Inhibitory effect of 17beta-estradiol in the parabrachial nucleus is mediated by GABA

Sex Hormones, Sleep, and Memory: Interrelationships Across the Adult Female Lifespan

As the population of older adults grows, so will the prevalence of aging-related conditions, including memory impairments and sleep disturbances, both of which are more common among women. Compared to older men, older women are up to twice as likely to experience sleep disturbances and are at a higher risk of cognitive decline and Alzheimer's disease and related dementias (ADRD). These sex differences may be attributed in part to fluctuations in levels of female sex hormones (i.e., estrogen and progesterone) that occur across the adult female lifespan. Though women tend to experience the most significant sleep and memory problems during the peri-menopausal period, changes in memory and sleep have also been observed across the menstrual cycle and during pregnancy. Here, we review current knowledge on the interrelationships among female sex hormones, sleep, and memory across the female lifespan, propose possible mediating and moderating mechanisms linking these variables and describe implications for ADRD risk in later life.

Contribution of sex hormones to the sexually dimorphic response of colorectal motility to noxious stimuli in rats

Our recent studies have shown that noxious stimuli in the colorectum enhance colorectal motility via the brain and spinal defecation centers in male rats. In female rats, however, noxious stimuli have no effect on colorectal motility. The purpose of this study was to determine whether sex hormones are major contributing factors for sex-dependent differences in neural components of the spinal defecation center. Colorectal motility was measured using an in vivo method under ketamine and α-chloralose anesthesia in rats. Capsaicin was administered into the colorectal lumen as noxious stimuli. Orchiectomy in male rats had no effect on the capsaicin-induced response of colorectal motility. However, in ovariectomized female rats, capsaicin administration enhanced colorectal motility, though intact female animals did not show enhanced motility. When estradiol was administered by using a sustained-release preparation in ovariectomized female rats, capsaicin administration did not enhance colorectal motility unless a GABA_A receptor antagonist was intrathecally administered to the lumbosacral spinal cord. These findings suggest that estradiol allowed the GABAergic neurons to operate in response to intracolonic administration of capsaicin. The operation of GABAergic inhibition by the action of estradiol could be manifested in male rats only when the effects of male sex hormones were removed by orchiectomy. Taken together, our results indicate that sex hormones contribute to the sexually dimorphic response in colorectal motility enhancement in response to noxious stimuli through modulating GABAergic pathways.NEW & NOTEWORTHY This study demonstrated that estradiol permits inhibitory regulation in the spinal defecation center not only in female rats but also in orchiectomized male rats. GABAergic pathways are likely involved in the effect of estradiol. This is the first report showing that sex hormones affect colorectal motility through the alteration of neural components of the regulatory pathways. Our findings provide a novel insight into pathophysiological mechanisms of defecation disorders related to changes in sex hormones.

Estrogenic impregnation alters pain expression: analysis through functional neuropeptidomics in a surgical rat model of osteoarthritis

PURPOSE

Several observational studies suggest that estrogens could bias pain perception. To evaluate the influence of estrogenic impregnation on pain expression, a prospective, randomized, controlled, blinded study was conducted in a Sprague-Dawley rat model of surgically induced osteoarthritis (OA).

METHODS

Female rats were ovariectomized and pre-emptive 17β-estradiol (0.025 mg, 90-day release time) or placebo pellets were installed subcutaneously during the OVX procedures. Thirty-five days after, OA was surgically induced on both 17β-estradiol (OA-E) and placebo (OA-P) groups. Mechanical hypersensitivity was assessed by static weight-bearing (SWB) and paw withdrawal threshold (PWT) tests. Mass spectrometry coupled with high-performance liquid chromatography (HPLC-MS) was performed to quantify the spinal pronociceptive neuropeptides substance P (SP), calcitonin gene-related peptide (CGRP), bradykinin (BK), somatostatin (SST), and dynorphin-A (Dyn-A).

RESULTS

Compared to control, ovariectomized rats presented higher SP (P = 0.009) and CGRP (P = 0.017) concentrations. OA induction increased the spinal level of SP (+ 33%, P < 0.020) and decreased the release of BK (- 20%, (P < 0.037)). The OA-E rats at functional assessment put more % body weight on the affected hind limb than OA-P rats at D7 (P = 0.027) and D56 (P = 0.033), and showed higher PWT at D56 (P = 0.009), suggesting an analgesic and anti-allodynic effect of 17β-estradiol. Interestingly, the 17β-estradiol treatment counteracted the increase of spinal concentration of Dyn-A (P < 0.016) and CGRP (P < 0.018).

CONCLUSION

These results clearly indicate that 17β-estradiol interfers with the development of central sensitization and confirm that gender dimorphism should be considered when looking at pain evaluation.

