroids, similar effects in the SCN remain unclear. For example, there are conflicting data on whether the SCN is sexually differentiated. This study attempted to determine sex differences in the number of SCN cells generated during late gestation, and if testosterone mediates these differences. Pregnant female rats were treated with 5-bromo-2'-deoxyuridine (BrdU; 50 mg/kg) on gestational day 18 (E18), the day when aromatase activity peaks in the developing rat fetus. These animals were also given injections of oil or testosterone propionate (10 mg/0.1 mL peanut oil) from E15 until parturition. Litters were allowed to survive until adulthood and were killed on postnatal day 60 (PN60). Following fixation, brain sections containing the SCN from these rats were processed for BrdU immunocytochemistry. A second set of SCN sections was processed for immunocytochemistry detecting BrdU and some of the cell groups prevalent within the SCN. Data showed that female rats have a higher number of cells labeled with BrdU in the SCN, particularly in the medial and caudal SCN. This sex difference was abolished in animals treated with testosterone during late gestation. Double immunocytochemistry revealed that BrdU-labeled cells were neurons expressing calbindin-D28K, vasoactive intestinal peptide and, to a lesser degree, vasopressin. Our results unveiled a previously unknown effect of gonadal steroids on the developing SCN, which may contribute to the emergence of gender-specific circadian rhythms.

Immunocytochemical localization of aromatase in sensory and integrating nuclei of the hindbrain in Japanese quail (Coturnix japonica)

The distribution of the estrogen synthesizing enzyme (aromatase) in the hindbrain (rhombencephalon and mesencephalon) of male adult quail was investigated by immunocytochemistry. Aromatase-immunoreactive neuronal structures (perikarya and fibers bearing punctate structures) were observed in sensory (trigeminal, solitary tract, vestibular, optic tectum) and integrating (parabrachial, periaqueductal, cerulean, raphe) nuclei. Besides the expression of aromatase in these well-delineated nuclei, dense to scattered networks of immunoreactive fibers were found dispersed throughout the hindbrain and, in particular, in its rostral and dorsal parts. To a lesser extent, they were also present throughout the premotor nuclei of the reticular formation and in various fiber tracts. In contrast, no immunoreactive signal was found in motor nuclei, and in most of the statoacoustic (cerebellum, cochlear, olive, pontine, part of vestibular) nuclei. The expression of aromatase in perikarya and fibers in areas of the adult hindbrain where estrogen receptors have been identified previously suggests a role for estrogens locally produced in the regulation of sensory and integrating functions, contrary to the widespread assumption that these functions are regulated exclusively by steroids produced in the gonads.

Estrogens and ICI182,780 (Faslodex) modulate mitosis and cell death in immature cerebellar neurons via rapid activation of p44/p42 mitogen-activated protein kinase

Estrogen influences the development and function of the nervous system through estrogen receptor-dependent changes in gene expression and by rapidly influencing diverse intracellular signaling pathways. We have investigated the influence of estradiol on developing neonatal rat cerebellar neurons in primary culture and found that low concentrations of 17beta-estradiol (17beta-E2), 17alpha-E2, 17beta-E2-BSA, and ICI182,780 stimulated phosphorylation of the extracellular signal-regulated kinases 1/2 (ERK1/2) mitogen-activated protein kinases (MAPK). Neither testosterone nor progesterone increased ERK1/2 phosphorylation. The effects of the estrogens were specific to the ERK1/2 MAPK pathway and were blocked by U0126, an inhibitor of the ERK1/2 MAPK kinase (MEK1/2). Compared with control cultures, significant MAPK-dependent decreases in viable granule cell numbers were observed in dissociated explant cultures of developing cerebellar neurons 24-96 hr after pulse treatment with 10 pm 17beta-E2 or 10 nm ICI182,780. In contrast, continuous exposure to 10 pm 17beta-E2 significantly increased granule cell numbers. Analysis of bromodeoxyuridine incorporation revealed that a 15 min pulsed treatment with 10 pm 17beta-E2 increased mitogenesis, whereas continuous exposure to the same concentration of 17beta-E2 was anti-mitotic. Estradiol did not increase caspase activity; however, significant increases in cellular permeability and lysis were observed. Cell lysis and death were independent of the pan-caspase inhibitor zVAD-fmk but were blocked fully by the irreversible calpain inhibitor PD150606. These results indicate that rapid activation of the ERK1/2 MAPK pathway by low concentrations of 17beta-E2 induces oncotic/necrotic, but not apoptotic, programmed cell death in a subpopulation of developing granule cells and increased mitogenesis of the granule cell neuroblasts refractory to estrogen-induced neurotoxicity.

