Biomedical subjects

A M Etgen

Publications and source records attributed to A M Etgen.

At least 19 recordsLinked to original sources

PubMed2000

Tyrosine kinase effects on adrenoceptor-stimulated cyclic AMP accumulation in preoptic area and hypothalamus of female rats: modulation by estradiol.

These studies examined the functional interactions between adrenergic G-protein coupled receptors and protein tyrosine kinases in the preoptic area and hypothalamus, brain regions that regulate reproductive function in female rats, and evaluated whether in vivo treatment with estradiol for 2 days modulates the cross-talk between these two signaling pathways. In hypothalamic slices genistein, a general tyrosine kinase inhibitor, enhances norepinephrine-stimulated cAMP synthesis independent of estradiol treatment. Genistein appears to act by increasing beta-adrenoceptor signaling. At high norepinephrine concentrations, estradiol potentiates genistein enhancement of the cAMP response in hypothalamic slices. This interaction between estradiol and genistein appears to involve modification of alpha(2)-adrenoceptor signaling mechanisms. In preoptic area slices, genistein enhancement of norepinephrine-stimulated cAMP synthesis is only observed in estradiol-treated rats. In this brain region, genistein enhances cAMP accumulation by modifying alpha(1)- and/or alpha(2)-adrenoceptor rather than beta-adrenoceptor signaling. Genistein amplification of norepinephrine-stimulated cAMP synthesis is not mediated by interactions with estrogen receptors, or by regulation of adenylyl cyclase or phosphodiesterase activities. At the concentration used, genistein inhibits tyrosine phosphorylation in slices from both brain regions. Daidzein, an inactive analogue of genistein, fails to enhance the norepinephrine-stimulated cAMP response in either brain region independent of hormone treatment. These results suggest that protein tyrosine kinases regulate adrenergic responses in the hypothalamus and preoptic area. Moreover, the functional interaction between adrenergic G-protein coupled receptor signaling and protein tyrosine kinases is modified in a brain region and receptor subtype specific manner by estradiol.

Adrenergic alpha-Agonists↗

PubMed1999

Hormonal integration of neurochemical and sensory signals governing female reproductive behavior.

This review focuses on findings from our laboratory regarding mechanisms by which the ovarian steroid hormones, estradiol (E2) and progesterone (P), act in the hypothalamus (HYP) to regulate the expression of lordosis, an important component of female reproductive behavior in rats and many other species. The first section summarizes recent work suggesting that cGMP, perhaps via P-receptor activation, may be an intracellular mediator of the facilitatory actions of a variety of hormones and neurotransmitters on lordosis behavior in E2-primed rats. In the second section, we focus on E2 and P regulation of norepinephrine (NE) neurotransmission in the HYP. We review evidence that ovarian hormones act both peripherally and centrally to determine whether NE is released in the HYP in response to copulatory stimuli. We also suggest that the steroid milieu determines the cellular responses of hypothalamic neurons to released NE, favoring the activation of pathways implicated in the facilitation of both lordosis behavior and the preovulatory gonadotropin surge. It is likely that E2 and P have similar actions on other neurotransmitter and neuromodulator systems, thereby maximizing the probability that females are sexually receptive during the periovulatory period.

Animals↗

PubMed1999

Region-specific modulation of limbic seizure susceptibility by ovarian steroids.

Gonadal steroid hormones can markedly affect seizure susceptibility. Ovariohysterectomized female rats given ovarian steroid hormone supplements were used to evaluate the effects of ovarian steroids on epileptiform activity in hippocampal slices in vitro and on flurothyl-induced seizures in vivo. Seizure susceptibility was compared in the entorhinal cortex (EC) and CA1 regions of the hippocampus perfused with Mg(2+)-free medium, which leads to epileptiform discharges caused by a relief of voltage-dependent NMDA receptor block. After in vivo treatment with 500 microg of progesterone for 2 h prior to slice preparation, the latency to onset of low Mg(2+)-induced epileptiform activity of slices was significantly prolonged compared to slices from controls. In contrast, progesterone replacement accelerated the development of epileptiform activity in the CA1 region. Neither estrogen alone (2 x 2 microg of estradiol benzoate, 48 and 24 h prior to the experiment), nor a combined treatment with estrogen plus progesterone, significantly affected seizure susceptibility in either CA1 or the EC. There were no consistent effects of estrogen or progesterone, alone or in combination, on flurothyl-induced seizures in vivo. The data suggest that in vitro, progesterone alters seizure susceptibility in a site- and seizure model-specific fashion. The differential effects of progesterone may be due to differential expression of progesterone receptor isoforms or metabolites in specific brain areas suggesting that selective modulation of NMDA receptor-dependent epileptiform activity may play a role in hormonal effects on epileptogenesis.