Estradiol potentiates inhibitory synaptic transmission in the oval bed nucleus of the striaterminalis of male and female rats

17ß-Estradiol (E2) is a potent neuromodulator capable of producing changes in inhibitory synaptic transmission by either changing pre-synaptic GABA release or post-synaptic GABA_A receptor function. Physiologically, E2 is important for energy homeostasis, influencing food consumption, body weight, adipose tissue metabolism and energy expenditure. E2 may influence energy homeostasis through estrogen receptor-rich regions such as the oval bed nucleus of the stria-terminalis (ovBNST). However, the neurophysiological effects of estradiol within the ovBNST remain largely unknown. Understanding how E2 affects inhibitory transmission may elucidate the ovBNST's contribution to energy homeostasis. Here, using brain slice electrophysiology, we saw that E2 produced a long-term potentiation (LTP) of GABA_A synaptic transmission (LTP^GABA) in the ovBNST in male rats. E2 acted on estrogen receptors α and G-protein coupled estrogen receptors (GPER), involved protein kinase activation and required an intact endocannabinoid system. The effects of E2 in males were sensitive to 24 h of food deprivation. In females, E2 was 100-fold more potent at producing LTP^GABA ovBNST compared to male rats and involved all three known subtypes of estrogen receptors (ERα, ERß, and GPER). These results demonstrate that E2 is a potent neuromodulator of inhibitory synaptic transmission within the ovBNST of both sexes to potentially regulate energy homeostasis.

Analgesic Effect of 17β-Estradiol on Nucleus Paragigantocellularis Lateralis of Male Rats Mediated Via GABAA Receptors

Introduction:

Beside its autonomic functions, the nucleus paragigantocellularis lateralis (LPGi) is involved in the descending pain modulation. 17β-Estradiol is a neuroactive steroid found in several brain areas such as LPGi. Intra-LPGi microinjection of 17β-estradiol can elicit the analgesic responses. 17β-Estradiol modulates nociception by binding to estrogenic receptors as well as allosteric interaction with other membrane-bound receptors like GABA_A receptors. This study aimed to examine the role of GABA_A receptors in the pain modulating effect of intra-LPGi injection of 17β-estradiol.

Methods:

To study the antinociceptive effects of 17β-estradiol, cannulation into the LPGi nucleus of male Wistar rats was performed. About 500 nL of drug was administered 15 minutes prior to formalin injection (50 μL of 4%). Then, formalin-induced flexing and licking behaviors were recorded for 60 minutes. For evaluating the role of GABA_A receptors in the estradiol-induced pain modulation, 17β-estradiol was administered into the LPGi nucleus 15 minutes after the injection of 25 ng/μL bicuculline (the GABA_A receptor antagonist). Then, the formalin-induced responses were recorded.

Results:

The results of the current study showed that intra-LPGi injection of 17β-estradiol decreased the flexing duration in both phases of formalin test (P<0.001); but it only attenuated the second phase of licking behavior (P<0.001). 17β-estradiol attenuated the second phase of formalin test of both behaviors (P<0.001). Bicuculline prevented the antinociceptive effect of intra-LPGi 17β-estradiol in both first and second phases of formalin-induced responses (P<0.001).

Conclusion:

According to the results of this study, the analgesic effect of intra-LPGi 17β-estradiol on the formalin-induced inflammatory pain might be mediated via GABA_A receptors.

Olfactory Hallucinations without Clinical Motor Activity: A Comparison of Unirhinal with Birhinal Phantosmia

Olfactory hallucinations without subsequent myoclonic activity have not been well characterized or understood. Herein we describe, in a retrospective study, two major forms of olfactory hallucinations labeled phantosmias: one, unirhinal, the other, birhinal. To describe these disorders we performed several procedures to elucidate similarities and differences between these processes. From 1272, patients evaluated for taste and smell dysfunction at The Taste and Smell Clinic, Washington, DC with clinical history, neurological and otolaryngological examinations, evaluations of taste and smell function, EEG and neuroradiological studies 40 exhibited cyclic unirhinal phantosmia (CUP) usually without hyposmia whereas 88 exhibited non-cyclic birhinal phantosmia with associated symptomology (BPAS) with hyposmia. Patients with CUP developed phantosmia spontaneously or after laughing, coughing or shouting initially with spontaneous inhibition and subsequently with Valsalva maneuvers, sleep or nasal water inhalation; they had frequent EEG changes usually ipsilateral sharp waves. Patients with BPAS developed phantosmia secondary to several clinical events usually after hyposmia onset with few EEG changes; their phantosmia could not be initiated or inhibited by any physiological maneuver. CUP is uncommonly encountered and represents a newly defined clinical syndrome. BPAS is commonly encountered, has been observed previously but has not been clearly defined. Mechanisms responsible for phantosmia in each group were related to decreased gamma-aminobutyric acid (GABA) activity in specific brain regions. Treatment which activated brain GABA inhibited phantosmia in both groups.