Temperature influences the ontogenetic expression of aromatase and oestrogen receptor mRNA in the developing tilapia (Oreochromis mossambicus) brain

Water temperature has a differential influence on the development of central neurotransmitter systems according to the developmental period in tilapia (Oreochromis mossambicus). Aromatase and oestrogen receptors (ERs) represent important components of the mechanism of brain differentiation. Gene expression of aromatase and ERs is modulated by neurotransmitters in the developing brain. In the present study, the quantitative reverse transcription-polymerase chain reaction method was used to investigate the effects of temperature on the ontogenetic expression of aromatase and ERs in the developing tilapia brain. Before day 10 posthatching, exposure to a higher temperature (32 degrees C) resulted in a significant increase in the expression of brain aromatase; conversely, a lower temperature (20 degrees C) resulted in a decrease. ERalpha expression was depressed in accordance with the decrease of temperature, but ERbeta was unaffected by temperature. Between days 10 and 20, neither brain aromatase nor ERalpha expression was altered by temperature, whereas ERbeta expression was significantly enhanced by exposure to 32 degrees C. Between days 20 and 30, brain aromatase significantly increased at the higher temperature and decreased at 20 degrees C, but neither ERalpha nor ERbeta was affected by temperature. The expression of both brain aromatase and ERs, differentially regulated according to the temperature and to the developmental period, could be related to brain-sex differentiation.

Male rats require testosterone to develop contralateral digastric muscle activity in response to noxious stimulation of the temporomandibular joint

The influence of testosterone on the postnatal development of reflex electromyographic (EMG) jaw muscle activity evoked by injection of mustard oil (MO) into the temporomandibular joint region and the later recurrence of this EMG activity after intravenous injection of naloxone, was studied in male rats. MO-evoked EMG activity in the contralateral digastric muscle and naloxone-induced recurrence of this EMG activity were fully developed in intact, 8-week-old rats. Castration at 4 weeks of age inhibited the development of the contralateral MO-evoked EMG activity, but did not influence the naloxone-induced recurrence. Contralateral MO-evoked responses were observed in 8-week-old castrated rats if they received testosterone replacement therapy beginning at 4 weeks of age. These data suggest that testosterone is required for the development of a contralateral nociceptive reflex in the digastric muscle of male rats.

Mechanisms in tissue-specific regulation of estrogen biosynthesis in humans

In humans, aromatase P450, which catalyses conversion of C(19)-steroids to estrogens, is expressed in several tissues, including gonads, brain, adipose tissue, skin and placenta, and is encoded by a single-copy gene (CYP19); however, this does not hold true for all species. The human gene is approximately 130 kb and its expression is regulated, in part, by tissue-specific promoters and by alternative splicing mechanisms. Using transgenic mouse technology, it was observed that ovary-, adipose tissue- and placenta-specific expression of human CYP19 is directed by relatively small segments of DNA within 500 bp upstream of each of the tissue-specific first exons. Thus, the use of alternative promoters allows greater versatility in tissue-specific regulation of CYP19 expression. Characterization and identification of transcription factors and crucial cis-acting elements within genomic regions that direct tissue-specific expression will contribute to improved understanding of the regulation of CYP19 expression in the tissues that synthesize estrogens under both physiological and pathophysiological conditions.

Brain androgen and progesterone metabolizing enzymes: biosynthesis, distribution and function

This review summarizes the biosynthesis, cell type-distribution and function of brain aromatase cytochrome P450 (P450aro) and 5alpha-reductase enzymes. This overview covers the impact of the steroid products of the P450aro and 5alpha-reductase enzymes in establishing sexually dimorphic brain structures, specifically the sexually dimorphic nucleus of the preoptic area (SDN) and the anteroventral periventricular nucleus (AVPV). Additionally, since metabolites of the P450aro and 5alpha-reductase enzymes are known to regulate the calcium-binding protein, calbindin (CALB), CALB is reviewed in relationship to its potential role in determining sexually dimorphic brain structures. Finally, recent reports indicate that phytoestrogens inhibit P450aro and 5alpha-reductase activities in peripheral tissue sites, therefore, the effects of phytoestrogens on brain P450aro and 5alpha-reductase are briefly considered and the impact of consuming a high vs. a low phytoestrogen diet on visual spatial memory in male and female rats is presented.