Animals↗

PubMed1999

Ovarian hormone dependence of alpha(1)-adrenoceptor activation of the nitric oxide-cGMP pathway: relevance for hormonal facilitation of lordosis behavior.

The ovarian hormones estradiol (E(2)) and progesterone (P) facilitate rat lordosis behavior in part by regulating the expression of and signal transduction by adrenoceptors in the hypothalamus (HYP) and preoptic area (POA). The major adrenoceptor subtype mediating E(2) and P facilitation of lordosis is the alpha(1)-adrenoceptor. In the present studies, we tested the hypotheses that (1) alpha(1)-adrenoceptors in the HYP enhance lordosis responses by activating the nitric oxide (NO)-cGMP signaling pathway, and (2) coupling of alpha(1)-adrenoceptors to this signal transduction pathway is hormone-dependent. Basal levels of cGMP were significantly higher in HYP and POA slices from animals treated with E(2) and P when compared with slices from ovariectomized controls or females treated with only E(2) or P. When slices of HYP and POA from ovariectomized female rats were incubated with norepinephrine or the selective alpha(1)-adrenoceptor agonist phenylephrine, cGMP accumulation was observed only if slices had been derived from females treated with both E(2) and P before experimentation. Moreover, alpha(1)-adrenoceptor stimulation of cGMP synthesis was blocked by an inhibitor of NO synthase, confirming that these receptors act by NO-mediated stimulation of soluble guanylyl cyclase. Behavioral studies demonstrated further that the cell-permeable cGMP analog 8-bromoadenosine-cGMP reverses the inhibitory effects of the alpha(1)-adrenoceptor antagonist prazosin on lordosis behavior in E(2)- and P-treated female rats. Thus, the NO-cGMP pathway mediates the facilitatory effects of alpha(1)-adrenoceptors on lordosis behavior in female rats, and previous exposure of the HYP and POA to both E(2) and P are required to link alpha(1)-adrenoceptors to this pathway.

Adrenergic alpha-1 Receptor Agonists↗

PubMed1999

Localization of alpha1B-adrenergic receptor in female rat brain regions involved in stress and neuroendocrine function.

Activation of alpha1-adrenergic receptors has been linked to the control of blood pressure, neuroendocrine secretion, reproductive behavior and mood. The present study describes the distribution of alpha1B-adrenergic receptor immunoreactivity in female rat brain regions involved in stress and neuroendocrine function. The pattern of immunolabeling seen resembles that obtained in previous in situ hybridization studies. Several hypothalamic areas that control pituitary function showed intense fiber and/or cell immunolabeling, including the paraventricular nucleus of the hypothalamus, the supraoptic nucleus, and the median eminence. Some regions such as the arcuate nucleus, the median eminence, and dorsal hypothalamus exhibit intense labeling of axonal varicosities, while other regions exhibit only perikarya immunolabeling. alpha1B-adrenergic receptor immunoreactivity was also observed in large pyramidal neurons of layer V of the cerebral cortex, the frontal cortex showing a particularly strong immunoreactivity. Virtually all thalamic regions were labeled, especially the lateral and ventral areas. In addition, labeled cells were present in hippocampus, the medial septum, the horizontal and vertical limbs of the diagonal band of Broca, and the caudate putamen. Finally, some midbrain and hindbrain regions important for motor function were immunoreactive. Because ligands specific for alpha1-adrenergic receptor subtypes are not available, the present immunocytochemical study not only addresses the subcellular and regional distribution of alpha1B-adrenergic receptors but may also provide clues about receptor subtype-specific function.

Animals↗

PubMed1999

Cyclic GMP may potentiate lordosis behaviour by progesterone receptor activation.