17β-Estradiol alters the response of subfornical organ neurons that project to supraoptic nucleus to plasma angiotensin II and hypernatremia

This study was done in urethane anesthetized, ovariectomized (OVX) female rats that were either implanted or not implanted with silastic capsules containing17β-estradiol (E2) to investigate the effect of systemic changes in E2 on the discharge rate of subfornical organ (SFO) neurons that projected to supraoptic nucleus (SON) and responded to changes in plasma levels of angiotensin II (ANG II) or hypernatremia. Extracellular single unit recordings were made from 146 histologically verified single units in SFO. Intra-carotid infusions of ANG II excited ~57% of these neurons, whereas ~23% were excited by hypertonic NaCl. Basal discharge rate of neurons excited by ANG II or hypertonic NaCl was significantly lower in OVX+E2 rats compared to OVX only animals. The response of SFO neurons antidromically activated by SON stimulation to intra-carotid injections of ANG II or hypertonic NaCl was greater in the OVX only compared to the OVX+E2 rats. Intra-carotid injections of E2 in either group attenuated not only the basal discharge of these neurons, but also their response to ANG II or hypertonic NaCl. In all cases this inhibitory effect of E2 was blocked by an intra-carotid injection of the E2 receptor antagonist ICI-182780, although ICI-182780 did not alter the neuron's response to ANG II or hypertonic NaCl. Additionally, ICI-182780 in the OVX+E2 animals significantly raised the basal discharge of SFO neurons and their response to ANG II or hypertonic NaCl. These data indicate that E2 alters the response of SFO neurons to ANG II or NaCl that project to SON, and suggest that E2 functions in the female to regulate neurohypophyseal function in response to circulating ANG II and plasma hypernatremia.

Stroke-induced changes in estrogen release and neuronal activity in the parabrachial nucleus of the male rat

Recent investigations have provided evidence to suggest exogenous estrogen administration into autonomic nuclei prevents or reverses the autonomic dysfunction observed after middle cerebral artery occlusion (MCAO) in male rats. Because estrogen seems to be a potent neuroprotectant against autonomic dysfunction, it is our hypothesis that endogenous estrogen levels within autonomic nuclei will increase in response to stroke. Therefore, in this investigation, in vivo microdialysis was used to simultaneously measure the concentration of estrogen in the plasma and in the parabrachial nucleus (PBN) of male Sprague-Dawley rats after MCAO. Analysis of dialysate samples before MCAO and in sham-operated controls revealed a baseline concentration of estrogen in the PBN (38 +/- 3 pg/mL; n = 36), which was significantly greater than that found in plasma (22 +/- 6 pg/mL; n = 6; P < .05). The concentration of estrogen in the PBN was significantly increased immediately after MCAO (85 +/- 4 pg/mL; n = 7; P < .05) but then decreased to below pre-MCAO values (12 +/- 2 pg/mL; n = 7; P < .05) by 90 minutes after MCAO and remained below baseline levels until the end of the experiment (240 minutes post-MCAO). No changes in plasma estrogen levels were detected at any time point after MCAO. In addition, extracellular electrophysiological recordings from PBN neurons revealed that MCAO resulted in an immediate decrease in the activity of PBN neurons, which was completely blocked after systemic estrogen injection. These results suggest that estrogen is released into the PBN in response to MCAO and that the source of estrogen seems to be primarily caused by terminal release as opposed to increased local synthesis.

Role of oestrogen in the central regulation of autonomic function

1. In recent years, the role of oestrogen in women's health has been a subject of considerable scientific and popular debate. There is unquestionable evidence that oestrogen has both potent and long-lasting effects on several vital organ systems, including the cardiovascular system, the autonomic nervous system and, most recently, within the central nervous system itself. 2. The research and medical community continues to debate whether the benefits of oestrogen therapy outweigh the risks in the treatment of the symptoms of menopause, the attenuation of the risk for cardiovascular insults, such as stroke and heart disease, and even the retardation of the progression of Alzheimer's disease. 3. The recent evidence provided by the Heart and Estrogen/Progestin Replacement Study (HERS) II clinical trial suggesting that long-term exposure to combined oestrogen and progestin in post-menopausal women who have previously had a heart attack or stroke (for secondary prevention) may actually increase their risk of a subsequent cardiovascular insult has further fuelled the debate. However, there remain considerable gaps in our knowledge with respect to the actual mechanisms by which oestrogen exerts its various beneficial effects at the cellular level for the primary prevention of cardiovascular disease. This information is essential if we are to harness the positive aspects of oestrogen therapy in such a manner as to avoid or minimize the associated risks of increased oestrogen exposure in women who we know, with some certainty, to be at an increased risk of cancers of the uterus, cervix and breast tissue.

Oestrogen affects the cardiovascular and central responses to isoproterenol of female rats

This study examined the influence of oestrogen on cardiovascular responses to hypotension produced by administration of isoproterenol (Isop) and on neural activation in hindbrain nuclei mediating these responses. We first measured mean arterial pressure (MAP) and heart rate (HR) after administration of isoproterenol, a beta-adrenergic agonist that increases circulating levels of AngII, in ovariectomized (OVX) rats treated with oestradiol benzoate (EB). We then evaluated EB effects on Isop-induced Fos immunoreactivity (Fos-IR) in the hindbrain baroreflex circuit. To control for weight loss associated with oestrogen replacement in OVX rats, we food restricted a separate group of OVX rats and evaluated Isop-induced changes in MAP, HR and Fos-IR. The depressor response to Isop was significantly attenuated by EB, which also produced a disproportionate increase in HR. These effects were not secondary to loss of body weight after EB treatment, because cardiovascular responses to Isop in food restricted rats were similar to those in OVX rats treated with the oil vehicle. Isop significantly increased Fos-IR in the nucleus of the solitary tract (NTS), area postrema (AP), rostral ventrolateral medulla (RVLM), and lateral parabrachial nucleus (lPBN); however, EB significantly attenuated the increase in the AP and in the lPBN. Again, these effects were not secondary to body weight loss, because food restricted rats had the same pattern of Fos-IR as did rats treated with the oil vehicle. These results suggest that EB modifies cardiovascular responses to Isop, possibly by decreasing activation of the AP and lPBN.

