Noradrenergic regulation of cytosol estrogen receptors in female rat hypothalamus: possible role of alpha 2-noradrenergic receptors

Involvement of brainstem catecholaminergic inputs to the hypothalamic paraventricular nucleus in estrogen receptor alpha expression in this nucleus during different stress conditions in female rats

In the present study, we determined the involvement of brainstem catecholaminergic inputs to the paraventricular nucleus (PVN) on estrogen receptor alpha (ERalpha) expression in this nucleus during conditions of 48-h fasting, 2-deoxy-d-glucose (2DG)-induced acute glucoprivation and 1-h immobilization, in ovariectomized rats. Our approach was to examine the effect of lesioning catecholaminergic inputs to the PVN using DSAP [saporin-conjugated anti-DBH (dopamine-beta-hydroxylase)]. Bilateral injection of DSAP into the PVN, 2 wk before stress, prevented fasting-, glucoprivation-, and immobilization-induced increase in ERalpha-immunopositive cells in the PVN. The DBH-immunoreactive (ir) terminals in the PVN were severely depleted by DSAP injection in all experimental groups. Among the brainstem noradreneregic cell groups examined, DBH-ir cell bodies were significantly reduced in the A2 region of all experimental groups treated with DSAP compared with the saporin- and vehicle-injected controls. PVN DSAP injection caused a small, but not significant, decrease in A1 DBH-ir cell bodies in fasted and immobilized rats, and a significant, but slight, reduction in A1 DBH-ir cell bodies of iv 2DG- injected rats compared with PVN vehicle-injected or PVN saporin-injected controls. The A6 DBH-ir cell bodies in all experimental groups treated with DSAP, saporin, or vehicle did not show any significant difference. These results suggest that the brainstem catecholaminergic inputs to the PVN, especially from the A2 cell group, may play a major role in mediating the induction of ERalpha expression in the PVN by metabolic stressors such as fasting, acute glucoprivation, and less specific stressors, such as immobilization, in female rats.

Oestradiol increases phosphorylation of a dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32) in female rat brain

Recent studies suggest that oestrogen and progestin receptors may be activated by the neurotransmitter dopamine, as well as by their respective ligands. Because intracerebroventricular infusion of D(1), but not D(2), dopaminergic receptor agonists increases oestrous behaviour in oestradiol-primed rats, we wanted to determine if treatment with oestradiol alters the activity of D(1) receptor-associated processes in steroid receptor-containing areas in female rat brain. One D(1) receptor-associated phosphoprotein that may be influenced by oestradiol is a dopamine- and cyclic AMP-regulated phosphoprotein, Mr = 32,000 (DARPP-32). Because DARPP-32 is phosphorylated in response to dopamine acting via a cAMP-dependent protein kinase, it provides a useful marker to examine where in the brain a particular stimulus might be altering the activity of D(1) receptor-containing neurones. To determine if oestradiol alters the phosphorylation of DARPP-32, we stained immunocytochemically brain sections of female rats treated with behaviourally relevant doses of oestradiol or oil vehicle with an antibody that detects only the threonine 34-phosphorylated form of DARPP-32. Behaviourally effective doses of oestradiol increase the phosphorylation of DARPP-32 within the medial preoptic nucleus, bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus and the ventromedial nucleus of the hypothalamus, 48 h after treatment. These data suggest that oestradiol increases the activity of D(1) dopamine receptor-associated processes in oestrogen receptor-containing areas of female rat forebrain.

The effect of diabetes on alpha2-adrenergic receptor activity in the reproductive centers of the female rat brain

OBJECTIVE

We hypothesized that known diabetes-induced deficits in female rat reproduction may result in part from decreased central alpha(2)-noradrenergic receptor density or affinity.

STUDY DESIGN

Female rats were oophorectomized and divided into 2 groups; one group received streptozocin during the operation to induce diabetes, and the other served as a nondiabetic control group. Random blood glucose levels were measured. Half the rats in each group were killed on postoperative day 10, and half were killed on postoperative day 14. Direct radioligand binding assays were performed on tissue prepared from the hypothalamus, preoptic area, and cortex of each rat. Analysis of variance was used to evaluate intergroup differences in receptor concentration or equilibrium constant.

RESULTS

We detected no significant difference in the mean receptor concentration or equilibrium constant between the groups with and without diabetes in the hypothalamus, the preoptic area, and the cortex on postoperative day 10 or 14.

CONCLUSION

Diabetes-induced impairments in female rat reproduction do not involve alterations in alpha(2)-receptor density or affinity in the hypothalamus, preoptic area, or cortex.