The purpose of this study was to test the hypothesis that cGMP acts as a progesterone substitute to facilitate lordosis in oestrogen-primed rats. Female Sprague-Dawley rats underwent stereotaxic surgery to place a 26-gauge guide cannula into the third ventricle. Bilateral ovariectomy was done at the same time as stereotaxic surgery. Five days later ovariectomized rats were primed with 2 microg estradiol benzoate 24 and 48 h prior to behaviour testing. Some animals were further injected with 200 microg progesterone 4 h before behaviour testing. A nitric oxide synthase inhibitor infused into the third ventricle before progesterone administration significantly reduced lordosis performance. 8-Bromo-cGMP, a cell permeable cGMP analogue, or saline vehicle was infused into the third ventricle of hormone-primed animals approximately 4 h prior to the first of 3-h behaviour tests. This cGMP analogue facilitated lordosis behaviour. We next used KT5823, a highly specific inhibitor of protein kinase G (PKG), to test the hypothesis that cGMP action is mediated by this kinase. In this experiment, KT5823 was infused 15 min before progesterone. KT5823 significantly decreased lordosis behaviour. RU486, a progesterone receptor antagonist, was used to assess whether the stimulatory effects of cGMP are mediated through the progesterone receptor. Oestrogen-primed animals were injected with 5 mg of RU486 or vehicle 60 min before infusion with 8-bromo-cGMP. RU486 significantly attenuated cGMP-facilitated lordosis behaviour. These data show that cGMP facilitates lordosis through activation of PKG and the progesterone receptor.

Alkaloids↗

PubMed1998

Evidence that GABA augmentation of norepinephrine release is mediated by interneurons.

GABAA receptor activation augments stimulated release of 3H-norepinephrine (NE) in brain slices from female rats. This effect is not blocked by acetazolamide or MK-801, indicating that permeability of the GABAA chloride channel to bicarbonate ions and NMDA receptor activation do not mediate GABA-induced NE release. Furthermore, GABA augments 3H-NE release from slices, but not from isolated nerve terminals (synaptosomes), indicating that interneurons mediate GABA effects on 3H-NE release.

Acetazolamide↗

PubMed1998

Effects of diabetes and estradiol on norepinephrine release in female rat hypothalamus, preoptic area and cortex.

These studies determined whether diabetes and estradiol treatment altered norepinephrine (NE) release from hypothalamus, preoptic area (POA), and cortical slices from ovariectomized (OVX) female rats. Animals were sacrificed 12 days after the onset of streptozotocin-induced diabetes and 48 h following vehicle or estradiol injection. Brain slices were preloaded with 3H-NE, and release was evoked twice (S and S2) by electrical stimulation. Diabetes increased hypothalamic NE release during S1 regardless of the administration of vehicle or estradiol. Neither estradiol treatment nor diabetes alone affected NE release during S2 in the hypothalamus or POA. Estradiol treatment elevated NE release in the POA during S2 but only in diabetic animals. Moreover, estradiol elevated cortical NE release during S2 regardless of the presence or absence of disease. We also examined whether alpha2-adrenoceptor regulation of NE release was influenced by diabetes or hormone treatment. Enhancement of NE release by alpha2-adrenoceptor antagonism was evident in all 3 brain regions. However, alpha2-adrenoceptor regulation of NE release was unaffected by diabetes and hormone treatment. These findings suggest that diabetes alters NE release in the hypothalamus/POA of female rats. Additionally, this work identifies a novel action of estradiol to enhance stimulated NE release in the cortex of female rats.

Animals↗

PubMed1998

Estradiol elevates protein kinase C catalytic activity in the preoptic area of female rats.