17beta-estradiol attenuates excitatory neurotransmission and enhances the excitability of rat parabrachial neurons in vitro

The steroid hormone 17beta-estradiol and its respective receptors have been found in several cardiovascular nuclei in the central nervous system including the parabrachial nucleus. In a previous study, we provided evidence that 17beta-estradiol attenuated an outward potassium conductance in parabrachial neurons of male rats, using an in vitro slice preparation. In this study we sought to enhance the comprehensive information provided previously on estradiol's postsynaptic effects in the parabrachial nucleus by directly examining whether 17beta-estradiol application will modulate excitatory synaptic neurotransmission. Using a pontine slice preparation and whole-cell patch-clamp recording, bath application of either 17beta-estradiol (20-100 muM) or BSA-17beta-estradiol (50 muM) decreased the amplitude of evoked excitatory postsynaptic currents (from 30-60% of control) recorded from neurons in the parabrachial nucleus. The paired pulse ratio was not significantly affected and suggests a post-synaptic site of action. The inhibitory effect on the synaptic current was relatively long-lasting (non-reversible) and was blocked by the selective estrogen receptor antagonist, ICI 182,780. Furthermore, 17beta-estradiol reduced the maximum current elicited by a ramp protocol, increased the input resistance measured between resting membrane potential and action potential threshold and caused an increase in the firing frequency of the cells under current-clamp. In summary, 17beta-estradiol caused 3 effects: first, a depolarization; second, a reduction in evoked excitatory postsynaptic potentials; and third, an enhancement of action potential firing frequency in neurons of the parabrachial nucleus. These observations are consistent with our previous findings and support a role for estrogen in modulating neurotransmission in this nucleus.

Estradiol-associated variation in responses of rostral medullary neurons to somatovisceral stimulation

The lordosis posture and cervix stimulation during copulation are important reproductive events involving complex neural circuitries that are under hormonal influence. An important component of this circuitry, neurons within the medullary reticular formation (MRF), was examined in the present study using electrophysiological techniques. Single unit extracellular recordings were performed in the MRF of 27 urethane-anesthetized female rats. Using bilateral electrical stimulation of the dorsal nerve of the clitoris as the search stimulus, a detailed examination of the somatovisceral convergent responses of 585 individual MRF neurons was made. A total of 7 different groups of cycling and ovariectomized/hormone-supplemented rats were examined and their neuronal response properties to mechanical stimulation of various pelvic organs (cervix pressure, vaginal distension, colon distension) compared. The results indicate the existence of complex response properties as well as several variations in MRF response characteristics that are hormone-dependent. Specifically, estradiol is associated with hyposensitivity to cervix pressure and hypersensitivity to stroking the face. These opposing effects of estradiol in the same subset of neurons likely relate to lordosis behavior which can be either disrupted or elicited, depending on the area being stimulated (upper versus lower parts of the body, respectively).

Relaxin-induced reduction of infarct size in male rats receiving MCAO is dependent on nitric oxide synthesis and not estrogenic mechanisms

Relaxins are members of the insulin peptide superfamily. Previous evidence has shown that relaxin pretreatment reduces cortical infarct size in anesthetized, male rats receiving permanent middle cerebral artery occlusion (MCAO). Therefore, the current study was designed to determine if estrogenic mechanisms or nitric oxide production are involved in mediating this relaxin-induced neuroprotection. In separate groups of rats (n=4-6), the following drugs were injected directly into the cortex 30 min prior to MCAO: (a) relaxin, (b) relaxin and estrogen, and (c) relaxin and an estrogen receptor antagonist (ICI 182,780). To investigate the involvement of nitric oxide, relaxin or relaxin and an inhibitor of endothelial nitric oxide synthase (L-NIO) were injected i.v. 30 min prior to MCAO. Saline-treated rats (both intracortical (i.c.) and intravenously (i.v.)) served as controls. Brains were harvested 4h post stroke, coronally sectioned using a brain matrix and stained using 2,3,5-triphenoltetrazolium chloride (TTC). Digital photographs were taken of brain sections and the ratio comparing the area of the infarct to the area of the ipsilateral hemisphere was calculated. Mean ratios were compared using ANOVA and Tukey's test. Intracortical and intravenous relaxin pretreatment significantly reduced the infarct area in the cortex by 33.7 and 58.6%, respectively compared to saline-treated controls. This effect was not dependent on an interaction with estrogenic receptors as co-injection of relaxin and ICI 182,780 did not reverse this effect. However, inhibition of nitric oxide synthase significantly reduced the relaxin-mediated neuroprotection suggesting that relaxin may induce the endothelin-NOS cascade in cerebral vasculature causing vasodilation and improved perfusion of neural tissue.