Deficits in reproductive behavior in diabetic female rats are due to hypoinsulinemia rather than hyperglycemia

These studies determined whether deficits in reproductive behavior observed in streptozotocin (STZ)-induced diabetic female rats are caused by hyperglycemia or loss of insulin. Female Sprague-Dawley rats were ovariectomized and made diabetic by a single ip injection of STZ (75 mg/kg). Reproductive behavior was measured 12 days after the onset of hyperglycemia following the injection of estrogen and progesterone in doses known to restore reproductive behavior in nondiabetic rats. Rats in which STZ produced diabetes showed significantly reduced receptive and proceptive sexual behaviors. Normalization of blood glucose levels either by restricting diet or by phlorizin treatment failed to restore reproductive behavior in diabetic animals. However, even doses of insulin which were not fully effective in correcting peripheral hyperglycemia were able to prevent the STZ-induced behavioral deficit. No changes in general activity were observed in any experimental group as assessed by open field activity. The density of the norepinephrine transporter, as measured by [3H]nisoxetine binding, was reduced in the cortex but not in the brain stem, hypothalamus, or hippocampus of diabetic animals. Insulin treatment prevented the loss of cortical [3H]nisoxetine binding, and even partial normalization of blood glucose restored cortical [3H]nisoxetine binding to control levels. These findings suggest that diabetes-induced reproductive deficits are due to hypoinsulinemia and cannot be corrected simply by the normalization of blood glucose, whereas reductions in the density of cortical norepinephrine transporter result from hyperglycemia.

Novel estrogen feedback sites associated with stress-induced suppression of luteinizing hormone secretion in female rats

1. The fasting-induced suppression of LH secretion is totally dependent on steroidal milieu because the suppression is observed only in intact or ovariectomized estrogen-primed rats but not in ovariectomized animals. The following neural pathway mediating fasting-induced suppression of LH secretion has been suggested by a series of experiment: A neural signal emanating from the stomach during fasting reaches the medulla oblongata via afferent vagal nerve so as to activate the noradrenergic system projecting to the PVN: this results in an increased CRH release, and in turn the suppression of the LHRH release and then LH release. Estrogen seems to activate the neural pathway by acting on somewhere in the pathway. 2. We found that the paraventricular nucleus of the hypothalamus (PVN) and A2 region of the medulla oblongata is the estrogen feedback sites associated the dependence of the fasting-induced suppression of LH secretion on estrogen. The estrogen feedback action on the PVN does not involve an increase in norepinephrine release in the PVN. In addition, we also found that estrogen receptors are increased in the PVN and A2 region by acute fasting. Therefore, the following hypothesis is proposed: fasting first induces an transient increase in the activity of noradrenergic system at the beginning of the first dark phase after the food deprivation; this activation results in an increase in estrogen receptors in the PVN and A2 region; the increase in estrogen receptors leads to an increase in the sensitivity of noradrenergic systems to the neural inputs associated with fasting to these nuclei. 3. The response of the reproductive activity to various external stimuli including stress is modulated by ovarian steroids. The estrogen feedback action on the PVN and A2 is totally different from the so-called "negative feedback action" of estrogen that is for monitoring the ovarian condition. The novel estrogen feedback action may alter the response of neurons regulating gonadal axis to the signal associated with environmental cues such as stress.

Effect of fasting and immobilization stress on estrogen receptor immunoreactivity in the brain in ovariectomized female rats

The present study examined the effect of 48-h fasting and 1-h immobilization on estrogen receptor immunoreactivity in selected hypothalamic areas and the nucleus of the solitary tract (NTS) in ovariectomized rats. Fasting induced an increase in ER-immunoreactive cells in the paraventricular nucleus (PVN), periventricular nucleus (PeVN) and NTS compared with the unfasted control group. Similarly, immobilization caused an increase in ER-positive cells in the same areas, PVN, PeVN and NTS, versus the non-immobilized group. There was no significant increase in the number of ER-immunoreactive cells in the preoptic area (POA), arcuate nucleus (ARC) or ventromedial hypothalamic nucleus (VMH) following fasting and immobilization. Our previous work in ovariectomized rats with estrogen microimplants in the brain revealed that the PVN and A2 region of the NTS are the feedback sites of estrogen in activating the neural pathway to suppress pulsatile LH secretion during 48-h fasting. The result in the food-deprived rats suggests that estrogen modulation of the suppression of LH secretion during fasting is partly due to the increase in estrogen receptors in the PVN and A2 region. The physiological significance of the increase in neural ER following immobilization remains to be elucidated.