The contribution of adrenal and reproductive hormones to the opposing effects of stress on trace conditioning in males versus females

Exposure to an acute stressful experience facilitates classical conditioning in male rats but impairs conditioning in female rats (T. J. Shors, C. Lewczyk, M. Paczynski, P. R. Mathew, & J. Pickett, 1998; G. E. Wood & T. J. Shors, 1998). The authors report that these effects extend to performance on the hippocampal-dependent task of trace conditioning. The stress-induced impairment of conditioning in females was evident immediately, 24 hr and 48 hr after stress, depending on the stage of estrus. Moreover, the effect could be reactivated days later by reexposure to the stressful context. Corticosterone levels correlated with overall performance in males but not in females. Unlike the effect seen in males, adrenalectomy did not prevent the stress-induced effect on conditioning in females. These data indicate that exposure to the same experience can have opposite effects on learning in males versus females and that these opposing effects are mediated by differing hormonal systems.

Sexual dimorphism and aromatase in the rat retina

The present study: (a) determined the effects of both pre- and early postnatal androgen hormonal manipulation on retinal thickness, (b) examined the presence of the aromatase enzyme by immunocytochemistry in retinal tissue and (c) quantified aromatase activity in the rat retina. The results suggest that retinal thickness is influenced by perinatal hormone manipulation via aromatizable androgens that have implications for the sex differences seen in visual information processing and performance.

Effects of gonadal steroids on brain serotonergic and aromatase activity during the critical period of sexual differentiation in tilapia, Oreochromis mossambicus

The effects of gonadal steroids on brain serotonin (5-HT) and aromatase activity during the critical period of sexual differentiation were investigated in tilapia, Oreochromis mossambicus. Treatment of tilapia with 17beta-oestradiol (E2) between days 7 and 10 posthatching resulted in a significant increase in the female : male ratio as determined at day 90, and a significant reduction in brain 5-HT content. Treatment between days 10 and 20, or between days 20 and 30, had no significant effect. Since the 5-HT system may influence sexual differentiation, we examined the effects of para-chlorophenylalanine (p-CPA), a 5-HT synthesis inhibitor. As with E2, treatment of tilapia with p-CPA between days 7 and 10 posthatching resulted in a significant increase in the female : male ratio. Again, treatment between days 10 and 20, or between days 20 and 30, had no significant effect. Both p-CPA and E2 significantly depressed brain aromatase activity when administrated between days 7 and 10, but not subsequently. In tilapia treated between days 7 and 10, the brain 5-HT content was lowered by E2 to an extent similar to that seen with p-CPA, which is consistent with the suggestion that the effect of E2 on sexual differentiation may be mediated by the 5-HT system. Treatment of tilapia with 17alpha-methyltestosterone (MT), by contrast, resulted in a reduction in the female : male ratio, and treatment was most effective when given between days 10 and 20. The period of maximal effect of MT upon sex ratio appears to coincide with the ability of MT to induce an increase in brain aromatase activity.

Localization and controls of aromatase in the quail spinal cord

In adult male and female Japanese quail, aromatase-immunoreactive cells were identified in the spinal dorsal horns from the upper cervical segments to the lower caudal area. These immunoreactive cells are located mostly in laminae I-III, with additional sparse cells being present in the medial part of lamina V and, at the cervical level exclusively, in lamina X around the central canal. Radioenzyme assays based on the measurement of tritiated water release confirmed the presence of substantial levels of aromatase activity throughout the rostrocaudal extent of the spinal cord. Contrary to what is observed in the brain, this enzyme activity and the number of aromatase-immunoreactive cells in five representative segments of the spinal cord are not different in sexually mature males or females and are not influenced in males by castration with or without testosterone treatment. The aromatase activity and the numbers of aromatase-immunoreactive cells per section are higher at the brachial and thoracic levels than in the cervical and lumbar segments. These experiments demonstrate for the first time the presence of local estrogen production in the spinal cord of a higher vertebrate. This production was localized in the sensory fields of the dorsal horn, where estrogen receptors have been identified previously in several avian and mammalian species, suggesting an implication of aromatase in the modulation of sensory (particularly nociceptive) processes.