Estrogen in the parabrachial nucleus attenuates the sympathoexcitation following MCAO in male rats

Recent investigations have provided evidence to suggest systemic estrogen administration prevented or reversed the sympathoexcitation observed following middle cerebral artery occlusion (MCAO) in male rats. The present investigation sought to determine the role of estrogen injected directly into the parabrachial nucleus (PBN) on the MCAO-induced sympathoexcitation as well as the role of the rostral ventrolateral medulla (RVLM) in mediating the sympathoexcitatory response. Male Sprague-Dawley rats were anesthetized with sodium thiobutabarbitol (100 mg/kg) and were instrumented to continuously record blood pressure, heart rate and renal sympathetic nerve activity (RSNA). Following occlusion of the middle cerebral artery, there was a significant increase in RSNA (from 3.8 +/- 0.4 to 8.3 +/- 0.6 microV/s; P < 0.05) which was significantly attenuated by the prior bilateral injection of estrogen (0.5 microM in 200 nl) into the PBN. Pre-injection of lidocaine (5% in 200 nl) directly into the RVLM resulted in only a slight reduction in the magnitude of the MCAO-induced sympathoexcitation (P > 0.05). Extracellular electrophysiological recordings from RVLM neurons demonstrated that MCAO did not produce any significant change in neuronal activity over the experimental time course (P > 0.05). Also, bilateral injection of estrogen into the PBN prior to MCAO or sham conditions did not result in any significant change in RVLM neuronal activity. These results indicate that estrogen receptors in the PBN play a major role in modulating the sympathoexcitatory response from ischemic forebrain nuclei, and that the pathway from the PBN to sympathetic preganglionic nuclei may not involve a synapse in the RVLM.

17beta-estradiol inhibits outward potassium currents recorded in rat parabrachial nucleus cells in vitro

Evidence is increasingly accumulating in support of a role for the steroid hormone 17beta-estradiol to modify neuronal functions in the mammalian CNS, especially in autonomic centers. In addition to its well known slowly developing and long lasting actions (genomic), estrogen can also rapidly modulate cell signaling events by affecting membrane excitability (non-genomic). Little, however, is known regarding the mechanism(s) by which 17beta-estradiol produces its rapid effects on neuronal membrane excitability. As potassium channels play a crucial role in cell excitability, we hypothesized that 17beta-estradiol caused excitability by modulating potassium flux through the neuronal cell membrane. We tested this hypothesis by examining the effects of 17beta-estradiol on outward potassium currents recorded in cells from the parabrachial nucleus of rats, in vitro. Bath application of 17beta-estradiol (10-100 microM) reversibly reduced voltage-activated outward potassium currents in a concentration-dependent manner. This effect was mimicked by BSA-17beta-estradiol but not mimicked by 17alpha-estradiol and was significantly reduced by ICI 182,780, a selective estrogen receptor antagonist. The inhibitory effect of 17beta-estradiol was dependent on extracellular potassium concentration, with more profound effects observed at lower concentrations. The 17beta-estradiol-induced inhibition of the outward current was blocked by pretreatment with the potassium channel blockers tetraethylammonium and 4-aminopyridine. The time constants of deactivation of tail currents were decreased by 17beta-estradiol over a range of test potentials (-140 to -80 mV). Finally, the inhibitory effect of 17beta-estradiol on the outward potassium currents was blocked following pre-incubation of slices in lavendustin A, a tyrosine kinase inhibitor. Taken together, these results suggest that 17beta-estradiol acts rapidly at an extracellular membrane receptor to reduce tetraethylammonium- and 4-aminopyridine-sensitive outward potassium currents by accelerating the closure of potassium channels. This may be the ionic basis of 17beta-estradiol-induced enhancement of neuronal excitability.

Sympathoexcitatory effects of estrogen in the insular cortex are mediated by GABA

The current investigation examined the effect of estrogen in the insular cortex (IC) on autonomic tone and cardiac baroreceptor reflex function and sought to determine if modulation of neurotransmission was responsible for mediating this effect. Experiments were performed in Inactin-anaesthetized, male Sprague-Dawley rats. Animals were instrumented to record blood pressure, heart rate, vagal parasympathetic and renal sympathetic nerve activities, as well as cardiac baroreflex sensitivity (BRS). Direct, bilateral injection of 17beta-estradiol (0.5 microM; 200 nl/side) into the IC resulted in a significant increase in sympathetic tone (from 10 +/- 4 to 24 +/- 3) with no significant change in blood pressure, heart rate, parasympathetic tone or BRS measured at 30 min post-injection. This estrogen-induced effect was completely blocked by the co-injection of estrogen with the estrogen receptor antagonist, ICI 182, 780 (20 microM; 200 nl/side). Co-injection of estrogen with a GABA(B), NMDA or non-NMDA receptor antagonists did not effect the estrogen-induced increase in sympathetic tone. Co-injection of a sub-threshold dose of estradiol (0.125 microM; 200 nl/side) with the GABA(A) receptor antagonist, (+)-bicuculline (0.025 microM; 200 nl/side), resulted in an additive response to increase sympathetic nerve activity. These results suggest that estrogen acts on estrogen receptors to modulate GABA(A)-receptor-mediated neurotransmission within the IC to modulate sympathetic tone.