Vagus nerve mediates the increase in estrogen receptors in the hypothalamic paraventricular nucleus and nucleus of the solitary tract during fasting in ovariectomized rats

The effect of total subdiaphragmatic vagotomy on estrogen-receptor immunoreactivity (ERIR) in the paraventricular nucleus (PVN) and nucleus of the solitary tract (NTS) was examined in fasted ovariectomized rats to clarify the peripheral inputs mediating fasting-induced increase in ERIR in these two nuclei. Vagotomy abolished the effect of 48-h fasting on the expression of ER in these two areas. The result indicates that the neural signal(s) that increase the expression of ER in the PVN and A2 region of the NTS following 48-h fasting is transmitted through the vagus. The involvement of the vagus in the fasting-induced increase in ER in the PVN and A2 region may also be the same neural pathway involved in the suppression of pulsatile luteinizing hormone secretion in fasted female rats.

Effect of phenylephrine and prazosin on the somatostatinergic system in the rat frontoparietal cortex

Somatostatin (SS) and noradrenaline (NA) are distributed in the rat cerebral cortex, and seizure activity is one of the aspects of behavior affected by both neurotransmitters. Due to the possible interaction between both neurotransmitter systems, we studied whether phenylphrine, an alpha 1-adrenoceptor agonist, and prazosin, an alpha 1-adrenoceptor antagonist, can modulate SS-like immunoreactivity (SS-LI) levels, binding of [125I][Tyr11]SS to its specific receptors, the ability of SS to inhibit adenylate cyclase (AC) activity, and the guanine nucleotide binding regulatory protein G, and G., in the Sprague-Dawley rat frontoparietal cortex. An IP dose of 2 or 4 mg/kg of phenylephrine injected 7 h before decapitation decreased the number of SS receptors and increased the apparent affinity in frontoparietal cortex membranes. An IP dose of 20 or 25 mg/kg of prazosin administered 8 h before decapitation increased the number of SS receptors and decreased their apparent affinity. The administration of prazosin before the phenylephrine injection prevented the phenylephrine-induced changes in SS binding. The addition of phenylephrine and/or prazosin 10(-5) M to the incubation medium changed neither the number nor the affinity of the SS receptors in the frontoparietal cortex membranes. Phenylephrine or prazosin affected neither SS-LI content nor the basal or forskolin (FK)-stimulated AC activities in the frontoparietal cortex. In addition, SS caused an equal inhibition of AC activity in frontoparietal cortex membranes of phenylephrine-and prazosintreated rats compared with the respective control group. Finally, phenylephrine and prazosin did not vary the pertussis toxin (PTX)-catalyzed ADP ribosylation of Gi- and/or Go-proteins. These results suggest that the above-mentioned changes are related to the phenylephrine activation of alpha 1-adrenoceptors or to the blocking of these receptors by prazosin. In addition, these data provide further support for a functional interrelationship between the alpha 1-adrenergic and somatostatinergic systems in the rat frontoparietal cortex.

Changes in alpha 1-adrenergic neurotransmission alter the number of somatostatin receptors in the rat hippocampus

The administration of an i.p. dose of phenylephrine (2 mg/kg) increased the number of [125I]Tyr11-somatostatin ([125I]Tyr11-SS) receptors and decreased their apparent affinity in rat hippocampal membranes 7 h after its injection. Prazosin (20 mg/kg, i.p.) administered 1 h before phenylephrine reversed effects of the latter on SS binding. Prazosin alone decreased the number of SS receptors without changing the affinity. The addition of phenylephrine or prazosin (10(-5) M) to the incubation medium did not change the SS binding characteristics. The present results support the notion that the alpha 1-adrenergic system regulates the binding of SS to its specific receptors in rat hippocampus.

Somatostatin receptors coupled to the inhibition of adenylyl cyclase in the rat frontoparietal cortex are modulated by alpha 2 adrenoceptors

The administration of an alpha 2-adrenoceptor agonist, clonidine, increased the number of somatostatin (SS) receptors and the affinity constant in frontoparietal cortex membranes. In addition, in the clonidine group, the capacity of SS to inhibit basal and forskolin (FK)-stimulated adenylyl cyclase (AC) activity in the frontoparietal cortex was significantly higher than in the control group. Pretreatment with the alpha 2-adrenoceptor antagonist yohimbine prevented the clonidine-induced changes in SS binding and SS-inhibited AC activity. Yohimbine alone had an opposite effect from clonidine. These experiments provide further evidence that the alpha-adrenergic system modulates the rat frontoparietal cortex somatostatinergic system.