Estrogen acts in the brain to regulate female reproductive physiology and behavior, and protein kinase C (PKC) is estrogen-regulated in many estrogen-responsive tissues. We examined whether estrogen regulates PKC in the hypothalamus (HYP) and preoptic area (POA), brain regions which mediate estrogenic control of female reproductive function. PKC activity in tissue from hormone-treated and control female rats was measured, in the presence of phorbol ester and calcium, by quantifying 32p incorporation into a substrate peptide. PKC catalytic activity increased significantly in POA tissue extracts from estradiol-treated, ovariectomized (OVX) female rats but not in HYP or cortical extracts. Phorbol ester potentiation of cAMP accumulation also was examined to determine whether the ability of PKC to potentiate adenylyl cyclase activity was affected by estrogen. PKC stimulation potentiated forskolin-induced cAMP accumulation to a greater degree in POA, but not HYP, slices from estrogen-treated OVX female rats. PKC enzyme levels were examined using phorbol-12,13-dibutyrate binding assays and immunoblots. Estrogen treatment did not change phorbol ester binding affinity or the density of binding sites in the POA or HYP. Immunoblots for the alpha, beta, and gamma PKC isoforms combined, or the gamma PKC isoform alone, did not detect differences between hormone-treated and control OVX female rats. Therefore, estrogen treatment increased PKC catalytic activity in the POA of OVX female rats but not in the HYP. However, the increased PKC catalytic activity was not correlated with detectable changes in the level of the alpha, beta, or gamma PKC isoforms or in the density of phorbol ester binding sites.

Animals↗

PubMed1997

A potential role of cyclic GMP in the regulation of lordosis behavior of female rats.

Nitric oxide (NO) has been suggested to play a crucial role in the regulation of lordosis behavior via stimulation of guanylyl cyclase to synthesize cyclic GMP. Whalen and Lauber (1986, Neurosci. Biobehav. Rev. 10, 47-53) hypothesized that hormones and pharmacological agents known to facilitate lordosis in estrogen-primed rodents act through cyclic GMP. The compound 1H-[1,2, 4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) has been shown to selectively inhibit NO-stimulated cyclic GMP production. In the present study, we investigated the effects of ODQ on lordosis behavior. Female rats were implanted with a guide cannula aimed at the lateral or third ventricles by stereotaxic surgery, and their ovaries were bilaterally removed. Five days later, animals were injected subcutaneously with 2 microg estradiol benzoate at 48 and 24 hr, and 200 microg progesterone 4 hr before behavioral testing. ODQ or vehicle (1 microl) was administered at the time of progesterone treatment or 20 min before lordosis testing. ODQ significantly decreased lordosis quotients and the quality of lordosis at both intervals of drug infusion. Locomotor activities, measured by line crossing and rearing, were not affected by ODQ. ODQ also inhibited cyclic GMP accumulation in response to NMDA stimulation in hypothalamic and cerebellar slices in vitro. We conclude that cyclic GMP produced by NO generation is an important modulator of female rat sexual behavior.

Animals↗

PubMed1997

GABA augments basal and electrically stimulated 3H-norepinephrine release in hypothalamic, preoptic area and cortical slices of female rats.

These studies examined the regulation by GABA of norepinephrine release from hypothalamus, preoptic area and frontal cortex. Using superfused brain slices from female rats, we show that 100 microM GABA enhances both basal and electrically stimulated release of 3H-norepinephrine in all three brain regions. The GABAA agonist muscimol (100 microM) significantly augments 3H-norepinephrine release, but it is somewhat less effective than GABA. The GABAB agonist baclofen has little or no effect on basal 3H-norepinephrine efflux. GABA also augments both the magnitude and duration of electrically evoked 3H-norepinephrine release in slices from all three brain regions. GABA facilitation of electrically stimulated 3H-norepinephrine release is mediated through GABAA receptors as evidenced by its blockade by 10 microM bicuculline, a GABAA antagonist, but not by 200 microM 2-OH-saclofen, a GABAB antagonist. These data show that the inhibitory amino acid neurotransmitter GABA enhances both basal and evoked release of 3H-norepinephrine in brain slices from female rats. These effects are predominantly mediated by GABAA receptors. GABA modulation of hypothalamic norepinephrine release may play a role in the regulation of gonadotropin secretion and reproductive behaviors such as lordosis.

Animals↗

PubMed1997

Gender-related differences exist in cortical [3H]nisoxetine binding and are not affected by prenatal morphine exposure.