Estrogen synthesis in the central nucleus of the amygdala following middle cerebral artery occlusion: Role in modulating neurotransmission

Stroke-induced lesions of the insular cortex in the brain have been linked to autonomic dysfunction (sympathoexcitation) leading to arrhythmogenesis and sudden cardiac death. In experimental models, systemic estrogen administration in male rats has been shown to reduce stroke-induced cell death in the insular cortex as well as prevent sympathoexcitation. The central nucleus of the amygdala has been postulated to mediate sympathoexcitatory output from the insular cortex. We therefore set out to determine if endogenous estrogen levels within the central nucleus of the amygdala are altered following stroke and if microinjection of estrogen into the central nucleus of the amygdala modulates autonomic tone. Plasma estrogen concentrations were not altered by middle cerebral artery occlusion (22.86+/-0.14 pg/ml vs. 21.24+/-0.33 pg/ml; P>0.05). In contrast, estrogen concentrations in the central nucleus of the amygdala increased significantly following middle cerebral artery occlusion (from 20.83+/-0.54 pg/ml to 76.67+/-1.59 pg/ml; P<0.05). Local infusion of an aromatase inhibitor, letrozole, into the central nucleus of the amygdala at the time of middle cerebral artery occlusion prevented the increase in estrogen concentration suggesting that this increase was dependent on aromatization from testosterone. Furthermore, bilateral microinjection of estrogen (0.5 microM in 200 nl) directly into the central nucleus of the amygdala significantly decreased arterial pressure and sympathetic tone and increased baroreflex sensitivity, and these effects were enhanced following co-injection with either an N-methyl-D-aspartate or non-N-methyl-D-aspartate receptor antagonist. Taken together, the results suggest that middle cerebral artery occlusion resulted in synthesis of estrogen within the central nucleus of the amygdala and that this enhanced estrogen level may act to attenuate overstimulation of central nucleus of the amygdala neurons to prevent middle cerebral artery occlusion-induced autonomic dysfunction.

Estrogen-induced neurochemical and electrophysiological changes in the parabrachial nucleus of the male rat

Estrogen has previously been shown to significantly change sympathetic and parasympathetic system output via an action within the central nuclei responsible for regulating autonomic tone. These estrogen-induced changes were observed within 30 min of systemic administration and could be blocked by the direct microinjection of the estrogen receptor antagonist, ICI 182780, into the parabrachial nucleus (PBN) of the pons. In the present investigation, we sought to determine the possible mechanism(s) by which estrogen produced these rapid changes in autonomic tone by determining if estrogen modulates neuronal excitability within the PBN. Male Sprague-Dawley rats were anaesthetized with Inactin (sodium thiobutabarbitol, 100 mg/kg) and instrumented for the intravenous injection of estrogen and placed in a stereotaxic frame for the insertion of a microdialysis probe or glass recording electrode into the PBN. In the first experiment, we sought to determine the local concentration of estrogen in the cerebrospinal fluid in the PBN following systemic injection of estrogen. In the second experiment, we sought to determine the functional significance of systemic estrogen injection on neuronal activity and amino acid neurotransmitter levels in the PBN. Systemic estrogen injection resulted in a significant increase in local estrogen concentration in the PBN which corresponded to a decrease in neuronal excitability and extracellular glutamate levels while increasing GABA levels in the PBN. These results suggest that estrogen decreases neuronal excitability in the PBN by modulating synaptic transmission via an increased release of GABA and a decreased release of glutamate.

Oestradiol dampens reflex-related activity of on- and off-cells in the rostral ventromedial medulla of female rats

The present study was conducted to determine whether the ovarian steroid oestradiol alters the activity of nociceptive modulatory neurons in the rostral ventromedial medulla (RVM). Adult female rats were ovariectomized and implanted s.c. with an oestradiol-filled or placebo capsule. Sixteen to 37 days later, rats were anaesthetised for single unit recording from RVM neurons. On-cells were characterised by a burst of activity, and off-cells by a pause in activity immediately preceding reflexive withdrawal of the tail from 51 and 54 degrees C water. Although on- and off-cells were evident in both oestradiol- and placebo-treated rats, the reflex-related on-cell burst and off-cell pause were dampened in oestradiol-treated rats. On-cells from oestradiol-treated rats had a mean activity burst of 9.1+/-2.2 Hz in the 2 s preceding the tail withdrawal reflex to 51 degrees C water, compared with 17.9+/-4.3 Hz for on-cells in placebo controls. Off-cell activity during the 2 s preceding tail withdrawal was 4.8+/-2.2 vs. 0.1+/-0.1 Hz in oestradiol vs. placebo-treated females, respectively. Similar changes in on- and off-cell activity occurred when the tail was placed in 54 degrees C water. The present data demonstrate that oestradiol constrains the magnitude of the shift in RVM on- and off-cell activity associated with nociceptive reflexes.