Estrogen receptor immunostaining in the preoptic area and medial basal hypothalamus of estradiol benzoate- and prazosin-treated female guinea-pigs

Evidence has accumulated showing that the alpha 1-adrenergic antagonist prazosin decreases nuclear estrogen binding in the hypothalamus of the guinea-pig. In this study we asked if prazosin treatment alters estrogen receptor (ER) protein content as reflected by changes in ER-immunoreactivity. The monoclonal rat antibody H222 directed against ER was used to detect ER-immunoreactive (ER-ir) cells in eight specific preoptic and hypothalamic brain regions of ovariectomized Hartley strain guinea-pigs treated with estradiol benzoate and 1.0 mg/kg prazosin or vehicle. Immunocytochemical parameters which provided optimum conditions for detection of even modest changes in ER-immunoreactivity were first established. Then, using these optimum conditions, we compared 1) the mean number of ER-ir profiles, 2) the mean density of ER-ir staining, and 3) the distribution of ER-ir staining density readings, between conditions within each of the eight brain regions. No differences in any of these measures were found between prazosin- and vehicle-treated females. We also compared the percentage of ER-ir nuclear profiles across the eight cell groups investigated in estradiol benzoate- and vehicle-treated females. The medial preoptic area had by far the highest percentage (48%) of ER-ir profiles (P < 0.05) compared to all seven other brain regions (23% to 32% ER-ir cells). Our data, showing that ER-immunoreactivity is not reduced (6h) after prazosin treatment, suggests that mechanisms other than alterations in ER protein should be considered when interpreting the effects of prazosin on the retention of estradiol by nuclear or cytosolic extracts of hypothalamic lysates.

Benextramine, a putative neuropeptide Y receptor antagonist, attenuates the termination of receptivity

Sexual behavior in female rats is dependent on gonadal steroids. In ovariectomized rats, progesterone treatment typically exerts a biphasic effect on copulatory behavior induced by prior treatment with estradiol. Thus, there is an initial augmentation of the facilitative effect of estradiol occurring 4-10 h after progesterone. This is followed by a profound inhibitory effect, essentially terminating receptivity. We hypothesized that the receptivity terminating effect of progesterone could be due to increased synthesis and release of neuropeptide Y in relevant brain regions. Rats were tested for female sexual behavior 4 h after progesterone (52 h postestradiol). Immediately following this test, benextramine was administered (0, 3, or 15 mg/kg, IP). Subsequently, behavioral tests were administered 24, 48, 72, and 96 h postbenextramine. Benextramine treatment attenuated the inhibitory effects of progesterone on receptivity (lordosis quotients and percent of responding animals) without affecting either proceptive or rejection behaviors. These data suggest that blockade of NPY (and alpha-adrenergic) receptors is associated with selective enhancements of specific components of sexual behavior in female rats. Specifically, blockade of NPY receptors by benextramine is associated with continued receptivity.

Neonatal prazosin exposure reduces ovarian weight and estrogen receptor binding in adult female rats

Exposure of estrogen treated adult female guinea pigs to the alpha-1 antagonist prazosin has been shown to reduce levels of estrogen binding in the hypothalamus and preoptic area. To further investigate this interaction between the noradrenergic and neuroendocrine axes, newborn female rat pups received an s.c. implant of prazosin (0.0125 mg/day for 5 days) or placebo. In adulthood, subjects were sacrificed by perfusion with DMSO on the morning of proestrous. Tissue analysis of the medial preoptic area, corticomedial amygdala, and mediobassal hypothalamus revealed that cytosolic estrogen binding was significantly reduced in all three areas for the prazosin treated group as compared to controls. Ovarian weight was also significantly reduced in the prazosin treated group, although uterine weight was unaffected. Interestingly, prazosin treated females showed a post-pubertal increase in body weight characteristic of ovariectomized females, while controls showed no such increase. These results support the existence of a significant developmental interaction between the noradrenergic system and the neuroendocrine axis as measured by ovarian weight and estrogen binding in the brain.

Norepinephrine inhibits vasopressin-stimulated flank marking in the Syrian hamster by acting within the medial preoptic-anterior hypothalamus