The present study was designed to test the hypothesis that prenatal morphine, which differentially affects hypothalamic norepinephrine content and turnover in male and female rats, has sexually dimorphic effects on the density of hypothalamic norepinephrine uptake sites in adult offspring. The binding characteristics of norepinephrine transporters were examined in the hypothalamus, preoptic area and frontal cortex of adult male and female rats exposed to morphine (5-10 mg/kg, twice daily) or saline on gestation days 11-18. There was a gender-related difference in the density of norepinephrine uptake sites measured by [3H]nisoxetine binding in the frontal cortex of saline controls, with control males having significantly fewer binding sites than control females. Prenatal morphine administration did not reverse or eliminate this difference. Additionally, prenatal morphine exposure had no effects on either the binding capacity or the affinity of norepinephrine uptake sites in the hypothalamus, preoptic area or frontal cortex of adult progeny. Thus, alterations in hypothalamic norepinephrine content and turnover following prenatal morphine exposure are not reflected in alterations in norepinephrine uptake sites. However, recent immunocytochemical work in our laboratory correlated reductions in hypothalamic norepinephrine content and turnover rate with reductions in tyrosine hydroxylase and dopamine-beta-hydroxylase fiber density in the hypothalamus of morphine-exposed female rats. Therefore, the present results may suggest that compensatory mechanisms increase the density of norepinephrine uptake sites in hypothalamic terminal fields of morphine-exposed females.

Analgesics, Opioid↗

PubMed1997

Estradiol reduction of alpha 2-adrenoceptor binding in female rat cortex is correlated with decreases in alpha 2A/D-adrenoceptor messenger RNA.

We developed a quantitative RNAse protection assay for alpha 2A/D-adrenoceptor messenger RNA in rats to test the hypothesis that decreases in cortical alpha 2-adrenoceptor binding observed following estrogen treatment of ovariectomized rats correlate with reduced levels of messenger RNA encoding the predominant alpha 2-adrenoceptor subtype expressed in cortex. Estradiol treatment for 48 h reduced cortical alpha 2A/D-adrenoceptor messenger RNA by approximately 50% when compared to ovariectomized, oil-treated control animals. Estradiol down-regulation of alpha 2A/D-adrenoceptor messenger RNA was accompanied by a significant decrease in cortical alpha 2-adrenoceptor density, as measured by 3H-RX821002 binding. Estrogen treatment did not alter alpha 2A/D-adrenoceptor messenger RNA or alpha 2-adrenoceptor binding in female rat hypothalamus-preoptic area. This study provides the first evidence that estradiol regulates expression of postsynaptic alpha 2-adrenoceptors in female rat frontal cortex, suggesting a possible molecular substrate for hormonal modulation of cognitive function.

Adrenergic alpha-Antagonists↗

PubMed1996

Estrogen regulation of mu opioid receptor density in hypothalamic premammillary nuclei.

The effect of estradiol on opioid receptor density in the hypothalamus of female rats was examined by in vitro receptor autoradiography using [3H]naloxone as the ligand. Exposure of ovariectomized rats to estradiol for 48 h markedly increased the density of [3H]naloxone binding in both the ventral and dorsal premammillary nuclei but not in other regions of the hypothalamus or amygdala. Thus, estrogen selectively modulates opioid receptor binding in posterior hypothalamic regions involved in gonadotropin secretion and temperature regulation.

Animals↗

PubMed1996

Effects of nitric oxide on stimulated release of norepinephrine from female rat hypothalamic slices.

Norepinephrine (NE) is an important neurotransmitter involved in ovarian steroid hormone regulation of female reproductive function in rats. Nitric oxide (NO) has also been suggested to be an essential mediator of gonadotropin-releasing hormone release and of lordosis behavior of female rats. These studies used a superfusion system to investigate the hypothesis that NO regulates [3H]NE release in the preoptic area (POA) and hypothalamus (HYP), brain regions that mediate ovarian steroid effects on reproductive function. The NO synthase inhibitors N-nitro-L-arginine and NG-nitro-L-arginine methyl ester did not modify either basal or N-methyl-D-aspartate (NMDA)-stimulated NE release in either brain region of ovariectomized, hormone-treated or control animals. The NO precursor L-arginine (L-Arg) reduced NMDA-stimulated NE release in POA but had no effect on KCl- or electrically-stimulated release. L-Arg did not influence basal or evoked release of [3H]NE from HYP slices. Sodium nitroprusside (SNP), a NO-generating compound, blocked the release of NE in response to NMDA stimulation but not in response to KCl or electrical stimulation. Thus, SNP is probably reducing NE release by acting as an NMDA antagonist rather than via NO production. There was a tendency for administration of both estrogen and progesterone to ovariectomized females to facilitate NMDA-stimulated NE release, particularly in the POA. Our data suggest that NO does not mediate basal or NMDA-stimulated NE release in rat POA and HYP. Therefore, NO regulation of lordosis behavior and gonadotropin release in female rats is probably not exerted at the level of NE release.