Estrogen receptor β in the paraventricular nucleus of hypothalamus regulates the neuroendocrine response to stress and is regulated by corticosterone

The function of the second nuclear estrogen receptor, estrogen receptor beta (ERbeta), in the brain is largely unknown. The present study tested whether 1) ERbeta in the paraventricular nucleus (PVN) of the hypothalamus has a direct role in the hypothalamic-pituitary-adrenal (HPA) axis-mediated stress function, and 2) whether corticosterone (CORT) can regulate ERbeta gene expression in the PVN in the intact, cycling female rat. To test the first hypothesis a pure estrogen receptor antagonist, ICI182, 780, was microinjected into the PVN bilaterally and stress-induced CORT response to an acute stressor (15 min restraint) was measured at 0, 15, 30, 60 and 90 min time points. Estrogen antagonist-injected rats showed inhibited CORT levels at the peak (15 min) of the stress response compared with vehicle-injected animals. To test the second hypothesis, ERbeta mRNA levels were measured in the PVN using in situ hybridization histochemistry following sham surgery, adrenalectomy, and adrenalectomy with low or high CORT replacement. Adrenalectomy reduced ERbeta mRNA expression in the PVN, whereas CORT replacement fully reversed this effect in a dose-dependent fashion. Both antagonist inhibition of CORT response and CORT-mediated regulation of ERbeta mRNA were found to be estrus cycle-dependent in the intact, cycling female. These data suggest that ERbeta in the PVN may critically modulate the HPA axis response to stress and is, in turn, regulated by circulating CORT.

The influence of ovariectomy with or without estrogen replacement on responses of rat gracile nucleus neurons to stimulation of hindquarter skin and pelvic viscera

Responses of neurons in the gracile nucleus (NG) of female rats to tactile and visceral stimulation change across the estrous cycle [J. Neurosci. 20 (2000) 7722]. To investigate estrogen's role in these changes, responses of NG neurons to tactile and visceral stimuli were examined in three groups: ovariectomized (OVX), OVX with estrogen replacement (OVX+E2), or sham OVX (tested in diestrus; shamOVX-D). The stimuli were: gentle brushing of hindquarter skin, pressure on the cervix, and distention of the uterus, vagina, or colon. After OVX, the magnitude of multi-unit responses to brushing the perineum, hip and tail, but not the foot and leg, were significantly reduced relative to shamOVX-D. OVX+E2 restored this magnitude to the same level as shamOVX-D, but not, as expected, to levels as large as previously observed in proestrus. After OVX, responses of single neurons to stimulation of the uterus, cervix, and colon were more likely to be excitatory (versus inhibitory) than they had been in cycling rats in proestrus (uterus, cervix) or diestrus (colon); OVX+E2 did not restore the inhibitory responses. In contrast, whereas all responses to vaginal distention after OVX were also excitatory, OVX+E2 in this case significantly restored the inhibitory responses. These findings provide further support for the conclusion that response characteristics of NG neurons are influenced by the rat's hormonal milieu, but also indicate that the influences are not a simple reflection of estrogen levels. The findings further suggest that NG is a component of neural systems that contribute to both reproductive behaviors and vaginal nociception.

Role of the GABAA system in behavioral, motoric, and cerebellar protection by estrogen during ethanol withdrawal

Results of studies from our laboratory have shown that administration of 17beta-estradiol (E(2)) reduces cerebellar neuronal damage during ethanol withdrawal (EW). In the current study, we investigated whether the GABAergic system is involved in the protective effects of E(2) against the EW syndrome. To test this hypothesis, we examined the effects of GABAergic drugs, with and without E(2), on EW sign scores, motoric capacity, and caspase activation. Ovariectomized rats implanted with an E(2) or an oil pellet received liquid ethanol [7.5% weight/volume (wt./vol.)] for 5 weeks or dextrin diet, followed by 2 weeks of EW. A gamma-aminobutyric acid type A (GABA(A)) agonist, muscimol (0.125 or 0.25 mg/kg), and antagonist, bicuculline (1.25 mg/kg), were administered (intraperitoneally; three times a day for 4 days) starting 1 day before the onset of EW. On termination of chronic administration of ethanol diet, rats were tested for overt withdrawal signs and latency to fall from a rotarod. The initial latency was measured separately to assess motoric capacity before learning occurred. Cerebelli were subsequently collected for immunohistochemistry to detect caspase activation. Results showed that treatment with E(2) lowered EW sign scores and improved initial as well as subsequent rotarod latencies compared with findings without treatment with E(2) (control group). These effects of E(2) were enhanced by combined treatment with muscimol and diminished by bicuculline. Results also showed that ethanol-withdrawn rats had more caspase-3-positive cells than observed for the dextrin diet-fed group in a manner reversed by E(2) and exacerbated by bicuculline. Bicuculline also caused partial antagonism of the protective effect of E(2). These findings support the suggestion that GABA(A) agonists ameliorate, and GABA(A) antagonists exacerbate, EW signs, cerebellar neuronal damage, and motoric impairment in ethanol-withdrawn rats. Also, results of the current study provide indirect evidence that the GABAergic system is involved in protective effects of E(2) against the EW syndrome.