Syrian hamsters exhibit a form of scent marking behavior called flank marking. Flank marking, which is elicited during social contact with other hamsters and by the odors of other hamsters, communicates socially important information such as mate choice and dominance status. Vasopressinergic activity within the medial preoptic-anterior hypothalamic continuum (MPOA-AH) is essential for the expression of flank marking. In female hamsters, an inverse relationship exists between the expression of flank marking and sexual receptivity during the 4-day estrous cycle. Since norepinephrine (NE) appears to facilitate sexual receptivity, the present study investigated whether NE might have an inhibitory effect on flank marking by acting on Vasopressinergic activity within the MPOA-AH. Microinjection of 9.0 μM arginine vasopressin (AVP) into the MPOA-AH stimulated high levels of flank marking. Microinjection of 9.0 μM AVP combined with NE in concentrations of 4.0, 0.4 or 0.2 nM, drastically reduced or eliminated flank marking. In contrast, AVP in combination with 0.09, 0.04 or 0.004 nM NE produced no significant reductions in flank marking. In addition, microinjection of 9.0 μM AVP, in combination with epinephrine (4.0 nM), but not dopamine (4.0 nM), serotonin (4.0 nM) or neuropeptide Y (900 μM), significantly reduced AVP-induced flank marking. In male hamsters, microinjection of NE (4.0 nM) combined with AVP (9.0 μM) into the MPOA-AH was not found to inhibit AVP-stimulated flank marking. These results suggest that NE is involved in regulating the expression of flank marking during the estrous cycle by acting on Vasopressinergic activity within the MPOA-AH.

Estrogen receptor binding in regions of the rat hypothalamus and preoptic area after inhibition of dopamine-beta-hydroxylase

Previous studies have shown that administration of diethyldithiocarbamate (DDC), a dopamine-beta-hydroxylase inhibitor, results in a decreased concentration of estrogen receptors measured in the rodent hypothalamus and preoptic area. To determine if this modulation of receptor content is region-specific, in vitro estrogen binding assays were performed on cytosol and cell nuclear extracts of microdissected brain regions from female rats treated with DDC. For cytosol binding comparisons, ovariectomized (OVX) rats were treated with 550 mg DDC/kg b. wt. or the saline vehicle 12 h before sacrifice. For cell nuclear binding comparisons, OVX rats received a maximal dose of estradiol 12 h after DDC or saline treatment and 1 h before sacrifice. No region-specific decreases in estrogen binding were observed in either cytosol or nuclear extracts. To further examine possible regional specificity, quantitative autoradiographic analysis of the in vivo hypothalamic uptake of an iodinated analog of estradiol, 11 beta-methoxy-16 alpha-[125I]iodoestradiol (MIE2), in OVX rats treated with DDC was conducted. Animals received a saturating dose of [125I]MIE2 12 h after DDC or saline treatment and 1 h before sacrifice. DDC treatment resulted in higher background levels of radioactivity and a trend toward higher uptake levels in all brain regions, but with no evidence of marked regional specific effects in any area of the brain. In tissue uptake studies, DDC treatment resulted in higher levels of radioactivity recovered from serum and neural tissues of [125I]MIE2-injected rats, suggesting that DDC slows the clearance of MIE2.(ABSTRACT TRUNCATED AT 250 WORDS)

The pre- and postnatal influence of hormones and neurotransmitters on sexual differentiation of the mammalian hypothalamus

A number of brain structures and a great number of brain functions have been shown to be sexually dimorphic. It has also been shown that development and differentiation of these structures and functions proceeds during a critical pre- and postnatal period of increased susceptibility, and is controlled by gonadal steroids and neurotransmitter substances. The brain of male and female mammals seems to be still undifferentiated before the period of increased susceptibility to gonadal steroids and neurotransmitters starts. Feminization of brain structure and functions, e.g., establishment of the cyclic LH-surge mechanism and the expression of lordosis behavior, seems to depend on the moderate interaction of estrogens with the developing nervous system. Defeminization and masculinization of brain functions seem to be established during interaction of the developing nervous system with androgens, which have to be converted, at least in part, into estrogens. Structural differentiation of the male brain, e.g., the sexually dimorphic nucleus of the preoptic area (SDN-POA), seems to be exclusively estrogen-dependent, during differentiation of male brain functions, however, estrogens may be supportive, rather than directive, to the primary action of androgens. The molecular mechanisms of sexual differentiation of the brain are not yet fully understood. It seems, however, that the priming action of gonadal steroids during the period of increased susceptibility is either mediated by neurotransmitters, or neurotransmitters modulate the priming action of gonadal steroids. In particular, the adrenergic, the serotoninergic, the cholinergic, and possibly the dopaminergic system were shown to have strong influences on sexual differentiation of brain structure and functions. In contrast to the great number of available studies on the influence of gonadal steroids on sexual differentiation of the brain, there are rather few studies available concerning the influence of neurotransmitter systems. The available results are partly contradictory, so that an interpretation must be done with caution and will leave plenty of room for speculation. Postnatal application of compounds which stimulate or inhibit adrenergic activity mainly affected the neural control of gonadotropin secretion, and had only minor influences on differentiation of behavior patterns. It seems, however, that adrenergic participation in the differentiation of the center for cyclic gonadotropin release is very complex and stimulatory and inhibitory components may operate simultaneously. Activation or inhibition of beta-adrenergic receptors during postnatal development was shown to impair the responsiveness of the center for cyclic gonadotropin release to gonadal steroids, and impairs the expression of ejaculatory behavior in male rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Progestin receptors in the ventromedial nucleus of the hypothalamus and arcuate nucleus-median eminence are decreased by idazoxan