Adrenergic Uptake Inhibitors↗

PubMed1996

Global sex differences in stress-induced activation of cerebral metabolism revealed by 2-deoxyglucose autoradiography.

Although it is well known that there are sex differences in stress-induced activation of the hypothalamic-pituitary-adrenal axis, it is not known if there are also gender-related differences in stress-induced neural activity. In this study, restraint and formalin injections into a forelimb were used as stressors and 2-[14C]-deoxyglucose (2DG) autoradiography was used to evaluate regional brain glucose metabolism, an index of neural activity. Analysis of blood samples collected during the 2DG procedure confirmed that stress elevates plasma glucose levels signficantly more in females than in males. Moreover, females show higher brain glucose utilization in all regions examined, including sex hormone-responsive regions such as the medial amygdala, medial preoptic nucleus, ventromedial nucleus, and arcuate nucleus, as well as the CA1 layer and dentate region of the hippocampus, the posterior parietal (sensorimotor) cortex, medial and lateral habenula, and splenium of the corpus callosum. The sex differences are apparent regardless of whether animals were injected with saline or formalin. Interestingly, the medial preoptic area, which shows robust neuroanatomical sex differences, demonstrates greater activation in response to formalin than to saline only in females. In some regions of both males and females, glucose utilization was higher on the side of the brain contralateral to the saline or formalin injection site. These findings suggest that there are widespread, gender-related differences in neuronal as well as endocrine activation in response to highly stressful conditions.

Animals↗

PubMed1996

Transcranial magnetic stimulation downregulates beta-adrenoreceptors in rat cortex.

Recently, a method for transcranial magnetic stimulation (TMS) of the brain has been developed. Thus, it is possible to explore neurochemical and behavioral effects of TMS in rats. Repeated TMS (9 days) reduced beta-adrenergic receptor binding in cortex, as does electroconvulsive shock (ECS) and other antidepressant treatments. Thus TMS appears to be a potential antidepressive treatment.

Adrenergic beta-Agonists↗

PubMed1996

Estradiol regulation of alpha 1b-adrenoceptor mRNA in female rat hypothalamus-preoptic area.

Estradiol treatment for 48 h increases the density of alpha 1B-adrenoceptors in the hypothalamus-preoptic area of ovariectomized female rats by five- to six-fold. Present studies tested the hypothesis that estradiol elevation of hypothalamus-preoptic area alpha 1B-adrenoceptor density is correlated with increased levels of mRNA for this receptor. We developed a semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) protocol for measuring brain alpha 1b-adrenoceptor mRNA. The primers chosen yielded the predicted 409 base pair PCR product when used to amplify authentic alpha 1b-adrenoceptor cDNA. The identity of the RT-PCR products from rat brain was confirmed by restriction digest analysis and sequencing. Moreover, there was a good correlation between the levels of alpha 1b-adrenoceptor mRNA measured by RT-PCR in liver, whole brain and cerebellum with previous measurements using Northern blots and RNAse protection assays. We then performed RT-PCR on total RNA from hypothalamic-preoptic area tissue taken from ovariectomized control rats and from ovariectomized rats injected once or twice with 2 micrograms of estradiol benzoate at 24 or 24 and 48 h before sacrifice. Exposure to estradiol for either 24 or 48 h significantly increased levels of alpha 1b-adrenoceptor mRNA by 86-110% in the hypothalamus-preoptic area of ovariectomized female rats when compared to oil-treated controls. We also examined whether estradiol regulates alpha 1b-adrenoceptor mRNA in the cortex. Cortical alpha 1b-adrenoceptor mRNA levels were reduced to approximately 20% of control levels when measured 24 h after hormone injection. A similar decrease in cortical alpha 1b-adrenoceptor mRNA was observed 48 h after estrogen administration. In summary, estradiol treatment significantly increases the level of alpha 1b-adrenoceptor mRNA in the hypothalamus-preoptic area, a brain region involved in the control of reproductive function. In the cortex, a brain region with relatively few estrogen receptors, the same estrogen treatment reduces alpha 1b-adrenoceptor mRNA levels.

Animals↗