Estrogen-induced autonomic effects are mediated by NMDA and GABAA receptors in the parabrachial nucleus

The present study was done to determine if estrogen interacts with excitatory and/or inhibitory amino acid neurotransmitters to alter neuronal excitability within the parabrachial nucleus (PBN) and modulate autonomic tone. First, the role of estrogen in modulating autonomic tone was investigated in male rats anesthetized with Inactin (100 mg/kg). Animals were instrumented to record blood pressure, heart rate, vagal parasympathetic and renal sympathetic nerve activities as well as baroreflex sensitivity. Direct, bilateral injection of 17beta-estradiol (0.5 microM; 200 nl/side) into the PBN resulted in a significant decrease in blood pressure (17+/-4 mmHg), sympathetic tone (20+/-5%) and heart rate (22+/-5 beats/min) while increasing parasympathetic tone (34+/-4%) 30 min post-injection. These estrogen-induced effects were completely blocked by the co-injection of estrogen with the estrogen receptor antagonist, ICI 182,780 (20 microM; 200 nl/side). Co-injection of the NMDA receptor antagonist, (+/-)-3-(2-carboxypiperazine-4-yl) propyl-1-phosphonic acid (CPP; 10 microM; 200 nl/side), with estradiol resulted in complete blockade of the estrogen-induced decrease in heart rate and increase in parasympathetic tone only. Co-injection of estradiol with the GABA(A) receptor antagonist, (+)-bicuculline (0.1 microM; 200 nl/side), resulted in complete blockade of the estrogen-induced decrease in blood pressure and sympathetic nerve activity only. These results suggest that estrogen acts on estrogen receptors on neurons in the PBN to modulate GABA(A)-receptor mediated inhibitory neurotransmission to alter sympathetic tone and blood pressure and on neurons in a separate, parallel pathway to modulate NMDA-receptor mediated neurotransmission to alter parasympathetic tone and heart rate.

Gonadal hormones and sex differences in pain reactivity

BACKGROUND

Sex differences in the response threshold to painful stimuli and the higher number of chronic pain syndromes in women than in men have prompted a series of studies on lower animals and humans aimed at clarifying the role of gonadal hormones in pain.

OBJECTIVE

This article examines the morphologic and functional aspects of gonadal hormone systems and the relations between gonadal hormones and pain circuits, to identify areas deserving of increased attention in elucidating the endocrine mechanisms that contribute to abnormal pain states.

Central nuclei mediating estrogen-induced changes in autonomic tone and baroreceptor reflex in male rats

The current investigation examines the significance of estrogen in central cardiovascular regulatory nuclei in modulating autonomic tone and baroreceptor reflex function. Experiments were done in anaesthetized male Sprague-Dawley rats. Changes in autonomic tone were assessed by monitoring vagal and renal efferent nerve activities before and following bilateral injection of estrogen into select central autonomic nuclei. In the first study, selective blockade of neurotransmission through the central nucleus of the amygdala (CNA), lateral hypothalamic area (LHA) and ventral posteromedial thalamic nucleus (VPM) using the local anaesthetic lidocaine was done to determine which nuclei were involved in mediating the autonomic changes observed following bilateral injections of estrogen into the insular cortex (IC). In the second study, the role of the parabrachial nucleus (PBN) in mediating the autonomic changes observed following bilateral estrogen injections into the CNA, LHA, VPM and IC was determined by blocking neurotransmission through the PBN using lidocaine.Injections of estrogen into the IC produced a significant increase in renal sympathetic nerve activity (RSNA; from 10+/-2 to 24+/-4 microV/sec; p<0.05). This estrogen-induced increase in RSNA was significantly attenuated when lidocaine was pre-injected into the LHA, CNA or PBN (55+/-6, 33+/-4 and 91+/-7% decrease respectively; p<0.05) but not when injected into the VPM (16+/-6% decrease; p>0.05). Injection of estrogen into the CNA resulted in a significant decrease in RSNA (48+/-5%; p<0.05) whereas estrogen injection into the LHA resulted in a significant increase (28+/-4%; p<0.05) in RSNA. Pre-injection of lidocaine into the PBN resulted in complete blockade of the autonomic changes observed following estrogen injection into the CNA but did not affect the changes observed following estrogen injection into the LHA. These results suggest that estrogen acting in forebrain and midbrain cardiovascular nuclei activated efferent pathways which synapse in the LHA, CNA and/or PBN prior to projecting to autonomic preganglionic nuclei to affect autonomic tone. These nuclei may therefore provide an added level of processing and/or integration of the autonomic response(s) following activation by local or systemic estrogen.