Steroid-dependent lordosis behavior in ovariectomized (OVX) guinea pigs is attenuated by alpha 1- and/or alpha 2-noradrenergic (NE) receptor antagonists. Correlated with the decrease in lordosis after alpha 1-NE receptor blockade by prazosin is a decrease in 'cytosol' progestin receptors in the ventromedial hypothalamic nucleus (VMN). We examined whether a presumed alpha 2-NE receptor blocker (idazoxan, IDA) also affects progestin receptors. A decrease in 'cytosol' progestin receptors was found after IDA treatment of OVX, estrogen-treated guinea pigs in the VMN and the arcuate nucleus-median eminence (ARC-ME). Apparently, either prazosin or IDA can inhibit lordosis behavior and decrease 'cytosol' progestin receptors in the VMN. In contrast, idazoxan but not prazosin, decrease 'cytosol' progestin receptors in the ARC-ME.

Estradiol induction of proenkephalin messenger RNA in hypothalamus: dose-response and relation to reproductive behavior in the female rat

Hormone effects on proenkephalin (PE) mRNA allow an opportunity to compare a brain region-specific molecular change with a quantifiable behavior. Slot blots were used to measure PE mRNA levels in the ventromedial hypothalamus (VMN) and preoptic area (POA) as a function of the dose of estrogen administered to ovariectomized rats. Every rat used had been characterized for the ability to display lordosis behavior. Estradiol treatment led to a monotonic dose-dependent increase in PE mRNA level in VMN, while only a small effect was observed in POA at the higher estradiol doses. Lordosis behavior, assessed manually and in mating tests, also increased monotonically with estradiol dose. The data indicate that an apparent 'threshold' level of PE mRNA in VMN coincided wit display of behavior, and suggest further that high levels of PE mRNA, alone, are not sufficient for lordosis. While the exact relationship of the eventual product, Met-enkephalin, to female reproductive behavior remains to be determined, the parallel changes in PE mRNA level and behavior encourage further analysis.

Prazosin treatment does not affect progestin receptor induction in microdissected regions of the rat hypothalamus

Sex differences in estrogen-induced progestin receptors have been described in specific regions of the rat brain. To determine if alpha 1-neurotransmission plays a role in the expression of the sex differences in progestin receptor induction, the effects of the alpha 1-antagonist, prazosin, on progestin binding in microdissected regions of the rat brain was determined. Adrenalectomized/gonadectomized male or female rats were administered various doses of estradiol benzoate (EB) in combination with prazosin. With all treatment paradigms, and in both sexes, no significant effect of prazosin treatment on progestin receptor levels was observed. These results are consistent with the idea that sex differences in the estrogen-induction of progestin receptors in the rat hypothalamus are not due to sex differences in the alpha 1-adrenergic regulation of progestin receptor synthesis.

Differential alpha-1 and alpha-2 adrenergic effects on hypothalamic corticotropin-releasing factor and plasma adrenocorticotropin

There is presently no consensus as to the nature of the catecholaminergic influence on the regulation of corticotropin-releasing factor. The potential role that the alpha-adrenergic system plays was investigated by measuring hypothalamic corticotropin-releasing factor-like immunoreactivity and plasma adrenocorticotropin, following manipulation of alpha-1 and alpha-2 adrenergic receptor activation. Administration of the alpha-1 agonist methoxamine did not significantly alter either plasma adrenocorticotropin or hypothalamic corticotropin-releasing factor. Administration of the alpha-2 agonist clonidine resulted in a 24-fold increase in plasma adrenocorticotropin and a significant decrease in median eminence corticotropin-releasing factor, consistent with its release. Corticotropin-releasing factor in the remainder of the hypothalamus was not altered. Concurrent administration of clonidine with the selective alpha-2 antagonist yohimbine prevented the clonidine-induced changes in plasma adrenocorticotropin and hypothalamic corticotropin-releasing factor, consistent with the clonidine effect being mediated through alpha-2 receptors. Concurrent administration of clonidine with methoxamine did not prevent these effects, suggesting that the effect of clonidine was not mediated through presynaptic inhibition of noradrenergic adrenergic neurotransmission. Inhibition of protein synthesis by anisomycin induced changes in corticotropin-releasing factor and adrenocorticotropin which were not altered by combined treatment with methoxamine or clonidine. These data suggest differential roles for alpha-1 and alpha-2 systems in the regulation of corticotropin-releasing factor. Results from alpha-2 adrenergic activation were consistent with stimulation of corticotropin-releasing factor release, an effect mediated by a postsynaptic alpha-2 mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-adrenergic regulation of androgen receptor concentration in the preoptic area of the rat

We examined the effect of the pharmacological disruption of the catecholaminergic system on the concentration of nuclear androgen receptor, as measured by the in vitro binding of methyltrienolone ([3H]R1881) to salt extracts of anterior pituitary (AP), preoptic area (POA) and medial basal hypothalamus (MBH). Treatment of gonadectomized male and female rats with the dopamine-beta-hydroxylase inhibitor, diethyldithiocarbamate (400 mg/kg b. wt.), 30 min before treatment with dihydrotestosterone (1 mg/animal) produced a decrease in the number of nuclear androgen receptor compared with saline-treated controls (P less than 0.05). This effect was specific for the POA and was not present 15 h after DHT treatment. There was no effect on cytosolic androgen receptor nor was there a drug effect on the apparent dissociation constant (Kd) of [3H]R1881 binding to hypothalamus-preoptic area cytosols. Treatment of intact males and castrated, testosterone-treated males with the alpha 1- and alpha 2-adrenergic antagonists, prazosin (5 mg/kg b. wt.) and yohimbine (2 mg/kg b. wt.), respectively, resulted in a significant decrease in the number of nuclear AR 2 h following drug treatment (P less than 0.05). There was no effect of the beta-adrenergic receptor antagonist propranolol (10 mg/kg b. wt.) when given to intact animals, nor was there an effect of idazoxan (5 mg/kg) when given to testosterone-treated animals. The effects of yohimbine and prazosin were restricted to the POA. None of the drugs competed with the binding of [3H]R1881 for the androgen receptor nor did they alter the Kd of cytosol or nuclear androgen receptor. These data provide evidence for an adrenergic interaction with the POA androgen receptor and suggest a role for catecholamines in modulating androgen sensitivity in the rat brain.

Social stimuli augment estrogen receptor binding in preoptic area of female prairie voles

In female prairie voles ovarian estrogen secretion is stimulated by exposure to males. The present study determined that social stimuli can also enhance the neural response to estrogen. Ovariectomized female voles given a fixed amount of estradiol and exposed to males had higher levels of estrogen receptor binding in cell nuclei in the preoptic area than did females given estrogen and not exposed to males.

Muscarinic cholinergic modulation of hypothalamic estrogen binding sites

Studies from other laboratories have demonstrated that agents which interact with the dopaminergic and noradrenergic neurotransmitter systems alter the concentrations of cytosolic hypothalamic estrogen receptors. These results have led to the hypothesis that catecholamine systems are involved intimately with the regulation of brain estrogen receptors. The present study was undertaken to determine if agents from a different neurotransmitter system similarly affect [3H]estradiol binding. The data presented here show that the muscarinic cholinergic agonist, bethanechol, increases the number of cytosolic hypothalamic estradiol binding sites in ovariectomized female rats by as much as 38% above control values. Pretreatment with atropine sulfate, a highly specific muscarinic antagonist, blocked the bethanechol effect. Interestingly, bethanechol failed to alter the concentration of estradiol binding sites in castrated male rats. The results of the present experiments show not only that pharmacological modulation of cytosolic hypothalamic estradiol binding sites is not limited to drugs which interact with catecholaminergic systems, but that such effects may be sex-specific.

Atropine sulfate modulates estrogen binding by female, but not male, rat hypothalamus

Studies have shown that pharmacological manipulation of the dopamine, norepinephrine and muscarinic cholinergic neurotransmitter systems modulates the number of neural estrogen binding sites. Previously, we reported that the muscarinic agonist, bethanechol, increased estrogen receptor binding by hypothalamic cytosols from female, but not male, rats. Moreover, pretreatment with atropine prevented the bethanechol-induced effect. The experiments reported here were executed with the expectation that atropine alone would either decrease or fail to alter estrogen binding. However, the present data show that atropine increases estrogen binding by female, but not male, hypothalamic cytosols. Thus, it appears that a muscarinic antagonist and agonist can similarly affect the concentration of estrogen binding sites in female rat hypothalamus.

Further evidence of noradrenergic regulation of rat hypothalamic estrogen receptor concentration: possible non-functional increase and functional decrease

The regulation of estrogen receptors by the alpha 2-noradrenergic system was studied. A single injection of the alpha 2-noradrenergic antagonist, yohimbine, caused a biphasic effect on the concentration of cytosol estrogen receptors in the mediobasal hypothalamus and anterior pituitary gland. A short-latency increase was seen at 1.5-3 h, followed by a longer-lasting decrease at 8-16 h. Scatchard analysis revealed that the apparent, short-latency increase is in the concentration of binding sites, not in the affinity of the receptor for [3H]estradiol. The increase in the concentration of cytosol estrogen receptors is not blocked by pretreatment with the alpha 2-noradrenergic agonist, clonidine. In addition, no increase is detected in the concentration of cell nuclear estrogen receptors accumulating in response to a saturating dose of estradiol. Therefore, the apparent increase in the concentration of cytosol estrogen receptors may not represent a functional increase in receptors. The decrease in the concentration of estrogen receptors, which occurs 8-16 h after yohimbine treatment, is also seen after injection of the alpha 2-adrenergic antagonist, idazoxan, and is not due to a change in the in vitro rate of association of the receptors with [3H]estradiol. Furthermore, the decrease seems to be a functional decrease in the concentration of receptors capable of cell nuclear accumulation in response to estradiol injection, as indicated by the results of experiments in which the concentration of cell nuclear estrogen receptors was assayed after estradiol injection. These experiments provide further support for the hypothesis that the alpha-noradrenergic system, and perhaps specifically the alpha 2-subtype, is involved in decreasing the concentration of estrogen receptors in parts of the brain and pituitary gland. This interaction provides a mechanism by which the environment could regulate the sensitivity of certain neurons to estradiol. However, the finding that the initial increase in the concentration of cytosol estrogen receptors after yohimbine treatment is not followed by the predicted increase in cell nuclear estrogen receptors after estradiol injection raises questions about the physiological relevance of the apparent increase under some conditions.

Small apomorphine-induced increase in the concentration of cytosol estrogen receptors in female rat hypothalamus and pituitary

A series of experiments was performed to investigate the previously-reported modulation of estradiol binding in female rat brain and pituitary gland by drugs that influence the dopaminergic system. Injection of the dopamine agonist, apomorphine, at minimum doses of 1-2 mg/kg body weight caused slight increases (in most cases, less than 10%) in the concentration of cytosol estrogen receptors in the mediobasal hypothalamus and anterior pituitary gland without influencing the concentration in the preoptic area. However, after subsequent injection of a saturating dose of estradiol, the level of nuclear estrogen receptors accumulating did not differ significantly between apomorphine-treated animals and vehicle-injected controls. These results extend, in part, previous reports that have shown an apomorphine-induced increase in the concentration of [3H]estradiol in brain and pituitary cell nuclei after an injection of [3H]estradiol. However, we failed to observe differences in the concentration of cytosol estrogen receptors as large as would be expected by previous work, and we failed to observe differences in the concentration of nuclear estrogen receptors after estradiol injection.

The alpha 1-noradrenergic antagonist prazosin decreases the concentration of estrogen receptors in female rat hypothalamus

A series of experiments was performed to determine the effects of the alpha 1-noradrenergic antagonist, prazosin, on the concentration of estrogen receptors in female rat brain and pituitary gland. Prazosin caused a dose-dependent decrease in the concentration of cytosol estrogen receptors in mediobasal hypothalamus when injected 10 and 16 h prior to assay. The drug was without effect on the concentration of nuclear estrogen receptors in the absence of estradiol, indicating that the decreased concentration of cytosol estrogen receptors is not due to nuclear estrogen receptor accumulation. Scatchard analysis confirmed that prazosin treatment decreases the concentration of cytosol estrogen receptors without influencing the apparent affinity of the receptors for [3H]estradiol. The prazosin-induced decrease in the concentration of cytosol estrogen receptors in the mediobasal hypothalamus was transient with maximal effects occurring between 8 and 12 h after a single injection. Competition analysis confirmed that prazosin is not an effective competitor for binding to the estrogen receptor in vitro. The effects of prazosin on the estrogen receptor system could not be attributed to modulation of the levels of norepinephrine or dopamine. Assay of the levels of norepinephrine and dopamine in hypothalamus and preoptic area after prazosin injection revealed no effects of prazosin on the level of either of these catecholamines. An estradiol injection resulted in the predicted decrease in the concentration of estrogen receptors accumulating in hypothalamic cell nuclei, suggesting that the cytosol estrogen receptors that decrease in concentration are functional receptors. Prazosin treatment did not result in a decrease in the effectiveness of estradiol in the induction of cytosol progestin receptors in the mediobasal hypothalamus, suggesting that the cells are regulated by the alpha 1-noradrenergic system may not be those cells in which progestin receptors are also induced. These experiments provide further evidence that the noradrenergic system modulates the concentration of estrogen receptors, and perhaps sensitivity to estradiol, in some cells within the rat hypothalamus.